RESOURCE RECOVERY & REUSE SERIES 3 Olufunke Cofie, Josiane Nikiema, Robert Impraim, Noah Adamtey, Johannes Paul and Doulaye Koné 3 Co-composting of Solid Waste and Fecal Sludge for Nutrient and Organic Matter Recovery About the Resource Recovery and Reuse Series Resource Recovery and Reuse (RRR) is a subprogram of the CGIAR Research Program on Water, Land and Ecosystems (WLE) dedicated to applied research on the safe recovery of water, nutrients and energy from domestic and agro-industrial waste streams. This subprogram aims to create impact through different lines of action research, including (i) developing and testing scalable RRR business models, (ii) assessing and mitigating risks from RRR for public health and the environment, (iii) supporting public and private entities with innovative approaches for the safe reuse of wastewater and organic waste, and (iv) improving rural-urban linkages and resource allocations while minimizing the negative urban footprint on the peri-urban environment. This subprogram works closely with the World Health Organization (WHO), Food and Agriculture Organization of the United Nations (FAO), United Nations Environment Programme (UNEP), United Nations University (UNU) and many national and international partners across the globe. The RRR series of documents presents summaries and reviews of the subprogram’s research and resulting application guidelines, targeting development experts and others in the research for development continuum. Science with a human face IN PARTNERSHIP WITH: RESOURCE RECOVERY & REUSE SERIES 3 Olufunke Cofie, Josiane Nikiema, Robert Impraim, Noah Adamtey, Johannes Paul and Doulaye Koné Co-composting of Solid Waste and Fecal Sludge for Nutrient and Organic Matter Recovery ii Donors The authors would like to acknowledge the financial support provided by the CGIAR Research Program on Water, Land and Ecosystems (WLE), and Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung (BMZ) (Federal Ministry for Economic Cooperation and Development), Germany, to conduct the research review presented. The authors Dr. Olufunke Cofie is a senior researcher with a background in soil science. She is based at the IWMI-Ghana office (o.cofie@cgiar.org); Dr. Josiane Nikiema is a researcher in environmental sciences. She is based at the IWMI-Ghana office (j.nikiema@cgiar.org); Dr. Noah Adamtey is a senior scientist at FiBL (Research Institute of Organic Agriculture), Switzerland (noah.adamtey@fibl.org); Mr. Robert Impraim is a research officer at the IWMI-Ghana office (r.impraim@cgiar. org); Dr. Johannes Paul is senior researcher at IWMI with a background in integrated waste management. He is based at IWMI’s headquarters in Colombo, Sri Lanka (j.paul@cgiar. org); and Dr. Doulaye Koné is a Deputy Director, Transformative Technologies, Bill & Melinda Gates Foundation, Seattle, USA (doulaye.kone@gatesfoundation.org). Cofie, O.; Nikiema, J.; Impraim, R.; Adamtey, N.; Paul, J.; Koné, D. 2016. Co-composting of solid waste and fecal sludge for nutrient and organic matter recovery. Colombo, Sri Lanka: International Water Management Institute (IWMI). CGIAR Research Program on Water, Land and Ecosystems (WLE). 47p. (Resource Recovery and Reuse Series 3). doi: 10.5337/2016.204 / resource recovery / environmental effects / nutrients / solid wastes / recycling / composting / faecal coliforms / sewage sludge / urbanization / urban wastes / food wastes / waste management / developing countries / farmyard manure / excreta / soil organic matter / organic wastes / organic fertilizers / public health / health hazards / sanitation / moisture content / temperature / pH / microorganisms / aeration / pathogens / emission / livestock / heavy metals / ISSN 2478-0510 e-ISSN 2478-0529 ISBN 978-92-9090-835-7 Copyright © 2016, CGIAR Research Program on Water, Land and Ecosystems, International Water Management Institute (IWMI). Unless otherwise noted, you are free to copy, duplicate or reproduce, and distribute, display, or transmit any part of this paper or portions thereof without permission, and to make translations, adaptations or other derivative works under the following conditions: ATTRIBUTION. The work must be attributed but not in any way that suggests endorsement by WLE or the author(s). NON-COMMERCIAL. This work may not be used for commercial purposes. SHARE ALIKE. If this work is altered, transformed, or built upon, the resulting work must be distributed only under the same or similar Creative Commons license to this one. Front cover photograph: Taking temperature readings of windrows containing different combinations of fecal sludge based compost. Bangladesh. Photo: Neil Palmer/IWMI Editor: Robin Leslie Designer: Michael Dougherty Bundesministerium für wirtschaftliche Zusammenarbeit und Entwicklung (BMZ) (Federal Ministry for Economic Cooperation and Development), Germany Acknowledgments The research presented in this report benefited from various projects that were conducted in Ghana, West Africa, over the last decade. The authors would like to thank, in particular, the Department of Sanitation, Water and Solid Waste for Development (SANDEC) of the Swiss Federal Institute of Aquatic Science and Technology (EAWAG), Dübendorf, Switzerland, for pioneering the co-composting technology at Buobai (near Kumasi) together with us. Thanks are extended to the Waste Management Departments of the Kumasi and Tema Municipal Assemblies (both in Ghana), and also to our various national research partners, in particular, the University of Ghana, Accra, and the Kwame Nkrumah University of Science and Technology (KNUST), Kumasi. The authors are also grateful to Dr. Pay Drechsel (Theme Leader, Resource Recovery, Water Quality, and Health, IWMI), Dr. Philip Amoah (Senior Researcher – Regional, IWMI) and Dr. Surendra Pradhan (Postdoctoral Fellow, IWMI) for their valuable research inputs. iii CONTENTS List of Tables .................................................................................................................................................... iv List of Figures ...................................................................................................................................................v List of Boxes .....................................................................................................................................................v Acronyms and Abbreviations ..........................................................................................................................vi Summary .........................................................................................................................................................vii 1 Introduction ..................................................................................................................................................1 1.1 Brief on Solid Waste Management ........................................................................................................1 1.1.1 The Waste Situation in Developing Countries ...............................................................................1 1.1.2 Challenges of Solid Waste Management ......................................................................................2 1.2 Brief on Fecal Sludge Management .......................................................................................................3 1.2.1 Fecal Sludge Generation in Developing Countries ........................................................................3 1.2.2 Challenges of Fecal Sludge Management ....................................................................................5 1.3 Use of Animal Manure for Agriculture .....................................................................................................6 1.4 Global Trends of Fertilizer Application ...................................................................................................7 2 Co-Composting of Fecal Sludge and Other Organic Wastes .....................................................................8 2.1 General Overview on Co-composting ....................................................................................................8 2.2 Input Materials for Co-composting .........................................................................................................9 2.3 Health Risks Related to Co-composting ..............................................................................................11 2.4 Waste Pretreatment for Co-composting ..............................................................................................12 2.4.1 Fecal Sludge Pretreatment ........................................................................................................12 2.4.2 Solid Waste Sorting ..................................................................................................................12 2.5 Co-composting Technologies ..............................................................................................................12 2.6 Feedstock and Operation Requirements ..............................................................................................15 2.6.1 C:N Ratio and Other Nutrients ...................................................................................................15 2.6.2 Porosity and Particle Size ..........................................................................................................16 2.6.3 Moisture ....................................................................................................................................16 2.6.4 Temperature ..............................................................................................................................16 2.6.5 pH .............................................................................................................................................18 2.6.6 Microorganisms and Invertebrates .............................................................................................18 2.6.7 Aeration ...................................................................................................................................19 2.6.8 Nutrient Conservation ................................................................................................................20 iv LIST OF TABLES Table 1. Average Amounts of Human Excreta Generation and Nutrient Concentration. .......................................4 Table 2. Fecal Sludge Per Capita Contributions in Various On-site Containments. ...............................................4 Table 3. Average Manure Generation from Common Livestock. ..........................................................................7 Table 4. Examples of Biodegradable/Compostable Materials. ..........................................................................11 Table 5. Typical Characteristics of a Composting Feedstock in Ghana. .............................................................11 Table 6. Advantages and Limits of Composting Technologies. .........................................................................13 Table 7. Maximum Foreign Matter Particles Allowed in Composts in Various National Standards.......................22 Table 8. Indicators to Assess Compost Maturity Level.......................................................................................24 Table 9. Heavy Metal Limits in Compost Based on Standards from European Countries and Canada. ..............25 Table 10. Selected Compost Hygiene Standards from Various Countries. .........................................................26 Table 11. Common Concentrations of Pathogenic Organisms in Excreta and Wastewater. ...............................27 Table 12. Requirements for Compost Plants. ....................................................................................................29 2.7 Amendments and Additives .................................................................................................................21 2.8 Compost Quality .................................................................................................................................22 2.8.1 Stability and Maturity .................................................................................................................22 2.8.2 Enrichment of Compost ............................................................................................................23 2.8.3 Content of Contaminants .........................................................................................................24 2.8.4 Pathogens ................................................................................................................................25 2.9 Compost Post-treatment and Storage ................................................................................................26 2.10 Monitoring ........................................................................................................................................28 2.10.1 Raw Materials.........................................................................................................................28 2.10.2 Process Documentation ..........................................................................................................28 2.10.3 Compost Quality Control .........................................................................................................28 3 Environmental Considerations in Siting of a Composting Facility ............................................................28 4 Benefits of Co-composts ............................................................................................................................30 4.1 Effects on Soil Organic Matter and Physical Properties ........................................................................30 4.2 Effects on Soil Chemical and Biological Properties ...............................................................................30 5 Conclusions ................................................................................................................................................32 References ......................................................................................................................................................34 v LIST OF FIGURES Figure 1. Global Trends in Solid Waste Generation. .............................................................................................1 Figure 2. Average Changes in Waste Composition Related to National Income. ..................................................2 Figure 3. Waste Recycling Framework. .............................................................................................................10 Figure 4. General Material Flow and Main Process Components of Co-composting. .........................................10 Figure 5. Main Factors that Influence Decision-making to Select a Site Specific Composting Technology. .........13 Figure 6. Temperature/pH Curves and Stages of Biotransformation Observed During Composting. ..................17 Figure 7. Ammonia-ammonium Equilibrium as a Function of Different Temperatures and pH. ............................21 LIST OF BOXES Box 1. Farm application of FS and excreta. ........................................................................................................6 Box 2. Controlling Heap Moisture. ....................................................................................................................16 Box 3. Controlling Heap Temperature. ..............................................................................................................17 Box 4. Pathogen Control. .................................................................................................................................18 Box 5. Curing and Maturation Phase. ...............................................................................................................18 Box 6. Necessity of Compost Turning. .............................................................................................................19 Box 7. Aeration Frequencies. ............................................................................................................................20 Box 8. Standard Measurements in Composting. ..............................................................................................24 Box 9. Measuring Heavy Metals in Compost. ...................................................................................................25 Box 10. Electrical Conductivity Measurements. ................................................................................................25 Box 11. Helmith Egg Analysis. ..........................................................................................................................27 Box 12. Monitoring Requirements in Composting. ............................................................................................28 Box 13. Turning an Environmental Challenge into a Business Opportunity. ........................................................31 vi ACRONYMS AND ABBREVIATIONS AIT Asian Institute for Technology, Thailand BOD Biological Oxygen Demand DFS Dried Fecal Sludge EC Electric Conductivity ENC European Compost Network FS Fecal Sludge FSM Fecal Sludge Management GDP Gross Domestic Product (the monetary value of all finished goods and services generated within a country’s borders in a specific time period) HIC High Income Countries IDRC International Development Research Center IFA International Fertilizer Industry Association IWMI International Water Management Institute LFS Raw Fecal Sludge LIC Low Income Countries MIC Medium Income Countries MSW Municipal Solid Waste MSWM Municipal Solid Waste Management NPK Nitrogen, Phosphorus, Potassium SOM Soil Organic Matter SWM Solid Waste Management TKN Total Kjeldahl Nitrogen TOC Total Organic Carbon TS Total Solids TVS Total Volatile Solids UNEP United Nations Environment Programme USEPA Environmental Protection Agency of the United States of America WRAP Waste Resources Action Programme (www.environment-agency.gov.uk) vii SUMMARY Resource depletion, environmental degradation and climate change are among the greatest challenges we face today. In this context, the proper management of generated waste and efficient resource recovery become relevant aspects of environmental management systems that could support a circular economy and assist in addressing these global challenges. However, establishing sustainable waste management systems requires provision of technologies and capacities that fit into the specific socio-economic and geographical conditions of a country. Especially in low income countries, ongoing population growth and rapid urbanization exacerbate waste management issues, while poverty, lack of awareness and technology, expertise and funding constraints hinder the establishment of efficient waste management systems. Although emerging innovations such as mechanical-biological waste treatment, waste-to-energy technologies, engineered landfilling and so forth are available and have proven effective in industrialized countries, they are not ready for uptake in most low income countries yet. In this context, composting, as a low cost technology, remains a valid and relevant option to enhance waste management in developing countries where the bulk of collected solid waste is organic in nature but recycling rates are still low. Although composting reduces municipal efforts and costs, especially for waste disposal, compost operations are often unsustainable, because revenues from compost sales alone are insufficient to cover plant operation costs; hence, subsidy from the municipality served is needed. In this context the ‘value-adding’ aspect of recycling activities needs to be explored to identify innovations that could offer enhanced waste management services and more valuable products for potential users of compost. Biological treatment, in particular composting, is a relatively simple, durable and inexpensive alternative for stabilizing and reducing biodegradable waste. Co-composting is considered as a suitable, low cost, waste treatment option for developing countries that allows recycling of organic waste from various waste streams in a combined manner, e.g. from municipal solid waste and excreta, likewise manure from livestock production. In particular, integration of ‘biosolids’ from the sanitation sector as potential input material for co-composting would provide a solution for the much needed treatment of fecal sludge from on-site sanitation systems. So far, fecal sludge removed from pit latrines or septic tanks is often disposed of close to the points of generation instead of being recycled in a proper manner in many developing countries. By combining various waste streams, new opportunities arise that could not only increase resource recovery rates but also enhance the quality of compost products, e.g. through mixing of selected input materials and additives that increase the content of crop nutrients and enhance application properties. Whereas co-composting offers many benefits, it can also have negative side effects if not properly managed. These include bad odor, leachate and methane emissions, or microbial as well as heavy metal contaminations that decrease the value and applicability of compost products. Therefore special care is needed to treat potential pathogen contaminations that could occur if human excreta or manures are used as input materials for co-composting. This research paper elaborates in detail the main parameters that govern the co-composting process as well as factors that control the production of a safe and valuable quality compost. It further explains technological options and proper design, conduct and monitoring of the co-composting process, including the specific conditions that arise during the main stages of biotransformation until its final maturation phase that delivers a stable, humus-rich and soil-like substrate. viii RESOURCE RECOVERY & REUSE SERIES 3 Pit latrine emptiers in Bangladesh collect and transport human waste to a site where it is processed into fertilizer. Photo: Neil Palmer (IWMI) 1 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY 1 INTRODUCTION Composting provides many benefits. It not only diverts organic materials from disposal in landfills, it also helps to return nutrients and organic matter to the soil, providing a valuable material for agriculture, horticulture and landscaping. This research paper was prepared to provide practical guidance and the latest knowledge related to co- composting of organic waste from municipal waste streams, including human excreta, in order to support planners, researchers, development experts and practitioners in their work. It offers an in-depth review of framework conditions, methods and relevant process parameters that govern co- composting with special attention on the reuse of sensitive input materials such as fecal sludge, manure and municipal organic waste that influence compost quality and offer significant co-benefits for sanitation and agriculture. 1.1 Brief on Solid Waste Management 1.1.1 The Waste Situation in Developing Countries Solid Waste Management (SWM) and Fecal Sludge Management (FSM) are relevant public tasks to enable sustainable and healthy human settlements, but they are severely constrained by various issues in many developing countries. Waste generation and the complexity of waste composition is steadily increasing due to population growth, urbanization and economic development, especially in larger cities. Although emerging innovations such as mechanical and biological waste treatment, waste-to-energy technologies, engineered landfilling and others are available and have proven effective in industrialized countries, they are not ready for uptake in most low income countries. In this context, composting, as a low cost technology, is a valid and relevant option to enhance waste management in developing countries where the bulk of collected solid waste is organic in nature but recycling rates are still low (UNEP 2011; D-Waste 2013). Whereas composting offers many benefits, it can also cause negative side effects if not properly managed. Such negative effects include, offensive odor, leachate and methane emissions, or microbial as well as heavy metal contaminations that decrease the value and applicability of compost products. Current municipal solid waste generation on a global scale is estimated to be approximately 1.3 billion tons year-1, and is expected to increase to approximately 2.2 billion tons year in 2025 (Hoornweg and Bhada-Tata 2012; D-Waste 2013). Based on this forecast, a significant increase in per capita waste generation rates will occur within the next 15 years. At present, average waste generation in industrialized countries varies between 1 and 2 kg person-1 day-1, while waste generation in low income countries is usually much lower with generation rates of 0.4 to 0.8 kg person-1 day-1 (UNEP 2011; Simelane and Mohee 2012; Hoornweg and Bhada-Tata 2012; D-Waste 2013). Figure 1 summarizes relevant trends in global solid waste generation. FIGURE 1. GLOBAL TRENDS IN SOLID WASTE GENERATION. Sources: UN-ESA 2011, 2013; D-Waste 2013. ARGENTIN A SAUDI A RABIA SPA IN JA PA N UNITE D KINGDOM UNITE D ARAB EMIRAT ES GERMANY FIN LA ND UNITE D STA TE S BRAZIL MADAGASCAR INDIA GHANA PHILIP PINES EGYPT ALG ERIA SERBIA CHINA SOUTH AFR ICA MEXIC O SENEGAL HIGH INCOME COUNTRIES MIDDLE INCOME COUNTRIES LOW INCOME COUNTRIES GDP US$/YEAR (W AS TE G EN ER AT IO N IN K G /C PD ) ORG KG/CPD NON-ORG KG/CPD 2.5 2.0 1.5 1.0 0.5 0 50,000 40,000 30,000 20,000 10,000 2 RESOURCE RECOVERY & REUSE SERIES 3 Figure 1 shows that the overall waste generation rate correlates with the economic capacity of a country, whereby countries with lower Gross Domestic Product (GDP) are also lower in waste generation, if compared with higher developed countries. Similarly, if GDP decreases, the capacity to perform waste treatment and recycling activities likewise decreases, meaning the lower the GDP, the less recycling is formally conducted and reported by a certain country whereas the recovery rate from informal sector activities remains unknown. Figure 1 also indicates that municipal solid waste generation in low income countries (LIC) may increase over time due to changes of lifestyle, consumption patterns and extended use of disposable materials, e.g. from additional trade and excessive packaging, provided that economic development and population growth are increasing (Wilson et al. 2009; Hoornweg and Bhada- Tata 2012; Annepu and Themelis 2013). Related to this development trend, the composition of generated municipal solid waste may gradually change as displayed in Figure 2. As shown in Figure 2, changes in waste composition due to economic development also affect the amounts and types of organic waste fraction whereby the latter relatively decreases with economic development. Projections to forecast waste composition changes are relevant for planning and are especially needed to identify and decide on best suited waste treatment options that allow accommodation of expected changes over time with the made investments. According to the United Nations Population Division the global population is projected to increase from 6.9 billion in 2011 to 9.3 billion in 2050 with the highest growth trends in urban and peri-urban areas of LIC (UN-ESA 2011). The ongoing trend of urbanization is mainly driven by economic activities that increasingly place investments in larger cities where access to infrastructure and support mechanisms is highest. Consequently, most citizens perceive cities as more attractive habitations, probably due to increasing job opportunities with higher salary levels and other benefits. This perception becomes a relevant driver for population migration and urbanization and amplifies the role of cities as ‘engines of economic growth’ (Achankeng 2003; Otto et al. 2006; Hove et al. 2013). It is expected that the combined effects of rapid urbanization, population increase and economic development will result in additional waste generation in many developing countries and especially in urban areas, a trend that will most likely trigger environmental degradation and need for intervention (Wilson et al. 2007; UNEP 2011). 1.1.2 Challenges of Solid Waste Management In developing countries, amounts of collected solid waste are usually less than half of what is generated so most of it is neither contained nor recycled (Simelane and Mohee 2012). Instead, it is often disposed of indiscriminately at illegal dump sites, at the periphery of urban centers, buried or burned in backyards, along roads or thrown into drainage systems, idle land or waterways. The magnitude of such malpractices in waste management correlates with the efficiency of available waste collection services, whereas lack of service increases illegal waste dumping, scattered waste burning and pollution of land, drainage systems and waterways. As a result, the aesthetic value of settlements decreases (Simelane and Mohee 2012). Moreover, uncollected waste is a nuisance and could serve as breeding ground for various disease-causing vectors such as mosquitoes, insects and rodents. It also endangers public health, contaminates water sources, causes emissions of odors and greenhouse gases and discourages tourism (USEPA 2002; UNEP 2005). Furthermore, due to clogging of drainage systems through solid waste disposal, FIGURE 2. AVERAGE CHANGES IN WASTE COMPOSITION RELATED TO NATIONAL INCOME. Source: UNEP 2011. HIGH INCOME COUNTRIES MIDDLE INCOME COUNTRIES LOW INCOME COUNTRIES (% WEIGHT) ORGANICS GLASS MIXED RESIDUALSMETALSPLASTICPAPER/ CARDBOARD 3 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY local flooding might increase. Impacts from waste disposal operations especially concern residents who live in the vicinity of dump sites but likewise threaten informal waste pickers who work on these dumps. Waste pickers are recognized as a marginalized and vulnerable group of around 15 million people worldwide (Durand 2013). In total, developing nations presently spend about USD46 billion per year on managing municipal solid waste, whereas these investments could go beyond USD150 billion per year by 2025 (Durand 2013). Although many cities incur high costs for waste management, they achieve poor performances. Often, most of the available budget is spent alone on waste collection which restricts municipalities from establishing additional technologies that could reduce waste generation or increase reuse and recycling, for instance through enhanced material segregation and waste treatment technologies. Current failures in solid waste management (SWM) have been attributed to weak institutional set-ups, financial constraints, inadequate organizational structures and policy responses, low public awareness, poorly designed collection systems, lack of collection vehicles as well as insufficient management and technical skills (Adamtey et al. 2010). In addition, mobilization of the private sector and other stakeholders that could support municipal waste management (MWM) is usually low, as is public support and cost recovery through user fees that would be needed to enable sustainable operation of SWM systems (Flipo 2012). Although the informal sector supports waste management through unorganized material recovery, most work is performed by waste pickers who usually operate on a low organizational level without proper management and adequate work safety measures (Wehenpohl and Kolb 2007; Wilson et al. 2007). Driven by poverty and demand for livelihood, the informal waste sector handles between 15 and 20% of generated MSW without any formal agreement and recognition or support from the served municipalities (Durand 2013). Main actors of the informal sector are waste pickers, also called ‘scavengers’, itinerant or stationary waste buyers, who often act as intermediaries, and consolidators who trade larger amounts of recovered materials (Wilson et al. 2007). Both the informal and formal private sectors are relevant actors in supporting MSWM, particularly related to material recovery and recycling initiatives. Although public authorities are usually aware of their contribution to material recovery, they often neglect to integrate them and especially fail to recognize and reward their efforts at waste management (Wehenpohl and Kolb 2007; Paul et al. 2012). In general, organic wastes represent the main fraction of generated, collected and disposed municipal waste, in developing countries often to the magnitude of 50-70% (UNEP 2011; D-Waste 2013). In low income countries, where the agriculture sector provides the main source of income, composting has clear advantages for municipalities and farmers, but it may not automatically offer mutual win-win options for both parties unless it is properly planned and well-coordinated (Cofie et al. 2014). MSWM departments are mostly aware of the advantages they can gain from composting, especially by reducing cost and efforts for waste collection, transport and final disposal. On the other hand, the unavailability of land for waste disposal and high costs for construction and operation of engineered landfills call for alternative and cheaper management options and application of appropriate technologies (FAO/IWMI 2004). One hindrance for the set up and sustainable operation of composting projects is their dependency on subsidy. Although composting reduces municipal efforts and costs, especially for waste disposal, compost operations are often unsustainable, because revenues from compost sales alone usually cannot cover the plant operation costs. Composting could be performed ‘economically’ if the savings provided for reduced collection and disposal efforts through composting are made available for the project. However, most municipalities fail to provide an adequate processing fee for involved private operators who could secure ongoing recycling or composting operations. 1.2 Brief on Fecal Sludge Management 1.2.1 Fecal Sludge Generation in Developing Countries Globally, about 2.6 billion people do not have access to improved sanitation (WHO/UNICEF 2010; Kvarnström et al. 2012; Rose et al. 2015). Whereas most residents in industrialized countries enjoy flush-toilets and connection to public sewerage systems, most households in developing countries depend on on-site sanitation systems, need to share facilities such as public toilets or have no access to toilets and hence proceed with open defecation. Graham and Polizzotto (2013) estimate that around 1.77 billion people in developing countries depend on pit latrines as their primary means of sanitation. Excreta are the wastes produced from human bodily metabolism and consist of feces and urine (Daisy and Kamaraj 2010). Feces are usually fetid and mainly consist of water, bacteria, nutrients and food residues. They may also contain pathogenic viruses, protozoa cysts and helminth eggs; urine basically comprises of water and large quantities of nutrients that are mostly water-soluble (Vinnerås et al. 2006; Daisy and Kamaraj 2010). Research has shown that excreta generation rates may differ considerably in various regions. Rose et al. (2015) reported average feces generation rates for high income countries as being 126 g cap-1 day-1 (wet weight) and 250 g cap-1 day-1 (wet weight) for low income countries, whereas the main factor affecting fecal mass production is the fiber intake of the population. Feces generation rates show higher variations on a global scale with 100-200 g cap-1 day-1 for Europe, e.g. 140 g cap-1 day-1 for Sweden (Vinnerås et al. 4 RESOURCE RECOVERY & REUSE SERIES 3 2006) but 400 g cap-1 day-1 for Africa (Mann 1999), even up to 540 g cap-1 day-1 for Kenya (Pieper 1987). As for urine, most reviewed research indicates daily generation rates in the magnitude of 1,000 to 1,500 g capita-1 whereas the average dry solids content of urine is reported at 59 g cap-1 day-1 (Rose et al. 2015). The observed variability in the regions may reflect different dietary habits that result from various cultural, economic and climatic conditions. Human excreta are a rich source of organic matter and plant nutrients such as nitrogen (N), phosphorus (P) and potassium (K). About 30 grams (g) of carbon (51.7 g of organic matter), 10-12 g of N, 2 g of P and 3 g of K are produced every day through human excreta (IWMI/Sandec 2002). Most organic matter is contained in feces, while most of the N (70-80 %) and K (60-70%) is contained in urine. Phosphorus is equally distributed between urine and feces. The nutrient content of human urine (Table 1) varies with concentrations from 1.8- 2.6 g L-1 for N, 0.2-0.4 g L-1 for P and 0.9-1.3 g L-1 for K (Kirchmann and Pettersson 1995; Jönsson et al. 2005). Fecal sludge (FS) comprises all liquid and semi-liquid contents of pits and vaults accumulating in on-site sanitation installations, such as public and private latrines or toilets, aqua privies and septic tanks (Heinss et al. 1998; Niwagaba et al. 2014). About one-third of the world’s population (approximately 2.4 billion urban dwellers) relies on such installations (Koné et al. 2010). This situation is likely to last for decades to come, since citywide establishment of sewerage systems is neither affordable nor feasible for most urban areas in developing countries. FS represents a combination of human excreta more or less diluted with flush water and toilet paper, and sometimes other waste types such as tissue paper, food waste, sponges, bones, wood particles, textiles, plant seeds, stones, plastics and sand (Nikiema et al. 2014). Table 2 summarizes the main characteristics of human excreta and related values identified for the various FS containment systems. Raw liquid fecal sludge (LFS) typically contains 8.2 g L-1 of N, 1.1 g L-1 of P, 2.2 g L-1 of K and 21.3 g L-1 of organic carbon TABLE 1. AVERAGE AMOUNTS OF HUMAN EXCRETA GENERATION AND NUTRIENT CONCENTRATION. PARAMETER FECES URINE EXCRETA g cap-1 day-1 (wet) 250 1,200 1,450 g cap-1 day-1 (dry) 50 60 110 Water content (%) 50-95 NUTRIENT CONTENT % OF DRY SOLIDS Organic matter 92 75 83 Carbon (C) 48 13 29 Nitrogen (N) 4-7 14-18 9-12 Phosphorus (P205) 4 3.7 3.8 Potash (K2O) 1.6 3.7 2.7 FOR COMPARISON % OF DRY MATTER N P205 K2O Human excreta 9-12 3.8 2.7 Plant matter 1-11 0.5-2.8 1.1-11 Pig manure 4-6 3-4 2.5-3 Cow manure 2.5 1.8 1.4 Source: Strauss 1999. TABLE 2. FECAL SLUDGE PER CAPITA CONTRIBUTIONS IN VARIOUS ON-SITE CONTAINMENTS. PARAMETER FRESH EXCRETA SEPTAGE1 SLUDGE FROM PUBLIC TOILETS2 PIT LATRINE SLUDGE2 BOD (g cap-1 day-1) 45 1 16 8 TS (g cap-1 day-1) 110 14 100 90 TKN (g cap-1 day-1) 10 0.8 8 5 L cap-1 day-1 1.5 (feces and urine) 1 2 (Includes water for toilet cleansing) 0.15-0.20 1 Based on an FS collection survey conducted in Accra, Ghana. 2 Unsewered systems; only assuming the received portion from pit emptying. Source: Aalbers 1999. 5 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY (C) (Asare et al. 2003). LFS accumulation in septic tanks varies depending on water use habits, construction type, toilet use frequency and other features related to local geographical and socio-economic conditions. In Thailand for instance, LFS from septic tanks was found in variations from 135-180 L cap-1 year--1 (AIT 2012), whereas filling rates of pit latrines were lower in general if compared with septic tanks (Koottatep 2014). The differences are mainly caused by the variability in amounts of water used for cleansing, evaporation and infiltration rates for liquids into the soil and level of degradation during storage. LFS contains higher levels of pathogens (e.g. Ascaris, Trichuris) which could cause harmful diseases if inadequately treated before being released into the environment. Based on the discussed main parameters of human excreta generation, a city with 1 million residents would generate excreta in the magnitude of 45,000 tons feces year-1 and 510,000 m3 of urine. These excreta would contain valuable soil nutrients if recovered as LFS with around 1,200 tons N year-1, 170 tons P year-1 and 330 tons K year-1. 1.2.2 Challenges of Fecal Sludge Management Generally, when fecal sludge (FS) or excreta are collected from on-site sanitation installations, the extracted sludge would need to be treated prior to disposal. However, common practice in many developing countries is to transport FS from tank cleaning directly to dump sites or treatment plants or to dispose it in the vicinity into dug pits, drainage systems, natural depressions, rivers or other water bodies. FS is also being used without further treatment on farmlands, discharged into fish ponds and lakes or discarded on backyards in private compounds (Jiménez et al. 2010). These predominant methods of excreta disposal are applied by most urban dwellers in Africa and Asia as well as in many communities of Latin America. On a global scale, most people tackle sanitation via two approaches which are the ‘drop and store’ and ‘flush and forget’ attitudes (Winblad 1997; Esrey et al. 2001; GTZ 2003). Water-borne sanitation as used in conventional sanitation systems in developed countries is based on the collection and transport of wastewater via a sewer system, using valuable freshwater (often drinking quality), as a transport medium (Lettinga et al. 2001). The system mixes comparatively small quantities of potentially harmful substances with large quantities of water thereby increasing the magnitude of the problem. Thus, the construction, operation and maintenance of such costly flush and discharge systems (sewer, wastewater and sludge treatment) are usually not applicable in developing countries due to high investment and operation costs. Even in developed countries, the chances that such conventional wastewater treatment systems become financially sustainable are considered low, because many sewerage systems are still subsidized substantially (Hauff and Lens 2001). Onsite sanitation systems on the other hand include latrines, aqua privies and septic tanks and constitute the main options for capturing human excreta in low income countries. On a regular basis, they must be emptied either mechanically or manually and treated in a safe manner prior to disposal. However, most FS from tank cleaning is disposed into the environment without treatment in many developing countries. In larger cities, FS collection is more difficult and tank emptying and haulage can face severe challenges. Often, the vehicles used for emptying have no access to pits and traffic congestion prevents efficient emptying and haulage. Besides, the emptying services may be poorly managed and employed laborers are unskilled, and lack proper work safety instructions and equipment. In most cities, there are either no suitable sites for treatment and for final disposal or they are too far away and their use is avoided by haulers due to high transportation cost. To save time, cost and efforts, vacuum tankers discharge their load at the shortest possible distance from the points of collection. The malpractice of FS dumping especially affects squatter areas and low income settlements thereby worsening the health risks of those marginalized and vulnerable groups of urban residents. Children especially are at the greatest risk of coming into contact with indiscriminately disposed excreta (Ingallinella et al. 2002). The conventional forms of sanitation systems are based on the perception of excreta being considered repulsive and ‘not to be touched’ (Stenstroem 1997). Therefore, design of excreta or FS treatment technologies is based on the premise that such waste is only suitable for containment and disposal (Esrey et al. 2001). Proper FS treatment, either in combination with wastewater or separately, is being practiced only in a few developing countries to some extent (e.g. Argentina, Ghana, Benin, Botswana, South Africa, Thailand, Indonesia and China). Although application of untreated FS or excreta on farmlands is attractive because of it is simple and cheap availability of organic matter and plant nutrients, in general, FS has to be treated prior to reuse or disposal. This is because of the high pathogen load and the high water content which makes it difficult to transport the sludge. However, low cost technologies that do not require skilled staff, high investment and energy costs are negligible in low income countries although they are available and applied in industrialized countries (Aalbers 1999). Conventional low cost FS treatment options include batch-operated settling-thickening units; Imhoff tanks; non- aerated stabilization ponds; combined composting with municipal organic refuse or extended aeration followed by pond polishing and anaerobic digestion (Ingallinella et al. 2002). Unfortunately, even these low cost technologies are often considered as ‘too expensive’ in many developing countries. The use of FS and urine could result in various benefits within the urban context, especially for urban and peri-urban farming. Human urine for example significantly increases the yield of spinach, cabbage, tomato and cucumber compared to inorganic fertilizers (Heinonen-Tanski et al. 2007; Mnkeni 6 RESOURCE RECOVERY & REUSE SERIES 3 et al. 2008). Municipal organic residues (both solid and liquid) represent valuable nutrient sources that can be used to improve soil fertility and sustain crop production (see Box 1). Human excreta are a rich source of organic matter and nutrients. When excreta are dewatered, its average N, P and K content is around 2.1, 2.4 and 0.5%, respectively (Adamtey et al. 2010). The fertilizer value of untreated human excreta and urine in cereal, potato, cabbage, cucumber and tomato production has been extensively studied and documented (Cofie et al. 2005; Guzha et al. 2005; Mnkeni et al. 2008). Human excreta also improved maize yields, water productivity (Adamtey et al. 2010; Guzha et al. 2005) and soil nutrient status, especially P and K (Mwakangele 2008). Inappropriate disposal of wastewater and untreated human excreta or fecal sludge can contaminate water bodies and promote the spread of diseases, such as cholera and diarrhoea (MoH 2000; NESSAP 2008). IDRC (1998) estimated that 5.2 million people including 4 million children die every year, most of them living in cities, due to diseases caused by improper disposal of sewage and solid waste. Merchant et al. (2003) reported that child health and growth in developing countries is likely to improve if programs in water and sanitation are introduced to communities which lack these facilities. Providing safe water and adequate sanitation infrastructure as well as the practice of good hygiene are essential for protecting health and socio- economic development. Improvements in FS management can substantially reduce morbidity rates and enhance the quality of life of people living in developing countries, especially children (Mara et al. 2010). Safe FS and wastewater recycling is not only a viable way of tackling increasing urban waste management issues in developing countries, but it also provides additional job opportunities. To date, the upscaling of promising initiatives is hindered by various barriers such as poor planning, low market development, lack of expertise, equipment and funding as well as unhygienic conditions for waste workers. Furthermore, poor stakeholder participation, lack of sectoral policies and enforcement mechanisms, bureaucracy and weak government collaboration may hinder or delay replication of innovative FSM approaches. 1.3 Use of Animal Manure for Agriculture Historically, use of manure and other forms of organic fertilizers was the traditional practice and most widely applied method of nutrient replenishment for crop production worldwide. Although mineral fertilizer application is increasing in many countries, use of manure still contributes a very significant amount of crop nutrients on a global scale. In fact, the main crop nutrients provided by manure may reach a magnitude that is comparable to mineral fertilizer application. According to Potter et al. (2010), manure provided around 152.6 million tons (Mt) of N and P on a global scale in 2007, whereas mineral fertilizer use was reported to be 180.1 Mt (N,P,K) for 2011 (FAO 2012). Manure use is most commonly practiced in South America and Africa, whereas mineral fertilizer application prevails in North America, Europe and parts of China and India (Potter et al. 2010; FAO 2012). The latest global livestock statistics estimate total numbers to be 1.43 billion cattle, 1.87 billion sheep and goats, 0.98 billion pigs and 19.6 billion chickens (Robinson et al. 2014). Although most livestock is scattered over many smaller farms, a significant number is raised in central production facilities where manure management becomes a relevant issue. Use of manure may involve several activities such as collection, drying, treatment and blending with other organic wastes and transporting it to a treatment facility or a farm. Manure can also be used as an energy source, for example as a solid fuel or as biogas through anaerobic digestion that generates methane. A byproduct of anaerobic digestion is sludge which can be used as input material for composting. The choice of using manures for nutrient or energy recovery depends on specific economic conditions, geographical setting, energy and/ or fertilizer demand, market development and various other factors which cannot be generalized. Because of the rising cost of commercial fertilizer and increasing emphasis on sound manure management due to environmental concerns, there is renewed interest in optimizing manure use for farming (Barker et al. 2002). Lately this trend is shared with increasing demand for manure as input material for biogas generation, especially in industrialized countries. Drivers for this development are new options arising from the carbon market that promote offsetting greenhouse gas generation from fossil fuels through use of BOX 1. FARM APPLICATION OF FS AND EXCRETA. Many farmers in developing countries (Asia, Africa or Latin America) are keen to use FS as a readily available resource for agriculture. The usual practice in some parts of Ghana involves informal arrangements between farmers and people who clean latrines. Farmers invite these FS operators to empty their trucks on their farmlands during the dry season. The material is then allowed to dry for three to four months, before being used for the cultivation of cereals at the beginning of the farming season (Cofie et al. 2005). Although there may be monetary benefits for farmers, this practice raises concerns due to possible health risks if safe handling and processing procedures are disregarded. Moreover, the sludge itself can only be transported and placed in septic trucks, which limits its marketing potential. One better option to sanitize the sludge and to produce a safe and easy-to-handle fertilizer would be to apply a controlled treatment process such as co-composting or biodigestion. The possibility of recycling nutrients from human excreta and MSW for use in agriculture creates a unique opportunity to likewise enhance FS management and urban sanitation. 7 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY renewable energies, which can be rewarded through ‘carbon credits’ based on international agreements under the Kyoto Protocol (www.unfccc.org). In general, manure is valued by most farmers and considered as a low cost fertilizer. It could also be used as input material for co-composting depending on the local situation. In developed countries, livestock is kept in larger production sites with more than 70% of all livestock living in such facilities, while 45-80% of the livestock in low income countries is kept in smaller, resource-poor holdings (Herrero et al. 2013). Consequently, special composting projects or biodigesters are being designed to provide a suitable treatment facility for the handling of manure from large livestock production facilities in industrialized countries; however this is not so common in low income countries where compost operators could benefit from co-composting of manure that contains considerable amounts of crop nutrients as summarized in Table 3. Manure management has to consider sanitation aspects and health risks in a similar manner as discussed later in detail for fecal sludge management (FSM). Manure characteristics are influenced by several factors, the most relevant being water content. Dilution by water, such as through rainfall, can cause leaching of nutrients and negatively affect substrate behavior in terms of storage and transportation. Manure drying on the other hand can cause N loss through volatilization. Therefore sound protection from the weather with regard to manure storage by maintaining sufficient substrate water content is a relevant factor for nutrient conservation. TABLE 3. AVERAGE MANURE GENERATION FROM COMMON LIVESTOCK. LIVESTOCK AVERAGE ANIMAL WEIGHT (KG) TOTAL MANURE (T YR-1) (FECES+URINE) TS (%) NH4-N1) P205 2) K203) Cattle (meat) 360 8.3 14.7 1.8 3.3 4.0 Cattle (dairy) 630 22.3 13.9 0.9 2.3 3.7 Pigs 60 1.9 10.3 3.4 4.2 4.0 Sheep 30 0.4 28.1 2.6 4.3 8.6 Horses 450 9.2 29.6 1.1 2.9 5.4 Chickens 0.9 0.024 25.6 3.0 7.4 5.3 1 Nitrogen as ammonia in kg ton manure-1 2 P in kg ton manure-1 3 Potash in kg ton manure-1 Source: Barker et al. 2002. 1.4 Global Trends of Fertilizer Application In 2011, the Food and Agriculture Organization of the United Nations (FAO) reported annual global fertilizer consumption of nitrate-phosphorus-potassium (NPK as N+P2O5+K2O) to be 180.1 Mt (FAO 2012). In 1990, the figure of 143.7 Mt was recorded (Bumb and Baanante 1996). With 1990 as a baseline, this indicates average global increase of fertilizer use of around 1.2% per annum during the last two decades. In parallel, agricultural production has increased 2.5-3 times during the last 50 years, supported by the use of mineral fertilizer, enhanced irrigation, improved seeds and better farm management technologies (FAO 2011). Out of the 179 Mt of NPK fertilizer applied on 1,563 billion hectares of arable land in 2012, N, P2O5 and K2O constituted 109, 41 and 29 Mt respectively (Drechsel et al. 2015). Asia was the region with highest fertilizer demand (East Asia and South Asia accounting for 38 and 18% of global consumption respectively) while Africa only consumed 3% (IFA 2014). Chemical fertilizer application emerged as a new global farming practice following the ‘green revolution’ that developed yield enhancing techniques (Mann 1999). This was supported by development of the Haber–Bosch ammonia synthesis process and a worldwide trend of mining rock phosphate and potash. These technologies allowed easier access to crop nutrients and modified agricultural practices to increase crop productivity (Tilman 1998; Vitousek et al. 1997). While manure application was the traditional practice to provide soil nutrients on farms, mineral fertilizers became widely available only in the mid-twentieth century. Since then, the intensification of existing agricultural activity through increased fertilizer application, rather than cropland expansion, has been a primary driver to increase food production (FAO 2002). While the benefits of intensified mineral fertilizer use provided higher crop yields, it has also resulted in widespread degradation of soil fertility and water quality (Richter 2007; Vitousek et al. 2009). Nutrients applied to croplands can leach into aquatic systems and alter ecosystem functions (Smil 2002). For example, excess nutrients can stimulate the growth of algae and other aquatic plants, and consequently the natural decomposition of this additional organic matter in water bodies consumes dissolved oxygen and degrades growth conditions. Some regions of the world have ample access to soil nutrients, while many others are adversely impacted by declines in soil fertility, especially where farmers do not have the means to replace the nutrients removed through crop harvesting or residues (Vitousek et al. 2009). Sub-Saharan Africa (SSA), 8 RESOURCE RECOVERY & REUSE SERIES 3 for instance, suffers from low crop yields due to negligible replacement of crop nutrients and organic matter over the past decades (Smaling and Dixon 2006; Vitousek et al. 2009). In fact, average fertilizer use in SSA was only 11 kg NPK ha-1 year-1 compared to average fertilizer use of > 100 kg NPK ha-1 year-1 in other regions (Drechsel et al. 2015). In many countries, fertilizer management is significantly influenced by two mechanisms: i) fertilizer subsidy and ii) nutrient management practices. In developing countries, fertilizer subsidy may be required to support smallholders’ livelihoods and crop production for some decades to come, whereas in high income countries new quality standards are emerging to safeguard food production and environmental capital, which also influence fertilizer application. Furthermore, global market trends and price changes for fertilizer likewise affect fertilizer use (Heffer and Prud’homme 2014). As fertilizer demand often cannot be met in developing countries, the recycling of organic wastes could be a ‘window of opportunity’, if combined recovery and treatment of organic materials from various waste streams can be introduced and maintained on a larger scale. 2 CO-COMPOSTING OF FECAL SLUDGE AND OTHER ORGANIC WASTES 2.1 General Overview on Co- composting Composting is the biotransformation of organic substrates in the presence of oxygen. The composting of organic material or waste allows the recovery of nutrients and organic matter for use in agriculture. Composting is a biological transformation that includes mineralization and humification of organic materials under controlled conditions into humus, whereas the latter represents a complex group of macromolecular organic compounds with high stability for safe use in agriculture. The composting process also reduces the mass and volume of organic materials through microbial degradation of organic matter and C in the form of CO2 (Banegas et al. 2007; Gu et al. 2011; Shan et al. 2013). The composting process generates heat which creates an environment necessary for the deactivation of pathogens and seeds. The quality of the final compost depends on the control of various factors during composting which are: nutritional composition of the feedstock, C:N ratio, particle size, pH, temperature, moisture content, aeration and operational parameters such as turning frequency and monitoring. Understanding and appropriate application of these factors are major prerequisites for successful composting (UNEP 2005). Haug (1993) provided an often cited definition of composting as follows: “Composting is the biological decomposition and stabilization of organic substrates, under conditions that allow development of thermophilic temperatures as a results of biologically produced heat, to produce a final product that is stable, free of pathogens and seeds and that can be beneficially applied to land”. Composting can include a wide variety of biosolids and organic wastes. In farming, composting of crop residues mixed with manures from livestock production was and is a common practice on a global scale. However, co-composting of FS with organic solid wastes is less widespread to date and replication of this recycling option will depend largely on country-specific context and socio- cultural conditions. Co-composting of FS is considered as a low-cost and appropriate technology to enhance sanitation and waste management in low income countries, especially in urban areas where on-site storage of FS is the main sanitation option for most households but proper treatment of removed sludge is often lacking. As far back as 1987, Obeng and Wright of the World Bank and the United Nations Development Programme (UNDP) reviewed available literature and prevailing practices on the co-composting of human waste together with organic solid wastes. They highlighted the following key issues for consideration in planning for co-composting in developing countries: available waste materials, market for compost, type of technology, scale of composting, as well as benefits and justification for co-composting (Obeng and Wright 1987). Cofie and Koné (2009) conducted in-depth research on the process dynamics of co-composting of fecal sludge and organic solid waste for agriculture and presented various options and performance data for combined treatment of FS and municipal solid waste (SW) through co-composting. The objectives were to investigate the appropriate SW type, SW/FS mixing ratio and the effect of turning frequency on compost maturity and quality. Solid waste from markets (MW) and households (HW) was combined with dewatered FS in mixing ratios of 2:1 and 3:1 by volume and aerobically composted for 90 days. Four composting cycles were monitored; the results were used to establish appropriate SW types and mixing ratios. Another set of five composting cycles was monitored to test two different turning frequencies: (i) turning once in three to four days during the thermophilic phase and every 10 days during the maturation phase and (ii) turning once every 10 days throughout the composting period. Samples were taken at every turning and analyzed for total solids (TS), total volatile solids (TVS), total organic carbon (TOC), electrical conductivity (EC), pH, ammonium and nitrate nitrogen (NH4–N and NO3–N) and total Kjeldahl nitrogen (TKN). Temperature, C:N ratio, NO3–N/NH4–N ratio and cress planting trials were chosen as maturity indicators. Results showed a preference for MW over HW and a mixing 9 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY ratio of 2:1 over 3:1. There was no significant effect of different turning frequencies on the temperature changes and the quality of mature compost. The final compost product had a C:N ratio of 13:1 and a NO3/NH4-ratio of about 7.8, while TVS was about 21% TS and the NH4–N content was reduced to 0.01%. A co-composting duration of 12 weeks was indicated by the cress test for achieving a mature and stable product within this research whereas a turning frequency of 10 days was chosen that allowed safe compost production with fairly high nutrient content. In order to check options that could enhance co-composting with useful additives, Wong et al. (1997) conducted a series of co-composting tests that applied various ashes from coal power plants in China. In particular, this research was conducted to check the feasibility of using coal ash residues as additives to enhance co-composting with sewage sludge. Alkaline coal ash residues produced from a coal-fired power plant were co-composted with sewage sludge to evaluate their effects on heavy metal availability and the biological process of composting. Coal fly ash (FA) and lagoon ash (LA) were mixed with dewatered sludge adding 0, 10 and 25% (mass) to the dewatered sludge and the mixtures were composted for 100 days in laboratory batch reactors. The changes in pH, electrical conductivity (EC), CO2 production, microbial population, soluble and extractable heavy metal contents were measured during the composting period. Following an initial increase, pH started to decrease from day 7 onward till the end of the composting period for all treatments. Sludge with FA amendment had a higher pH and EC than that of the control and LA-sludge composts. Increasing FA amendment levels resulted in a significant reduction in DTPA-extractable Cd, Cu, Zn, Mn and Pb contents of the FA-sludge composts while the reduction was less obvious in the LA-sludge composts. No significant difference in CO2 production and number of thermophilic bacteria was observed for all treatments except for 25% FA-sludge compost which had reduced thermophilic bacterial growth and CO2 production. The inhibition, which was possibly due to the high pH of FA, decreased with an increase in composting time. It was concluded that the co- composting of coal ash residues with sewage sludge was suitable for reducing metal concentrations in the compost product but did not exert a significant inhibition on the biological process of composting, except for 25% FA-sludge compost. 2.2 Input Materials for Co- composting As a relevant initial step to assess the options and dimensions of a potential co-composting project—and later on to select the best suited co-composting technology—a thorough assessment of the waste generation situation and availability of suitable input materials and additives for co-composting is needed. Based on recommendations from prior research, an approach for identification of best possible technology options should consider and analyze the following key aspects of recycling (Drechsel and Kunze 2001; Cofie et al. 2008): � Waste generation (quantitative and qualitative waste supply analysis); � Compost demand by potential users (market analysis, willingness and ability to pay); � Waste processing and scales (technical options considering supply vs. demand); � Economic analysis (competing products, collection and processing costs, best locations, economies of scale, and subsidy sourcing); and � Options and constraints related to legal, institutional and local communal settings. The following discussion focuses on relevant features and parameters for selection of input materials that significantly influence co-composting as well as the pre-treatment for the composting process itself; other criteria mentioned in the waste recycling framework of Figure 3 are not discussed in detail in this publication. Figure 4 displays the general material flow in a co-composting process. It starts with input materials on the upper left and progresses to pre-treatment activities such as sorting, drying and mixing, the co-composting process and final product distribution for either farm application or for other uses. Related details and relevant parameters for composting will be discussed in the following subsections. Feedstock materials for composting should be selected according to availability, cost and quality aspects and properties that favor the biotransformation process such as carbon and water content and appropriate C:N ratio. Carbon content should be at least 50% dry weight. Preferably, the material should be amenable to microbial decomposition and cost effective to use (e.g. locally available), but also suited to the proposed or applied composting technology. Although the composting of unconventional waste such as used disposable diapers together with yard waste has been reported (Espinosa-Valdemar et al. 2014), such waste would require specific adjustment and additional pretreatment technology. Manure and sludge can be processed through composting. However, due to their compactness and high moisture content, in most cases addition of a bulking agent is required to provide structural support, e.g. to create voids between particles that facilitate the composting process (Doublet et al. 2011). The types of bulking agent used have little effect on the level of organic matter stabilization and N availability in the final compost, but the time to reach organic matter stability is significantly influenced by the type of bulking agent used (Doublet et al. 2011). Additionally, the particle size of the bulking agent in the final mixture is an important factor to enhance the sludge composting process and mainly controls aeration (Wong et al. 1995; Larsen and McCartney 2000). The bulking agent may also have 10 RESOURCE RECOVERY & REUSE SERIES 3 a diluting effect on toxic substances present in waste, e.g. sewage sludge. The type and proportion of bulking agent used will also influence the rate of decomposition, nutrient, carbon and water content and the final compost quality (Banegas et al. 2007). The most commonly used bulking agents are fibrous carbonaceous materials with low moisture content (Miner et al. 2001). Examples include cereal straw, cotton waste, husks, wood chippings or leaves, fruit pods or sawdust and materials that usually have a C:N ratio in the range of 50:1 to 80:1. In order to minimize additional operational cost incurred by purchasing, transportation and storage of the bulking agent, this input material may be adjusted to the lowest possible level but added to provide sufficient pore space in the compost matrix during the compost process (Ponsa et al. 2009). According to Gea et al. (2007), bulking agents with low particle size may offer FIGURE 4. GENERAL MATERIAL FLOW AND MAIN PROCESS COMPONENTS OF CO-COMPOSTING. ORGANIC WASTE FROM MSW FECAL SLUDGE REJECTS TO LANDFILL MANURE SORTING MIXING, BLENDING AERATION, TURNINGDRYINGDRYING ADDITITVES QUALITY CONTROL FARM APPLICATION OTHER MARKETS COMPOST PRODUCT DRYING PACKING CO-COMPOSTING SHREDDING (OPTIONAL) FIGURE 3. WASTE RECYCLING FRAMEWORK. 1. WASTE SUPPLY ANALYSIS PLANNING A CITY OR COMMUNITY BASED COMPOSTING PROJECT 2. COMPOST DEMAND ANALYSIS 3. ASSESSING COMPOSTING TECHNOLOGY AND VOLUME 4. ECONOMIC ANALYSIS AND SUBSIDY SOURCING 5. LEGAL, INSTITUTIONAL AND COMMUNAL FACTOR ANALYSIS Source: Cofie et al. 2008 11 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY better homogenous pore size distribution which acts as an efficient oxygen diffuser as well as an effective water absorber compared with bulking agents of larger particle size. Appropriate volumetric ratios for bulking agents with dewatered sludge were reported (1:1 to 3:1) by Gea et al. (2007) and Ponsa et al. (2009). In the selection of feedstock, both green and brown organic waste should be considered in order to create optimum conditions for microbial activity during composting. Green or fresh waste or feedstock are in general higher in N content (a few percent in dry weight) or have a low C:N ratio (<30:1). Examples include fruits, vegetables, manure, fresh yard waste and kitchen scraps. Brown waste on the other hand usually have a high carbon content or higher C:N ratio (>30:1). Examples of brown feedstock are straw, rice husk, maize stalks, sawdust and other woody residues (Willson 1989; Bayard and Gourdon 2010). TABLE 4. EXAMPLES OF BIODEGRADABLE/COMPOSTABLE MATERIALS. SOURCE OF MATERIALS TYPE OF WASTE Residences and gardens Garden trimmings, leaves, grass cuttings Restaurants and canteens Raw peelings and stems, rotten fruits and vegetables and leftover food Market Organic waste of vegetable and fruit markets Agro-industries Food waste, bagasse, organic residues Parks and road verges Grass clippings, branches, leaves Municipal areas Residential solid wastes, human and animal excreta Dumping sites Decomposed garbage Animal excreta Cattle, poultry, pig dung from urban and peri-urban farms Slaughterhouses Contents of digestive system TABLE 5. TYPICAL CHARACTERISTICS OF A COMPOSTING FEEDSTOCK IN GHANA. PARAMETERS UNIT HOUSEHOLD WASTE MUNICIPAL ORGANIC WASTE DEWATERED FECAL SLUDGE pH 8.44 ± 0.68 9.04 ± 0.37 6.21 ± 0.99 Acidity cmol kg-1 1.03 ±1.27 2.15 ± 1.48 2.30 ± 1.61 Moisture % 50.65 ± 0.92 68.05 ± 1.34 42.30 ± 0.42 Carbon % 30.20 ±14.90 32.81 ± 19.08 11.39 ± 7.70 Nitrogen % 1.43 ± 0.33 1.25 ± 0.93 1.05 ± 1.02 C:N 31.44 ± 6.93 28.49 ± 6.00 18.22 ± 11.12 K % 1.30 ± 0.64 0.94 ± 0.03 0.39 ± 0.41 Ca % 5.37 ± 3.77 6.17 ± 2.64 0.76 ± 0.54 Mg % 2.32 ± 0.73 3.20 ± 2.93 3.29 ± 3.07 P % 0.46 ± 0.30 0.54 ± 0.07 1.02 ± 0.36 E. coli 108 CFU g-1 5.03 ± 0.91 5.70 ± 3.54 4.07 ± 2.04 Total bacteria 108 CFU g-1 7.17 ± 2.75 2.71 ± 2.40 6.10 ± 1.05 Total fungi 106 CFU g-1 5.10 ± 0.87 5.75 ± 5.02 4.67 ± 1.54 Clostridium 108 CFU g-1 5.30 ± 1.30 4.50 ± 3.82 4.93 ± 1.48 Helminth Eggs Gts-1 25-83 Source: Cofie and Koné 2009. Table 4 summarizes potential materials that could be used for composting. Organic waste input materials must be free from chemical contaminants. Consequently, input materials that contain potential hazardous wastes (e.g. hospital waste) should not be used for composting. In general, source separated materials are better and less prone to contamination. If mixed waste from MSW is used as input material, appropriate technologies for sorting and removal of hazardous materials should be provided. The material quality of various feedstocks was tested in Ghana based on relevant parameters as shown in Table 5. 2.3 Health Risks Related to Co- composting Pathogenic organisms in wastes can cause diseases. Various studies have reported microbial risks from excreta 12 RESOURCE RECOVERY & REUSE SERIES 3 use in agriculture (Feachem et al. 1983; Hussain et al. 2002). The survival of excreted pathogens in soils and crops is an important factor in determining the risk related to reuse or recycling of human waste. Factors that may affect the survival time of enteric bacteria in soil are numerous. In general, a greater survival time is observed at low temperatures (i.e., winter versus summer), in the presence of high water/ moisture levels (i.e., in moist soils, during times of high rainfall) or in soils with greater water-holding capacity (versus sandy soils). Increased survival and possible regrowth are also observed when sufficient amounts of organic matter are present. However, survival time is lower in acidic soils (pH 3-5) than in alkaline soils (Westcot 1997). Chemical contamination is a potential risk associated with waste recycling, especially if input materials are of industrial origin. As organic solid waste is often stored and collected together with other waste fractions, contamination of the organic fraction by chemical constituents or heavy metals in particular is possible. When applying contaminated compost, these constituents can accumulate in soils and the potential uptake by crops would result in chronic and long-term toxic effects in humans (Singh and Kalamdhad 2012). Metals in municipal waste come from a variety of sources. Batteries, consumer electronics, ceramics, light bulbs, house dust, street sweepings, paint chips, used motor oils, plastics and some inks and glass can all introduce metal contaminants into the solid waste stream (Smith 2009). Composts may inevitably contain these elements, although mostly in low concentrations, even if foreign elements have been removed through sorting. In small amounts, many of these trace elements (e.g. boron, zinc, copper, and nickel) are essential for plant growth. However, in higher amounts they may decrease plant growth. Other trace elements (e.g. arsenic, cadmium, lead and mercury) are of greater concern primarily because of their potential to harm soil organisms or plants or by entering the food chain. The impact of these metals on plants grown in compost-amended soils depends not only on the concentration of metals and soil/compost properties as mentioned above, but also the kind of crop grown. Different types of plants can absorb and tolerate metals differently. Special care might be needed if, for example, mushrooms are cultivated on soil ameliorated with compost that contains mercury or cadmium. The application of composts might, however, increase the metal content of uncontaminated soils. This could also pose a risk to animals in the area who might ingest the composted soil directly. Further nonpathogenic risks result from impurities of non- biodegradable origin such as glass splinters or other sharp objects contained in the compost product. Such impurities can result from insufficiently sorted municipal solid waste before or after the composting process. These risks also include indirect health risks due to the attraction and proliferation of rodents and other disease-carrying vectors (Furedy and Chowdhury 1996). Health risks can be minimized if adequate control measures are consistently practiced, and co-composting workers adopt basic precautions and hygienic practices (Keraita et al. 2006). As most risks are related to the composition of the waste material, the quality of separation is a crucial indicator for risk reduction. The second factor is the composting process. If correct compost temperatures can be obtained in all parts of the pile (e.g. through turning), risks related to pathogens will be minimized as reported by various research (Cofie and Koné 2009; Koné et al. 2007). Another strategy for risk reduction is the continuous monitoring of compost quality and the provision of sanitation facilities for compost workers. 2.4 Waste Pretreatment for Co- composting 2.4.1 Fecal Sludge Pretreatment Depending on the source of FS, some form of pretreatment will be needed prior to co-composting. Usually human excreta from public toilets and septic tanks are too high in moisture content (95-97%) and need to be dewatered prior to composting with organic solid waste to ensure aerobic composting. This requires the use of solid-liquid separation systems such as unplanted drying beds, constructed wetlands or thickening/settling tanks. The effluent from these systems must be treated (for example in facultative and maturation ponds, constructed wetlands) to meet discharge guidelines before being discharged into receiving water bodies. The effluent can also be used for watering the compost windrows at the early stages of composting or as irrigation water in peri- urban farming provided its quality meets the standards set for unrestricted irrigation. Nikiema et al. (2014) provide more information on selected solid-liquid separation technologies that can be used prior to co-composting. 2.4.2 Solid Waste Sorting As solid wastes could have negative impacts on the final compost quality, it is important to ensure proper separation of organic from inorganic and especially hazardous materials. Usually an organic fraction of household waste, market waste or agro-industrial waste is recommended for use in co-composting. The solid waste should be mixed with the pretreated (e.g., dewatered FS) in the appropriate proportion to ensure an optimal composting process (Cofie et al. 2009). 2.5 Co-composting Technologies Two main types of composting systems are generally distinguished: 1) open systems such as windrows and static piles and 2) closed ‘in-vessel’ systems. These in- vessel or ‘reactor’ systems can be static or movable closed structures where aeration and moisture are controlled by mechanical means. Such systems usually require an external energy supply, either by electricity or through decentralized electricity generators, whereas the latter is often provided by diesel engines. In general, in vessel or reactor systems 13 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY require higher investment compared with static systems and are also more expensive to operate and maintain. Static composting systems on the other hand, require much lower investments and are hence the preferred option for composting in developing countries. Among them, windrow composting is the most commonly applied system. The identification of the best-suited option for composting depends on numerous parameters. The main choices to be made are related to a) scale (household, community, commercial), b) input materials, c) business models (public, private or combined), d) demand and market situation, e) investment and operation cost, f) technology option and equipment, f) standards and legal framework and g) environmental and health concerns as shown in Figure 5. Decision-making has to be done on a case-by-case basis aiming at the highest possible cost- and co-benefits and sustainability level for the operator, community, stakeholder and the environment. Different technological options are available to establish a specific composting project, as presented in Table 6. TABLE 6. ADVANTAGES AND LIMITS OF COMPOSTING TECHNOLOGIES. KEY FEATURES ADVANTAGES DISADVANTAGES St at ic p ile a, 1 � Static piles are the simplest form of composting � Typically larger than heap size whereas heaps are usually not turned � Generally ideal for feedstock with larger particle size and higher porosity � Requires minimal management and equipment � Aerobic conditions can be achieved if the porosity in the initial pile is high (>60%) and if there is a high proportion of bulking materials to keep pores open for air exchange � While simple, this method takes longer to produce matured compost; the final product is often quite heterogeneous due to the lack of mechanical treatment and physical breakdown of feedstock during the process. � Anaerobic conditions can occur in the core of the heap which can also result in odor emissions Tre nc h an d pi t c om po st ing a,2 � Characterized by heaps which are partly or fully contained under the soil surface � Structuring the heap with bulky material or turning is usually the choice for best aeration � In some cases, composting materials are completely buried in the trench which then serves as a planting bed � Requires low capital investment � Requires less moisture, thus suitable for dry areas � Control of leaching is difficult in trench or pit composting � Monitoring the composting process is difficult � The process is labor-intense, especially digging of the pit and emptying it FIGURE 5. MAIN FACTORS THAT INFLUENCE DECISION-MAKING TO SELECT A SITE SPECIFIC COMPOSTING TECHNOLOGY. SELECTING A COMPOSTING TECHNOLOGY TECHNOLOGY SCALE ST AN DA RD S LEGAL FRAMEWORK LO C AL SETTIN G BUSINESS M ODEL IN PU T M AT ER IA LS O & M COST D EM AN D INVESTMENT HOUSEHOLD COMMUNITY COMMERCIAL PRODUCT QUALITY WORK SAFETY HEALTH ENVIRONMENT PUBLIC PRIVATE COMBINED CONTINUED 14 RESOURCE RECOVERY & REUSE SERIES 3 KEY FEATURES ADVANTAGES DISADVANTAGES Ae ra te d st at ic p ile /h ea p b, 3 � Aerated static pile (ASP) composting is comprised of forcing (positive) or pulling (negative) air through the pile. � In a static aerated pile, a 15-30 cm thick layer of finished compost or wood chips is placed all around the MSW pile to provide insulation. This arrangement minimizes odor generation and also leads to uniform sustained heating of waste leading to destruction of plant pathogens and weed seeds � The ASP can be used together with other composting technologies at the curing stage � The land requirements for this method are lower than that of windrow composting � The technology allows for capturing and treating air to reduce odor generation � Large volumes of feedstock can be treated with the help of aeration systems � The primary disadvantage of using this technology is the lack of mechanical agitation, which slows down physical breakdown of materials � Usually suitable for feedstock of similar consistency and homogenity � The compost pile/heap can dry out quickly and therefore requires regular monitoring � The aeration system may require capita- intensive installations W in dr ow c om po st in g a, b, 4 � The material is piled up in heaps or elongated heaps (called windrows) � Suitable for outdoor composting in piles that rely on passive, manual or mechanical aeration � Some portions of waste piled up in the windrows may not be exposed sufficiently to a temperature of over 55 °C for a period of 7-10 days � Can be low cost � Windrow composting produces the highest volume reduction compared to static piling (passively aerated with minimum turning) and forced aeration (static aerated pile) � Introducing air mechanically speeds up the composting process and greatly reduces emissions of methane � Methane emissions from windrow composting are comparably lower, e.g. passively aerated piles produce higher methane emissions (x100) than windrow turned piles whereas forced aeration piles produced even 1,000 times greater methane emissions � Anaerobic conditions could occur in the core of large piles or windrows, and together with a larger emitting surface, could result in odor generation � Such plants often experience resistance from the community where they are set up � Should be sited with consideration of the risk of odor � Workers are in close contact with material during composting � The minimum windrow/pile size must be 3 m3 In -v es se l ( En cl os ed ) c om po st in g a, b, 5 � Refers to a group of composting systems, which range from enclosed halls to tunnels and containers, rotary drum or bins � Often have one exhaust air outlet � Allows easy collection and discharge (through a chimney) or treatment of air (e.g. biofilter) to minimize emissions of odors and greenhouse gases � Operating temperature is uniform, more efficient in sterilizing the compost compared to open composting techniques � Production of leachate is low (can be recycled if any) � Requires less processing time (2-3 weeks) and less labor � Less land requirement � Effect of weather on the composting process is limited � Public acceptance of the facility is higher � More costly than other units and, in addition, more equipment maintenance is required � Skilled labor required for operation and maintenance � Comparable higher investment cost and energy consumption � Additional cost for operation and maintenance � There is a need to treat exhaust air Ve rm ic om po st in g a, b, 6 � A non thermophilic, biooxidative process that uses earthworms and associated microbes to transform organic waste into rich humus, similar to compost � Local varieties of both surface and burrowing earthworms can be used � In broad-scale vermiculture7, the earthworms are introduced to organic waste piled in elongated rows that are covered with protection layers to prevent water logging1 � Appropriate process indicators are survival rate, biomass production and reproduction of earthworms � Both pathogens and weed seeds can be destroyed in the intestines of worms during vermicomposting. Protozoa and fungi are important parts of their diet � The earthworms mix, grind, aerate, fragment and digest waste � Vermicomposting hastens the decomposition process by 2-5 times � Produces much more homogeneous materials compared to thermophilic composting � It is particularly suited to urban agriculture because it can be applied in a variety of settings and at different scales � A pre-composting may be required before earthworms are added to the mixture � Tolerates temperatures between 0 and 40 °C with pH of 7, while optimal growth is at temperatures from 25-40 °C � Optimal moisture content: 40-45%. Higher moisture content may result in the death of earthworms � Organic matter is rich in nitrogen � Sorting is required after composting to allow removal of earthworms � Earthworms may die when conditions are unfavorable; e.g. anaerobic � They may be affected by pests/mites a decentralized; b centralized Sources: 1 Cooperband 2002; Hansen et al. 1995 (available at http://ohioline.osu.edu/). 2 Strauss et al. 2003. 3 Nema 2009; Composting Council of Canada 1999. 4 Cooperband 2002; Gruneklee 1998; Nema 2009; Lopez-Real and Baptista 1996; Brinton 1998. 5 Litterick et al. 2003; Singh et al. 2012; Cooperband 2002; USEPA 2000. 6 Adi and Noor 2009; Pathma and Sakthivel 2012; Bhatnagar and Palta 1996; Atiyeh et al. 2000. 7 Mid-to-large-scale vermiculture is an emerging composting approach that is being increasingly applied in developing countries (Guerro and Guerro-del Castillo 2005; Sherman-Hunloo 2000). TABLE 6. ADVANTAGES AND LIMITS OF COMPOSTING TECHNOLOGIES. (CONTINUED) 15 CO-COMPOSTING OF SOLID WASTE AND FECAL SLUDGE FOR NUTRIENT AND ORGANIC MATTER RECOVERY All composting technologies allow production of a safe recycling product but require variable processing time, process control, human and financial resources while having different impacts on the environment and health. The degree of compost stability attained within a certain time is a key indicator which can be used to compare different composting techniques (Singh et al. 2012). Decomposition of organic matter through composting can be achieved in the presence or in the absence of oxygen. Therefore, different composting methods involve either aerobic (with oxygen), anaerobic (without oxygen) phases and sometimes even alternate between the two during the decomposition process. Under anaerobic conditions, composting is often achieved at mesophilic temperatures with the disadvantage that the process temperature may be too low to efficiently eliminate pathogens that are especially present if organic input materials from municipal waste management, manures and fecal sludge are utilized for composting. Anaerobic conditions may also generate strong odors which could pose a major nuisance in urban areas. Conversely, under aerobic conditions, composting is achieved at thermophilic temperatures due to the accelerated growth rate of bacteria that results in a higher biodegradation rate of the waste. As a result, pathogens are more quickly eliminated. A composting facility which is not well managed could generate odor that can expand over a radius of 2 to 3 kilometers (km) around the plant and bother residents. This could even be a reason for plant closure for example the compost plant at Thane (near Mumbai) in India that had to be dismantled in 2002- 2003 after court intervention (Nema 2009). In a similar case, the Woodhue composting facility in New Jersey, USA in 2004, had to divert food residues used as input material for composting for 2 months following complaints from people living nearby the site (Goldstein and Goldstein 2005). 2.6 Feedstock and Operation Requirements In the following section, key factors affecting the biological decomposition process and resulting compost quality are discussed. The formulation of a feedstock largely influences C:N ratio, porosity, moisture content, pH and nutrient content, while other factors actively influence but also change during the composting process such as moisture content, microorganisms involved, temperature, aeration and nutrient loss. 2.6.1 C:N Ratio and Other Nutrients Carbon, water, nitrogen, phosphorus and potassium are the main substances needed to enable optimum microbial activity during composting besides sufficient aeration. Their availability during the process significantly influences the compost product and its value (Cooperband 2000; Turovskiy and Westbrook 2002; Tognetti et al. 2007). Carbon is the primary energy source for microorganisms while N, P and K are the primary nutrients. During composting, even though P is sometimes limited, C and N (which serve for building cell structure) are the main elements to be monitored closely. Although bacteria also need trace amounts of micronutrients such as sulphur (S), sodium (Na), calcium (Ca), magnesium (Mg) and iron (Fe), these elements are usually present in waste at sufficient quantities and therefore often have a limited impact on microbial growth but do not limit bacterial activity (Hoornweg et al. 2000). The ideal ratio of C:N to enable composting should fall between 25:1 and 35:1 (Strauss et al. 2003; Bernal et al. 2009; Guo et al. 2012). This ratio corresponds with the fact that most bacteria need approximately 30 g of C for 1 g of N uptake. Insufficient N (i.e., C:N ratio > 35) will hinder microbial growth, which will slow down the composting process because microorganisms are forced to go through additional cycles of carbon consumption, cell synthesis, decay, etc., in order to burn off the carbon (GTZ 2000; Bernal et al. 2009). In contrast, too much N (i.e., C:N ratio < 20) allows rapid microbial growth through fast consumption of carbon and this accelerates decomposition and quick oxygen depletion. But quick oxygen depletion may cause anaerobic conditions. In addition, the composting process will experience higher losses of N as ammonia and nitrogen oxides because inorganic N is generated in excess (Bernal et al. 2009; Zigmontiene and Zoukaite 2010). Both phenomena are the main reason for odor generation due to wrongly set C:N ratios of input materials in compost plants. Therefore it is essential to ensure that the feedstock used for composting is chosen carefully and the C:N ratio adjusted before the composting process starts (Bernal et al. 2009; Nema 2009). Mixing various feedstock allows control of the average C:N ratio as some raw materials are high in C while others are high in N. In practice, the ideal combination of different feedstock types can be determined by experimentation and experience (Guo et al. 2012; Ch’ng et al. 2013). As a rule-of-thumb, the mixture of equal volumes of ‘green’ materials (rich in N, e.g., fresh grass clippings, manure, garden plants or kitchen scraps) and ‘brown’ input materials (high C content, e.g., dried leaves and plants, branches and woody materials) provides an appropriate C:N ratio (Willson 1989; Bayard and Gourdon 2010). Furthermore, it should be considered that carbon-rich input materials may differ considerably related to the bioavailability of contained C. This is of particular concern because some C-rich materials (wood and other lignified plant materials such as sawdust) are known to be more resistant towards biodegeneration than many other organic materials (Cooperband 2000). In such cases, the C:N ratio must be above 30:1 as recommended earlier because a certain portion of the carbon is not easily available for microbial activity. After completion of the composting process, around 50% of the C from the organic matter of input materials is lost as CO2. The higher the organic matter loss, the higher the 16 RESOURCE RECOVERY & REUSE SERIES 3 temperature is increased during the process (Singh et al. 2012). Whereas the input volume is mostly reduced to around 50% after compost maturation, the weight of the final product will be reduced by 30-40% if compared to the inputs (Bayard and Gourdon 2010). 2.6.2 Porosity and Particle Size Porosity is a key composting factor determined by the particle size distribution, the shape, the texture and the moisture content of the feedstock. It determines air distribution during the composting process, i.e., the maximum amount of O2 available during the composting period (Bernal et al. 2009). Feedstock with larger particle size may not be decomposed adequately in a reasonable time. In addition, when porosity is > 50% for an open composting system (e.g., a windrow), the temperature may not increase sufficiently to the expected values within the pile because of energy losses since the heat easily escapes through the larger pores. Microorganisms need to attach to the particle surface to grow; consequently a higher surface area is preferable because it favors bacterial growth. Larger particle size of input materials will result in lower total surface area compared with smaller particle size. Consequently, the smaller the particle size of organic waste components (e.g., achieved through shredding, chipping or mixing), the faster the biodegradation process. This is significant, especially to accelerate composting of slow degradable woody materials. However, feedstock with very small particle sizes can reduce the porosity within the compost heap too much, which especially hinders the early stages of composting. Besides, it also increases the tendency to compact, which could further negatively affect the aeration of the material (Cooperband 2000; Bernal et al. 2009). Based on the discussed requirements, optimum particle size ranges from 1 to 2.5 cm for composting methods that apply forced aeration systems, and from 5 to 10 cm for methods that use passive aeration combined with mechanical or manual heap turning (Obeng and Wright 1987; GTZ 2000). With this approach a porosity level of 35-50% may result in the pile, which from experience was found to be satisfactory to enable aerobic conditions. To compost input materials with very small particle sizes and high tendency for compaction (e.g., manures or fecal sludge), C-rich materials such as sawdust, rice husks or similar materials were found to be very useful as a bulking agent, to adjust mixture quality, to increase porosity and to optimize the composting process (Chen et al. 2010; Singh et al. 2012). 2.6.3 Moisture Regarding biological activities, the presence of water is also essential for microbial growth during composting and needs to be maintained at the proper level in order to achieve optimum degradation. It also serves as a means to convey crucial nutrients and assists in the dissipation of heat (Strauss et al. 2003; Bernal et al. 2009). Optimum moisture content varies with the type of feedstock used (e.g., the coarser the material the higher the moisture content could be) as well as with the composting method. Optimal moisture content lies between 40 and 65 percent by weight (Bernal et al. 2009; Kumar et al. 2010). Moisture levels > 65% hinder the decomposition process, promote nutrient leaching and may trigger anaerobic degradation because interparticles air spaces within the compost are filled with water and cannot be supplied with oxygen. This can result in foul smell, especially for materials with low C:N ratio (Cooperband 2000; Bernal et al. 2009). However, moisture levels < 40% reduce microbial activity and could even lead to their inactivation or decay. Due to changes in temperature, microbial growth/decay and volume losses related to the ongoing biodegeneration in the compost heap, moisture varies as well and needs to be adjusted to maintain an efficient composting process. Adjusting the moisture level is often achieved simult