(Co-) composting (Large-scale)

Solid waste composting for use as a soil amendment, fertiliser or growth medium is of prime importance in many countries (EAWAG/SANDEC 2008). Asian countries in particular have a long-standing tradition of making and using compost. In Western Europe, a range of modern technologies is applied to produce compost (EAWAG/SANDEC 2008). Organic matter such as food waste and yard waste are the main components of municipal waste in the developing world. Faecal sludge can also be composted after it has been dewatered (e.g. drying beds, thickening ponds, mechanical dewatering).

For effective management and sustainability, decentralised composting systems should be treated like business ventures that use waste as raw materials and produce compost as final product that needs to be sold to different market segments. Therefore, before starting a composting project, the existing political/legal, social, technical, institutional and economic environment that can affect the operation of the compost plant and the market for compost should be carefully analysed to prepare a business plan and financial projections.

There are two types of co-composting designs: open and in-vessel(bin or box composting) . Open composting can be separated in windrow composting where air exposure is enhanced, or tranch or pit composting (STRAUSS et al. 2003).

Open Composting

Open composting is a slow but simple process (DULAC 2001). In open windrow composting, the mixed material (sludge and solid waste) is piled into long heaps called windrows and left to decompose. Windrow piles are periodically turned to provide oxygen and ensure that all parts of the pile are subjected to the same heat treatment. Aeration is also ensured by the addition of bulky materials and passive or active ventilation (STRAUSS et al. 2003). Systems with active aeration by blowers are usually referred to as forced aeration systems and when heaps are seldom turned they are referred to as static piles. Sloped and sealed or impervious composting pads (the surface where the heaps are located) control the leachate with a surrounding drainage system.

Trench and pit systems are characterized by heaps, which are partly or fully contained under the soil surface (STRAUSS et al. 2003). This allows to save space and to reduce construction cost (in comparison to boxes). Structuring the heap with bulky material or turning is usually the choice for best aeration, although turning can be cumbersome when the heap is in a deep pit and leachate control is difficult in trench or pit composting (STRAUSS et al. 2003).

In-vessel composting 

The compost is placed in boxes made out of bricks, wood or mesh boxes with holes in between and a screen at the bottom. In-vessel composting requires controlled moisture and air supply, as well as mechanical mixing. Therefore, it is not generally appropriate for decentralized facilities but is less labour intensive than open composting . It also needs less space. But the the initial capital cost required for a box system is slightly higher.

Although the composting process seems like a simple, passive technology, a well-working facility requires careful planning and design to avoid failure. For effective management and sustainability, decentralized composting systems should be treated like business ventures that use waste as raw materials and produce compost as final product that needs to be sold to different market segments. Therefore, before starting a composting project, the existing political/legal, social, technical, institutional and economic environment that can affect the operation of the compost plant and the market for compost should be carefully analysed to prepare a business plan and financial projections.

Vermi-composting, which involves using special types of earthworms to convert organic waste into worm casting, can also be done in decentralized composting (ZURBRUGG et al., 2002). In a vermi-composting plant, the waste is first composted aerobically for about two weeks as in an ordinary plant. Then, the semi-decomposed waste is put in boxes with special types of worms, such as Eisenia foetida, Lumbricus rubellus, and Eisenia hortensis. Vermi-composting results in better quality compost, but the worms need more care than aerobic composting. For vermi-composting, the pile does not need turning, but the temperature and moisture needs to be suitable for the worms at all times to ensure their survival.

Decentralised composting

Decentralized waste management systems that maximize recycling can be appropriate systems for managing municipal waste in developing countries because waste minimisation and managing of waste as close to the source as possible are the two most important tools for reducing cost and improving efficiency of waste management systems. Scrap dealers generally collect inorganic waste such as plastics, metals, and glass. But organic waste is difficult to handle, store and transport. Therefore, decentralized community-based systems, such as small compost plants and biogas plants (see also anaerobic digestion of solid organic waste) are suitable for managing organic waste. Decentralised composting facilities will generally be in the range of 2 to 50 tons per day, depending on the size of the community and volume of compostable materials in the waste stream (UNEP/IETC 1996).

Centralised composting at municipal scale

Centralised composting facilities can receive up to 10 to 200 tons per day (UNEP/IETC 1996). Differences in scale, management, financing, and site selection distinguish municipal-scale composting form decentralised composting. Centralised composting plants require thus the involvement of engineering and professional designers and the involvement of all stakeholders.

Waste collection

Decentralised and centralised composting starts with waste collection. As waste collection is generally the most expensive and visible part of the waste management system, it needs to be properly planned and executed. Waste for compost plants can be collected from households, markets or institutions that generate organic waste. If possible, waste should be segregated at source before collection as to reduce the chances of having contaminants such as glass particles and heavy metals in the compost. Glass particles in the compost can severely hamper marketing of the compost (TULADHAR and BANIA, 1997) while toxic chemicals or heavy metals in the compost can damage the soil and contaminate the food chain as well. Along with waste collection, a service fee should also be charged to the waste generators so as to ensure the sustainability of the waste collection system as well as the compost plant. The amount of fee charged should depend on the cost of waste collection and composting and the willingness to pay.

Marketing of compost

In order to ensure the sustainability of the compost plant it is necessary to identify the market segments where the compost can be sold and then develop a marketing strategy to cater to these segments. The strategy should address the “4 Ps” of marketing — Product, Price, Place, and Promotion. “Product” refers to the characteristics and quality of the compost and the packaging, “price” refers to the pricing structure at the wholesale and retail levels, “place” refers to the distribution channel that connects the plant to the customers and “promotion” refers to the activities that makes the product visible and establishes it as a credible brand.

 

The facility should be located close to the sources of organic waste and faecal sludge to minimize transport costs, but still at a distance away from homes and businesses to minimize nuisances. The total land area required for a 3 ton per day compost plant is about 810 m2 for an open system and 770 m2 for an in-vessel system. This includes space for sorting, composting, screening and bagging, storage of compost and rejects, office sanitary facilities and a buffer zone (ROTHENBERGER et al., 2006). Depending on the climate and available space, the facility may be covered to prevent excess evaporation and/or provide protection from rain and wind.

At the compost plant, the waste is first sorted to remove contaminants, mixed with additives if necessary to keep the carbon to nitrogen ratio (C/N) between 25 and 40, and then placed in the windrows, boxes or trenches. Key factors affecting the biological decomposition processes of composting and/or the resulting compost quality (STRAUS et al. 2003) are: 

• Carbon to nitrogen ratio (C/N)
• Moisture content
• Oxygen supply, aeration
• Particle size
• pH
• Temperature
• Turning frequency
• Microorganisms and invertebrates
• Control of pathogens
• Degree of decomposition
• Nitrogen conservation

Ratios for co-composting: For dewatered sludge, a ratio of 1:2 to 1:3 of sludge to solid waste should be used. Liquid sludge (TS of 5 %) should be used at a ratio of 1:5 to 1:10 of sludge to solid waste (STRAUSS et al. 2003; OLUFUNKE & KONE 2009).

Windrow piles should be at least 1 m high and insulated with compost or soil to promote an even distribution of heat inside the pile. To ensure aerobic conditions, the pile should be turned twice a week for the first two weeks and then once every 10 days. The temperature of the pile should increase to about 65ºC in the first week and then go down to 40 ºC over the next few weeks. After about 21 to 60 days (UNEP/IETC 1996), the composting process enters the maturing or curing phase when the pile should be left without turning for some weeks up to a year depending on the local conditions. Then the compost is screened and packed. Nutrients or other additives may be added to the compost to improve its quality or make it more attractive for the buyers.

Vermi-composting, which involves using special types of earthworms to convert organic waste into worm casting, can also be done in decentralized composting (ZURBRUGG et al., 2002). In a vermi-composting plant, the waste is first composted aerobically for about two weeks as in an ordinary plant. Then, the semi-decomposed waste is put in boxes with special types of worms, such as Eisenia foetida, Lumbricus rubellus, and Eisenia hortensis. Vermi-composting results in better quality compost, but the worms need more care than aerobic composting. For vermi-composting, the pile does not need to be turned, but the temperature and moisture needs to be suitable for the worms at all times to ensure their survival.

Maintaining the temperature in the pile between 55 and 60°C can reduce the pathogen load in sludge to a level safe to touch and work with. Although the finished compost can be safely handled, care should be taken when dealing with the sludge, regardless of the previous treatment. If the material is found to be dusty, workers should wear protective clothing and use appropriate respiratory equipment. Proper ventilation and dust control are important.

The mixture must be carefully designed so that it has the proper C:N ratio, moisture and oxygen content. If facilities exist, it would be useful to monitor helminth egg inactivation as a proxy measure of sterilization. 

A well-trained staff is necessary for the operation and maintenance of the facility. Maintenance staff must carefully monitor the quality of the input material, and keep track of the inflows, outflows, turning schedules, and maturing times to ensure a high quality product. Forced aeration systems must be carefully controlled and monitored.

Turning must be periodically done with either a front-end loader or by hand. Robust grinders for shredding large pieces of solid waste (i.e., small branches and coconut shells) and pile turners help to optimize the process, reduce manual labour, and ensure a more homogenous end product.