Processes

The text and videos below illustrates ten steps involved in operating an IRRC with a capacity of 3-20 tons. For managers of an IRRC who has received training at the regional training centre in Dhaka, the information below is complimented with an operational guideline developed by Waste Concern with the support of ESCAP.


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Collection of waste




The collection of fresh organic waste that has been separated at the source (homes, offices, businesses) is necessary for the composting process. Already in the collection and transportation it is important to keep the organic waste separated from inorganic waste to avoid contamination. Once collected from communities, markets and other sources, the organic waste is transported to a compost plant.


Weighing the Incoming Waste


Incoming organic waste must be weighed using a manual or electronic digital weighing machine. Typically, carts or trucks loaded with waste enter the compost plant and unload the waste at an unloading platform. Depending on the size of the plant, the weigh bridge capacity can be from 250 to 500 kg. For smaller plants incoming waste can instead be weighed using a 50–100 litre bucket or basket. Keeping an accurate record of incoming waste in a compost plant is required for claiming carbon trading credits as well as a critical component of running the business.

Sorting




Compost quality is mainly determined by the quality of the input material. Hence, sorting and segregating the waste is vital. Substances that are not biodegradable need to be separated from what is biodegradable. If households segregate their waste, it certainly saves a tremendous amount of time and expense for the composting plant. More importantly, though, it increases the quality of both the biodegradable waste and the recyclable material. Thus the long-term goal of a vibrant IRRC should be the introduction of source segregation of waste in households.

Sorting is especially crucial where there are hazardous materials. They must be removed before they are loaded into the compost box. Otherwise they will contaminate the entire pile of waste and severely compromise the final compost quality.

Mixing




Balancing the carbon and nitrogen ratio is necessary for the composting process. The ratio of carbon (C) to nitrogen (N) – also called the C/N ratio – is very important for the biological degradation of organic waste. Both carbon and nitrogen are feedstock for the microorganisms that break down the organic matter: carbon is important for the cell proliferation, while nitrogen is the nutrient.

Loading the Box




The compost box is constructed with perforated walls, a perforated bottom grid and vent pipes that are also perforated, which allow air to circulate through the waste. Oxygen is necessary for the degradation of organic waste because it enables the microorganisms that breakdown the waste to survive. The construction of the compost box in combination with a layering technique ensures sufficient aeration (air circulation) and thus does not require any additional turning. Air is supplied to the organic material through the holes in the box walls and through the perforated vertical pipes embedded into the waste. The perforated bottom of the box allows excess water to drain out.

The plastic perforated pipes increase the oxygen supply into the waste by what’s known as “passive aeration”. The box should not be filled within a day; filling of box with organic waste should be done incrementally in layers to give it time for oxygen to seep through. If the box is filled in a day, the pile may become anaerobic (lacking oxygen). The depth of each layer should be kept within the range of 30–45 cm so that it allows additional oxygen into the pile when the waste comes into contact with fresh air. The presence of steam is a good indicator of the effectiveness of the composting process.

Assuming an input load (or the “piling” of the waste) of 3 tons per day, with a density of 800 kg per m3 (and a 15-ton capacity), one box may be filled within 5 or 6 days. The box receives one layer (up to 3 tons) of waste per day. Every time a layer is added, it is loosely mixed with the previous layer (using a fork or shovel). When the box is full, the waste is left for 45–50 days so that it can go through the thermophilic composting process.

Temperature Control




Because natural microorganisms, such as bacteria, actinomycetes, fungi and some protozoa, are already present in organic waste, the degradation phase starts directly after the piling. These aerobic microorganisms decompose or break down the carbohydrates and amino acids present in food and yard waste into simpler compounds, carbon dioxide and water. Under favourable conditions, the microorganisms multiply exponentially and generate a large amount of heat as a result of the oxidative process. The temperature within the heap rises to over 60°C. If high temperatures are maintained for one week and all of the material experiences such temperatures (the cooler outer layer being relocated to the warm interior as a result of the turning process during the piling step), pathogens and weed seeds are destroyed. After about a month, the process slows down and temperatures drop slightly.

Moisture Control




For measuring the moisture weekly, dig several holes into the compost and check the moisture content. If the material is too dry, spray water over the compost and level the material again.

Maturing




In the box system, the temperature within the mass increases within a few days, up to 60°C; this is the ideal process to ensure that the final compost product is free of harmful pathogens or weed seeds. Typically, a compost box is filled within 5 or 6 days, and the waste in the box decomposes aerobically for 45–50 days. At that point, the material in the pile (in the box) changes colour resembling soil and the pile temperature falls below 50°C. This indicates that the process has entered the “curing” or “maturing” phase. This decomposed product needs another 15 days for maturing in the “maturing box”. All the compost boxes are placed under a covered roof to protect the composting process from rain and excessive sun.

The maturing box is a specially designed box with a drying bed (with layers of coarse sand and brick or stone ships) to absorb extra moisture from the pile. This box could have a blower to control the moisture content before screening (making smaller particles). Compost with high moisture content does not comply with compost standard regulations and is difficult to screen. If the moisture is too high during this stage, drying or forced aeration can be done with a mobile blower (1 horse power). Run the blowers for hours with interval, till the piles have the required amount of moisture.

Screening




Mature compost has a rather coarse texture. The particle size of the compost strongly depends on the size of the individual pieces and the composition of the input material In many cases, customers require “fine” compost, and thus the compost must be “screened”. The screening is accomplished with either a flat-frame sieve or a rotating drum sieve.

Storage, Bagging and Sales




Depending on your customers, you might store compost in bulk or pack it into smaller bags of different volumes. Before bagging it, however, check the temperature; if the compost reheats above the ambient temperature after the screening process, it is not completely mature. In such a case, sprinkle a little water and let the compost rest for another week. Check the temperature again before you start bagging it. The compost should be relatively dry when it is bagged to avoid transporting large amounts of water with the compost – the moisture content needs to be less than 40 per cent).

A proper compost marketing strategy is necessary to market compost products to farmers and other crop growers. Compost can be produced to comply with local quality standards and enriched with necessary nutrients to meet the demand of different types of soil and crops.