(Co-) composting (Small-scale)

Municipal solid waste management starts at the household level as households are the main source of waste in most urban areas. 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. Thus, waste recycling at the household level is very important.

Organic matter such as food waste and yard waste are the main components of household waste. Other types of waste generated by households include inorganic components plastics, metals, glass, and inert materials such as soil. The portion of organic waste in household waste is generally higher in developing countries. In Nepal, for example, over 65 % of the total waste is organic waste.

Organic waste can be recycled at the household level to produce animal feed, biogas (i.e. anaerobic digestion small-scale; anaerobic digestion of organic waste) or compost. Among these options, composting is probably the most simple and common method for recycling household organic waste. Composting is the process of optimising the environment in the waste for microbial activity to decompose organic matter into valuable nutrients for the soil. Household composting involves the following three stages: waste preparation, degradation of waste and finishing of waste.

Waste needs to be sorted and prepared for rapid degradation. This includes the following steps:

• Mix the waste to ensure that the carbon to nitrogen ratio (C:N) is close to 25 – If the waste has too much carbon (normally brown waste such as dried leaves) it should be mixed with waste that has high nitrogen content (also known as green waste such as green grass clippings). Kitchen waste only normally has a suitable C:N ratio for composting.
• Chop the waste into small pieces. Large pieces of organic waste should be cut to small pieces to accelerate the composting process.
• Adjust the moisture content to about 50 per cent. A compost pile with 50% moisture should feel moist but water should not be dripping from it.
• Add a starter such as mature compost or effective microorganisms (EM) to speed up the composting process.

Once the waste is converted into raw material for composting, the waste can be degraded using one of the methods mentioned below. The degradation process should be controlled by maintaining adequate temperature, moisture and aeration. There are different types of vessels for composting at this stage: composting in pits, composting in piles, in-vessel composting and vermi-composting.

Compost pits

One simple method for composting organic waste is to put it in pits and let it turn into compost over a period of six months or more. This process requires some space and time but the main benefit is that the waste is not visible as it is buried in the pit.

Compost piles

In this method, the waste is put in piles on the ground and regularly turned to allow aeration. The size of the pile may vary depending on the amount of waste and available space, but generally, it should be 1 to 2 m on each side and not more than 1.5 m in height. Chicken wire or wooden planks can be used to keep the pile together.

In-vessel composting

Compost can also be made in made in bags or bins. Normally holes are made in the bin or bags to allow aeration. Compost bins can be made from 100 to 200 litre plastic bins or barrels. In order to allow proper aeration, the bin is normally divided into two sections with a grill separating the two sections. Organic waste is put into the top section and allowed to degrade and once the compost is prepared it is removed from the bottom section. Because of the natural draught created in the bin by the grill and the holes, frequent turning of the waste is not required in this method. Municipalities and NGOs in countries such as Nepal, Bangladesh and Sri Lanka have been distributing compost bins to residents at subsidised rates to promote household composting (TULADHAR 2004; ALI 2004; LEKAMMUDIYANSE et al. 2009).

Vermi-composting

This involves the use of special types of earthworms to convert organic waste into worm casting, which is better than ordinary compost in improving soil’s structure and fertility. Vermi-composting is a simple and effective process that can be done indoors in the kitchen itself as it requires very little space and does not cause problems such as odour. Most common worm species for vermi-composting are Eisenia foetida, Lumbricus rubellus, and Eisenia hortensis (KOFF et al. 2007). The worms are placed in bedding made of loose materials such as coconut husk or shredded paper in a shallow box or tub and fed with soft organic matter such as kitchen waste. After about two months the compost can be harvested by putting the compost in the shape of a cone and scrapping off the top layers. The worms go to the bottom of the cone as they run away from light.

Once the compost is prepared, it needs to be cured and screened before it is applied in the soil. The composting process is complete when (KOFF et al. 2007):

• The compost pile’s internal temperature has decreased to within 5 degrees of outside temperature.
• There is little or no discernible plant material left in the compost
• The compost has a dark brown, crumbly and rich look (similar to chocolate cake)

Once the compost is ready, it should be left for about a month for curing and then screened using a fine screen. The end product can now be used as soil amendment (use of compost). Any material that has not yet decomposed can go back into the compost pile or bin.

Besides organic waste, composting can also be applied as a secondary treatment for faeces and excreta collected in UDDTs (urine diversion dehydration toilets) or vault composting toilets. The process is the same as the one that takes place in any pit-composting toilet (e.g. fossa alterna, arboloo). Composting can also be applied for the secondary treatment of sludge from twin-pits for pour-flush toilets or digest from anaerobic digesters or any other pre-treated sludge. Even better results can be achieved when organic wastes are composted together with faeces in order to optimise the C/N ratio. Co-composting can be done either in the toilet or off-site similar to pure household waste (in compost, compost piles, In-vessel composting Vermi-composting). See also VALLEY VIEW UNIVERSITY (2008) for more information.