Composting takes place within two temperature ranges known as mesophilic (100 – 400 C) and thermophilic (over 420C). It is generally accepted that maintaining temperatures between 430C and 650C allows for effective composting. The thermophilic temperatures are favored in the composting materials, because they destroy more pathogens, weed seeds and fly larvae. In some composting processes, Temperatures can continue to rise above 700C due to insulation effects and on-going microbial activity. At these temperatures many micro-organisms die or become dormant and the process effectively stops until the micro-organisms can recover.
Nutrients and the Carbon Nitrogen (C:N) ratio
The microbes involved in composting use carbon for energy and nitrogen for proteinsynthesis. The proportion of these two elements required by the microbes averages about 30 parts carbon to 1 part nitrogen. Accordingly, the ideal ratio of Carbon to Nitrogen (C: N) is 30 to 1 (measured on a dry weight basis). This ratio governs the speed at which the microbes decompose organic waste. Most organic materials do not have this ratio and, to accelerate the composting process, it may be necessary to balance the numbers by mixing different substrates. (eg.. kitchen waste is rich in nitrogen while garden waste is in poor)
Particle size, porosity, structure and texture
The ideal particle size is around 2 to 3 inches. In some cases, such as in the composting of kitchen waste, the raw material may be too dense to permit adequate air flow or may be too moist. A common solution to this problem is to add a bulking agent (straw, dry leaves) to allow for proper air flow. Mixing materials of different sizes and textures also helps aeration the compost pile.
(a) Home composting methods
Large scale, commercialized composting facilities (Ex. Windrows, Static piles and Reactor systems) use sophisticated technologies and equipments to handle the composting process. Home composting depends on traditional or small scale simplified composting technologies at low cost. (Ex. simple pit method, heap method, bins, rotating drums). But each method uses the same scientific principle though it differs in procedures and equipments used. Appropriate/most suitable home composting systems may differ from place to place depending on, climate conditions, economic conditions and social factors of the people. In Sri Lanka, space limitation has been a critical issue for many local authorities that have crippled the implementation of home composting programs.
Most common home composting systems in Sri Lanka
1. Heap method
2. Pit method
3. Traditional Jeewakotu and basket methods
4. Rotating drums
5. Composting bin systems
Composting Bin System
Bin composting is the most popular and advance version of home composting system that
overcomes problems experienced in other composting systems. There are different types of bins available for home composting and generally it varies from 200- 300L in size. These are from different materials such as cement/concrete, plastic, metal, etc. The bins allow higher stacking of composting materials and better use of floor space than free-standing piles. Bins can also eliminate weather problems and reduce problems of odors, and provide better temperature control. At present, most bins are designed to suit the urban landscape as well.
The bin design should facilitate the composting process with easy management practices.
Therefore, standard bin design should focus on;
- Easy transportation and installation
- Good aeration (sufficient aeration for all parts of the bin – aeration hole diameter – <>
- Drain the excess moisture (porous bottom pad)
- Protect from external whether conditions, mainly rain, winds (proper cover or lid)
- Retain the temperature inside (dimensions of the bin and material used for constructing)
- Easy adding and mixing of waste ( proper height ,easy handling lid)
- Easy removal of compost (size and number of compost removing doors)
- Keeps away from the pest like rats, dogs crows etc.
- Durability of the bin (material used, strength etc)
As management practices, more care should be taken on selecting suitable materials for composting. In most systems, 98% of the biodegradable can be composted without much problem. But in urban areas, as there are space limitations any form of malfunction in the composting process can lead to environmental issues in the surroundings. Malfunctions are caused primarily when non-degradable materials are added to the composting bin. (Table: 1)
Table: 1 Things to be composted and excluded from composting bin
Materials to include:
- Vegetables/kitchen refuses
- Garden trimmings, grass clippings
- Leaves, dry leaves (straw)
- Twigs and shredded branches
- Food refuses: bread, buns etc
- Egg shells
- Farm animal manure (e.g. Cow, Sheep, Goat , Poultry)
- Fruit refuses
- Wood ash
Materials to exclude:
- Non biodegradable waste: polythene, plastics, glass, metal etc.
- Human feces, pet manure(e.g. dog, cat)
- Dairy Products
- Diseased plants
- Fish , meat scraps and bones
- Slow degradable materials like coconut shells, coconut husk, etc.
- Fats/cooking oils
- Hazardous material like batteries, bulbs, electronic components, chemicals
source: www.practicalaction.org, photo from www.eawag.ch