Composting is a biological process of aerobic decomposition for converting organic waste materials (feedstocks) to stable value-added compost suitable for nutrient supplement to crop. Because composting is environment friendly and allows reuse of natural resources, it is becoming a popular waste management option. Compost increases the water and nutrient retention of the soil, provides a porous medium for roots to grow in, increase organic matter and decreases the bulk density or penetration resistance.
The composting process produces a final product that is stable, free of pathogens and viable plant seeds, and can be beneficially applied to the product stabilizes, odours are reduced and pathogens eliminated.
A Case study on ‘Development of a Farm-Scale System to Compost Liquid Pig Manure’ will give an insight for process, product and economics of the composting system from swine manure. A documentation entitled ‘Composting Hog Manure – Is it Right for Your Farm?’ (Agriculture and Agri-Food Canada) elaborates the perspective of composting swine waste with an explanation of different machineries used in the composting process.
Under controlled conditions, composting is accomplished in two main stages: an active stage and a curing stage. In the first stage, while most of the degradable organic matter is decomposed, microorganisms consume oxygen (O2) while feeding on organic matter in manure and produce heat, carbon dioxide (CO2) and water vapour.
Appropriate management plan is needed to maintain proper temperature, oxygen and moisture for the organisms. Finished compost has 20 to 60% volume reduction, 40 to 50% moisture content reduction. Composting may contribute greenhouse effect because carbon dioxide (CO2), methane (NH4) and nitrous oxide (NO2) are emitted to the atmosphere during composting
Composting can be done in one of the following methods:
- Bin composting
- Passive windrow composting
- Active windrow composting (turned)
- Aerated static pile composting
- In-vessel composting
The most important factors influencing a quality compost are moisture content, carbon to nitrogen ratio (C:N ratio) of the raw materials, and the temperature achieved during the composting process. A check of C:N ratio of base material (eg. animal waste) helps determine what proportion of carbon rich materials need to be added for composting. During composting process microbes need C:N ratio of about 20:1 to 25:1. The microorganisms digest carbon as an energy source and ingest nitrogen for protein and reproduction. Generally sources of carbon include shavings, sawdust, straw, municipal shredded newsprint or cardboard.
The minimum desirable oxygen concentration in the composting material is 5%. Greater than 10% is ideal to avoid conditions and high odour potential. Aeration adds fresh air in the center of the composting material. Rapid aerobic decomposition can only occur in the presence of sufficient oxygen.
Moisture plays an essential role in the metabolism of microorganisms and indirectly in the supply of oxygen. Microbes can utilize only those organic molecules that are dissolved in water. Moisture content between 50 and 60% provided moisture without limiting aeration.