Reduction of ammonia emissions during composting by Uncoupling of oxygen demand and heat removal
Intensive pig production systems result in high emissions of ammonia, odour and greenhouse-gases (methane and laughing gas) by exhaust ventilation air. Moreover, large amounts of manure (faeces and urine) are produced with insufficient nearby land for applications (Imbeath, 1998). These outputs lead to environmental problems related to acidification and euthrofication of natural ecosystems, pollution of groundwater reservoirs and global warming. However, pig manure can also be seen as a highly valuable resource through the recycling of nutrients and organic matter to soil systems, thus reducing the use of artificial fertilisers and peat.
Large quantities of diluted pig manure are produced in traditional pig production systems leading to high costs in treatment and transport. Valorisation and application of pig manures can only be made possible if specific products are produced in an economical way without negative environmental impacts. A novel approach is the separate collection of faeces and urine by a convex conveyer belt (Kroodsma et al., 1998). The separation creates options for separate treatment of the liquid and solid fraction into a nitrogen rich liquid fertiliser (containing 60-70% of total N) and a stabilised organic fertiliser (containing 95-98% of the total P), respectively.
Faeces can be converted into a stabilised organic fertiliser through composting. Composting will result in breakdown and stabilisation of organic matter, mass reduction and removal of pathogens and plant seeds (Haug, 1993). However, composting of N-rich and less structured substrates such as pig faeces results in high ammonia emission and production of odours. Porosity and compactability of less structured substrates can be improved through mixing with a bulking agent such as straw or wood chips. Ammonia and odour emissions can be reduced by the application of absorbing materials such as zeolite or peat (Witter and Lopez-Real, 1988) or treating waste-gases with biofilters (Haug, 1993). These approaches are too expensive to give an economically feasible composting process.
A composting system is proposed, composed of a tunnel reactor and cooler, to reduce total waste-gas flow and ammonia emissions during composting of straw-amended pig faeces (Haug, 1993). As the recirculation air passes a cooler and removes the heat from the system, there is no longer the need to supply fresh outside air. Fresh air is only needed to supply sufficient oxygen for aerobic degradation. The system thus enables to uncouple the cooling function and the oxygen supply of the air. The process control of the system changes principally. In a conventional composting system air supply is regulated for temperature-control and the oxygen supply is automatically assured. By separating both functions, both temperature-control and oxygen supply have to be controlled, separately. The proposed system could help in managing ammonia emissions in two ways:
The exit air amount may be reduced, decreasing cost of odour treatment
Part of the ammonia may be removed in the cooler with the condensate
This study presents a balance model and experimental work of the composting system to determine the feasibility of such a system.