Influence of Aeration Rate on Transformations of Separated Pig Manure and Gaseous Emissions during Composting

In Brittany, pig slurry may be submitted to centrifugation in order to trap phosphorus in solid fraction whereas nitrogen will remain mostly in liquid fraction. The solid fraction, separated pig manure (SPM), will be composted in order to be exported outside Brittany. SPM composts have to respect Animal By-Products Regulations (1774/2002) which impose to demonstrate that composting has an equivalent hygienic effect to that of 70°C maintained for one hour (free of Salmonella in 25 g wet weight). They also have to comply with organic fertilizers standard specifications of NF U 42001 i.e. dry matter (DM) and total nitrogen (TN) should be superior to respectively 40% and 1,5% wet weight. From another point of view, concerns with ammonia emissions are growing in the European Union leading to new regulations to reduce these emissions (IPPC and NEC Directives). Compost quality standards and ammonia emissions impose to improve composting processes of SPM.The aim of this study was to investigate the influence of aeration rate on transformations of SPM and gaseous emissions during composting. Fresh SPM was introduced into six composting reactors (300L). Six aeration rates were applied ranging from 3,5 to 43,4 L/h/kg DM and aeration rate was maintained constant during the composting treatment. The mass and the temperature of the composting material and the concentrations of oxygen, carbon dioxide, methane and nitrous oxide, in both the incoming and the outgoing airflows, were measured in continue. Ammonia and hydrogen sulphide emissions were measured daily thanks to accumulation in sulphuric acid and mercuric chloride. SPM and composts were characterized with regard to their DM content, stability (oxygen consumption and carbon and COD removals), nitrogen content and sanitary quality. Mass balances in carbon, COD and nitrogen were established. The results showed that low aeration rates led to the highest temperatures. As a consequence, E. coli and enterococci were detected only in composts obtained with the two highest aeration rates. In return, composts DM contents were 10% lower at low aeration rates. Oxygen consumptions and carbon and COD removal rates showed that organic matter stabilisation was much better at low aeration rates. However oxygen consumptions were quite low. Total nitrogen removal rate was poor influenced by aeration rate. Ammonia emissions occurred sooner and were higher at high aeration rate. Nitrous oxide and hydrogen sulphide emissions also increased with aeration rate. Defaults in nitrogen mass balances were all the higher as aeration rate was low. This was interpreted as a higher conversion of NH4+/NH3 to N2, through nitrification and denitrification, at low aeration rate. An aeration strategy was proposed. This strategy consists in applying a low aeration rate for ten days to reach temperatures higher than 60°C and remove most of the biodegradable matter. This biodegradation leads to the release of NH4+/NH3 by ammonification. The low aeration rate will increase retention of NH4+/NH3 in composting material and thus will favour its conversion through nitrification to the detriment of ammonia emissions. Nitrous oxide and hydrogen sulphide emissions will be also reduced. After ten days under low aeration, the aeration rate should be increased. 4 to 10 days at high aeration rate will be enough to reach dry matter levels obtained with the strategy under high constant aeration rate.