Continuous aerobic treatment of piggery slurry for odour control scaled up to a farm-size unit

Effective aerobic treatment of piggery slurry was achieved in a continuous farm scale process sited at a small piggery (2000 pigs). The plant design and the operating conditions were based on data obtained from pilot-scale studies with the aim of preventing or reducing odours from pig slurry at a minimum cost. Raw slurry was separated before passing into the main treatment vessel. Aeration was achieved by recirculating of slurry (achieving jet mixing of the tank contents) through a venturi where air was entrained. Aeration was controlled to maintain redox potential between 100 and 200 mV E h . Slurry was added and withdrawn at hourly intervals, to provide nominal residence times of 1, 2 and 4 days. The performance of the aerobic treatment process in terms of COD reduction could be predicted using data from laboratory and pilot-scale experiments. Some allowance could be made for minor feed fluctuations but a more comprehensive model will be necessary for predictions where larger variations occur in operating conditions. Assessments were made of odour quality using dynamic dilution olfactometers for odour strength, and volatile fatty acids (VFA) concentration as an indicator of odour offensiveness. Results indicate that pilot-scale experiments may overestimate by 10–20% what can be achieved with the farm-scale plant. Nitrogen losses were greatest in the 4-day residence time trial with 56% lost in the form of N 2 gas following a nitrification/denitrification process. Conversely, in the short residence time trials, virtually all of the nitrogen was conserved.