Continuous aerobic treatment of pig slurry: Evaluation of options based on long-treatment time and two-stage processing

Aerobic treatment can be very effective in dealing with a wide range of pollution problems from pig slurry, but it is often not popular due to the high energy costs implied. Improvement in the process performance and lower costs are key requirements in ensuring greater uptake of this technology. Two options in particular which may achieve this have been studied; long treatments of 20 d duration and two-stage processing. The long-treatment option offered major advantages when compared with a 2 d treatment process. The higher breakdown of organic matter, as indicated by the observed reduction in chemical oxygen demand (COD) of 43% compared with 30% for the short process was a significant improvement. This was achieved at a lower energy cost per m(3) of slurry processed. The improved efficiency of aeration was a consequence of the much lower aeration intensity (kgO(2)/m(3) h) required in the long-treatment reactor which enabled the use of high-efficiency diffusers. However, the reduced agitation also resulted in a tendency for particles to settle out, leading to the need for some additional mixing; this partly negated the energy saving from the efficient aeration process. Emissions of the air pollutant, nitrous oxide, accounted for 12% of the original nitrogen. Similar emissions also occurred in the two-stage and the 2 d treatment processes. A two-stage treatment comprising a 2 d reactor followed by a 20 d reactor achieved a COD reduction of 40%; this was no better than the 20 d reactor alone. Most of the microbial activity occurred in the first stage leaving the second stage under-utilized. The energy cost of operation of the two-stage process exceeded the sum of the two separate stages making this an expensive option with little tangible benefit over the single stage. By contrast, the long-treatment option, with lower running costs, emerges as an improvement over a short intensive treatment. Thus, the long-treatment strategy is recommended for aerobic treatment of pig slurry. (C) 1998 Silsoe Research Institute.