Cost-effective treatment of piggery wastewater

Abstract

Nine complete wastewater and sludge treatment trains are presented for a case of two industrial piggeries, with unit water uses of 28 and 20 litres d −1 per hog and respective COD f concentrations of 14 and 20 g litre −1. Assuming two energy cost levels of 4 and 12 ¢ kWh −1, full economic effectiveness analyses are presented. All wastewater unit processes are based on actual laboratory-, pilot- or full-scale results. The treatment systems analyzed combine both practiced aerobic and novel anaerobic technology. Analysis is based on the ratio of the effectiveness index defined as total annual cost in dollars per kilogram COD removed for each system to the index for a series of lagoons (SystemI). The lagoon system has been regarded as standard by the industry, which traditionally consisted of mostly small piggeries of less than 2000 animals. It has been found that some of the practiced systems: extended aeration; chemical coagulation; series of lagoons; and systems featuring land disposal, are not cost-effective. With the increase in concentration of wastes and inflationary energy costs, the systems combining high-rate anaerobic and aerobic unit processes can do the same job at half or even a third of the total annual cost, expressed as cost per unit COD removed. Although numerous simplifications had to be made in the analysis, the results are conservative. The organic loads used for anaerobic reactors were lower than actually achieved; heat losses were assumed higher than normal. On the other hand cost of land, a significant factor in traditional Systems I and VII, was not included. Existence of, or lack of, odor problems were not factored into the comparison. To compensate for the higher level of skill required to operate anaerobic reactors, their gas production rates were lowered to 0·25 m 3 CH 4 per kilogram COD removed.