Reducing dairy lagoon organic loading rates with high-rate anaerobic digesters

In a two-stage study, the possibility of using high-rate anaerobic digesters to reduce dairy lagoon organic loading rate was explored. Four anaerobic sequencing batch reactors (ASBR) and four downflow anaerobic filters (DFAF) were tested, with two of each type operated at 25 degreesC, and the other two at 35 degreesC. The first stage of the experiment demonstrated that settling processes, rather than biodegradation, accounted for most of the organic matter reduction in both reactor types, when operated at a 0.5-d hydraulic retention time. Specifically, settling accounted far an average of 100% of the chemical oxygen demand (COD) removal in the ASBR reactors, and for an average of 72% of the COD removal in the DFAF reactors. The second stage of the experiment explored the possibility of removing volatile fatty acids (VFA) from synthetic dairy lagoon supernatant. At low VFA loading rates (1.0 and 3.0 g L-1 d(-1)), both reactor types achieved moderate VFA removal. However, at high loading rates (6.0 g L-1 d(-1)), the ASBR reactors were ineffective, whereas the DFAF reactors - at both 25 and 35 degreesC - removed more than 80% of the influent VFA. Based on these experiments, it appears that the DFAF reactors are better suited to the task of rapidly removing accumulated VFA from lagoon supernatant.