Algal abundance, organic matter, and physico-chemical characteristics of dairy farm facultative ponds: Implications for treatment performance

Abstract

Six Waikato (New Zealand) dairy farm facultative ponds (DFPs), which met the larger sizes specified in recent dairy industry guidelines, were sampled monthly over an annual period. Median wastewater BOD5 was 65 g m-3, suspended solids (SS) 206 g m-3, ammoniacal N 37 g m-3, total nitrogen 69 g m-3, and faecal coliforms 24 000 (100 ml)-1. This was 20–70% better than reported for DFPs built to previous guidelines, except for SS levels which were within reported ranges. However, performance was highly variable and only ½ of the DFPs studied consistently met an effluent standard of ≤ 100 g m-3 BOD 5 and only one reached ≤ 150 g m-3 SS. Removal of BOD 5 was much lower than recorded for SFPs in New Zealand with equivalent BOD 5 loading. Although the mean euphotic depth was only 0.11 m, algal biomass in DFPs was similar to that recorded for SFPs. Low phaeophytin concentrations and daytime oxygen exceeding 200% saturation in the shallow epilimnion on sunny days suggested a relatively healthy photosynthetic algal population was present in the DFPs. However, wastewater entering DFPs showed high median COD levels (1420 g m-2). COD:BOD 5 ratios of c. 12.1 (compared with 1.5–1.8 for SFPs) and BOD 10 :BOD 5 ratios of c. 2 indicated the presence of a large pool of slowly degradable organic matter in the wastewater. This resulted in sustained exertion of BOD in the pond, explaining the “apparent” poor removal of BOD 5 by DFPs. Conductivity was found to be a useful single-measure indicator of overall pond performance and management of sludge levels in the preceding anaerobic pond was identified as a key factor affecting DFP performance. Further improvements in dairy farm stabilisation pond performance are likely to be required on many farms to meet receiving water guidelines for the protection of water quality and aquatic life. © 2001 Taylor & Francis Group, LLC.