Effluent characteristics of dairy shed oxidation ponds and their potential impacts on rivers

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Abstract
The effluent characteristics of 11 dairy shed oxidation ponds designed to national specifications were examined. Measurements covering a wide range of parameters were made monthly over at least 1 year in ponds from two regions (Manawatu and Southland) and covering two types of farms: town milk supply (non‐seasonal) and daily factory (seasonal). There was considerable variation in effluent composition within ponds with time and between different ponds. Biochemical oxygen demand (BOD) concentrations (overall median 98 g m‐3; 3‐fold range of individual pond medians) showed less variation between ponds than suspended solids (SS) concentrations (median 198 g m‐3; 9‐fold range). Available nutrient levels were very high (e.g., an overall median of 12.2 g m‐3 for dissolved reactive phosphorus and 75.0 g m‐3 for ammonium (NH4‐N)). The ammonia represents a 4‐fold higher level of potential oxygen demand than the measured BOD. Faecal coliforms (median 70 000 (100 ml)‐1) showed large variability both within and between ponds, with higher levels in town milk supply ponds. Differences in pond effluent characteristics could not be attributed to influent loading as apercentage of design or to the significant temperature difference between regions. Maintenance of receiving waterconcentrations below existing criteria for 95% of the time would require > 2 700‐fold dilution for faecal coliforms (bathing criterion), > 67‐fold dilution for coliforms (post treatment drinking criterion), and > 2700‐fold dilution to prevent nuisance levels of algal proliferations below discharges. Most uses are accommodated provided dilution exceeds 250‐fold requiring a minimum stream flow of 0.058 m3 s‐1 for a 220‐cow herd.