Farm dairy effluent: A review of published data on chemical and physical characteristics in New Zealand

Increased concern about environmental degradation and a move towards sustainable farming systems has lead to closer attention being paid to farm dairy effluents (FDE). Treatment of FDE in New Zealand is mainly through land application, or through oxidation ponds. Since the introduction of the Resource Management Act, 1991, regional councils require dairy farmers to be more accountable for the management of effluent from their dairy farms. Regulations have been imposed to limit the application of nitrogen (N) to land from FDE, and these limits range from 150 to 200 kg N ha(-1) yr(-1). Farmers, consultants, and regional councils require information on the chemical composition, particularly N content, of effluents, so that land effluent application systems can be designed and managed within the guidelines or regulations imposed. Data gathered from previous investigations on effluents in New Zealand found an average solids content of 0.9% dry matter. Between 1977 and 1997 the mean N content of FDE doubled from approximately 200 to 400 mg N l(-1) The trend for higher N concentrations is likely to continue as dairy herd numbers increase. The most likely reason for the increase in N is that volume of washwater used per cow has proportionately decreased as herd size has increased, thus, FDE has become more concentrated with levels above 400 mgN l(-1) increasingly common. Average values of phosphorus (P) and potassium (K) in FDE were 70 and 370 mg l(-1), respectively. Slurries obtained from anaerobic ponds, which require periodic desludging about every 5 years, had average nutrient concentrations of 1650, 290, and 510 mg l(-1) for N, P, and K, respectively, representing an accumulation of minerals over the 5 years.