Treatment of dirty water from dairy farms using a soil-based batch recirculation system

“Dirty water”, a wastewater produced on dairy farms, is typically disposed of by application to land with no prior treatment. Pollution can occur if the dirty water reaches a watercourse following an inadequate period of retention in the soil. This paper describes experiments using a novel, soil-based batch recirculation system for pre-treating dirty water prior to land application. Three polythene-lined, vegetated soil-based treatment planes (23 m long, 1 m wide, 0.25 m deep) were constructed. Each treatment plane was supplied with approximately 1 m³ of dirty water which was recirculated until a clear treatment pattern had emerged. Five batches were treated over a six-month period. The soil-based treatment system could typically be expected to achieve a 90% removal of key pollutants in approximately two weeks for BOD₅ and NH₄-N, and three weeks for MRP and total solids. An exponential trendline gave a good fit to the treatment curves for BOD₅, NH₄-N and MRP after the first day or two of batch treatment. The data for total solids removal were more variable. Treatment rates were sustained throughout the five runs for BOD₅ and NH₄-N, indicating no apparent effect of seasonal weather on the treatment process. The apparent progressive slowing of the MRP removal rate throughout the treatment of the five batches may have implications for the sustainable use of this technology for phosphorus control.