Comparison of a Stratified and a Single-Layer Laboratory Sand Filter to Treat Dairy Soiled Water from a Farm-Scale Woodchip Filter

Washing-down parlours and standing areas, following milking on dairy farms, produce dairy soiled water (DSW) that contains variable concentrations of nutrients. Aerobic woodchip filters can remove organic matter, nutrients and suspended solids (SS) in DSW, but the effluent exiting the filters may have to be further treated before it is suitable for re-use for washing yard areas. The performance of a single-layer sand filter (SF) and a stratified SF, loaded at 20 L m-2 day-1, to polish effluent from a woodchip filter was investigated over 82 days. Average influent unfiltered chemical oxygen demand (CODT), total nitrogen (TN), ammonium-N (NH4-N), ortho-phosphorus (PO4-P) and SS concentrations of 1,991 +/- 296, 163 +/- 40, 42.3 +/- 16.9, 27.2 +/- 6.9 and 8430 mg L-1 were recorded. The single-layer SF decreased the influent concentration of CODT, TN, NH4-N, PO4-P and SS by 39, 36, 34, 58 and 52 %, respectively. Influent concentrations of CODT, TNT, NH4-N, PO4-P and SS were decreased by 56, 57, 41, 74 and 62% in the stratified SF. The single-layer SF and the stratified SF were capable of reducing the influent concentration of total coliforms by 96 and 95 %, respectively. Although a limited amount of biomass accumulated in the uppermost layers of both SFs, organic and particulate matter deposition within both filters affected rates of nitrification. Both types of SFs produced final water quality in excess of the standards for re-use in the washing of milking parlours.