Study on removal efficiency of residual milk from milk line for wastewater purification at milk house

Milking centers at dairy farms generate wastewater through the washing of milking units, milk line and bulk cooler. High-performance treatment systems for water purification, such as activated sludge systems, have been used at some large-scale dairy farms but are generally not feasible at almost all dairy farms because of the high costs of construction and maintenance. The aim of this study is reducing the contamination of raw wastewater from milking centers by methods for efficient draining of residual milk in milk lines. Residual milk in the milk lines was drained into the pail after milking by letting it flow down the pipeline naturally or by forcing out the line using compressed air. Removing or reducing residual milk from milk lines as much as possible will be beneficial for low-cost treatment systems for milking center wastewater. Investigation of 7 tie-stall dairy barns (TS) with a pipeline milking system and 10 free-stall dairy barns (FS) with a milking parlor revealed the following. At the TS, the average gradient of the milk lines was 0.34+-0.14 deg, and the average time for capturing residual milk was 14+-11 min. At the FS, the average gradient of the milk lines was 0.67+-0.19 deg, and the average time for capturing residual milk was 12+-5.2 min. Contamination of milking center wastewater with milk was lower for milking pipelines with a high gradient. On the basis of these results, milking pipeline gradient has more effect on the amount of captured residual milk than the capturing time when capturing residual milk after allowing it to flow naturally down the pipeline. In addition, a laboratory experiment revealed that capturing time has more effect on the amount of captured residual milk than pipeline gradient when removing residual milk with compressed air. Therefore, a high gradient and a long drainage time with compressed air will provide the most effective low-cost treatment system for removing or reducing contaminants in milking center wastewater. On average, 51 % of BOD in milking center effluent was reduced when extending the drainage time of residual milk with compressed air to 10 min at 3 dairy farms where 5 min of drainage using compressed air was normally conducted. At dairy farms, therefore, extending the drainage time for residual milk with compressed air might be a feasible means to improve the removal efficiency of residual milk while reducing the contaminants of milking center effluent.