On many dairy farms, the water used to wash milking equipment is contaminated with bacteria and has to be disinfected. Often, the water requires a coagulation–flocculation (CF) pre-treatment to reduce turbidity and remove dissolved organics prior to disinfection. This paper examines the effect of temperature and water characteristics on the efficiency of an on-farm CF treatment using polyaluminum chloride (PACl) as coagulant. Since the CF process concentrates suspended solids and bacteria in a sludge that will be land-applied, Escherichia coli survival and gene transfer occurrences in the sludge were also determined. Coagulant dose was highly correlated to water UVA254 nm, but not turbidity. For water with variable UVA254 nm, exceeding 0.85 cm−1, the coagulant dose could be adjusted using a simple online UVA254 nm sensor, while settling time should be increased when water temperature drops below 10 °C. E. coli survived a 2-h PACl exposure at a dose of 0.05 mL ClearPAC/L. There was no difference in conjugative transfer of a multi-drug resistance conferring plasmid in water without PACl and in the PACl-derived sludge over a 2-day period. However, since bacteria remained viable in sludge and genetic conjugation may occur, sludge residues should be stored in the manure tank prior to land application.

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