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Regulatory limits for soil P, significant P concentration in wastewater, lack of nearby pasture or cropland, and uneconomical transport costs have created a wastewater disposal dilemma for many dairies. A simple on-site alum injection system was used to determine the alum requirement to reduce soluble reactive P (SRP) to 1.0, 0.5, or 0.25 mg P L -1 . Alum solution was injected at target rates of 15, 37, 73, and 110 mg Al L -1 wastewater during transfer by centrifugal pump to a cone tank which provided for solids separation and sample collection. SRP was reduced from 11.3 (untreated) to 1.0 mg P L -1 in the supernatant by treatment with 0.47 ml liquid alum L -1 lagoon water (27.6 mg Al L -1 ; 95% CL=0.38 - 0.58 ml L -1 (22 - 33.8 mg Al L -1 )). Solution of the inverse quadratic equation SRP= 1/0.435 + 0.964x + 2.65x 2 (r 2 = 0.96) for SRP = 0.5 and 0.25 resulted in the alum requirement of 0.72 and 1.07 ml L -1 . Supernatant chemical oxygen demand, pH, total P, total N, total solids, total suspended solids, and turbidity decreased steadily (p< 0.0004) with increasing alum rate. Initial electrical conductivity (0.359 S m -1 ) was greater than that of quality irrigation water and linearly increased (p=0.0093) with increasing alum. Estimated increase in soil aluminum content in the surface 15 cm during 20 yr of supernatant application at 39 600 L/day to 8.1 ha is only 0.33%. Estimated chemical cost for reduction of SRP to 1.0 mg P L -1 was $US 11.10/cow/yr or $US 8.04/kg P removed. Results indicate potential for economic and effective reduction of SRP with collateral improvement of other water quality factors.