The concentrations of PO₄-P, NH4-N and certain minerals (Mg, Ca, K, Cu, Zn) in effluents of bio-gas digesters were determined in 6 swine farms in Tan Phu Thanh and Hoa An villages in Can Tho Province, Vietnam, for the evaluation of water pollution from swine husbandry with the incorporation of a bio-gas digester into the farming system. Phosphate, NH₄-N, Mg and Ca occurred at high concentrations in the effluents. However, the phosphorus and nitrogen balance associated with the cycling of the components in the farming system was considered to be satisfactory, because the total volume of effluents was small compared to the area of the ponds and rivers where the effluents were discharged. It was demonstrated that sludge in a bio-gas digester contains high concentrations of P, Mg and Ca, which are considered to be effective fertilizers. Artificial crystallization reactions (MAP and HAP reactions) were examined for the effluents of the bio-gas digester with the pH adjusted by NaOH to confirm the possibility of removing and recovering PO₄-P, Mg and Ca from the effluent. Because of the high density of crystals, the components could be removed and recovered easily by sedimentation.