Assessment of current status of zinc in wastewater treatment plants to set effluent standards for protecting aquatic organisms in Japan.

New environmental standards for protecting aquatic organisms for zinc (e.g., 0.03 mg/L) in surface waters were set in Japan in 2003. Although wastewater effluent might be one of the major pathways of zinc to public water bodies in Japan, current status of concentration of zinc in wastewater effluent was not clear due to higher detection limits (e.g., 0.5 mg/L) than the level required by the new regulations. This study aims at assessing current status of zinc in wastewater effluent in Japan to revise wastewater effluent standards for protecting aquatic organisms. Survey of zinc in wastewater treatment plants (WWTPs) was carried out in Japan in 2005, setting the detection limits at least 0.01 mg/L. The results of the survey suggested the difficulty to remove zinc (especially dissolved zinc) with conventional activated sludge treatment if concentration of zinc in influent was relatively low. And it was suggested that high concentration of dissolved zinc might be derived from some industries discharging high concentration of zinc. The concentration of zinc in wastewater influent without industrial discharges was about 0.1 mg/L which might be lower than that in wastewater from industries discharging high concentration of zinc. Finally, effluent standards for point sources including WWTPs to public water bodies were set at 2 mg/L in 2006. Based on the results of the survey that it was difficult to remove dissolved zinc discharged from industries at WWTPs, the effluent standards from industries to sewerage were set at the same value of the effluent standards from WWTPs to public water bodies.