WHY BE CONCERNED OVER END-OF-PIPE METAL WATER QUALITY STANDARDS?

For wastewater treatment plants that discharge to small streams or are denied a mixing zone, meeting metals limits based upon water quality standards will be a difficult, if not impossible challenge. While the actual numerical limits may vary from state to state, most of the metals limits are in the low ug/L range. In the early 1980's, when the industrial pretreatment limits were promulgated, many industries argued that these limits could not be met. It was found that in most cases, the limits could be easily met by alkaline or sulfide precipitation, however, when the metals were chelated or complexed, it was difficult to meet the pretreatment standards. The limits set in the water quality standards are anywhere from 1 to 3 orders of magnitude lower than the pretreatment limits.Can a conventional municipal wastewater treatment plant that relies on co-precipitation and bioadsorption for metals removal comply with these new standards? Jar tests were performed on wastewater effluent from a treatment plant in South Carolina to investigate the possibility of meeting a permit requiring 2.9 ug/L copper and 37 ug/L zinc at end-of-pipe. This particular treatment facility served only domestic users. The copper and zinc present in the wastewater came solely from domestic sources.The influent concentrations of copper and zinc in 1999–2000 averaged 108 ug/L and 394 ug/L respectively. The effluent concentrations averaged 11 ug/L for copper and 79 ug/L for zinc. The treatment plant was routinely removing 90 percent of the copper and 80 percent of the zinc. These removal efficiencies are not only within the expected range for metals removal across a WWTP, they are above average. The effluent quality from this facility was very good; however, even with the above-average removals for copper and zinc, the numerical water quality standards were not met.How can a conventional Publicly-Owned Treatment Works (POTW) comply with these strict standards? One of the first things to consider is moving the point of discharge to another receiving stream to obtain a mixing zone. If the new stream is not impaired and has a significant 7Q10 low flow, the allowable mixing zone may be sufficient to ensure compliance. Other alternatives that may need to be evaluated include enhanced metals removal, reducing the metals load to the treatment plant by more restrictive pretreatment limits, or zero discharge (no discharge to a receiving stream). This paper presents the findings of a treatability study that looked at enhanced treatment of metals in the wastewater treatment plant effluent.