WASTEWATER TREATMENT

摘要

Today, some 15,000 companies in the United States perform electroplating and metal finishing operations. These firms discharge their spent process wastewaters either directly to rivers and streams, or indirectly to Publicly Owned Treatment Works (POTWs). Metal finishing, by far, comprises more individual wastewater discharges than any other industrial category. Typically, pollutants contained in metal finishing process waters are potentially hazardous, therefore, to comply with Clean Water Act requirements, the wastewaters must be treated, or contamination otherwise removed, before being discharged to waterways or POTWs. Regulations, in general, require oxidation of cyanides, reduction of hexavalent chromium, removal of heavy metals, and pH control. Understandably, for companies discharging wastewater directly to waterways (direct discharges), regulations promulgated through the years require attainment of the more stringent concentration-based limitations for toxic wastewater constituents necessary for protection of aquatic life. These stream standards were developed from Federal Water Quality Criteria and limit in-stream pollutant concentrations to levels that will not adversely affect drinking water quality and aquatic life. Since the mid 70s, state agencies have continued to drive direct discharge limitations downward to levels well below water-quality-based stream standards, using antidegration, antibacksliding, and existing effluent quality (EEQ) policies, and the number of direct dischargers has dropped precipitously. Implementation of biological-based criteria through biomonitoring and bioassay testing will continue to force direct discharging facility closures and relocation to POTWs.