In order to maintain compliance when disposing waste, facilities must follow specific criteria for finding metal levels in waste discharge. These findings result from toxicity tests in diluted water and there are four strategies available for dischargers to achieve compliance: source reduction, treatment facility design, operational strategies, and application of Environmental Protection Agency (EPA)-approved procedures.

Source Reduction

In many cases, source reduction strategies—such as product substitution—can be effective in reducing metal contaminant loading to a treatment facility.

Treatment Facility Design

The following design features can result in enhanced metal removal or more lenient permit limits at secondary treatment facilities:

• Chemical additions can accelerate how certain metals dissolve or alter charge stabilization to encourage adsorption and co-precipitation with biosolids
• Tertiary filtration maximizes the removal of liquid waste
• Extend outfalls to larger receiving streams or provide diffusers to obtain improved mixing and dilutional values

Operational Strategies

Simple operational controls—sludge age, managing decant operations, filtering dewatering operations, and optimizing biological nutrient removal (BNR) systems to maximize alkalinity recovery—can be implemented to minimize metal concentrations.

EPA-Approved Procedure

Since many metals are considerably less toxic in most effluents than laboratory-diluted water, the EPA and some state authorities have recognized that values may be overly conservative for many facilities.

To address this issue, several states have allowed the use of some EPA-published procedures—such as the Water-Effect Ration (WER)—which involve the derivation of revised toxicity limits based on site-specific conditions. In other words, facility permit limits can be raised to account for lower toxicity exhibited when metals are present in wastewater rather than pure laboratory water.

Metals such as copper respond very well to the WER procedure, providing a cost-effective way for facilities to obtain metals permit limits which are both protective to the environment and reasonable for helping facilities maintain compliance.

Often, the most practical approach to control metals in the discharge of wastewater treatment facilities requires an integration of more than one of the approaches noted above. Careful consideration of each, along with long-term operational goals and cost consideration, is important to properly addressing metals.