Metals Effluent Limitations: Pathways Toward Compliance - PUB2710

It is not uncommon for a wastewater treatment facility to have metals monitoring or limitations included in its Missouri State Operating Permit. Usually these monitoring requirements or limitations are included as a result of known industries identified in an operating permit application or as the result of expanded effluent testing where required.

Municipal wastewater treatment facilities are often not designed for metals removal as metals are not a significant constituent in domestic or municipal wastewater streams. However, metals can make their way into a wastewater collection system through a variety of sources such as industrial and commercial connections, through inflow and infiltration in the collection system, source water contamination and even drinking water distribution systems. It is important to identify the sources of metals in the collection system in order to effectively mitigate the problem before the metals reach the wastewater treatment facility. Effectively managing and removing the metals prior to the water treatment plant is generally more affordable than the construction and operation of metals removal technologies prior to discharge. 

Sources and Mitigation

In most cases, metals contribution to a wastewater treatment plant is the result of industrial processes such as a metal plating facility. Routine evaluation of all commercial and industrial users in the community should be conducted to determine what pollutants may be introduced to the treatment system. This may include metals monitoring throughout the collection system to possibly identify metals sources, industrial or not, that may not be obvious from the outside. The development of local and enforceable ordinances that prohibit these discharges to the sanitary sewer or a pretreatment program are other options for eliminating metals from the waste stream before they are introduced to the system.

As Inflow and Infiltration (I&I) into the sewer collection system allows excess water to make its way into collection system through cracks, crevices and damaged parts of infrastructure, contaminants present in the soil and surrounding environment may be introduced to the system that otherwise would not be. Being able to quantify metals concentrations of a treatment plant’s influent and correlating those concentrations to periods of wet weather may solidify I&I as being the contributing factor. In these cases, a facility may wish to prioritize available funds or funding opportunities to reduce I&I verses treatment technologies.

In certain parts of the state, drinking water sources for the area served by the wastewater facility should be evaluated for any metals present. Metals present in drinking water wells should be addressed as these metals will inevitably end up being received by the wastewater system. Drinking water sources need to then be evaluated for the possibility of treatment prior to distribution or in some cases perhaps the construction of a new well may be a more financially achievable option than wastewater treatment. Additionally, and although the source water may not have significant metals concentrations, its characteristics may increase the leaching of metals from drinking water distribution pipes which then makes their way to the wastewater treatment facility after use. In these cases, treatment of the drinking water in combination with a routine distribution system maintenance program, including periodic or increased flushing of the system, may be sufficient to reduce metals loading to the wastewater treatment facility.

If mitigating the issue at the source is determined not to be a practical solution, the development of site specific water quality criteria may be of some benefit. Toxicity or bioavailability of a specific metal is based on several factors including, but not limited to hardness, pH and conductivity. 

Missouri Regulation 10 CSR 20-7.031 allows for the development of site-specific criteria, including but not limited to, water effects ratio and metal translator studies.

A water affects ratio compares the toxicity of a material obtained from onsite water divided by the same measure of the toxicity of the same material obtained simultaneously in a laboratory setting.  A metal translator study may also be utilized to establish site-specific criteria.  A translator study is used to translate the dissolved fraction of a metal to total recoverable.  This conversion is necessary as water quality criteria are reflective of the dissolved portion a better representation of aquatic life toxicity in a receiving stream’s water column, while effluent limitations are, for the most part, required to be represented as total recoverable per 40 CFR 122.45(c). 

Guidance documents for establishment of site specific criteria include but are not limited to the following: 

• U.S. Environmental Protection Agency Interim Guidance on Determination and Use of Water-Effect Ratios for Metals (EPA-823-B-94-001) and subsequent 1997 modifications
• U.S. Environmental Protection Agency Streamlined Water –Effect Ratio Procedure for Discharges of Copper (EPA-822-R01-005)
• U.S. Environmental Protection Agency Water Quality Standards Handbook, Second Edition, August 1994
• U.S. Environmental Protection Agency Aquatic life ambient freshwater quality criteria- Copper 2007 Revision (EPA-822-R-07-001)
• U.S. Environmental Protection Agency The Metal Translator: Guidance for Calculating a Total Recoverable Permit Limit From a Dissolved Criterion June 1996 (EPA-823-B-96-007)

In order to develop site specific water quality criteria the permittee must submit a Quality Assurance Project Plan (QAPP).  Prior to conducting a study, the QAPP must be submitted for review and approval by the Department of Natural Resources' Water Protection Program.