A used oil re-refining facility in the Midwest discharges to a municipality that has strict cyanide pre-treatment local limits based on strict NPDES Permit conditions for the municipality. Local limits, which are usually distributed equally for all of a municipality’s industrial dischargers, are required to be imposed by municipalities for those parameters that are not directly treated by the municipality’s Publicly-Owned Treatment Works (POTW), such as organics and suspended solids, but still could cause other issues with the municipality’s POTW operations, such as health and safety of the POTW’s operators, disposal restrictions of the sludge produced as a result of the POTW operations, or in the case of this case study, compliance with the municipality’s NPDES Permit limits.
Given the low cyanide pre-treatment limits, the used oil re-refining facility had difficulty in complying with their municipality pre-treatment permit. The challenge was compounded by the following factors:
- The primary source of the cyanide to the municipality’s POTW was from the used oil re-refining facility (i.e., no other sources of cyanide were present for the municipality);
- The used oil re-refining facility’s pre-treatment wastewater treatment process was not designed for treatment of cyanide;
- The municipality did not have an approved cyanide analytical method that would result in accurately evaluating the amount of cyanide in their incoming sources to the POTW; and,
- The local limits for cyanide were uniformly applied for by the municipality for all incoming industrial sources and were originally developed using overly conservative assumptions.
Given the complexity of the challenge, a multi-tiered approach was utilized to assist the used oil re-refining facility to address the cyanide compliance issues. The multi-tiered approach consisted of:
- Conducting treatability studies to evaluate if improvements to the used oil re-refining facility’s pre-treatment wastewater treatment process can be implemented that would result in additional cyanide removal before discharge to the municipality’s POTW;
- Performing a cyanide analytical methods evaluation study to determine the most effective analytical method for the measurement of cyanide; and,
- Approaching the municipality with suggestions for re-deriving the local limits for cyanide.
As a result of the approach:
- Minor operational improvements such as improving equalization via the cleaning of tank sediments and the improvement of aeration capacity by utilizing overflow pipes were implemented to improve cyanide removal efficiencies;
- A more effective cyanide analytical method was successfully recommended to the municipality for future municipality-wide cyanide measurements; and,
- The municipality was successfully approached to derive the local limits utilizing more reasonable assumptions and a different re-allocation method other than uniform.
The municipality has re-developed the local limits for cyanide utilizing the more realistic assumptions and is in the process of re-allocating the cyanide local limits so that the used oil re-refining facility has a bigger share of the cyanide being discharged to the municipality’s POTW. The greater share will have no impact to the municipality’s POTW operations.