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This section addresses municipal and industrial WWTPs that either create saline water in their operations or receive saline water and discharge it. The concentration of chloride present in the waste stream will vary for every facility and is dependent on the source of chloride. The major source of chloride to municipal WWTP is from residential water softeners (>90% in some municipalities).
If WWTPs effluent chloride concentrations demonstrate a reasonable potential to exceed 230 mg/L, the MPCA will work with the permitted entity to include appropriate permit conditions, including monitoring requirements, compliance schedules, and possible effluent limits. If a permitted facility receives a chloride limit they will be required to identify sources of chloride.
For municipal wastewater facilities, technologies capable of removing chloride from wastewater are either cost-prohibitive, technologically infeasible, or a mix of the two. The RO and evaporation of the resulting brine is the most viable option for removal of chloride at the WWTP. The MPCA analyzed the cost and implementation concerns of using RO treatment and evaporation to remove chloride for WWTPs in 2012 (Henningsgaard 2012), which is also summarized in the section addressing costs. Based on the assessment, RO treatment and evaporation are cost prohibitive and pose significant implementation concerns.
The most feasible option for reducing chloride loading to the WWTPs is upstream source reduction. The two primary sources of chloride to WWTPs are industrial users and residential water softeners. If a facility has a chloride limit or would like to voluntarily reduce chloride, WWTPs should work through their Industrial Pretreatment Program (IPP) to identify significant users who may be contributing chloride. The WWTPs can review existing data from industrial users or can require industrial users to collect chloride data. If industrial users are identified as a significant source of chloride, the WWTP can work with the industrial user through the IPP to develop and implement a plan to reduce chloride loads