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Marshall WTP Softening Enhancement Project


Marshall, Minnesota

Water Quality Regulations Drive Collaborate Approach for Treatment

The City of Marshall owns and operates the Wastewater Treatment Facility (WWTF) that serves the City’s municipal, commercial, and industrial customers. Treated wastewater effluent from the WWTF is discharged to the Redwood River which is an impaired water body.

Based on monitoring and evaluation by the Minnesota Pollution Control Agency (MPCA), it was determined that reissuance of the permit would require compliance with more stringent water quality parameters.

One of the WWTF effluent contaminants of concern was chloride. After further review it was determined that the removal of chloride at the wastewater treatment facility would be costly and other solutions were reviewed.

Project Details

Date: 2018

Client: City of Marshall

Key Project Elements
  • Comprehensive Water/Wastewater System Approach
  • Funding
  • Softening

The City of Marshall and Marshall Municipal Utilities (MMU) joined together to evaluate how they could collaborate on a cost effective and sustainable solution that would best serve the community. After in-depth analysis of their water and wastewater systems, as well as discussions with local industrial users, it was determined that the most financially and operationally feasible solution would be to provide additional softening at the MMU Filtration Plant (MMUFP).

Additional softening at the MMUFP would reduce the water hardness from 600mg/l to 100mg/l and significantly reduce the use of in-home water softeners that were identified as the main contributor to chlorides in the residential wastewater stream.

City and Municipality Join Forces and Commit to Additional Lime Softening

MMU owns and operates a Water Filtration Plant that provides high quality drinking water to the City of Marshall. The MMUFP has a treatment capacity of 8.5 million gallons per day (MGD), and currently provides potable water service to approximately 13,000 residential, commercial, and industrial customers throughout the City.

Approximately 65 to 70 percent of the MMU finished water supply is provided to industrial customers. MMU has historically only targeted removal of the carbonate hardness through the usage of lime softening. This approach allowed them to reduce their total hardness from 900mg/l down to 600mg/l. MMU and the City of Marshall’s new goal of softening down to 100mg/l would require the use of Soda Ash to remove the remaining noncarbonate hardness.

In addition to the enhanced softening MMU would be required to increase their CO2 feed system size and add a gravity thickener to assist with sludge accumulation and handling.

Collaborative Solution Leads to Successful Implementation

Following the identification of a feasible solution, AE2S began the design of a 2,900 SF addition to the existing WTP and select improvements to the existing facility. The modifications to the water treatment plant to accommodate the new softening process included:
  • Replacement of the existing lime slakers with two (2) 2,000pph batch slaking systems
  • Installation of one (1) new 2,000pph batch soda ash prep system
  • Installation of 2 new dry chemical storage silos and refurbishment of two (2) existing storage silos for a total storage capacity of 250 tons of pebble lime and 300 tons of soda ash.
  • Installation of a two (2) 200pph CO2 feed systems
  • Installation of an additional 26-ton CO2 storage tank
  • Installation of a new 55-foot diameter solids thickener
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Marshall WTP Softening Enhancement Project