Membrane filtration requires frequent backwashing to remain clean and functional, a process that uses a lot of water. In the case of the City of Mankato, Minn., it was 450,000 gallons every day of treated drinking water. That water was immediately discharged to the sanitary sewer system, which meant it had to travel back to the drinking water facility and be retreated. As a result, the city pumped an additional 450,000 gallons of raw water from its wells every day, which increased costs, and there was unnecessary hydraulic loading on the wastewater side, which also increased costs.
A shortcut devised by the Public Utilities Department and consulting engineers Bolton & Menk Inc. earned an American Public Works Association (APWA) Project of the Year award in the Environment (less than $5 million) category.
The backwash-reclamation project converted holding tanks to reclamation tanks to return the 450,000 gallons back to the drinking water supply. A major design consideration was ensuring the converted tanks and return system met regulations for potable water storage and conveyance. According to the July 2017 issue of the APWA Reporter, this required:
- 50-foot setbacks of all sanitary and storm sewer lines to avoid potential contamination
- Installing curbs around all tank openings
- Installing access hatches rated for potable water storage Modifying drain piping to include double-wall piping inside the reclaim tanks for drinking water protection
- Modifying building plumbing and process piping routing to meet all state health department requirements for cross connections
- Installing an air break manhole outside of the 50-foot setback to provide positive backflow prevention to the sanitary sewer discharge
- Disinfection of the tanks and return piping before use in accordance with potable standards.
The project finished on schedule and under budget. After six months of operation, the city is projected to save nearly 180 million gallons in the first 12 months. This has significant benefits to the sustainability of the city’s raw water resources, as well as improving operations at the water and wastewater treatment facilities.
The city was managing agency, Gridor Construction was primary contractor, and Bolton & Menk was primary consultant.
