The Water Reclamation Plant is staffed 24 hours per day and has 30 full-time employees. The site is situated on 32 acres with buildings covering 8 acres. Two parallel treatment processes are used on-site. The High Purity Oxygen (HPO) plant began operations in 1983, and the Aeration Basin Complex (ABC) began operations in 2007. The Rochester Water Reclamation Plant currently treats an average of 13 million gallons of wastewater per day, using a combination of physical, chemical, and biological processes to remove pollutants. The wastewater undergoes at least 10 steps, which take approximately 24 hours to complete, before being discharged to the South Fork of the Zumbro River.
Water Reclamation Plant
The Water Reclamation Plant treats wastewater for the City’s residents, industries, and numerous visitors. The original plant was built in 1926 and was one of the first wastewater treatment plants to operate in Minnesota. Since that time, five additional major upgrades have occurred to comply with effluent regulations and meet the needs of the growing Rochester community. The Water Reclamation Plant is located at 37th Street NW and West River Parkway.
Plant Processes
Both the HPO and ABC plants use an activated sludge process, which relies on biological growth to remove pollutants that would otherwise be extremely difficult to remove by chemical or physical means. The general concept is the same for both plants; however, there are some differences, primarily that the HPO plant uses high-purity oxygen (92%) to meet the microorganisms' aeration requirements. The HPO plant is also divided into two stages: the first for organic removal and the second for ammonia removal. ABC plant removes the same pollutants but uses conventional air (21% oxygen) and removes organics and ammonia in the same tank.
The Rochester Water Reclamation Plant uses several advanced treatment technologies, including biological nutrient removal. Both plants perform nitrification, and the ABC plant has the ability to denitrify (convert nitrates to nitrogen gas) and to perform biological phosphorus removal. Both plants inject ferric chloride into the primary clarifiers and can inject alum into the final clarifiers to assist in precipitating phosphorus.
Between April and October, chlorine is added to the treated wastewater to kill most remaining microorganisms from the activated sludge that weren't captured in the clarifiers. This protects any people or animals that come in contact with the reclaimed water after it is released to the Zumbro River. Sodium bisulfite is used to neutralize residual chlorine and prevent negative impacts on downstream aquatic life. The reclaimed water is also aerated during conveyance to the river to increase dissolved oxygen levels for aquatic life.
Wastewater enters the plant where centrifugal pumps lift the flow 65 feet to the headworks for preliminary treatment. This is the only time the liquid stream is pumped; thereafter, it will gravity flow through all the remaining processes. The first process encountered is the fine screens, which allow liquids to pass while catching any debris larger than 6 mm. After the screens, a vortex grit unit removes grit, sand, and heavier particulates. Flow is then directed to the HPO Plant, the ABC Plant, or the equalization basin. Equalization is a storage tank that allows flow to be stored during high-flow periods and released during low-flow periods, reducing strain on the plant.
Primary clarification is used to separate out solids that will fall out of suspension when the flow velocity is reduced. Solids are collected at the bottom of the tank and sent to the plant's solids treatment section. Approximately 50% of total suspended solids and 40% of organic matter are removed during primary clarification.
Waste activated sludge from both stages in the HPO plant and the ABC plant is thickened on the gravity belt thickeners (GBTs) prior to being combined with primary sludge. Blended sludge is anaerobically digested in a single-stage digester with a detention time of approximately 30 days, which produces a methane-rich gas byproduct. The digested sludge is thickened using GBTs to reduce volume before being stored and ultimately land-applied seasonally.
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