Wastewater Treatment Process

The St. Cloud Wastewater Treatment Facility (WWTF) was put in service in 1976. The City of St. Cloud owes and operates the WWTF. Treatment is shared contractually among the area cities including St. Cloud, St. Augusta, St. Joseph, Sartell, Sauk Rapids, and Waite Park. The facility is designed to treat an average flow of 13 million gallons per day generated from a population of over 120,000 people.

Process Walk Through 

There are 5 main steps to treating domestic and industrial wastewater.
  1. Preliminary treatment
  2. Primary treatment
  3. Secondary treatment
  4. Final clarification
  5. Solids processing

Preliminary Treatment

The majority of the wastewater flows to the main lift station. The main lift station pumps the water to the treatment facility.

Primary Treatment

Primary treatment is the separation of liquids and solids. The system consists of 4 large rectangular tanks that allow the solids to settle.

Secondary Treatment

Secondary treatment is an activated solids process used to remove suspended solids, dissolved solids, nutrients, and metals. The system is a continuous flow, biological process where microorganisms use the organic material in the wastewater as an energy source. During secondary treatment, organic matter is reduced to carbon dioxide and water.

Final Clarification

Similar to primary treatment, final clarification is where solids are allowed settle out. A portion of the solids are used for "seed" and recycled back to the activated solids process. The remaining solids are further processed. The liquid portion is disinfected using ultraviolet light and then discharged to the Mississippi River.

Solids Processing

Solids are generated at 2 stages during wastewater treatment; primary treatment and secondary treatment.

The 2 solids are mixed together and dewatered. The liquid portion is returned to the beginning of the treatment process and the solids portion is sent to an anaerobic digester.

During the digestion process, solids are heated and mixed for a minimum of 15 days. The digestion process produces liquid, gas and solids. The liquid is returned to the head of the treatment process, the gas is used to heat the digestion process and the solids portion, called biosolids, are land applied.