Wastewater Treatment Facility Improvements
HR Green was retained by the City of Fairmont, Minnesota to complete a Facility Plan, final design, and construction phase services for the Fairmont Wastewater Treatment Plant (WWTP). The Facility Plan report included a thorough evaluation of existing treatment processes and structures as well as recommended improvements to meet future conditions.
The existing site footprint limited the size and orientation of new structures and required that existing facilities be incorporated into the plan where possible.
Specific concerns facing the 2.35 MGD plant included the following:
- More stringent effluent limits with the reissued NPDES permit including phosphorus and ammonia nitrogen limits.
- Industrial flow and load increases and their impact on WWTP operations.
- Evaluation of disinfection alternatives, which would eliminate the need for chlorine gas and corresponding safety requirements.
- New solids treatment processes to replace the existing aerobic digester and sludge drying beds. Recommendations included a Class A biosolids dryer to make the WWTP less dependant on land application issues.
- Wastewater user charge evaluation to determine the rate increase necessary to pay for the necessary improvements.
Preliminary and Primary Treatment
The historical peak flow events were analyzed as part of the Facility Plan. The preliminary and primary treatment processes were sized for the peak hourly flow of 11.5 MGD with secondary process handling the maximum day flow of 7.57 MGD. Wet weather flows in excess of the maximum day were diverted around the secondary process and through disinfection prior to discharge.
A new Headworks Building was designed with mechanical fine screening, flow measurement and two 7.0 MGD induced vortex grit chambers. Each grit chamber included grit pumps to move the grit to a cyclone and inclined screw for grit separation and further dewatering.
Two new primary clarifiers were designed to improve primary treatment. The clarifiers provide greater removal of suspended solids and reduce nutrient loading to the secondary process.
The aeration basin improvements include converting the existing aerobic digester into an additional aeration basin, upgrading the exiting aeration basins with a fine-bubble membrane diffuser aeration system, new blowers, and the existing aeration basins were modified with baffles to separate them into anaerobic and anoxic zones for biological nutrient removal. The improvements provide increased solids retention time and improved oxygen delivery in order to achieve ammonia removal. Two additional secondary clarifiers were recommended along with continued use of the two existing secondary clarifiers.
Disinfection alternatives considered the use of liquid chemicals for chlorination and dechlorination as well as ultraviolet (UV) disinfection. One concern was the anticipated higher oil and grease levels from industrial sources and its effect on the UV process. Samples of a similar wastewater source were analyzed for UV transmissivity during schematic design phase. The results were found to be above 70%. Based on the favorable transmissivity results and operator preference to eliminate the use of chemicals, UV disinfection was incorporated into the final design.
The new sludge quantities were developed based on a higher solids production due to chemical phosphorus removal. The solids processing evaluation focused on alternatives that relied upon land application for final disposal of Class B sludge. Waste activated sludge (WAS) is gravity thickened and pumped to a new rotary drum thickener for additional thickening. Primary sludge is blended with thickened WAS (6 percent solids) and pumped to two primary anaerobic digesters. The digesters are heated and mixed with external draft tube mixers/heat exchangers. The resulting stabilized biosolids flows to a secondary sludge storage tank and stored for dewatering. Digester gas is collected from the floating cover sludge storage tank and used to fuel one of the two 375,000 Btu boilers.
Stabilized sludge is dewatered to approximately 24 percent solids with a single one-meter high-solids belt filter press. The dewatered biosolids is conveyed by a belt conveyor either to a covered cake storage area or for further processing by a sludge dryer. The heat drying process produces a Class A biosolids product. Class A biosolids has the advantage of fewer monitoring requirements and more disposal options.
The Facility Plan included an evaluation of the City’s wastewater rates. The expected cost of improvements, administrative costs and anticipated operation and maintenance (O&M) costs for both the WWTP and collection system were evaluated. A 25 percent rate increase was recommended starting in 2003. The improvements project will be funded with a low interest loan from the State Revolving Fund (SRF). HRG provided assistance to the City in applying for and working with regulatory officials to comply with the requirements for this loan.