2017 Phosphorus Removal and Digester Improvements

The City's EPA NPDES Discharge Permit to the Fox River imposes an annual average effluent total phosphorus concentration of 1.0 mg/L and a monitoring requirement for total nitrogen. The permit also includes several special conditions; one of which requires that the City study, design and construct improvements which will allow the plant to achieve a 1 mg/L effluent phosphorus limit within 54 months after issuance of the permit.  The City must have a phosphorus removal process in operation by June 1, 2019.

The existing anaerobic digesters and digested sludge storage tank have been in operation for 25 years. Corrosive conditions within the anaerobic digestion process impact valves, piping equipment and controls. Several emergency repairs have been needed to repair major system components. Other improvements are for code compliance, such as access, fire protection, and gas safety.

Project Phase:  Construction
Proposed Improvements:  Phosphorus and Digestion Improvements
Contractor:  IHC Construction Companies, Elgin, IL
Construction Engineer:  Trotter and Associates Inc., St Charles, IL
Estimated Timeline:  September 2017 – March 2019
Project Location:  Main Wastewater Treatment Plant, 200 Devereaux Way
Project Financing:  ILEPA Clean Water State Revolving Fund (SRF)
Project Cost:  $15,032,396   

Construction of the City of St. Charles’s Main Wastewater Treatment Facility Phosphorus Removal and Digester Improvements is financed by the Clean Water State Revolving Fund (SRF). The SRF program is administered by the Illinois Environmental Protection Agency and receives a portion of its money to fund wastewater projects from the U.S. Environmental Protection Agency.  

The City’s Improvements Project consists of:
•             Construction of biological phosphorus removal facilities, including a primary sludge fermenter and conversion of existing aeration
               basins to anoxic and anaerobic reactors;
•             Construction of two internal pump stations;
•             Installation of a chemical feed system for phosphorus removal, including chemical pumps and chemical storage
•             Rehabilitation of the existing anaerobic digestion system; and
•             Construction of a new sludge storage tank

The City proposes implementing biological nutrient removal using the A2/0 process configuration to address effluent phosphorus limits and future effluent total nitrogen limits. The A2/0 configuration of the biological process utilizes three zones; an anaerobic zone, followed by an anoxic and an aerobic zone.   This configuration can be implemented within the existing concrete basins, requiring only the construction of baffle walls for zone isolation and the conversion of existing aeration basin volume to anoxic and anaerobic basins. A proposed internal recycle pump station will return approximately two times the design flow from the end of the aerobic zones to the head of the anoxic zones. Operation and maintenance will be simplified by having all internal recycle pumping equipment in one location. A primary sludge fermenter for carbon augmentation will be constructed adjacent to the biological process.

The proposed improvements include appropriate detention time to meet effluent standards at average loading and design flow conditions. Under maximum day demand loading the biological process will revert back to single-stage nitrification. In this mode, the City will utilize chemical precipitation for phosphorus removal to achieve the proposed NPDES permit effluent limits. Chemical feed systems will be constructed as part of the proposed improvements.

The proposed improvements to the anaerobic digestion system include rehabilitation of the existing digesters, increasing the digested sludge storage capacity, and increasing the digester gas storage. Rehabilitation work includes installation of a new mixing system, replacement of the gas safety equipment, and other architectural, structural, mechanical and electrical improvements.

The above improvements are required to replace aging infrastructure and equipment as well as achieve current and future EPA mandated effluent limits.  SRF programs operate in each state to provide communities the resources necessary to build, maintain, and improve the infrastructure that protects one of our most valuable resources of water.


Update - 12/08/17

Construction is currently underway with progress moving forward on schedule. The South egg-shaped anaerobic digester has been drained and cleaned. Construction crews have set up scaffolding systems so they can safely work inside for the next couple of months as they rehabilitate the tanks, piping and walls.