Amendment No 2 to PSA - WWTP Hydraulic & Clarifier Proj No. 119-069

Home Browse Search

STANDARD AGREEMENT FOR PROFESSIONAL SERVICES Page 2 of 7 Between City of South Bend and ARCADIS U.S., Inc. Project Number: 30029091 AMENDMENT No. 2 119-069 WWTP Secondary and Disinfection Improvements Amendment No. 2 Schedule A-1 Scope of Amended Services and Related Matters ENGINEERING SERVICES BY ARCADIS WWTP Hydraulic Improvements. The City of South Bend desires to continue its efforts to rehabilitate, modify and expand the WWTP to work towards achieving the CSO Long-Term Control Plan requirement, which requires an increase in the peak wet weather capacity from 77 MGD to 100 MGD. The South Bend WWTP is a Class IV activated sludge treatment facility required to provide preliminary treatment, primary and secondary treatment, phosphorous removal, and disinfection throughout all seasons. The NPDES permit lists a rated average flow of 48 MGD and a rated peak flow of 77 MGD. The plant’s current average flow is approximately 36 MGD and it can treat a peak flow up to 77 MGD. Liquid treatment processes include screening, grit removal, raw sewage pumping, primary treatment, activated sludge, final clarifiers, and chlorination / dechlorination. Sludge treatment processes include gravity thickening of primary sludge, dissolved air floatation thickening of waste activated sludge, two- stage anaerobic digestion, and belt filter press dewatering. Class B biosolids are reused by land application on farmland. As part of Arcadis’ prior planning projects, a hydraulic model was created for the plant and used to model different flow conditions through each process at the plant to achieve the plant’s future 100 MGD peak flow requirement as part of the CSO Long-Term Control Plan. The hydraulic model was calibrated using recorded plant flows and water surface elevations around the plant. Limiting hydraulic elements, also called bottlenecks, were identified and analyzed further under peak flow and other hydraulic conditions. The City has addressed all bottlenecks except two major areas require hydraulic improvements at the WWTP to achieve the future 100 MGD flow: (1) Primary Effluent and (2) Final Clarifier Nos. 6 and 7 Influent, as further described below. In addition, there are abandoned structures within these areas that may be demolished. Primary Effluent Hydraulic Improvements. The preliminary and primary treatment process consists of the headworks facility (screening, grit removal), raw sewage pumping and the primary clarifiers. After the plant flow leaves the headworks, the raw sewage pumps (located in the Compressor Building) pump flow into the primary influent channel which discharges into eight primary clarifiers. The flow from each individual primary clarifier goes into a common primary effluent conduit and the common primary effluent conduit is a hydraulic bottleneck. The primary clarifier effluent conduit restricts flow leaving the primary clarifiers such that at flows equal to or greater than approximately 65 MGD, the primary clarifier effluent flumes are submerged. At 100 MGD flow, the model predicts that this bottleneck results in no freeboard in the primary clarifiers. Modifying the effluent conduit and primary clarifier effluent Parshall flumes would alleviate this bottleneck. The abandoned in place 48-inch primary effluent meter has also been identified as a hydraulic bottleneck in the primary effluent piping. The following is proposed to alleviate the primary effluent bottleneck. A new open channel located parallel to the existing primary effluent conduit along Primary Clarifier Nos. 1 through 4 only, north of the Primary Scum Building, and utilizing the abandoned east-west bypass channel located above the primary effluent conduit is recommended. The new channel and the existing conduit will flow into the existing east-west conduit and into the re-utilized abandoned bypass channel. Flow from the bypass channel will flow into the primary effluent structure, similar to the existing primary effluent conduit, by removing the bypass channel gates and demolishing a portion of the primary effluent structure wall. With all primary clarifiers in service (with a return sludge flow of 72 MGD and all other tanks in service), these improvements will un- submerge the weirs up to 80 MGD and the flumes up to 100 MGD and provide approximately 12-inches of freeboard in the primary clarifiers up to 100 MGD. The abandoned in place 48-inch primary effluent meter and 48”x72” reducers will be replaced with 72-inch pipe. Final Clarifier Nos. 6 and 7 Influent Hydraulic Improvements. The Final Clarifier Nos. 6 and 7 influent pipe was identified as another plant hydraulic bottleneck. The 60-inch pipe located between the south end of the Final Clarifier Influent Channel and Junction Box NT1 that feeds Final Clarifier Nos. 6 and 7 is the source of the hydraulic bottleneck under peak flow conditions. At the current peak capacity of 77 MGD and at the FEMA 100-year flood elevation, the 60- inch pipe has inadequate flow capacity and can cause flow overtopping upstream within the plant. This pipe segment has been modeled to show inadequate capacity at the future peak flow of 100 MGD under normal river conditions. Replacing the existing 60-inch pipe with a larger size pipe would alleviate this hydraulic bottleneck. As part of the prior planning, upsizing the existing 60-inch pipe in the segment indicated above to 84-inch pipe is recommended. However, another alternative for consideration is instead to provide a new channel from the south end of the Final Clarifier Influent Channel to Junction Box NT1 or potentially to Junction Box NT. This would require channel mixing. The benefits include