SITE CONSTRAINTS + AMMONIA LIMITS + PEAK WET WEATHER FLOWS = HIGH RATE TREATMENT & BIOSOLIDS TECHNOLOGY FOR THE HEART OF THE VALLEY METROPOLITAN SEWERAGE DISTRICT

The Heart Of The Valley Metropolitan Sewerage District (HOVMSD), located in Kaukauna, Wisconsin, treats wastewater from five communities with a 6.5 mgd pure oxygen activated sludge Wastewater Treatment Facility. Wastewater treatment was originally provided at the site in the 1930's for the City of Kaukauna. The plant was modified in the late 1970's as a regional facility consisting of primary treatment, pure oxygen activated sludge, filtration and anaerobic digestion. The plant was again modified in the mid-1990's to process a peak flow of 30 mgd using primary effluent filtration during peak flow events. Continued growth in the service area (60% increase in loadings), persistent peak wet weather flows in excess of 50 mgd, and the need to meet an ammonia nitrogen limit of 3.6 mg/l during the summer, precipitated the need for plant modifications.Use of conventional technology, such as nitrifying activated sludge, was not possible due to limited site space and severe site restrictions, including shallow bedrock and space constraints (paper mill to the south and west of the triangular site, with a canal on the north). A new site was considered, but was cost prohibitive.Creative application of emerging technology, using high rate treatment and biosolids processing systems, was necessary to expand on site cost effectively. The flow sheet chosen is the first of its kind in the United States. Construction of the new facilities was initiated in January 2006, and will start-up in Spring/Summer 2007.Ballasted sedimentation was selected as the 24/7 primary treatment system to treat normal dry weather flows, as well as peak wet weather flows. This system was coupled with a Biological Aerated Filter (BAF) for BOD removal, phosphorus removal to 1 mg/l, and nitrification. For peak flows above 25 mgd, blended ballasted sedimentation (up to 35 mgd) and BAF effluent (up to 25 mgd) is disinfected prior to discharge. This combination of unit processes is able to achieve effluent limits of 15/15 CBOD/TSS, 3 to 6 mg/l ammonia nitrogen, and 1 mg/l phosphorous in one-tenth of the area required for conventional activated sludge.The exiting tertiary filters were also eliminated, allowing for pumping once through the plant instead of twice, as is the current system. The new solids processing facilities consist of converting the existing anaerobic digesters into Auto-Thermal Thermophilic Aerobic Digesters (ATAD) with sludge nitrification to reduce ammonia laden sidestreams from decanting. This system reused all existing tankage and used one-half the space of conventional anaerobic digestion.The entire project was constructed on the existing site, while still allowing room for a doubling of the organic removal capacity in the future through expansion of the ballasted sedimentation system, BAF and ATAD. Benefits of the project include: Reuse of existing site, wet weather peak flow, blending of effluent in compliance with the intent of the USEPA's new blending policy, economic expansion, reuse of all existing tankage, advance treatment without secondary clarifiers, and Class A pathogen reduction for biosolids processing.