Water Environment Federation (WEF)

Side By Side By Side: The Evaluation of Three High Rate Process Technologies for Wet Weather Treatment

Most large combined sewer communities face significant financial investment to address wet weather compliance issues. Innovative, cost-effective methods of dealing with wet weather flows is a must to prevent spiraling rate increases for rate payers. High rate clarification and high rate filtration systems are becoming the technology of choice for treating wet weather flows. The City of Akron, Ohio selected three high rate treatment technologies to pilot test and evaluate for incorporation into their wastewater treatment plant as a Secondary Bypass process treatment train. The technologies were evaluated based on their ability to reduce pollutant parameters contained in their NPDES Permit. The pilot units were operated at unit flow rates commensurate with full-scale units. Treated effluent was also tested for potential impacts to ultraviolet disinfection systems. The results of these evaluations establish a baseline expectation for fullscale performance of each process technology if applied for treating Secondary Bypass flow at the Akron Water Pollution Control Station.

The purpose of this Study is to evaluate high rate process technologies for treatment of wet weather flows at either the Akron Water Pollution Control Station (WPCS) for treatment of Secondary Bypass flow or in Akron’s collection system for treatment of combined sewer overflow (CSO). To achieve this objective, three different pilot-scale process units were operated at the Akron WPCS in three phases. They included the Krüger ACTIFLO process, the Infilco Degremont, Inc. (IDI) DensaDeg process and, the Wet Weather Engineering & Technology, LLC (WWETCO) compressed media filter (CMF) process. Other pilot studies for each technology were also reviewed, and existing full-scale processes in the United States were assessed. Evaluation of process treatment performance for treating primary treated municipal wastewater flow (Secondary Bypass) was emphasized in the evaluations.

The Akron WPCS screening and grit removal systems have a maximum capacity (with all units in service) of 280 MGD. Akron’s primary settling tanks (PSTs) have a capacity of 150 MGD while the secondary treatment process has a capacity of 110 MGD. Wastewater flows exceeding the PSTs’ capacity are directed to a Storm Retention Tank (SRT). Once the SRT is full, it overflows (up to a 130 MGD rate) and the overflow combines with PST effluent that exceeds the secondary treatment capacity (up to a 40 MGD rate). Subsequently, during the heaviest rain events, up to 170 MGD can flow through Secondary Bypass piping once the plant’s 60 MG SRT has been filled. The Secondary Bypass flow is typically of primary effluent quality, having been screened and degritted and settled in the PSTs or SRT. It is disinfected during summer months prior to being discharged into the Cuyahoga River.

The WPCS currently operates under a 1998 NPDES permit, which establishes separate limits for the final effluent flow versus the Secondary Bypass flow during wet weather events. However, it is not known whether, upon renewal of this permit, limits will change to become more stringent for the Secondary Bypass limits or if the “split permit” will be replaced with one blended final effluent permit that will need to be met even during wet weather events. To address this concern, the city could: 1) Construct an equalization basin to store wet weather flow for secondary treatment once the storm event subsides, 2) Construct additional capacity in their secondary treatment process, or 3) Treat the Secondary Bypass with a high rate process. This Study is limited to evaluating the high rate process alternative.

The City of Akron has studied their combined sewer collection system and is planning construction of storage basins at several CSO locations. A high rate treatment process could be added to the overflow of such basins in the future. While there are currently no specific effluent quality limits established for combined sewer overflows, the Ohio Environmental Protection Agency (OEPA) has established a “primary treatment” level as a general benchmark with the possible additional requirement of disinfection. A high rate treatment process may be a viable treatment alternative at CSOs. This Study also relates pilot test results to potential use at Akron CSO locations.

The Krüger ACTIFLO Process
ACTIFLO is a micro-sand ballasted clarification process that may be used to treat water or wastewater. By adding ballast to a chemical mixing and flocculation process, wastewater flow can be treated in high rate sedimentation tanks that are a fraction of the footprint and costs of conventionally designed settling tanks. The process, shown in Figure 1, begins with the addition of a coagulant to destabilize suspended solids. The flow enters the coagulation tank for flash mixing to allow the coagulant to take effect, then overflows into the injection tank where microsand is added. The micro-sand serves as a “seed” for floc formation, providing a large surface area for suspended solids to bond to, and is the key to ACTIFLO. It allows solids to settle out more quickly, thereby requiring a smaller footprint than conventional clarification. Polymer may either be added in the injection tank or at the next step, the maturation tank. Mixing is slower in the maturation tank, allowing the polymer to help bond the micro-sand to the destabilized suspended solids. Finally, the settling tank effectively removes the floc with help from plate settlers, allowing the tank size to be further reduced. Clarified water exits the process by overflowing weirs above the plate settlers. The sand and sludge mixture is collected at the bottom of the settling tank with a conventional scraper system and pumped to a hydrocyclone, located above the injection tank. The hydrocyclone converts the pumping energy into centrifugal forces to separate the higher-density sand from the lower-density sludge. The sludge is discharged out of the top of the hydrocyclone while the sand is recycled back into the ACTIFLO process for further use. Screening is required upstream of ACTIFLO so that articles larger than 3 to 6 mm do not clog the hydrocyclone.

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