Maximize Plant Capacity, Minimize CSOs

July 15, 2013
Wastewater treatment facility improvements for CSO reduction

About the author: Paul F. Birkel, P.E., is senior vice president of Wright-Pierce. Birkel can be reached at [email protected] or 207.523.1400.


Many communities across the U.S. have combined sewer systems that transport both storm water and wastewater, leading to combined sewer overflows (CSOs) that activate during wet weather. Communities with CSOs have had to develop mitigation strategies to abate them and comply with U.S. Environmental Protection Agency mandates and the National CSO Control Policy established in 1994.

Traditional approaches to CSO abatement include sewer separation, inline storage, offline storage facilities and satellite treatment facilities.  Sewer separation is a common technique that provides separate pipeline for storm water and wastewater. Major drawbacks to this approach are high initial costs and loss of captured pollutants in runoff when storm water is directed to nearby water bodies. In addition, much of the extraneous flow into combined systems is from private inflow and infiltration sources that are, at best, difficult to control and remove. 

Recognizing that a majority of storm water pollutants occur within the “first flush” (defined as flow from the first 1 in. of rainfall), many communities have considered methods such as inline storage (oversized conduits in lieu of pipe) and offline storage (tanks, banks of conduits or underground tunnels). Another “in system” alternative is to provide satellite treatment systems at critical, or worst offending, CSO points that screen floatables, remove primary solids and disinfect combined flows before release to the adjoining water body.

At the Facility

While many CSO abatement efforts have focused on what can be done within collection systems to reduce CSOs, much can be done at wastewater treatment facilities. Increasing treatment plant capacity provides numerous environmental benefits. Conventional activated sludge plants have significant limitations to treating peak flow. There are several process configurations and/or modifications, however, that can significantly improve peak flow handling capability at treatment facilities thereby reducing untreated CSO activity and providing water quality benefits. The following case studies outline how three communities incorporated process configuration/modifications within their existing activated sludge facilities to treat increased wet weather flows and the benefits received.

Lewiston-Auburn Water Pollution Control Authority

The Lewiston-Auburn Water Pollution Control Authority owns and operates a 14.2-million-gal-per-day (mgd) wastewater treatment facility that serves the cities of Lewiston and Auburn, Maine. Peak secondary flows were limited to less than 20 mgd, and the facility experienced numerous suspended solids violations during wet weather events associated with proliferation of filamentous bacteria and zoogleal bulking. As part of a long-term CSO control plan, maximization of flow to the treatment facility was desirable to minimize costly sewer separation. Wright-Pierce modeled the authority’s activated sludge system and concluded that the conventionally configured process could be expanded substantially by reconfiguring to the selector contact stabilization (SCS) process. In this process, an anaerobic or anoxic selector zone is placed between the stabilization and contact zones. It was the first application of this process in New England. 

With minor reconfiguration of the existing aeration basins and fine bubble diffuser system, and with no increase in tankage, the secondary capacity of the plant was expanded from less than 20 mgd on a peak daily basis to more than 32 mgd. The SCS process dramatically improved settle-ability (sludge volume index of 100 or less), controlled filamentous organisms, reduced zoogleal bulking, reduced operating expenditures and brought the facility back into compliance. Being able to expand secondary treatment capacity significantly reduced sewer separation work, saved the communities millions of dollars, and contributed to improved water quality in the Androscoggin River.   

Greater Lawrence Sanitary District 

The Greater Lawrence Sanitary District (GLSD) owns and operates a 30-mgd wastewater treatment facility that serves the communities of North Andover, Andover, Methuen, Lawrence, and Dracut, Mass., and Salem, N.H. Some of these communities have combined sewer systems causing a need to mitigate CSOs. The mechanically aerated, complete mix activated sludge system had a peak wet weather capacity of 110 mgd. The long-term control plan called for increasing flow to the treatment facility and construction of a secondary bypass system. Wright-Pierce modeled the system and determined  that the process capacity could be expanded substantially with the addition of an anaerobic selector incorporated at the head end of, and within, the existing aeration basins.  

The process was upgraded with a new fine bubble aeration system, anaerobic selector and secondary bypass. Similar to the SCS process, the anaerobic selector enhances sludge settle-ability thereby increasing process capacity. The selector also creates optimum conditions for phosphorus-accumulating organisms for enhanced biological phosphorus removal, allowing the facility to reliably achieve an effluent concentration of 0.3 mg/L. With these improvements, the facility increased its wet weather capacity significantly (up to a peak daily flow of 135 mgd), reduced annual electrical costs and minimized use of the new secondary bypass system.

Fitchburg, Mass.

The city of Fitchburg, Mass., has a combined sewer system and is in the middle of a major sewer separation program to mitigate CSO activity. Peak flows of up to 40 mgd reach its two-stage, activated sludge treatment facility. The facility, which has stringent seasonal limits on biochemical oxygen demand (BOD), total suspended solids (TSS) and ammonia, could only treat 12.5 mgd through the secondary system. The remaining flow was bypassed around the secondary process and blended with secondary treated effluent. This practice resulted in numerous combined effluent discharge permit violations. Wright-Pierce modeled this activated sludge system and determined that converting the two-stage process into a parallel train operation could achieve the stringent permit limitations while increasing secondary treatment capacity to approximately 25 mgd. Minor modifications were made to the flow distribution structure to split flows between the two different-sized trains. In addition, chlorination system changes were made. 

After a year, success was achieved, exceeding all expectations. With the success of the trial, the city is moving forward with the addition of a selector in the head end of each aeration basin to improve sludge settle-ability and to promote a further increase in peak flow capacity. To further reduce effluent violations during wet weather with stringent seasonal limits, the city added chemically enhanced primary treatment. By reducing BOD and TSS levels in the primary effluent, blended effluent can better meet higher seasonal standards. The changes to plant capacity have made a positive influence on the health of the receiving body and allowed a reduction in CSO removal efforts within the collection system.              

A variety of process configuration adjustments can transform process control at plants and lead to increased secondary treatment capacity.  Increasing flow to the treatment facility and the portion of the flow receiving secondary treatment can have a major effect not only on general process control considerations, but on handling wet weather flows. More flow through secondary treatment means better water quality in receiving waters. It is another tool for aiding in the reduction of CSO-related flows. In short, maximizing flow through secondary treatment is a vital part of an economical and sustainable CSO control program. 

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