Traditional approaches to CSO abatement and their drawback
CSO abatement includes sewer separation, in-line storage, off-line storage facilities, and satellite treatment facilities. Sewer separation is a common technique that provides separate pipelines for stormwater and wastewater. Major drawbacks to this approach are high initial cost and loss of captured pollutants in runoff when stormwater 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/remove. Recognizing that a majority of stormwater pollutants occur within the “first flush” (defined as flow from the first 1” of rainfall) many communities have considered methods such as in-line storage (oversized conduits) 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 receiving water body.
Opportunities to reduce CSO at the treatment facility
While many CSO abatement efforts have focused on what can be done within collection systems to reduce CSOs, much can be done at our wastewater treatment facilities. Increasing treatment plant capacity provides numerous environmental benefits. Conventional activated sludge plants frequently have significant limitations to treating peak flow. However, there are several process configurations and/or modifications 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 two communities incorporated process configuration modifications within their existing activated sludge facilities to treat increased wet weather flows and the benefits received. A variety of process configuration adjustments can transform process control at your plant 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 in handling wet weather flows. More flow through secondary treatment means better water quality in our receiving waters. It’s another tool in our toolbox for aiding in the reduction of untreated CSOs. In short, maximizing flow through secondary treatment is a vital part of an economical and sustainable CSO control program.