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Abstract
Maximizing treatment of wet weather flows through existing wastewater treatment plants is one if New York State's thirteen Combined Sewer Overflow (CSO) Best Management Practices (BMP's). This can be a relatively low cost BMP if wet weather operating procedures are established and followed. Applying this practice successfully is a challenge in balancing maximum CSO capture with preserving the integrity of the treatment process. One of the obstacles in applying this BMP is the fear of destabilizing or "losing" the treatment process due to excessive solids loads, washouts of biological solids and flooding damage to critical equipment. Properly developed wet weather operating plans consider and address these issues and establish practical and engineering based limitations for wet weather treatment capacities.
Treatment plant capacities have been traditionally characterized as design capacity and maximum hydraulic capacity. The design capacity is generally based on providing the required level of treatment under design loads and conditions which usually don't consider wet weather. The maximum hydraulic capacity is the maximum flow that can pass through the plant without causing flooding or damage to equipment. Treatment performance has not traditionally been a factor in determining maximum hydraulic capacity. CSO issues have generated a new capacity consideration for WPCP's, the wet weather capacity. This should be a value between the design capacity and the maximum hydraulic capacity. The wet weather capacity should be the maximum flow that can be handled without upsetting plant operations or inhibiting the plants ability to meet its discharge criteria during or after a wet weather event.
Maximizing treatment of wet weather flows requires a working knowledge of treatment plant operations and an understanding of how plant performance responds to wet weather conditions. The operator has to anticipate wet weather conditions and ready the plant for higher than normal flows. Simple steps can be taken such as bringing spare pumps and processes online, increasing bar screen cleaning frequency, drawing down sludge blankets, altering sludge recycle and wasting rates and ensuring that adequate disinfection capacity is available. Some plants are equipped with secondary treatment bypass channels which allow higher flows to be processed through primary treatment and disinfection while preserving the secondary treatment process. More complex treatment options are also available such as instituting chemical treatment or changing the mode of operation of the secondary treatment process. Increased process monitoring is also sometimes necessary to monitor the plant's response to high flows. This includes monitoring wet well and channel water surface elevations, sludge blankets levels and chlorine residuals. Reliable plant SCADA systems can help the operator monitor plant conditions and make timely decisions in controlling wet weather flows.
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