DISINFECTION UPGRADES AT THE BIG SISTER CREEK WWTP

The Erie County Sewer District (ECSD) No. 2, Big Sister Creek Wastewater Treatment Plant (WWTP) is located in Town of Angola, Erie County, New York. The WWTP has an average design flow capacity of 7.68 million gallons/day (MGD), and a maximum design flow capacity of 11.52 MGD. Prior to the completion of the disinfection upgrade project, the plant utilized chlorine gas to disinfect its effluent. Chlorine gas was used at two points: the plant effluent and Overflow Retention Facility (ORF) effluent. Plant effluent is continuous, and therefore needs a disinfection system which runs 24 hours a day. The ORF is only used during periods of high flow, so the disinfection system for this application is intermittent.Due to the age of the gaseous chlorine equipment and new requirements of dechlorination, new disinfection alternatives were evaluated. Alternatives evaluated included: chlorine gas, sodium hypochlorite, and ultraviolet (UV) disinfection. The analysis of the sodium hypochlorite alternative included a comparison of generating the chemical onsite versus installing bulk delivery and storage facilities. Dechlorination with sodium bisulfite and sulfur dioxide were also evaluated. The study concluded that a UV system for plant effluent combined with a sodium hypochlorite system for ORF effluent would be most effective.The Contractor had to take special care to not impact the plant effluent during construction. During periods of high flow, when plant performance is severely limited, the Contractor was not allowed to perform any bypasses. In addition, the Contractor was required to notify the New York State Department of Environmental Conservation (NYSDEC) prior to any planned bypasses. It was critical to maintain treatment and the Contractor would have been fined had any permit limits been violated as a result of his operations. The most challenging aspect of this project was the work sequence and scheduling required. There were modifications made to the two reaeration tanks so they could accept flow from the new UV channels. The new UV channels disinfect wastewater on its flow path from the filters to reaeration. Portions of the tank were to be cut to accept the new flows, and weir troughs were installed at the effluent end of the reaeration tanks. However, during construction, at least one tank needed to remain in service at all times.Since startup in the fall of 2005, the UV and hypochlorite systems have both continued to meet disinfection permit requirements set forth by the NYSDEC.