DECHLORINATION SYSTEM IMPROVEMENTS AT THE EAST BAY MUNICIPAL UTILITY DISTRICT

The East Bay Municipal Utility District (District) Main Wastewater Treatment Plant (MWWTP) is a high-purity oxygen activated sludge plant located in Oakland, CA, with an average daily flow of 3.5 m3/s (80 MGD). In late 1996, the District converted the existing chlorination and dechlorination facilities from gaseous chlorine and sulfur dioxide to liquid sodium hypochlorite and sodium bisulfite (SBS), respectively. Following conversion, an excessive, nonstoichiometric amount of SBS in the range of 15 to 30 mg/L was often required to dechlorinate a chlorine residual of 1 to 3 mg/L. The following contributing factors were identified: 1) SBS consumption by biological slime growth in the final effluent sample line tubing, 2) ineffective initial mixing, and 3) the absence of an accurate final effluent flow value to allow automated, flow-paced dose control. In July 1999, the District began construction of a full-scale pilot dechlorination system located immediately upstream of the Effluent Pump Station (EPS) inside the plant boundary, approximately 9,000 feet upstream of the existing off-site dechlorination facility. Although the pilot facility significantly reduced the available chlorine contact time, it offered a number of significant advantages, including provisions for enhanced mixing capabilities and process control automation. In addition, the EPS location eliminated travel time to and from the existing off-site dechlorination station for hourly grab sample residual monitoring. During pilot facility operation, the required a chlorine and SBS doses were 3.5 mg/L and 7.0 mg/L, respectively. Despite the advantages and demonstrated performance, two significant developments reduced the cost-effectiveness of the pilot facility: 1) routine replacement of the final effluent sample line at the existing dechlorination facility resulted in a sharp drop in the average SBS dose, and 2) the sodium hypochlorite unit price increased 45 percent during the course of the pilot evaluation. Based on the economic analysis presented, the District shifted its focus toward upgrading the existing, off-site dechlorination facility. This paper presents the results of the pilot system evaluation, including disinfection performance under reduced chlorine contact time conditions, and an economic analysis of project alternatives.