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Page 24 <br />tors. Chlorinators deliver me- <br />tered chlorine gas to the chlo- <br />rine contact basins. <br />The chlorine feed system is con- <br />trolled by computer. The pro- <br />gram determines feed rates <br />.through a combination of <br />setpoints and plant flow moni- <br />toring. Plant operators define <br />residual setpoints, and the pro- <br />gramautomatically increases or <br />decreases feed rates based on <br />plantflow fluctuations and chlo- <br />rine demand. <br />Until 1989, chlorine residual <br />analyzers -which measure <br />chlorine by the amperiometric <br />method -were used to moni- <br />tor basin and final effluent re- <br />siduals. The analyzers required <br />considerable maintenance and <br />chemicals. In 1989, the old ana- <br />lyzers were replaced by newer <br />technology, using units that <br />measure chlorine residuals by <br />conductivity. The new analyz- <br />ershave provided consistent and <br />reliable readings with minimal <br />maintenance and no chemicals. <br />• Dechiorination <br />The plant discharge permit requires dechlorina- <br />tionfromMay through October. Dechlorination is <br />done with sulfurdioxide, which is received, handled, <br />and applied with the same type of equipment used <br />for chlorine. Because dechlorination occurs in- <br />stantaneously, the sulfur dioxide injected solution <br />can be discharged into the plant flow just before it <br />reaches the outfall structure. When sulfur dioxide <br />is injected into the flow, sulfurous acid is formed. <br />The sulfurous acid reacts with various chlorine <br />residuals forming a weak concentration of sulfuric <br />acid which is buffered by the alkalinity of the <br />wastewater. <br />As with chlorine, sulfur dioxide feed rates are <br />controlled by computer. The program determines <br />sulfur dioxide solution feed rates based on a plant <br />flow setpoint. As the computer monitors changes <br />according to plant flow, the feed rate is raised or <br />lowered accordingly. <br />Section D: PLANT OPBRATIONS <br />• Turbidimeter <br />A turbidimetermonitors turbidiryofplanteffluent <br />immediately prior to the outfall structure. Because <br />they can be correlated to TSS concentrations, <br />turbidity readings provide a quick check of treat- <br />ment quality. <br />• W2 Pumping Station <br />The W2 pumping station system has three verti- <br />callymounted,variable speed (via magdrives) line <br />shaft turbine pumps, each with a 2.9 MGD capac- <br />ity. Just downstream from the pumps are two 5.8 <br />MGD strainers that reduce the number of particu- <br />late solids. <br />The pumps and strainers are controlled by com- <br />puter and are based on a predetermined pressure <br />setpoint-normallyat90psi. Pressure is occasion- <br />ally run higher for major activities such as basin <br />wash downs. <br />The chlorine contact chamber insures adequate disinfection. <br />