SITE INFORMATION AND CORRESPONDENCE

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INFORMATION SHEET -4- CITY OF STOCKTON REGIONAL WATER QUALITY CONTROL PLANT SAN JOAQUIN COUNTY meet the limitation for ammonia without proceeding with plant improvements. Until such time as other sources of oxygen demand impacting the San Joaquin River in the vicinity of Stockton have been identified and addressed, the Board does not believe that it is reasonable to require reductions in effluent CBOD and ammonia beyond the levels required by the new final effluent limitations. Based on the City's model, three different CBOD and ammonia limitations are prescribed for different times of the year. - For the months when water temperatures are cool (1 December through 31 March) a monthly average of 20 mg/l CBOD is prescribed. The model found that ammonia did not exert a measureable oxygen demand when water temperatures are low because of significantly reduced metabolic rates in nitrifying organisms at these lower temperatures. Therefore no ammonia limitation is prescribed for these months. A CBOD limitation was prescribed rather than a BOD limitation, because the BOD test is conducted at a standardized warm temperature at which the oxygen demand of the ammonia would be exerted, showing an apparent oxygen demand in the laboratory test which does not properly predict the oxygen demand on the receiving water, which is cooler. - For the months when water temperatures are warm ( 1 April through 31 October), a monthly average of 10 mg/l CBOD and a monthly average of 2 mg/l ammonia are prescribed. Due to the warmer water temperatures, the metabolic rates of organisms oxidizing carbonaceous and nitrogenous compounds are increased, increasing the impact of the effluent on the receiving water. Also, during these months, there are frequent, sustained periods with low net flow or reversals of the flow in the River due to pumping of water from the South Delta. The low net flow and flow reversals result in very little dilution of the effluent in the receiving water. Tides move a mixture of wastewater and receiving water back and forth past the discharge point, so that the "receiving water" for the discharge already contains significant portions of wastewater. - For November, intermediate effluent limits of 15 mg/l CBOD and 10 mg/l ammonia are prescribed. The proposed nitrifying treatment facility to be built by Stockton will use nitrifying bacteria to reduce the ammonia concentrations in the effluent. The nitrifying bacteria in the plant can not be instantaneously turned on and off, but will slowly change the effluent quality as process modifications are made and the temperature of the wastewater changes seasonally. The City's model has demonstrated that an intermediate level of treatment during this transition period in the wastewater treatment plant will not increase the impact of the wastewater discharge during November. Daily maximum effluent limitations for BOD and TSS are established in this permit as allowed under state regulations. The daily maximums for BOD and TSS are based on professional judgement, having been found to be technically achievable at a properly operating