AWWA WQTC62532

Maerkle Reservoir, the primary potable water storage reservoir for the City of Carlsbad, California, is used to maintain storage capacity for daily operational needs. In addition, when imported water is not available due to aqueduct and treatment plant shut-downs, the reservoir is used in maintaining sufficient local storage to meet the City's "Growth Management Requirement", which requires Carlsbad Municipal Water District (CMWD) to maintain 10 days of storage to supply water to the distribution system. However, these storage requirements are affecting CMWD's ability to maintain water quality in Maerkle Reservoir. The finished water purchased from MWD uses chloramines as the secondary or residual disinfectant, and thus also contains low concentrations of ammonia. The presence of ammonia serves as a potential source of reduced nitrogen for nitrification and the long residence times in Maerkle Reservoir require special treatment of this water to avoid water quality complications due to nitrification. Historically, CMWD practiced breakpoint chlorination to remove the ammonia and produce a free chlorine residual in the reservoir influent water. However, this practice formed high concentrations of disinfection byproducts (DBPs) and may have adverse consequences for water system operation and compliance with the newly effective Stage 1 Disinfectants and Disinfection Byproducts (D/DBP) Rule and the soon to be proposed Stage 2 D/DBP Rule. A previously conducted compliance assessment based on historical data conducted by McGuire Malcolm Pirnie, Inc. (MMP) concluded that CMWD should be able to comply with the Stage 1 D/DBP Rule, primarily due to the system-wide method of compliance calculation. However, the impending Stage 2 D/DBP Rule is projected to change the method of compliance calculation and require a calculation based on location-specific averages. Because breakpoint chlorination produces high concentrations of TTHMs in the Maerkle effluent (sometimes greater than 180 µg/L), some locations in the CMWD distribution system have exhibited TTHM concentrations approaching 70 µg/L. MMP, Inc. and CMWD concluded that current operations might result in a level of DBP formation that represents an unreasonable risk of Stage 2 D/DBP Rule violation and embarked on a program to evaluate options for enhanced control of DBP formation. Several bench scale experiments were performed in preparation for and in conjunction with the full-scale Maerkle demonstration. Four sets of bench-scale experiments have been performed to-date: a trial nitrification experiment using chloraminated tap water from Costa Mesa, California, that is known to nitrify and this experiment was performed to establish if nitrification could be observed in a bench-scale, laboratory setting; trial nitrification experiments using influent water from Maerkle Reservoir, with split samples spiked with chlorite to control nitrification. These experiments were aimed at obtaining insight on potential behavior of water in Maerkle Reservoir once breakpoint chlorination ceased and whether chlorite, and at what levels, would be capable of controlling/preventing nitrification; a control experiment using influent water from Maerkle Reservoir, conducted in parallel with the full-scale demonstration for comparative purposes; and, trial experiments using Maerkle Reservoir influent water and the addition of aliquots of water known to contain nitrifying bacteria (Costa Mesa tap water) that spiked spilt samples with chlorite to validate the concept that chlorite acts to inhibit the nitrifying bacteria responsible for symptoms of nitrification. Includes tables, figures.