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. Inaddition, when imported water is not available due to aqueduct and treatmentplant shut-downs, the reservoir is used in maintaining sufficient local storage to meet theCity's "Growth Management Requirement", which requires Carlsbad Municipal Water District (CMWD) to maintain 10 daysof storage to supply water to the distribution system. However, these storagerequirements are affecting CMWD's ability to maintain water quality in MaerkleReservoir. The finished water purchased from MWD uses chloramines as the secondaryor residual disinfectant, and thus also contains low concentrations of ammonia. Thepresence of ammonia serves as a potential source of reduced nitrogen for nitrification andthe long residence times in Maerkle Reservoir require special treatment of this water toavoid water quality complications due to nitrification. Historically, CMWD practicedbreakpoint chlorination to remove the ammonia and produce a free chlorine residual inthe reservoir influent water. However, this practice formed high concentrations ofdisinfection byproducts (DBPs) and may have adverse consequences for water systemoperation and compliance with the newly effective Stage 1 Disinfectants and DisinfectionByproducts (D/DBP) Rule and the soon to be proposed Stage 2 D/DBP Rule. A previously conducted compliance assessment based on historical data conducted byMcGuire Malcolm Pirnie, Inc. (MMP) concluded that CMWD should be able to complywith the Stage 1 D/DBP Rule, primarily due to the system-wide method of compliancecalculation. However, the impending Stage 2 D/DBP Rule is projected to change themethod of compliance calculation and require a calculation based on location-specificaverages. Because breakpoint chlorination produces high concentrations of TTHMs inthe Maerkle effluent (sometimes greater than 180 µg/L), some locations in the CMWDdistribution system have exhibited TTHM concentrations approaching 70 µg/L. MMP,Inc. and CMWD concluded that current operations might result in a level of DBPformation that represents an unreasonable risk of Stage 2 D/DBP Rule violation andembarked on a program to evaluate options for enhanced control of DBP formation. Several bench scale experiments were performed in preparation for and in conjunctionwith the full-scale Maerkle demonstration. Four sets of bench-scale experiments havebeen 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 toestablish if nitrification could be observed in a bench-scale, laboratory setting;trial nitrification experiments using influent water from Maerkle Reservoir, withsplit samples spiked with chlorite to control nitrification. These experiments wereaimed at obtaining insight on potential behavior of water in Maerkle Reservoironce 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 inparallel with the full-scale demonstration for comparative purposes; and,trial experiments using Maerkle Reservoir influent water and the addition ofaliquots of water known to contain nitrifying bacteria (Costa Mesa tap water) that spikedspilt samples with chlorite to validate the concept that chlorite acts toinhibit the nitrifying bacteria responsible for symptoms of nitrification. Includes tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2005

Number of Pages:

21

File Size:

1 file , 570 KB