Guidance Document on the Microbiological Quality and Biostability of Reclaimed Water Following Storage and Distribution
Project: 05-02
Type: Decision Making Tool
Year Released: 2010
Program: Principal
Funding Partners: Bureau of Reclamation, California State Water Resources Control Board, South Florida Water Management District
Total Investment: $484,990.89 (Cash: $297,601.89, In-Kind: $187,389)
Principal Investigator: Patrick K. Jjemba, PhD
Background
Freshwater is becoming increasingly scarce as a result of increasing populations, changing precipitation patterns, and/or degradation of existing sources of water, making water reuse a necessity. While much attention to reclaimed water has focused on the quality of the water at the treatment plant, that quality can degrade by the time it gets to the point of use. Therefore, a comprehensive understanding of the physical, chemical, and biological factors that affect the microbial quality of reclaimed water within distribution systems is necessary.
Goals and Objectives
The project identifies the key chemical and physical water quality parameters that influence changes in microbial water quality in reclaimed water distribution systems and evaluates a novel assimilable organic carbon (AOC) assay for analysis of reclaimed waters. The results from this study were used to develop recommendations for system operators to control microbiological growth in reclaimed water systems.
Research Approach
Four reclaimed water plants with different treatment technologies (trickling filters, activated sludge, or membrane bioreactors), production capacity, disinfection, storage (open or closed reservoirs), water end-use (irrigation, toilet flushing, cooling towers), and location (California, Florida, Massachusetts, and New York) were intensively surveyed for one year. Nutrients (nitrite, nitrate, ammonia, phosphates, sulfide, etc), physical (turbidity, color, alkalinity, and temperature), biochemical (TOC, BDOC, and AOC) and microbiological (bacteria, phage, phytoplankton, protozoa, and viruses) parameters were determined. Pipe loops were established at each location to study the efficacy of disinfectants on biofilm bacteria and formation of AOC. To understand how different treatment technologies influenced the level of biodegradable organic carbon, 21 plants in 12 States, with treatments including MBR, activated sludge, sequence batch reactor, and rotating biological contactors were surveyed once.
Findings and Conclusions
All four treatment systems effectively removed bacteria in treated effluents but re-growth occurred in the distribution systems due to the (i) high AOC and BDOC levels; (ii) rapid dissipation of the disinfectant in the system; (iii) intermittent flow of water in some systems which causes stagnation of the water and depressurization of the pipeline; and (iv) the presence of open reservoirs that promoted algal growth, increasing the organic carbon levels, bacterial loading to the system, turbidity, and hydrogen sulfide – a precursor for objectionable odors. The concentration and frequency of occurrence of most of the monitored bacteria (HPC, coliforms, E. coli, Pseudomonas, Aeromonas, Enterococci, Legionella, and Mycobacterium) increased in the distribution systems. The absence of coliform bacteria and E. coli did not preclude the presence of potential pathogens such as Legionella, Mycobacterium, Giardia, Cryptosporidum and enteric viruses. Re-growth of bacteria was especially prevalent in systems that lacked a disinfectant residual and had high levels of AOC. Experiments performed in pipe loop systems showed that free chlorine was more effective than chloramines in controlling HPC and Legionella but in some instances, chlorination ironically increased AOC and BDOC levels – leading to more nutrients for downstream bacterial growth. The MBR systems showed better removal of enteric viruses possibly due to the cake development on the membrane. In the broad survey of 21 systems, membrane bioreactors consistently prodcued the lowest levels of AOC, but other technologies (like activated sludge) were also capable of achieving low levels of AOC; suggesting that there is a possibility of optimizing treatment operations to enhance AOC removal. To minimize degradation in distribution systems, utilities should: (i) maintain a constant flow in the system; (ii) control algal growth in reservoirs; (iii) routinely flush the network; (iv) evaluate treatment strategies that could improve removal of AOC, and (vi) institute post-treatment disinfection with UV, or maintain a monochloramine residual in the system.
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