Approaches to Maintaining Consistently High Quality Recycled Water in Storage and Distribution Systems
Year Released: 2014
Funding Partner: Bureau of Reclamation, California State Water Resources Control Board
Total Investment: $712,477.59 (Cash: $300,000, In-Kind: $412,477.59)
Principal Investigator: Timothy M. Thomure, PE, HDR
Increasing demands on limited water resources have made water recycling an attractive option for stretching water supplies. Despite the growth of the recycled water industry, there remain some public health concerns about the potential risks of human contact with recycled water used for irrigating public and recreational areas. Most of the water quality testing that is performed in recycled water systems is conducted at the treatment facility where the water enters the distribution system. Little prior study has been done on the temporal variations in the microbiological and nutrient/chemical composition of recycled water through distribution and storage.
Goals and Objectives
The project investigates the treatment, disinfection, and operational practices that will help ensure that high quality recycled waters are maintained through storage and distribution. An ability to identify and predict water quality changes within the system will guide remediation programs (e.g., placement of intermediate chlorine boosters), with the ultimate objective of enhancing the success of recycled water management.
The study was divided into five phases involving a questionnaire, case studies, storage tank experiments, distribution system sampling, and hydraulic modeling. It focused on temporal and spatial variations of biological and chemical parameters in recycled water following entry into the distribution system. This work defined water quality markers by identifying and quantifying both viral and bacterial indicator organisms, traditional and nontraditional pathogens, and nutrient and chemical constituents in the recycled water. Water quality information relating to water samples collected at the point of compliance for the wastewater treatment facilities (before recycled water enters the distribution system) was compared to that of samples collected at various time points and distances along the distribution system.
Findings and Conclusions
- Regardless of the initial level of treatment, the microbial quality of the recycled water sampled from the utilities monitored for this research study deteriorated with increased residence time in the water distribution system.
- Water-based pathogens, including Legionella, Mycobacterium, and Aeromonas, were routinely found in recycled waters beyond the point of compliance.
- All water-based pathogens demonstrated the ability to grow within the distribution systems. Furthermore, although water-based pathogen concentrations were reduced following chlorination boosters, pathogens showed a potential to regrow following disinfection within the distribution system.
- Fecal indicator organisms, including E. coli and Enterococcus, were rarely detected in either distribution system, suggesting that treatment effectively eliminated waterborne pathogens.
- In this study, fecal indicators had no correlation with the presence of waterbased pathogens and are not a reliable indicator of microbial water quality.
- Amoebic activity was detected in approximately one-third of all water samples collected, with frequency of detection being similar despite the variety of treatment technologies.
- Microbial water quality changes over time in correlation to the rapid dissipation of chlorine concentrations. Within a matter of days, organism levels stabilize as assimilable organic carbon levels decrease; this suggests that an equilibrium level of available carbon is reached during growth and subsequent death of organisms.
The application of these findings is dependent on the uses of recycled water in the system. If the recycled water is only used for turf irrigation, the fecal indicators may be adequate to measure quality at the point of entry and within the distribution system. If the recycled water is to be used indoors or as a source for potable reuse, then the microbial quality of the waterbased pathogens, along with the fecal pathogens, is critical in determining the quality.