Role of Retention Time in the Environmental Buffer of Indirect Potable Reuse Projects: An Investigation on Managed Aquifer Recharge
Year Released: 2015
Type: Scientific Investigation
Funding Partners: Bureau of Reclamation, California State Water Resources Control Board
Total Investment: $698,189 (Cash: $300,000, In-Kind cash and service: $398,189)
Principal Investigator: Jörg E. Drewes, Ph.D., Colorado School of Mines
In many regions experiencing water scarcity, there is increasing interest in water reuse offering opportunities to augment local drinking water supplies with highly treated recycled water. Indirect potable reuse (IPR) is referred to as the intentional augmentation of drinking water supplies with recycled water via an environmental buffer such as a surface water body (e.g., surface water reservoir) or a groundwater aquifer. An environmental buffer provides retention time which can serve two purposes: (1) provide time to respond to potential treatment failures or upsets and (2) allow an additional opportunity for attenuation of microbial and chemical contaminants. This study focused on managed aquifer recharge (MAR) with recycled water. It developed relationships between the removal of microbial contaminants (e.g., bacteria, protozoa, viruses) and attenuation of chemical contaminants (e.g., pharmaceutical residues, personal care products, household chemicals) and retention time, system characteristics, and operating conditions to provide better guidance for design and operation of MAR systems.
Goals and Objectives
The goals and objectives of the project are to:
- Develop relationships between the removal of pathogens and attenuation of chemical contaminants and retention time, system characteristics, and operating conditions to provide better guidance for design and operation of MAR systems.
- Develop predictive models to assess the attenuation of microbial and chemical contaminants in MAR facilities.
- Validate these relationships during MAR field monitoring efforts and through an assessment of historic water quality monitoring data from full-scale MAR installations representing various conditions.
In order to derive relationships between the attenuation of chemical and microbial contaminants and retention time, system characteristics, and operating conditions, controlled laboratory-scale soil-column experiments were conducted. Feed water compositions, travel time, and redox conditions were altered in these experiments. Microbial contaminants (i.e., viruses) were quantified with quantitative PCR and cell culturing techniques. Trace organic chemical contaminants (i.e., pharmaceuticals, personal care products) were quantified using liquid chromatography with tandem mass spectrometry utilizing the isotope dilution method. In addition, findings were validated through monitoring campaigns at three full-scale managed aquifer recharge facilities in Arizona, California, and Colorado.
Findings and Conclusions
A literature review revealed that adenoviruses, coliphages ΦX-174 and PRD-1 are among the longest surviving viruses in groundwater. Inactivation rates of coliform bacteria and Cryptosporidium parvum during MAR, however, appeared to be much higher than virus inactivation rates. Inactivation rates are often not constant and may slow down with distance. Laboratory and field data suggest that linear-log functions best describe pathogen removal. Removal rates are specific to the physical and chemical properties of the microbes, subsurface media, solution chemistry, transport scale, type of feed water, and duration of contamination. Where proper soil conditions are met, an inactivation of at least 2-log can be expected within a travel time of 5 to 10 days.
Findings from controlled laboratory studies and field monitoring campaigns revealed that reducing travel time in SAT to less than 30 days does not seem to result in compromised water quality regarding trace organic chemicals. During this study, a subsurface travel time of 8 days in the aquifer was sufficient to remove the biodegradable portion of DOC. If denitrification during SAT is desired, slightly longer travel times (10 to 30 days) might be needed where anoxic conditions can prevail. In general, biotransformation of trace organic chemicals under carbon-starving and oxic redox conditions was more favorable. Moderately degradable compounds were removed significantly better under carbon-starving (~0.15–0.25 mg/L) than under high BDOC (>2 mg/L) conditions. Partial treatment by nanofiltration or reverse osmosis can reduce DOC concentrations, creating carbon-starving conditions after blending with a conventionally treated (tertiary effluent) reclaimed water.