Q: From where can we obtain water when the nearby natural sources are inadequate to meet increased demand?
A: The recycling of wastewater into safe drinking water is technologically feasible and growing in applications.
Water availability is limited in many parts of the U.S., so do not blame it all on climate change. Southern California, the Southwest and parts of the South are prone to drought conditions, and these are greatly exacerbated by the influx of population occurring in those regions. Dams and reservoirs have been traditional ways of storing water for later use, but in many locations, they have not been sufficient to compensate for the increased water demand.
Desalination of seawater and brackish water is a growing choice in the U.S., as it has become the principle source of fresh water in the Middle East for 50 years. Desalination is expensive and not without environmental, physical and political concerns. Agricultural reuse of wastewater has been practiced for millennia. Potable reuse of wastewater is a logical option for increasing access to and the more efficient use of available water in some areas.
Like it or not, potable reuse of wastewater has been with us in some form for millennia. Many surface water supplies draw upon rivers and lakes that have upstream wastewater discharges for what is called de facto or unplanned wastewater reuse. Not long ago, that wastewater may have been untreated or only received primary treatment before entering the river. Since the 1970s and 1980s, that wastewater has been treated to secondary or tertiary levels before being discharged to the river. It undergoes further natural treatment by sedimentation, sunlight, oxidation, biological activity and dilution. It arrives at the downstream drinking water intake where it undergoes drinking water treatment by coagulation, flocculation, sedimentation, filtration and disinfection.
The quality of wastewater has improved during the past 30 to 40 years. The Clean Water Act requires that discharges to wastewater systems be pretreated to meet standards, and chemical industry discharges are subject to effluent guidelines as well as National Pollution Elimination Discharge System permits. In addition, heavy industry has declined in the U.S., so fewer wastes are discharged from those sources.
Drinking water suppliers have been bound by Safe Drinking Water Act (SDWA) regulatory requirements for more than 35 years, so drinking water quality has also improved significantly because of national legislation. Waterborne disease rates are extremely low, less than foodborne diseases, and they have declined consistently since the implementation of the SDWA requirements beginning in about 1980. All this has occurred while detecting and identifying the causes of waterborne diseases have improved.
What are drinking water quality and safety concerns about? With the onset of major advances in analytical chemistry, detecting many substances at parts per trillion levels or even less is possible, so the public is constantly bombarded with reports of detections of chemicals at trace levels. The lower the concentration, the less the potential for health concern. Many of those substances have probably been there but are just now being detected.
Types of potable reuse
As stated above, de facto or unplanned potable reuse is common practice. The two forms of planned potable reuse are direct potable reuse (DPR), in which very highly treated wastewater is introduced into an existing water supply system, and indirect potable reuse (IPR), in which highly treated wastewater is discharged to an environmental buffer (for example, groundwater or a surface water reservoir, lake or river) and the blended water is often further treated at a drinking water plant and introduced to the water system.
DPR or IPR is generally considered a supplement to the existing natural supply, which might actually be an unplanned reuse system. Many people, including some regulators, have the impression that the presence of a natural surface or groundwater buffer in the process before entering a drinking water system provides a greater comfort that the water will be safe and more natural.
Soil aquifer treatment is a well-known process in which secondary or tertiary treated wastewater is percolated through the soil, where it is filtered and biologically treated before it reaches the aquifer. Highly treated water can be injected into an aquifer. Groundwater storage and distribution of highly treated water is a beneficial part of a water system, although it can potentially also be a source of additional contamination from the aquifer.
Surface water passage is a valid part of a treatment train for lesser quality water, that is, unplanned potable reuse, but much less desirable for highly treated water unless the reservoir is covered and protected from contamination or unless the water will be processed in a drinking water plant before distribution. A third alternative is the use of an engineered storage buffer, a tank that retains the highly treated water for a short time that would allow a diversion in the event of a serious treatment failure.
Enhanced water treatment technology
Numerous treatment technologies are serially employed in an advanced water treatment plant to assure that the finished water is safe and high-quality. A possible treatment train could include different combinations of the following:
- Membrane filtration (microbial and organic carbon removal)
- Reverse osmosis (inorganics, organic carbon and microbial removal)
- Ozone (disinfection and oxidation)
- Advanced oxidation (trace organics removal)
- Ultraviolet (UV) light (disinfection and some chemical degradation)
- Granular activated carbon adsorption (organics removal)
- Biological activated carbon (organics and microbial)
The treatment trains utilized for IPR or DPR have been optimized to remove specific classes of contaminants, and they include redundancy that is provided by multiple barriers. This means that more than one process in the train is capable of removing the same contaminants, so if a temporary performance excursion in a process occurs, the finished water is still safe because another process also operates to control the same substance. For example, several disinfectants as well as membranes might be used to remove microbial contaminants.
Monitoring for assured performance
The technologies are designed and verified to remove specific and broad classes of contaminants. Operational performance verification is essential. Hazard assessment, critical control point (CCP) or water safety plan (WSP) management methods are employed to assure continuous performance to specifications. Another key element of advanced water treatment systems is that chemical and microbial controls are monitored more frequently than most conventional drinking water facilities. Whenever the technology exists, on-line, real time monitoring is used for process management with automatic alarms when levels are outside the normal parameters.
For example, on-line technologies exist for process performance that measure total organic carbon, turbidity, electrical conductivity (inorganics), disinfectant residuals and UV transmittance. Pressure decay measurements assess membrane performance. Direct microbial indicators still require a minimum of about 16 hours. When real-time data and monitoring are not readily available, relying on disinfectant residuals and contact times (CT values) for bacteria and virus removal and microfiltration, ultrafiltration or reverse osmosis membrane filtration for turbidity control and for the removal of bacteria, viruses and protozoa (cryptosporidium) is sufficient.
Drinking water is drinking water regardless of its origin. It all should meet drinking water standards and be safe for lifetime consumption. Planned recycled water reuse is actually higher quality than most other sources because the treatment processes employed are much advanced from conventional drinking water treatment. Its processing has been specifically designed to remove contaminants with large margins of process performance and safety, and it is more closely monitored.
If wastewater is the proximate source of drinking water, advanced technologies must be employed to be sure that the finished water will be safe and high-quality 24/7/365. The most essential concern is to assure that the finished water is microbiologically safe and free of pathogens. Chemical quality is also important, but not quite as critical as protection from acute microbial disease risk.
Several direct potable reuse drinking water systems are installed and in operation, and more are being designed. Comprehensive federal drinking water regulations are in place, but neither states nor the U.S. Environmental Protection Agency (EPA) have regulations specifically directed to DPR, although several are working in that direction. Generally, very protective design and performance specifications are being considered — for example, 12 logs of removal for enteric virus, 10 logs for cryptosporidium and 9 logs for coliform bacteria.
Costs and the employed treatment train vary by location, but production costs and energy demand for potable reuse water is about half the cost of desalinated water.
Wastewater is now considered a resource to be exploited rather than something to be sufficiently treated and disposed
to the environment. Municipal wastewater is virtually 100 percent recoverable, including as a source of grease for biofuel, nitrogen and phosphorous fertilizer, biosolids as soil conditioners, methane gas as fuel, and the water, of course.
Ideally, we would all probably prefer to drink protected, naturally mineralized spring water that has been filtered by passage through the ground. This ideal is possible by buying it in bottles for a price, but it cannot be provided to everyone at the tap in urbanized society. Tap water is well-treated and safe to drink, but it might contain some traces of products of disinfection or other natural substances.
Public acceptance is not universal for consumption of drinking water by potable reuse of wastewater. However, once agreement is in place that this water source is needed, and reuse is the best option, the public does support the construction of the expanded water supply.
An expert committee organized by the National Water Research Institute, WateReuse Research Foundation, American Water Works Association and Water Environment Federation recently completed a comprehensive treatise entitled “Framework for Direct Potable Reuse.” It provides a theoretical and practical basis for determining the proper applications and specifications for the design and performance of direct potable reuse systems. It is available at no cost from the WateReuse Association.
Dr. Joseph Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.