De Facto Water Reuse Reveals Nationwide Treatment Gaps
by Dr. Paul Westerhoff and Dr. Jinyue Jiang
Picture this, you hold a bottle of water coming out from your tap—the same water molecules were in the sewer of the city that is upstream of yours just a few days ago.
Surprised? Yuck? This phenomenon, known as de facto water reuse, might be more common than you think.
Because surface water streams like rivers are frequently both the receiver of treated wastewater and provider of drinking water, anytime this type of hydraulic connections happens unintentionally, we are actually, or “de facto,” reusing the wastewater for potable purpose. Maybe this makes you squeamish, but while the wastewater provides a reliable and constant source of water, it may be a potential source of pathogens or unknown chemicals. Unlike highly engineered planned or indirect and direct wastewater reuse that is regulated, de facto reuse relies upon existing potable drinking water treatment plants to purify your water.
We recently published a study in Environmental Science & Technology,1 providing the most comprehensive assessment of de facto reuse in contiguous United States. We found modeled wastewater contributions at 37 percent of the surface water intakes evaluated.
For water treatment professionals, these findings point to a practical need: understand upstream wastewater influences alongside the treatment available downstream and use that information to guide local testing and investment.
Downstream Wastewater
Treated wastewater is part of the supply for many river systems. Conventional wastewater treatment removes suspended solids and biodegradable organic matter and, depending on the facility, reduces nutrients and pathogens. However, some pharmaceuticals, personal care products, food residuals (e.g., artificial sweeteners, etc.), per- and polyfluoroalkyl substances (PFAS), and other wastewater-associated contaminants can remain in the discharge.
Nutrients (nitrogen and phosphorus) in wastewater effluents can also affect algae growth, leading to taste and odor issues in drinking water and increase downstream treatment needs.
De facto reuse describes the contribution of this effluent to a drinking water source. A modeled value of 5 percent, for example, means that upstream treated wastewater accounts for an estimated 5 percent of the water at the intake under the modeled flow conditions. It does not mean that the finished drinking water contains 5 percent untreated sewage, nor does it establish a contaminant concentration or health risk.
Our team expanded the De Facto Reuse Incidence in our Nation’s Consumable Supply model, known as DRINCS, to better capture small systems previously excluded from national assessments. DRINCS2.0 connects 16,225 wastewater outfalls with 6,881 surface water intakes belonging to 4,087 public water systems. Including utilities that purchase water from these systems, the assessment covers supplies serving approximately 232 million people.
National Patterns & Local Differences
Under mean annual streamflow conditions, roughly 2,500 of the 6,800 intakes had modeled upstream wastewater contributions. Intakes belonging to large systems, defined as serving more than 10,000 people, were affected more frequently than those belonging to small systems: 41 percent versus 34 percent.
Estimating how many people receive water influenced by de facto reuse is more complicated. A utility may use several intakes or purchase water from another system, and the available data do not show exactly how those sources are blended and distributed. Assigning populations to the lowest or highest modeled contribution among their connected supplies produced a range of 60-170 million people potentially served by affected sources.
Local hydrology also matters. Large rivers receive discharges from many upstream communities, making wastewater influence common, but their larger flows can provide substantial dilution. Smaller streams are less frequently affected; where an upstream discharge is present, limited dilution can produce a much larger wastewater fraction. A national average therefore offers only a starting point for understanding an individual intake.
Wastewater contributions change as river flow rises and falls. Across much of the eastern United States, spring rainfall and snowmelt increase dilution. Wastewater makes up a larger fraction of source water during the lower flows of late summer and early autumn.
The same temporal pattern does not apply everywhere. Some central and western intakes had their highest modeled contributions in winter, when precipitation stored as snow provides little immediate streamflow. At most affected intakes, the highest monthly wastewater fraction was four to ten times the lowest monthly value.
Small System Treatment Gaps
We reviewed federal records for four treatment processes: granular activated carbon (GAC), powdered activated carbon (PAC), ozonation, and reverse osmosis (RO). These processes offer different capabilities for addressing wastewater-associated contaminants. Of the public water systems affected by modeled de facto reuse, roughly two-thirds had none of these processes reported. The gap was substantially larger for small systems: about 70 percent of the affected small systems lacked all four, compared with around 40 percent of affected large systems.
Having a process listed does not establish its performance. The treatment categories also should not be interpreted as interchangeable protection against every contaminant. GAC performance depends on the compounds present, competing organic matter, contact time, and media condition. Short-chain PFAS can be more difficult to remove with activated carbon than longer-chain compounds.2 Ozonation may address certain organic contaminants and taste-and-odor problems, but its inclusion in the study is not evidence of PFAS removal. Treatment selection requires contaminant-specific evidence.
PAC was the most frequently reported advanced process, present in approximately 32 percent of affected systems. Its availability can support responses to seasonal water quality problems, but a record of PAC use does not establish continuous removal of wastewater-associated contaminants.
Actionable Findings
For utilities and the professionals who support them, the assessment offers a way to focus limited monitoring resources. An intake with substantial modeled wastewater influence, pronounced seasonal variation, and limited treatment capability is a reasonable candidate for closer source-water and finished-water testing.
Local verification is essential. The model assumes wastewater plants discharge at their design capacity, which often exceeds actual flow. It also assumes complete mixing and no loss of wastewater as it moves downstream. Reservoir stratification, intake depth, and local flow patterns can alter the contribution reaching an intake. These assumptions make the model useful for screening, while detailed operating data and field measurements are needed before choosing infrastructure upgrades.
Upstream wastewater and downstream drinking water are already connected in many communities. Mapping those connections can help utilities anticipate seasonal challenges and direct testing where it is most informative. Pairing that knowledge with verified treatment performance and support for communities with limited resources offers a practical route toward closing the gaps identified in this study.
WCP Online.
