On-site alternate water treatment systems have been a growing national interest since the National Blue Ribbon Commission (NBRC) launched a series of publications in 2017. During the 2019 Water Reuse Summit in San Francisco, the U.S. Environmental Protection Agency (EPA) announced the development of a Water Reuse Action Plan that would collaborate with partners across the water sector.

Prior to this, San Francisco Public Utilities Commission (SFPUC), convened a meeting in 2014 to discuss on-site water systems involving participants across the country representing local and federal agencies. A 10-step program was developed and published for local governments to manage on-site water systems (Blueprint, 2014). 

This created a paradigm shift in the United States for new water management strategies that integrate smaller, decentralized on-site water systems to reuse and diversify water supply by using alternate water sources. In 2016, SFPUC, the US Water Alliance and the Water Research Foundation (WRF) jointly accelerated on-site water reuse projects by launching the NBRC for on-site nonpotable water systems (ONWS). 

The NBRC collaborated with the US Water Alliance, the Water Environment & Reuse Foundation (WE&RF), and WRF and published five reports in 2017. Four of the five reports are guidance manuals to establish a consistent local, state and national approach for regulation and management of ONWS. All the guidance manuals replicate the water quality requirements based on a fifth report, “Risk-Based Framework for the Development of Public Health Guidance for Decentralized Non-Potable Water Systems” (Sharvelle et al., 2017).

The risk-based framework report established a scale-appropriate water quality criteria and monitoring for decentralized nonpotable water systems (DNWS). These systems collect, treat and reuse water from alternate water sources: blackwater, greywater, rainwater and stormwater. The water quality criteria are pathogen targets applicable to the type of alternate water needing treatment. 

The treatment of pathogens is performance-based log10 reduction targets (LRTs) necessary to reduce the risk of infections associated with exposure to alternative ONWS source waters down to a threshold of 1 in 10,000 (10^-4) infections/person/year. The LRTs were developed for three target pathogens: virus, protozoa and bacteria. The recommended reduction targets were produced in tables for each type of alternate water. The DNWS design would need to achieve the LRTs listed in these tables.

In 2018, the International Association of Plumbing and Mechanical Officials (IAPMO) initiated a task group to incorporate and codify the risk-based framework into the American National Water Efficiency and Sanitation Standard (WEStand) by including the LRT recommendations for on-site blackwater and stormwater treatment in the 2020 WEStand, and for on-site greywater treatment in the 2023 WEStand. 

Although this is the only American national standard that has codified the risk-based framework for ONWS, the U.S. regulatory landscape for on-site alternate water treatment methods is not uniform or standardized. Twelve states have on-site nonpotable water reuse regulations or guidelines (On-site Water Reuse Summit, 2024; Health Risk-Based Benchmarks, 2023). California is proposing updated LRTs for treatment trains by the end of 2024 that are stricter than the NBRC (CA State Water Resources Control Board, 2024). 

The EPA continues research in reevaluating and comparing different sets of risk-based LRTs based on different benchmarks (Schoen et al., 2017; Pecson et al., 2022; Schoen et al., 2023). The benchmark of quantitative microbial risk analysis (QMRA), which is a probability of annual infections per person per year, is compared with the benchmark of disability-adjusted life year (DALY). This measure of the health burden of a disease accounts for the severity and duration of illness (Schoen et al., 2023). An EPA LRT harmonization paper is forthcoming.

Treatment Train Requirements to Achieve LRTs

The treatment train describes the performance of multibarrier processes to target and remove enteric viruses (norovirus), parasitic protozoa (cryptosporidium), enteric bacteria (campylobacter), particulates and organics from alternate water sources used in DNWS. Typically, one technology is not able to remove the wide diversity of contaminants. Pathogens and other contaminants can be reduced by natural and biological processes, filtration processes and disinfection processes. 

Common treatment processes include microfiltration (MF), ultrafiltration (UF), membrane biological reactor (MBR), ultraviolet light (UV) disinfection, ozone disinfection and chlorination (A Guidebook, 2017). 

The required LRT benchmarks for nonpotable reuse in the following Tables 1, 2 and 3 were derived using QMRA expressed as LRTs to reduce the risk of infections down to an annual probability of infection threshold of 1 in 10,000 (10^-4) infections/person/year. To meet the LRTs for the range of alternate water sources and end uses, treatment processes are designed in series to create an effective treatment train.

To assess the suitability of the treatment processes to meet the LRTs for target pathogen groups, expected performance values (called log reduction values, LRVs) can be assigned to each barrier treatment (Sharvelle et al., 2017). The LRVs are established by the removal efficiency of the pathogen demonstrated during challenge testing (EPA, 2005). 

The risk-based framework report provides five tables of observed values (or credits) for pathogen reduction for various treatment processes. The regulatory authority will determine what data is permissible for use in assigning LRV credits.

1. Blackwater Treatment Train

Blackwater is wastewater containing bodily or other biological wastes originating from toilets, kitchen sink and dishwasher waste. Figure 1 shows an example of a multiple barrier treatment train for blackwater consisting of an MBR, UV disinfection and chlorination. Evaluating the treatment train in Figure 1, LRVs are assigned for each process step seen in the black oval under each treatment barrier. The black oval shows the LRVs in the following order for virus/protozoa/bacteria. 

The LRVs are summed up for each pathogen group to determine if they meet the required LRTs. The summed total in Table 1 for each pathogen group shows that this treatment train has achieved the LRT benchmarks for indoor and outdoor reuse.

2. Greywater Treatment Train

Greywater is wastewater that has not come into contact with toilet or kitchen waste. It includes wastewater from bathtubs, showers, lavatories, clothes washers and laundry sinks. Figure 2 shows an example of a multiple barrier treatment train for greywater treatment consisting of MBR and UV disinfection for nonpotable indoor reuse. Evaluating the treatment train in Figure 2, LRVs are assigned for each process step seen in the black oval under each treatment barrier. 

The LRVs are summed up for each pathogen group to determine if they meet the required LRTs. The summed total in Table 2 for each pathogen group shows that this treatment train has achieved the LRT benchmarks for indoor and outdoor reuse.

3. Stormwater Treatment Train

Stormwater is precipitation from rain or snowmelt events that has contacted a surface at grade, belowgrade or aboveground impervious surfaces (e.g., streets, parking lots and rooftops). Figure 3 shows an example of a multiple barrier treatment train for stormwater treatment consisting of micro filtration and UV disinfection for nonpotable outdoor reuse. 

Evaluating the treatment train in Figure 3, LRVs are assigned only for UV disinfection. Microfiltration does not treat the pathogen targets. The total LRVs for the UV disinfection in Table 3 for each pathogen group show that this treatment train has achieved the LRT benchmarks for indoor and outdoor reuse. 

Effluent Water Quality 

The effluent water quality parameters in Table 4 were recommended by Plumbing Manufacturers International and are applicable for plumbing product performance to ensure that products function as intended by the manufacturer. The effluent water quality parameters are not intended to serve as guidance for public health and safety or for plumbing products used for bathing, cooking, drinking or hygiene purposes (e.g., faucets, showerheads, personal hygiene devices and bidets). 

The effluent water quality parameters are intended to protect the product parts in water closets and urinals (rubber flappers, seals, ballcocks and flushometer valve) to maintain proper function. 

Monitoring Requirements 

IAPMO IGC 324 in Table 5 — now the American National Standard for Alternate Water Source Systems for Multi-Family, Residential and Commercial Use (IAPMO/ANSI Z1324, 2022) — is required for validation procedures for blackwater, greywater and stormwater treatment systems in WEStand. The monitoring parameters are based on the risk level whether the reuse water is outdoors (Risk Category 1) or indoors (Risk Category 2). 

They are monitored by sensors placed on the system’s effluent and connected to a smart controller, which activates an alarm when the parameters in Table 5 are outside the specifications and shut the system down when the alarm is not acknowledged after a period of eight hours has lapsed. Continuous monitoring is recommended to ensure that the treatment processes are continuously meeting their performance goals.

ONWS treatment trains are emerging technologies in the United States without standardization. The risk-based framework to determine LRTs is guiding current state regulations for treatment methodologies. However, there is no U.S. regulatory consensus for LRVs and LRTs to evaluate the treatment trains. 

WEStand and IAPMO/ANSI Z1324-2022 are American national consensus standards incorporating LRTs that can be adopted by state regulators for ONWS performance requirements. The EPA is attempting to harmonize the benchmarks for LRTs that may further help industry consensus and standardization. 

U.S. treatment trains are used for treating blackwater, greywater, rainwater and stormwater for nonpotable water reuse. LRVs are used to evaluate the performance of multibarrier processes to target and remove pathogens, particulates and organics from alternate water sources. The multibarrier processes may be combined variously using MF, UF, MBR, UV disinfection, ozone disinfection or chlorination to meet the LRTs. 

Additional treatment parameters are needed to protect product components in water closets and urinals subject to treated water reuse. Continuous monitoring using sensor validation is recommended to ensure the treatment processes are continuously meeting performance goals. 

Daniel Cole is the senior director of technical services and research at the International Association of Plumbing and Mechanical Officials. He was a licensed journeyman plumber, contractor, plumbing inspector and plans reviewer in Illinois. Cole is a member of the American Society of Plumbing Engineers, serving on the ASPE Main Standards Committee. He received the ASPE Scientific Achievement Award in 2018. Cole served as liaison in the development of WEStand and the working groups for onsite non-potable water systems.