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Article

Augmentation of Reclaimed Water with Excess Urban Stormwater for Direct Potable Use

by
Keisuke Ikehata
1,*,
Carlos A. Espindola, Jr.
1,
Anjumand Ashraf
1 and
Hunter Adams
2
1
Ingram School of Engineering, Texas State University, San Marcos, TX 78666, USA
2
City of Wichita Falls, Wichita Falls, TX 76307, USA
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(18), 7917; https://doi.org/10.3390/su16187917
Submission received: 2 August 2024 / Revised: 8 September 2024 / Accepted: 8 September 2024 / Published: 11 September 2024
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Review Reports Versions Notes

Abstract

:
Groundwater and surface water have been the primary sources of our public water supply around the world. However, rapid population and economic growth, as well as global climate change, are posing major threats to the quality and quantity of these water resources. Treated wastewater (reclaimed water) and stormwater are becoming more important water resources. Use/reuse of these unconventional water resources can enable a truly sustainable, closed-loop, circular water system. However, these two sources are not usually mixed with each other. In this study, we propose the use of combined excess urban stormwater and reclaimed water as a source of potable water supply. One of the most pronounced benefits of this proposed scheme is the possible elimination of costly and energy-intensive processes like reverse osmosis. Reclaimed water tends to have high concentrations of dissolved solids (>500 mg/L) and nitrate-N (>10 mg/L), which can be lowered by blending with stormwater or rainwater. Despite technical and engineering challenges, this approach can benefit various communities—small, medium, large, upstream, downstream, urban, and rural—in diverse climates. Our study suggests that this new holistic approach is feasible, enabling the combined water to be directly used as a sustainable drinking water source.

1. Introduction

Water scarcity is a growing issue, especially in the Southwestern and Southern parts of the United States of America (USA) and many parts of the world [1]. This is due to frequent, prolonged droughts and climate change, as well as rapid population growth, mass migration, and urbanization [2,3]. It is unlikely that conventional water resources, including surface water and groundwater, alone can support future population and economic growth, and long-term water supply availability has been one of the most important issues for water utilities in the USA [4]. Alternative water resources, such as brackish water and reclaimed water, have been used to supplement freshwater resources [5,6]. However, the presence of high salinity and undesirable contaminants dictate the use of advanced processes such as membrane filtration, including reverse osmosis (RO), ozonation, and ultraviolet (UV) advanced oxidation to treat those alternative water sources for potable use and reuse [7,8,9]. The ongoing challenges of brackish- and reclaimed-water utilization include the trend of increasing salinity and the management of concentrate from RO-based schemes, especially for the facilities in inland communities, including Central Texas [10].
A possible solution to the above challenges is the capture and use of stormwater, which is another alternative water source that is currently underutilized. Stormwater management is another major challenge in urban and suburban communities because of the detrimental impacts of heavy rainstorms on infrastructure, businesses, and the environment. Global climate change has affected local and regional weather patterns, apparently intensifying and exacerbating the severity of both rainstorms and droughts [11,12]. It is crucial to develop and implement effective strategies for buffering the impacts of too much rain and too little rain and re-balancing the temporal and spatial water availability. Traditionally, rainwater capture and use have been practiced in relatively small and remote areas where centralized water supply systems are not available [13,14,15]. Yet, rainwater harvesting for varieties of potable and non-potable uses is becoming more and more popular in suburban and urban cities such as Austin, Texas, USA and Colombes, France [16,17,18]. Integrating excess urban stormwater into urban water supply systems is becoming a viable option [17,19]. However, combining excess urban stormwater with reclaimed water for potable applications has not yet been discussed fully.
Therefore, more study is required to understand the potential and feasibility of blending excess urban stormwater with treated wastewater at advanced water purification facilities (AWPFs) for potable reuse to increase recovery and improve water quality. This is a novel approach and differs from conventional combined sewer systems, in which untreated wastewater and stormwater meet in a municipal sewer system before treatment. Those combined sewer systems often experience combined sewer overflow (CSO) during heavy rainfall or snowmelt, which overwhelms the sewer system and wastewater treatment plant and discharges excess wastewater directly into nearby water bodies with little to no treatment [20]. CSO is known to pose many public health risks and create environmental damage including contamination of surface water with pathogens and toxic chemicals as well as nutrients [21]. Instead, by combining treated wastewater with excess stormwater in a controlled manner, it is possible to alleviate the negative impacts of flooding and to reduce wastewater discharges, while also increasing the volume of water available for urban demand. If properly collected and handled, stormwater is often better in water quality, including lower salinity and contaminant concentrations, and can be blended with impaired water such as brackish water and reclaimed water as a diluent. This blending can provide various benefits, such as salinity reduction, water quality improvement, and water recovery enhancement, as well as possible elimination of RO from AWPF schemes. This approach will enable the construction of a truly sustainable, circular water system using two underutilized local water resources. This paper explores the feasibility, potential benefits, and challenges of blending excess urban stormwater with treated wastewater at AWPFs for potable reuse, which is a novel concept. We hypothesize that this approach is conceptually feasible and enables the construction of AWPFs without RO by improving water quality and reducing salinity across diverse climate regions. It should be noted that the capture and use of rainwater and stormwater may be restricted depending on the jurisdiction [22,23]. Water samples were collected in Central Texas and analyzed as part of the feasibility study. The water quality data and representative climate data were used to design a conceptual AWPF scheme. Subsequently, opportunities and challenges of the proposed approach in different types of communities in various climates are discussed.

2. Materials and Methods

Reclaimed-water samples were collected in 2021–2023 at a wastewater treatment plant (WWTP) in Central Texas that employs conventional activated sludge followed by anthracite filtration and UV or chlorine disinfection. Stormwater samples were collected at outfalls in the Texas State University Campus, San Marcos, Texas, USA, in Spring 2021. Rainwater samples were collected at a residential rainwater harvesting system in San Marcos, Texas, USA in Spring–Summer 2021. Table 1 summarizes the sampling dates and times. Upon collection, water samples were immediately analyzed for selected general chemistry parameters at Texas State University. In addition, reclaimed-water samples were shipped and analyzed for additional drinking water parameters at Cypress Environmental Laboratory (Wichita Falls, TX, USA) and/or for trace organic contaminants at Eurofins Eaton South Bend Lab (South Bend, IN, USA).
A Hach DR-1900 spectrophotometer (Loveland, CO, USA) and Hach 2100 Q Turbidimeter was used for colorimetric and turbidity analyses, respectively, while Hach Digital Titrators and Hach PocketPro Testers were used for titration analyses and pH-conductivity analysis, respectively, at Texas State University. A Hach IntelliCAL ISENA381 probe with a Hach HQ40d meter was used for sodium analysis. A Thermo Evolution 201 UV–Vis spectrophotometer (Waltham, MA, USA) was used for UV254 analysis. See Table S1 for the general chemistry methods used. General chemistry parameters tested at Cypress Environmental Laboratory include pH by Standard Method (SM) 4500H-B [24], conductivity by SM 2510B, free ammonia using an ion selective electrode by SM 4500NH3-E using a Thermo Orion Versa Star meter (Waltham, MA, USA), and turbidity using a Hach TL2300 (Loveland, CO, USA) by SM 2310B. Alkalinity and hardness titration analyses were performed using KEM AT-710 auto-titrators (Kyoto, Japan) by SM 2320B and SM 3500Ca-B, respectively. Other parameters tested at Cypress Environmental Laboratory include total organic carbon (TOC) using a Shimadzu TOC-L analyzer and autosampler (Columbia, MA, USA) using high temperature combustion by SM 5310B, anions and cations using a Thermo Scientific ICS5000 ion chromatograph and an ASAP autosampler by the United States (US) Environmental Protection Agency (EPA) Method 300.0, heavy metals using a Thermo Scientific iCAP RQ inductively coupled plasma mass spectrometer and an Elemental Scientific PrepFAST SC-4 DX (Omaha, NE, USA) autosampler (by EPA 200.8), total trihalomethanes using a Teledyne Tekmar Lumin P&T (Mason, OH, USA) and Thermo Scientific Trace 1300-ISQ LT GC-MS with an AquaTek LVA autosampler by EPA 524.2, and haloacetic acids using a Thermo Scientific Trace 1310 gas chromatograph–electron capture detector with an AS1310 autosampler by EPA 552.3. At Eurofin South Bend Laboratory, Eurofin, PPCP NEG and POS methods SM 5310C, EPA 524.2, EPA 522, EPA 525.2, EPA 533, EPA 537.1, EPA 200.7/8, and EPA 245.1 were used for pharmaceuticals and personal care products (PPCPs), TOC, 1,4-dioxane, volatile organic compounds, semivolatile organic compounds, per- and polyfluoroalkyl substances (PFAS), perfluorinated alkyl acids, metals, and mercury, respectively.
Based on the water quality data, a hypothetical one million gallons per day (1 MGD or 3800 m3/d) system was designed using reclaimed water and stormwater without RO based on a mass balance approach to achieve finished purified water that meets the US EPA National Primary and Secondary Drinking Water Regulations. For example, to eliminate RO, it is important to reduce the total dissolved solids (TDS) concentration to below 500 mg/L. The following equation was used to calculate the blend ratio:
[TDS (mg/L)]Reclaimed Water × (1 − x) + [TDS (mg/L)]Stormwater × x = 500 mg/L
where [TDS (mg/L)]Reclaimed Water and [TDS (mg/L)]Stormwater are the average TDS concentration of reclaimed water and stormwater, respectively, and x is the proportion of stormwater to be added. Then, the estimated concentration of water constituent A in blended water can be determined by the following equation:
[A (mg/L)]Reclaimed Water × (1 − x) + [A (mg/L)]Stormwater × x = [A (mg/L)]Blended Water
This 1-MGD facility can support approximately 6400 people, assuming a per capita municipal water demand of 157 gallons (594 L) per day in Texas [25]. The average annual rainfall of 35.5 inches (902 mm) in Central Texas [26] was used to estimate the stormwater availability and impervious surface requirement. Stormwater storage capacity requirements were calculated based on the daily, weekly, monthly, and yearly stormwater requirements. Subsequently, an AWPF scheme was constructed based on the existing literature on contaminants of concern, such as nitrate-N [27,28,29], color [30,31,32,33], PFAS [34,35,36], and sucralose [37,38,39]. The pathogenic-microorganisms removal requirement was assessed based on the Texas Commission on Environmental Quality (TCEQ) guidelines [40], which require a minimum of 8, 5.5, and 6 log removal of enteric viruses, Cryptosporidium oocysts, and Giardia cysts for DPR.

3. Results

3.1. General Water Quality of Reclaimed Water, Stormwater, and Rainwater

Table 2 shows the general water quality of the reclaimed water, stormwater, rainwater samples collected in Central Texas in 2021. Reclaimed water exhibited reasonably good water quality except for nitrate-N, TDS, and color, which consistently exceeded the drinking water standards of 10,500 mg/L, and 15 PtCo Unit, respectively. Among them, nitrate-N is a primary drinking water contaminant, which represents a public health risk if exceeded, while TDS and color are secondary drinking water contaminants (i.e., aesthetic parameters). The high nitrate-N concentration is not surprising because the wastewater treatment plants employ full nitrification and partial denitrification processes, which convert ammonia-N into nitrate-N, then remove some of the nitrate-N within the treatment plants. TDS is also typically higher in wastewater than drinking water due to several domestic sources such as cooking, laundry, water softeners, as well as industrial discharges [10]. The color in reclaimed water was likely derived from the microbial decomposition of organic matter in the wastewater during the biological treatment in the WWTPs. Removal of these parameters would be required for safe and palatable potable reuse of these reclaimed waters. To reduce the excessive nitrate-N, several approaches may be considered, such as upgrading WWTPs to full denitrification, using biological denitrification filters, anion exchange, or RO in AWPFs. Color could be removed by either granular activated carbon (GAC), ozonation, nanofiltration (NF), or RO. The removal of TDS is more difficult as compared with the other two parameters because it requires NF or RO. Heterotrophic bacteria, which represent general microbial quality and are expressed as heterotrophic plate count (HPC), were generally present in the reclaimed water and stormwater. This highlights and confirms the requirement of additional disinfection to make the reclaimed water safe for potable reuse. Combinations of various disinfection and membrane processes such as ozone, UV, chlorine, MF, UF, NF, and RO could be used to achieve the required virus and protozoa removal.
This study shows that stormwater can have better water quality compared with reclaimed water with respect to general chemistry parameters, while rainwater exhibits superb water quality (Table 2). The only parameters that exceeded the drinking water standards in stormwater were turbidity, apparent color, and HPC. Treatment processes such as ozonation, MF/UF, and UV could be used for turbidity and color removal, as well as pathogen removal/inactivation, from stormwater combined with reclaimed water. As expected, stormwater has much lower nitrate-N (1.1 mg/L) and TDS (122 mg/L) than reclaimed water because there are not many sources of these contaminants in the stormwater collection system. Obviously, the first flush would be dirtier and more contaminated [41]; however, if the stormwater collection systems were properly cleaned and maintained, the water quality could be further improved. It should be noted that the application of road salts may compromise the stormwater quality in terms of TDS in northern states and countries [42,43]. It is also known that rainwater tends to have higher concentrations of TDS in coastal communities due to salt aerosol from seawater [44].

3.2. Assessment of Metals and Trace Organics in Reclaimed Water

Further analysis of selected reclaimed-water samples revealed the presence of trace organics such as pesticides, volatile organic compounds, PPCPs, as well as some trace metals (Tables S2 and S3), albeit none of them exceeded current drinking water standards in the USA, except perfluorooctanoic acid (PFOA, 5.2 ng/L). Removal of PFAS such as PFOA can be achieved by GAC, anion exchange, or RO. A notable compound that has been detected at high concentrations in the reclaimed-water samples is sucralose (73.5 ± 6.4 µg/L), which is an artificial sweetener known to be highly resistant to conventional water and wastewater treatment. It is also resistant to chemical oxidation using ozone. Sucralose is often used as a chemical marker in surface and groundwater that can be influenced by wastewater discharges. Although the taste threshold value for sucralose is 3.49 mg/L [45], which is 47 times higher than the concentrations detected in this study, sensitive populations may possibly taste slight sweetness if the concentration of this increased in the future.

3.3. Approaches for DPR of Combining Reclaimed Water and Stormwater

Based on the water quality analysis, it is apparent that the reclaimed-water samples tested in this study will require RO or NF to meet the TDS standard. Assuming the average TDS concentrations in Table 2 are representative, a minimum stormwater–reclaimed-water blend ratio of 35:65 would be required to bring the TDS concentration to less than 500 mg/L, based on Equation (3):
700 mg/L × (1 − x) + 122 mg/L × x = 500 mg/L
x = 0.346 = 34.6% ≅ 35%
This blend ratio could be reduced to 30:70 if little to no contamination occurred during the stormwater collection and storage processes (i.e., like the rainwater quality, TDS = 23 mg/L). After the blending, the combined reclaimed water and stormwater can be treated by an AWPF without RO. Please note that the water quality varies in different seasons, weather, and other natural and anthropogenic factors and conditions. More detailed water quality analysis, as well as application of safety factors, would be required for actual facility design.
Assuming the average TDS concentrations in Table 2, 484,000 m3 (128 million gallons) of stormwater will be needed annually to produce 3800 m3/d (1 MGD) of drinking water from reclaimed water without RO (Figure 1). The required impervious surface area of the urban stormwater catchment will be 536,000 m2 (132 acres), which is large, albeit not impractical. Assuming the volume requirement of 484,000 m3 per year, the stormwater storage capacity for one week and one month supply will be 9300 m3 (2.5 million gallons) to 40,300 m3 (10.6 million gallons), respectively.
Assuming the blend ratio of 35:65, the nitrate-N concentration can be reduced to 8.2 mg/L based on Equation (5), as shown below:
12 mg/L × 0.65 + 1.1 mg/L × 0.35 = 8.2 mg/L
This is lower than the primary drinking water standard for nitrate-N (10 mg/L, Table 1), which suggests that the stormwater blending with reclaimed water can produce drinkable water with respect to this primary contaminant. Yet, close monitoring of nitrate-N would be still required to ensure no violation of the standard. Additional nitrate removal may be required if fluctuation in reclaimed water quality is anticipated as well.
Consequently, the non-RO based AWPF can be proposed by a combination of ozonation, biological activated carbon (BAC), MF or UF, GAC, anion exchange (IX), and UV disinfection or AOP to achieve desired pathogens, turbidity, color, nitrate-N, total organic carbon, PFAS and other trace organics removal as shown in Figure 2. All these unit processes have been used at conventional water treatment plants and AWPFs, and the construction and operation of a facility are technically feasible [7]. Actual facility design must be carefully conducted based on a detailed water quality analysis, considering regulatory standards and guidelines specific to the jurisdiction.
Obviously, securing enough stormwater stored (Figure 1) to have a consistent supply to blend with reclaimed water will be a major challenge due to the unpredictable nature of rainfall frequencies and intensities, including ongoing climate change. The size of the storage facilities can be decided depending on the frequency of consistent precipitation in the area. Larger facilities will be needed if the precipitation occurs only in certain months. A backup freshwater supply (e.g., groundwater, surface water, heating, ventilation, and air conditioning condensate) and/or emergency RO may be needed if an extended period of drought is expected. The blend ratio may need to be adjusted based on the alternative freshwater supply. The storage facilities need to be carefully designed to accommodate the site-specific conditions.

4. Discussion

This study evaluated and demonstrated the preliminary feasibility of the use of a stormwater–reclaimed-water blend for DPR using the water quality data collected in Central Texas as an example. Central Texas can be characterized as an inland, sub-humid to semi-arid climate and as prone to drought [26]. This innovative approach can be applicable to other types of climates and geographical regions. This section discusses and compares the applicability of this proposed approach in different types of communities, including upstream (inland), downstream (coastal), small, large, rural, and urban communities in various climates.

4.1. Upstream (Inland) vs. Downstream (Coastal) Communities

A community both upstream and downstream of a river basin can benefit from this approach because it will reduce raw water withdrawal from conventional water sources and reduce the environmental discharge of the treated wastewater into the water body. The upstream community will be able to preserve and improve the stream water quality and aquatic ecosystem from immediately downstream all the way to the outlet, while the downstream community will see the impact on a larger receiving water (e.g., estuaries, bays, oceans, large lakes) only. However, an upstream community may have an obligation to discharge treated wastewater to downstream to maintain the base flow of the river [22].
Flood and erosion risks may be mitigated by creating large-scale stormwater collection and storage facilities in both types of communities. While any excess stormwater can be used to recharge groundwater basins in upstream communities, coastal communities may utilize it for groundwater recharge only if the groundwater level is not very high. Coastal communities have a large body of water (i.e., sea or lake) nearby, which can be another alternative water source and can be utilized with seawater desalination or conventional surface water treatment, while upstream, inland communities may have limited water sources and do not have convenient access to seawater or lake water. The convenient access to seawater in coastal communities may encourage the use of RO process because the RO concentrate can be discharged through ocean outfalls.
Inland communities often have varied terrains and multiple elevations/pressure zones within their service areas. This creates a more challenging design requirement for stormwater collection, storage, and delivery systems compared to downstream, coastal communities where the terrain is typically flat. However, stormwater in a marine coastal community may be influenced by seawater through groundwater infiltration, as well as salty aerosols and their deposits, which will influence the blend ratio. Wastewater can also be affected by high salinity coastal groundwater infiltration in sewer systems [46]. Thus, it is important to have proper lining in the sewer systems to prevent such infiltration, any detrimental corrosion issues, and high TDS in reclaimed water.

4.2. Small vs. Large Communities

A small community may embrace greater benefits from this approach because the overall water demand is smaller and the infrastructure development cost can be more reasonable, whereas a larger community would require larger infrastructure, and the cost can be prohibitively high. Especially, finding locations for building/installing large water storage facilities can be a major challenge in a large, already built-out city. Moreover, it might be more challenging to reach consensus to implement new systems among the stakeholders in a larger community compared to a smaller one. On the contrary, it can be relatively easy to implement stormwater capture and storage systems in a smaller community. It is also possible to reuse the entire wastewater effluent and create a completely closed-loop, circular water supply system, if sufficient stormwater is secured to replace and eliminate surface water or groundwater withdrawals. This can be a reality in new subdivisions in suburban areas where stormwater management systems and wastewater collection and treatment systems are being designed and implemented at the same time. However, a small community may lack a sufficient capital and operational budget and technical expertise to design, construct, operate, and maintain the facilities, including AWPFs. Also, building and operating a small-scale AWPF may not be particularly cost-effective [47]. The risk of system failure can be higher in a smaller community, if their water supply system is fully relying on this system, while a larger community tends to have a more diverse water portfolio, and the impact of system failure can be mitigated.

4.3. Rural vs. Urban Communities

Rural communities have similar characteristics to small communities in terms of limited availability of budget and technical expertise, unfavorable economies of scale, and higher failure risks. In addition, these communities typically have large land availability, which is both advantageous and disadvantageous. Since the population density is very small and residential buildings are not clustered together in rural areas, building centralized wastewater and stormwater collection and treatment facilities are not very realistic. A household-scale rainwater harvesting system is available and becoming popular in rural areas [48,49]. Although adding a household-scale DPR system to the rainwater system is possible, it must be very simple to operate, inexpensive, very robust, and easy to maintain. This represents a significant challenge in a rural community. Urban communities also share very similar characteristics to large communities. A smaller urban or suburban community may be a more promising place to implement a stormwater–reclaimed-water DPR system because of the benefits of having urban characteristics along with the likely availability of resources, including land, budget, and technical expertise.

4.4. Arid, Semi-Arid, and Humid Climates

Since this stormwater–reclaimed-water blend approach relies on natural precipitation, the climate has a major impact on its feasibility and applicability. It is needless to state that ongoing climate change, including both more serious and prolonged drought and more frequent, unpredictable, and intense storms, would have impacts on any stormwater management practices. Arid, desert areas probably already have a limited availability of stormwater to be used to supplement the water supply. Securing enough stormwater to bring reclaimed water TDS down below its drinking water standard (typically 500 mg/L) throughout the year would be very challenging. The salinity of reclaimed water tends to be higher in arid areas as well. RO-based AWPFs complemented with brackish/sea water desalination are likely more reasonable alternative water resources in such areas. However, climate change has caused unusual weather patterns in those historically very dry regions such as Las Vegas, Nevada, and Saudi Arabia [50,51]. Additional stormwater collection and storage facilities may be still beneficial to mitigate flood risks and supplement already limited water resources. The inclusion of urban stormwater into indirect potable reuse projects has been implemented in several cities in California, such as Santa Monica [52] and Monterey [53]. Due to the unpredictable and sporadic nature of precipitation, as well as a state-specific requirement, the RO process is incorporated in the AWPF designs in these projects. Wet, humid areas probably have enough precipitation and stormwater to augment reclaimed water. Flood and erosion risk mitigation can be a major benefit in such areas, whereas the need for supplemental water can be small or marginal. However, rapidly growing metropolitan areas in humid areas will still benefit from this additional water source. Very wet and humid areas tend to experience extreme storm events such as tropical storms and hurricanes. This would require special attention to infrastructure design, construction, and operation. Semi-arid areas, such as Central Texas, obviously are situated between dry and wet climates and probably have the best suitability for the stormwater–reclaimed-water DPR approach due to relatively predictable precipitation and rapidly growing population, as demonstrated in this feasibility study (Figure 1). Yet, climate change has altered the weather patterns in semi-arid areas as well, and predicting the rainfall and properly sizing stormwater collection and storage systems can be a major challenge.

4.5. Cold vs. Warm Climate

Cold regions tend to have lower population density and smaller water demand as compared to warm regions. The incentive to reuse treated wastewater is probably relatively small in cold regions. Also, precipitation in cold regions occurs in the form of snow during the winter months, and stormwater flow occurs only in warmer months. Spring runoff is also a characteristic of cold regions, as well as mountainous areas. Therefore, many additional factors and conditions need to be considered for implementing this concept in cold regions. The impact of de-icing road salts [43] to the quality of stormwater needs to be considered as well. A warm climate is generally favored for economic growth and migration. Thus, the need for additional water resources is typically higher in warm, temperate areas, and the proposed approach is probably more suitable there. Again, the risks of extreme drought and storm events are highly prevalent in warm regions, as well as in cold regions, which represent both opportunities and challenges. Table 3 shows the summary of the discussion.

5. Conclusions

This research paper evaluated and revealed the preliminary feasibility of the use of excess urban stormwater to augment reclaimed water for DPR for the first time. Conceptual guidelines for designing stormwater collection and storage systems, as well as AWPFs using a combination of existing advanced unit processes, are provided. This holistic approach to supplement existing water resources such as groundwater and surface water with currently underutilized stormwater and reclaimed water can be widely applicable. Medium-size suburban communities, especially newly developed subdivisions, in a warm and semi-arid climate such as Central Texas appear to be the most promising locations to implement stormwater–reclaimed-water blending for potable reuse. This innovative approach can achieve a completely closed-loop, circular water system with minimal or no water withdrawal and environmental discharge, supporting both sustainable development and environmental preservation. Since water quality and availability vary significantly, any design and implementation of new water infrastructure systems needs to be carefully studied, planned, and executed with site-specific considerations. Further research is needed and currently underway to develop quantitative models that predict the quantity and quality of stormwater–reclaimed-water blends at various locations and assess their techno-economic viability, including their energy requirements and management. This research provides a critical foundation for future regulatory frameworks and policy development by demonstrating the potential of integrating stormwater into potable water reuse systems, paving the way for innovative water management strategies in regions facing water scarcity. It also offers new insights into the practical application of stormwater and reclaimed-water blending, contributing to the development of sustainable, closed-loop water systems.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su16187917/s1; Table S1: General chemistry analysis methods; Table S2: Additional drinking water analysis results on reclaimed-water samples; Table S3: Metals and trace organics analysis results on reclaimed-water samples.

Author Contributions

Conceptualization, K.I. and C.A.E.J.; methodology, K.I. and H.A.; formal analysis, C.A.E.J., A.A. and H.A.; investigation, K.I., C.A.E.J., A.A. and HA.; resources, K.I. and H.A.; data curation, K.I., C.A.E.J. and A.A.; writing—original draft preparation, K.I.; writing—review and editing, K.I., C.A.E.J., A.A. and H.A.; visualization, K.I.; supervision, K.I.; project administration, K.I.; funding acquisition, K.I. and C.A.E.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Texas State University Research Enhancement Program (PI: K.I.), as well as the Texas State University STEM Undergraduate Summer Research Experience (SURE) Program (SURE Scholar: C.A.E.J.).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Supplementary data for this article can be found at (data provided as Supplementary Materials).

Acknowledgments

The authors thank the technical assistance of Sam Reeder at Cypress Environmental Laboratory, City of Wichita Falls, Wichita Falls, TX. We thank Eva Steinle-Darling, Carollo Engineers, Austin, TX for her technical advice. We also thank Paul Kite with the City of San Marcos, San Marcos, TX, and Paul Shropshire and Andrea Alexander with Jacobs Solutions, San Marcos, TX, for their technical assistance and access to the reclaimed-water samples. We also thank Carla Batey at Texas State University for providing rainwater samples for this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Conceptual schematic of stormwater and reclaimed-water blending and treatment system for 1-MGD (3800 m3/day) potable purified water production.
Figure 1. Conceptual schematic of stormwater and reclaimed-water blending and treatment system for 1-MGD (3800 m3/day) potable purified water production.
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Figure 2. Conceptual schematic of a non-RO AWPF for combined stormwater and reclaimed water for DPR.
Figure 2. Conceptual schematic of a non-RO AWPF for combined stormwater and reclaimed water for DPR.
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Table 1. Water sampling dates and times.
Table 1. Water sampling dates and times.
Type DateTimeNote
Reclaimed Water3 February 20211:40 PMChlorinated
3 February 20211:55 PMUV disinfected
5 March 20214:10 PMUV disinfected
14 May 20219:00 AMChlorinated
14 May 20219:30 AMUV disinfected
9 August 20229:00 AMUV disinfected
16 October 20235:00 PMUV disinfected
17 December 20236:00 PMUV disinfected
18 December 20234:30 PMUV disinfected
Stormwater11 February 202110:05 AMOutfall 1-20 (Middle of rain)
11 February 202110:10 AMOutfall 1-32 (Middle of rain)
23 April 202111:10 AMOutfall 1-20 (First flush)
23 April 202111:15 AMOutfall 1-32 (First flush)
23 April 20213:00 PMOutfall 1-32 (End of rain)
Rainwater17 March 20215:00 PMFrom a storage tank
9 August 20214:00 PMFrom a storage tank
Table 2. General chemistry of reclaimed water, stormwater, and rainwater samples collected in 2021. Averages and standard deviations are shown.
Table 2. General chemistry of reclaimed water, stormwater, and rainwater samples collected in 2021. Averages and standard deviations are shown.
ParameterReclaimed Water
(n = 5)		Stormwater
(n = 5)		Rainwater
(n = 2)		USEPA Drinking Water Standard
Sodium (mg/L)233 ± 3465 ± 48<10n/a
Potassium (mg/L)17 ± 127 ± 250.4 ± 0.1n/a
Calcium (mg/L)96 ± 76.6 ± 0.51.5 ± 0.1n/a
Magnesium (mg/L)18 ± 3 0.5 ± 0.10.5n/a
Iron (mg/L)0.06 ± 0.010.03 ± 0.010.020.3 *
Manganese (mg/L)0.032 ± 0.0050.015 ± 0.0070.0220.05 *
Copper (mg/L)0.06 ± 0.020.06 ± 0.03<0.041.0 *
Ammonia-N (mg/L)0.6 ± 0.90.42 ± 0.030.85 ± 0.78n/a
Chloride (mg/L)167 ± 57 ± 318250 *
Sulfate (mg/L)55 ± 1<2<2250 *
Bicarbonate (mg/L)274 ± 1570 ± 553n/a
Nitrate-N (mg/L)12 ± 21.1 ± 0.10.810
Reactive Silica (mg/L)13 ± 19.4 ± 4.4<1n/a
Orthophosphate (mg/L as PO43−)0.8 ± 0.20.16 ± 0.070.08 ± 0.01n/a
Total Dissolved Solids (mg/L)700 ± 19122 ± 9723 ± 15500*
Turbidity (NTU)0.38 ± 0.1310.3 ± 6.70.44 ± 0.16TT 1
Alkalinity (mg/L as CaCO3)225 ± 1372 ± 592 ± 1n/a
Ca Hardness (mg/L as CaCO3)241 ± 1715 ± 12n/a
Total Hardness (mg/L as CaCO3)313 ± 1519 ± 113n/a
Conductivity (µS/cm)1094 ± 30 191 ± 15231 ± 25n/a
pH7.6 ± 0.28.9 ± 0.98.2 ± 0.96.5–8.5 *
Chemical Oxygen Demand (mg/L)14 ± 215 ± 4<10n/a
Total Organic Carbon (mg/L)4.7 ± 0.8 8.4 ± 1.2Not testedTT 2
UV254 (OD)0.115 ± 0.0600.046 ± 0.0400.019n/a
Apparent Color (PtCo Unit)26 ± 8104 ± 674 ± 115 *
Heterotrophic Plate Count (CFU/mL)1.4 × 104 ± 1.2 × 1051.6 × 105 ± 1.2 × 105Not tested500
Notes: * secondary standard. 1 At no time turbidity can be higher than 1 NTU. 2 Removal is required depending on raw water concentration and alkalinity. Bold values indicate higher than standard values. Abbreviations: NTU = nephelometric turbidity unit, TT = treatment technology, OD = optical density, PtCo = platinum-cobalt, CFU = colony forming unit.
Table 3. Summary of community/climate comparisons.
Table 3. Summary of community/climate comparisons.
TypeProsCons
Upstream/InlandReduce raw water withdrawal; improve receiving water quality and aquatic ecosystem in the entire river basin; flood and erosion risk mitigation; groundwater recharge with excess stormwater; better stormwater quality.Water rights agreements may prevent the use of stormwater and/or wastewater to preserve the base flow; challenging infrastructure design due to varied terrains and multiple elevations in the area.
Downstream/ CoastalReduce raw water withdrawal; protect the ecosystem in the receiving water (estuaries, bays, large lakes); flood and erosion risk mitigation; generally flat terrain.Does not improve the water quality and ecosystem in the entire river basin; compete/complement with seawater desalination and RO-based AWPF; stormwater quality can be affected by the seawater infiltration and salty aerosols; salinity of wastewater can be higher due to infiltration; corrosion risk due to higher salinity; groundwater recharge potential may be limited.
SmallSmaller-size facilities more manageable; easier to reach project consensus; completely closed water supply system possible.Limited capital and operational budgets; limited technical expertise; unfavorable economies of scale for AWPFs; higher failure risks.
Large Larger capital and operational budgets; technical expertise likely available; better economy of scale for AWPFs; more diverse water portfolio.Limited land availability for large stormwater storage facilities in a built-out city; larger capital cost; harder to reach project consensus.
RuralSimilar to small communities; land availability.Centralized stormwater–wastewater systems unrealistic; household-scale DPR system can be costly and challenging.
UrbanSimilar to large communities; smaller urban and suburban communities may have access to resources (e.g., land, budget, expertise).Similar to large communities.
AridNot very suitable, but stormwater still constitutes additional water supply when it rains (RO is likely required).Limited availability of natural precipitation; high TDSs in reclaimed water; prolonged drought.
Semi-aridMost suitable due to the moderate availability of rainwater and increased water scarcity.Prone to climate change impact and extreme drought/storm events.
HumidFlood and erosion risk mitigation; rapidly grown metropolitan areas would benefit. Better availability of conventional surface water and groundwater; extreme storm events (e.g., tropical storms and hurricanes) require special attention.
ColdApplicable only in warmer months.Likely lower incentive for water reuse; de-icing salt contributes to potentially high TDS in stormwater, spring runoff.
WarmSuitable due to favorable climate for economic and population growth; flood risk mitigation.Prone to climate change impact and extreme drought/storm events.
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Ikehata, K.; Espindola, C.A., Jr.; Ashraf, A.; Adams, H. Augmentation of Reclaimed Water with Excess Urban Stormwater for Direct Potable Use. Sustainability 2024, 16, 7917. https://doi.org/10.3390/su16187917

AMA Style

Ikehata K, Espindola CA Jr., Ashraf A, Adams H. Augmentation of Reclaimed Water with Excess Urban Stormwater for Direct Potable Use. Sustainability. 2024; 16(18):7917. https://doi.org/10.3390/su16187917

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Ikehata, Keisuke, Carlos A. Espindola, Jr., Anjumand Ashraf, and Hunter Adams. 2024. "Augmentation of Reclaimed Water with Excess Urban Stormwater for Direct Potable Use" Sustainability 16, no. 18: 7917. https://doi.org/10.3390/su16187917

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