ABSTRACT
Climate change is causing increased flooding and droughts. Droughts can cause drinking water sources to run dry. Therefore, recycling water from the wastewater plant effluent to the water treatment plant influent, also called ‘direct reuse’ is becoming necessary. A connection between a city's wastewater treatment plant (WWTP) effluent and water treatment plant (WTP) influent via pipe was simulated to provide an understanding of the capital costs and feasibility of execution. Hydraulic pipe-flow and pump equations were used to calculate the pipe and pump sizes needed for various flow rate and elevation head values. Various flow rate recycle rates were modeled by taking the WWTP effluent flow and in percentages ranging from 5 to 100% to estimate the pipe and pump requirements to send that amount to the WTP influent, including greenhouse gas emissions from pumping. The costs of installation were then determined using current market values for parts and trenching. Data analysis showed that the cost of this project is driven by the pump size requirements.
HIGHLIGHTS
The hydraulics of direct reuse is important to its feasibility.
The pipe price is usually the main thing considered.
But the greenhouse gas emission of the pumping must also be considered.
Direct reuse will be more important in the future due to increasing drought.
Using a larger pipe and a smaller pump might be worth it over the life cycle of the project.
LIST OF ABBREVIATIONS
INTRODUCTION
The climate is changing, with one result being more drought and flooding. Warmer temperatures due to greenhouse gases being emitted into the atmosphere stabilize the jet stream that conveys moisture in the atmosphere. Therefore, rainfall patterns will fall more over the same geographical area and result in more flooding. Conversely, more droughts will result since the jet stream will not bring moisture over lands that are not under it.
For drought-stricken areas or areas with fast-growing populations, direct potable water reuse is becoming an increasingly more prominent technology as areas are seeing a drop in surface- and ground-water source levels due to this drought (Rodriguez et al. 2009; Lahnsteiner et al. 2018; Tortajada 2020). There are two types of water reuse, direct (DPR) and indirect (IPR) (Jeffrey et al. 2022). IPR is the use of wastewater for drinking with an environmental buffer in place, like discharge to a lake, for example, prior to intake to a water treatment plant. The focus of this study is on DPR, which is the recycling of wastewater for potable use without discharge to the environment (Jeffrey et al. 2022).
Indirect and direct potable reuse schematics (dashed lines indicate additional required piping and possible pump(s), to recycle all or a portion of the wastewater effluent.).
Indirect and direct potable reuse schematics (dashed lines indicate additional required piping and possible pump(s), to recycle all or a portion of the wastewater effluent.).
This is vitally important, according to the United Nations Sustainability Goals. Namely Goal 1 No Poverty, since potable water is essential to allow people to work to earn a living, Goal 3 Good Health and Well-Being since water is essential for life, Goal 4 Quality Education since students cannot study if they do not have clean drinking water, Goal 6 Clean Water and Sanitation (self-explanatory), Goal 8 Decent Work and Economic Growth since essential services such as drinking water allow people to be able to work, Goal 9 Industry, Innovation, and Infrastructure since drinking water and sewer systems are essential infrastructure, Goal 11 Sustainable Cities and Communities since all people are entitled to clean drinking water and sanitation.
Water reuse does not only function to conserve drinking water resources; it also has other impacts. Reuse decreases the draw of water and the discharge of WWTP effluent into sensitive habitats (EPA). When water is drawn from a waterbody for drinking water demands, it can create an imbalance in the ecosystem due to a loss in water volume for the environment (EPA 2023). The WWTP effluent that is discharging into the waterbody can also be harmful (American Water Works Association 2016). In addition, the need for more dams and reservoirs is reduced. Direct reuse, however, means less water withdrawn from nature from the water treatment plant influent but then also less water discharged to nature from the wastewater plant effluent.
There are other relevant aspects of direct reuse that are related to this work. These include educational aspects (Rosenberg Goldstein et al. 2024), emerging contaminants (Han et al. 2023), lessons from earlier civilizations (Angelakis et al. 2023), water system supply resilience (Porse 2023), political issues (Procházková et al. 2023), public perception (Scruggs et al. 2020; Barnes et al. 2023; Fu et al. 2023; Pathiranage et al. 2024), management planning (Bunney et al. 2023), effect on the environment (Wang & Gu 2022; Munné et al. 2023), regulatory and operational scenarios (Cagno et al. 2022), relation to a circular economy (Urrea Vivas et al. 2023), demonstrating safety (Ness et al. 2023), monitoring (Jiang et al. 2022), and legal issues (Santos et al. 2022). Treatment processes such as nanocomposite ultrafiltration (Ayyaru & Ahn 2023), bromate formation (Babcock et al. 2023), biomanipulation (Du et al. 2023), electrochemical advanced oxidation (Forés et al. 2023), and identified bacterial consortium in slow-sand filters (Ibrahim et al. 2020) are also of prime interest.
This study focuses on the hydraulics, costs, and greenhouse gas emissions to directly connect a city's wastewater treatment plant (WWTP) effluent to the city's water treatment plant (WTP) influent, compared to the do-nothing case of continuing to use the conventional method of using water from nature and not recycling it. This analysis does not explore any treatment process adjustments required or the impacts of this on the current WTP performance. This study is unique since it is the first to quantify the hydraulic and economic cost aspects of the transition to DPR.
METHODS
Hydraulic pipe and pump equations were used to calculate the diameters of the return pipe and pump size. These results could apply to any type of water flowing through a pipe, not only through reclaimed water, thereby making these results even more broadly applicable. Key assumptions made are as follows in Table 1.
Assumptions
Parameter . | Metric units . | English units . |
---|---|---|
Velocity | 1.83 m/s | 6.0 ft/s |
Water specific gravity (γ) | 1,000 kg/m3 | 62.4 pounds per foot cubed |
Viscosity ![]() | 0.001001607 kg*s/m2 | 0.000020919 lbf*s/ft2 |
Pipe material | Galvanized iron | |
Pipe roughness, ε | 0.00015 m | 0.0005 ft |
WWTP effluent flow rate recycle | 5–100% | |
Distance (L) | 4,828 m | 15,840 ft |
Flow type | Completely turbulent | |
Elevation head | 3–15 m | 10–50 ft |
Parameter . | Metric units . | English units . |
---|---|---|
Velocity | 1.83 m/s | 6.0 ft/s |
Water specific gravity (γ) | 1,000 kg/m3 | 62.4 pounds per foot cubed |
Viscosity ![]() | 0.001001607 kg*s/m2 | 0.000020919 lbf*s/ft2 |
Pipe material | Galvanized iron | |
Pipe roughness, ε | 0.00015 m | 0.0005 ft |
WWTP effluent flow rate recycle | 5–100% | |
Distance (L) | 4,828 m | 15,840 ft |
Flow type | Completely turbulent | |
Elevation head | 3–15 m | 10–50 ft |
The pipe diameter, D, was calculated from the continuity equation assuming the water velocity, V = 6 ft/s, which is midway between the lower limit to avoid sedimentation of 0.61 m/s (2 ft/s) and the upper limit to avoid particle scouring of the pipe walls of 3.05 m/s (10 ft/s). After this value of D was calculated, then the pipe diameter was rounded up to the next commercially available size. Independent variables used were WWTP effluent flow rate and elevation head between the WWTP and WTP.
Costs of installing a direct reuse pipe and pump from the WWTPP effluent to the WTP influent were estimated, compared to doing nothing. Based on the pipe diameter, pipe length, and pump horsepower, values of the cost of installation were determined from current equipment and construction costs (Table 2).
Cost sources
Ductile iron pipe | Fairfax (2024a), Fairfax (2024b), CCWA (2024) Michigan (2024)) |
Pump | Inverter (2024a), Inverter (2024b), Absolute (2024) Dultmeier (2024) |
Excavation | Iowa (2024) |
Maintenance |
Ductile iron pipe | Fairfax (2024a), Fairfax (2024b), CCWA (2024) Michigan (2024)) |
Pump | Inverter (2024a), Inverter (2024b), Absolute (2024) Dultmeier (2024) |
Excavation | Iowa (2024) |
Maintenance |
To analyze the effect of increasing the percentage of WWTP effluent percentage to be recycled on the cost, the recycle flow was altered from 5 to 100% of the WWTP flow. Similarly, the elevation head (H) was varied from 10 to 50 feet to show the impact the variable would have on overall cost.
The overall maintenance cost per year was estimated to be $760–$4,000 per year (Xylem 2015). These estimates are based on both planned and unplanned maintenance and are hard to estimate. They depend on how many clogging events occur, which depends on the water quality in the influent. Influent quality can vary considerably. These costs were increased by 2% each year due to inflation.
The carbon emission costs were calculated using 0.39 kg (0.86 lb) of CO2 per kWh of electricity usage (EIA 2024).
RESULTS AND DISCUSSION
Total friction headloss, hL (ft), for a given flow rate and elevation head. (1 ft = 0.305 m; 1 ft3/s = 0.305 m3/s).
Total friction headloss, hL (ft), for a given flow rate and elevation head. (1 ft = 0.305 m; 1 ft3/s = 0.305 m3/s).
Figure 2 shows that as the percentage of the WWTP stream set to reuse increases, the head loss decreases. This can be attributed to the pipe diameter. As the elevation head increases, the total head loss increases; this is due to the elevation head being added to the head loss through the pipe. Even if the head loss in the pipe is zero, the elevation head will be attributed to the overall head loss. This is shown within the total head loss equation discussed in the Methods section above. Since V is kept near 1.83 m/s (6 ft/s), as Q increases, then D also increases, thereby resulting in a lower frictional headloss.
Change in all components of LCC as flow increases for the case when head, H = 50 ft (1 ft3/s = 0.305 m3/s).
Change in all components of LCC as flow increases for the case when head, H = 50 ft (1 ft3/s = 0.305 m3/s).
Normalized change in installation costs as normalized recycle rate increases, showing a big increase when a larger pump was required at Q/Qmax = 0.6 and that there is little variation with H values (Q is flow rate and H is head; 1 ft3/s = 0.305 m3/s).
Normalized change in installation costs as normalized recycle rate increases, showing a big increase when a larger pump was required at Q/Qmax = 0.6 and that there is little variation with H values (Q is flow rate and H is head; 1 ft3/s = 0.305 m3/s).
LCC for a given percent of WWTP recycle flow and head (1 ft = 0.305 m).
The flux in LCC is caused by the pumping requirements and pipe diameters. As the flow rate increases, the equipment costs increase, resulting in a higher LCC. For the conditions entered in this example, the cheapest course of action is to recycle the lowest percentage of flow at an elevation head of 3.05 m (10 ft) or 6.10 m (20 ft). The most expensive method would be 100% of the flow and an elevation head of 12.19 m (40 ft) or 15.24 m (50 ft).
Carbon emissions for a given percent of WWTP recycle flow and head (1 ft = 0.305 m; 1 lb = 0.45 kg).
Carbon emissions for a given percent of WWTP recycle flow and head (1 ft = 0.305 m; 1 lb = 0.45 kg).
A sensitivity analysis was performed on the variables of pipe length, L, and pipe roughness, ε. The results show that a 10% change in L resulted in a 21% change in LCC resulted in a 10% change in ε resulted in a 28% change in LCC, both at H = 15.24 m (50 ft) and Q = 0.28 m3/s (10 ft3/s). These are both significant, thereby showing that their values need to be carefully considered when designing direct reuse.
This study does not include additional treatment costs for bringing the WWTP effluent quality to the drinking water supply requirements or its operation and maintenance costs. These should be added as well. These costs are dependent on the WWTP effluent quality, which could vary widely depending on the influent quality and the treatment processes employed.
CONCLUSIONS
The LCC of direct potable reuse recycle piping varies depending on the flow rate to be recycled. If drought conditions are persistent, then more water must be recycled due to the scarcity of water sources. Cost and risk must be weighed in this case to make the best decision. There is an optimal value of the recycle rate that results from the use of a smaller pump since it costs less and uses less electricity. For the cases studied here, that was about 60% recycled. No optimal value of recycling exists to minimize LCCs since the slope of the curve is monotonically and linearly proportional. Carbon emissions increase linearly with the recycle rate and head but have a drop at 70% recycling due to the use of a smaller pump.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.