Urban Water Reuse Applications
Urban water reuse systems provide reclaimed water for various nonpotable purposes including:
- irrigation of public parks and recreation centers, athletic fields, school yards and playing fields, highway medians and shoulders, and landscaped areas surrounding public buildings and facilities
- irrigation of landscaped areas surrounding single-family and multi-family residences, general wash down, as well as other maintenance activities
- irrigation of landscaped areas surrounding commercial, office and industrial developments
- irrigation of golf courses
- commercial uses such as vehicle washing facilities, laundry facilities, window washing and mixing water for pesticides, herbicides and liquid fertilizers
- ornamental landscape uses and decorative water features, such as fountains, reflecting pools and waterfalls
- dust control and concrete production for construction projects
- fire protection through reclaimed water fire hydrants
- toilet and urinal flushing in commercial and industrial buildings
Urban reuse can include systems serving large users. Examples include parks, playgrounds, athletic fields, highway medians, golf courses and recreational facilities. In addition, reuse systems can supply major water-using industries or industrial complexes as well as a combination of residential, industrial and commercial properties through dual distribution systems. A two-year field demonstration/ research garden compared the impacts of irrigation with reclaimed versus potable water for landscape plants, soils and irrigation components. The comparison showed few significant differences; however, landscape plants grew faster with reclaimed water. But such results are not a given. Elevated chlorides in the reclaimed water provided by the City of St. Petersburg, Fla., have limited the foliage that can be irrigated. In dual distribution systems, the reclaimed water is delivered to customers through a parallel network of distribution mains separate from the community's potable water distribution system. The reclaimed water distribution system becomes a third water utility, in addition to wastewater and potable water.
Reclaimed water systems are operated, maintained and managed in a manner similar to the potable water system.
In assessing the reuse needs of an urban system, demands for uses other than irrigation also must be considered. These demands are likely to include industrial, commercial and recreational uses. Demands for industrial users, as well as commercial users, such as car washes, can be estimated from water use or billing records. Demands for recreational impoundments can be estimated by determining the volume of water required to maintain a desired water elevation in the impoundment. For those systems using reclaimed water for toilet flushing as part of their urban reuse system, water use records can again be used to estimate demand. Toilet flushing can account for up to 45 percent of indoor residential water demand. When the Irvine Ranch Water District began using reclaimed water for toilet flushing in high-rise office buildings, potable water demands in these buildings decreased by as much as 75 percent due to the reclaimed water use.
Reliability and Health ProtectionIn the design of an urban reclaimed water distribution system, the most important considerations are the reliability of service and protection of public health. The following safeguards must be considered during the design of any dual distribution system:
- assurance that the reclaimed water delivered to the customer meets the water quality requirements for the intended uses
- prevention of improper operation of the system
- prevention of cross-connections with potable water lines
- prevention of improper use of nonpotable water
To avoid cross connections, all aboveground appurtenances and equipment associated with reclaimed water systems must be clearly marked. Additional designations include using the international symbol for “Do Not Drink” on all materials, both surface and subsurface, to minimize potential cross connections.
Urban water reuse systems have two major components - water reclamation facilities and reclaimed water distribution system, including storage and pumping facilities.
Reclamation FacilitiesWater reclamation facilities must provide the required treatment to meet appropriate water quality standards for the intended use. In addition to secondary treatment, filtration and disinfection generally are required for reuse in an urban setting. Because urban reuse usually involves irrigation of properties with unrestricted public access or other types of reuse where human exposure to the reclaimed water is likely, reclaimed water must be of a higher quality than may be necessary for other reuse applications. In cases where a single large customer needs a higher quality reclaimed water, the customer may have to provide additional treatment onsite, as is commonly done with potable water.
Distribution SystemReclaimed water operational storage and high-service pumping facilities usually are located onsite at the water reclamation facility. However, in some cases, particularly for large cities, operational storage facilities may be located at appropriate locations in the system and/or near the reuse sites. When located near the pumping facilities, ground or elevated tanks may be used; when located within the system, operational storage generally is elevated.
Sufficient storage to accommodate diurnal flow variation is essential to the operation of a reclaimed water system. The volume of storage required can be determined from the daily reclaimed water demand and supply curves. Reclaimed water normally is produced 24 hours per day in accordance with the diurnal flow at the water reclamation plant, and may flow to ground storage to be pumped into the system or into a clear well for high-lift pumping to elevated storage facilities. In order to maintain suitable water quality, covered storage is preferred to preclude biological growth and maintain chlorine residual.
Since variations in the demand for reclaimed water occur seasonally, large volumes of seasonal storage may be needed if all available reclaimed water is to be used, although this may not be economically practical. The selected location of a seasonal storage facility also will have an effect on the design of the distribution system. In areas where surface storage may be limited due to space limitations, aquifer storage and recovery (ASR) could prove to be a viable enhancement to the system.
The design of an urban distribution system is similar in many respects to a municipal potable water distribution system. Materials of equal quality for construction are recommended. System integrity should be assured; however, the reliability of the system need not be as stringent as a potable water system unless reclaimed water is being used as the only source of fire protection.
No special measures are required to pump, deliver and use the water. No modifications are required because reclaimed water is being used, with the exception that equipment and materials must be clearly identified. For service lines in urban settings, different materials may be desirable for more certain identification.
The design of distribution facilities is based on topographical conditions as well as reclaimed water demand requirements. If topography has wide variations, multilevel systems may have to be used. Distribution mains must be sized to provide the peak hourly demands at a pressure adequate for the user being served. Pressure requirements for a dual distribution system vary depending on the type of user being served. Pressures for irrigation systems can be as low as 10 psi if additional booster pumps are provided at the point of delivery, and maximum pressures can be as high as 100 psi to 150 psi. The peak hourly rate of use, which is a critical consideration in sizing the delivery pumps and distribution mains, may best be determined by observing and studying local urban practices and considering time of day and rates of use by large users to be served by the system.
For reclaimed water systems that include fire protection as part of their service, fire flow plus the maximum daily demand should be considered when sizing the distribution system. This scenario is not as critical in sizing the delivery pumps because it likely will result in less pumping capacity, but is critical in sizing the distribution mains because fire flow could be required at any point in the system, resulting in high localized flows.
Generally, there will be “high-pressure” and “low-pressure” users on an urban reuse system. The high-pressure users receive water directly from the system at pressures suitable for the particular type of reuse. Examples include residential and landscape irrigation, industrial processes and cooling water, car washes, fire protection and toilet flushing in commercial and industrial buildings. The low-pressure users receive reclaimed water into an onsite storage pond to be repumped into their reuse system. Typical low-pressure users are golf courses, parks and condominium developments that use reclaimed water for irrigation. Other low-pressure uses include the delivery of reclaimed water to landscape or recreational impoundments, or industrial or cooling tower sites that have onsite tanks for blending and/or storing water.
Typically, urban dual distribution systems operate at a minimum pressure of 50 psi, which will satisfy the pressure requirements for irrigation of larger landscaped areas such as multi-family complexes, offices and commercial and industrial parks. A minimum pressure of 50 psi also should satisfy the requirements of car washes, toilet flushing, construction dust control and some industrial uses. Based on requirements of typical residential irrigation equipment, a minimum delivery pressure of 30 psi is used for the satisfactory operation of in-ground residential irrigation systems.
For users who operate at higher pressures than other users on the system, additional onsite pumping will be required to satisfy the pressure requirements. For example, golf course irrigation systems typically operate at higher pressures (100-200 psi), and if directly connected to the reclaimed water system, likely will require a booster pump station. Repumping may be required in high-rise office buildings using reclaimed water for toilet flushing. Additionally, some industrial users may operate at higher pressures. The design of a reuse transmission system usually is accomplished through the use of computer modeling, with portions of each of the sub-area distribution systems representing demand nodes in the model. The demand of each node is determined from the irrigable acreage tributary to the node, the irrigation rate and the daily irrigation time period. Additional demands for uses other than irrigation, such as fire flow protection, toilet flushing and industrial uses, also must be added to the appropriate node.
Maintaining PressureThe two most common methods of maintaining system pressure under widely varying flow rates:
1. constant-speed supply pumps and system elevated storage tanks, which maintain essentially consistent system pressures
2. constant-pressure, variable-speed, high-service supply pumps, which maintain a constant system pressure while meeting the varying demand for reclaimed water by varying the pump speed
While each of these systems has advantages and disadvantages, either system will perform well and remains a matter of local choice. The system should be designed with the flexibility to institute some form of usage control when necessary and provide for the potential resulting increase in the peak hourly demand. One such form of usage control would be to vary the days per week that schools, parks, golf courses and residential areas are irrigated.
In addition, large users, such as golf courses, will have a major impact on the shape of the re-claimed water daily demand curve, and, hence, on the peak hourly demand, depending upon how the water is delivered to them. The reclaimed water daily demand curve may be “flattened” and the peak hourly demand may be reduced if the reclaimed water is discharged to golf course ponds over a 24-hour period or during the daytime hours when de-mand for residential landscape irrigation is low.
These methods of operation can reduce peak demands, thereby reducing storage requirements, pumping capacities and pipe diameters. This, in turn, can reduce construction costs.
Fire ProtectionReclaimed water may be used for fire protection, but this application requires additional design efforts. Urban potable water distribution systems typically are sized based on fire flow requirements. In residential areas, this can result in 6-inch diameter pipes to support fire demands where 2-inch diameter pipes may be sufficient to meet potable needs. Fire flow requirements also increase the volume of water required to be in storage at any given time. While this results in a very robust distribution system, the increased pipe size and storage required for fire flows results in increased residence time within the distribution system, and a corresponding potential reduction in reclaimed water quality.
In some cases, municipalities may be faced with replacing existing potable water distribution systems, because the pipe material is contributing to water quality problems. In such instances, consideration could be given to converting the existing network into a nonpotable distribution system capable of providing fire protection and installing a new, smaller network to handle potable demands. Such an approach would require a comprehensive cross-connection control process to ensure all connections between the potable and nonpotable system were severed. Color-coding of belowground piping also poses a challenge. More often, the primary means of fire protection is the potable water system, with reclaimed water systems providing an additional source of water for fire flows. In some cases, site-specific investigations may determine that reclaimed water is the most cost-effective means of providing fire protection.