Irrigating with reclaimed water through permanent subsurface drip irrigation systems
In Hawaii, reclaimed water has been used in agricultural irrigation and the irrigation of golf courses and other large landscaped areas. However, the Hawaii Department of Health's new 'Guidelines for the Treatment and Use of Reclaimed Water' published in November 1993, limits uses of reclaimed water through overhead sprinkler irrigation systems. The subsurface drip irrigation concept provides a unique opportunity to effectively address the issues of reclaimed water management and dispersal while providing for irrigation needs. This paper discusses the design, implementation and management or two permanent subsurface drip irrigation projects in Hawaii using reclaimed water ill accordance with the state's new reuse guidelines.
KEYWORDS. Reclaimed water, Wastewater, Reuse, dispersal, Subsurface drip irrigation.
The continuing effort to conserve and preserve potable water resources emphasizes using alternative water sources for irrigation. Potable water is less available for such non-potable activities as irrigation and is being replaced with alternative sources such as non potable fresh water, brackish sources, or treated effluent. Agriculture, golf courses. highways, parks, playgrounds and cemeteries are among the heaviest users of water due to the large areas requiring irrigation.
Reclaimed water for irrigation has been the least desirable alternative primarily due to public perception of using treated effluent. Reclaimed water has been a last resort onlv when other water sources were unavailable and/or unsuitable. Furthermore, when primary options for effluent dispersal preclude ocean discharge, injection wells, or other dispersal methods, then irrigation as a primary dispersal component becomes very attractive.
Using reclaimed water for irrigation requires closer examination of current irrigation design practices and current irrigation technologies for landscape and agricultural applications. Where conventional irrigation systems are designed and built to complement the skill level of operators, reuse irrigation systems must be designed, built and managed in accordance with reuse regulations which require higher operating and management skills
HAWAII REUSE GUIDELINES
In Hawaii, reclaimed water is viewed as a great potential resource for irrigation. As such, irrigation is construed to be the primary component for effluent reuse and is the focus of Hawaii's reuse guidelines. The Hawaii State Department of Health released the Guidelmes for the Treatment and Use of Reclaimed Water on November 22, 1993. These reuse guidelines promote water reuse and supplement, clarify and provide further guidance in promulgating existing rules which refer to wastewater effluent dispersal systems. While the guidelines are not rules, they set the framework upon which administrative rules will be implemented in the future. The guidelines provide more specifics in evaluating reuse projects to safeguard public health and preclude environmental degradation of aquifers and/or surface waters while seeking to maximize the potential of this relatively new resource.
However, the guidelines limit the use of reclaimed water through sprinkler and other above ground irrigation Systems. One promising alternative is subsurface drip irrigation. Subsurface drip irrigation provides a unique alternative that speaks directly to the issues of safe wastewater effluent dispersal, increasing landscape and agricultural irrigation requirements and conserving and extending our potable water resources.
Specific Uses For Reclaimed Water In Non–Agricultural Irrigation Applications
Hawaii reuse guidelines define reclaimed water as water which, as a result of domestic wastewater treatment, is suitable for a direct beneficial use or a controlled use that would not otherwise occur. The guidelines classify and identify reclaimed water as R–1 (Significant reduction in viral and bacterial pathogens), R-2 (Disinfected Secondary–4 Reclaimed Water), and R–3 (Undisinfected Secondary Reclaimed Water) with R–1 water being of the highest quality.
R–1 water may be used in any form of irrigation served by fixed irrigation Systems supplied by buried piping for turf and landscape irrigation of golf courses, parks, elementary school yards, athletic fields, road sides and medians, and residential property where managed by an irrigation supervisor. Spray irrigation of R–1 water does not have restricted times of operation. In Hawaii, there are no wastewater treatment facilities currently classified as producing R-1 water.
R–2 water may be used for subsurface irrigation of landscape and turf on golf courses, parks, elementary school yards, and residential property where managed by an irrigation supervisor. R–2 water may be used in any form of irrigation for landscape on cemeteries and around freeways. Spray irrigation of R-2 water is limited such that the outer periphery of the irrigated area must not be within 500 feet of a residential property or a place where public exposure could be similar to that at a park, elementary school yard or athletic field. Overspray or runoff of R–2 water must be confined to the approved use area and is not permitted to come in contact with the public or equipment, facilities and areas with frequent human contact. Spray irrigation of R–2 water shall only occur when the area is closed to the public and end at least one hour before the area is open to the public.
R–3 water is not permitted for use in the irrigation of landscape and turf.
SUBSURFACE DRIP IRRIGATION
Drip irrigation has been around for over 30 years. In Hawaii, over 40,500 hectares of sugar cane and pineapple are subsurface drip irrigated of which several thousand hectares are subsurface drip irrigated with reclaimed water. A high percentage of the drip irrigation development work in Hawaii was done in sugar cane through the Hawaiian Sugar Planters' Association and the University of Hawaii.
In recent years, subsurface drip irrigation systems have gained greater acceptance in landscape and commercial turf applications. Hawaii’s new reuse guidelines have advanced subsurface drip irrigation to the forefront as a significant technology that transcends the problems and perceptions associated with wastewater dispersal and reuse.
Subsurface Drip Irrigation Advantages
Conventional effluent dispersal in Hawaii generally has been via ocean outfalls, injection wells, sprinkler irrigation, surface drip irrigation, and various other land application methods. A properly engineered and managed subsurface drip irrigation System using reclaimed water offers many advantages over conventional dispersal methods:
1. Minimizes health risks associated with exposure to reclaimed water
2. Minimizes liability exposure associated with overspray and aerosol drift from conventional sprinkler Systems especially near residential properties
3. Psychological and politically a very acceptable method of dispersal
4. Eliminates odor, ponding, and runoff problems
5. More manageable, balanced water distribution throughout a relatively shallow soil profile by applying reclaimed water directly into plant root zones, thereby minimizing potential ground water degradation and/or near seashore pollution
6. Unrestricted irrigation scheduling
7. Low pressure requirements, 138 to 240 kPa average operating range
8. Extends and conserves potable water resources
9. Virtual elimination of vandalism and damage to subsurface dnp System compared to conventional sprinkler Systems
10. Cost versus benefit of a subsurface drip System compares very favorably to other methods of dispersal
Emitter Plugging And Root Intrusion Prevention
The primary challenges of utilizing subsurface drip irrigation for long term applications have been the potential for internal plugging of the emitter devices and external root intrusion into the drip tubing. Internal plugging, caused primarily by sediment, algae, and bacterial slime has been virtually eliminated by proper filtration, disinfection of the drip irrigation System, and emitter designs with larger discharge orifices and flow paths.
Currently, there appears to be two accepted means of preventing root intrusion – physical and chemical. One type of pressure compensating emitter device with a physical barrier design appeared to have no initial root intrusion problems. However, recent reported evidence of root intrusion into the physical barrier emitter device has prompted further study and evaluation. The chemical barrier emitter device incorporates Treflan into the emitter during the manufacturing process. There has been no evidence of root intrusion into the Treflan impregnated emitter.
In Hawaii, the primary components of a reclaimed water subsurface drip irrigation System are supply lines, filtration station and automation controls, booster pumps, flow meters and monitoring sensors, chlorination system, main lines, submains, and polyethylene lateral drip tubing with emitters.
Supply lines transport reclaimed water from the wastewater treatment facility to the irrigation filter station. The filter station, the heart of the System, consists of filters, flow meters and sensors, and automatic valves and controls to effectively manage and monitor the subsurface drip irrigation System. Chlorine, the primary disinfectant used to preclude biofouling of the drip tube emitters, is injected at the filter station. The main lines transport filtered and treated reclaimed water from the filter station to various irrigation blocks controlled by automatic valves. Submains within these irrigation blocks further distribute water to individual drip tube laterals. The drip tube laterals uniformly distribute and emit reclaimed water into the soil matrix for distribution within the plant root zones.
LAIE WATER RECLAMATION FACILITY IRRIGATION DEMONSTRATION PROJECT – LAIE, OAHU, HAWAII
The Laie Demonstration Project (LDP) is a cooperative venture between Hawaii Reserves, Inc. (HRI) and Brigham Young University–Hawaii (BYUH) that seeks to provide substantive data for use in the planning, design and implementation of a larger scale reuse project that may use up to 3.4 million liters per day of reclaimed water. Reclaimed water for the LDP is produced by the Laie community which is currently served by the Laie Water Reclamation Facility (Laie WRF). The Laie WRF is privately owned and operated by HRI. Reclaimed water generated by the Laie WRF is filtered and classified as R–2 water. Only domestic wastewater from residences, commercial enterprises and institutions enters the Laie collection System. No industrial waste is treated by the Laie WRF.
Reclaimed water is being applied to the approximate 3 hectare project site on land owned by BYUH. The project site is divided into eight separate subsurface drip irrigated blocks consisting of three banana plots, two papaya plots, two bermuda turf grass plots, and one giant bermuda pasture grass plot. The banana and papaya crops were selected on the basis of marketability, the bermuda grass turf for use in landscape applications and giant bermuda grass as a cattle forage crop.
Approximately 120,000 liters per day is expected to be required for irrigating the project site during the peak utilization month of August. The design application rate for the project area is based primarily on pan evaporation data conservatively estimated to be 1650 mm per year and crop utilization data for the selected crop types. Crop factors of 1.00 times pan evaporation is used for the banana block, 0.90 for the papaya block and 1.10 for the turf and pasture blocks. One turf plot is currently irrigated at a rate in excess of four times pan evaporation. This high rate experiment is to determine the upper limits of the soil's intake rate of reclaimed water through drip irrigation in turf grass without ponding. A rain gauge and evaporation pan have been established at Laie to record local datas which will be used to factor new irrigation application rates.
Water demand for the selected crops is equal to or slightly greater than pan evaporation, based on sources from the University of Hawaii Water Resources Research Center and the College of Tropical Agriculture of the University of Hawaii. The design application rates vary seasonally based on the variation in pan evaporation. The application rate varies from 0 to 122 mm per month for banana and papaya, and 0 to 140 mm per month for bermuda turfgrass and giant bermuda pasture grass.
The automated LDP reuse system is a flow-based managed System controlled by a central irrigation computer. A flow meter, soil moisture sensors and a rain shutoff sensor provide additional input into the computer.
The subsurface polyethylene drip tubing has a 40 mil wall thickness with Treflan® impregnated emitters rated at 2.46 liters per hour at 103 kPa spaced 61 cm in the tubing. The tubing is buried 10 to 15 cm deep and spaced 38 cm from each side of the banana rows, 61 cm to one side of the papaya rows, 61 cm on center in the turf plots, and 122 cm on center in the pasture plot. Drip tubing was installed via subsurface injection. Suction lysimeters are incorporated throughout the LDP for shallow horizon monitoring of any nutrient leaching. Monitoring wells are utilized for groundwater testing and observation.
Another area to be incorporated in the ongoing HRI subsurface drip irrigation reclamation project is the BYUH 1.8 hectare rugby field and adjacent baseball field. This area will utilize the same reclaimed water source as the existing Laie Demonstration Project. Ultimately, a total area of approximately 22 hectares will be subsurface drip irrigated with reclaimed water from the Laie WRF.
KUKUIULA DEVELOPMENT, PHASE 1– KOLOA, KAUAI, HAWAII
The Kukuiula Development is a planned 405 hectare residential community and golf course project on the island of Kauai. Phase 1 addresses turf and landscape irrigation needs by reclaiming wastewater through a subsurface drip irrigation system. Initially, 21.9 hectares are being subsurface drip irrigated. Ultimately this project will accommodate 3.8 million liters per day of secondary–disinfected effluent (R–2 Water) for reuse from the new mechanical wastewater treatment facility (WWTF).
Reuse irrigation is designated as the primary dispersal component for this project. As required by Hawaii's reuse guidelines, a 100 per cent backup dispersal System is provided for by two onsite injection wells when the irrigation system is not in operation or when effluent quantities exceed irrigation requirements.
Two fully computerized subsurface drip irrigation Systems accommodate all turf and landscape water requirements. One irrigation system blends R–2 water from the WWTF with non–potable reservoir fresh water. The second irrigation System uses only non-potable fresh water from the reservoir source. Reclaimed water from the WWTF is the primary source for irrigating landscaped areas around the WWTF and adjacent roads. When reclaimed water flows are insufficient to meet irrigation requirements, as expected during initial construction of the Phase 1 development, portions of the irrigation system are supplemented by the non–potable fresh water reservoir source.
The reuse System incorporates computerized operation and management, storage/blending tanks, a multi–media filtration System, chlorination system, booster pumps, flow monitoring, and various safeties and controls. A weather station provides daily evapotranspiration data for determining application rates and updating irrigation scheduling. A phone modem hook–up allows for additional daily monitoring of the reuse irrigation System from the island of Oahu. This System is a flow–based managed system in that water requirements of the various irrigation blocks are calculated from the weather station automatically and metered water is provided accordingly. The System is not operated on the conventional irrigation time clock basis.
The subsurface polyethylene drip tubing has a 40 mil wall thickness with Treflan¨ impregnated emitters rated at 2.46 liters per hour at 103 kPa spaced 61 cm in the tubing. The tubing is buried 10 to 15 cm deep and spaced 61 cm on center.
Mass grading of the project site included blasting and fracturing of the underlying basaltic bedrock. These site conditions precluded conventional injection methods of the more than 358,000 meters of drip tubing. The drip tubing was laid on, and secured to, an erosion control netting base mat which prevented tube movement during the soil importation, spreading and covering operation.
The WWTF and reclamation System are each managed by separate outside contractors who are responsible for operating, maintaining and managing the Systems in accordance with Chapter 62 of Title 11 Hawaii Administrative Rules and the Reuse Guidelines. Maintenance of the reuse System has been very minimal with no evidence of emitter plugging or flow reduction.
We face the immediate challenges of safely and properly disposing wastewater effluent and the challenges of managing and extending our limited potable water resources. Reclaimed water is a growing potential resource that can address both the concerns of safe effluent dispersal and our ongoing irrigation water needs. A well engineered subsurface drip irrigation System provides a very viable and attractive alternative to conventional effluent dispersal methods as well as provide an alternative water source for various landscape and agricultural irrigation needs.