Distributing treated wastewater evenly over a field is the purpose of the irrigation system. A variety of system types and components are available. A detailed design of the necessary pumps, pipes, valves, and emitters must be developed by an irrigation professional. Because irrigation systems are not commonplace in Ohio, finding the right professional may be a challenge.
The society for engineering in agricultural, food, and biological systems (ASAE) in St. Joseph, Michigan, maintains a list of its members that specialize in irrigation systems. The Irrigation Association offers training and certifications for irrigation designers, contractors, and water auditors and also provides a list of certified irrigation system designers. Companies that sell irrigation systems may also provide design services. See the references in this bulletin for the addresses and phone numbers of the ASAE and the Irrigation Association.
Irrigation systems fall into three categories; sprinkler, surface, and drip. Sprinkler systems are the most common. They work on slopes with up to 30 percent grade and are not limited by wastewater quality. All types of crops can be irrigated using sprinkler systems. Solid set sprinkler systems are most often used in wastewater reuse systems; center pivot, traveling gun, and traveling lateral systems also have applications.
Some limitations to the use of sprinkler systems are the purchase, placement costs, and field space for the equipment. Uncultivated tracks must be maintained for traveling systems. Field operations must maneuver around solid set systems. Another limitation of sprinkler systems is spray drift. Setbacks must be included in the field layout to minimize spray drift onto roads and dwellings.
Surface systems rely on surface grade and channels to help distribute the wastewater. Gated pipes discharge the wastewater at one end of a field and gently sloping furrows carry the wastewater throughout the field. Row crops and plant nurseries on level terrain are well suited for surface irrigation. Surface irrigation systems require less equipment than sprinkler systems and are not subject to spray drift problems. However, surface irrigation systems do not uniformly distribute the wastewater; the heaviest applications occur near the discharge points.
Drip irrigation systems use low-rate emitters to deliver wastewater slowly to the plant. Wastewater must be very low in solids, and disinfection may be required to reduce biofilms that can clog emitters. Drip systems can be used on any slope and are well suited to permanent planting, such as landscaping. The equipment and installation costs for drip systems may be high, but they do not create spray drift problems and, if buried, do not interfere with agricultural operations.
Some operations may wish to combine irrigation system types. Some of the wastewater may be reused for the landscaping around buildings through a drip irrigation system. Turf and field crop areas may be irrigated with sprinklers. The areas close to roads may rely on surface irrigation to minimize spray drift (Figure 5).
|
| Figure 5. Sample irrigation system plan. |
Irrigation systems are often designed to operate in zones across the field area. Zone operation has several advantages. Pumping capacity can be lower, which results in energy savings. Zone operation also allows wastewater and air to infiltrate the soil.
Control valves are needed throughout the system to accommodate field management and system maintenance. While it is tempting to consider the use of automatic valves, extensive use is not recommended. Operators need to attend to irrigation operations to monitor weather conditions, check for system clogging, and observe ponding and possible runoff. Automatic valves not only give the illusion that the system is completely automatic, they can make it difficult for the operator to make necessary adjustments. Consider valves that are manual on with an automatic shut-off. In this way the operator can check field conditions before irrigation. If automatic valves are used, they should be controlled from a central location and be easy to override with manual controls.