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Drip irrigation systems are an efficient and proven technology to recycle and dispose of wastewater. A drip irrigation system receives high quality wastewater from an advanced onsite wastewater treatment facility and dispenses it through a shallow network of drip tubes and emitters. The system is designed to disperse the wastewater into the soil under unsaturated conditions by pressure distribution and timed dosing. The system reduces the downward percolation of the wastewater by enhancing evapotranspiration to the atmosphere.
Drip irrigation can easily be installed on sites with large undulating slopes. It can be installed without disturbing the site as much as trench or bed construction does.
A properly engineered and managed subsurface drip irrigation system using reclaimed water offers many advantages over conventional disposal methods it minimizes health risks associated with exposure to reclaimed water by distributing the water below grade. It directly applies the water to plant root-zones and achieves a more manageable balanced water distribution throughout a relatively shallow soil profile.
Drip irrigation systems are considered when high groundwater, shallow soils slowly permeable soils or highly permeable soils are present at the site or when water conservation is needed.
In the past, 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.
Drip irrigation is the slow and precise application of water, in this case treated wastewater, into a plants root-zone. Drip systems for subsurface wastewater disposal typically consists of flexible polyethylene dripline with large turbulent flow emitters regularly spaced. The line is buried from 6 inches to 12 inches below grade. Wastewater is pumped through the drip lines under pressure, but discharges slowly from a series of evenly spaced drip emitters. They are fitted with turbulent flow and sometimes pressure-compensating emitter devices.
Systems are engineered to maintain a relatively consistent pressure inside the drip lines, usually about 20 psi. The pressure-compensating emitters allow drip irrigation lines to be installed at different elevations at a site while maintaining a uniform flow.
Because subsurface drip irrigation systems release the wastewater below ground and directly into the plant roots, they irrigate very efficiently and there is little opportunity for the wastewater to come in contact with humans or animals.
By discharging the wastewater into the root zones of plants, the water is delivered to the most biologically active soil horizon, which enhances treatment and minimizes the possibility of groundwater contamination. The constant moisture in the root zone also increases the availability of nutrients to plants, reducing the delivery of nitrogen to groundwater.
The wastewater must be pretreated prior to drip irrigation to protect public health and the environment and to prevent systems from clogging. Settleable and floatable solids are removed by primary treatment (septic tank or pretreatment tank). Secondary treatment is also required to remove BOD and more solids. In most systems, the treated effluent collects in a pump chamber where it is stored until a predetermined dosing volume or time is reached. All drip systems are equipped with a filtration system before the distribution system to remove small-suspended solid materials that may clog the drip lines or emitters. The size of materials that may pass the filter should be four to six times smaller than the emitter orifice size. The system may include disinfection.
Placement of the drip irrigation lines and emitters are placed at twelve inches below grade, they can be placed as shallow as six inches if there is not a concern of freezing and the effluent was disinfected. Regulations typically require that drip systems be installed an minimum distances from structures, property lines, wells, water resources and groundwater.
The distribution system includes a discharge manifold, a return manifold, drip lines and emitters. A pressure regulator assures that excessive operating pressure or surges do not damage the drip irrigation system. If the system is built on a steep site, several pressure regulators may need to be used to assure equal pressures at all drip emitters. Air vacuum release valves are installed to prevent water and soil being drawn back into the emitters. The drip lines and emitters are spaced at 12 to 24 inches apart, giving each emitter one to four square feet to disperse water. As the drip lines are placed slight variations allow preservation of existing trees and shrubs. The emitters discharge from 0.5 to 2.0 gallons per hour. The system must be designed to flush the irrigation components with wastewater. Piping and valves allow the wastewater to be pumped in a line flushing the system and discharging back to the treatment system. The system can be designed and constructed to automatically back flush the system after a specific number of doses or the system can be built to manually back flush the system when necessary.
The EPA approves the use of chemical trifluralin to prevent root intrusion into the emitters. Some manufactures of drip irrigation systems incorporate the root intrusion chemical barrier directly into the tubing material itself.
Drip Irrigation Field
Root intrusion into the drip lines and internal clogging from the buildup of sediment, suspended solids, algae and bacterial slime have been diminished greatly by better pretreatment, filtration disinfection, and new tubing and emitter designs. Most systems allow for flushing at scouring velocity to remove slime and sediment buildup. If scale buildup develops on emitters and acid treatment may be required.
When emitters clog it affects the pressure inside the drip lines and wastewater distribution in the field. It is very difficult to identify and service buried emitters that clog. Saturation of the soil around emitters can eventually lead to the formation of a biological clogging mat, which can cause system failure.
Filters on drip systems need to be checked and periodically back flushed or cleaned. Back flushing reverses the water flow through the lines and filters to release trapped sediments. As mentioned above systems can be set up to backwash automatically at preset intervals, or operators can do it manually as needed. The wastewater flow needs to be checked periodically to determine if any emitters are plugging.
The impregnated herbicides have a life-span of approximately 15 years. When the herbicides no longer control the growth of algae or stop root penetration the system will have to be removed and replaced.
Some Advantages and Disadvantages are listed below.
Advantages:
· Can be used in adverse terrain conditions.
· Is associated with water reuse because emitters are typically placed in the root-zone.
· Wastewater is distributed more evenly than spray systems or trenches.
· Ease in construction.
Disadvantages:
· Effluent to drip disposal must be filtered to remove all solids to avoid clogging.
· No storage in trench.
· Emitters can potentially clog, affecting the uniformity of application.
· It is difficult to monitor and correct potential emitter clogging.
· System must be back flushed routinely to avoid clogging of the emitters.
· May be sensitive to extremely cold temperatures.
· The impregnated herbicides have a life span of approximately 12 to15 years.