Drip Irrigation Leach Field Shallow, slow, pressure-dosed release of pretreated wastewater directly at or above the surface of the soil  This technology is a specific application of Pressurized Leach Field Dosing, which is fully described in another Technology Inventory article. From the dosing tank, filtered effluent is distributed close to the surface of the ground, through small plastic tubing with holes every one or two feet. Leachate is preferentially taken up and processed by the surface planting, and evapotranspiration is a significant factor. According to the National Small Flows Clearing House (NSFCH), which uses the term Low Pressure Pipe (LPP) for this system, it originated in North Carolina and Wisconsin as an alternative to conventional soil absorption systems to eliminate problems such as clogging, mechanical compaction of the soil, and anaerobic conditions due to continuous saturation or a high water table.
See the article "On-Site Sewage Disposal Systems - Overview" for an overview that will help relate this technology to other options and to the overall system. Drip "irrigation" is the shallow, slow, pressure-dosed release of pretreated wastewater directly at or above the surface of the soil. Common to all pressurized dosing systems is the uniform distribution of effluent and periodic dosing and resting cycles. Particular to drip irrigation or LPP systems is the use of small diameter piping with underground drip emitters placed about 6" under the surface. Effluent must be filtered and treated before distribution. Effluent is applied at a controlled rate in the plant root zone, which tends to minimize percolation and enhance evapotranspiration (the evaporation of water from soils, plants and surface waters). For a typical home, the pump (which might be rated at 30 GPM) will only operate for 10 to 15 minutes during each operating period. The level controls in the dosing chamber are set for a specific pumping sequence, depending on the design, which might be as seldom as once a day. The laterals are placed in narrow trenches that allow enough storage volume so that the depth of the wastewater does not exceed 2 or 3 inches of the total trench depth during each dosing cycle. Hydraulic loading rates may vary between 0.01 and 0.4 gallons per day per square foot. The primary concern is the intrusion of roots into the drip piping, and clogging of orifices in the tubing. This system should not be used if there is any chance of vehicles crossing the field. Additional pretreatment may be necessary, and filtration is essential. Because of limited storage capacity around their laterals, the soil may become saturated around orifices. Periodic backflushing is necessary to clear out any solid material in the piping. Regular monitoring and maintenance of the system is required; lack of maintenance is a sure precursor to failure. Like any pump-operated system, a reliable source of power is required, and an electrician may be needed to hook up the pump. Routine maintenance is necessary to ensure the proper functioning of these systems. Many system components supplied from local commercial sources are proprietary. 
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Code officials may not be familiar with the system. As with any innovative design, it may be allowed only as an experimental system, with extensive monitoring, until the jurisdiction is satisfied with testing results.
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The system must be designed by a registered professional engineer. Small vibratory plows or trenchers may be used to install drip emitter lines. The system should definitely be arranged so that it drains by gravity. All components except the piping should be protected from freezing. Because of the small size of the orifices, an effective effluent filter system is needed, of the reusable cartridge type. Some means, such as the regular injection of herbicide into the piping, must be provided to inhibit root growth into the orifices. A control panel and an elapsed time counter are essential accessories. The pump chamber (as well as the septic tank) should have easily accessible access risers with child-proof and slip-resistant lids.
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This system offers three primary benefits. First, it is a "green" technology, because the water, instead of disappearing into the ground, is usefully recycled to sustain necessary plant growth. Second, because the tubing is in shallow trenches and is flexible, the field can be woven around existing trees and shrubs, areas that could not support any other type of effluent disposal except a spray system (see On-Site Sewage Disposal Systems - Overview). Third, the system does not have to be level or even, although there are limits on the amount of slope allowed. Although reports indicate that the discharge of nitrates is reduced, Massachusetts has not rated the system as nitrogen-reducing in lieu of further data. According to NSFCH, In a 1989 study of LPP use among different counties in North Carolina, it cost an average of $2,600 to install an LPP system for a three-bedroom house. They also estimated the average installation cost of LPP systems ranging from $1,500 to $5,000, inversely related to the extent of its use within a county. The Texas Cooperative Extension quotes costs between $4,000 and $10,000, while the City of Austin describes a system costing $15,000 and requiring nearly $50 a month maintenance (size not specified). |
