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 (about 6 to 8 inches below the surface) , through small diameter (about ½ inch) plastic tubing with flow controlling apertures called "emitters" spaced every one or two feet. Leachate is preferentially taken up and processed by the roots of grass or other non food chain plants growing at the surface over the drip irrigation field. Evapotranspiration (combined effects of evaporation from the soil surface and transpiration of moisture from the soil to the air through plant leaves) is a significant factor in removing effluent water from the drip irrigation field. . 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 shallow depth to groundwater.
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.
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" depth 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 gravel filled 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 tubing, and clogging of orifices in the tubing emitters. 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 the laterals, the soil may become briefly saturated around orifices.
Periodic back flushing of the pressure distribution tubing is necessary to clear out any solid material from the effluent deposited 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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See Benefits/ costs section
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See Benefits/ costs section
See Benefits/ costs section
See Benefits/ costs section
Approval usually comes from the state or local health department. NSF/ANSI 40 - 2004 Standard for Residential Wastewater Treatment Systems details requirements for approval of the system. The NSF/ANSI 40-2004 standard applies to on site wastewater recycling systems with capacities of up to 1,500 gallons per day, and leads to approval as a Class I or Class II plant. A Class I certification indicates performance to EPA Secondary Treatment Guidelines for three parameters: BOD, suspended solids and pH. Noise levels, odors, oily films and foaming are also measured. The Class II criteria require that not more than 10% of the effluent CBOD5 values exceed 60 mg/L and that TSS not exceed 100 mg/L..
Class I and Class II systems are required to include two years of manufacturer maintenance service and renewal options, and to have alarms to alert the homeowner of malfunctions.
If the local health department does not recognize the technology or will not allow adequate reduction or elimination of the drainage field, costs will increase. Local policy may require a drain field size that makes the total treatment system larger than if a conventional system was used.
A database including references to state by state sanitary code requirements is located at:
http://www.nesc.wvu.edu/nsfc/nsfc_regulations.htm
Innovative ISDS Evaluation including drip irrigation
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 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.
Not Applicable
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).
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