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Utilizing Drip Irrigation Technology
for Onsite Wastewater Treatment
By Thomas A. Sinclair, Dr. Bob Rubin, & Richard J. Otis P.E.
Onsite wastewater treatment systems have traditionally been viewed as the septic tank/soil absorption system. These traditional systems have been utilized with reasonable effectiveness where site and soil conditions permit, but where conditions are unfavorable, these systems have a high failure rate. Consequently, advanced onsite wastewater treatment technologies were developed to address opportunities presented by the more challenging situations.
Advances to onsite system technology include the use of pump dosed systems which place wastewater in the specific zone of the soil that offers best potential for treatment and renovation. Drip treatment systems offer wastewater generators the ability to place domestic or industrial wastewater in the soil at very slow and controlled rates.
Land Treatment by subsurface slow rate disposal is considered to be an innovative technology which successfully answers the challenge of proper environmental management. Subsurface slow rate treatment is achieved through an underground drip absorption system serving as a slow rate biofilter. This system achieves organic and facilitating phosphorous fixes to the soil. The loading rate is designed according to soil characteristics with data such as soil permeability rates of the more restrictive layers, rainfall, evaporation and evapotranspiration rates, and nutrient balances taken into careful consideration.
Subsurface slow rate disposal systems for land treatment consists of the following components.
Pretreatment: All types of pretreatment systems can be used. Check with the manufacturer for recommendations.
Pumping and Control: Required to operate the TIMED dosing cycles, zone selection, filter backflushing, lateral flushing, flow control and other system vitals.
Filtration: A fully automatic filter system prevents the solids from entering into the delivery system. Filters should be backwashed on a preset frequency and/or by sensing pressuring differential through the filters.
Control Valves: A drip soil absorption field is divided into zones, with only one zone being activated with each dose cycle. The delivery network includes automatic zone control valves and air release valves that allow air to escape, but more importantly allows air into the lines at the end of each cycle.
Dripperline: A drip soil absorption field consists of polyethylene tubing laterals installed in parallel lines on contour within each zone. The dripperline's distal end is connected to a collecting manifold allowing automatic monthly line flushing. Good design will achieve at least 2 ft per second flushing velocities at distal end. There are 2 types of drip emitters available, pressure compensating and non pressure compensating. The use of subsurface drip ensures excellent distribution uniformity and maximizes conditions for biological reduction.
Drip system design begins with an assessment of the site and soil resources available to assimilate wastewater and wastewater constituents. Generally, drip systems are used to overcome limitations associated with slow permeability, seasonal high water tables, or severe slopes. Hydraulic loadings to wastewater receiver sites can be designed at rates of 0.05 to 0.15 gallons per square foot per day. One of the biggest advantages to the drip system is the ability to provide the dose/rest cycles which facilitate wastewater infiltration into the soil. Time dosed rather than demand dosed systems allow for the even spacing or dosing of effluent through the day rather than just at the time generated. The process of spreading the wastewater over the demand day allows liquid to infiltrate between doses.
Time dosing is one of the most powerful of the wastewater management technique that drip systems allow. The drip lines can be charged with a very low prescribed volumes of liquid and in a very short period of time. This promotes distribution of liquid to each line and optimizes treatment. The rapid charging time associated with drip systems optimizes the treatment in the soil and minimizes the time wasted to charge the entire wastewater or effluent distribution network.
Drip treatment systems offer another alternative for managing wastewater. When drip treatment systems are used, a comprehensive site evaluation and design by onsite professionals familiar with drip treatment technology, careful and precise designation of each required system component, installation by qualified contractors and operation by competent individuals is imperative. Drip treatment systems are not irrigation systems. Drip treatment technology offers wastewater managers an excellent alternative. Each component of this wastewater management alternative requires input from qualified specialists skilled in the areas of wastewater characterization, site evaluation, system design, installation and system management.
Presently there are hundreds of drip systems in operation, and most industry professionals are observing the effects of time and continued use on system performance. Among these effects are, kind of maintenance needed, and frequency of mechanical breakdowns. With overall system evaluations designers are finding out a lot about installation and design techniques. Development of more sophisticated management tools offered by the decreasing cost of electronic instrumentation i.e. remote sensing systems, modem links, datalogging of systems operation, etc., is allowing greater flexibility in operation and maintenance reporting of historical data for long term system performance.
In the hierarchy of wastewater management alternatives, drip treatment systems are precise and cost effective effluent delivery systems.
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