A reader recently asked how drip irrigation systems operate to treat septic tank effluent. In general, a drip irrigation system is nothing more than a type of pressure distribution system where the goal is to spread the septic tank effluent out over both space and time. This means that effluent is applied across the entire soil treatment area (space) and at certain times of the day (time).

A drip irrigation system consists of some level of pretreatment, a dosing tank, pump controls, flowmetering device, filtration headworks and the dripfield for final dispersal and treatment. The dripfield has 1/2-inch-diameter tubing with a pressure compensating emitter to provide uniform flow.

Minimum pretreatment for drip irrigation is a septic tank; but many areas and some products used require aerobic treatment. As always, state and local requirements should be checked before servicing or installing a system.

The dosing tank stores the treated wastewater until it is dosed to the soil treatment area. Typically, a high-head multistage turbine effluent pump delivers effluent through the filtration headworks to the drip distribution field.

ON THE CONTOUR

A drip distribution field consists of the drip tubing placed along the contour to form a run of tubing. These can be connected directly to the supply and return manifold, which forms a ladder-shaped drip zone. Individual runs can be looped together to form a lateral. Supply and return manifolds run up and down the slope to allow installation of the driplines on the contour.

By distributing small doses of effluent to the soil spread over a day, the soil is able to accept the effluent, maintaining aerobic conditions and allowing treatment of the contaminants in the soil. Instantaneous loading to the soil causes moist conditions around the emitter, but time between doses allow aerobic conditions to be maintained. Aerobic soil conditions and unsaturated flow through soil are the conditions we want present for optimum soil treatment efficiency.

Conventional gravity-fed trenches rely on development of a biomat to control flow from the trench into the soil under unsaturated conditions in the presence of oxygen to provide aerobic conditions for treatment.

In drip irrigation or any other pressure system, flow is controlled in the soil by use of a pump and timing of applications to make sure flow is not saturated (or too rapid) and oxygen is available to aid aerobic organisms in soil to break down and treat the effluent added. Since drip irrigation systems are typically installed no deeper than 1 foot (and often less) in the soil, permeability is higher for both water movement and availability of oxygen.

Early in use of drip irrigation for wastewater treatment, the question was whether some type of equivalent biomat or clogging zone would occur around the emitters, thus reducing their effectiveness in distributing effluent. Research done on drip irrigation systems using “treated” effluent showed that while there were changes in moisture retention, pore size distribution and saturated conductivity in line with the emitters, a severely clogged layer or area did not develop. Any hydraulic effects of effluent application were reduced farther away from the emitter.

FILTERING IS KEY

This research was done on systems that had additional pretreatment of the effluent beyond a septic tank. Additional pretreatment can consist of aerobic treatment units, media filters (sand, fabric, etc.) or constructed wetlands after a septic tank. The main purpose of the additional pretreatment is to reduce the organic loading of the system as indicated by BOD and fewer suspended solids to reduce the potential to clog the tubing or emitters. Increased BOD to the soil will also cause further reduction in conductivity and pore size and increase moisture retention.

In addition to more pretreatment, the filtration headworks has a disc filter, screen filter or sand filter. The primary purpose of the filters is to remove larger particles from the wastewater so they do not plug the emitters. Concern about solids collecting in the tubing — as well as growth within the tubing — means that all drip irrigation fields are built to either manually, automatically or continuously flush the drip tubing. This regular maintenance activity must be performed. Flushed solids and effluent are delivered back to the septic tank or the dosing chamber to cycle to the dripfield.

Research has shown that treatment efficiency for a range of soil conditions is very good. A study was conducted in Wisconsin under cold weather conditions for drip systems using only septic tank effluent and effluent after more pretreatment. Soil textures in the areas of the driplines ranged from loamy sand to clay. The tubing was installed at depths of 4 to 20 inches. The presence of fecal and total coliforms was evaluated up to 2 feet below the tubing depth.

For the systems using septic tank effluent, fecal coliforms were below detection limits at a 2-foot depth and less than that for the systems with the greater pretreatment. Based on the study results, the researchers suggested separation distances for drip systems using septic tank effluent could be reduced from 3 to 1.5 feet and 1 foot for more highly pretreated effluent. The caveat being that using this reduction would be in areas where other high-risk factors were not involved.

Drip irrigation systems provide quality treatment. They are an excellent choice to use in very tough soil conditions, such as high water tables, shallow to bedrock and tight clay soils. There are additional maintenance and management requirements and in cold climates additional design requirements to avoid freezing; but they can perform very well.