Important information for laying out your next drip distribution installation
Drip distribution is no different than any other type of pressure distribution system where the length of laterals is dependent on the number of orifices (in this case, emitters), friction loss in the pipe (tubing), and pump capacity. The emitters have specific flow rates and the total number of emitters determines the flow rate necessary to fill the tubing and provide the flow through the emitters to the soil.
Typically, the maximum lateral length for non-pressure-compensating emitters will be about 200 feet. Tubing with pressure-compensating emitters will allow laterals of 300 feet. These lengths provide the opportunity to flush the tubing either automatically or manually. The number of laterals in a zone will determine the volume of water necessary to flush the zone. With a velocity of 1-2 feet per second to flush the laterals (using 1/2-inch-diameter tubing) each lateral requires a 1.6 gpm flushing flow. Therefore, the pump capacity required to flush the field is estimated by the number of emitters and laterals in the zone.
The supply and return manifolds run up and down the slope so runs and laterals can be laid on the contour. Draindown in the supply and return manifolds should be minimized. This is another reason for the small-diameter flexible feeder tubes running to the laterals. They are installed to top-load the laterals. In cold climates like mine, the supply and return manifolds should drain back to the pump tank to avoid freezing.
Most drip systems will have more than one zone. Hydraulic or electronic switching devices can be used to dose the zones. Isolation of individual zones can be accomplished through the use of check valves. Usually the return line will drain back to the pump tank. This is an absolute must if the system uses an ATU as part of the treatment process. Drainback can exceed the unit capacity and result in solids moving into the pump tank and potentially plugging filters, pumps and tubing.
One final piece is necessary. Air release valves need to be installed at the highest point of each zone. They allow air to enter the piping when the system shuts down. This allows the effluent to drop out of the piping into the soil and eliminates soil being “sucked” into the emitters, plugging them. These valves need to be protected, so they are usually placed in a valve box. This provides space for air exchange while protecting them from freezing or damage. In cold climates, this box may need to be insulated.
A word of caution: If you find a plugged air release valve, resist the temptation to use a screwdriver or other pointy object to remove the blockage or unstick the ball that moves up and down in the valve. Any slight scratch or indentation will cause the ball to not seat properly and the result will be a valve that leaks when the system is pressurized and effluent will leak out of the valve. This will cause freezing in cold climates and a wet area around the valve box in other locations.
About the Author
Jim Anderson is connected with the University of Minnesota onsite wastewater treatment education program and is an emeritus professor in the university’s Department of Soil Water and Climate. Send him questions about septic system maintenance and operation by email to firstname.lastname@example.org.
This article is part of a series on drip distribution.