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emitter. This emitter discharges at nearly a constant rate over a wide range of in-line pressures (fig- ure 4-18). Head losses through driplines are high because of the small diameter of the tubing and its in-line emitters, and therefore dripline lengths must be limited. Manufacturers limit lengths at various emitter spacings. With turbulent flow emitters, the discharge from each successive emitter diminishes in response to pressure loss created by friction or by elevation changes along the length of the dripline. With pressure -compensating emitters, the in-line pressure should not drop below 7 to 10 psi at the final emitter. The designer is urged to work with manufacturers to ensure that the system meets their requirements. Pressure -compensating emitters are somewhat more expensive but offer some important advantages over turbulent -flow emitters for use in onsite wastewater systems. Pressure -compensating dripline is better suited for sloping sites or sites with rolling topography where the dripline cannot be laid on contour. Turbulent -flow emitters dis- charge more liquid at lower elevations than the same emitters at higher elevations. The designer should limit the difference in discharge rates between emitters to no more than 10 percent. Also, because the discharge rates are equal when under pressure, monitoring flow rates during dosing of a pressure -compensating dripline network can provide an effective way to determine whether leaks or obstructions are present in the network or emitters. Early detection is important so that simple and effective corrective actions can be taken. Usually, injection of a mild bleach solution into the dripline is effective in restoring emitter perfor- mance if clogging is due to biofilms. If this action proves to be unsuccessful, other corrective actions are more difficult and costly. An additional advan- tage of pressure -compensating emitters is that pressure regulators are not required. Finally, when operating in their normal pressure range, pressure - compensating emitters are not affected by soil water pressure in structured soils, which can cause turbulent -flow emitters to suffer reduced dosing volumes. Figure4-18. Turbulent -f low and pressure -compensating emitter discharge rates versus in-line pressure 2.00 1.80 a 1.60 °1 1.40 1.20 1.00 F 0.80 M 0.60 0 0.40 0.20 0.00 0 5 10 15 20 25 30 35 45 60 In -Line Pressure (psi) Controlling clogging in drip systems With small orifices, emitters are susceptible to clogging. Particulate materials in the wastewater, soil particulates drawn into an emitter when the dripline drains following a dose, and biological slimes that grow within the dripline pose potential clogging problems. Also, the moisture and nutrients discharged from the emitters may invite root intrusion through the emitter. Solutions to these problems lie in both the design of the dripline and the design of the distribution network. Emitter hydrodynamic design and biocide impregnation of the dripline and emitters help to minimize some of these problems. Careful network design is also necessary to provide adequate safeguards. Monitor- ing allows the operator to identify other problems such as destruction from burrowing animals. To control emitter clogging, appropriate engineer- ing controls must be provided. These include prefiltration of the wastewater, regular dripline flushing, and vacuum release valves on the net- work. Prefiltration of the effluent through granular or mechanical filters is necessary. These filters should be capable of removing all particulates that could plug the emitter orifices. Dripline manufactur- ers recommend that self-cleaning filters be designed to remove particles larger than 100 to 115 microns. Despite this disparate experience, pretreatment with filters is recommended in light of the potential cost of replacing plugged emitters. Regular cleaning of the filters is necessary to maintain satisfactory performance. The backflush water should be returned to the head of the treatment works. USEPA Onsite Wastewater Treatment Systems Manual 4-29