hat it cannot pass the last dose of sewage before a new dose arrives, the other filter section should be brought into service and the first reconditioned. The same mechanisms used to divert the flow into alternating fields (Figure 6.2) can be used to switch this flow. A high water alarm system can be installed in each filter sec- tion to notify you of the clogged filter, and if you wish, you can wire the alarm to an electric valve to divert the flow automatically. The maintenance intervals can be increased by sizing the filter for septic tank effluent and using an aerobic treatment unit. As shown in Figure 6.26, the distribution system for an intermit- tent sand filter can be quite simple. The sewage is directed onto a stone or masonry splashplate from the influent pipe. A sprinkler system has been used for distribution, but this simply adds mainte- nance problems when the holes in the pipe become clogged with algae or other material. Since the sand should be flooded to several inches above the sand surface by each dose, even distribution over the sand surface should not be a problem. The underdrain system is designed the same way as the underdrain for the collected subsur- face sand filter. An overflow pipe can be added, but its opening should be above the high water alarm level. The recirculating sand filter is somewhat more complicated than the ONSITE WASTEWATER DISPOSAL Sand Filter Septic Float Valve Underdrain Drain Recirculation Field Recirulation Pump Figure 6.27 Recirculating sand filter. intermittent sand filter. The housing for the filter sand and the sand and gravel layers are the same as used for the intermittent sand filter, except that sand up to an effective size of 1.5 mm is acceptable and sand below 0.6 mm effective size should not be used. The distribution system can be the same as used in the intermittent filter. Although a dosing trough (overflowing rain gutter) and a pressure dosing system of small-diameter perforated pipe have been used with some success, you should keep the system simple. T