k and filter can drain by gravity to the recirculation tank. This means that the outlet of the filter unit must be higher than the inlet to the recirculation tank. The recirculation tank should have a riser above its access port for easy inspection and maintenance. Locate ONSITE WASTEWATER DISPOSAL the float valve or other flow control mechanism and the pump SO that they are accessible from this port. Locate the time clock outside the tank away from corrosive gases. Both the filter housing interior and the recirculation tank interior should be coated with a water- proofing compound. Operation and Maintenance The collected and uncollected subsurface sand filters are not accessible for maintenance, SO satisfactory performance depends on proper maintenance of the septic tank and operation well within the filter design limits. Remember to have the collection or dosing chamber pumped out when the septic tank is pumped, and follow the main- tenance schedule suggested by the pump manufacturer if you are using a pumped dosing chamber. Elevated sand filters require much more attention. In addition to the previously mentioned maintenance procedures, the filter bed itself requires periodic maintenance to overcome the progressive clogging that will occur. With fine-grained sand and a high application rate, the bed may clog and effluent rise to 12 in. above the sand as frequently as every 30 days. Coarse sand with low loading rates may last a year or more without need for maintenance. Maintenance consists of inspections every three months or as the clogging rate dictates. Initial inspections should be made at least monthly until the clogging rate is established. During maintenance, remove any weeds that may take root in the sand, and perhaps rake the sand surface to a depth of several inches. This last procedure is often recommended, although its usefulness is limited. Even with raking, the top layer of sand will soon have to be replaced when clogging becomes extensive. There are two techniques for a better maintenance proced