ance compared with other treatment and disposal systems, and they can be obtrusive. Because of their limitations, aboveground sand filters are not popular for treating sewage from single residences where other alternatives are available. Process Properly sized and graded sand makes a fine filter. This sand will allow the formation of a biological mat on its surface and a flow beneath the surface mat which is rapid and unsaturated. Its pore spaces are small enough to provide excellent mechanical filtration. The loading rate is critical for proper operation of these filters. Sub- 156 ONSITE WASTEWATER DISPOSAL surface filters are not easily accessible for maintenance and repair, SO they can be loaded with much less effluent per square foot of surface area than can aboveground filters. The aboveground filters are typically loaded at a rate which is high enough to result in progressive clogging, but through frequent maintenance, failure of the system is prevented. Some means of disposing of the effluent from the aboveground filter is necessary. If the reason for using an aboveground filter is because of a very high ground water table, the effluent, being suffi- ciently cleansed, can safely be discharged into a standard bed or trench with reduced clearance between the trench bottom and the water table. Assuming that there is sufficient soil above bedrock downhill from the disposal field to prevent effluent from surfacing, clearance to bedrock can be reduced as well. But check with the proper regulatory authorities before counting on any reduction in requirements. If the reason for using an aboveground sand filter is to cleanse the effluent for reuse, then the effluent is normally collected in pipes under the sand and allowed to flow into a dosing chamber. From there it can be pumped to a land application system. The effluent may or may not have to be disinfected before it is released to the environment. If there is no regulation specifying disinfection, then use the recommendations given in the section on land applica