 such as a high ground water table. In areas where very tight soil would require an extremely large and costly conventional drain field, a collector pipe can be placed below the sand in the trench to recover the cleansed effluent. The cleansed effluent can then be disposed of on the land surface after disinfection and receipt of health department permission, or reused through other land appli- cation techniques (also with health department permission). If the ground water table is very near the surface, or if you have little or no soil above bedrock, the best solution is to build some- where else. But if you must build in such an area, then one way to provide adequate soil for cleansing of sewage effluent is to install an elevated sand filter. The filter sand can be enclosed in a container and the container walls constructed to whatever height necessary to provide sufficient depth of sand and degree of cleansing capacity (Figure 6.21). The aboveground or superficial sand filter can be made smaller than the subsurface filter, and assuming you've got somewhere to put the treated effluent, this system can be useful in cramped quarters. There are two types of aboveground sand filters used for small-scale residential sewage treatment: intermittent and recirculating. The intermittent is SO named because there are actu- ally two identical filters in the filter unit. When one filter bed CONVENTIONAL SYSTEM MODIFICATIONS Figure 6.21 Elevated sand filter. becomes clogged to the extent that sewage is continually ponded on the surface, the other is brought into service and the clogged filter cleaned and allowed to rest. Thus, each bed is used intermittently. The recirculating sand filter was developed to overcome some of the problems encountered with the intermittent filter, the main problem being odors. The recirculating filter recycles the effluent collected from under the filter sand back to the filter surface and the extra cycles serve to reduce or eliminate odors. Both filter types require a tremendous amount of mainten