tter in the grey water as from the bacterial growth on the filter cloth or medium. In the case of the cartridge filter, washing with water may be inadequate to renovate it, but strong solvents may dissolve the filter cloth. Buy several filter inserts SO the dirty ones can soak in dilute bleach for a period of time. DISINFECTION Application As a rule of thumb, disinfection of sewage is needed whenever sewage is exposed at or above ground level. Also, as a rule of thumb, home sewage disinfection systems are not particularly effec- tive at preventing potential health hazards. The reasons for this lack LOW-IMPACT SYSTEMS 211 Hypochlorite Tablets Figure 7.15 Disinfection unit. of effectiveness are that there is no simple and reliable method for determining how well the system is doing its job, and many home- owners do not want to be bothered with the maintenance which disinfection units require. One possible conclusion drawn from these two rules is that sewage from home sewage treatment units should not be disposed of at or above ground level, except under conditions that negate one or the other of the rules of thumb. If disinfection of septic tank or aerobic treatment unit effluent is necessary, such as with some of the systems discussed in the section on land application, there is a potentially effective method. Process There are two basic types of disinfection units used in home swimming pool disinfection which are applicable to home sewage disinfection. One operates by liquid passing through a tube, where the liquid dissolves and releases disinfectant from tablets stacked in the tube (Figure 7.15). This is called a dry feed unit. The other type of unit pumps a solution of disinfectant into the liquid with a small pump. Both types work well on backyard swimming pools, and have been in use for many years. Other swimming pool and spa disinfection units, such as ozonation and ultraviolet irradiation, are probably less amenable to onsite sewage disinfection because of 212 ONSITE WASTEWATER DISPOSAL questionable effecti