 beneficial to drain fields because, over a period of several months of not being used, the clogging layer begins to break up and the infiltration rate is increased. The resting drain field serves as a backup system should the operational drain field need repairs. When replacing a drain field which has become clogged, it is wise to keep the clogged drain field as an alternate. This alternating of drain fields is most beneficial in soils with slow percolation rates and where the application rate of effluent into the soil is very high. Where soils with percolation rates faster than about six minutes per inch are used for disposal, alternating fields is not a good idea because a continuous clogging layer in these soils is nec- essary for adequate filtration. Process As a drain field which has built up a thick layer of slime and small particulate matter along its bottom and sidewall surfaces is allowed to rest, air invades, bacteria continue to digest what food remains, and organic chemicals are decomposed. This process continues over several months, depending upon the thickness of the clogging layer and the climatic conditions. In general, systems left to rest for 9 to 12 months are considered to be rejuvenated. There is speculation that 100 ONSITE WASTEWATER DISPOSAL Alternating Fields diversion box Figure 6.1 Alternating fields. two full-size drain fields, if properly designed, installed, operated, and maintained, will last forever if used only in alternate years. Design Many combinations of alternating fields have been suggested. Two full-size fields; two half-size fields; three half-size fields; three one-third-size fields used two at a time; and so on. The data on these various combinations is too sparse and the conditions too varied to be able to say that one combination is best. Two half-size fields should be considered as the minimum design, and two full- size fields as more than sufficient. The drain field to be used is fed through a diversion box (Figure 6.2) or a diversion valve. Inexpensive plastic diver