A drain field may be designed to offer several separate disposal areas for effluent from a single septic tank.  One area may be "rested" while effluent is routed to a different area.  The nematode community in the resting drain field continues feeding on the accumulated biofilm and fats when the anaerobic septic tank effluent is no longer available.  This natural cleansing process may reduce bioclogging to improve the hydraulic capacity of the field by increasing the available interstitial area of the soil as the accumulated organic material is oxidized.  The percolation rate after resting may approach, but is unlikely to match, the original clean water percolation rate of the site.