Abstract: An underground structure having at least one uncovered windowed wall, has side walls and roof constructed of corrugated sheet metal, the roof being constructed as a single or multi-arch in order to support the overlying earth, both the roof and base being drained of water which collects at the subsurface levels.

Abstract: A waste treatment apparatus operated either by gravity or by positive displacement pump in which a variable inflow of contaminated liquid is aerated, clarified, and filtered. The filtered liquid is used to backwash one of at least a pair of rapid sand filters, while the other rapid sand filter is being utilized. The alternating cycle of backwashing and filtering by the respective rapid sand filters, all of which is integrated with overall filtering operation, insures that at least one filter is fully operative at any time.The apparatus is adapted for variable inflow and can adapt to such unfavorable exigencies as overtaxing inflow and clogging at the rapid sand filter or at an earlier stage of contaminated liquid treatment. Sensors are located suitably throughout the apparatus to adjust the system to overflow conditions or clogged conditions, and the flows can be redirected through a preferred stage.