Keeping groundwater and surface water out of the sand bioreactor is the single most important goal in construction. Liners, or cast or precast tanks, are required with sand bioreactors to keep out groundwater. Liners and tanks are also used to keep untreated wastewater from entering groundwater. Berms and surface grading must be provided in such a way as to keep surface water from draining into a sand bioreactor.
Liners are an effective way to prevent infiltration of groundwater into the system. A PVC liner of at least 20 mil is recommended in an earthen basin with provisions for UV protection. As with any liner system, it is important to use as few seams as possible. If an outlet must penetrate the bottom of the liner, great care must be taken at this point to prevent leaks.
Precast, or poured in place, concrete tanks are an alternative to lined earthen basins. Small bioreactors have been constructed in the bottom section of precast septic tanks. The tanks from an existing aeration basin can be retrofitted with a drain, filled with suitable sand, and a distribution system for use as a sand bioreactor.
As with plastic lined earthen basins, all connections through a concrete tank wall must be watertight. Linkseals are effective in eliminating infiltration for connections into concrete structures (Figure 11).
Short circuiting through or around the sand is another concern with sand bioreactors. Careful sand placement to avoid internal layering is necessary to avoid forming internal clogging and channels. Dropping the sand from a height of several feet should be avoided, because it promotes segregation by grain size. If sand layering is suspected, plow or rototill to redistribute the sand.
Draining renovated wastewater from the bottom of the bioreactors is accomplished by simply sloping the bottom of the structure to an outlet. A layer of gravel should be placed at the bottom of the bioreactor to support the sand and promote the free flow of effluent to the outlet.
Four-inch diameter, perforated vent pipe, on six-foot centers should be positioned in the gravel layer and brought up to the ground surface. Bringing air into the bottom of the bioreactor promotes aerobic conditions.