The discharge of saltwater complicates placement and increases the operational costs of both coastal and inland marine RAS. In salt tolerant coastal areas, operations are increasingly challenged with nutrient discharge limitations, a logical consequence to highly visible coastal eutrophication issues. Inland areas, particularly in the western US, are sensitive to both salt and nitrate because of their adverse impact on heavily taxed groundwaters.
The quantity of water physically lost is ultimately limited by the sludge that must be removed from the system when the solids capture device is backwashed. The development of pneumatically washed bead filters that inherently recycle backwash waters allows the sludge discharges to be reduced, so water (salt) reuse can approach several months. Coupling a pneumatically washed floating bead filter with a localized sludge digestor can facilitate nitrate control and further reduce sludge mass by about 50 percent. Secondary consolidation coupled with recycling of supernatants off the surface of the mineralization basin can result in another twofold reduction in sludge discharge volumes potentially extending water reuse beyond a year.
Mineralization of sludge is a straight-forward operation as the degradation process is spontaneous. If the only objective is reduction of sludge discharge volumes, then a continually mixed, aerated basin is the most efficient. AST uses a sizing criteria of 83 liters/kg fed-day (10 gals/lb fed daily) @20oC to reduce the sludge mass by about 50 percent. Discharge concentrations of 4-6% solids are readily obtained with good reactor designs. Nitrate removal requires anaerobic digestion. With some intermittent mixing, a sludge sizing of 170-250 liters/kg fed-day (20-30 gals/lb fed daily) can approach a mass reduction of about 40% while substantially reducing nitrate accumulations. Carbon addition can be used to enhance the denitrification that benefits from high levels of readily digestible organic carbon.
With cumulative feed burdens (CFBs) more than 100,000 mg/L, closed RAS for some marine species may warrant further treatment or supplements to adjust for mineral drifts or dissolved organic accumulations. An extended water (salt) reuse strategy provides an avenue for RAS expansion without the environmental conflict that is plaguing more open approaches.