The National Pollutant Discharge Elimination System regulations require that any discharge of process water into an open stream, river or lake must be done with a permit. Because returned concrete is a major source of substances found in plant wastewater, effective wastewater management requires evaluation of the several options for handling returned concrete.
Methods of handling leftover concrete include: discharging directly on the ground on plant property; discharging into a washout pit; casting large concrete blocks; reusing leftover concrete by delaying hydration with admixtures; using reclaiming machines that separate aggregate and slurry for recycling. Each of these has its own advantages, disadvantages and limitations.
Methods used to handle returned concrete must also be integrated with those used to control stormwater runoff at the plant. A separate basin can be constructed to collect stormwater, which can then be reused in batching, lowering water costs. Keep in mind that without an NPDES permit, stormwater that has been mixed with process water cannot leave the property.