nonpotable purposes groundwater recharge collected rainwater greener tomorrow recharge pits advanced purification capacity requirements designed based downpipes direct rooftops terraces catchment surfaces sustainable method filtered rainwater utilizing rainwater underground tanks reduces dependency industrial processes contaminants common filters irrigation industrial processes rainwater harvesting solutions reduces external supply storage tanks aboveground allegiance water management prevents depletion benefits irrigation cleaning flushing sand gravel charcoal remove debris leaves promotes water conservation collecting filtering storing