ABSTRACT
The feasibility of recycling filter backwash water and alum sludge generated from water purification plants has been investigated. Actual wastewater and alum sludge used in this study were collected from a water plant employing water treatment processes including chemical addition, mixing, flocculation, clarification, filtration, and chlorination. Wastewater and sludge are generated mainly from the clarifier and the filter backwash. The waste recycle system presented here consists of (a) recycling the filter backwash water to the intake system for the reproduction of potable water, (b) dividing the combined sludge into two fractions for alum solubilization, separately, in an acid reactor and an alkaline reactor, (c) removing the inert silts from alum solutions by two separate water–solids separators for ultimate disposal, and (d) returning the solubilized alums from the two separate water–solids separators in proper proportions for reuse as flocculants.
The proposed recycle process was designed to provide a cost-effective system for achieving “zero” wastewater discharge and alum recovery from a water purification plant. Recommended process design parameters necessary to achieve the above stated goals have been established. Experimental results tend to suggest that practical designs based on the proposed water recycle and sludge thickening and alum recovery (STAR) system are technically feasible. Additional conclusions drawn from this research are (a) discharging raw alum sludge from a water treatment plant to a nearby wastewater treatment plant through a sewer system is a viable means of sludge disposal for the water utility; (b) the thickened raw alum sludge can be disposed on land as a soil amendment without adverse effect on soil if the pH of the disposed alum sludge is near neutral; (c) recycling the recovered alum for water purification within the water treatment plant is technically feasible the problem of impurity concentration (heavy metals and soluble organics) can be met by a scheduled recycling application or an automatic blowdown; (d) the U.S. Federal and the Commonwealth of Massachusetts prohibit recycling of the recovered alum for water purification within the water treatment plant because its long-term health effect is unknown; (e) employing recovered alum from a water treatment sludge as a precipitant for phosphate removal in a wastewater treatment plant is technically feasible although its economic feasibility needs to be studied; and (f) direct recycle of filter backwash water from a dissolved air flotation–filtration (DAFF) water treatment plant to the plant’s intake unit is both technically and economically feasible. The recovered alum (either aluminum sulfate or sodium aluminate) can be effectively used for removal of heavy metals and phosphorus from wastewater.50
