Until recently, increased wet-weather treatment capacity has been limited to traditional approaches such as additional primary and secondary biological treatment or equalization basins. These approaches often incur significant capital investment and despair over limited space. Yet many small entities need cost-effective alternatives to meet permit requirements during limited-duration wet-weather events without investing a large sum of capital.
Ballasted flocculation was traditionally used throughout Europe to treat water. Ballasted flocculation is a high-rate, physical-chemical clarification process involving the fixing of flocs, or suspended solids, onto ballast (sand) with the aid of a polymer. A combination of a metal-salt coagulant, micro-sand (or sludge recycle), and enhanced clarifier features (such as lamella settlers) increase settling velocities by a near factor of 10.
With ballasted flocculation, screened sewage is combined with a coagulant--typically ferric chloride or alum-in a flash mix tank, where the micro-sand and polymer are added. The micro-sand provides a large number of particles (contact area), enhancing the flocculation rate, and acts as ballast to accelerate the settling of the floc.
The suspended solids in the wastewater, which have been destabilized by the addition of coagulant, bind to the micro-sand particles via polymer bridges. These large particle agglomerates grow into high-density flocs with-in maturation tanks. There the conditioned wastewater experiences a gentle mixing energy and increased retention time, allowing the agglomerates to trap random flocs and settle faster in the sedimentation tank.
The resulting sludge, which contains the micro-sand mixture, collects at the bottom of the sedimentation basin for pumping to hydrocyclones, where the sludge is centrifuge-separated from the micro-sand. The residual solids are sent through a sludge processing system and the recovered micro-sand is recycled to the injection tank.
The ballasted flocculation process yields significant benefits, including:
- relatively small site requirements with a footprint 5 to 15% that of conventional treatment trains (making it attractive to utilities with limited space);
- rapid startup times for reaching peak efficiency in fewer than 20 minutes (making it suitable for handling storm-induced peak flows in combined and separate sewer systems); and
- a highly clarified effluent in terms of turbidity and total suspended solids (TSS).
Pilot studies conducted by CDM in Galveston, TX, and Fort Smith, AR, indicate that the process removes more than 85% TSS and 65% biochemical oxygen demand (BOD). In fact, the pilot studies have so succeeded that the Fort Worth (TX) Village Creek wastewater treatment plant will be the first such plant in the U.S. incorporating this technology to obtain a National Pollutant Discharge Elimination System permit.
Performance data indicate that the process removes 80 to 90% of extent phosphorus and 25 to 35% of nitrogen present. Also, unlike conventional biological treatment trains that are sensitive to large increases in flow, the ballasted flocculation process has the ability to treat a wide range of flows with sustained pollutant removal efficiencies.*
The ballasted flocculation process is integrated as a parallel treatment train into existing and new wastewater-treatment facilities to the biological process to treat a more consistent flow and at a reduced flow. The ballasted flocculation process also has use during non-storm-induced peak flows. It can serve as a chemically enhanced primary clarifier or for flow maintenance during construction periods at the head of the plant. In addition, the process can be used within the collection system at permitted sanitary-sewer overflow sites to provide greater treatment than with standard stormwater clarifiers.
Ballasted flocculation is attractive as a wet-weather treatment train because capital costs are relatively low and because oper-ational costs are incurred only during peak-flow events. For a 100-MGD facility, capital and operational costs associated with ballasted flocculation are calculated to be U.S.$0.20 per gallon and $90 per million gallons per day, respectively.
Contact: Camp Dresser & McKee Inc., One Cambridge Center, Cambridge, MA 02142, Richard Sawey 817.332.8727
*Process capacity ranges from 25 to 60 GPM/sq. ft.