BALLASTED FLOCCULATION: A WET-WEATHER TREATMENT SOLUTION?

Three large wastewater treatment plants in the United States have been managing their storm flows by either diverting excess flows around the main treatment train for discharge to the receiving stream, or storing the excess flow for future treatment through the biological process during lower flow conditions. Neither approach is ideal, as the first has obvious negative impacts on the receiving stream, and the second tends to be labor and space intensive and often involves odor issues. Ballasted flocculation was examined in each case as a more practical and economical storm water management approach.Based on pilot and bench scale testing performed at the three facilities, it was determined that the ballasted flocculation technology would be compatible with client needs. In all cases, the ballasted flocculation system was capable of removing 75 to 95 percent of influent storm flow TSS at hydraulic loading rates varying from 60 to 80 gpm/sf when ferric chloride was used as the coagulant. Other contaminants were also monitored and removal rates were impressive; however, these results could not be guaranteed as representative of actual storm flow conditions in cases where storm flows were simulated due to lack of precipitation. Cost savings varied significantly between facilities depending on the details of the existing storm flow treatment systems, though ballasted flocculation was generally the least expensive alternative. In the case where ballasted flocculation was about the same cost as the current storm flow management approach, several non-economic issues, such as reduced labor requirements and odor generation, still made it the more attractive option.