Integration of high-rate DAF technology within a municipal biofiltration plant for the treatment and thickening of backwash wastewaters

Industrial full-scale application of high-rate dissolved air floatation (DAF) in the municipal wastewater treatment plant (WWTP) of Grenoble (France) has highlighted outstanding performance results leading to new design-to-cost perspectives. The integration of DAF technology to treat the returns from the backwash waters of submerged biological aerated filters (BAF) (nitrification stage) has demonstrated removal efficiencies that allow further room for global process optimization. The results obtained on nitrifying BAF backwash water showed a DAF outlet water concentration of less than 25 mg.L⁻¹ of total suspended solids at 25 m.h⁻¹, with only polymer conditioning. Such high clarification performance allows leveraging of valuable cost optimization of global process design integration. Direct discharge from DAF's outlet into the receiving body can be implemented. Hydraulic and solid return loads can therefore be significantly reduced at the inlet of the WWTP. Moreover, floated sludge extracted from the DAF units achieved 4.4% dryness on average. The high thickening operational performance of this DAF technology is able to produce sludge directly compatible with anaerobic digestion. These full-scale results demonstrate that Suez's GreenDAF™-BWW technology in such application can leverage new rooms for design improvement for BAF treatment and total cost optimization of both the mainstream water treatment line and sludge line.