Patent ID: 7311841

Claim:
A ballasted flocculation process for purifying water comprising: a. mixing a coagulant with the water; b. selecting a ballast in the form of insoluble granular material where the insoluble granular material has an effective diameter between approximately 40 and 300 micrometers; c. mixing the selected ballast with the water and mixing a flocculant with the water to form flocs and microflocs around the ballast; d. introducing the mixture of water and flocs into a decantation zone; e. in the decantation zone, separating purified water from a mix of sludge and ballast; f. directing the mix of sludge and ballast from the decantation zone to a separator that is operative to separate the sludge and ballast; g. separating the ballast from the mix of sludge and ballast and recycling the separated ballast to or upstream mixing step c. wherein the separated ballast recycled constitutes 20% or more by volume of the mix of sludge and ballast directed to the separator; h. separating sludge from the mix of sludge; i. concentrating the separated sludge, recycling at least a part of the sludge to or upstream mixing step c., and wasting a portion of the sludge wherein the concentration of the sludge wasted is at least 5 g/L; and j. directing at least some of the mix of sludge and ballast from the decantation zone to an intermediary mixing zone; directing at least some of the sludge separated by the separator to the intermediary zone and mixing the mix of sludge and ballast with at least a part of the separated sludge; directing the mixture of the mix of sludge and ballast and the separated sludge from the intermediary mixing zone to the separator and continuing to return separated sludge from the separator to the intermediary mixing zone.