Tertiary Waste Water Treatment with Flotation Filters

Abstract

Tertiary waste water treatment has been usually carried out by adding filtration after an activated sludge process, which can be fitted with pre- or simultaneous precipitation for removal of phosphorus. Filtration is operated either as a mechanical unit operation for upgrading the removal of suspended solids only, or as contact filtration where the precipitation of phosphorus is boosted with a minute dose of trivalent iron or aluminium. Instead of using conventional downflow filters, tertiary treatment can also be carried out with flotation filters where flotation is accomplished in the head above the filter, necessary for filtration in any case. The possibility of using flotation between secondary clarification (sedimentation) and filtration, when it is needed, results in a very good load bearing capacity and in a high degree of removal of solids. This means that excellent operational results can be achieved with an almost complete reliability in all operational circumstances at a very reasonable cost. Long experience in Finland from a medium size (30 000 pe) activated sludge plant with simultaneous precipitation fitted with flotation filter tertiary treatment in 1984, shows that values of BOD7 ≤ 5 mg/l, Ptot ≤ 0.3 mg/l NH4-N ≤ 1 mg/l and of suspended solids ≤ 5 mg/l can be continuously achieved, in the treatment of municipal waste water where total nitrification is applied.