| Abstract: |
Flotation has considerable promise as a rapid separation process and is particularly effective for water heavily laden with algae or for low turbidity, highly colored water that produce light flocs. Flotation has several process advantages compared with sedimentation, including better treated water quality, rapid start up, a high rate of operation and the production of a thicker more easily treatable sludge. For these reasons, dissolved air flotation was selected to be applied with Nile water as an innovative treatment facility.The major conclusions of the research are: Flotation was applied successfully with Nile water producing satisfactory results even during flood time.• The DAF unit provided excellent algal removal capability, low residual turbidity and low final suspended solids concentrations which minimize the load on filter.• The great ability of DAF to separate algae efficiently eliminates the need for pre-chlorination during flood time, thus, decreasing potential for THM formation.Poor flotation performance occurred without coagulant addition or at non-optimum coagulant dosages.• Alum proved to be a suitable and effective coagulant when used with flotation. It produced excellent finished water quality with desages lower than conventional treatment with settling since production of large floc is not necessary.• The use of polymer (i.e:Nalco S600) as a coagulant aid with alum does not improve DAF efficiency, but it leads to 20% reduction in alum dose (i.e. 20% reduciton of. alum cost ) , versus a slight additional cost due to polymer addition.• The work indicated that good flocculation was as important in flotation as in sedimentation but long flocculation periods are not necessary since there is no need for large floc formation. Flocculation time of only 10 min was found to be the optimum.• The short residence time, which is chosen to be 15 min., from results of the experimental work, implies a lowering plant costs and a less floor space.• Application of flofilter concept adds a further reduction to the area requirement since it combines the flotation and filtration processes in one compact unit.• Saturator pressure and recycle ratio are very important operating parameters which control DAF performance. Typical conditions of 8%recycle rate and 70 psi (483KPa) saturator pressure produce the best treatment results in all cases.• The flotation tank could be designed with high overflow rates of 5-10 or 12m3/m2/hr, which means a rapid separation processand a feasibility to overcome the shortage of the existing water treatment plants in the future.• Flotation process starts up rapidly. It needs a maximum of 45 min to achieve steady water quality. This fact allows intennittent operation.• DAF is a flexible process since it can face the deterioration in raw water quality either by increasing the recycle flow or decreasing the overflow rate.• Application of flotation prior to filtration yields excellent filtered water quality. With DAF, filter runs are longer (i.e.4048hr), consequently, less backwash water is needed.DAF system has high operating costs principally due to the need for extra pumps for recycled flow and compressors for air saturation, however, savings in other operation, capital and maintenance costs because of the lower chemical costs for coagulants and flocculant aids, smaller tanks, less solids disposal costs and longer filter runs make the process more cost effective .• Since flotation has proven to be an important technique in water treatment technology, it must be tested in larger pilot plants as well as full scale plants which must be constructed to gain wider experience. Results also indicates the need to
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