Bench‐scale and pilot‐plant studies were used to investigate the effects of flocculation time and floe size on the performance of dissolved air flotation and to determine air requirements for three different types of water supplies. The results show that long flocculation periods are not needed and suggest that floe particles of 10–30 μm should be prepared for flotation. Two measures of the air supplied were examined: percentage recycle and the bubble volume concentration or the volume of air released per volume of water treated. Bubble volume concentrations of 4,600 ppm or 8 percent recycle were sufficient to effectively treat all the waters tested.

This research examined removals of Giardia and Cryptosporidium by clarification (either dissolved‐air flotation [DAF] or plate sedimentation) and by dual‐media filtration following clarification. Studies were conducted for challenge conditions of cysts and oocysts (i.e., high influent concentrations) and for process design detention times and hydraulic loadings. To evaluate the effect of water temperature on performance, sets of experiments were performed for winter and spring seasons. DAF clarification performed better than plate sedimentation and consistently resulted in lower turbidity levels and particle counts. Overall log removals of Giardia and Cryptosporidium by clarification and filtration were about the same for DAF versus plate sedimentation. Although DAF is not recognized under the Surface Water Treatment Rule or the Interim Enhanced Surface Water Treatment Rule, the authors argue that based on these findings, DAF plants should receive Giardia and Cryptosporidium removal credits at least equal to those received by sedimentation plants.

No abstract

No abstract

This article provides the results of studies of full-scale waste filter backwash water (FBW) recycle practices and water quality effects at six water treatment plants as well as recommendations for evaluating FBW recycle practices. Water quality parameters assessed included suspended solids, turbidity, particle counts, total and dissolved organic carbon, aluminum, and manganese. Results showed insignificant effects on plant influent water quality for recycle of clarified FBW and for a plant with continuous recycle. The FBW recycle quality was highly temporally variable for plants with only flow equalization (no solids removal), exhibiting significant peaks in solids levels that caused significant short-term increases in plant influent turbidity. However, the performance of downstream clarification (dissolved-air flotation or plate sedimentation) and filtration were not affected by FBW recycle for these plants. Evaluation of recycle effects requires knowledge of the site-specific details of plant operation (generation of recycle streams, flow control) and the chemistry of process waters (raw, recycle, and coagulated). FIGURE 2S St ta am mf fo or rd d W Wa at te er r T Tr re ea at tm me en nt t P Pl la an nt t s sc ch he em ma at ti ic c