Sixteen full-scale tracer studies were completed at two water treatment plants to assess disinfection performance under the concentration-time (CT) concept. The step residence time distribution (F RTD) was developed for each case. The value of the effective contact time, t10, in the CT concept was then obtained. For reservoirs without baffles, the t10 values were found to be much smaller than the expected values, indicating poor performance under the CT concept. Several models were used to interpret the F RTD characteristics, but the results were unsatisfactory. The standard jet model was then applied and was able to match the field data F RTD curve up to the relative concentration c/co [Formula: see text] 0.2. This showed that the momentum causing jet was responsible for the rapid movement of water through the system causing small t10 values. The work shows the importance of the momentum causing jet in reservoirs, and that in addition to traditional criteria it should be considered in the evaluation of water treatment component design. Other models that are commonly used to predict the t10 value should be applied carefully as a result of this jet effect.Key words: tracer study, F RTD, t10, CT concept, jet, water treatment component design.
A flocculation study was completed by means of a vertically oscillating grid mixing device. Five types of single grids with different solidity ratios were vertically oscillated inside a 2 L jar to promote floc aggregation. Kaolin was used as the simulated turbid particles and alum was applied as the chemical coagulant. The method of measurement was completed based on the particle reduction represented by the settled water turbidity. It was found that low turbidity readings could be achieved at a wide range of average volume velocity gradient [Formula: see text], especially in the case of high solidity ratio types of grids. This indicates that the grid mixing had a stable performance and was not greatly affected by mixing variations in the vessel. The floc aggregation and erosion rate coefficient analysis showed that the grid mixing produced particle contacts with low break-up rate. A general relationship among [Formula: see text], flocculation performance parameters, and grid physical characteristics was found, indicating that the flocculation performance was easily controlled. This study has shown the potential of grids as a mixing device to produce an excellent mixing environment for floc aggregation.Key words: flocculation, grid, solidity ratio, kaolin, alum, [Formula: see text], turbidity.
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