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Water treatment

DyGF-A had flow

DyGF-A had flow reductions in the range of 41 to 57 % during the first three filtration runs shown, and 98 % during the fourth, in the presence of several turbidity peaks. After this fourth filtration run, DyGF-A required total cleaning activities (as summarised in table 3.2). This unit required these activities again 65 days later. DyGF-C had flow reductions in the range of 65 to 91% during the first four (figure 3.13) filtration runs shown but having limitations in recovering headlosses after partial cleaning activities. These limitations became clear during the fifth filtration run, lasting only two days and making total cleaning activities necessary at the end of this run (this unit required these activities again 71 days later). During the sixth filtration run, DyGF-C presented a maximum flow reduction of 99%, at the time of several turbidity peaks. At the end of this sixth filtration run partial cleaning activities were sufficient to recover filtration capacity of this treatment unit. Figure 3.14 shows headloss development in the top and lower (intermediate and bottom) gravel layers and flow reduction during the sixth filtration run of DyGF-C already included in figure 3.13. The headlosses in figure 3.14 correspond to each measured flow; in other words, they have not been adjusted to any particular flow value. Then the filtered flow (Q f ) reduction explains the headloss reduction in the intermediate and bottom (lower) gravel layers during the 2 nd half of the filter run. The plotted results also show that headlosses in the lower gravel layers hardly increase during the 1 st half of the filter run when Q f was fairly constant. In contrast, headlosses gradually concentrate in the top gravel layer made of filter media in the smallest size range of those used to pack the DyGF unit (table 3.1). Therefore, partialcleaning activities with surface raking complemented with bottom drainage, allowed the filtration capacity of the DyGF unit to be recovered. These results seem to support the stratified gravel bed option in this filtration stage. 100 1 Headloss (cm) 10 1 0.1 0.01 Flow (ls -1 ) 0.1 119 120 121 122 123 124 126 Running time (days) Top. Layer Intermediate and bottom layer (cm) Filtered flow (ls ) -1 -1 0.001 Headlosses correspond to each flow measurement. They are not normalised. Figure 3.14 Headloss development in the top and lower (intermediate and bottom) gravel layers in DyGF-C during the filter run covering the period April 29-May 06, 1991. Peaks of SS and DyGF. Reduction of SS and turbidity by the DyGF units as represented by mean values is also accompanied by smaller values of their corresponding SD (table 3.8). Besides, DyGF units contribute to reduce the impact of the SS and turbidity peaks (maximum values) presented in the raw water during the test periods. Results show a major reduction of 97

these peaks when filtration rates are lower. Considering their high capacity to reduce SS (mean and peak values), it seems that this strength of DyGF units could play a major role in the integrated water treatment concept (section 1.2), to protect subsequent treatment stages and to improve the overall treatment plant performance. This protection capacity is further improved when the filtered effluent of this treatment stage is allowed to decline, particularly during an abrupt water quality change, as shown in the following section. Protection capacity of DyGF units. The “ dynamic” operational characteristic of DyGF units allows a double protection effect for the subsequent treatment stages. In fact, besides the effect due to the removal of contaminants in the gravel bed, there is a 2 nd effect due to flow reduction through the filter bed. The combination of these two effects constitutes the “protection capacity” of the DyGF units. This is illustrated with experimental data from the sixth filtration run of DyGF-C included in figure 3.13. During this run, raw water and filtered water turbidities were in the ranges of 73 to 600 and 35 to 220 NTU respectively, meanwhile the filtered flow declined from 0.44 to 0.0 ls -1 , as further illustrated in figure 3.15. The following equation is proposed to estimate protection capacity with DyGF units P = 1 – (C f /C i ) R Q (3.5) In which P is the protection capacity, C i and C f are the contaminant concentrations in the influent (Q i ) and filtered effluent (Q f ) respectively, and R Q is the ratio between Q f and the filtered effluent at the beginning of the filtration run or maximum filtered flow Q fm . 100 80 Fraction (%) 60 40 20 0 0 1 2 3 4 5 6 7 8 9 Runing days Ci / Cimax RQ P Running Turbidity (UNT) Flow Day C i C f Q f (m 3 h -1 ) R Q (%) P (%) (1-C i / C f )*R Q 1 82 40 1.58 100 51.2 2 73 50 1.58 100 31.1 3 105 65 1.51 95.5 40.9 4 277 183 1.44 90.9 39.9 5 75 45 1.15 72.7 56.4 6 163 134 0.11 6.8 94.4 7 600 220 0.02 1.4 99.5 8 300 35 0.01 0.7 99.9 Figure 3.15. Example of protection capacity (P) due to cumulative effect of turbidity and flow reduction in DyGF-C. April 29-May 06, 1991. 98

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