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Water treatment

The experimental data

The experimental data used to produce figure 3.15 show that turbidity reduction in the following treatment stages can be as high as 100% in the case of turbidity peaks thanks to the turbidity removal in the gravel bed combined with the effect of declining filtered flow. Based on data included in figure 3.13 and using equation 3.2, P values at the end of the filtration runs (without including the last ones with the highest values of R Q ) were in the range of 67 to 73% and 57 to 93% for DyGF units A and C respectively. O&M guidelines for a treatment plant with DyGF units (two being the minimum recommended number) should include criteria to deal with this concept of protection capacity, especially during the rainy seasons. In the Colombian Andean Region, water supply in small systems is usually in the range of 100 to 200 ld -1 per person. In situations like this, the treatment plant could work at declining rate for some hours or exceptionally few days without putting at risk public health or having social conflicts due to quantity requirements. The potential of this technological concept to reduce capital and running costs seems to be high in mountainous areas, with small rivers having abrupt changes in SS concentrations. In this situation the flow reduction at the DyGF stage, could represent better treatment efficiencies and lower O&M requirements in the following bigger and more costly treatment stages. It is important to be aware that CGF lines included in the MSF pilot system did not benefit from the declining rate filtration effect of DyGF since they were set to work at constant filtration rates in this study. 3.2.2.4 Total cleaning cycle lengths and productivity in DyGF units. Total cleaning activities summarised in table 3.2 include excavation, washing, and repositioning of gravel bed layers. These activities are labour demanding and may play a significant role in the acceptance and sustainability of this treatment stage at local level. Productivity is understood here as the amount of filtered water produced per m 2 of surface filtering area through a total cleaning cycle. Table 3.12 presents examples of total cleaning cycle lengths and productivity for each testing period and DyGF unit. Table 3.12 Total cleaning cycle lengths and productivity in DyGF units Total cleaning cycles Dates Length (days) Turbidity (NTU) Filtration rate (mh -1 ) Run lengths (days) Mean ± SD Mean ± SD Mean ± SD Total cycle productivity Mean daily (m 3 /m 2 /d) Mean total (m 3 /m 2 ) DyGF A May 6 – July 9 /1991 65 113 ± 85 0.93 ± 0.13 5.3 ± 1.03 22.5 1,505 March 8 – May 16 /1992 71 51 ± 40 1.93 ± 0.08 3.1 ± 0.24 45.4 3,321 October 13/1992 – January 23/1993 104 72 ± 62 1.88 ± 0.19 2.9 ± 0.19 44.1 4,724 DyGF B May 1 – July 10/1991 71 119 ± 89 1.35 ± 0.22 5.3 ± 1.01 32.4 2,245 March 8 – May 16/1992 71 51 ± 40 1.47 ± 0.05 3.0 ± 0.24 35.4 2,477 October 11/1992 – January 26/1993 108 72 ± 62 1.73 ± 0.11 3.0 ± 0.28 41.5 4,435 DyGF C May 1 – July 10/1991 71 119 ± 89 1.41 ± 0.59 5.5 ± 1.15 33.4 2,592 March 8 – May 16/1992 71 51 ± 40 2.68 ± 0.47 3.0 ± 0.17 64.5 4,534 October 10 – December 26/1992 78 64 ± 47 2.61 ± 0.84 3.1 ± 0.49 62.2 4,852 In spite of the fact that the experimental design was not oriented to compare total cleaning cycles the results summarised in table 3.12 seem to suggest that shorter partial cleaning cycles (filtration runs) produce longer total cleaning cycles. This tendency is observed even 99

at the higher mean filtration rates in some of the examples included in table 3.12. In harmony with the previous observations, higher filtration rates and shorter partial cleaning cycles tend to produce higher productivity values during a total cleaning cycle. These observations could be explained for the smaller probability of having high flow reduction (R Q ) and sludge accumulation values in the gravel bed of the DyGF units. 3.2.3 Specific studies on dynamic gravel filtration units Based on these initial results more specific studies were made to improve our understanding and design criteria for DyGF units. They were carried out with the participation of M.Sc. students. Parts of these studies are included in this section. One of them covered research to identify the influence of overflow changes on the performance of DyGF units and the sludge retention distribution over the height of the filter bed working at different filtration rates (Latorre, 1994). Two DyGF pilot units running in parallel with declining rate filtration were organised to investigate this. Both had hydraulic structures and gravel layer specifications similar to those summarised for DyGF-C in table 3.1. The filtering box had the following dimensions: 1.5m long, 0.5m wide, and 0.7m high (0.1m as freeboard). Influence of overflow changes on the performance of DyGF units. During a filtration run in a DyGF unit only part of the influent (Q i ) is filtered (Q f ) and the excess water (Q e ) constitutes the overflow which is usually channelled or piped back to the water source (section 2.8.3 and figure 2.19). To contribute to clarify the role of Q e on the performance of DyGF one Q f value was tested with different values of Q e . Based on practical observations, Q e values, in the range of 2.5 to 9.4 m 3 h -1 (0.7 to 2.6 ls -1 ), produce horizontal surface velocities on the pilot units in the range of 0.05 to 0.2 ms -1 . After these observations, the initial filtration rate (Vf) was kept at about 2.0 mh -1 and the influent (Q i ) was programmed to have values in a range close to 2.5 to 9.4 m 3 h -1 . Several test runs were implemented with the two units working in parallel, until final values of Vf ≤ 0.2 mh -1 ensured high overflow (Q e ) values. The results for all filtration runs but one (2B) showed that a net positive scouring effect of the removed material on top of DyGF units was not taking place, even at the Q e value of 7.3 m 3 h -1 (2.7 ls -1 ). Consistently the SS concentrations were observed to be lower in the overflow in comparison with the influent. The results are summarised in table 3.13. Table 3.13. Performance of DyGF units with similar filtration rates but changing overflow values. Filtering surface area of 0.75 m 2 (Based on Latorre, 1994). Run (filter unit) Raw water SS (mgl -1 ) Mean ± SD Run length (days) Q i Flow (m 3 h -1 ) Q e (overflow) Filtration rate (mh -1 ) (range) Initial Final Initial Final Filtered SS (mgl -1 ) Mean ± SD Overflow SS (mgl -1 ) Mean ± SD 1.A 188 ± 50 5 3.8 – 4.1 2.5 3.7 2.0 0.2 30 ± 17 163 ± 54 1.B 232 ± 50 4 7.1 – 7.3 5.8 7.0 2.1 0.0 36 ± 27 212 ± 36 2.A 229 ± 56 4 5.5 – 5.7 4.0 5.5 2.1 0.0 39 ± 31 196 ± 50 2.B 148 ± 103 4 8.1 – 9.7 7.3 9.7 1.8 0.0 19 ± 15 152 ± 122 3.A 145 ± 111 4 2.3 – 2.6 1.0 2.3 2.0 0.1 18 ± 16 127 ± 108 Testing period: January 4 – April 7 1994. 100

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