Results And Discussion

3.1 Process Stability

Figure 1 shows the raw TMP data on the Bethune polder water treatment; results on ARC-water are similar. Each run can be clearly identified by the "saw teeth" in the graph. It is observed that for each operating condition (flux, ferric dosage), different fouling rates exist. In general it is found that fouling rate increases with flux and decreases by addition of ferric chloride to the raw water. Furthermore, it can be seen that most lines are more or less straight filtration curves, indicating the occurrence of 'normal' cake filtration.

Figure 1 shows the raw TMP data on the Bethune polder water treatment; results on ARC-water are similar. Each run can be clearly identified by the "saw teeth" in the graph. It is observed that for each operating condition (flux, ferric dosage), different fouling rates exist. In general it is found that fouling rate increases with flux and decreases by addition of ferric chloride to the raw water. Furthermore, it can be seen that most lines are more or less straight filtration curves, indicating the occurrence of 'normal' cake filtration.

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Figure 1. TMP for the runs on Bethune polder water.

Using these TMP data to calculate the membrane fouling rate and plotting them as function of the applied flux gives good opportunities for optimising the membrane operation. The results of this exercise are shown in Figure 2. It was found that the membrane fouling rate increases exponentially with the applied flux. Figure 2 shows that for ARC-water, the ferric addition reduces the fouling rate by a factor of approximately 2. For the Bethune polder water the effect of ferric addition is less clear, probably because of the natural iroq content of the water.

The optimum flux can be found by selecting a chemical cleaning interval, e.g. 7 days. Using the maximum allowable TMP (180 kPa) and the clean membrane TMP (approx. 40 kPa), one can than calculate the fouling rate (20 kPa/d). Figure 2 than gives the flux, which can be applied: for the ARC a flux of 85 l/m2h and 75 l/m2h can be reached for operation with and without ferric chloride addition respectively. For the BP-water, the flux for stable operation is around 73 l/m2h.

The results shown if Figure 2 indicate that a stable operation with the microfiltration can be achieved at relatively high fluxes and with low chemical demand. With ultrafiltration the same flux levels could be achieved, but with chemically enhanced backwashes every 3 hours and membrane disinfection every 24 hours 1 microfiltration however has higher energy consumption due to high pressure needed for the air back wash. Exact figures for the energy consumption however cannot be given on the basis of this study, because the further optimisation of the air backwash time interval is than necessary2.

Bethune Polder

Amsterdam Rhine Canal

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