Conclusion

Three out-to-in microfiltration systems were compared with one another. Zee Weed and CMF-S were submerged systems; the CMF is a pressurised system.

All three systems reached recoveries over 85 % and could produce for at least one week without the need to be chemically cleaned. The filtrate produced with all systems was comparable and free of bacteria.

All MF systems that were tested proved capable of producing a filtrate quality ready to feed a reverse osmosis installation.

The submerged systems showed some advantages over the pressurised system. The footprints are smaller and the concept is less complicated compared to the pressurised air backwash of CMF; a 500nm strainer is no longer needed so coarser strainers could be sufficient. This could mean that investment and maintenance costs would be smaller. As the transmembrane pressures are smaller the initial pressure could be restored very easily. A disadvantage is that submerged systems use filtrate for the backwash that means that the risk of recontamination of those systems is higher. Care should be taken to prevent recontamination of the filtrate. However chloramination not only proved to enhance the performance of CMF5 but also showed to be an efficient way to prevent recontamination of the filtrate.

Using chlorine resistant membranes is an advantage as the risk on damage is smaller and the chlorine and ammonia consumption for bio-fouling prevention could be reduced. However the hydrophobic PP membranes seem to have a slower TMP increase which means that chemical cleanings should be performed less frequently.

Standardisation of the submerged modules, which would make them interchangeable, could in the future benefit to the customers, as the investment risks would be reduced.

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