Ultrafiltration (UF) and Microfiltration (MF) are pressure driven membrane processes that are used to remove solids from water. Ultrafiltration membranes are slightly “tighter” with a pore size in the range of 0.001 to 0.1 micron whereas microfiltration membranes have a pore size range of 0.1 to 1 micron. Consequently both types of membranes are capable of removing bacteria and cryptosporidium oocysts with ultrafiltration membranes having the additional capability of removing virus.

Neither membrane will remove dissolved salts from the water.

Ultrafiltration and microfiltration membranes have been used on a wide range of different waters. In one case water from a shallow borehole had turbidity spikes following heavy rain and the Water Supply Company concluded that there was a risk that the chlorine resistant oocyst, cryptosporidium, may be present.

Biwater have supplied a 7,000 m3/d microfiltration plant that will remove the turbidity spikes and consequently the risk of cryptosporidium. In some cases the water is too turbid or coloured to treat directly with the membrane. In these cases a small dose of coagulant can greatly improve the performance of the membrane system. The coagulant dose is generally about ¼ of the dose required when a conventional clarification or sand filter is used.

Ultrafiltration or microfiltration has also been used as a pre-treatment stage for reverse osmosis plants as the land area required can be smaller than that required for “conventional” processes. Furthermore the excellent water quality will reduce membrane fouling on the reverse osmosis membranes which results in significant power savings.

UF and MF membranes are incorporated into systems and these can take many different forms. For example the membranes may be incorporated into a pressurised system where a pump drives the water through the membrane at around 1 bar which is housedinside a pressure vessel. Alternatively the membranes may be suspended within a tank and the treated water or permeate is sucked through the membrane using a pump or even a syphon arrangement. This is known as an immersed system and has the advantage that no pressure vessels are required and it may be possible to retrofit a membrane system inside existing concrete structures. Biwater have experience with both pressurised and immersed systems.