Cleaning microfiltration water treatment

Eor water-treatment processes such as drinking water or potable water production, reverse osmosis (desalination), nanofiltration, and ultrafiltration are mainly used. In these processes often a microfiltration stage is implemented as the first cleaning stage for the removal of dissolved organic matter, colloids and particles from the source. 

The water to be filtered by these water-treatment plants is relatively clean, so a microfiltration system called semi-dead-end filtration is often used. In these systems, the membrane unit is operated as an inline (dead-end) filter until the pressure required to maintain a useful flow across the filter reaches its maximum level. At this point, the filter is operated in a crossflow mode, while concurrently backflushing with air or permeate solution. After a short period of backflushing in crossflow mode to remove material deposited on the membrane, the system is switched back to dead-end operation. This procedure is particularly applicable in microfiltration units used as a final bacterial and virus filter for municipal water treatment plants. A photograph of a municipal microfiltration plant of this type is shown in Fig. 7.5. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

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