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Ultrafiltration provides the end user with the ability to consistently and efficiently remove suspended solids, bacteria, yeast, many viruses, and harmful biological contaminants such as Giardia Lamblia and Cryptosporidium from feed water. Typical fiber material is dependant on the manufacturer and application, but can be commonly found in PVDF (Polyvinylidenefluoride), Polyacrylonitrile or Polyethylene. Material choice can be dictated by the Molecular Weight Cutoff of the membrane material. A membrane containing a smaller MWCO provides better overall filtration.

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There are a multitude of applications for ultrafiltration ranging from pretreatment for the offshore industry to the pharmaceutical industry. This filtration method is commonly used at municipal drinking and wastewater treatment plants as both a pretreatment and polishing mechanism.

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Ultrafiltration can be very cost effective in comparison to multi-media and cartridge filtration systems due to having the ability to regularly clean the membrane and continue service for much longer periods of time. Filter cartridges typically must be replaced quite often depending on the level of suspended solids and do not provide the level of filtration equal to ultrafiltration. This lack of filtration can lead to more frequent downstream equipment maintenance due to faster foulant build-up.

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A typical ultrafiltration system consists of a skid frame containing racks that can hold several membranes with valves and instruments to help monitor system properties such as pressure, flow, and backwash frequency. Typical feed pressures for ultrafiltration systems are from 30 to 50 psi. Filtration can occur in one of two ways with a hollow fiber membrane: outside to in, or inside to out. Ultrafiltration systems are usually operated in a cross flow mode so a small amount of concentrated dirty water is continually sent to waste.

The total amount of filtrate (filtered water) that the membrane makes each day per square foot of membrane surface area is know as flux. Flux is measured in gallons of filtrate per square foot of membrane surface per day. Membrane manufacturers have a ceiling value for the acceptable flux range of any certain element.

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While a ultrafiltration system is in operation, it is often flushed and backwashed to remove deposits from the fiber surfaces. However, the system will eventually need to be cleaned to caked solids that flushing and backwashing can no longer remove. This returns the system to its most effective level of performance.

The main parameter used to show the effectivity and operational cleanliness of a system is transmembrane pressure, or TMP.When this pressure rises to 30 psi or higher, the suspended solids and other impurities in the water have "caked" on the membrane surface. The loss in pressure as the water moves across the membrane surface exemplifies the level of this caked material. The pores in the membrane wall are effectively clogged and filtrate flow is drastically reduced when the TMP has risen to the upper end of the allowable range.

At this point the unit can be backwashed with permeate for a specified amount of time and at a particular flow rate. This basically works in reverse of the normal flow path and forces filtrate at a higher flow rate back through the pores, dislodging the cake on the membrane surface. The resulting freed particulate is sent to drain.

To further aid in cleaning the membrane surface, a pressurized air scour can also be utilized to help dislodge additional caked-on material. Low-pressure air is injected into the feed flow at the normal flow rate. This air helps to agitate the membrane fibers, further loosening any remaining cake. Filtrate is not made during this scouring. The air scour can be performed on a less frequent basis depending on how severe the turbidity of the water is.

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Although backwashing and air scouring cleans the membranes effectively, there comes a point when it will take a chemical cleaning to restore the membranes to their normal filtration capabilities. Here, a closed loop is formed and cleaning chemicals are circulated through the system. The filtrate is dumped to drain and the reject is recirculated back into the cleaning tank.

The cleaning is stopped, the unit is put back into normal operating mode, and all filtrate is dumped to drain for a set amount of time. Normal operation can then be resumed and the new TMP should read near the level of a new membrane. When backwashing, air scouring and chemical cleaning will no longer reduce the TMP, the membrane is considered to be fouled and must be replaced.

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There are several factors that affect the cleaning frequency for a membrane-based system. The main ones are operational recovery and the filtrate flux rate. These system factors are optimized to produce the maximum amount of filtrate with the smallest possible cleaning frequency per the end user's needs.

As a rule of thumb, the higher the system recovery and flux rate, the shorter the productive run time will be. Subsequent cleaning frequency will also increase. By increasing the reject rate, you can allow more of the concentrated, rejected material to be removed via the higher flow rate.

In doing so, it will take a longer time to grow a large film of reject material on the membrane surface, thus improving the membrane's productive life between cleanings. Operational recovery is the ratio of filtrate made to the feed flow rate expressed as a percentage. Therefore, if the recovery is raised, permeate output increases accordingly, as does the flux rate.