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ULTRAFILTRATION (UF) Plants for pure water

Theory of Ultra Filtration

Ultra filtration is a unique pressure-driven physical separation process in which the water is passed through porous (partially permeable) membranes with pore sizes in the range of 0.1 to 0.001 micron. The pore size of UF membranes is much larger than the porocity of RO membranes. Typically, UF membranes will remove high molecular-weight substances, colloidal materials, silt, turbidity, organic / inorganic polymeric molecules, pollen, algae, particulate matters, suspended solids and all micro-organisms like parasites, bacteria, cryptosporidium, giardia lamblia, viruses, endotoxins and other pathogens. These membranes are in the form of long hollow fibres. The result is high pure water free from physical and biological contaminations.

UF also provides a means for concentrating bacteria, some proteins, some dyes, and constituents that have a larger molecular weight of greater than 10,000 daltons. However, UF is typically not effective at separating organic streams.

UF systems are most efficient and eliminate the use of conventional treatments, chemical agents for coagulation, flocculation and in some cases carbon and sand filters. UF also removes colloidal silica and other colloids which cause fouling, scaling and poor performance of the plant equipments.

Low molecular-weight organics and ions such as sodium, calcium, magnesium and sulphate are not removed by UF membranes. Since only high-molecular weight species are removed, the osmotic pressure differential across the UF membrane surface is negligible. Low-pressure applications are therefore suitable to achieve high flux rates from an UF membrane. Flux of a membrane is defined as the amount of permeate produced per unit are of membrane surface per unit time. Generally flux is expressed as gallons per square foot per day (GFD) or as cubic meters per square meters per day. UF membranes can have extremely high fluxes but in most practical applications the flux varies between 50 and 200 GFD at an operating pressure of about 50 psig. In contrast, RO membranes only produce between 10 and 30 GFD at 200-400 psig.

The cleaning intervals for the UF are controlled on a load-dependant basis. During the cleaning process, the water flows through the capillaries unobstructed at a high flow speed and in the process removes any deposits from the inner surface. This process may be supported by an additional backflushing process that uses purified water taken from parallel in-line modules. UF hollow fibre membranes are hot water sterilizable, chlorine and alkali tolerant and is highly fouling-resistant due to its hydrophilic nature.

Unlike RO, the pretreatment requirement for UF normally is quite low. Fortunately, due to the chemical and hydrolytic stability of UF membrane materials, some of the pretreatments essential for RO membranes (e.g., adjustment of pH or chlorine concentration levels) do not apply. However, it may be necessary to adjust the pH to decrease the solubility of a solute in the feed so that it may be filtered out apart from pre-screening and macrofiltration.

MORF INDIA offers state-of-the-art skid mounted UF systems with hollow fibre modules having pore sizes in the range of 0.01 - 0.10 microns and with molecular weight cutoffs (MWCO) from 10,000 to 2,00,000 daltons and an operating pressure of approximately 30 to 100 psi. UF modules are sourced from global players like Hydranautics-USA, Norit- Netherlands and GE-ZENON-USA.

Pre-engineered and packaged units come with an auto-cleaning prefilter, raw water pump, recirculation pump, backwash pump, chlorine dosing pump for the backwash water, air compressor (valve actuation), chlorine tank, chemical tank (detergent), programmable logic controller with security sensors.

UF module consists of bundled, tube shaped UF membranes which are poured into an encased tube at both ends. The untreated water flows into the membranes as a result of the types of overall construction used for the module. UF membranes have both type of filtration modes. ie., inside-out and outside-in.

These compact and modular systems are designed according to the requirements of the customers and strictly as per the guidelines stipulated by the membrane manufacturers.

Usage of UF technology results in maximum product recovery, high solid compatibility, consistently low SDI in filtrate, enhanced life of RO membrane due to reduced fouling and cleaning frequency, higher RO membrane flux, smaller plant footprint, lower consumption of operational and cleaning chemicals and lesser energy costs.