Ultrafiltration (UF) Systems | Ultrafiltration Plants

Technology Overview

Ultrafiltration is a distinctive pressure-driven physical separation process in which the water is passed through porous (partially permeable) UF membranes with pore sizes in the range of 0.01 to 0.10 micron and with a molecular weight cut-offs from 10,000 to 2,00,000 daltons at an operating pressure of approximately 30 – 100 psi. These membranes are either in the form of long hollow fibres or spiral wound with both types of filtration modes. ie. Inside-out and outside-in

The pore size of the UF membranes is much larger than the porosity of RO membranes. Typically, UF membranes are designed to remove particulate matters, suspended solids, turbidity, colloidal materials, silt, high molecular-weight substances, organic / inorganic polymeric molecules, pollen, algae, and all microorganisms like parasites, bacteria, cryptosporidium, giardia lamblia, viruses, endotoxins and other pathogens.

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

UF systems are very efficient and eliminate the use of conventional filtration plants, chemical agents for coagulation, flocculation and in some cases carbon and sand filters. UF also removes colloidal silica and other colloids that cause fouling, scaling and poor performance of the treatment plant/ equipment.

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 Ultrafiltration membrane.

Flux of a membrane is defined as the amount of permeate produced per unit area 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 10-30 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 pre-treatment requirement for UF normally is quite low. Fortunately, due to the chemical and hydrolytic stability of UF membrane materials, some of the pre-treatment essential for RO membranes 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.

System Comparisons

While all types of UF membranes work well under prescribed conditions, choosing the most appropriate membrane for a specific application still remains crucial. In many cases, selection is complicated by the availability of new types of membranes, complex applications or site-specific conditions. Bench and pilot tests are powerful tools for situations where process risks and uncertainties exist or the cost impacts from problems are potentially high.

Membrane classification standards vary considerably from one filter supplier to another. What one supplier sells as a UF membrane, another manufacturer calls a NF membrane. It is better to look directly at pore size, molecular weight cut-off (MWCO) and applied pressure needed when comparing two membrane systems. MWCO (a measure of membrane pore dimensions) is a specification used by membrane suppliers to describe a UF membrane’s retention capabilities.

Ultrafiltration vs Conventional Filtration

Ultrafiltration, like reverse osmosis, is a cross-flow separation process. Here liquid stream to be treated (feed) flows tangentially along the membrane surface, thereby producing two streams. The stream of liquid that comes through the membrane is called permeate. The type and amount of constituents left in the permeate will depend on the characteristics of the UF membrane, the operating

conditions, and the quality of feed. The other liquid stream is called concentrate and gets progressively concentrated in those species removed by the membrane. In cross-flow separation, therefore, the membrane itself does not act as a collector of ions, molecules, or colloids but merely as a barrier to these species.

Conventional filters such as media filters or cartridge filters, on the other hand, only remove suspended solids by trapping these in the pores of the filter-media. These filters therefore act as depositories of suspended solids and have to be cleaned or replaced frequently. Conventional filters are used in the upstream of UF systems to remove relatively large suspended solids and to let the UF System do the job of removing fine particles and dissolved solids. In ultrafiltration, for many applications, no prefilters are used and ultrafiltration modules concentrate all of the suspended and emulsified materials.

Concentration Polarization

When a membrane is used in the separation process, the concentration of any species being removed is higher near the membrane surface than it is in the bulk of the stream. This condition is known as concentration polarization and exists in all ultrafiltration and reverse osmosis separation processes. The result of concentration polarization is the formation of a boundary layer of substantially higher concentration of substances being removed by the membrane.

The thickness of the layer and its concentration depend on the mass of transfer conditions that exist in the membrane system. Membrane flux and feed flow velocity are both important in controlling the thickness and the concentration in the boundary layer. The boundary layer impedes the flow of water through the membrane and the high concentration of species in the boundary layer produces permeate of inferior quality in ultrafiltration applications relatively high fluid velocities are maintained along the membrane surface to reduce the concentration polarization effect.

Concentration polarisation plays a dominant role in ultrafiltration as compared to microfiltration because of the small pore size membrane.

Ultrafiltration Membrane Modules

Ultrafiltration Membrane modules come in plate-and-frame, spiral-wound, and tubular configurations. All configurations have been used successfully in different process applications.

Each configuration is specially suited for some specific applications and there are many applications where more than one configuration is appropriate. For high purity water, spiral-wound and capillary configurations are generally used. The configuration selected depends on the type and concentration of colloidal material or emulsion. For more concentrated solutions, more open configurations like plate-and-frame and tubular are used.

In all configurations the optimum system design must take into consideration the flow velocity, pressure drop, power consumption, membrane fouling and module cost.

General Applications

• Surface Water Clarification
• Drinking Water Treatment
• Pre-treatment to Reverse Osmosis plants (Sea Water / Surface Water)
• Tertiary treatment to STPs and ETPs
• Waste water Treatment and Recycling Plants
• Industrial Uses - Power Generation, Food & Beverage Processing, Pharmaceutical Production, biotechnology and semiconductor manufacturing, Boiler Feed Water, Cooling Tower Blow down, Protein-Cheese Concentration for Dairy Industry, Oil-Water-Emulsion Separation, Textiles and chemicals, Enzyme Recovery