February 15, 2003

One of the fast growing disciplines, and an exciting area of science with application in water desalination technology is membrane technology. This proposition, though a little cumbersome, is not as uneconomical as it was thought earlier, and it has several benefits.

When process engineers undertake separating effluent streams, clarifying or fractionating, and when they demand reliable and repeatable performance, memberance filtration systems are often their first choice. Because of energy costs and maintenance difficulties associated with evaporation methods, membrane technology proves to be a more practical approach.

Membrane treatment essentially involves separation of the components of the pressurized fluid by a semi-permeable membrane, one based on cross flow filtration and one on electrodialysis, both of which produce less expensive separation than evaporation. Hence choosing the most suitable membrance for an application is of vital importance — the nature of the process fluid being one of the main deciding factors when selecting which membrane is right to use.

Membrane technology has developed both in the way membranes are packaged and in the type of materials used. The result is a wide range of module configuration and membrane gemoetries suited to a variety of applications.

Spiral-wound membranes are grouped in a variety of ways (series, parallel, one or two passes, reject-staged, etc) to form a complete system. The chemistry of the influent feed water determines the maximum salt concentration possible, hence the water recovery rate. HF elements used for UF comprise enclosures housing hundreds of fibres. Fibre bundle are oriented in parallel, and plotted at both ends to separate product water permeate from feed water. This provides high packing density and fair resistance to channel blockings and feed flows through fibre interiors and permeate is collected on the exteriors, so cleaning may be possible by back-washing. A major limitation is low inherent fibre strength, limiting transmembrane pressure to 25psi.

The hollow fibres used in RO may be as thin as hair. These hollow fine fibres provide much greater total surface area, although their advantage is offset by their lower inherent flux.

Because of the latter, designs place the feed streams on the outer side of the fibres, the product permeating to the interiors. Poor fluid dyanmics and plugging potential necessitate a very high degree of pretreatment.

1. The filtration spectrum starts with the smallest molecular level with reverse osmosis. This provides the finest degree of separation followed by nano filtration, ultra filtration and micro filtration. Between them, these processes separate particles that differ in size from a few angstroms (10-10m) up to a few microns (10-6m). The internal pressure range from 1-5 bar in low pressure MF units to 30-50 bar in high pressure systems. The filtration process relies on this pressure forcing the liquid through a physical barrier, i.e., the membrane.

The separation of suspended and dissolved material in the incoming feed produces the desired concentrated end-product. By choosing the correct configuration, specific sized particles can be isolated or allowed to permeate through membrane, according to the membrane type.

2. RO uses a light membrane that holds back most dissolved ions including molecules and ionic salts. The pressure in this process is greater than the osmotic pressure, thereby forcing the liquid fraction across the semi permeable membrane. RO membrane classes are cellulosic and non-cellulosic. The former are made of either cellulose acetate or triacetate cellulose or a blend of the two. They are low in cost but feed water must be dechlorinated to prevent biodegradation and held in a narrow pH range to avoid hydrolysis in alkaline solutions.

Non-cellulosic membrane are formed from a variety of chemical polymers and have relatively wide pH compatibility. Most common group is aromatic polyamide membrane, which has the advantages of high rejection capability for salts and organic and resistance to bio-degradation. Aromatic polyamide membranes are also attacked by oxidizing agents such as ozone, chlorine, bromine, iodine and permanganate, etc, so that feed water streams must be dechlorinated.

Polymeric membrane account for the biggest preparation of membrane currently in use. Several types of polymers are available to suit the molecular cut-off required, or to achieve the desired resistance to fouling or performance when in contact with specific fluids. Common polymers include polysulphone and polyethersulphone which are used for full range of UF membranes.

Polyvinylidene fluoride is often used for open UF membranes, and polyamide used as thin film membrane layer in NF and RO membranes. Cellulose acetate, the first used polymer, still used in certain applications where it exhibits superior fouling characteristics, though its use is limited due to its tendency to hydrolyze in alkaline solution.

Although membrane systems for the separation of liquid from solids have grown in popularity over the last 20 years, the technology has an inherent flaw — the membrane fouling. The long-term loss in through-put capacity is due primarily to formation of a boundary layer that builds up on the surface during filtration. In addition to cutting down on the flux performance of the membrane, this gel layer acts as a secondary membrane reducing the selectivity of the original one.

In order to help to minimize this boundary layer build-up, membrane designers use a method known as tangential flow or cross flow filtration that relies on high velocity fluid flow being pumped across the membrane surface to minimize boundary formation. However, it is not economical to create shear forces greater than 10000-15000/second, which limits the use of these systems to low velocity fluids. In addition, increased cross flow velocities results in a significant pressure drop near the filter outlet, which leads to premature fouling that eventually spreads to words the front of the filter, causing the permeate rates to drop to unacceptably low levels.

Now according to new logic international, it has developed an alternative method for intense shear waves at the surface of a membrane that not only prevents the boundary layer build-up, but also allows membranes to process liquids that have a much higher solid content.

Cleaning the membrane: The cleaning of membranes is an important part of optimizing a system performance. The type and frequency of cleaning is a function of both the membrane and the process fluid. In the food industry, for example, it is common practice to clean membrane once daily, whilst in certain water applications, cleaning may only be necessary on every or/after three months or so. The most widely applied cleaning technique is to circulate an appropriate chemical solution around the plant at low pressure to remove the soils from the membrane surface. Acids are used for removal of mineral foulants, while caustic detergents or enzymes detergents remove proteinaceous soil. Oxidizing agents such as sodium hypochlorite can be used to remove organic fouling.

A non-ionic aqueous cleaner is recently introduced that gently and effectively cleans filter membranes. This non-ionic cleaner is pH neutral when mixed with water at (2%w/w). It can remove a variety of soil from different surfaces. The cleaner, designed for critical cleaning, is self rinsing so it can accommodate situations where residue from a cleaner could interfere with the operation of apart.

The performance specifications for any membrane are nominal values and individual element permeate flows may vary ± 15% from the nominal values. In seawater desalination process, the rejection limits for chloride of high rejection premium. RO membrane model TCF 2822SS (polyamide), is 99.8% minimum. Other operating limits for pressure, temperature are also will defined.

TFC-SS membrane may be invariably fouled if exposed to cation (positively charged) polymers or surfactants. Exposure to these chemicals during operations or cleaning is not recommended. For the elements loading use of silicon lubricant (or approved equivalent), water or glycerin to lubricate o-rings or brine seals is recommended. The use of petroleum based lubricants or vegetable based oils may damage the elements.

The combination reverse osmosis/ion exchange water demineralization (RO/IX) system can provide a practical and economical solution to many water treatment problems. In those situations where it can be applied, it provides highly visible benefits.

The chemical usage and resulting water load of an ion exchange system are directly related to the influent TDS. So any reduction in TDS will reduce the cost and waste. By removing 90% or more of the TDS in the waste before it enters an ion exchange system, the user of combination RO/IX system can expect an approximate 90% reduction in both regenerant chemicals and the resulting regeneratant waste.

Where RO is added in front of an existing ion exchange system, the large runs that come from reduced TDS influent to the IX system will result in better product water quality and reduced nee for standby deminrelizers to handle the job while the main system is down for regeneration.

The ion exchanger size under these new facilities, may be reduced considerably. Waste levels are appreciably reduced when the roughing demineralization is done by reverse osmosis. Since the chemical regenerants for the ion exchange system are minimized, a substantial percent of the dissolved solids is kept out of the waste streams.

The writer is a scientist specializing in chemistry control of water

Please Visit our Sponsor (Ads open in separate window)