What is ultrafiltration of water?

An effective way of purifying water is to push it through semipermeable membranes. The filtration processes are classified according to the size of the particles to be separated:

• Microfiltration through membranes with pore sizes from 0.05 to 10 μm;
• Ultrafiltration - pores from 0.001 μm to 0.05 μm;
• Reverse osmosis and nanofiltration - pores 1 nm and below.

Ultrafiltration of water is designed to remove from it microorganisms and macroscopic inclusions that do not pass through the pores of the membrane.

The traditional mechanism of the action of filters with backfilling is based on gravitational purification. Ultrafiltration is carried out like sifting through a porous screen, where all particles of a larger diameter are separated.

Types of membranes

Filter elements are flat sheets or fibers with capillaries. Through the first is produced mainly ultrafiltration of waste water, and the latter are intended for water treatment.

Fibers are mainly made by single-channel, with an internal diameter of about 0.8 mm. They are subjected to frequent loads and can be destroyed during backwashing. Multichannel fibers contain several capillaries and have much greater strength.

The membranes are made of polymers, for example, polyester sulfone. Its parameters can be changed by adding other synthetic materials. The wide pH range of the liquids being treated makes it possible to efficiently clean the filter elements.

Polymer membranes should be periodically disinfected, because microbes like to eat organic matter and form colonies on it.

Long serves as a ceramic membrane, which is well washed with detergents. Its price is higher, but the service life reaches 10 years.

Methods of filtration

The system of ultrafiltration of water consists of modules filled with hollow porous fibers. The initial liquid enters the capillaries, after which the filtration through the side walls occurs. Feed in the opposite direction is also possible.

Washing is carried out by a filtrate with its submission by a counter flow. Uniform distribution of liquid outside the fibers ensures the removal of deposits from the capillaries. It is important to choose the correct washing mode in order to remove the contaminant layer more easily.

Filters work in two modes, one of which is pressure: water is fed into the casing of the device under pressure. The submerged method is carried out using membranes lowered into an open vessel. On the output side, a vacuum is created and the liquid is sucked through the filter material.

The modules are arranged vertically. The water in them comes from one end, and is diverted from the other. The number of modules in one filter usually does not exceed two units. This requires less gaskets, which reduces the likelihood of leaks. Vertical modules are convenient to maintain and test. They are easy to install and remove.

Filtering Modes

When ultrafiltration of water is performed, the filters can work in deadlock and tangential modes. In the first case, all the supplied water is purified. Deposits from the membrane are periodically removed during the washing process or with a drainage stream. The membrane quickly becomes dirty, and the pressure drop across it must be kept small, which reduces the productivity of the device. The method is used for water treatment, with a small concentration of suspensions.

In the tangential mode, the medium to be filtered circulates along the surface of the membrane and deposits are formed on it a little. The turbulence of the flow in the supply channel makes it possible to purify water with a high concentration of suspended solids. Disadvantages of the method are an increase in energy costs for creating a high flow rate and the need to install additional pipelines.

Parameters of ultrafiltration

The main parameters of ultrafiltration are:

1. Selectivity is the ratio of impurity concentrations in contaminated water ( _Cin ) and in the filtrate (with _outflow ): R = (1 - C _out / C _input ) ∙ 100%. For the process of ultrafiltration, it is large, which allows to retain the smallest particles, including bacteria and viruses.
2. The rate of filtrate is the amount of purified water per unit time.
3. The specific consumption of the filtrate is the amount of product passing through 1 m ^{2 of} the membrane area. Depends on the characteristics of the filter element and the purity of the source water.
4. Differential pressure on the membrane - the difference between the pressure on the supply side and the side of the filtrate.
5. Permeability is the ratio between the specific consumption of the filtrate and the pressure drop across the membrane.
6. Hydraulic efficiency is the ratio between the flow rates of the filtrate and the feed water supplied.

Ultrafiltration for water disinfection

Traditional methods for the removal of microorganisms include technologies using reagents. Ultrafiltration of water consists in the physical separation of microorganisms and colloids from it due to the small pore size of the membrane. The advantage of the method is the removal of corpses of microorganisms, algae, organic substances and mechanical particles. There is no need for special preparation of water, which in other cases is mandatory. It is only necessary to pre-pass it through a 30-micron filter of mechanical cleaning.

When buying filters, you need to determine the pore size of the membranes. To completely remove viruses, the hole diameters should be at the level of 0.005 μm. With a large pore size, the disinfection function will not be performed.

In addition, ultrafiltration technology provides for water clarification. All suspensions are completely removed.

The installation of ultrafiltration of water contains parallel devices, which ensures the necessary process performance and the possibility of replacing them during operation.

Water purification in front of ion-exchange filters

The resin is effective in delaying colloid particles of 0.1-1.0 μm in size, but they quickly clog the granules. Washing and regeneration are of little help here. Particularly difficult to remove particles of SiO ₂ , which are especially numerous in wells and river water. After clogging, the resin begins to build up with microorganisms in places that are not washed with cleaning solutions.

Ionites are also actively clogged with emulsified oils that can not be removed. Clogging is so strong that it is easier to replace the filter than to separate the oil from it.

Filtering resin beads are actively clogged by high-molecular compounds. They are well removed by activated carbon, but it has a short service life.

Ion exchange resins are effective together with ultrafiltration, which removes more than 95% of the colloids.

Purification of water - ultrafiltration before reverse osmosis

Operational costs are reduced with the stepwise installation of filters with a consequent reduction in the size of the delayed particles. If more coarse cleaning is established before the ultrafiltration module, then it improves the efficiency of reverse osmosis systems. The latter are sensitive to anionic and non-ionic flocculants, if in the preliminary stage coagulation of contaminants is carried out.

Large-molecule organics quickly clog the pores of the reverse osmosis membranes. They quickly grow with microorganisms. Pre-ultrafiltration of water solves all problems and is economically feasible when used with reverse osmosis.

Treatment of waste water

Wastewater treatment with ultrafiltration makes it possible to reuse them in industry. They are suitable for use in engineering, and the technogenic load on open reservoirs for drinking purposes is reduced.

Membrane technologies are used for cleaning of electrochemical and textile waste, in the food industry, deironing, removal from carbamide solutions, electrolytes, heavy metal compounds, petroleum products, etc. At the same time, the cleaning efficiency is increased and the technology is simplified.

With a low molecular weight of impurities, ultrafiltration can produce concentrates of pure products.

Especially important is the problem of separating emulsified oils from water. The advantage of membrane technology is the simplicity of the method, low energy consumption and no need for chemicals.

Surface water treatment

Precipitation and filtration were previously effective methods of water purification. Admixtures of natural origin are effectively removed here, but now technogenic pollutants have appeared, for the removal of which other methods of purification are required. Especially many problems are created by the primary chlorination of water, which forms chlororganic compounds. The use of additional stages of cleaning with activated carbon and ozonization increases the cost of water.

Ultrafiltration allows drinking water to be obtained directly from surface sources: algae, microorganisms, suspended particles and other compounds are removed from it. The method is effective with preliminary coagulation. This does not require a long settling, since it is not necessary to form large flakes.

The installation of ultrafiltration of water (photo below) allows to achieve stable good quality of purified water without using complex equipment and reagents.

The use of coagulation methods becomes inefficient, since many organic compounds in water are not determined by the traditional method of oxidation with potassium permanganate. In addition, the content of organic matter varies widely, which makes it difficult to select the required concentration of reagents.

Conclusion

Ultrafiltration of water through membranes makes it possible to achieve its necessary purity with minimal consumption of reagents. Sewage after treatment can be used for industrial purposes.

Ultrafiltration is not always effective. The method does not allow the removal of certain substances, for example, organochlorine compounds and certain humic acids. In such cases, multi-stage cleaning is used.