An effective way to purify water is to force it through semipermeable membranes. Filtration processes are classified by the size of the particles to be separated:
- microfiltration through membranes with pore sizes from 0.05 to 10 microns;
- ultrafiltration - pores from 0.001 microns to 0.05 microns;
- reverse osmosis and nanofiltration - pores of 1 nm or lower.
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 action of backfilled filters is based on gravity cleaning. Ultrafiltration is done like sieving through a porous sieve, where all particles of a larger diameter are separated.
Membrane types
The filter elements are flat sheets or fibers with capillaries. Through the former, predominantly ultrafiltration of wastewater is carried out, and the latter are intended for water treatment.
Fibers are mainly single-channel, with an inner diameter of about 0.8 mm. They are subjected to frequent loads and can be destroyed by backwashing. Multichannel fibers contain several capillaries and have much greater strength.
Membranes are made of polymers, for example, polyester sulfone. Its parameters can be changed due to the addition of other synthetic materials. The wide pH range of the processed fluids makes it possible to efficiently clean the filter elements.
Polymeric membranes should be periodically disinfected, since microbes like to eat organics and form colonies on it.
The ceramic membrane, which is well washed with detergents, serves for a long time. Its price is higher, but the service life reaches 10 years.
Filtering methods
The ultrafiltration system of water consists of modules filled with hollow porous fibers. The initial liquid enters the capillaries, after which filtration occurs through the side walls. Reverse feed is also possible.
Rinsing is carried out by filtrate with its supply in the opposite stream. A uniform distribution of fluid from the outside of the fibers ensures removal of deposits from the capillaries. It is important to choose the right flushing mode so that the contaminant layer is more easily removed.
Filters operate in two modes, one of which is pressure head: water is supplied to the device casing under pressure. The submersible method is carried out using membranes lowered into an open container. A vacuum is created from the outlet side and the liquid is sucked through the filter material.
Modules are arranged vertically. Water enters them from one end, and is diverted from the other. The number of modules in one filter usually does not exceed two units. Due to this, less gaskets are required, 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 operate in deadlock and tangential modes. In the first case, all supplied water is purified. Deposits from the membrane are periodically removed during washing or with a drainage stream. The membrane is quickly contaminated, and the pressure drop across it should be kept small, which reduces the performance of the apparatus. The method is used for water treatment, with a small concentration of suspensions.
In the tangential mode, the filtered medium circulates along the membrane surface and a little is formed on it. The turbulence of the flow in the feed channel allows you to purify water with a high concentration of suspended matter. The disadvantages of the method are the increase in energy consumption to create a high flow rate and the need to install additional pipelines.
Ultrafiltration options
The main parameters of ultrafiltration are:
- Selectivity is the ratio of the concentrations of impurities in polluted water (C in ) and in the filtrate (C out ): R = (1 - C out / C in ) ∙ 100%. For the ultrafiltration process, it is large, which allows you to retain the smallest particles, including bacteria and viruses.
- The filtrate consumption is the amount of purified water per unit time.
- Specific filtrate consumption is the amount of product passing through 1 m 2 of membrane area. Depends on the characteristics of the filter element and the purity of the source water.
- The pressure drop across the membrane is the difference between the pressure on the supply side and the filtrate side.
- Permeability - the ratio between the specific flow rate of the filtrate and the pressure drop across the membrane.
- Hydraulic efficiency - the ratio between the flow rate of the filtrate and the supplied source water.
Ultrafiltration for water disinfection
Traditional methods for removing microorganisms include reagent technology. 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 this method is the removal of the corpses of microorganisms, algae, organic substances and mechanical particles. Moreover, there is no need for special water treatment, which in other cases is mandatory. It is only required to first pass it through a 30-micron filter for mechanical cleaning.
When buying filters, it is necessary to determine the pore sizes of the membranes. To completely remove viruses, the diameters of the holes should be at the level of 0.005 microns. With large pore sizes, the disinfection function will not be performed.
In addition, ultrafiltration technology provides clarification of water. All suspensions are completely removed.
The ultrafiltration water installation contains devices connected in parallel, which ensures the necessary process performance and the possibility of their replacement during operation.
Water purification before ion-exchange filters
The resin is effective in retaining colloidal particles with a size of 0.1-1.0 microns, but they quickly clog the granules. Flushing and regeneration do not help much here. It is especially difficult to remove SiO 2 particles, which are especially abundant in wells and river water. After clogging, the resin begins to grow with microorganisms in places not washed with detergent solutions.
Ionites are also actively clogged with emulsified oils that cannot be removed. Corking is so strong that it is easier to replace the filter than to separate the oil from it.
The filtering granules of resins are actively clogged with high molecular weight compounds. Activated carbon removes them well, but it has a short service life.
Ion exchange resins are effective together with ultrafiltration, which removes more than 95% of colloids.
Water purification - ultrafiltration before reverse osmosis
Operating costs are reduced by stepwise installation of filters with a sequential reduction in the size of the trapped particles. If a coarser cleaning is installed in front of the ultrafiltration module, then it increases the efficiency of reverse osmosis systems. The latter are sensitive to anionic and nonionic flocculants if contamination is coagulated at the preliminary stage.
Large-molecular organics quickly clog pores of reverse osmosis membranes. They quickly become overgrown with microorganisms. Preliminary ultrafiltration of water solves all problems and is economically feasible when used with reverse osmosis.
Sewage treatment
Ultrafiltration wastewater treatment makes it possible to reuse them in industry. They are suitable for use in technology, and the technogenic load on open reservoirs for drinking purposes is reduced.
Membrane technologies are used for wastewater treatment of galvanic and textile production, in the food industry, iron removal systems, when carbamide, electrolytes, heavy metal compounds, oil products are removed from solutions. At the same time, the cleaning efficiency is increased and the technology is simplified.
With a low molecular weight of impurities by ultrafiltration, concentrates of pure products can be obtained.
Especially important is the problem of separation of emulsified oils from water. The advantage of membrane technology is the simplicity of the method, low energy consumption and the absence of the need for chemicals.
Surface Water Treatment
Precipitation and filtration were previously effective ways of purifying water. Impurities of natural origin are efficiently removed here, but technogenic pollutants have now appeared, the removal of which requires other cleaning methods. The primary chlorination of water, which forms organochlorine compounds, creates especially many problems. The use of additional purification stages with activated carbon and ozonation increases the cost of water.
Ultrafiltration allows you to get drinking water directly from surface sources: algae, microorganisms, suspended particles and other compounds are removed from it. The method is effective with preliminary coagulation. At the same time, prolonged settling is not required, since the formation of large flakes is not mandatory.
The ultrafiltration of water (photo below) allows you to achieve a consistently good quality of purified water without the use of sophisticated equipment and reagents.
The use of coagulation methods becomes ineffective, since many organic compounds in water are not determined by the traditional method of oxidation with potassium permanganate. In addition, the organic content varies widely, which makes it difficult to select the required concentration of reagents.
Conclusion
Ultrafiltration of water through membranes allows to achieve its necessary purity with a minimum consumption of reagents. After treatment, waste water 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 some humic acids. In such cases, multi-stage cleaning is used.