Water Purification

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by Alexander PERVEYEV, Director of Aquaros Ltd., and Yelena BORISOVA, expert, FiD Company

No living creature, man including, can do without water. It's an absolute fact. Recall that water makes up as much as eighty percent of the human organism. But gone are the times when the water of rivers, lakes and other bodies was potable without special treatment-beware of pollution! The more so as this liquid is consumed on a mass scale by all the various industries. Most of the water available today is contaminated by every kind of pollutants (chemical, radioactive agents and so on) present in industrial effluents. That is why high- performance water purification is a must; what we need are low-cost and ecologically safe methods of purification.

The quality of water is a function of two interdependent characteristics. First, the concentration of salts in grams per liter. This is a good indicator if large amounts of minerals are present - from 1 g/l to 40 g/l and more, which is very dirty water. Second, the electric resistance of water. This is a generalized parameter of specific resistance (measured in ohms, kQ or MQ x cm): the higher the resistance, the lower concentration of impurities. As we have just said, these two characteristics are interdependent; for example, the value 200-250 kQ x cm is grossly correlated with the presence of 1 mg/l of salts. The ideally clean water correlates with 18 MQ x cm, a value obtained after high-degree purification.

Today we know of four purification methods assessed by the purity of the end liquid.

One of the best-tested methods is that of back (reverse) osmosis * . This technique works as follows: a definite amount of water (with a salt concentration of up to 40 g/l) is pumped under high pressure through a tube, or pipe. A double distillate (resistance equal to 200-250 kQ x cm) permeates across the walls of the tube; the thus distilled water is used in food and chemical industries, in medicine, electronics and so forth, with the impurities remaining in the effluents.

This technique enables high-performance purification by eliminating mechanical particles, organic impurities and toxic agents. Yet the service life of the filters used is rather short, not above fifteen months. Another drawback: nearly half of the purified water goes into effluents, i.e. the efficiency of such units is a mere 50 to 60 percent.

Another common method is that of distillation. Its advantage: simplicity and efficiency (the purity of the liquid thus obtained is characterized by the resistance 100 kQ x cm). However, it is a very energy-consuming technique (over 0.7 kW/l); and then the operating unit builds scale rather fast, while descaling is quite laborious and costly.

The third technique involves the use of ionites, or ion exchangers ** . Applying this method, moderately mineralized water under 4 g/l is purified with much efficiency (18 MQ x cm). And only 3 to 5 percent of the stock water is lost in the process. For all these merits, the ionite technique is not ecology- friendly: the ionite resins have to be regenerated now and again with the use of harmful chemicals; besides, the purification units are too sophisticated for commercial production.

* Osmosis-transfer through a permeable membrane of a more concentrated solution toward a less concentrated one.- Ed.

** Ionites (ion exchangers)-hard insoluble resins capable of exchanging ions with those in the ambient medium.- Ed.

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And last, electrodialysis. Most widely used in our country, this technique of water purification is gaining ever broader application elsewhere. It involves a three-chamber electrodialytic unit in which the space between the anode and the cathode is partitioned by two membranes into three parts (the membranes are cationic and anionic respectively). Direct current is used as a power source. As power is on, cations of the potassium, sodium and magnesium contained in the water start moving from the middle chamber to the cathode (it is a one-way traffic secured by a proper membrane), while the anions of other harmful substances, such as chlorine and bicarbonate sulfates, are transported toward the anode. The water in the middle chamber is cleansed from pollutants (metal hydroxides, acids) eliminated in the two other chambers.

Heterogeneous partitioning membranes are the main elements of the unit. They are formed into sheets from a mixture of ionite and polyethylene powders, and are reinforced with kapron or lavsan mesh.

Using this technique, we can purify dirty water (12 g/l) to actually ideal conditions (18 MQ x cm); true, it should be pretreated before such purification. Say, the presence of sluggish ions of some metals (iron, aluminum, manganese) or colloidal particles (clay, sand, hydroxides of heavy metals, microorganisms, putrid organic matter) contributes to a significant increase in the electrical resistance of membranes and thus brings down the efficiency of the process. That is why for stable performance of an electrodialytic apparatus the concentration of iron in the water should not exceed 50 mg/kg, that of manganese - 50, and of suspended particles - 2 mg/kg.

The Aquaros company, founded in 1994, specializes in the development and manufacture of this type of apparatuses. Two units designed and built by this firm - the electrodialytic (EDA) and the electroionite (EIA) apparatus - are the world's unique. The former, EDA, cleanses softened water (without hardness-of-water salts) to a specific resistance of 20-250 kQ x cm; and the latter, EIA, brings this indicator to 2-16 MQ x cm.

Both apparatuses are unipolar units of continuous action that do not let hydrogen, oxygen and chlorine escape into the environment. For technical reasons the previous models had to be reswitched periodically from the plus to the minus sign and conversely. But this is not needed in the modified versions, a factor that makes the whole process much easier. Besides, considerable parsimony of energy (just 0.5 W/l against 3-10 W/l in the conventional units) and of purified water (a mere 5 percent of the original volume is lost against 30-40 percent before) is achieved thereby. The rated service life of EDA and EIA is more than ten years. Both are protected by patents.

These two units boast of other advantages too: they can be employed both autonomously and in complexes - say, in purification lines, making it possible to obtain water of desired quality. High-degree purification is one such important technique.

First, ordinary tap water is cleansed of mechanical impurities by means of a filter with 10-20 mkm pores. Then the liquid is decontaminated by an ultraviolet radiation source, UVR-1 (UFO-1 in Russian transcription), developed by us. Thereupon organic impurities are removed with the aid of special filters. These enable multiple regeneration through annealing at temperature 300 o C; the purification factor is 98 percent. This done, the water is softened, i.e. sodium and chlorine are substituted for hardness-of-water salts (rather, their ions). At this stage EDA and EIA are turned on to bring the purification factor to 99.9 percent. Finally, the water is passed through a stack of fine filters (with pores 1-2 mkm in diameter) and checked on purity with the use of a conductivity apparatus equipped with a built-in processor likewise developed by us.

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The unit can operate in the automatic mode too, it is easy to run and to maintain. The dimensions of EDA and EIA modules with an output of 250 l/h are nearly identical - 630(620)x430x330 mm.

To up the efficiency of the unit and bring down its energy consumption, it is best to assemble it from modules. Demineralization and concentration chambers are connected in parallel or in series with the water flow. This way one can build a high-performance unit multiple of 250 l/h.

A combined employment of various units is effective for purification of industrial and domestic sewage alike. The degree of purification can be regulated - from 1 g/l (what is needed for fish farming) to 18 MQ x cm (high-degree purification) for medicine and microelectronics. Our Aquaros LTD has supplied the design and equipment for a purification complex built by the Angstrom Company (at Zelenograd near Moscow) for the treatment of industrial sewage with a capacity of 16 m/h. This purification complex is used by an enterprise involved with the production of microchips and it can get back as much as 90 percent of recycled water for reuse.

Electrodialysis of water has many merits other than economy and longer service life. We can dispense with chemical reagents, and then the hydraulic system is quite simple, it needs no high pressure and fine pretreatment. And last but not least, a high percentage of water is recovered for reuse.

Something very important just as well: all the materials, modules and parts used in the above apparatuses are made in Russia.

Prepared by Arkady MALTSEV

Опубликовано 09 сентября 2018 года

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© A. Perveyev, Ye. Borisova • Публикатор (): БЦБ LIBRARY.BY

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