Disinfection technology using Chlorine

CONDRILIA offers a modern, innovative, patented technology for disinfection: of drinking water, household and infectious wastewaters, swimming pools, pipelines, etc. — by decomposing the common salt NaCl in the MBE plant to produce active chlorine (Cl2), alkali (Sodium hydroxide (NaOH)) and hydrogen (H₂) by membrane electrolysis directly at the site of the reagent use.

In accordance with developed TS 4859-002-71153463-2003 our company supplies a product range of water disinfection stations based on membranous bipolar electrolyzers of MBE type with the productivity of 1.56, 3.12, 6.25, 12.5 and 25 kg of active chlorine per day.

Here the issue of a rational combination of the positive properties of the known «disinfectant» — active chlorine (98.5%) is solved and the negative moments of this reagent are eliminated. That is, the possible formation of chlorination by-products is shut out.

Using the technology of membrane electrolysis, on the MB electrolyzers for water disinfection, allows you not only to get rid of unwanted organic and biological impurities, but also completely remove dissolved salts of divalent ferrous iron and manganese, destroys phenols (a source of unpleasant odor and taste), improves flocculation and coagulation of impurities, destroys all microorganisms, including cysts, viruses, microbacteria, spores, microbial toxins, provides removal of turbidity from water, removes biofilms from internal surfaces of water supply networks along the entire length, including the most distant point, and prevents their subsequent appearance.

An important advantage of using the membranous bipolar unit MBE in the disinfection of water is the ability to provide microbiological safety of water when it is transported to the user due to the aftereffect.

The technology of water disinfection at MBE stations:

  1. Purification and softening (PVC) of water for chemical reaction (electrolysis).

Water from the fire and utility water pipe (hard water) undergoes the stage of cleaning and softening using ion-exchange resins, where it is purified from the hardness salts. Hardness ions (such as calcium (Ca), magnesium (Mg), iron (Fe), etc. are replaced by Na+ ions) contained in it. After passing through the softening unit, distilled water is obtained, i. e. chemically pure product (popular name — «dead water», which is used only as a reagent for electrolysis). Such water is absolutely undesirable for human consumption because ions of Ca, Mg, Fe and other mineral substances taken away from it, distilled (dead) water compensates them from the human body, which will only harm the latter).

  1. Preparation of extremely saturated sodium chloride solution (NaCl + HO).

The threshold saturation is about 330 grams (NaCl) / 1 liter(НО)

Purified and softened water (chemically pure product (H₂O)) is supplied to storage tanks and solution tanks with table salt (NaCl). In the MBE plants, only «Extra» table salt is used (with a mass fraction of not less than 99.8%), since impurities (≤ 0.2%) of GOST 13830-97 are excluded in its composition. To meet the needs of salt supply for the MBE water disinfection plants should be guided by the specific salt consumption to obtain 1 kg of active chlorine.

Such consumptions amount to 3 kg for 1 kg of active chlorine.

For comparison: according to «Physico-chemical parameters of the table salt without additives», the content of impurities (Ca, Mg, Fe, etc.) in edible salt: of premium, first and second grade, is 1.6% to 4% of mass fraction.

The use of chemically pure reagents (NaCl + H₂O) during electrolysis is of great importance, because in this process free chlorine (Cl₂⁻) is formed — which is the strongest oxidant. Chlorine (Cl₂⁻) is very active — it directly combines with almost all elements of the periodic system, metals and non-metals (except for carbon, nitrogen, oxygen and inert gases). To eliminate unplanned side compounds with chlorine (for example, chlorides, salt chlorates, etc.), our water disinfection process at the MBE plants uses: purified softened water and edible salt Extra (evaporated), which provides the production of chemically pure products (Cl₂⁻) and (NaOH) and also the increase of the life of the sulfacathionite membrane of the electrolyser (MB).

Edible salt is loaded into solution tanks, manually or by means of an automatic feeder. The softened water flows into the solution tanks for the threshold saturation of NaCl (≤ 330 g/liter).

At the same time softened water flows into the storage tanks. The level in the tanks is controlled by floating level sensors. Maximally saturated water (saline solution) is fed to the anode chamber of the MB electrolyser by a dosing pump. In its turn, the cathode space is filled with softened water from the storage tank, using a dosing pump. To prevent the solid phase from getting into the cell of the electrolyser filtering elements are installed in the solution tanks (slot filters).

  1. Method of membrane electrolysis of edible salt NaCl in the MBE plant to produce active chlorine (Cl), alkali (sodium hydroxide (NaOH)) and hydrogen(Н)

Active chlorine (Cl₂⁻) is obtained in membranous bipolar electrolysers (MBE) during the reaction of electrochemical decomposition of the edible salt solution.

The block of electrode elements consists of two monopolar (anode and cathode) elements, forming one electrolytic cell (bipolar) during assembly and installation of membranes.

An ion-exchange sulfa cation selective membrane which has a unidirectional conductance is installed between the electrode elements, whereby Na + ions penetrate into the cathode space.

The sulfa cation selective membrane (electronic sieve) is impenetrable for gases, which completely excludes the mixing of gases of hydrogen H₂ ↑ and chlorine Cl₂ ↑ released during the electrolysis (chlorine ions are much larger than the membrane openings, when sodium ions freely and smoothly pass through the «sieve»).

The membrane method is the most energy efficient in comparison with other methods of electrolysis of edible salt. The consumption of electricity per 1 kg of active chlorine is not more than 3.0 kW.

In terms of electrochemical processes, the anode and cathode spaces are completely separated by a cation-exchange membrane impenetrable for anions. Due to this property it becomes possible to obtain chemically pure chlorine (Cl₂) and liquor (NaOH). Therefore, there are two steams in membrane electrolysis.

A salt solution steam flows into the anode space, and softened water — into the cathode one, both spaces are completely filled with the liquid phase, which completely excludes (when mixing the gas phase in the case of mechanical damage to the membrane) the possibility of an explosion of the mixture of gases (Cl₂, O₂ and Н2) formed during the electrolysis of NaCl when voltage is supplied.

The stream of depleted anolyte returns from the anode space to the solution tanks, and chlorine gas Cl ₂ (active chlorine) 98.5% is discharged into the anolyte separator. Sodium hydroxide (NaOH) and hydrogen (H₂) — from the cathode one.

Systems of circulation of anolyte and catholyte serve to supply and distribute the edible solutions and softened water among the cells and to discharge the products of electrolysis.

When the voltage is applied to the electrolyser, the process of electrolysis of the edible salt solution with the ion exchange sulfacationite membrane on the electrodes begins and the following basic reactions occur in the electrolyte volume:

NaCl+H₂O = Cl⁻↑+(NaOH+ H₂↑)

  1. On anode:      NaCL-2e→ CL⁻↑ + 2 Na⁺

During the electrolysis, the ions of chlorine and sodium are ruptured, which results in the migration of Na+ ions to the cathode space (reductant), and chlorine gas Cl₂⁻↑ is formed in the anode (oxidizer+) and discharged into the anolyte separator equipped with a waterlock. The chlorine gas is suck out from the anolyte separator by an ejector.

Chlorine gas (Cl₂) is completely absorbed by water and forms «chlorine water». Chlorine water has increased oxidizing ability. This is achieved due to the appearance of an additional substance — active chlorine in chlorine. As a consequence, reduces the dose of chlorine during the water disinfection and minimizes the side effects of the use of chlorine.

  1. At the cathode 2N₂O + 2e → N₂ ↑ + 2ON⁻

Hydrogen (H₂) is discharged to the atmosphere from the catholyte separator through the waterlock. Hydrogen in its composition does not contain impurities of oxidants (О₂ and Сl₂), but it contains water vapor (at electrolysis temperature (60 ÷ 85 ° С — up to 293 g/m3), which are a phlegmatizing agent.

Let’s give an example: Considering the operation scheme of electrolysers in a flowing electrochemical reactor (non- diaphragm method) and also the diaphragm method (using a mechanical diaphragm), it should be noted that during the electrolysis of NaCl, not pure hydrogen but an explosive mixture consisting of hydrogen, oxygen and chlorine is formed which also provides the intense destructive effects on process equipment (corrosion).

Only the ventilation of explosive electrolysis gases results in uncontrolled dispersing of chlorine in the atmosphere, which is not permissible, and therefore the use of these electrolysers at the facility should provide devices neutralizing chlorine emissions.

This again proves the advantages of using membrane bipolar electrolysis.

The MBE plants exclude the possibility of formation of explosive chlorine-hydrogen mixtures in process equipment and facilities during operation;

ОН⁻ ions in the cathode space are compounded with Na+ ons migrating from the anode space under the action of an electric current to form alkali.

Na⁺ + OH⁻→ NaOH

The concentration of alkali in the cathode space is maintained at the required level due to the adding of catholyte with softened water.

Electrolytic alkali (sodium hydroxide) with a concentration of up to 20%, formed during the electrolysis, is discharged from the catholyte separator by gravity through the jet interrupter to the alkaline storage tank.

The membrane method of electrolysis of edible salt at MBE stations does not require acid and other washes. MBE plant is of continuous-action and operates in automatic mode.

Let’s give an example: Electrolysis plants of low-concentration hypochlorite (0.6%) include the following process units: salt dissolver (sakurator), decarbonizer, electrolyzer, storage tanks for prepared sodium hypochlorite; Acid household for washing the electrode system and water decarbonization. From this technology it follows that the chemical components that do not improve the quality of water quality and substances that are the impurities of the salt and reagents used, as well as the products of their interaction are added to the water. And how much of this uncontrollable ballast is introduced into the clean water?

The use of our technology completely excludes the import, transportation and storage of hazardous chemicals, which means that no by-products, impurities and other chemical compounds deteriorating the quality of disinfected water get into disinfected water, except for the chlorine water (obtained from 98.5% Cl in the MBE plant).

Technological scheme