Reverse osmosis systems in Ukraine

The principle of operation of reverse osmosis technology is based on the phenomenon of osmosis – the spontaneous transition of a solvent through a semi-permeable membrane into a solution at different concentrations of substances in solutions on both sides of the membrane. When the concentration of solutions is equalized, equilibrium occurs. The pressure at which equilibrium is reached is called osmotic pressure. If a pressure greater than osmotic pressure is applied from the side of the solution, then water molecules will move through the membrane in the direction opposite to direct osmosis. In this case, demineralized water and concentrate can be obtained. This process is called reverse osmosis.

Reverse osmosis technology uses membranes with a minimum pore size commensurate with the size of the ions. The efficiency of removing ions from water depends on their charge and size, which determine the degree of their hydration.

The efficiency of the reverse osmosis process is mainly determined by the properties of the membranes. The main characteristics of membranes are selectivity, specific productivity (permeability), conversion (recovery).

Selectivity for separating components is defined as the ratio of the concentration of the solute in the original solution to the concentration of the solute in the purified water, expressed in fractions of unity or percent.

Проницаемость мембраны при постоянном давлении характеризуется объемом прошедшего фильтрата в единицу времени через единицу поверхности и выражается в л/м2*час.

Conversion – the degree of water use, expressed as a percentage of the output of permeate (purified water) from the consumption of the initial solution, expressed as a percentage.

Membrane permeability and selectivity depend on operating pressure and solution temperature. With increasing pressure, productivity and selectivity increase. An increase in temperature leads (due to a decrease in the viscosity of water and a corresponding increase in the mobility of ions) to an increase in the specific productivity of the membrane. In this case, the selectivity is reduced.

The increase in conversion is possible up to a certain limit. This is due to the fact that with an increase in the salt content in the concentrate, the osmotic pressure of the concentrate increases. If the osmotic pressure of the concentrate approaches the pressure of the source water, the transfer process stops.

Increasing the concentration of salts in the source water reduces the permeate yield and selectivity.

Reverse osmosis filtration process

To ensure long-term and stable operation of the membrane, it is necessary to reduce the polarization effect, which occurs due to an increase in the concentration of salts in the near-membrane layer. This will prevent contamination of the membrane pores by precipitation. This is achieved by creating an intense turbulent flow of the treated liquid above the membrane surface, and only part of it is filtered through the membrane. As a result, two constant streams are formed: permeate (purified water), which has passed through the membrane and concentrate, which moves along the membrane. This process is called tangential filtration.

At present, in Ukraine, and in such large cities as Kyiv, Dnipro, Zaporozhye, Kharkov, Lvov, Odessa, etc., rolled membrane elements, in which a membrane made of polyamide fiber is used, have found the greatest application. These elements are the most effective in terms of efficiency / cost. Such elements are placed in special high-pressure housings. The number of membrane elements in one housing is from 1 to 6 (rarely up to 8).

Almost all manufacturers produce membrane elements of standardized sizes: length – 40 inches, diameter – 2.5; four; 8 and 16 inches.

Benefits of reverse osmosis technology:

  1. High efficiency of the desalination process.
  2. There is no need to use large amounts of acids and alkalis.
  3. The compactness of the equipment.
  4. Ease of automation.

Disadvantages of reverse osmosis technology:

  1. Increased electricity consumption.
  2. High requirements for pre-cleaning.