ro membrane The blockage on the membrane surface includes minerals, proteins, sugars, etc. the object of membrane separation is generally the mixture of components, and there are complex physical and chemical interactions between components. The blockage on the membrane surface is often the result of the joint action of various components, so it is difficult to determine its type. Physical and chemical methods are often used for cleaning after blockage, and the two methods can be used interchangeably when necessary.
1. Common physical cleaning technology
Hydraulic cleaning focuses on removing reversible pollutants and some irreversible pollutants.
(1) Clean water or air-water mixture is used to flush the water out of the membrane at high speed and low pressure. Hydraulic shear action, reduce the blockage on the membrane surface, and restore the water permeability. The effect is poor when used alone.
(2) The backflushing with clean water or air-water mixture can choose different backflushing pressure, flow rate and duration according to the degree of blockage, which is effective for the initial blockage.
(3) The diameter of the hydraulic delivery sponge ball is slightly larger than the diameter of the membrane tube to remove the soft blockage. It flows through the blocked membrane surface under the pressure of water for forced flushing, which is suitable for the blockage dominated by organic colloids. The operation is complex, which often damages the membrane surface.
2. Chemical cleaning technology
Commonly used acid cleaning agent, alkali cleaning agent, enzyme cleaning agent, surfactant, complexing agent, disinfectant, compound agent cleaning. The cleaning effect is related to the type, concentration, temperature and pH value of cleaning agent.
(1) Alkali cleaner sodium hydroxide dissolves silica, soluble protein and saponified lipids; Carbonate adjusts pH value; Phosphate is often used as dispersant, dissolving carbonate, adjusting pH value, etc
(2) Acid cleaner sulfuric acid is dangerous when used, non-volatile, and its salt forming solubility is small, so it is rarely used for membrane cleaning; Hydrochloric acid is the most commonly used, which is suitable for low-temperature operation and widely removes almost all obstructions except silicon dioxide, but it may produce HCI and corrode steel, so its application is limited to a certain extent; Nitric acid has strong chemical reaction, high salt forming solubility and wide application, but it has slight corrosion to low carbon steel; Hydrofluoric acid has strong chemical reaction and high solubility, which can better dissolve silica blockages. It is highly volatile, corrosive, toxic and difficult to deal with, and is rarely used; Sulfamic acid is powdery, easy to handle, reacts violently with plugging substances such as carbonate and hydroxide, has weak solubility in iron oxide, and its calcium salt solubility is large, which is suitable for cleaning the membrane surface pollution with calcium salt and iron oxide hydrate as the plugging main body; Citric acid is a solid, which is less dangerous and easy to deal with. It forms a salt with blockage and has a large solubility. It is often used as an auxiliary of cleaning agents.
(3) Enzyme cleaning agent is effective for the blockage of organic matters, especially proteins, Oils and polysaccharides, but it is expensive, slow in reaction, and takes a long time. The residual enzyme will affect the growth of microorganisms.
(4) Surfactants mainly include anionic, cationic and non-ionic surfactants, which can improve cleaning agents and membranes
Face to face contact, reduce water consumption and shorten time. The pH value of anion is neutral. It is an organic foaming agent. The cation is composed of quaternary amino compounds, which has poor activity, but it can also inhibit the growth of microorganisms at a very low concentration. Non ionic surfactants are composed of concentrated products, which have the characteristics of low foam, easy elution and are not limited by pH value.
(5) The complexing agent is complexed with almost all metal ions, the reaction speed is fast, and most of the generated chelates are water-soluble,
And relatively stable.
(6) Disinfection cleaning agent has strong oxidation ability. It can effectively remove the organic matter in the blockage on the membrane surface and restore the membrane flux at the same time of disinfection. Sodium hypochlorite can effectively remove the blockage dominated by organic matter. It has fast chemical reaction and short cleaning time, but it is highly corrosive. Especially when the pH value is low, it has obvious corrosive effect on stainless steel. When the temperature is high, the dissolved chlorine will escape and cause harm to people. Hydrogen peroxide is a binary weak acid, which can decompose into a large amount of foam in the presence of light, oxides and reducing substances. It is a mild disinfectant, and 1.2% aqueous solution can remove blocked organic matter.
(7) The mixture of compound detergent, alkaline detergent, phosphate, complexing agent, enzyme detergent, etc. is better than a single detergent. Products have been sold, but the price is high.
The most commonly used acid cleaning agents are nitric acid, citric acid and hydrochloric acid; The alkali cleaner is sodium hydroxide and potassium hydroxide, and the disinfectant is sodium hypochlorite. Compounding agents are commonly used in European and American countries.
The ideal cleaning agent has the following characteristics: loosening or dissolving film surface pollutants; Disperse or dissolve the blockage on the membrane surface; Cannot cause new blockage; There shall be no corrosion to its own devices; It can disinfect the membrane surface and pipeline system at the same time.
In actual cleaning, different cleaning agents and
Cleaning procedures, especially the resistance of the whole pipeline system and components of the circulating pump must be considered. Generally, the blockage caused by oil is cleaned with alkali + surfactant; Use acid + surfactant to clean calcium, iron, manganese and other sediments; It is best to use alkali + oxidant to clean gel, slime and other organic substances.
The environmental conditions of cleaning are also important. When other conditions remain unchanged, the higher the temperature, the faster the reaction speed and the more serious the corrosion; The higher the velocity of membrane surface is, the more obvious the physical peeling caused by hydraulic shear is; The price and operation steps of cleaning agents should also be optimized.