Analysis of pollution

(1) Calcium carbonate scale:

Calcium carbonate scale is a kind of mineral scaling. When the scale inhibitor / dispersant adding system fails, or the acid pH regulating system fails and the pH of the feed water increases, calcium carbonate scale may deposit. It is very necessary to detect calcium carbonate scale as early as possible to prevent crystal damage of film elements deposited on the surface of the film. The calcium carbonate scale detected in the early stage can be removed by reducing the pH value of feed water to 3-5 and running for 1-2 hours. For the calcium carbonate scale with long deposition time, it can be cleaned and removed by citric acid solution with low pH value.

(2) Calcium sulfate, barium sulfate, strontium sulfate scale:

Sulfate scale is a mineral scale much harder than calcium carbonate scale, and it is not easy to remove. Sulfate scale can be deposited when the antiscale / dispersant addition system fails or when sulfuric acid is added to adjust pH. It is very necessary to detect sulfate scale as early as possible to prevent crystal damage of film elements. Barium sulfate and strontium sulfate scale are difficult to remove because they are difficult to dissolve in almost all cleaning solutions, so special attention should be paid to prevent the formation of such scaling.

(3) Metal oxide / hydroxide pollution:

The typical metal oxides and metal hydroxides are polluted by iron, zinc, manganese, copper, aluminum, etc. The formation of this scale may be caused by corrosion products of the unit pipeline, vessel (tank / tank), or metal ions, chlorine, ozone, potassium, permanganate oxidized in the air, or the use of iron or aluminum coagulant aids in the pretreatment filtration system.

(4) Scale of polysilicon:

Silicon gel layer scale is caused by supersaturated state and polymer of soluble silicon, and it is very difficult to remove. It is important to note that this pollution of silicon is different from that of silica gel. The contamination of silica gel may be caused by association with metal hydroxide or organic matter. It is difficult to remove silicon scale, and traditional chemical cleaning method can be used. The existing chemical cleaning agents, such as ammonium hydrogen fluoride, have been successfully used in some projects, but the operation hazards and equipment damage of this method must be considered in the use, and protective measures should be taken into account.

(5) Colloidal pollution:

Colloid is an inorganic substance suspended in water or a mixture of organic and inorganic particles, which will not precipitate due to its own gravity. Colloids usually contain one or more of the following major components, such as iron, aluminum, silicon, sulfur, or organic matter.

(6) Non soluble natural organic matter pollution (NOM):

Non soluble natural organic matter (NOM) is usually caused by the decomposition of nutrients in surface water or deep well water. The chemical mechanism of organic pollution is very complex, the main organic component is humic acid or griseofulvic acid. Non soluble NOM adsorbed onto the surface of the membrane can cause rapid pollution of RO membrane elements. Once absorption occurs, gradually forming a gel or blocky pollution process will start.

(7) Microbial deposition:

Organic sediment is produced by bacteria slime, fungi, mould, etc. this kind of pollutant is difficult to remove, especially when the water supply channel is completely blocked. The blockage of the feed water passage will make it difficult to fully and evenly enter the membrane element with clean water. To inhibit the further growth of this sediment, it is important not only to clean and maintain the RO system, but also to clean the pretreatment, pipeline and end heads. When using oxidizing sterilization for membrane components, please use approved fungicide.

Selection and use of cleaning solution

There are many factors involved in choosing suitable chemical cleaning agent and reasonable cleaning scheme. First, the main pollutants should be determined and the appropriate chemical cleaning agent should be selected. Sometimes, for a special pollutant or pollution situation, special chemical cleaning agent from ro pharmaceutical manufacturer shall be used, and the product performance and operation instructions provided by the pharmaceutical supplier shall be followed when applying. Sometimes, the polluted single membrane elements can be taken out from the reverse osmosis device for testing and cleaning test according to the specific conditions to determine the appropriate chemical agents and cleaning scheme.

In order to achieve the best cleaning effect, sometimes some different chemical cleaning agents are used for combined cleaning.

The typical procedure is to clean at low pH, remove the mineral scale pollutants, and then clean with high pH value to remove organic matter. Some cleaning solutions are added with detergents to help remove serious biological and organic debris scale, while other agents such as EDTA chelates can be used to assist in the removal of colloid, organic matter, microorganism and sulfate scale.

It is important to consider that if improper chemical cleaning methods and reagents are selected, the pollution will worsen.

Cleaning and flushing procedure of RO membrane element

RO membrane elements can be placed in pressure vessels, and in the case of high flow rate, clean water (RO product water or clean water without free chlorine) flows through the membrane element for cleaning. Ro cleaning procedure is entirely dependent on the specific situation and replace the clean water for circulation if necessary.

The routine cleaning procedure for RO membrane elements is as follows:

Low pressure flushing at 60psi (4bar) or lower, i.e. pump clean water from the tank (or equivalent water source) into the pressure vessel and drain it for several minutes. The flushing water must be clean, hardness free ro product water or deionized water without transition metal and residual chlorine.

Prepare specific cleaning solution in the cleaning tank. The preparation water must be ro product water or deionized water that is hard to remove, free of transition metal and residual chlorine. The temperature and pH shall be adjusted to the required value.

Start the cleaning pump to pump the cleaning fluid into the membrane assembly, and cycle the cleaning for about an hour or the required time. In the initial stage, before the cleaning solution returns to ro cleaning tank, the initial reflux liquid shall be discharged to avoid dilution of the cleaning solution caused by the water trapped in the system. Slowly adjust the flow rate to 1/3 of the maximum design flow rate in the first 5 minutes. This can reduce potential fouling due to the large amount of deposition of dirt. In the second 5 minutes, increase the flow rate to 2/3 of the maximum design flow rate, and then increase the flow rate to the maximum design flow rate. If necessary, when the pH change is greater than 1, it is necessary to reset to the original value.

Cycle cleaning and soaking procedures can be used alternately as required. The soaking time is recommended to be 1 to 8 hours. Be careful to maintain the appropriate temperature and pH.

After the chemical cleaning, the cleaning water (RO product water or deionized water without metal ions such as iron and chlorine) shall be used for low pressure flushing. The residual part of chemical agent shall be removed from the cleaning device / component, the remaining part of chemical agent shall be discharged and washed, and then the cleaning tank shall be filled with clean water for flushing. Pump all flushing water from the cleaning tank to flush the pressure vessel to discharge. If necessary, a second cleaning may be carried out.

Once the RO system has been completely washed with clean water from the water storage tank, the final low pressure flushing can be performed with pretreatment water supply. The feed water pressure should be lower than 60psi (4bar), and the final flushing will continue until the rinse water is clean and contains no foam and detergent residues. Usually it takes 15-60 minutes. Operators can use clean flasks to sample and shake well to monitor the residue of detergent and foam in the washing water at the outlet. The removal of washing solution can be carried out by measuring conductivity. For example, the conductivity from flushing water to discharge water is within 10-20% of the water supply conductivity, it can be considered that the washing has reached the end point; The pH meter can also be used to determine whether the pH value of the flushing water to the discharge water is close to the feed water.

Once all stages have been cleaned and chemical agents have been washed out, RO can be put into operation procedure again, but the initial product water shall be discharged and monitored until RO water production can meet the process requirements (conductivity, pH value, etc.). In order to obtain stable RO water quality, this recovery time sometimes takes from hours to days, especially after high pH cleaning.