For the selection of reverse osmosis equipment pipeline materials, internal and external problems shall be considered. The internal is the corrosion of water quality to the pipeline, and the external is the operating environment. The operating environment of the reverse osmosis equipment determines the external corrosion of the pipeline. If the operating environment is good, the surface coating (such as paint or galvanized, etc.) is generally used. Residual chlorine, chemicals, pH value, temperature, etc. shall be considered for the corrosion of water quality to the pipeline. We generally use UPVC, stainless steel, etc.

Stainless steel pipe

The basic advantage of stainless steel is that it has good resistance to general corrosion. Stainless steel rarely produces current corrosion and stress corrosion damage, but stainless steel is prone to pitting corrosion and crevice corrosion. Pitting corrosion represents local corrosion of metal, causing concave holes on its surface. If the chromium oxide passivation layer is damaged, chloride ions will attack the bare metal to form pitting corrosion. Crevice corrosion is related to pitting corrosion caused by a small amount of static water stop at small holes, gasket surfaces, around sediments and cracks under screws. In order to avoid pitting corrosion and crevice corrosion of high-pressure pipeline of membrane system, the suggestions are as follows:

304 stainless steel can be selected for general water source;

When the salt content of raw water is 2-5000ppm, 316 stainless steel with carbon content less than 0.08% is recommended;

When the salt content of raw water is 5-7000ppm, 316L stainless steel with carbon content less than 0.03% is recommended;

When the salt content of raw water is 7-30000ppm, 904L stainless steel with molybdenum content of 4% - 5% is recommended.

UPVC pipe

The characteristics are as follows:

a. Strong corrosion resistance: compared with general cast iron pipe and galvanized pipe, PVC pipe has very strong corrosion resistance, can resist strong acid and alkali, and will not rust and scale. There is no need to worry about "red water" when using.

b. Small fluid resistance: the inner wall of PVC pipe is very smooth. The surface roughness coefficient is only 0.009, the fluid resistance is very small, and the water pressure will not be excessively reduced.

c. High mechanical strength: UPVC pipe for water supply has good water pressure resistance, impact resistance and tensile strength. It can withstand 110 atmospheric pressure for 1 hour at room temperature without rupture.

d. Sanitary and non-toxic: the UPVC pipe for water supply adopts a unique green environmental protection lead-free formula system to replace the traditional compound lead salt formula system, so it will not damage the water quality and affect human health.

e. Light texture and convenient installation and construction: the density of PVC pipe is one fifth of that of general cast iron, which is convenient for transportation, loading and unloading. It is pasted with special adhesive or sleeved with elastic seals, which makes the installation and construction simple and fast.

f. Suitable temperature performance of water supply pipe: the temperature range of UPVC pipe for water supply is the best at room temperature. The temperature is low and the brittleness increases, which is unfavorable for installation and construction; When the temperature is high, the tensile strength decreases and the water pressure resistance decreases.

g. Good water tightness: after 15 minutes of adhesive bonding, the bonding strength can reach more than 12.5/c, which will not decrease significantly after years of use. The sealing ring at the socket joint of elastic seal has the same service life as PVC pipe, and there is no worry of aging and water leakage.

Maintenance chapter

Generally, the main reason for the decline of RO reverse osmosis membrane performance is that the membrane surface is polluted, such as surface scaling, blocking membrane pores or physical and chemical changes. The physical change is the decrease of water permeability of reverse osmosis membrane caused by compaction effect. The chemical change is mainly caused by the fluctuation of pH value. For example, the hydrolysis of cellulose acetate membrane and free chlorine will also degrade the performance of directional polyamide membrane. The blockage of reverse osmosis membrane pores is mainly caused by membrane surface deposition and microbial growth. Microorganisms not only block membrane pores, but also erode cellulose acetate.

A certain residual chlorine of 0.1 ~ 0.5mg/l must be maintained in RO reverse osmosis membrane, and the residual chlorine in front of aromatic polyamide membrane shall be less than 0.1mg/l. The cleaning treatment of RO reverse osmosis membrane is a subtle and complicated work. At present, the quality of domestic membrane is not high enough, and more cleaning membranes are easy to be damaged. In order to reduce the cleaning work, we must do a good job in pre-treatment and strictly control the water quality, otherwise "there will be endless trouble".

RO reverse osmosis membrane treatment method:

Periodically wash with 0.1% formaldehyde solution for at least 1 hour. The contaminated membrane shall be cleaned with 2% ammonium citrate solution (pH = 4 ~ 8), or sodium bisulfite, sodium hexametaphosphate and dilute hydrochloric acid shall be used to prevent the scaling of manganese, iron and carbonate. Sometimes the organic matter is also cleaned with enzyme detergent. The cleaning pressure is 0.34 ~ 0.98mpa (3.5 ~ 10kgf / cm2), and the cleaning flow rate is 2 ~ 3 times of the original water treatment flow rate.

How should reverse osmosis membrane be maintained

During the operation of reverse osmosis membrane system, pollution and cleaning are always a dynamic balance process. Only by maintaining the maximum cleanliness every time, can the membrane run more efficiently.

After a period of operation, a large number of pollutants are adsorbed and deposited on the membrane, which needs to be cleaned in time for the next high-efficiency operation. If the filter is carried out after each cleaning is not in place, the pollution will further accumulate until the flow channel on the membrane surface is blocked and the cleaning agent cannot penetrate. At the same time, due to long-term operation under pressure, pollutants will be dense, which will lead to extremely difficult cleaning and recovery of the membrane, and finally lead to the scrapping of the membrane.

Rome wasn't built in a day. If the previous membrane has not been cleaned efficiently and has been polluted to a very low flux, it must be the result of accumulation over time. Don't expect that all pollutants can be removed at one time with a certain efficient membrane cleaning agent. It's just like having a serious illness. It's completely impractical to expect to recover immediately after taking a medicine. Reverse osmosis membrane should be cleaned regularly, so as to ensure the health and safety of our drinking water.

Generally, when the water production decreases more, the water quality and desalination effect become worse, and the pressure difference between various sections of the system becomes larger, it indicates that the reverse osmosis membrane is seriously polluted and needs to be cleaned up. Generally used chemical cleaning solutions include citric acid, hydrochloric acid, sodium bisulfite, etc. different solutions are used for different pollution. The tools are very simple, such as water tank, water pump, filter, pressure gauge and flowmeter. If the reverse osmosis is not used for a long time (up for 3 days), sodium bisulfite solution shall be injected into the reverse osmosis membrane module to inhibit microbial growth.

Shutdown of reverse osmosis membrane system for more than one week (ambient temperature above 5 ℃): before shutdown, flush the system with low pressure (0.2-0.4mpa) and large flow (about equal to the water production of the system) for 14 ~ 16 minutes; Carry out chemical cleaning according to the method of system chemical cleaning in the operation manual of reverse osmosis system; After chemical cleaning, flush the reverse osmosis membrane; Prepare 0.5% formalin solution, input it into the system at low pressure and circulate for 10 minutes; Close the valves of all systems for sealing; If the system is shut down for more than 10 days, the formalin solution must be replaced every 10 days.

Short term shutdown of reverse osmosis membrane system (1-3 days): before shutdown, flush the system with low pressure (0.2-0.4mpa) and large flow (about equal to the water production of the system) for 14 ~ 16 minutes; Keep the normal natural water flow and let the water flow into the thick channel.

The ambient temperature is below 5 ℃: before shutdown, flush the system with low pressure (0.2-0.4mpa) and large flow (about equal to the water production of the system) for 14 ~ 16 minutes; Where conditions permit, the ambient temperature can be raised above 5 ℃, and then the reverse osmosis membrane system can be maintained according to the above method.