Reverse osmosis membrane cleaning plan

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Cleaning Plan 1. Introduction to cleaning: The pretreatment process design for reverse osmosis systems aims to remove as many contaminants as possible from the raw water, but it is not possible to eliminate them all; therefore, contamination in reverse osmosis systems is a common phenomenon. Fortunately, most pollutants can be removed through regular chemical cleaning. As long as the pretreatment process operates properly and there are no uncontrollable issues such as changes in the quality of the raw water or unavoidable microbial contamination, the cleaning frequency can be kept as low as possible. Sometimes operational errors can cause membrane contamination, such as too high a recovery rate or problems with the chemical dosing system. When the membrane becomes contaminated, the water production rate decreases, the desalination efficiency drops, and the pressure difference between the feed water and the concentrated water increases. Membrane cleaning can be carried out using various acid and alkali agents (within pH 2–pH 12), with the temperature of the cleaning solution reaching up to 45°C. Much of the pollution, especially the sludge-like substances, accumulates over time, gets compressed and thickens, which makes cleaning more difficult and prolongs the cleaning time. If the symptoms match the following characteristics, clean the membrane elements immediately: a 10–15% decrease in the standardized water production rate, a 0.5% drop in the desalination efficiency, and a 15% increase in the pressure difference ΔP (compared to the pressure difference at the initial 24–48 hours). 2. Cleaning chemicals and other equipment are required. 1. Cleaning chemical tank: The chemical tanks used for cleaning are usually made of PE or FRP materials and can withstand a pH range of 1–12. 3. Clean the heating device: Use a high cleaning temperature, such as 20–40°C. Chemical cleaning should not be carried out at temperatures below 10°C, as the cleaning efficiency is very low; therefore, a heating device is required during cleaning. 3. Cleaning agents: Strong acids and bases are used for cleaning (such as hydrochloric acid, phosphoric acid, citric acid, sodium hydroxide, etc.). Therefore, it is important to choose the right chemical agents; sometimes, an inappropriate choice of agent can lead to contamination or even damage of the membrane. 4. Cleaning pump: A pump resistant to acids and alkalis, with a flow rate of 12–15 m3/h and a head of 20–30 meters. 5. Appropriate pipes, valves, and pressure gauges must be installed during cleaning. Connect the pump’s outlet to the inlet of the membrane, and connect the concentrate pipe to the chemical tank; install a ball valve before connecting it to the raw water tank. Then connect the chemical tank to the pump. 4. Cleaning procedure 1. The cleaning procedure is a cyclic cleaning process, with the outlet pipe for the concentrated water connected to the cleaning solution tank. 5. Fill the cleaning water tank with pure water; the cleaning solution should be in sufficient quantity to meet the requirements of the pressure vessel and pipelines. Add a dosage of 1%–2% of the chemical to the cleaning tank; the pH value should be above 1 in acidic conditions and not higher than 13 in alkaline conditions. 4. It is best to heat the cleaning solution during cleaning to improve efficiency. 6. The heated solution should be pumped into the RO membrane at a flow rate of 10–15 m3/h and a head pressure of 20–30 meters; these values must meet the specified requirements, without exceeding them. When cleaning, do not recycle the bucket of solution back to the solution tank; instead, dispose of it directly. It contains a large amount of contaminants that can directly affect the effectiveness of the solution, resulting in inadequate cleaning. After there is no significant contamination, the liquid medicine returns directly to the original medicine tank for cyclic cleaning. During cleaning, fully open the concentrated water discharge valve to minimize pressure, and close the product water valve to prevent the chemical solution from entering the pure water tank. The 6.1 cleaning procedure generally involves circulating the liquid for 1–2 hours first, followed by a soaking period of half an hour; sometimes a soaking time of 1 hour or longer can be very effective. Recycle and clean for 1-2 hours. The entire cleaning process took over 4 hours. 7. After cleaning is complete, rinse the membrane using reverse osmosis water or clean water (filtered with SDI < 3, free of residual chlorine and microorganisms, and having a conductivity of < 10,000 μs/cm); the rinsing time should be at least half an hour. 8. After the cleaning is complete, the system is restarted, but the water at the beginning may need to be drained until the conductivity and pH value of the produced water return to normal. Note that if the system is shut down for more than 24 hours after cleaning, it should be soaked in a protective solution (1% sodium bisulfite solution). 9. When the system is restarted, scale inhibitors must be added. Due to the high conductivity of the water before this system, which exceeds 1000 μs/cm, such water quality can easily cause fouling of the membranes, leading to an increase in conductivity, a decrease in water production, and an increase in pressure. Therefore, when the system is operated again, scale inhibitors must be added to reduce membrane fouling and minimize the number of cleaning cycles. It keeps the equipment free from contamination for extended periods of time. The dosage of this scale inhibitor is 2-6PPM. (Add 2–6 grams of scale inhibitor per ton of water) Cleaning formula, types of contaminants, and effectiveness of the cleaning formula: Inorganic salts (CaCO3, CaSO4, BaSO4): 1% hydrochloric acid, 0.5% phosphoric acid, 2.0% citric acid – Best, Good, Average. Metal oxides (iron): 0.5% phosphoric acid, 1.0% sodium bisulfite – Very good, Good. Inorganic colloids (sediment particles): 1% sodium hydroxide (NaOH) at 30°C, 0.025% sodium dodecyl sulfate/0.1% NaOH at 30°C – Very good. Biofilms: 2% sodium hydroxide at 30°C. 1.0% sodium ethylenediaminetetraacetate (sodium EDTA) and 0.1% NaOH, 30°C is optimal (when the biofilm contains inorganic scaling); 0.025% sodium dodecyl sulfate/0.1% NaOH, 30°C for organic matter. 0.1% sodium triphosphate/1% sodium EDTA, 1% silica, 1% sodium hydroxide, 30°C. 1.0% sodium ethylenediaminetetraacetate (sodium EDTA) and 0.1% NaOH.