Water desalination plant maintenance steps
Water desalination plant maintenance steps of seawater vary according to the system used. In this article, we will talk about the maintenance steps of a reverse osmosis desalination plant. Regular maintenance of a desalination plant is always one of the most important steps that must be taken care of. You can read more about the importance of desalination plant maintenance from this article.
Beginning with the knowledge of the different types of water according to its salinity.
Types of water according to salinity
Water on Earth is divided according to its salinity into two main parts, freshwater, and saltwater. Freshwater has a salinity ranging from zero (distilled water) to 1000 parts per million. And the salty water is either medium-saline wells water, or highly saline seawater. As for the low salinity well water, its salinity ranges from 1000 to 5000 parts per million, while the high salinity well water ranges from 5000 to 10,000 parts per million, while the salinity in seawater ranges from 30000 to 100000 parts per million. Salinity in bays and dead seas (where there are no tributaries to rivers and rain).
The waters of the Mediterranean Sea range in salinity from 33,000 to 37,000 parts per million.
The salinity of the Red Sea water ranges from 40,000 to 45,000 parts per million.
As for the waters of the Arabian Gulf, its salinity ranges from 50,000 to 55,000 parts per million.
As for the water of the Dead Sea, its salinity ranges from 65,000 to 75,000 parts per million.
Water properties
Water properties are divided into:
Characteristics are natural: include temperature, turbidity, color, taste, and odor.
Chemical properties include pH, total hardness, and dissolved oxygen – total dissolved salts (T.D.S).
Biological properties: It includes all life forms contained in water such as bacteria, viruses, and algae.
Natural properties of water
Temperature
It affects the purification processes, as it helps the rapid dissolution of additive chemicals and the speed of sedimentation of fine particles.
Turbidity
Turbidity may be organic matter such as algae, which can cause major problems unless treated chemically to stop its growth. It may be inorganic materials such as silt and sand and may reach several thousand parts per million in surface water, and be much less in groundwater because the latter was subjected to filtration during its passage in the soil layers.
Turbidity may be colloidal substances, and under this name are very small substances that cannot be seen with an ordinary microscope. These substances exist in a state between attachment and solubility but can be purified by filtration.
The color
The water in surface waters becomes discolored as a result of the decomposition of organic matter or the presence of inorganic substances such as iron and manganese. The discoloration of the water is one of the most indications that it is not suitable for human use and for most industrial uses.
Taste
Sometimes the water has an unpleasant taste as a result of it containing algae and organic matter, or mixing it with wastewater or industrial waste before purification.
Odor
The presence of an unpleasant taste is usually associated with the presence of an unpleasant odor at the same time.
Chemical properties of water
pH
It is symbolized by the symbol pH, which expresses the degree of the tendency of the substance (water) to acidic or alkaline if not neutral, and it is divided from zero to 14.
The number 7 indicates neutrality, which is the pH of pure water.
If it is less than 7, this indicates the acidity of the water, and acidic water causes erosion of the surfaces containing it.
If it exceeds 7, it indicates its alkalinity, and alkaline water deposits scale on those surfaces.
Total HARDNESS
It results from the presence of calcium and magnesium salts dissolved in the water, and the hardness results in an increase in the pH of the water and the deposition of scales on the surfaces, in addition to giving the water an unpleasant taste and it is difficult with hard water to use soap.
Dissolved oxygen
Oxygen is permanently dissolved in water, and its percentage is higher in cold water than in hot water. The presence of algae in the water leads to the production of oxygen during the day, and its percentage increases during the day, while it takes it at night and its percentage decreases at night. The increase in the percentage of dissolved oxygen in the water leads to corrosion of the metal surfaces in contact with it.
Total Dissolved Salts (T.D.S)
Sodium chloride salts dissolved in the water represent from 85% to 90% of the total dissolved salts, and their quantity in natural water reaches 500 parts per million or more. It contains a percentage of sodium salts. There is a maximum amount of dissolved solids in water, which is 1000 parts per million, so as not to cause health problems to consumers or give water an unacceptable taste and odor. In addition, some dissolved substances are harmful to human health, so it is necessary to give special care to get rid of them during purification processes.
Biological properties of water
It is the bacteria and viruses contained in water that are harmful to the health of consumers. The detection of these bacteria and viruses leads to the establishment of sound systems for purification and sterilization to ensure the killing of these disease-causing organisms.
Sterilization is an important step in water desalination plant maintenance
The sterilization system for high-quality sterilization membranes and reverse osmosis is the application of effective solutions or by hot water to these membranes when the operating system is closed, that is, it is not in the production model and biological chemicals are injected during operation so that no biological precipitation occurs.
Membranes systems are sterilized in order to keep the number of microorganisms living at a low level and therefore there are two main reasons for the sterilization process.
In the case of normal operation, microorganisms may grow into a biofilm on the surface of the membranes and deposition takes place on the surface of the nutrient membranes and causes a biological deposition to occur. Regular sterilization methods are part of the strategy to control biological sedimentation.
Biological precipitation poses a major threat to the operating system and traditional sterilization methods help reduce the growth rate of biological precipitation to avoid operating problems.
In reverse osmosis systems during biological precipitation activity. A biofilm can appear within 3 to 5 days after inoculation with viable organisms. Thus, the most common frequency of sterilization is within 3 to 5 days during peak biological activity in the summer and every 7 days in the winter. The optimum frequency for the sterilization process to be in the ideal location must be determined by the operational characteristics of the reverse osmosis systems.
Desalinated water quality
It is used in some applications, such as the food and pharmaceutical industries. The produced water requires very high quality in terms of purification and sterilization and the absence of biological pollutants. As reverse osmosis and high-quality purification membranes refuse the passage of biological pollutants.
When any defect occurs in the membrane system, this leads to the desalinated water becoming contaminated. The risk of contamination is much higher than that of biofilm in terms of the feed line. And for this, the state of the membranes must be kept stable. And conventional sterilization methods must be done in these applications to obtain the microbiological quality of the desalinated water, even if we encounter any operational problems.
Hydrogen peroxide and peroxide acid
Hydrogen peroxide or a mixture of hydrogen peroxide and peroxide acid. It has been used successfully to treat biologically contaminated reverse osmosis membranes.
Two factors that greatly affect the rate of hydrogen peroxide attack on the membrane are temperature and iron.
The presence of iron and other transition metals in conjunction with the hydrogen peroxide solution can also cause membrane degradation. Samples of some membranes were tested using 0.15% hydrogen peroxide solution. After 150 hours, the passage of salt to the membrane begins to increase very significantly. Continuous exposure at this concentration may eventually damage the membrane.
Instead, it is recommended to use periodic operations.
A biologically contaminated RO water desalination plant maintenance the following procedure is recommended for the application of hydrogen peroxide solutions
- Any kind of deposit on the film or any other impurities should be removed using an alkaline wash before sterilization. Removing these sediments will double the degree of sterilization.
- After the alkaline washing process, the membranes are rinsed with desalinated water.
Clean the membrane surface using an acid washing process to remove iron from the membrane surface. After that, the membranes are rinsed using desalinated water. - Submerge the membranes with a chemical solution of 0.2% hydrogen peroxide added to desalinated water at a temperature of less than 77°F (25°C) for 20 minutes. When the pH value is 3-4, this gives better results and longer film life.
Sterilization with chlorine and other biological products
Crude chlorine and chlorine dioxide are not recommended for disinfecting membranes. Quaternary biocides and phenolic compounds reduce membrane productivity and therefore are not recommended for use as biological agents in sterilization processes.
Heat sterilization
Heat sterilization is the preferred method in food and pharmaceutical applications. The advantages of hot water as a sterilizing agent are:
- Reaches areas where chemicals cannot reach.
- Simpler to monitor heat than chemical concentrations.
- Easier to demonstrate the complete distribution of heat.
- No need to rinse off chemicals.
- No need to store chemicals.
- Reduces waste disposal issues.
- No need to approve chemicals.
The steps of heat sterilization consist of the following:
- Rinse the concentrated water using sweetened water of low pressure and low flow rate.
- Recycle warm water (below 45°C) across the membranes through very low pressure (1.5 bar) with the highest flow pressure that can reach (3bar).
- Raise the water temperature to reach 80 degrees Celsius.
- Maintain membrane-permeable pressure (1.7 bar) when fed with warm water or hot water (45°C or more).
- Maintain the temperature for 60-90 minutes.
- Complete system cooling at 45°C or less.
- The concentrated water is rinsed to the lowest possible membrane pressure (1.7 bar) with the highest source water flow pressure (3 bar).
