What is a reverse osmosis membrane washing plant?
Membrane washing of the reverse osmosis plant is a chemical process in which the salt deposits formed inside the membranes during operation are disposed of through the Cleaning In Place Unit attached to the desalination plant.
Some stations do not have their own washing unit, so a mobile unit is used for this.
When should the decision be taken to discontinue the reverse osmosis plant so that the plant membrane washing?
There are two ways to do this: the first is through the instrumentation of the station. The second: depends on examining the membrane practically to be sure, and the two methods must be done in succession
First: by instrumentation
In it, three important elements can be noted:
- Differential pressure or the pressure difference between the feed pressure and the pressure of the rejected water. Reject pressure by monitoring the pressure gauges. The pressure difference is usually between 1.5 and 2.5 bar. But the pressure is allowed to reach a certain limit, after which the station must be stopped and the washing started. Which is 4 bar. The greater the pressure difference over 4 bar. The more difficult the washing process.
- Total salinity of the produced water TDS: When the salts are on the surface of the membranes covering part of its surface. This leads to an impediment to this part of its ability to separate the salts present in the water. thus affecting the produced water by increasing the salinity in it.
- The capacity of the station or its production capacity: The formation of the scale inside the membranes reduces the chance of the station producing the quantity designed for it. So the flowmeter reading will be less than the required value.
Second: Checking the membranes
After making sure through the previous steps that there is actually a change in the value of the parameters on which the station is designed, we unpack one of the end caps of the pressure vessel and check the membranes (Visual Inspection).
Why is this step important?
- Ensure that the deposits inside the membranes are scale, fouling, or both. In order to determine the type of chemical washing that will be carried out, the selection of the appropriate chemicals for it, and the order of their use.
- Taking a sample of the formed scale and analyzing it qualitatively and quantitatively to find out the type and quantity of sedimented materials.
The last step in the examination process is to weigh the membrane a quarter of an hour after it has been removed from the container. Normally the normal weight of an 8-inch membrane is 16-17 kilos. Where washing is easier if it weighs less than 20 kg. And the higher the weight, the more difficult the process of washing, and if the weight of the membrane reaches 30 kg, the process is almost impossible.
The reasons that lead to the formation of these sediments inside the station?
Causes of scaling on the membranes
It depends on the design of the station. If there are some errors in the design, crusts will continue to form, and their treatment will be expensive, and the operation will be more expensive. Because the cost of solving problems during operation is much greater than the modification.
1- Do not add acid before the water enters the RO membranes
Or not adding it in the required dose (and as we said, it lowers the pH to 5.5 – 6.5 and converts the hard salts from precipitated to dissolved salts that are disposed of with the Concentrate.
2- Not adding antiscalant
Not adding it from the start or not adding it in the appropriate dose recommended by the supplier and appropriate for the analysis of the feed water or not using a suitable antiscalant or not taking into account the pH at which the antiscalant works.
3- Leakage of salts in the initial treatment area
As an infusion of Softener or sand or carbon filters by any means.
4- Change in the values of salts, specifically hard salts, in the feeding water
Especially well water, which is sometimes characterized by the change of salts in it from time to time. So the feeding water must be monitored and the periodic analyzes that we mentioned should be carried out even if there is a change in the composition. It must be followed by a change in the doses of acid and anti-sedimentation.
5- A common mistake is to operate the station above its designed capacity
So that by increasing the percentage of the recovery or water produced than the permissible limit in order to obtain higher productivity.
It is recommended that the recovery be between 50-80% of the volume of the feeding water. This percentage depends on the salts in the feeding water…the more it increases, the recovery should be reduced. the lifespan of membranes.
6- Sometimes we resort to the initial treatment to add acid to reduce the pH.
Some water sources have a pH above 8.5. which causes calcium carbonate to be poorly soluble in water. It is easily deposited on the membranes. As for adding acid and reducing the pH and thus reducing the value of LSI. It follows chemically that calcium salts (such as carbonate Calcium) turn into soluble bicarbonate with low pH and do not settle on the membranes.
7- There are other reasons such as concentration polarization, β-factor, and Flux.
Components of a CIP Unit in desalination plants
It is a tank connected by pipes with the desalination plant. So that the water is circulated in a closed circuit. That includes the tank with all the pressure vessels inside the R.O station. Through a pump, the water moves from the tank through the pump to the station and back again to the tank. With the assurance that the station’s production line and the rejected water line flow into this tank.
Precautions to be observed before and during the process of membrane washing of the station
- Observing the safety conditions for the workers and technicians of the place regarding dealing with chemicals, and the presence of a safety and security bulletin for all the materials used.
- Ensure that the water used in the washing process is completely free of free chlorine.
- During the water recycling process, make sure that the production and return lines flow from it into the tank.
- The water pressure on the line does not exceed 4 bar by adjusting the valve of the circulation pump.
- Ensure that all valves for feeding and rejecting water are fully open.
- Ensure that the water flow rate does not exceed 40 gallons per minute.
- The pH value of the solution prepared inside the tank for all types of chemicals used must be between 2 – 12, as well as at all flow points.
Remove any float switch on the tank to avoid any back pressure on the membrane. - Fill the washing tank with the required amount of water according to the calculations agreed upon in the washing operations.
If the station consists of 2 or 3 stages, it is preferable to separate each stage and wash it separately.
Calculate the amount of water required for one wash:
Each 8-inch membrane needs 32-35 liters, so we count the number of membranes in the station multiply it by 35 liters plus tank volume and add 20% to it.
Method of membrane washing of the Reverse osmosis station chemically
From what we mentioned previously, we determine exactly what the problem is by analyzing the scale and knowing its type and quantity. And if it is fouling, and thus we can determine the materials required for washing.
General, different and potential cases of problems when membrane washing plant:
1- If the problem is fouling, the washing should be done with alkaline cleaners such as caustic soda, along with detergents.
In the case of fouling, the use of any acid is cautious because the acid causes all the fine particles to stick to the surface of the membranes. This leads to the difficulty of removing them except by removing the effects of the membrane. So it must be checked if there is fouling, whether biofouling or normal fouling
2- In cases of salt deposits resulting from the accumulation of carbonate salts. It is preferable to use acids such as citric acid or sulfamic acid and sometimes hydrochloric acid.
3- In cases of metal oxides and sulfate salts, of course, it is preferable to use chelating agents
4- In cases of the presence of silica, of course. It is a very difficult salt, HF can be used, but with caution.
What are silica husks:
The element silicon Si is present in groundwater in a dissolved or insoluble form…
If it is in the dissolved form, it will be in the form of silica (SiO2) … silicon bound to two oxygen atoms … and its percentage in the feed water ranges between 1 – 100 parts per million (and in seawater, it is between 0.4 – 8 parts per million).
The chemical form of silica is determined by its pH.
The predominant form of silica at pH 7 or less is Meta Silicic acid with the chemical symbol (H2SiO3)n. It is a weak acid in one of these forms:
SiO2 – H2SiO3 – H4SiO4 (-Si(OH)4)
We note that the N denotes the number of particles.
If the saturation of this acid occurs in water and exceeds it, it means a Supersaturation event, meaning above saturation, then it is a polymer and the value of n increases to form a long chain of insoluble colloidal silica or amorphous silica gel is formed … and these compounds cause crusts on membranes;
Dangerous to form silica crystals
If the pH increases above the neutral point of 7. the concentration of hydrogen ions decreases and the silicic acid disintegrates into anion silicate and its symbol (HSiO3-)n. It combines with positive cations such as calcium, magnesium, iron, manganese, and aluminum to form insoluble silicate salts. Which also causes crust formation.
Silica Fowling usually appears in the front membranes. Unfortunately, silica crystals cause deformation of the membranes, and chemical washing usually does not work.
However, silica remains more soluble at high pH after eight. But it is active in interaction with positive cations as we have explained. Its solubility decreases until it is absent between 8.2 – 7 and then rises slightly at a lower pH.
Ph affect
Also, in a lot of research, aluminum and iron are present in the feed water. It precipitates with silica even if it is undersaturation. Therefore, iron and aluminum should be less than 0.05 parts per million in the feed water even if the silica is less than saturated. As we can see The pH factor is also important. All of this is taken into account when chemically analyzing raw water. Especially when using broths or coagulants that consist of iron or aluminum. From tracking the pH in the osmosis unit, we find that it is less than 7.5. For example, to 7, 6.5, or less, and here lies the risk, based on what we have said. The formation of colloidal or amorphous silica with increasing concentration and the formation of sediments.
5- It is possible to have barium, strontium, and fluoride salts, which are the most difficult to remove. Therefore, we always prefer that the water be pre-treated well, as we mentioned in this article before the water enters the station and it accumulates problems. Therefore, it is very important to analyze the water in the first place and draw a visualization of the problems and how to overcome them in the design.
The degree of difficulty and severity of the formed scales can be classified according to the following order, from left to right:
CaCO3 ==˃ CaSO4 ==˃ Silica ==˃ SrCO3 ==˃ BaSO4 ==˃ SrSO4 ==˃ CaF2 ==˃ CaSiO3 ==˃ MgSiO3 ==˃ Ca3(PO4)2
How are deposits formed?
The first stages of sedimentation begin where crystals (crystals) of the salts are initially formed in a stage we call pre-clustering. Or the stage before the formation of crystal clusters. These crystals are a trap for other particles of salt above the saturation stage. They accumulate on top of each other to form huge crystals. That is deposited on the surface of the membranes and causes tragedy.
In fact, the dynamics of the sedimentation process are still mysterious and not fully understood. And it is affected by several factors such as temperature, pH, and the chemical composition of the salts themselves. And the time required to form the crystals of these different salts. Which we call in the year the induction time.
Pictures of chemical deposits that appear during membrane washing:
- Silica.
- Positive cations: calcium, magnesium, barium, and strontium.
- And negative cations: carbonates, sulfates, and phosphates.
The formation of scales causes important symptoms. The most important of which is the heavyweight of the membrane and a decrease in the flow of primates due to the occurrence of sediments blocking the membrane holes.
