Reverse osmosis

Reverse osmosis

The natural osmosis is the physico-chemical process by which the salts transfer from a more diluted to another more concentrated solution across a semipermeable membrane takes place. If this process is done by means of a pump or a pumping equipment it is when the concept of reverse osmosis appears, The separation media is what is called membrane and the two solutions are called permeate (low percentage in salts contents) and brine (high percentage in salts contents).

To carry out the process of reverse osmosis it is necessary driving the water supply, what we call feed solution, using what is known as the high pressure pump. So that, it is required to provide an approximate value of 70 kg/cm2 pressure with what will be two flows from the first: a permeate - with optimum quality characteristics and leaving virtually no pressure--and brine or concentrate- laden salts with a pressure almost equal to the current of the power supply. The diagram scheme may be represented as follows:




But... What is a membrane? and what is it known with the name of membrane?. The answer is "a membrane is nothing more than a medium separator between the water supply to a seawater desalination plant and the water product". This separating medium has had many shapes throughout the History (from flat-up membranes to hollow-fiber membranes and, also, the spiral-wounded ones) and have been built based on several materials including cellulose acetate and aromatic polyamide. They are the key elements where the reverse osmosis takes place and to know them more deeply, it is necessary to delve a little bit into what are the guts of the desalination process and hence easily understand the true architects of the salts removal process from the water supply to permeate.

As shown in the figure, a membrane is a set of rolled sheets so that, with the passage of the feeding stream through it, two new streams are produced:

- on the one hand, a permeate, -low salts concentration- and

- on the other hand, a stream called rejection or concentrate -high salts concentration.( from now on we will call it brine).

It must be said that, currently, aromatic polyamide material has been imposed on the cellulose acetate even though personally, I also like the behavior that presents the cellulose acetate in comparison with aromatic polyamide as it has some advantages: it can bear the chlorine effects better than aromatic polyamide and, for purposes of carrying out the removal of organic matter by means of the injection of chlorine it has been proved to be the most effective one. The problem, as It was already treated in the chapter referring to the pre-treatment, is that bacteria are got used to live together in a state of unease and finally become immune to a shock provided chlorine to produce biofouling elimination. For this reason, there are several membrane manufacturers that advocate the chlorine injection permissiveness to a dose of sodium bisulphite supply, which inhibits the effect of chlorine during thirty minutes a day so bacteria do not become accustomed to the presence of chlorine as a bactericidal agent.

Although it was already mentioned in previous articles, to improve both the quantity and optimize the process quality what we have to do best is pumping a first stage of an osmosis concentrate to a new rack so a Booster pump is required to be included in the process. This configuration is deprecated nowadays with the configuration of the Pressure Center Models that were already explained in the previous article, but it was very useful to the current knowledge of the technique of the brine energy recovery after the inclusion of reverse pumps, on the one hand, and Pelton turbines on the other hand.

Similarly, settings for high pressure pump that feeds to a single frame of reverse osmosis membranes is considered deprecated and currently the concept that is imposed is the pressure center with a single pump that feeds several frames simultaneously. This configuration, as explained in the previous chapter, is which is imposed in the current design for the large desalination plants, whose capacity is greater than 100,000 m3/day and is based on the power through a common manifold as shown in the image below. The supply line is not observed since it is buried under the process plant to not subtract operation to it, although it has a number of branches willing to feed the different frames of the desalination plant.

Initially, reverse osmosis desalination was associated with the membranes of 4 inches for household plants. Subsequently, it was expanded the use of the same industry to impose the 8-inch-diameter membrane. For some time, it was scheduled working in 16 inches models but it has the disadvantage that are not easily manageable when it comes to placement in the pressure vessels, and also their price is maintained even high so I think that it will take even enough time to produce its widespread use.

The membranes are connected to each other via so-called connectors which can be telescopic type although there are commercial houses that have opted for a solution type perfect mating (without connectors) so that this union is perfect and with minimal loss of transfer between consecutive elements interconnected.

There are also commercial houses that proposed an elements orientation vertically instead of horizontally. The problem I see in this type of configuration is the difficulty for placing membranes inside them when start-ups or while changing membranes, forcing a complex operation to do so, although cranes or hoists from the own process building can be used to easy it.

Another variant of the pressure vessels configuration is multiport feed vs the mono-port feed. In the first one, a considerable saving in the configuration of the pressure vessels rack occurs in terms of cost per linear metre of steel structure, while in the mono-port the flow is distributed in a more simple and homogeneous way. As it can be seen, everything has its pros and cons.However, I personally think that membrane racks should be configured in the same way as currently are in order to obtain optimum facility production. Before placing the membranes, it is necessary to proceed to the pressure vessel cleaning for which a ball of polyurethane as a sponge. It must be dipped in glycerin to remove remains of cuttings that may have been formed inside the pressure vessel after the total rack assembly conformation. Once clean, it is necessary to close it up by placing a lid with caps at its ends so system will be insulated and ready for the startup. There are pressure vessels manufacturers that, anticipating this situation, have made a variant with dummy cover pressure vessels so that the load of the membranes is done on one side only.

Another possibility that has been adopted is transporting assembled membrane racks and place them in the installation on its corresponding layout point to, subsequently, proceed to start up the desalination plant.

Normally, it stands a rack of high rejection of boron membranes followed by a another rack with high rejection of salts membranes with the result of an excellent matrix. Also, with the same previous purpose, membranes of different types within the same pressure vessel forming what is called a hybrid system with high boron rejection and high salt rejection membranes. In general, for a pressure vessel containing seven membranes, four membranes must be salts high rejection along with three membranes of boron high rejection although the variability of the model greatly depends on the quality of product water in accordance with the established criteria in the project specification.

Another possibility is to raise the pH of the power to a value of 10,5 or 11,0 to help boron removal with a double pass system in which the permeate passes through another rack of membranes for Boron rejection. Other possible solution is passing the permeate through what is called Boron removal resins. Unfortunately, the cost of this kind of solutions is quite high.

Other variants with which we can find in the facility operation is to modify the so-called conversion which is defined as the ratio between the current product and the main inlet stream. Normally, for the purpose of achieving better energy efficiency, an optimal conversion is set to 45%; i.e. outlet flow as permeate represents 45 percent of the inlet stream to the membrane, rejecting the rest of it as brine. In addition, it is also helpful modifying the so-called flux (flow of water per square metre of membrane surface) which varies depending on the membranes manufacturer although it is usually around a (12-15) L/m2-h value.

Despite everything that has been already written, in terms to ensure the recovery of this stream to improve the system performance, It is necessary to know the ions analysis within the inlet stream to the sea water desalination plant first and prior to be installed. To do so, it is recommended to take some samples throughout the day and at different seasons of the year analysing hence the variability of both physical and chemical parameters. This is essential to avoid problems that may appear during the system operation throughout the forthcoming years, what comes to be about 25 years, on average.

The truth is that, unfortunately, there is not always time to carry out this test sample or that the operator is not the same as the constructor of the desalination plant, so these aspects are often overlooked. These arguments have already been exposed in the first article of these series whose titled is "The importance of a good sea water intake".