Stages of reverse osmosis systems
The reverse osmosis system is easy and simple, and it consists of a group of tubes that bear high pressures and contains reverse osmosis membranes, which carry out the process of separating water ions as a result of water pressure by means of a pump.
The process results in two water flow, the first is the produced water with low salts that do not exceed 4% of the salts of the feed water. As for the second flow, it has a high salt concentration, which is generally thrown and sometimes re-entered to the beginning of the system and to pass in a second stage of the membranes, which allows raising the efficiency of the system and its production capacity.
The main consideration for designing an industrial reverse osmosis system is to determine the quantity and quality of water required, which ensures a successful design that matches the requirements of the end-user.
The industrial reverse osmosis system consists of three main stages:
- Pre-Treatment
- Treatment or desalination stage
- Post-Treatment Or Adjustment
1 – Initial or pre-treatment stage:
The components of this stage are chosen very carefully, and they play an important role in preventing the later stage from serious damage that costs a lot of effort and money.
This stage aims to reduce the possibility of corruption, calcification and dissolution of the osmosis membranes during operating periods, and most membrane manufacturers, through scientific publications or computer programs, rationalize the designer to the requirements of primary treatment and determine the types of chemicals to be used, all based on detailed chemical analyzes of samples of raw water.
It should be noted here that Ground Water is usually characterized by stability in components and temperature, and this is useful in an initial treatment that is less expensive and complex than that which will be used with surface water, which is undergoing clear changes with the change of climatic seasons and natural factors.
Silt Density Index is the method that will determine the number of particles in the feed water and therefore the means to be used in the initial treatment such as clarification, filtration, or addition of polymer materials.
And because the after-treatment unit will result in a stream containing a high salt concentration that is separated by membranes, the solvent materials for salts such as Calcium Carbonate CaCO3 – Calcium Sulfite CaSo4 – Barium Sulfite BaSo4 – Silica SiO2 must be taken into consideration as the System Designer depends on the procedures next:
- Calcium removal by Softening Ion Exchange unit.
- Infusion or glaucoma of the following substances:
Acid.
Antiscalant material.
Acid and anti-scale together.
- Reducing the recovery rate.
Once again, we reiterate that the designer will choose the possibilities of the initial treatment based on the data of each project. Adding acid instead of the anti-calc may be an excellent option if you want to get rid of hydrogen sulfide gas as well, but this same option may be bad in another application because of the formation of Carbon dioxide, which requires the installation of an additional system to get rid of it, called Polishing Ion Exchange Unit, after the stage of treatment or desalination.
2 – The stage of treatment or desalination:
Film manufacturers at this stage have a major influence by determining the type and number of membranes used in the system.
The ideal flux rate is 15 gallons per square foot/day for surface water.
This percentage may change to other amounts if the feeding water is from sources such as groundwater, seawater, or wastewater.
After calculating the number of membranes, the required number of vessels will be determined. Most industrial systems use pressure vessels containing 6 membranes, but pressure pipes containing between 3 and 7 membranes can be used.
It remains to point out that the space allocated to the desalination system sometimes plays a role in determining the number of pressurized tubes.
The next task revolves around the appropriate arrangement of these compressed tubes and the membranes inside them, where the tree arrangement prevails most of the time, and this means that the first stage contains the largest number, then the second stage less, and then the third stage, if there is the least number of the first two.
The purpose of this arrangement is to reduce the possibility of membrane failure after extended periods of operation.
In industrial systems that use 6-membrane carrier tubes, the following considerations are taken with regard to recovery:
- The first: 45 to 55 percent of the recovery
- Second: from 70 to 80 percent of the recovery
- Third: 80 to 90 percent of the recovery
As for the typical design, it stops at the two stages and has a 75% refund.
Benefits of using design software:
The computer programs that membrane manufacturers distribute to water treatment companies for free or at nominal prices aim, in addition to marketing encouragement, to provide advanced service, necessary technical support and to estimate the expected performance of membranes, as well as provide warnings to designers when they violate manufacturers’ instructions and directions.
These programs also help designers quickly experience the options available to designers and make decisions effectively and economically at the same time.
3 – The final stage or post-treatment:
At this stage, the water produced permeate from the membranes is examined and its pH is adjusted, which usually decreases as a result of the osmotic pressure and is raised to its neutrality again, i.e. 7.5 by adding chemicals such as caustic soda or others.
Chlorine is also added at a rate ranging between 0.1 and 0.5 parts per million to the water flow destined for storage or distribution through networks, in order to protect the water from bacteria when exposed to natural factors and to protect users.
The reverse osmosis units are available in different sizes, some of them are very small for domestic use, whose production capacity ranges between 100 and 3000 liters per day, up to the huge uses for feeding factories, villages, and cities, where the production capacity in some of them reaches more than one hundred thousand cubic meters per day.
Types of reverse osmosis membranes:
Reverse osmosis membranes come in several types, the most important of which are Spiral Wound membranes and Hollow Fine Fiber membranes.
Reverse Osmosis Membranes Materials:
All these films are made of Cellulose Acetate, Aromatic Polyamides, or, as is the case nowadays, Film Polymer compounds.
Benefits of reverse osmosis membranes:
These membranes remove more than 75 percent of the salts.
Remove most types of organisms, viruses, germs, and other chemical contaminants.
The pores of different types of membranes vary in size, ranging from less than 10 angstroms to 100 microns.
A brief description of the features of each type:
1 – CELLULOSE ACETATE:
Tolerance to chlorine.
Not resistant to bacteria.
The pH ranges between 6 and 8.
Good water production rate.
It should be used with water containing a certain percentage of chlorine.
One of the most common types of membranes in the market.
2- CELLULOSE TRI-ACETATE:
Tolerant of chlorine.
It tolerates many types of bacteria.
The pH ranges between 4 and 8.
Excellent water production rate.
The use of water containing chlorine prolongs its life.
3 – THIN-FILM COMPOSITE:
Sensitive to chlorine, which needs to be removed before the water reaches the membranes.
Resistant to bacteria.
The pH ranges between 3 and 11.
One of the most types of membranes that produce water.
High salt rejection property.
Long years of service if the correct operating specifications are available.
