10 Steps of How To Design Seawater Reverse Osmosis Systems

Seawater reverse osmosis systems, SWRO is a special and independent process system in the reverse osmosis water treatment technology. Typical seawater reverse osmosis systems are mainly composed of four major parts: seawater intake, seawater pre-treatment, reverse osmosis treatment, and post-treatment for production. The design method of seawater reverse osmosis is introduced in the following ten steps.

1. Preliminary Determination of Technological Process

The process design must be conducted in any design method, and SWRO process design is more important, which includes the selection of following mentioned equipment type, determination of the parameters and calculation of the equipment in the whole system. Therefore, when the reverse osmosis desalination system is designed according to customer’s requirements, a preliminary technological process is first determined. And then determine the type and parameters of the equipment according to the process.

It should be pointed out here that this initial process and the final determined process may be very different. Because in the design process, according to the calculation, it is probably to adjust equipment selection, parameters, and equipment layout, etc.

After the initial process is determined, the system can be designed according to the following steps, but there is a certain connection between each step of the design process. The final technological process will be revised through comprehensive design.

2. Determination of Hydraulic Equilibrium Diagram

With an initial process, the hydraulic equilibrium diagram can be determined and calculated. Hydraulic equilibrium diagram is very important in seawater reverse osmosis systems. It can be said that it is the verification and validation of preliminary process, and also provides the data for the next step of the equipment selection, parameter determination and calculation. The factors to be considered include the chemical composition of seawater, seawater intake method, system recovery rate, system energy recovery, the loss of water in the process, and the chemical composition of production water.

3. Determination of Seawater Intake

In seawater reverse osmosis systems, seawater intake has a great influence on the pre-treatment of seawater. It will seriously affect the efficiency of the reverse osmosis if factors are not considered well, and will increase the working load of the security filter and reverse osmosis membrane. The following factors should be considered in water intake: water intake location, the impact of the typhoon on the intake facilities, delivery method from intake location to pre-treatment system, intake pump type(diving pump or sucking pump), seawater corrosion, and the microorganism, bacteria in sea water, etc. After considering the above factors, in general, there are three kinds of water intake: beach well intake, surface seawater intake, and seabed filter intake. How to select which water intake method in detail, it should be determined by considering all kinds of factors.

4. Pre-treatment Determination

Multi-media filter is mostly used in the pre-treatment in domestic and international seawater desalination system nowadays, which function is to remove impurities in raw water, colloid, microorganism and other suspended solids, and make the pollution index of filtered seawater less than or equal to 5, meeting the pre-treatment requirements.

To the large-scale seawater desalination system, in order to protect RO membrane, and prolong membrane lifespan, ultra-filtration membrane is also necessary. Production SDI of ultra-filtration can be less than 3, and its production quality is better than that of the sand filter.

5. Selection and Calculation of Security Filter

Security filter: it is the last line to enter the reverse osmosis system. The minimum filter accuracy is 1um. According to hydraulic equilibrium diagram, it can know the water flow of security filter QT, select filter type and determine each filter flux Q, so the number of security filter. Security filter usually select bag or micro-filtration membrane kind. If you want to know more about the security filter, you can check How Multi Medium Filter Works in Desalination Systems.

6. Determination and Calculation of Equipment Types and Parameters in the Main RO System

Reverse osmosis system is the core of the whole project, mainly composed of the reverse osmosis membrane module and a high-pressure pump. The core of the design is to arrange different water recovery rate according to different raw water quality and to make the system economic as far as possible through the process. Reverse osmosis is the reverse movement of permeation, it is a kind of solution that separates solute and solvent by semi-permeable membrane under the driven pressure. Seawater desalination technology is to remove inorganic ions in seawater, bacteria, viruses, organic matter and colloid and other impurities by reverse osmosis, producing high-quality desalinated water. Reverse osmosis membrane module is the key part of the seawater desalination system. Seawater is lifted by the high-pressure pump into the membrane module and water is desalinated through membranes. Nowadays RO membrane is applied widely, and there are lots of its manufacturers abroad, such as DOW made in USA, Hydranautics, and Toray in Japan. Different manufacturers of reverse osmosis membrane have different characteristics, and different membrane manufacturers have their own RO membrane calculation software, such as Dow Rosa, Japan Toray RO. To the membrane module arrangement, the software will calculate according to the design.

The high-pressure pump is used to lift seawater into reverse osmosis membrane with 6~8MPa pressure of the seawater, which is the required pressure for reverse osmosis system to achieve desalination. One of the most important characteristics of reverse osmosis seawater desalination is to adopt HP pump to delivery seawater into membrane module, where fresh water is permeated into another side of the membrane through high pressure, so as to achieve RO desalination. It is also easy to select HP pump, and there are lots of HP pump manufacturers abroad, such as famous Danfoss. In general, the multi-stage centrifugal pump is selected in large seawater desalination system, and plunger pump is selected in small seawater desalination system. It has no special demand for HP pump type and its calculation, but there is a big difference among materials. Here the HP pump is duplex stainless steel for the seawater corrosion is great.

Energy recovery device is a kind of device which can recover the pressure of concentrated seawater. It can transfer high pressure of concentrated seawater to the raw water, so as to reduce the flow of the high-pressure pump. PX series energy recovery device produced by USA ERI company is applied widely in seawater desalination reverse osmosis system, accounting for more than 90 percent of the whole desalination, and its recovery rate can reach to more than 95 percent.

7.Production Post-treatment

The production desalinated by reverse osmosis can meet the standard of drinking water of GB5749-85 standard. And most of the indicators are beyond mineral water. In the process of pre-treatment, pre-filtered water is acidic because of reductant. In order to adjust PH, it is required to add alkali for neutralization. And then delivery into the use of point according to customer’s requirement. If production is required to be treated further, it can adopt 2nd RO or ion exchange process.

8. Selection and Calculation of Dosing Device

Dosing system in seawater desalination is an indispensable part. Therefore, the chemical dosing system configuration and dosing kinds, and the calculation of the amount is very important. First of all, NaClO or chlorine is added into the intake wells or intake pipelines to kill bacteria and algae. Before the sand filter, the flocculant is then added into the pipelines to condense colloid and impurities in the water which is conducive to the filtration. At last, antiscalant is sometimes added to the filtered seawater according to the situation of water quality, preventing smaller colloid and scaling in the RO membrane. And reductant is added to restore the left free chlorine when adding bactericide, for reverse osmosis membrane has very strict requirements for the free chlorine, which will have very strong oxidation on the reverse osmosis membrane.

Here we take NaClO as the calculation example as follows:
(Ps: Q is dosing pump flow, X is added dosage, q is water flow and w is dosing density.)

9. Material Selection Principle of Seawater Desalination Pipeline

1)PVC, UPVC pipe, or carbon steel lined pipe can be selected in water intake and pre-treatment part.
2) Duplex stainless steel is adopted in the main part of reverse osmosis system.
3) PVC or UPVC pipe can be also selected in the post-treatment part.

Read more about how to choose pipelines for seawater desalination plants.

10. Control Design Principle

In seawater reverse osmosis systems, PLC is used as the controller to control the on-line signal and control the interlocking signal. The industrial control computer is used as the monitor, and the “configuration” software is used to develop a unique “seawater desalination treatment technology process monitoring picture”, which shows the operation condition in the whole plant.

11. Conclusion

The seawater reverse osmosis systems design method described in this paper is a brief summary of the present water treatment industry materials and process development. With the development of technology, the development, and utilization of new energy sources, the design method will change accordingly.