The design of a reverse osmosis system is based on original data such as raw water quality, product water quality and production volume requirements, discharge volume requirements, and site conditions. This involves selecting a reasonable water treatment process flow, choosing appropriate membrane elements, determining the number of membrane elements and their arrangement, and selecting high-pressure pumps. The goal is to minimize operating pressure and membrane element costs while maximizing recovery rates and reducing system fouling rates. This extends the system cleaning cycle, reduces cleaning frequency, improves long-term system stability, and lowers cleaning and maintenance costs and reagent expenses. Therefore, the design work is a crucial step in the construction of a water treatment system, playing a decisive role in project quality, investment, and water production cost control.
The basic process of a reverse osmosis system includes three steps: pretreatment, reverse osmosis, and posttreatment.
Pretreatment typically employs processes such as sterilization, coagulation and sedimentation, multi-media filtration, activated carbon filtration, and microfiltration. After pretreatment, the pollutants in the raw water are reduced, meeting the membrane element’s requirements for feed water quality. The reverse osmosis unit is the core component of the reverse osmosis desalination process. In the reverse osmosis unit, most of the salt in the feed water is removed, along with organic matter, bacteria, and other contaminants.
Post-processing steps are set according to the application requirements, such as CO2 removal , ion exchange desalination, and electro-deionization (EDI).
1. Inlet Water Source
The water quality requirements for our company’s reverse osmosis feed water are shown in the table below :
Table 5.1 Water Quality Requirements for Reverse Osmosis Feed Water
|
Index |
Unit |
Allowed values |
|
Turbidity |
NTU |
<1, ideally kept below 0.2. |
|
SDI15 |
|
<5, it is recommended to keep it below 3. |
|
Particulate matter |
pcs/ml |
Substances larger than 5µm are not allowed to enter the RO system. |
|
Microorganism |
pcs/ml |
<1 |
|
Iron Fe3+ |
ug/L |
When dissolved oxygen is >5 ppm, it should be <50 ppm; the lower the better. |
|
Manganese |
ug/L |
<50, the lower the better |
|
Al |
ug/L |
<50, the lower the better |
|
Oils and fats |
|
Must not be detected |
|
TOC |
ppm |
<5, it is recommended to keep it below 3. |
|
COD |
ppm |
<10, the lower the better. |
|
BOD |
ppm |
<5, it is recommended to keep it below 2. |
|
H2S |
ppm |
<0.1, the lower the better. |
|
Alcohol |
% |
<10, the lower the better. |
|
Surfactants |
|
Must not be detected |
|
Residual chlorine |
ppm |
<0.1 |
|
Ozone |
ppm |
0 |
|
CaCO3 |
|
LSI<0 |
|
CaSO4 |
% |
<230 |
|
BaSO4 |
% |
<6000 |
|
SrSO4 |
% |
<800 |
|
CaF2 |
ppm |
Concentration on the concentrate side <1.7 |
|
CaPO4 |
|
The concentration of the concentrated solution must not exceed its solubility. |
|
SiO2 |
% |
<100, empirically recommended to keep the influent ppm below 20 ppm. |
|
ph value |
|
3-10, it is recommended to keep it between 6.5-8.3. |
|
Temperature |
℃ |
5-45, it is recommended to keep it around 25℃. |
|
Hardness |
ppm |
Unless otherwise specified, if the concentration exceeds 50 ppm, acid should be added to adjust the pH to around 5, and a scale inhibitor should be added. |
|
Flocculants/coagulants |
ppm |
Must not be detected |
2. Reverse Osmosis System Design
Reverse osmosis membrane system design procedure
The general procedure for designing an RO system is as follows:
- The type of membrane element to be used is determined based on the water source and water quality.
- Based on the requirements for permeate volume and quality, determine the number of membrane elements, the arrangement of membrane modules, and the system recovery rate;
- Select a high-pressure pump based on the calculated driving pressure required for the membrane module;
- Configure instruments, valves, and other accessories;
- Select the pressure piping; determine the system control method; and select the electrical components used in the system.
Selection of Membrane Housing
Regardless of the type of membrane element, it must be placed in a pressure vessel before it can be used. Because each membrane element has a different size, the size of the pressure vessel used to house them also varies. Common pressure vessels are 4 inches or 8 inches in diameter, but their basic construction is the same.
In each pressure vessel, one membrane element can be installed, or several membrane elements can be installed in series. Typically, 1 to 7 membrane elements can be installed in each pressure vessel.
When selecting the pressure specifications of a pressure vessel, the required water supply pressure in the system calculation and analysis should be met, and the pressure increase required due to contamination during operation should also be taken into account (generally, the design is based on at least three years of contamination, i.e., the pressure should be increased by 15%).
Pressure vessels are made of either stainless steel or fiberglass reinforced plastic (FRP). FRP membrane shells, especially 6-core 8-inch membrane shells, should be used whenever possible. Stainless steel pressure vessels vary significantly in processing precision, and if the raw water has a high chloride ion content, leakage or even pitting and perforation can easily occur after a period of use.
The number of elements connected in series within each pressure vessel is proportional to the system recovery rate. Typically, the recovery rate of a single membrane element is between 8% and 15%, a single-stage system with six elements connected in series can achieve a recovery rate of 50%, a two-stage system can achieve a recovery rate of around 75%, and a three-stage system can achieve around 85%.
Table 5.2 Relationship between Component Quantity and Recovery Rate
|
System recovery rate (%) |
Number of series components |
Pressure vessel section containing 6 components |
|
40~60 |
6 |
1 |
|
70~80 |
12 |
2 |
|
85~90 |
18 |
3 |
Selection of Membrane Elements
Membrane elements are selected based on the salinity of the influent, the risk of fouling, the required desalination rate, the permeate volume, and the energy consumption requirements. They are generally categorized into brackish water, ultra-low pressure, fouling-resistant, and seawater desalination types. For details, please refer to the membrane element selection method described above.
System permutations and series
Depending on production needs, multiple membrane modules can be arranged into single-stage, two-stage, or even multi-stage configurations. Within each stage, the membrane modules can be further arranged into single-stage, two-stage, or even multi-stage configurations. A single-stage configuration refers to the feed solution undergoing one pressurized reverse osmosis separation, a two-stage configuration refers to two pressurized reverse osmosis separations, and so on. Within the same stage, identical membrane modules form a single stage. Single-stage with two-stage and single-stage with three-stage configurations are commonly used in water treatment.
(1) First-level multi-segment arrangement
This arrangement is a multi-stage concentration system. The concentrate from the first stage serves as the feed water for the second stage, and the concentrate from the second stage serves as the feed water for the next stage. In this case, the permeate from each stage is discharged directly. Therefore, as the number of stages increases, the overall recovery rate of the system increases. The concentrate flow rate decreases with the number of stages. To maintain the same concentrate flow velocity on the membrane surface of each stage, the number of membrane modules connected in parallel in each stage can be gradually reduced, making the feed water flow rate into each module equal. This is the common conical arrangement in water treatment. This process is suitable for applications with large treatment capacities and high recovery rates, and is most widely used in industrial water desalination.
Characteristics of multi-segment systems:
- Improve system recovery rate without exceeding the recovery limit of individual membrane elements.
- A typical arrangement ratio is 2:1 (the ratio of the number of upstream to downstream pressure vessels).
Spiral wound RO/NF membrane systems typically have a two-stage arrangement
(2) Multi-level arrangement
A multi-stage reverse osmosis system refers to a system where the permeate from the first stage of RO is used as feed water for the second stage of reverse osmosis, and the permeate from the second stage is used as feed water for the next stage. After several stages of desalination, high-purity fresh water can be produced.
A multi-stage reverse osmosis treatment system can generally be considered in the following situations:
- To achieve higher product water quality;
- Ion exchange cannot be used as a post-treatment.
- High levels of bacterial, pyrogen, and organic matter removal are required (e.g., for pharmaceutical water).
- Higher reliability is required.
Determination of Average Flux
The selection of the average flux design value is generally based on field test data, experience, or by referring to the typical design flux value recommended by the design guidelines.
Number of Components
Once the design permeate flow rate Qp and the design flux f are obtained, the number of elements Ne can be calculated .
Ne=Qp/f×S
Where: Ne — theoretical number of membrane elements
Qp — Water production (GPD, gallons/day)
f — Water production flux per unit area (GFD, gallons/square foot*day)
S — Membrane element area ( in.^2, square feet )
Number of pressure vessels
Dividing the number of membrane elements Ne by the number of elements Ve that can be installed in each pressure vessel yields the number of pressure vessels Nv, rounded to the nearest integer.
Nv=Ne/Ve
Analysis and optimization of membrane systems
The system design can be optimized by changing the number, type, and arrangement of membrane elements based on the set flux, recovery rate, and operating parameters of the membrane elements.
