What are the different methods of pretreatment technology before running a reverse osmosis plant?

Permeate pressure of reverse osmosis plant is a function of the concentration of salt or organic matter contained in the water, the higher the salt content, the higher the permeate pressure will increase, the net pressure will decrease and the water production will be reduced.

If the feed water pressure remains unchanged, The salt permeability is proportional to the difference in salt concentration between the front and back sides of the reverse osmosis membrane.

The higher the salt content of the feed water, the greater the concentration difference, and the higher the salt permeability, which leads to a decrease in the desalination rate.

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For the same system, the salt content of feed water is different, and the operating pressure and product water conductivity also differ, for every l00ppm increase in feed water salt content, the feed water pressure needs to be increased by about 0.007MPa, while the product water conductivity also increases accordingly due to the increase in concentration.

Suspended matter in the water is the material that remains on the surface of the filter material while the water is filtered, with the particle component as the main body.

The high content of suspended matter will lead to serious clogging of the reverse osmosis and nanofiltration system soon, affecting the water yield of the system and the quality of produced water.

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All kinds of reverse osmosis plants have a permissible pH range, and the pH value of feed water has almost no effect on water production; but even within the permissible range, the pH value has a great influence on the desalination rate.

The pH value has a direct effect on the form of impurities in the feed water, for example, the retention rate of dissociable organic matter decreases with the decrease in pH value. The recovery rate has a great influence on the pressure drop of each section.

Under the condition that the total flow rate of feed water is maintained at a certain level, the recovery rate increases, and the total pressure drop decreases due to the reduction of the flow rate of concentrated water flowing through the high-pressure side of the reverse osmosis plant.

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The recovery rate decreases and the total pressure drop increases, and the actual operation shows that even a small change in the recovery rate, such as 1%, will cause a change of about 0.02 MPa in the total pressure difference.

As the dissolved CO₂ in water is influenced by pH value, it exists in the form of gaseous CO₂ when the pH value is low and can easily pass through the reverse osmosis membrane, so the desalination rate is low when pH is low, and the desalination rate rises gradually.

When pH increases and gaseous CO₂ is converted into CO3⁻ and CO3²⁻ ions, the desalination rate reaches its highest when pH is between 7.5 and 8.5.

The effect of the recovery rate on the conductivity of the product water depends on the amount of salt permeation and the amount of product water.

What are the characteristics of pre-treatment technology before the reverse osmosis plant?

In order to improve the operational performance of the reverse osmosis system, some of the following agents can be added to the feed water: acids, bases, biocides, scale inhibitors, and dispersants.

♦ Add acid – prevent scaling

Hydrochloric acid (HCl) and sulfuric acid (H2SO4) can be added to the feed water to reduce pH. sulfuric acid is cheaper, does not fume and corrode the surrounding metal components, and the membrane has a higher removal rate of sulfate ions than chloride ions, so sulfuric acid is more commonly used than hydrochloric acid.

Industrial grade sulfuric acid without other additives is suitable for reverse osmosis use and is available in two concentrations, 20%, and 93%. 93% sulfuric acid is also known as 66 BoM sulfuric acid. Care must be taken when diluting 93% sulfuric acid, as heat can raise the temperature of the solution to 138°C when diluting to 66%.

Be sure to add the acid slowly to the water with stirring to avoid localized boiling of the aqueous solution. Hydrochloric acid is used primarily when calcium sulfate or strontium sulfate scaling is likely to occur.

The use of sulfuric acid increases the concentration of sulfate ions in the reverse osmosis feed water, which directly leads to an increased tendency for calcium sulfate scaling. Industrial-grade hydrochloric acid (without additives) is very easy to purchase, and commercial hydrochloric acid typically contains 30-37%.

The primary objective of lowering pH is to reduce the tendency of calcium carbonate scaling in RO concentrated water, i.e., to reduce the Langley Index (LSI), which is the saturation of calcium carbonate in low salinity brackish water and indicates the potential for calcium carbonate scaling or corrosion.

In reverse osmosis water chemistry, LSI is an important indicator to determine whether calcium carbonate scaling will occur. When the LSI is negative, the water will corrode metal pipes, but no calcium carbonate scaling will form. If the LSI is positive, the water is not corrosive, but calcium carbonate scaling will occur.

LSI is the pH of calcium carbonate saturation minus the actual pH of the water. the solubility of calcium carbonate decreases with increasing temperature (this is how scale forms in kettles) and decreases with increasing pH, the concentration of calcium ions, i.e. alkalinity.

The LSI value can be adjusted by injecting acid (usually sulfuric or hydrochloric acid) into the reverse osmosis feed water, i.e. by lowering the pH. The recommended LSI value for reverse osmosis concentrated water is 0.2 (indicating a concentration of 0.2 pH units below the saturation concentration of calcium carbonate).

Polymeric scale inhibitors can also be used to prevent calcium carbonate precipitation, and some scale inhibitor suppliers claim that their products can achieve LSIs as high as +2.5 (a more conservative design is an LSI of +1.8) in reverse osmosis concentrated water.

♦ The addition of alkali – increase the removal rate

Alkali addition is less used in primary reverse osmosis. The only alkali agent generally used is sodium hydroxide (NaOH), which is easy to purchase and soluble in water. Industrial-grade sodium hydroxide with no other additives is generally sufficient for this purpose.

Commercial sodium hydroxide is available in 100% flake soda, as well as 20% and 50% liquid soda. Care must be taken when adding alkali to raise the pH. Increased pH increases LSI, and decreases calcium carbonate and the solubility of iron and manganese.

The most common application for alkali addition is the secondary RO plant. In a two-stage reverse osmosis plant, the primary RO-produced water is supplied to the secondary RO as raw water.

The secondary RO “polishes” the primary RO-produced water, and the secondary RO-produced water can reach 4 MΩ. There are four reasons for adding alkali to the secondary RO feed water.

①  Above pH8.2, all carbon dioxide is converted into carbonate ions, which can be removed by reverse osmosis. And carbon dioxide itself is a gas, which will enter RO-produced water freely with the permeate, causing the undue load to the downstream ion exchange bed polishing treatment.

② Some TOC components are more easily removed at high pH.

③ The solubility and removal rate of silica is higher at high pH (especially above 9).

④ Boron removal is also higher at high pH (especially above 9).

There is a special case of alkaline addition application, often called the HERO (High-Efficiency Reverse Osmosis System) process, which adjusts the feed water pH to 9 or 10.

Primary reverse osmosis is used to treat brackish water, which can have contamination problems (e.g., hardness, alkalinity, iron, manganese, etc.) at high pH. Pretreatment usually uses a weakly acidic cationic resin system and a degassing unit to remove these contaminants.

♦ Dechlorination agent – Eliminate residual chlorine

The free chlorine in RO and NF feed water should be reduced to less than 0.05 ppm to meet the requirements of polyamide composite membranes. There are two pretreatment methods for chlorine removal, granular activated carbon adsorption and the use of reducing agents such as sodium sulfite.

Activated carbon filters are generally used in small systems (50-100 GPM), where the investment cost is more reasonable. It is recommended to use acid-washed and treated high-quality activated carbon to remove hardness and metal ions, and the fines content should be very low, otherwise, it will cause pollution to the membrane.

The newly installed carbon filter media must be fully drenched until the carbon powder is completely removed, which usually takes several hours or even days. We cannot rely on a 5-μm security filter to protect the reverse osmosis membrane from contamination by carbon fines.

The advantage of carbon filters is that they remove organic matter that can cause membrane contamination and are more reliable for all influent water treatment than adding chemicals.

However, the disadvantage is that carbon can become a feed for microorganisms, breeding bacteria in the carbon filter, with the result that it causes biological contamination of the reverse osmosis membrane.

Sodium bisulfite (SBS) is the typical reducing agent chosen for larger RO plants. Solid sodium metabisulfite is dissolved in water to make a solution with a commercial sodium metabisulfite purity of 97.5-99% and a dry storage period of 6 months.

SBS solution is unstable in air and reacts with oxygen, so the recommended use period is 3-7 days for a 2% solution and 7-14 days for a solution of less than 10%. Theoretically, 1.47ppm of SBS (or 0.70ppm of sodium bisulfite) can reduce 1.0ppm of chlorine.

The design takes into account the safety factor of the industrial brackish water system and sets the addition of SBS at 1.8-3.0 ppm per 1.0 ppm chlorine. the injection port of SBS should be upstream of the membrane element, and the setting distance should be such that there is 29 seconds reaction time before entering the membrane element. A proper in-line stirring device (static stirrer) is recommended.

♦ Multi-media filter

A common method of removing suspended solids from water is multi-media filtration. Multi-media filters are layered with anthracite, quartz sand, finely crushed garnet, or other materials as the bed.

The top layer of the bed consists of light and coarse-grade materials, while the heaviest and finest-grade materials are placed at the bottom of the bed.

The principle is depth filtration – the larger particles in the water are removed in the top layer and the smaller particles are removed in the deeper part of the filter media.

RO reverse osmosis plant operation process requires attention to several matters

1, The hydrolysis of cellulose acetate membrane is easy to cause the performance of the reverse osmosis device to deteriorate, for this reason, the PH value of water must be strictly controlled, the PH value of feed water must be maintained at 5-6, while the composite membrane can operate under the range of feed water PH3-PH11.

2, When the amount of injected sodium hypochlorite is insufficient and the free chlorine in feed water cannot be measured, the slime will occur on the membrane module of the reverse osmosis unit and the pressure difference of the reverse osmosis unit will increase.

But for composite membrane and polyamide membranes, the amount of free chlorine entering the membrane module must be strictly controlled, exceeding the specified value will lead to the oxidation and decomposition of the membrane.

3, If the FI value exceeds the standard water supply to the reverse osmosis plant as feed water, the surface of the membrane module will be attached to the dirt, so it must be cleaned to remove the dirt.

4, Excessive feed water flow will make the membrane component deteriorate in advance, so the feed water flow should not exceed the design standard value. In addition, the flow rate of concentrated water should avoid less than the design standard value as much as possible.

Operating under the condition that the flow rate of concentrated water is too small will cause uneven flow in the pressure vessel of the reverse osmosis device and precipitate fouling on the membrane module due to excessive concentration.

5, The high-pressure pump of the reverse osmosis plant may cause the unit to malfunction even if there is a very short interruption in operation.

6, The reverse osmosis inlet pressure should be maintained with proper margin, otherwise, the desalination rate will be reduced due to the lack of proper compaction.

7, The reverse osmosis plant is stopped when the application of low-pressure feed water replaces the water in the reverse osmosis unit. This is to prevent the precipitation of silica during the shutdown (in winter when the water temperature drops).

8, Need to pay attention to the pressure validity of the precision filter. The reason for the sharp rise of the pressure difference is mainly the leakage of the turbidity of the precision filter. On the contrary, the reason for the sharp decrease in pressure difference is the breakage of the precision filter element and the loosening of the precision filter element fastening screw, etc.

9, When the pressure difference between the inlet and outlet of the reverse osmosis plant exceeds the standard, it means that the membrane surface has been contaminated or the feed water flow is above the design value. If the differential pressure problem cannot be solved by flow adjustment, the membrane surface should be cleaned.

10, In the summer, when the feed water temperature is high, the flow of produced water is too much, and sometimes the operating pressure has to be reduced, which will lead to a decrease in the quality of produced water. In order to prevent this, the number of membrane components can be reduced, while the operating pressure is still maintained at a high level.