1 Working Principle of Dosing in Reverse Osmosis
Various raw waters contain certain concentrations of suspended matters and dissolved substances. Suspended matters mainly consist of inorganic salts,
colloids, and microorganisms, algae, etc. biological particles. Dissolved substances mainly consist of easily soluble salts (such as chlorides) and insoluble
salts (such as carbonates, sulfates, and silicates) metal oxides, acids and bases.
In the reverse osmosis process, the volume of the feedwater is decreasing, while the concentration of suspended particles and dissolved substances is
increasing. Suspended particles will deposit on the membrane and block the feedwater passage, increasing frictional resistance (pressure drop).
Insoluble salts will precipitate out of the concentrate when exceeding their saturation limit, forming scale on the membrane surface, reducing the RO
membrane's flux, increasing operating pressure and pressure drop, and causing a decline in product water quality. This phenomenon of depositing a
layer on the membrane surface, which is called membrane fouling, results in the deterioration of system performance.
Pretreatment is needed before raw water enters the reverse osmosis membrane system to remove suspended solids, dissolved organic matter, and
excess insoluble salt components that may cause membrane fouling, and to reduce the tendency of membrane fouling. The purpose of pretreating
the feedwater is to improve the feedwater quality and provide reliable operation assurance for the RO membrane.
The effect of pretreatment on raw water is reflected by the reduction of TSS, TOC, COD, BOD, LSI, and iron, manganese, aluminum, silicon, barium,
strontium, etc. pollutant water quality indicators. In the previous chapter, there was a detailed description of these pollutant water quality indicators.
Another important water quality index that characterizes membrane pollution tendency is SDI. In addition to reducing the above indicators to the
required range for feeding water in the reverse osmosis membrane system by pretreatment, it is also important to reduce SDI as much as possible.
The ideal SDI (15-minute) value should be less than 3.
2 Chemical Pretreatment
In order to improve the operating performance of the reverse osmosis system, some chemicals can be added to the feedwater, including acids, alkalis,
disinfectants, scale inhibitors, and dispersants.
3 Adding Acid to Prevent Scale Formation
Hydrochloric acid (HCl) and sulfuric acid (H2SO4) can be added to the feedwater to reduce the pH.
Sulfuric acid is cheap and can’t corrode metal components nearby with fume. The membrane has a higher removal rate of sulfate ions than chloride ions,
so sulfate acid is more commonly used than hydrochloric acid.
Industrial-grade sulfate acid without other additives is suitable for reverse osmosis use. Commercial sulfate acid comes in two concentration specifications
of 20% and 93%. 93% sulfate acid is also known as 66° Baume sulfate acid. Be careful when diluting 93% sulfate acid, as the heat generated by diluting it to
66% can raise the solution temperature to 138°C. Be sure to add the acid slowly into the water while stirring to avoid local overheating and boiling of the
water solution.
Hydrochloric acid is mainly used when calcium sulfate or strontium sulfate scale may form. Using sulfuric acid will increase the concentration of sulfate ions
in the RO feedwater, directly leading to an increased tendency for calcium sulfate scaling. Industrial-grade hydrochloric acid (without additives) is readily
available, and commercial hydrochloric acid typically has a content of 30-37%.
The primary goal of lowering pH is to reduce the tendency for calcium carbonate scaling in the RO concentrate, i.e., to lower the Langlier index (LSI). The LSI
is the saturation of calcium carbonate in low-salinity brackish water, indicating the likelihood of calcium carbonate scaling or corrosion.
In RO water chemistry, the LSI is an important indicator of whether calcium carbonate scaling will occur. When the LSI is negative, the water will corrode
metal pipes but will not form calcium carbonate scale.
When the LSI is positive, the water is non-corrosive but will form calcium carbonate scale. The LSI is calculated by subtracting the pH of calcium carbonate
saturation from the actual pH of the water.
The solubility of calcium carbonate decreases as the temperature rises (that's how water scale form in teapots), and it decreases as the pH, calcium ion
concentration, and alkalinity increase.
The LSI value can be lowered by injecting acid (usually sulfuric or hydrochloric acid) into the RO feedwater to lower the pH. Recommended LSI value for the
concentrated water from reverse osmosis is 0.2 (indicating a concentration that is 0.2 pH units below the saturation concentration of calcium carbonate).
Polymer scale inhibitors can also be used to prevent calcium carbonate precipitation. Some suppliers of scale inhibitors claim that their products can raise the
LSI of the concentrated water from reverse osmosis up to + 2.5 (a more conservative design would have an LSI of + 1.8).
4 Add lye to improve salt rejection
Less lye is used in the primary reverse osmosis system. Lye is injected into the feedwater of the reverse osmosis to raise the pH. Only sodium hydroxide (NaOH)
is commonly used as an alkali agent, which is readily available and soluble in water. Industrial-grade sodium hydroxide that contains no other additives is
sufficient for most needs.
Commercial sodium hydroxide comes in 100% flake form, as well as 20% and 50% liquid form. When adding lye to raise the pH, it is important to note that an
increase in pH will increase the LSI, decrease the solubility of calcium carbonate and iron and manganese.
The most common application of adding lye is in secondary RO systems. In a secondary reverse osmosis system, the permeate from the primary RO system is
used as the feedwater for the secondary RO system. The secondary RO system performs a "polishing" operation on the permeate from the primary RO system,
producing water with a resistivity of 4 megohms.
There are four reasons to add lye to the feedwater of the secondary RO system: a. At pH 8.2 and above, all the carbon dioxide is converted into carbonate ions,
which can be removed by reverse osmosis. Carbon dioxide itself is a gas that freely enters the RO permeate with the permeable liquid, causing an inappropriate
load for the downstream ion exchange bed polishing treatment.
b. Some TOC components are more easily removed at high pH.
c. The solubility and removal rate of silica are higher at high pH (especially above 9).
d. Boron removal rate is also higher at high pH (especially above 9). There is a special case of adding alkali, which is usually called the HERO (High Efficiency
Reverse Osmosis System) process, where the feedwater pH is adjusted to 9 or 10. The primary reverse osmosis is used to treat brackish water, which may have
pollution problems at high pH (such as hardness, alkalinity, iron, and manganese). Pretreatment usually uses a weak acid cation resin system and a degasser to
remove these contaminants.
5 Chlorine Removal Agent - Eliminate Residual Chlorine
Free chlorine in RO and NF feedwater must be reduced to 0.05ppm or less 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.
In small systems (with a flow rate of 50-100gpm), a activated carbon filter is usually used, which has a reasonable investment cost. Recommended use of acid-washed
high-quality activated carbon, which removes hardness, metal ions, and very low fine powder content to avoid membrane contamination. New activated carbon
filter material must be thoroughly flushed until all carbon fines are removed. This can take several hours or even days. We cannot rely on 5μm security filters to protect
reverse osmosis membranes from carbon fines contamination.
The advantage of carbon filter is that it can remove organic matter that causes membrane fouling, which is more reliable than adding chemicals for the treatment of
all feedwater. However, its disadvantage is that carbon becomes a food source for microorganisms, which breed bacteria in the carbon filter, resulting in biological
fouling of
the reverse osmosis membrane.
Sodium bisulfite (SBS) is a typical reducing agent used in larger RO systems. Solid sodium bisulfite is dissolved in water to form a solution, commercial sodium bisulfite
with a purity of 97.5-99% and a shelf life of 6 months when dry stored.
SBS solution is unstable in air and reacts with oxygen, so the recommended usage period for a 2% solution is 3-7 days, and for a 10% solution or lower is 7-14 days. In
theory, 1.47ppm of SBS (or 0.70ppm of sodium metabisulfite) can reduce 1.0ppm of chlorine.
Considering the safety factor for industrial brackish water systems in the design, SBS is added at a rate of 1.8-3.0ppm per 1.0ppm of chlorine. The injection point of SBS
should be upstream of the membrane element, and the distance should be set to ensure that there is a reaction time of 29 seconds before entering the membrane element.
A suitable online mixing device (static stirrer) is recommended.
SBS chlorine removal reaction: Na2S2O5 (sodium metabisulfite) + H2O = 2 NaHSO3 (sodium bisulfite) NaHSO3 + HOCl = NaHSO4 (sodium bisulfite) + HCl (hydrochloric acid)
NaHSO3+ Cl2 + H2O = NaHSO4 + 2 HCl. The advantage of SBS dechlorination is that it requires less investment than carbon filter in large systems, and the reaction by-products
and residual SBS are easily removed by RO.
The disadvantage of SBS dechlorination is that it requires manual mixing of small volumes of chemicals, and increases the threat of chlorine to the membrane when the
dechlorination system is not equipped with sufficient monitoring and control instruments. In addition, in some cases, sulfur-reducing bacteria (SBR) are found in feedwater,
and sulfite will become a nutrient to help the bacteria reproduce.
SBR is usually found in shallow well water in anaerobic environments, and hydrogen sulfide (H2S) as a metabolic product of SBR will also be present. Monitoring of the
dechlorination process can be done using free chlorine monitors to monitor the concentration of residual sulfite, and ORP monitors can also be used.
The recommended method is to monitor the concentration of residual sulfite to ensure that there is enough sulfite to reduce chlorine. The detection concentration of most
commercial chlorine monitors is 0.1 ppm, which is the upper limit of residual chlorine for CPA membrane. Directly using ORP monitors to monitor the concentration of sulfite
is not reliable, as the baseline drift of this type of instrument that measures the oxidation-reduction potential of water is difficult to predict.
The CPA membrane's chlorine tolerance is approximately 1000-2000 ppm per hour (where the salt passage doubles), which equals running at 0.038 ppm residual chlorine
for 3 years. It is important to note that in some cases, the chlorine tolerance has been found to decrease significantly due to increased temperature (above 90 degrees Fahrenheit),
increased pH (above 7), and the presence of transition metals such as iron, manganese, zinc, copper, and aluminum. The CPA membrane's chloramine tolerance is approximately
50,000-200,000 ppm per hour (where there is a significant increase in the salt passage), which corresponds to 1.9-7.6 ppm chloramine in the RO feedwater, allowing the membrane
to run for 3 years. Similarly, the membrane's chloramine tolerance can change in the presence of increased temperature, decreased pH, and the presence of transition metals.
In a California tertiary wastewater treatment device, the salt rejection rate of the membrane decreased from 98% to 96% within 2-3 years under conditions of chloramine
concentration 6-8 ppm in feedwater. Designers should keep in mind that dechlorination is still necessary after chloramination. Chloramine is a product of mixed chlorine and ammonia,
and free chlorine has a much stronger degrading effect on the membrane than chloramine. If ammonia is deficient, free chlorine may exist. Therefore, using excess ammonia is critical,
and system monitoring is necessary to ensure this.
6 Scale Inhibitors and Dispersants
Many manufacturers of scale inhibitors and dispersants offer products for improving the performance of reverse osmosis and nanofiltration systems. Scale inhibitors are a series of
chemical agents used to prevent the precipitation and scaling of crystalline mineral salts. Most scale inhibitors are specialized organic synthetic polymers (such as polyacrylic acid,
carboxylic acid, polymeric maleate, organic metal phosphates, polymeric phosphates, phosphinic acid, anionic polymers, etc.) with molecular weights ranging from 2000 to 10,000
Dalton. The technology of reverse osmosis system scale inhibitors evolved from cooling circulation water and boiler water chemistry. There are many different types of scale inhibitors,
and the effects and efficiency achieved with different organic compounds in different applications vary greatly.
Special care must be taken when using polyacrylic acid-based scale inhibitors, as high iron content may cause membrane fouling, which increases the operating pressure of the
membrane and requires acid washing to effectively remove this type of fouling.
Special care must also be taken when using anionic scale inhibitors if a cationic flocculant or filter aid was used in the pretreatment. A sticky viscous adhesive contaminant will be
produced, which can cause an increase in operating pressure and make cleaning of this type of contaminant very difficult.
Sodium hexametaphosphate (SHMP) was a common scale inhibitor used in early reverse osmosis, but its use has been greatly reduced with the appearance of specialized scale
inhibitors. There are some limitations to the use of SHMP. The solution should be prepared every 2-3 days because it will hydrolyze when exposed to air. After hydrolysis, not only
will the scale inhibiting effect be reduced, but there is also a possibility of calcium phosphate scale formation.
Using SHMP can reduce calcium carbonate scale, and the LSI can reach + 1.0. Scale inhibitors prevent the growth of salt crystals in the RO feedwater and concentrate, so it is
possible to allow insoluble salts to exceed their saturated solubility in the concentrate. Scale inhibitors can replace acid addition, or they can be used in conjunction with acid addition.
There are many factors that affect the formation of mineral scale. Lowering the temperature will reduce the solubility of scale-forming minerals (calcium carbonate is excluded, unlike
most substances, whose solubility decreases as temperature increases). Increasing TDS will increase the solubility of insoluble salts (because high ionic strength interferes with the
formation of seed crystals).
