Response measures for microbial contamination in reverse osmosis operation

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23 Feb 2023

Response measures for microbial contamination in reverse osmosis operation

Response measures for microbial contamination in reverse osmosis operation

01 Chlorine sterilization

The effectiveness of chlorine depends on the concentration of chlorine, the contact time and the pH of the water.

It is often used to sterilize drinking water, and the general residual chlorine concentration is 0.5ppm.

In industrial water treatment, microbial contamination on heat exchangers and sand filters can be prevented by maintaining the residual chlorine concentration in water above 0.5-1.0ppm. The amount of chlorine dosing depends on the content of organic matter in the influent, because organic matter will consume chlorine.

Surface water treatment usually requires chlorine disinfection in the reverse osmosis pretreatment part to prevent microbial contamination. The method is to add chlorine at the water intake and maintain a reaction time of 20-30 minutes to keep 0.5-1.0ppm residual chlorine in the entire pretreatment pipeline concentration.

However, it must be thoroughly dechlorinated before entering the membrane element to prevent the membrane from being oxidized and damaged by chlorine.

(1) Chlorination reaction

Commonly used chlorine-containing disinfectants are chlorine gas, sodium hypochlorite or calcium hypochlorite. In water, they rapidly hydrolyze to hypochlorous acid.
Cl2 + H2O → HClO + HCl (1)
NaClO + H2O → HClO + NaOH (2)
Ca(ClO)2 + 2H2O → 2HClO + Ca(OH)(3)

Hypochlorous acid in water decomposes hydrogen ions and hypochlorite ions:
HClO←→ H+ + ClO- (4)

The sum of Cl2, NaClO, Ca(ClO)2, HClO and ClO– is called free chlorine (FAC) or residual residual chlorine (FRC), and is expressed in mg/LCl2.

Chlorine reacts with ammonia in water to form chloramines, which are called combined chlorine (CAC) or combined residual chlorine (CRC), and the sum of residual chlorine and combined chlorine is called total residual chlorine (TRC)

The bactericidal efficiency of residual chlorine is directly proportional to the concentration of undecomposed HClO. The bactericidal effect of hypochlorous acid is 100 times higher than that of hypochlorite, and the proportion of undissociated hypochlorous acid increases with the decrease of pH value.

At pH=7.5 (25°C, TDS=40mg/L), only 50% of residual chlorine exists as HClO, but at pH=6.5, 90% is HClO.

The proportion of HClO also increases with the decrease of temperature. At 5°C, the molecular fraction of HClO is 62% (pH=7.5, TDS=40mg/L). In high salinity water, the proportion of HClO is very small (when pH=7.5, 25°C, 40000mg/L TDS, the ratio is about 30%).

(2) Dosing amount of chlorine

A part of the added chlorine reacts with ammonia nitrogen in the water to form combined chlorine according to the following reaction steps:

HClO + NH3 ←→NH2Cl (monochloramine) + H2O (6)
HClO + NH2Cl ←→ NHCl2 (dichloramine) + H2O (7)
HClO + NHCl2 ←→ NCl3 (Trichloramine) + H2O (8)

The above reactions mainly depend on the pH and the mass ratio of chlorine/nitrogen. Chloramine also has a bactericidal effect, but it is lower than that of chlorine.

The other part of the chlorine gas is transformed into inactive chlorine. The amount of chlorine required for this part depends on reducing agents such as nitrite, chloride, sulfide, ferrous iron and manganese. The oxidation reaction of organic matter in water also consumes chlorine.

(3) Chlorination of seawater

Different from the situation in brackish water, seawater usually contains about 65 mg/L of bromine. When seawater is chemically treated with chlorine, bromine will quickly react with hypochlorous acid to produce hypobromous acid

Br- + HClO → HBrO + Cl- (9)

In this way, when the seawater is treated with chlorine, the bactericidal effect is mainly HBrO instead of HClO, and the hypobromous acid will be decomposed into hypobromite ions.

HBrO ←→ BrO- + H+ (10)

The degree of decomposition of HBrO is lower than that of HClO. At pH=8, only 28% of HClO will not decompose, but 83% of HBrO will not decompose.

For seawater under high pH conditions, the bactericidal effect is still better than that in brackish water. Hypobromous acid and hypobromite ions will interfere with the determination of residual chlorine, which is included in the measured value of residual chlorine.

02 Impact sterilization treatment

Shock treatment involves the addition of biocide to reverse osmosis or nanofiltration feedwater for a limited period of time and during normal operation of the water treatment system.

Sodium bisulfite is often used for this treatment purpose. Generally, 500-1000ppm of NaHSO3 is added for about 30 minutes.

Shock treatment can be carried out periodically at regular intervals, for example, once every 24 hours, or when biological growth is suspected. The product water produced during this shock treatment will contain 1-4% of the added sodium bisulfite concentration.

Depending on the use of the product water, it can be decided whether the product water during shock sterilization should be recycled or discharged. Sodium bisulfite is more effective against aerobic bacteria than anaerobic microorganisms. Therefore, the use of shock sterilization should be carefully evaluated in advance.

03 Periodic disinfection

In addition to continuously adding fungicides to the raw water, the system can also be regularly sanitized to control biological contamination.

This treatment method is used on systems with a moderate biofouling hazard, but in systems with a high biofouling hazard, disinfection is only an adjunct to continuous biocide treatment.

Preventive disinfection is more effective than corrective disinfection because isolated bacteria are easier to kill and remove than thick, aged biofilms.

The general disinfection interval is once a month, but systems with strict hygiene requirements (such as pharmaceutical process water) and highly polluted raw water (such as wastewater) may be once a day. Of course, the life of the membrane is affected by the type and concentration of the chemicals used. After Intense disinfection may shorten membrane life.

04 Ozone sterilization

It is more oxidizing than chlorine, but it decomposes quickly, so it needs to be maintained at a certain level to kill microorganisms. At the same time, the ozone resistance of the equipment used should also be considered, and stainless steel should usually be used.

In order to protect the membrane elements, ozone must be carefully removed, and UV irradiation can successfully achieve this goal.

05 UV irradiation

254nm UV light is proven to be bactericidal. It has been used in small water plants. It does not require chemicals to be added to the water. The maintenance requirements of the equipment are low. Only periodic cleaning or replacement of mercury vapor lamps is required.

However, the application of UV irradiation treatment is very limited and only suitable for cleaner water sources, because colloids and organic matter will affect the penetration of optical radiation.

06 Sodium bisulfite

When its concentration reaches 50mg/L in the influent of the seawater desalination system, it is effective in controlling biological pollution. In this way, colloid contamination can also be reduced.

An added advantage of sulfurous acid is that it does not require the addition of acid to control calcium carbonate due to the acidic reaction of sulfurous acid to generate hydrogen ions.
HSO3- → H+ + SO42-

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