Why do reverse osmosis equipment need to be equipped with scale inhibitors?

Author: Hongjie Water

Causes and types of scaling

Reverse osmosis membrane separation is a precise membrane liquid separation technology that can separate most soluble salts and organic compounds with molecular weight greater than 100 in aqueous solutions. Currently, the standard sodium chloride desalination rate of market-oriented industrial reverse osmosis composite membranes is generally greater than 99%.

The composition of dissolved salts in different types of water sources is diverse. After desalination and concentration by reverse osmosis systems, the salt concentration on the concentrated water side increases exponentially, and there is also a concentration polarization factor. When the solubility limit is exceeded, they will form scaling on the membrane surface. The higher the recovery rate and concentration polarization factor, the greater the risk of scaling. In reverse osmosis systems, common insoluble salts include CaSO4, CaCO3, BaSO4, CaF2, silicates, etc.

The reasons for scaling in reverse osmosis usually include the following: 

(1) The concentration of insoluble salt in feed water exceeds solubility product, but no scale inhibitor is added.

(2) Improper dosing of scale inhibitor.

(3) Excessive recovery rate.

(4) Unsuitable operating parameters, such as excessive pH control of silica scale leading to CaCO3 precipitation.

Scaling hazards and prevention and control measures

After scaling pollution occurs in the reverse osmosis system, it usually manifests as a decrease in water production, a decrease in desalination rate, an increase in operating pressure, and an increase in section pressure difference. Scaling usually occurs first in the last membrane element of the final segment, and then gradually spreads forward. Raw water containing calcium, heavy carbonate or sulfate may block the membrane system due to scaling within a few hours, and scaling with barium and fluorine is generally slow, because their common raw water concentration is usually low.

By disassembling the last branch membrane element at the end of the end section, scaling substances can usually be observed at the end face of the membrane element, the end plate of the membrane shell, etc. After the membrane element is weighed after water control, the weight of the membrane element will be significantly increased than that of the new membrane (the weight of the new membrane element is generally about 13.5KG).

Anatomy of the final membrane element, analysis of the scaling on the membrane surface, and observation of the crystal structure of the sediment under a microscope. Chemical analysis or X-ray analysis can identify the type and substance of scaling.

It is necessary to take targeted and effective prevention and control measures based on different types of water sources. Common prevention and control measures include pre-treatment, adding acid to adjust pH, adding scale inhibitors, pre-treatment to remove or reduce insoluble salts (such as resin softening to remove hardness, chemical softening to remove hardness, silicon removal, iron and manganese removal, etc.).

Scale inhibitor dosing for scale prevention and control technology

When the solubility product of insoluble salt at the concentrated water side of the reverse osmosis system reaches or exceeds saturation, adding scale inhibitor is a common scale inhibition measure. The correct selection and dosing of scale inhibitors are crucial.

1、 Selection of scale inhibitors

Currently, the specialized scale inhibitors for reverse osmosis provided by legitimate manufacturers in the market are mostly organic acid salts and polyacrylate salts, with good scale inhibition effects. The addition amount and scale inhibition effect can be calculated based on the design software provided by the scale inhibitor manufacturer.

The organic scale inhibitor may precipitate with the cationic polyelectrolyte or multivalent cations (such as Fe and Al) used for coagulation in the pretreatment and cause pollution, so excessive use should be avoided.

When adding anionic scale inhibitors, it is necessary to avoid adding or over adding cationic polymers in the pre-treatment to prevent precipitation.

Regardless of which brand and model of scale inhibitor is selected, it is necessary to provide raw water quality analysis data and design parameters of the reverse osmosis system to ensure that the scale inhibitor matches the actual operating conditions.

2、 Scale inhibitor dosing device 

Correct design and regulation

(1) Prevent dosing bias

Scale inhibitors are usually added in front of the security filter, and it is recommended to set up a pipeline mixer to ensure that the agent is fully mixed with the raw water and prevent dosing bias.

(2) Rationality of scale inhibitor dosing

The dilution ratio of the scale inhibitor strictly follows the guidance and recommendations of the scale inhibitor manufacturer. Excessive dilution can cause bacterial growth and blockage of the metering pump and membrane components.

(3) Flexible control of dosing devices

When the operating conditions of the reverse osmosis system are adjusted, the scale inhibitor dosing device can be flexibly adjusted to ensure that the dosage meets the current system requirements. To avoid scaling caused by insufficient dosing of chemicals, as well as to prevent excessive dosing of chemicals from polluting membrane components and increasing operating costs.

3、 Interlocking control of scale inhibitor metering pump

The scale inhibitor metering pump should be interlocked with the high-pressure pump. When the high-pressure pump starts, the scale inhibitor metering pump should be started, and when the high-pressure pump stops, the scale inhibitor metering pump should be turned off. Common faults in the dosing system include:

(1) Inlet filter screen blocked

(2) There are bubbles blocking the dosing pipeline

(3) Incorrect installation method of metering pump

(4) The output pressure of the metering pump is less than the system back pressure

(5) Diaphragm rupture of metering pump

(6) The actuator fails to reach the nominal pressure

During on-site operation, it is necessary to carefully inspect and maintain the dosing device.

4、 System maintenance recommendations 

(1) During daily operation, detailed records should be kept of the operation ledger, instruments should be calibrated regularly, and the output of the calibrated metering pump should be checked.

(2) Each team shall inspect and check the liquid level of the scale inhibitor tank to ensure normal dosing, and calculate the dosing concentration based on the treatment water volume.

(3) Do a good job in data monitoring and regularly organize preventive cleaning.

(4) Timely shut down and investigate the cause of abnormal data, and organize targeted chemical cleaning.

(5) When adjusting the operating conditions of the reverse osmosis system (especially when adjusting the parameters of water production and recovery rate), it is necessary to first confirm the feasibility of the adjustment and adjust the dosage of the scale inhibitor accordingly.