01 causes and types of scaling
Reverse osmosis membrane separation is a precise membrane liquid separation technology, which can separate most dissolved salts and organic compounds with molecular weight greater than 100 in aqueous solution. at present, the standard sodium chloride desalination rate of reverse osmosis composite membrane in market chemical industry is generally more than 99%.
The composition of dissolved salts in different types of water sources is diverse. after desalination and concentration by reverse osmosis system, the salt concentration on the concentrated water side increases exponentially, and the factor of concentration polarization exists at the same time, when it exceeds its solubility limit, they will scale on the membrane surface. the higher the recovery rate and the greater the concentration polarization factor, the greater the risk of scaling. The common insoluble salts in reverse osmosis system include CaSO4, CaCO3, BaSO4, CaF2, silicate and so on.
The causes of fouling in reverse osmosis usually include the following:
The main results are as follows: (1) the concentration of insoluble salt in feed water exceeds the solubility product, but no scale inhibitor is added.
(2) the addition of scale inhibitor is inappropriate.
(3) too high recovery rate.
(4) unsuitable operating parameters, such as too high pH value to control silica scale and CaCO3 precipitation.
02 scaling harm and its prevention and control measures
After the fouling pollution occurs in the reverse osmosis system, it is usually characterized by the decrease of water production, the decrease of desalination rate, the increase of operating pressure and the increase of section pressure difference. Scaling usually occurs first in the terminal branch membrane element, and then gradually spreads to the front. Raw water containing calcium, bicarbonate or sulfate may clog the membrane system within hours, and scale containing barium and fluorine generally forms more slowly because their common raw water concentrations are usually low.
By disassembling the terminal branch membrane element, scale substances can usually be observed at the end face of the membrane element and the end plate of the membrane shell. After controlling the water of the membrane element, the weight of the membrane element will be significantly higher than that of the new membrane (the weight of the new membrane element is generally about 13.5KG).
The terminal branch membrane element is dissected and the scale on the membrane surface is analyzed. The crystal structure of the sediment can also be observed under the microscope. The scaling type and scaling material can be identified by chemical analysis or X-ray analysis.
It is necessary to take effective prevention and control measures according to different types of water sources. Common prevention and control measures include pretreatment and acid regulation of pH, addition of scale inhibitors, pretreatment to remove or reduce insoluble salts (such as resin softening, chemical softening, silicon removal, iron and manganese removal, etc.).
03 scale prevention and control technology
-addition of scale inhibitor
When the solubility product of insoluble salt on the concentrated water side of the reverse osmosis system reaches or exceeds saturation, the addition of scale inhibitor is the most commonly used scale inhibition measure. The correct selection and addition of scale inhibitors is very important.
A) selection of scale inhibitors
At present, most of the scale inhibitors for reverse osmosis provided by regular manufacturers in the market are organic acid salts and polyacrylic acid scale inhibitors, and the scale inhibition effect is better. The addition amount and its scale inhibition effect can be calculated according to the design software provided by the scale inhibitor manufacturer.
Organic scale inhibitors may precipitate with cationic polyelectrolytes or multivalent cations (such as Fe, Al) used for coagulation, resulting in pollution, so it is necessary to avoid overuse.
When adding anionic scale inhibitor, it is necessary to avoid adding or excessive addition of cationic polymer in the pretreatment to prevent precipitation.
No matter which brand or type of scale inhibitor is selected, it is necessary to provide raw water quality analysis data and design parameters of reverse osmosis system to ensure that the scale inhibitor matches the actual operating conditions.
B) scale inhibitor dosing device
Correct design and regulation
(1) to prevent the deviation of dosing current.
Scale inhibitors are usually added in front of the security filter. It is recommended to set up a pipe mixer to ensure that the agent is fully mixed with the raw water and prevent the drug from deviating from the flow.
(2) the rationality of scale inhibitor dispensing.
The dilution multiple of scale inhibitor strictly follows the guidelines of scale inhibitor manufacturers. Excessive dilution will cause bacteria to breed and block metering pumps and membrane components.
(3) flexible control of dosing device
When the reverse osmosis system adjusts the operating conditions, the scale inhibitor dosing device can be flexibly adjusted to ensure that the dosage is in line with the needs of the current system. Avoid fouling caused by insufficient dosage, and put an end to the fouling of membrane components caused by excessive dosage and increase the operating cost.
C) interlock control of scale inhibitor metering pump
The scale inhibitor metering pump should be interlocked with the high-pressure pump, open the scale inhibitor metering pump when the high-pressure pump starts, and close the scale inhibitor metering pump when the high-pressure pump stops. Common failures in the dosing system include:
(1) the import filter is blocked.
(2) there is a bubble barrier in the dosing pipeline.
(3) the installation mode of metering pump is wrong.
(4) the output pressure of the metering pump is less than the system back pressure.
(5) rupture of diaphragm of metering pump
(6) the failure of the actuator can not reach the nominal pressure.
The dosing device needs to be carefully inspected and maintained during on-site operation.
D) recommendations for system maintenance
(1) during daily operation, the operating account should be recorded in detail, the instrument should be calibrated regularly, and the output of calibrated metering pump should be checked.
(2) each team inspects the liquid level of the scale inhibitor box to ensure the normal administration, and calculates the concentration according to the treated water volume.
(3) do a good job of data monitoring and organize preventive cleaning regularly.
(4) stop the machine in time when finding abnormal data, investigate the cause, and organize targeted chemical cleaning.
(5) when adjusting the operating condition of the reverse osmosis system (especially when adjusting the water production and recovery parameters), it is necessary to confirm the feasibility of the adjustment and adjust the dosage of scale inhibitor accordingly.
(source: new Horizon of Water treatment)
