[Abstract] Reverse osmosis desalination technology has become the main process in water treatment systems, and the stable operation of the membrane system is particularly crucial for the entire water treatment system. In this case, the desalination rate of the new membrane elements in the reverse osmosis system of a certain steel plant showed a rapid decline within just two months of operation. After inspection, it was found to be a problem with the operation sequence and operation mode of the membrane system.

[Keywords] Reverse osmosis; Operation sequence; Desalination rate

This case is a water treatment system of a certain steel plant. The design water source is pit water (groundwater), which enters this unit after coagulation and sedimentation treatment by the water plant.

The designed output of the primary reverse osmosis system is 2×80m3/h, with a recovery rate of 75%. It adopts a two-stage (left and right) arrangement of 12:6 (6 cores). Each reverse osmosis system is equipped with 108 membrane elements, using DOW's BW30FR-400.

Process flow:

Raw water → Raw water tank → Raw water pump → Multi-media filter → Self-cleaning filter → Ultrafiltration device → Ultrafiltration water tank → UF product water pump → Security filter → Primary high-pressure pump → Primary reverse osmosis device → Primary water tank → Secondary high-pressure pump → Secondary reverse osmosis device → Secondary water tank → EDI feed water pump → EDI device → Deionized water tank

Primary reverse osmosis concentrate → Concentrate RO booster pump → Security filter → Concentrate high-pressure pump → Concentrate RO → Primary water tank

Since the commissioning began in early January 2021, the operating pressure, pressure difference, and product water flow rate data of the primary reverse osmosis system have been relatively stable. Except for the relatively stable desalination rate in the first 15 days, it subsequently showed a rapid decline (especially the #2 reverse osmosis system).

Figures 1 and 2 are the trend charts of the desalination rate changes of the #1 and #2 reverse osmosis systems, respectively:

Figure 1: Desalination rate change curve of the #1 reverse osmosis system

Figure 2: Desalination rate change curve of the #2 reverse osmosis system

The operation data of the #1 and #2 primary reverse osmosis systems within two months after the commissioning of the new system were recorded and sorted out to understand the changes in the desalination rate of these two systems. Since January 19, after commissioning, the desalination rate of the two reverse osmosis systems has begun to show a significant downward trend, and the decline has certain stages. The #2 reverse osmosis system is particularly serious, with its desalination rate dropping from 99.3% to 93.2%, which means that in just two months of operation, the desalination rate of this system has dropped by 6.1%, and the desalination rate decay speed is getting faster and faster.

3.1 Reverse osmosis product water quality

Due to the small water consumption at this stage of the water treatment system

Inspection of reverse osmosis product water quality

Regarding the problem of the rapid decline in the desalination rate of the #1 and #2 reverse osmosis systems, the system operation process and chemical addition method were comprehensively checked: After the water source is treated by the water plant, it passes through a multi-media filter + ultrafiltration filter before entering the primary reverse osmosis system. A reducing agent, sodium bisulfite, is added to the primary reverse osmosis inlet pipeline, and the reducing agent dosing pump frequency is automatically adjusted according to the ORP data monitoring results. Since there is basically no residual chlorine in the incoming water, the reducing agent dosing pump is basically in a shutdown state. At the same time, judging from the relatively stable operating pressure, pressure difference, and product water flow rate of the #1 and #2 reverse osmosis systems, it does not meet the situation of desalination rate decline caused by chlorine oxidation of the membrane system, so the possibility of chemical damage to the membrane system due to oxidation can be ruled out.

The single-tube product water conductivity of the two reverse osmosis systems was tested (the raw water conductivity is 1600~1800μs/cm). The specific data are shown in Tables 1 and 2:

From the data in the above two tables: The single-tube product water conductivity on both sides of the membrane shell of the two reverse osmosis systems was tested (water samples could not be taken from the single container located at the top of the device, which is caused by negative pressure in the product water). The conductivity of the first-stage product water is generally higher than that of the second-stage product water, while under normal circumstances, the conductivity of the second-stage product water should be about twice that of the first-stage product water, indicating that the first-stage membrane elements have been seriously damaged, and mainly caused by mechanical damage (#2 reverse osmosis damage is particularly serious).

3.2 Inspection and analysis of reverse osmosis operation sequence

Regarding the problem of the rapid decline in the desalination rate of the #1 and #2 reverse osmosis systems, the system operation process and chemical addition method were comprehensively checked.

After inspection: When the reverse osmosis device is started, after the low-pressure flushing is completed, the low-pressure flushing pump is stopped first, the flushing solenoid valve and concentrate discharge solenoid valve are closed, then the high-pressure pump outlet solenoid slow-opening valve is opened, and then the (power frequency) high-pressure pump is started. When the reverse osmosis is stopped, the high-pressure pump is stopped first, then low-pressure flushing is performed, and then the flushing water pump is stopped and the flushing solenoid valve and concentrate discharge solenoid valve are closed.

There are several problems in this start-stop sequence:

a. The high-pressure pump outlet solenoid slow-opening valve did not play its due role. When automatically started, the solenoid slow-opening valve is fully opened before the high-pressure pump is started at power frequency.

b. When the system is shut down, the high-pressure pump also stops instantly before the solenoid slow-opening valve is closed.

During the automatic start-stop process of this system, there is a sudden rapid increase and decrease in system operating pressure, which will cause a certain pressure impact on the membrane elements on both the inlet and outlet sides of the system, causing mechanical damage to the membrane elements and affecting the desalination rate of the reverse osmosis system.

3.3 Inspection of reverse osmosis product water quality

Due to the small water consumption at this stage of the water treatment system, and the primary and secondary reverse osmosis product water tanks are both 50m3 stainless steel tanks, the tank volume is also small. When the system is running, the automatic operation mode of the two reverse osmosis systems is used simultaneously, resulting in the reverse osmosis system being in a state of too frequent start-stop.

From the historical operation curve of the reverse osmosis system: The primary reverse osmosis system is in a state of repeated start-stop, starting for about ten minutes and then stopping, and stopping for about twenty minutes before starting again. Such repeated and frequent system start-stop increases the degree of high-pressure impact on the reverse osmosis membrane elements, and the more serious the impact on the desalination rate of the membrane elements.

4.1 Adjustment of reverse osmosis operation mode

To address the insufficient workshop water supply and the small capacity of the primary water tank, the operation mode of the reverse osmosis system was adjusted to a one-working-one-standby mode. Simultaneously, the high and low liquid level settings of the reverse osmosis water tank were widened to increase the buffer capacity of the reverse osmosis water tank, extend the start-up time of the reverse osmosis system, and minimize the number of starts and stops of the reverse osmosis system.

4.2 Adjustment of Reverse Osmosis Operation Sequence

The start-up and shutdown sequence of the reverse osmosis system was slightly adjusted. Due to the high difficulty of overall adjustment, only minor adjustments were made to the current automatic operation sequence. The details are as follows:

When the reverse osmosis unit starts up, after the low-pressure rinsing is completed, the low-pressure rinsing pump is stopped first, the rinsing solenoid valve is closed, then the high-pressure pump outlet slow-opening valve is opened, then the (frequency) high-pressure pump is started, and then the concentrate discharge solenoid valve is closed, and the reverse osmosis is put into operation. When the reverse osmosis stops, the concentrate discharge solenoid valve is opened first, then the high-pressure pump is stopped, then the rinsing solenoid valve is opened, and the low-pressure rinsing water valve is started for low-pressure rinsing, and then the rinsing water pump is stopped, and the rinsing solenoid valve and concentrate discharge solenoid valve are closed.

After the adjustment of the reverse osmosis operation sequence, the desalination rate of the #2 reverse osmosis system gradually stabilized. Simultaneously, as the operating time of the reverse osmosis system increased, the desalination rate of the membrane system also showed a slow upward trend, indicating that the initial desalination rate attenuation problem of the primary reverse osmosis system was mainly caused by the operation sequence and operation mode of the reverse osmosis system. After the corresponding adjustments, the system desalination rate has stabilized, but because the membrane elements have suffered mechanical damage, which is irreversible, the desalination rate of the membrane system cannot be restored to the new membrane state.