Reverse osmosis flushing is the use of low-pressure and high-flow permeate or influent water to flush the membrane element to flush out the pollutants or accumulations attached to the membrane surface.

Flush Flow Rate:

As the plant is operating, particulate contaminants gradually accumulate on the surface of the membrane. If the flow rate of the flush is equal or slightly lower than the flow rate of the water production, it is difficult to flush the contaminants out of the membrane element. Use a higher flow rate than normal operation for flushing. Typically, the flushing flow rate in a single pressure vessel is:

8-inch elements: ≤ 10.9 m3/h (28 mil) or 12.1 m3/h (34 mil) 4-inch elements: ≤ 2.7 m3/h

Flushing pressure:

During normal high-pressure operation, contaminants are pressed against the membrane surface to cause contamination. Therefore, if the same high pressure is applied during rinsing, the contaminants will still be pressed against the membrane surface, and the cleaning effect will not be satisfactory.

Therefore, during flushing, the shear force in the horizontal direction should be increased by low pressure and high flow rate as much as possible to flush the contaminants out of the membrane element. Generally, the inlet water pressure should not be greater than 0.4 MPa.

Frequency of flushing:

Frequent flushing of the system is recommended when possible. Increasing the number of rinses is more effective than performing a chemical clean. In general, the frequency of flushing is recommended once a day. Depending on the situation, the frequency of flushing can be controlled by yourself.

Notes:

(1) The inlet water pump needs to meet the inlet flow rate during normal operation (inlet flow = produced water flow + concentrated water flow), and must be considered to meet the requirements of flushing flow.

(2) The selection of concentrated water pipelines and valves should also consider the large flow rate during flushing. When producing water, the flow rate of concentrated water is relatively small due to the high recovery rate. During the flushing operation, low pressure and high flow are required, and almost all the inlet water is excluded from the concentrated water pipeline, so when designing the concentrated water pipeline and valve, it is necessary to consider not only the flow rate during water production but also the flow rate during flushing.

If the pipes and valves are designed only with the flow rate at the time of water production, the pressure drop at the brine line and the brine valve will increase during flushing, and the required flow rate may not be reached or the required flushing pressure may be exceeded. Of course, you can also consider setting up a separate flushing line.

(3) The flow meter should be selected taking into account the maximum flow rate that can be read at the time of flushing.

For a multi-stage reverse osmosis system, as shown in the figure below, in order to flush the membrane elements more efficiently, it is necessary to design the system in such a way that it can be flushed in stages.

If the entire section is flushed, the rinsing water and contaminants from the front section will flow into the latter section together, which can easily cause blockage in the back section.

An increase in the number of segments also means an increase in the number of membrane elements through which the rinse water flows. In order to be able to meet the flow requirements, the inlet water pressure needs to be increased. The allowable value of the flushing pressure may be exceeded, resulting in an increase in the pressure on the membrane surface and reducing the effectiveness of the flushing.

When the first stage of flushing is carried out, the valve of the first stage of the concentrated water drainage pipeline is fully opened, and the valves between the first stage of concentrated water and the second stage of the inlet water, and the inlet flushing valves of the second and third stages are closed.

When the second stage of flushing is carried out, the valve of the second section of the concentrated water drainage pipeline is fully opened, the inlet flushing valve of the first and third sections is closed, the valves between the first stage of concentrated water and the second stage of water inlet are closed, and the valves of the second stage of concentrated water and the third stage of water inlet are closed.

When the third stage of flushing is carried out, fully open the valve of the third stage of flushing concentrated water drainage pipeline, close the inlet flushing valve of the first and third sections, and close the valves between the second stage of concentrated water and the third stage of water inlet.