In the field of reverse osmosis water treatment, back pressure refers to the situation where the pressure on the product water side is greater than the pressure on the feed water side. The roll type membrane element is similar to a long envelope shaped membrane pocket, with the open side bonded to the product water center tube containing the opening. Wrap multiple membrane pockets onto the same product center, allowing the water flow to pass through the outside of the membrane. Under the pressure of the water supply, fresh water enters the membrane pockets through the membrane and converges into the product water center pipe. In order to facilitate the flow of product water within the membrane bag, a fabric support layer for guiding product water is sandwiched within the envelope shaped membrane bag; In order to ensure uniform flow of water over the surface of the membrane bag and to disturb the water flow, a mesh layer is sandwiched in the water supply channel between the membrane bags. The three sides of the membrane pocket are bonded together with adhesive. If the pressure on the water side of the product is greater than that on the water side, these adhesive lines will break, causing a loss or significant decrease in the desalination rate of the membrane element. Therefore, from a safety perspective, reverse osmosis systems cannot have back pressure. Due to the fact that reverse osmosis membrane filtration is driven by pressure, there is no back pressure during normal operation. However, if the system is shut down due to normal operation or malfunction, or if the valve settings or opening and closing are improper, back pressure may exist. Therefore, it is necessary to properly address and solve the problem of back pressure.
How is back pressure generated? How to avoid back pressure? Generally, technical manuals related to reverse osmosis membranes have back pressure limit parameters. Assuming that the maximum back pressure that the reverse osmosis membrane can withstand is 0.1 MPa, which is about 1 bar of pressure, approximately equal to the pressure generated by a 10 meter water column. When reverse osmosis is shut down, the water potential in the water production pipe will inevitably exert a certain pressure on the water production side of the reverse osmosis membrane. The higher the water production pipe climbs, the greater the pressure. If the water production pipe climbs more than 10 meters, the back pressure of the water produced on the water production side will be sufficient to cause membrane damage. Therefore, in the process of reverse osmosis design, the maximum climbing height of the production water pipe should be set at 8 meters, and a check valve should be installed on the production water pipe to prevent pressure from being generated in the water. Sometimes there may be situations where the check valve is not tight or the height of the production water pipe must be raised. In this case, the method of installing an exhaust pipe after the check valve of the production water pipe can be adopted to prevent the generation of back pressure. Another type of back pressure is caused by operational errors. In the process of reverse osmosis, if there is an operational error. Starting the water pump without opening both the water production valve and the water discharge valve can cause the system pressure to continuously increase, and the pressure on the water production side to continuously increase without producing water. The water production pipeline is generally made of UPVC material. If the pressure is too high and the pipeline bursts, it may cause safety accidents. If the operator urgently opens the delivery valve to release pressure at this time, accidents can be avoided. If the operator stops the water pump, the pressure on the inlet side will suddenly decrease, and the pressure on the production side will be very high. Such high back pressure will immediately cause membrane damage. So, designers should also consider this during the design process. A bypass can be installed next to the production and discharge valve, and a rupture disc can be installed on the bypass. Assuming the rupture pressure of the rupture disc is 0.3 MPa, once the system is pressurized and the pressure on the production side exceeds 0.3 MPa, it will automatically burst and release pressure to prevent greater losses. Due to non-standard blasting membranes, they may explode randomly before reaching the blasting pressure, or fail to explode when reaching the blasting pressure. Sometimes, on-site personnel may replace the blasting membrane with iron plates or other materials to save time. In this way, once pressure build-up occurs, it is really irreversible. So it is necessary to follow the specifications and conduct experiments on the bursting film. On site debugging personnel of water treatment equipment should also pay attention to checking whether the production and discharge valves are open before starting up the reverse osmosis device. Before shutting down, they should also prioritize opening the production and discharge valves to prevent back pressure accidents.
