Membrane fouling is the biggest problem during the operation of ultrafiltration device, and it is also one of the most common problems of ultrafiltration device. The pollution is mainly manifested in the reduction of water yield and the deterioration of water quality. At the same time, with the increase of water inlet pressure, the transmembrane pressure difference increases, and there is a certain phenomenon of wire breakage. The greater the transmembrane pressure difference, the more serious the wire breakage phenomenon will be.
Causes of fouling in ultrafiltration system:
(1) The inlet water temperature of ultrafiltration is too low, resulting in the increase of operation resistance;
(2) Ultrafiltration membrane fouling is caused by raw water pollution, too many bacteria and algae, that is, the SDI of influent is too high, resulting in ultrafiltration fouling;
(3) The influent flow of ultrafiltration increases, which exceeds the water production capacity of the filter membrane;
(4) The inlet pressure is too high. In order to increase the water production, artificially increase the inlet pressure of ultrafiltration.
When ultrafiltration is polluted, it will have a great impact on RO system and increase pollution blockage, mainly in the following aspects:
(1) The water quality deteriorates, the SDI value becomes higher or exceeds the standard, and the color of the SDI value test diaphragm becomes darker;
(2) The reverse osmosis membrane is polluted and blocked, resulting in the rapid decline of water production rate of the reverse osmosis system;
(3) When the UF is seriously polluted, a layer of "sponge" will be generated in the UF. During chemical cleaning, the residual NaOH or sodium hypochlorite can not be washed in time, which will be released slowly, which will bring serious harm to the subsequent RO system.
Therefore, in ultrafiltration operation, it is generally designed with forward flushing, backflushing and conventional on-line automatic pH adjustment immersion cleaning. Generally, it is backwashed once from 20min to 1H; Conduct a low pH or high pH cleaning for more than 8h. When adjusting the pH, generally soak for 10min, and then conduct physical flushing. When soaking with alkaline solution, a certain amount of NaClO is often added to enhance the cleaning effect.
The filtration mode of membrane is divided into dead end and cross flow. Dead end filtration, also known as full flow filtration, means that the flow direction of the liquid is perpendicular to the membrane surface. After the filtered liquid passes through the membrane, all the intercepts are deposited on the membrane surface. This filtration mode has a high probability of blockage and is generally suitable for water quality with low turbidity and low suspended solids content. Cross flow filtration can realize the continuous operation of ultrafiltration device and meet the scale requirements of large flow. In cross flow filtration, the water flow is parallel to the surface of the filter membrane, which can form a large shear force on the surface of the filter membrane, resulting in the membrane pollutants always in a suspended state, reducing the impact of concentration polarization, reducing the deposition of particles and solutes on the membrane surface and reducing membrane pollution. Therefore, the cross flow mode has strong adaptability to the water quality of feed water. In addition, increasing the cross flow velocity can also improve the critical flux.
When the TMP increases to a certain level and cannot be recovered through cross flow and backwashing, chemical cleaning can be used to recover the membrane flux. Through contact with a certain dose of chemicals and under the action of large flow cleaning, the pollutants on the membrane surface and membrane pores can be removed, so as to restore the flux of the membrane.
Most ultrafiltration equipment adopts hollow fiber membrane wire. It is a common phenomenon that a small amount of wire breakage occurs during operation. Wire breakage is generally caused by membrane wire fouling, back impact and other reasons. Tens of thousands of membrane wires are installed in one element, and a small amount of broken wires will not immediately affect the water quality. Therefore, it is not easy to detect during operation. However, once the change of water production quality is found, the accurate position of broken wire should be determined quickly in order to take corresponding measures.
The treatment of broken wire is generally to find out the damaged membrane wire and block it. If the amount of broken wire or blocked membrane wire increases to a certain extent (more than 10%), the whole element needs to be replaced.
