By: Kate
Email:kate@aquasust.com
Date: 17th January 2025
1. Control of MBR influencing factors
In the membrane bioreactor process, the operating conditions of membrane separation are similar to those of traditional membrane separation. The main control factors are influent water quality, membrane surface flow rate, temperature, operating pressure, pH value, MLSS, etc.
1) Temperature
The membrane bioreactor system should be operated at 15℃～35℃. Usually, the membrane flux increases with the increase of temperature, which is mainly because the viscosity of the activated sludge mixed liquor is reduced after the temperature increases, thereby reducing the permeation resistance.
2) Operating pressure
When the characteristics of the activated sludge mixed liquor are basically unchanged, the membrane flux increases with the increase of pressure; but when the pressure reaches a certain value, that is, the concentration polarization makes the solute concentration on the membrane surface reach the limit concentration, continuing to increase the pressure can hardly increase the membrane flux, but aggravates the membrane fouling. The transmembrane pressure difference of the submerged MBR should not exceed 0.05MPa.
3) Dissolved oxygen
Dissolved oxygen is an important factor affecting the removal effect of organic matter. Especially in the case of phosphorus removal and nitrogen removal, the concentration control of dissolved oxygen is particularly important. In different membrane bioreactor process types, the mixed liquid forms aerobic, anoxic and anaerobic sections in the bioreactor in various forms. The control range of DO in each section of the reactor is: anaerobic section is below 0.2mg/L, anoxic section is between 0.2mg/L and 0.5mg/L, and the dissolved oxygen concentration in the aerobic section should not be less than 2mg/L.
4) Membrane surface flow rate
The effects of membrane surface flow rate and pressure on membrane flux are interrelated. When the pressure is low, the membrane surface flow rate has little effect on the membrane flux, and when the pressure is high, the membrane surface flow rate has a great effect on the membrane flux. As the membrane surface flow rate increases, the membrane flux also increases, especially when the pressure is relatively high. This is because the increase in membrane surface flow rate can increase the shear force of the water flow and reduce the deposition of pollutants on the membrane surface on the one hand; on the other hand, the increase in flow rate can increase the convective mass transfer coefficient, reduce the thickness of the boundary layer, and reduce the influence of concentration polarization. In addition, the degree of influence of the membrane surface velocity on the membrane surface deposition layer is also related to the sludge concentration in the feed liquid. When the sludge concentration is low, the membrane permeation rate increases linearly with the membrane surface velocity. However, when the sludge concentration is high, after the membrane surface velocity increases to a certain value, the influence on the deposition layer weakens, and the rate of increase of the membrane flux decreases. For external MBR, the operating conditions should be controlled at low pressure and high flow rate as much as possible, and the membrane surface velocity should be maintained at 3m/s~5m/s. This is not only conducive to maintaining a high water flux, but also conducive to the maintenance and maintenance of the membrane, reducing the cleaning and replacement of the membrane.
5) MLSS
The sludge concentration in the aerobic zone (tank) of the submerged MBR should be controlled at 3000mg/L~20000mg/L. Generally speaking, at a certain membrane surface velocity, when the sludge concentration in the feed liquid increases, due to the high sludge concentration, the sludge is easy to deposit on the membrane surface to form a thick sludge layer, resulting in an increase in filtration resistance and a decrease in membrane flux. However, the sludge concentration in the feed liquid cannot be too low, otherwise the degradation rate of pollutants will be low, and the adsorption and degradation ability of activated sludge on soluble organic matter will be weakened, which will increase the concentration of soluble organic matter in the supernatant of the mixed liquid and easily be adsorbed by the membrane surface, resulting in increased filtration resistance and decreased membrane flux. Therefore, a moderate sludge concentration in the feed liquid should be maintained. Too high or too low will reduce the water flux.
6) pH value
The pH value of the influent of the membrane bioreactor should be 6-9.
2. MBR biochemical process control
When the inlet water temperature is lower than 8℃, the activity of the activated sludge is affected to a certain extent. At this time, the effluent volume should be appropriately reduced to ensure that the organic matter in the sewage is fully degraded in the reaction tank, thereby ensuring the effluent quality. Slow down the membrane blockage.
In the season when the temperature changes suddenly, it is especially important to observe the effluent quality. If the effluent quality changes suddenly, the appropriate effluent volume should be reduced and the aeration time should be increased.
During normal operation, disinfectants and disinfectants that inhibit the metabolism of microorganisms should be avoided from being mixed into the bioreactor. Prevent the normal biological mechanism of microorganisms in the equipment from being destroyed, resulting in deterioration of the effluent.
When the sewage contains a large amount of synthetic detergents or other foaming substances, a large amount of foam will appear in the membrane bioreactor. At this time, water spraying can be used to solve the problem, but do not add defoamers containing oily substances to the reaction tank to remove the foam. Silica gel series defoamers are also not allowed to be used. Silica gel series defoamers are adsorbed on the membrane surface, which will accelerate the rise of the differential pressure between the membranes and cause the membrane to block. At this time, it is difficult to restore the pressure difference even if the liquid is used for cleaning, and the membrane needs to be replaced.
The MBR process system should discharge a certain amount of residual sludge regularly. The amount of sludge discharged can be determined based on the sludge settling ratio, the sludge concentration of the mixed liquor, the organic load of the activated sludge or the sludge age.
3. Control of MBR membrane pollution and cleaning
Membrane pollution is the phenomenon that suspended particles and colloids in sewage are deposited on the membrane surface, causing the membrane pores to be blocked. Once the membrane comes into contact with the feed liquid, the pollution begins, and the adsorption between the solute and the membrane begins to change the membrane properties. For microfiltration membranes, this effect is not very obvious, mainly the aggregation and pore blocking of solute particles; for ultrafiltration, if the membrane material is not properly selected, the impact is quite large, and it can be reduced by 20% to 40% compared with the initial pure water flux. Especially under the conditions of low flow rate and high solute concentration, when the solute reaches or exceeds the saturated solubility on the membrane surface, a gel layer is formed, causing the membrane permeability to be independent of the applied pressure, resulting in a sharp decrease in the membrane permeability. Therefore, the membrane operating in this state must be cleaned after use to restore its performance.