Membrane bioreactor (MBR) has been widely used in wastewater treatment and resource engineering due to its high efficiency, energy saving, no phase change, no secondary pollution, good water quality, less land occupation and high degree of automation. Membrane bioreactor is used to treat municipal wastewater and industrial organic wastewater with an investment of 2000 yuan   Yuan to 4000 yuan   The operating cost is less than 1.50 yuan per ton of water   Yuan / ton of water, environmental benefits and economic benefits are very significant!

1. Control of influencing factors of MBR

In MBR process, the operation conditions of membrane separation are similar to traditional membrane separation, and the main control factors are influent water quality, membrane surface velocity, temperature, operating pressure, pH value, MLSS, etc.

1) Temperature

The MBR system should be operated at 15 ℃ ~ 35 ℃. Generally, the membrane flux increases with the increase of temperature, which is mainly due to the decrease of the viscosity of the activated sludge mixture and the decrease of the osmotic resistance.

2) Operating pressure

The results show that the membrane flux increases with the increase of pressure when the characteristics of activated sludge mixture remain unchanged; However, when the pressure reaches a certain value, that is, concentration polarization makes the solute concentration on the membrane surface reach the limit concentration, increasing the pressure can hardly improve the membrane flux, but aggravate the membrane fouling. The transmembrane pressure difference of submerged MBR should not exceed 0.05Mpa.

3）   dissolved oxygen

Dissolved oxygen is an important factor affecting the removal efficiency of organic matter. Especially for the purpose of phosphorus and nitrogen removal, the control of dissolved oxygen concentration is particularly important. In different types of MBR process, the mixed liquid forms aerobic, anoxic and anaerobic sections in various forms in the bioreactor. The control range of do in each section of the reaction tank is: the anaerobic section is below 0.2mg/l, the 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 velocity

The effects of velocity and pressure on membrane flux are interrelated. When the pressure is low, the surface velocity has little effect on the membrane flux, while when the pressure is high, the surface velocity has great effect on the membrane flux. With the increase of membrane velocity, the membrane flux also increases, especially when the pressure is high. On the one hand, it can increase the shear force of water flow and reduce the deposition of pollutants on the membrane surface; On the other hand, the increase of flow velocity can improve the convective mass transfer coefficient, reduce the thickness of boundary layer and reduce the influence of concentration polarization. In addition, the influence of membrane velocity on the 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 velocity. However, when the sludge concentration is high, the effect on the deposition layer will be weakened and the increasing speed of membrane flux will be reduced when the membrane velocity increases to a certain value. For external MBR, the operating conditions should be controlled at low pressure and high flow rate as far as possible, and the membrane flow rate should be kept at 3m / S ~ 5m / s. This is not only conducive to maintaining a high water flux, but also conducive to the maintenance of the membrane, reducing the cleaning and replacement of the membrane.

5） MLSS

The sludge concentration in aerobic zone (pool) of submerged MBR should be controlled between 3000mg / L and 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 the increase of filtration resistance and the decrease of membrane flux. However, the sludge concentration in the feed liquid should not be too low, otherwise the degradation rate of pollutants is low, and the adsorption and degradation ability of the activated sludge to dissolved organic matter is weakened, which makes the concentration of dissolved organic matter in the supernatant of the mixed liquid increase and easy to be adsorbed by the membrane surface, resulting in the increase of filtration resistance and the decrease of membrane flux. Therefore, it is necessary to maintain a moderate sludge concentration in the feed liquid. If the sludge concentration is too high or too low, the water flux will decrease.

6) PH value

The pH value of influent water should be 6-9.

2. MBR biochemical process control

When the influent water temperature is lower than 8 ℃, the activity of activated sludge will be affected to a certain extent. At this time, the water yield should be reduced appropriately to ensure that the organic matter in the wastewater can be fully degraded in the reaction tank, so as to ensure the effluent quality. Slow down membrane clogging.

In the season of sudden change of temperature, we should pay special attention to observe the effluent quality. If there is a sudden change of effluent quality, we should reduce the appropriate water yield and increase the aeration time.

During normal operation, the disinfectant and disinfectant which have inhibitory effect on microbial metabolism should be avoided to mix into the biological reaction pool. To prevent the normal biological mechanism of microorganisms in the equipment from being destroyed, resulting in the deterioration of the effluent.

When there is a lot of synthetic detergent or other foaming substances in the sewage, there will be a lot of bubbles in the membrane bioreactor. At this time, water spray 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. When silica gel series defoamers are adsorbed on the membrane surface, the rise of differential pressure between membranes will be accelerated and the membrane will be blocked. At this time, it is difficult to restore the pressure difference even with the use of liquid medicine cleaning, and the membrane needs to be replaced.

The MBR process system should discharge a certain amount of excess sludge regularly. Sludge discharge can be determined by sludge settling ratio, sludge concentration in mixed liquor, organic loading of activated sludge or sludge age.

3. MBR membrane fouling and cleaning control

Membrane fouling is the deposition of suspended particles and colloids on the membrane surface, resulting in the blockage of membrane pores. Once the membrane comes into contact with the feed solution, the fouling begins. Due to the interaction between solute and membrane, adsorption occurs, and the membrane characteristics begin to change. For microfiltration membrane, this effect is not very obvious, and mainly depends on the aggregation and pore plugging of solute particles; For ultrafiltration, if the membrane material is not selected properly, the influence is quite large, which can be reduced by 20% ~ 40% compared with the initial pure water flux. Especially in the case of low flow rate and high solute concentration, when the solute reaches or exceeds the saturation solubility on the surface of the membrane, the gel layer is formed, resulting in the membrane's transmittance does not depend on the pressure, resulting in a sharp decrease in the membrane permeability. Therefore, the membrane used in this condition must be cleaned after use to restore its performance.

The measures to control membrane pollution are as follows

1）   The influent of MBR system was pretreated to remove the coarse particles;

2）   Select the appropriate operating pressure;

3）   Shortening the suction time of the outlet pump or prolonging the suction stop time and increasing the aeration rate are beneficial to slow down the membrane pollution.

The impurities on the surface of the membrane can be removed by air cleaning, and the impurities in the pores can be discharged by water backwashing. Water backwashing is to pump filtered water from the backwashing tank to the pumping pipe. According to the different types of membrane, it is generally backwashed every 10 minutes to 24 hours.

In order to maintain the good performance of the membrane, it is necessary to use chemical cleaning method to remove pollutants when water backwashing has no effect. The chemical cleaning of membrane is different according to the specific situation of pollutants, and the cleaning agents used are also different. During chemical cleaning, the principles of selecting chemicals are as follows: first, no chemical reaction with membrane and other component materials; second, no secondary pollution caused by the use of chemicals.

The categories and properties of commonly used cleaning agents for membrane cleaning are shown in the table below:

Types and properties of surface cleaning agents

type

major function

Typical compounds

Types of pollutants

Alkaline substance

Hydrolysis and solubilization

NaOH

Organic matter, polysaccharide, protein and microorganism

Oxidant and bactericide

Oxidation degradation, sterilization and disinfection

NaClO, H2O2, ozone, peracetic acid

Humus, protein and microbial contamination

acid

Solubilization

Citric acid, nitric acid

Fouling, scaling, metal oxides

Chelating agent

Chelation and solubilization

Nitric acid / EDTA

Fouling, scaling, metal oxides

surface active agent

Emulsifying, dispersing, adjusting surface properties

Surfactants, detergents

Fat, oil and protein, microbial contamination

Enzyme preparation

Degradation of polymer chain and solubilization

Enzyme detergent

Protein, extracellular polymer, microbial contamination