The principle of sewage biological treatment

The method of MBR integrated sewage treatment system includes physical treatment, physicochemical treatment and biochemical method. Among them, the pre-treatment is a physical treatment method, the treatment stage of defecation, hypoxia and aerobic is a biochemical method, the membrane separation (pool) is a physical method, and the auxiliary phosphorus removal process belongs to the physicochemical method.

Sewage from the septic tank enters the equalization tank. Through the bar screen remove the visible inorganic garbage, plastic, rags, and various types of slag in the sewage are isolated to protect the subsequent treatment facilities.  Directly into the anoxic pool of the MBR treatment system section, the dissolved oxygen of the Anaerobic pool is generally within 0.5mg/L. A mixer is installed in this tank to make the activated sludge and sewage fully mixed and in contact. A large amount of mixture containing nitrates and nitrites is returned from the aerobic tank. In the absence of oxygen, denitrifying bacteria reduce nitrates and nitrites to gaseous nitrogen (N2). The reaction formula is:

2NO2-+6[H] (electron donor) — N2 + 2H2O+2OH-

2NO3-+6[H] (electron donor) — N2 + 2H2O+2OH-

The main function of the Anaerobic pool is to remove total nitrogen from the wastewater by denitrification.

The sewage from the Anaerobic tank flows into the aerobic tank, Aerobic tank is the main place to remove organic pollutants, under the action of microorganisms, complete the carbonization reaction (organic matter is decomposed into water and carbon dioxide under the action of aerobic bacteria) and nitrification reaction (ammonia nitrogen is oxidized to nitrate nitrogen under the action of autotrophic bacteria). The biochemical reaction equations of the aerobic biological treatment process are as follows:

1. Decomposition reaction (also known as oxidation reaction, foreignizing metabolism, catabolism)

CHONS + O2    =       CO2 + H2O + NH3 + SO42- +…..+ Energy

(constituent elements of organic matter)

2, synthetic reaction (also known as anabolism, assimilation)

C, H, O, N, S + energy =      C5H7NO2

3. Endogenous respiration (also known as self-oxidation of cellular material)

C5H7NO2 + O2    =      CO2 + H2O + NH3 + SO42- +…….+ Energy

4. Ammonification reaction

RCHNH2COOH+O2    =        RCOOH+ CO2  + NH3

5. Nitration reaction

NH4+ + O2 + HCO3-   =         NO2- + H2O + H2CO3 + C5H7O2N

NO2- + O2 + H2CO3 +NH4+   =         NO3- +H2O +C5H7O2N

The aerobic biochemical tank is provided with a microporous aerator at the bottom, and the air is provided by the air blower for oxygen, and the dissolved oxygen concentration is generally 2-3mg/l. In the aerobic tank, BOD and COD are mainly removed; At the same time, nitration is performed to remove ammonia nitrogen and also provide conditions for the elimination of total nitrogen.

The effluent from the biochemical tank directly enters the MBR Tank, Membrane Bioreactor (MBR). MBR is a new sewage treatment technology which combines high efficiency membrane separation technology and activated sludge process. In the membrane separation part, polyvinylidene fluoride (PVDF) hollow fiber membrane is used for efficient solid-liquid separation, trapping microorganisms and filtering suspended solids and colloids in sewage. Membrane bioreactor (MBR) technology uses ultrafiltration membrane separation process to replace the gravity sedimentation separation process of sludge and water in the traditional activated sludge treatment process for solid-liquid separation. Due to the use of membrane separation, high biological phase concentration and excellent effluent effect can be maintained. It can effectively remove pollutants such as organic matter and ammonia nitrogen in water, and MBR has the following characteristics:

• MBR can efficiently separate solid and liquid, the separation effect is far better than the traditional sedimentation tank, the effluent water quality is good, the effluent suspended matter and turbidity are close to zero, can be directly reused, and realize the sewage resources.
• The efficient interception effect, enables the microorganisms to be completely trapped in the reactor, realizes the complete separation of hydraulic retention time (HRT) and sludge age (SRT) of the reactor, and makes the operation control more flexible and stable.
• High microbial concentration in the reactor, resistant to impact load.
• MBR is conducive to the interception, growth and propagation of nitrifying bacteria with slow growth and proliferation, and the nitrification efficiency of the system can be improved. By changing the operation mode, it can have the function of removing nitrogen and phosphorus.
• The Sludge age is longer. Membrane separation makes the macromolecular refractory components in wastewater have sufficient residence time in the bioreactor with limited volume, which greatly improves the degradation efficiency of refractory organic matter. The reactor is operated under high volume load, low sludge load and long sludge age, and the residual sludge discharge is less. The MLSS in MBR Tank will be around 6000—–8000mg/L.
• High degree of automatically operation, simple and easy managed.
• Smaller area, concentrated process equipment. Modular, easy to expand.

The permeate water of MBR, it can used for irrigation, and cannot be used as domestic drinking water.

The biological treatment requirements and preparations

1) All kinds of pumps /Valves/ air blower/control system /instrument /water level detector have been tested and has good performance, and meet the process requirements

2) The MBR System running by automatically, and it is confirmed that there are no problems.

3) Ensure that the incoming water quality and quantity can meet the commissioning requirements.

4) Professional of commissioning person: the operator is trained and qualified to the operation of MBR System.

Prepare some COD, PH test strip or instrument.

6) Material preparation:

Biological treatment procedure:

1). The first 2 days (the first 2 days of biological treatment)

• If possible, it’s best to do a water quality test and know the COD,BOD5 and ammonia nitrogen, then know input wastewater condition that know how to determine the nutrients required for good biochemical needs. In general, the nutrient ratio of the anaerobic tank is maintained at C: N:P=200:5:1; Aerobic tank to keep C: N:P=100:5:1
• enter the wastewater level of anaerobic tank about 1.5 meters, aerobic tank and MBR Tank water level around 1.5m and start the air blower running 24hours continuous, 2 air blower one working one standby.

2). Day 3 —- Day 6

• Enter fresh wastewater around 5m3 every day and keep the air blower running.
• Add the anaerobic bacteria (1kg) in Anaerobic tank in the third day, and add Glucose or white sugar 5kgs /every day.
• Add the aerobic bacteria (1kg) in aerobic tank in the third day, and add Glucose or white sugar 5kgs /every day.
• And it will have some bubbles in the Fifth days.

3)    Day 7 —- Day 15

• Manual open the Vacuum pump around 5 minutes, take the air out of Suction pipeline, open the Permeate Electrical valve and suction pump, running for 1.5 hours, outlet around 8m3 wastewater.
• Enter fresh wastewater around 8m3 and keep the air blower running.
• Check the output COD /BOD by Test Strip if possible and add Glucose or white sugar in Anaerobic tank and Aerobic tank.
• And it will have lots of bubbles during those days.

3)    Day 16 —- Day 20

• Manual open the Vacuum pump around 5 minutes, take the air out of Suction pipeline, open the Permeate Electrical valve and suction pump, running for 4 hours, outlet around 20m3 wastewater.
• Enter fresh wastewater around 20m3 and keep the air blower running.
• Check the output COD /BOD by Test Strip if possible and add Glucose or white sugar in Anaerobic tank and Aerobic tank.

And it will have lots of bubbles during those days.

5). Normal operation

• Enter fresh wastewater according to the designed 90m3 per day,
• The amount of dissolved oxygen (DO) should be strictly controlled DO of anaerobic process should be controlled within the range of less than 0.5mg/L; The aerobic process should ensure that the DO is not less than 2-4mg /L.
• According to the regulations to take water samples ,testing every day (mainly COD, PH, SS), after passing the self-test qualified