MBR process for sewage treatment

4. Drainage system

(1) The supernatant removal device should be able to discharge the supernatant within the set drainage time without the activated sludge floating, and the discharge methods are gravity discharge and pump discharge.

(2) In order to prevent the failure of the supernatant discharge device, an accident drainage device should be set up.

(3) In the supernatant discharge device, a mechanism to prevent scum outflow should be provided.

The sequencing batch activated sludge discharge device should discharge the supernatant separated from the activated sludge during the precipitation and drainage period, and has the following characteristics:

1) Should be able to neither disturb the settling sludge, nor make the sludge float, according to the specified flow rate discharge supernatant. (quantitative drainage)

2) In order to obtain clear treated water after separation, the water collection mechanism should be as close to the water surface as possible, and it can be drained with the water level change after the supernatant discharge. (Water level tracking performance)

3) The action of drainage and stopping drainage should be smooth, accurate, durable and reliable. (Reliability)

The structural form of the drainage device, according to the different ways of lifting, there are float type, mechanical type and fixed type without lifting.

5. Mud discharge equipment

Design sludge dry solid quantity = design sewage quantity × design influent SS concentration × sludge yield /1000

Sludge production is calculated at 1 kg per 1 kgSS inflow in high load operation (0.1 ~ 0.4 kg-bod /kg-ss·d) and 0.75 kg per 1 kgSS inflow in low load operation (0.03 ~ 0.1 kg-bod /kg-ss·d).

The sludge concentration of 2 ~ 3% can be obtained by setting a simple sludge concentration tank in the reaction tank. Because the sequencing batch activated sludge method does not have a primary sedimentation tank, it is easy to flow more debris, and the sludge pump should be a pump type that is not easy to plug.

Main parameters of SBR design

The design parameters of the sequencing batch activated sludge method must be properly determined considering the regional characteristics and design conditions of the treatment plant (land area, maintenance management, treatment water quality indicators, etc.).

The following values shall prevail in the design parameters for facility design:

Parameter number of the entry

BOD-SS load (kg-BOD/kg-ss·d) 0.03 ~ 0.4

MLSS(mg/l) 1500 ~ 5000

Discharge ratio (1/m) 1/2 ~ 1/6

Safe height ε(cm)(minimum water depth above the activated sludge interface) 50 or more

Sequencing batch activated sludge process is a process that can operate in the range of low load (equivalent to oxidation ditch method) to high load (equivalent to standard activated sludge method) depending on the organic load. The BOD-SS load of the sequencing batch activated sludge process is defined as follows because the aeration time is considered as the reaction time:

QS: Sewage intake (m3/d)

CS: Average influent BOD5(mg/l)

CA: Average MLSS concentration in aeration tank (mg/l)

V: aeration tank volume

e: aeration time ratio e=n·TA/24

n: number of cycles TA: aeration time of a cycle

The loading conditions of the sequencing batch activated sludge method are determined according to the ratio of the volume of the reaction tank to the amount of sewage water in each cycle and the number of cycles per day. In addition, in the sequencing batch activated sludge method, because the concentration of MLSS in the tank is easy to maintain a good concentration, the organic load can also be adjusted through the change of the concentration of MLSS. Furthermore, because the aeration time is easy to adjust, the organic load can also be adjusted by changing the aeration time.

In nitrogen removal and desulphurization, in addition to organic load, it is necessary to study the discharge ratio, the number of cycles, and the daily aeration time.

In facilities with limited land area, it is suitable for high-load operation and small-scale facilities with small inlet flow and large load changes, it is best to run at low load. Therefore, the effective way is to operate at low load in the early stage of production, and with the increase of water volume, it can also be operated at high load.

Characteristics under different load conditions

Organic load conditions (water inlet conditions) High load operation low load operation

Intermittent water intake Intermittent water intake, continuous

Operating conditions BOD-SS load (kg-BOD/kg-ss·d) 0.1 ~ 0.4 0.03 ~ 0.1

Number of cycles large (3 ~ 4) Small (2 ~ 3)

Discharge is smaller than big

Treatment characteristics Organic matter removal treatment water BOD<20mg/l removal rate is relatively high

Nitrogen removal is low and high

Dephosphorization high is low

How much sludge production

Maintenance management is more adaptable to load changes and more flexible in operation than low load difference

The volume of land area reaction pond is small, and the volume of provincial reaction pond is large

Scope of application can effectively treat sewage above medium scale, suitable for facilities with a treatment scale of about 2000m3/d or more suitable for small sewage treatment plants, the treatment scale of about 2000m3/d or less, suitable for facilities that do not require nitrogen removal.