Aeration tank is the core structure of activated sludge treatment system. Its volume is not only related to the purification effect of the whole treatment system, but also related to the construction cost. Therefore, it is necessary to analyze the calculation method of aeration tank volume, so as to get a better design value. For a long time, the method of BOD - sludge loading rate has been widely used to calculate the volume of aeration tank, but recently it has been suggested to use sludge age method. So, what are the similarities and differences between the two? Is there some internal connection between them? Can they be organically combined? This paper makes the following analysis and discussion.

1 BOD - sludge loading rate (NS) aeration tank volume calculation method

1.1 BOD - physical concept of sludge loading rate (NS)

The unit weight (kg) of activated sludge in the aeration tank, which can be accepted and degraded to a specified amount of BOD5 weight value in unit time, is called BOD sludge loading rate (NS). [1][2]:

Where ns - BOD - sludge loading rate,

kg BOD5/kgMLSS·d

Q - design flow of sewage, m3 / D

SA - BOD5 value of raw sewage, mg / L

X -- suspended solid concentration of mixed liquid in aeration tank

(MLSS), mg/l

V - volume of aeration tank, M3

1.2 material balance equation of aeration tank

As shown in Figure 1, the material balance diagram of the fully mixed activated sludge system [1] [4].

Under stable conditions, material balance of organics in the system is as follows:

Collate:

From the inference of Monod equation [1] [4]:

Substituting (3) and sorting out:

or

also

Substitute (6) to get:

or

In the formula, XV -- volatile suspended solid of mixed liquid in aeration tank

Concentration (MLVSS), mg / L

Se -- organic matter concentration of treated water effluent, mg / L

——Degradation rate of organics,

K2 -- degradation constant of organic matter.

1.3 volume calculation of aeration tank

The formula (1) includes:

Substituting formula (9) into formula (10) can get:

Equation (10) is the calculation formula of the volume of the aeration tank according to the BOD sludge load rate method, and equation (11) is the calculation formula after transformation.

Calculation method of aeration tank volume for sludge age (θ C)

2.1 physical concept of sludge age (θ C)

The ratio of the total amount of activated sludge in the aeration tank to the amount of sludge discharged per day is called sludge age (θ C). That is, Lawrence McCarty's "average residence time of biosolids" [1]. Namely:

Where θ C -- sludge age, D

Δ XV - daily increased volatility in the aeration tank

Sludge volume (VSS), KMG / L

Others - same as before

2.2 basic equation of biological growth

In the aeration tank, the proliferation of activated sludge microorganisms is the result of the joint activities of microbial synthesis and internal metabolism. Namely:

or

This equation (14) can be used to deduce the inference of Lawrence mccayty equation the relationship between the concentration of activated sludge and sludge age in the aeration tank [1]

This formula is the recommended formula for calculating the volume of aeration tank according to sludge age in data [1] [2]:

Substituting EQ (5) and EQ (12) into EQ (4), we can get:

Where y -- microbial yield coefficient, kgvss / kgbod5

KD - microbial attenuation coefficient, D-1

Others - same as before.

2.3 volume calculation of aeration tank

By way of style

Put formula ⒄, formula ⒀ and formula ⒀ into formula 18, and sort out:

Equation (16) or (18) is the calculation formula of sludge age method to calculate the volume of aeration tank. Equation 19 is the transformed calculation formula.

3 discussion on two calculation methods

3.1 similarities between the two calculation methods

Although the original concepts of the two calculation methods are different, it can be seen from the analysis that the formula (11) and (19) of the two calculation methods are exactly the same. In other words, following the inference of Monod reaction kinetics equation, there is no essential difference between the two calculation methods, which can be expressed by the same formula: Eq.