A method for controlling an aeration volume, which is a volumetric flow rate of an aeration gas flowing into an activated sludge treatment apparatus, configured to remove BOD in wastewater or simultaneously remove BOD and nitrogen components in wastewater by performing aeration treatment while a dissolved oxygen concentration (hereinafter referred to as DO) of an activated sludge mixed liquor in an aeration tank is maintained at 0.5 mg/l or lower (hereinafter this treatment is referred to as ultralow DO treatment),
- the method comprising:
- (1-0) acquiring an aeration volume correlation,
- wherein an aeration volume measuring device (hereinafter referred to as the measuring device) draws the activated sludge mixed liquor in the aeration tank into the measuring device and aerates the activated sludge mixed liquor to determine a measuring device aeration volume G,
- so as to acquire a correlation (G=F(Gr), meaning G is a function of Gr) between an aeration volume Gr in the aeration tank and the aeration volume G in the measuring device at which a DO value of the aeration tank equals a DO value of the measuring device;
- (1-1) performing an aeration operation in the measuring device during ultralow DO treatment operation,
- wherein the activated sludge mixed liquor is sampled into the measuring device from the aeration tank in an ultralow DO treatment operation and aerated at an aeration volume G1 (G1>G2) to increase DO of the activated sludge mixed liquor, and then aeration is stopped,
- where G2 represents an aeration volume of the measuring device corresponding to an aeration volume Gr2 of the aeration tank at the time of the ultralow DO treatment operation and G2 is calculated from the correlation (G2=F(Gr2));
- (1-2) acquiring oxygen consumption rate Rr and equilibrium DO value C1,
- where a temporal change in DO value (C), (dC(t)/dt), is expressed by formula (1):
- dC(t)/dt=KLa(Cs−C(t))−Rr formula(1)
- where KLa is an overall mass transfer coefficient, Cs is a saturated dissolved oxygen concentration, and Rr is an oxygen consumption rate of the activated sludge mixed liquor under an aerobic condition,
- (1-2-1) Rr is obtained on a basis of changes in C(t) during a DO decreasing process when aeration is stopped, and
- (1-2-2) an equilibrium DO value C1 (dC(t)/dt=0, C(t)=C1) under temporarily intensified aeration is obtained by repeat calculation by varying KLa or C1 on a basis of changes in C(t) during a DO increasing process under intensified aeration and a relationship of formula (2):
- KLa(Cs−C1)=Rr formula(2);
- (1-3) acquiring an appropriate aeration volume G2* of the measuring device,
- wherein, an aeration volume G0 at which an oxygen supply capacity by aeration equals an oxygen consumption rate of the activated sludge mixed liquor is obtained by formula (3′) where G1 represents an aeration volume under temporarily intensified aeration and Ea1 represents an oxygen dissolution efficiency during the temporarily intensified aeration, the appropriate aeration volume G2* in the measuring device during the ultralow DO treatment operation is obtained by repeat calculation using formula (3):
- G0=((Cs−C1)/Cs)·(Ea1/Ea0)·G1 formula(3′)
- G2*=k·((Cs−C1)/Cs)·(Ea1/Ea2)·G1 formula(3)
- where Ea0 and Ea2 are respectively oxygen dissolution efficiencies at aeration volumes G0 and G2* and are acquired from a relationship formula of Ea and G obtained by experiments, and
- where k is a proportionality factor that derives the appropriate aeration volume G2* under an ultralow DO treatment condition from the calculated appropriate aeration volume based on formula (3′), an optimum value of k is acquired by measuring aeration volumes under one or more treatment conditions, and an optimum value of k corresponding to the ultralow DO treatment condition is substituted into formula (3);
- (1-4) acquiring an appropriate aeration volume Gr2* of the aeration tank,
- wherein the aeration volume Gr2* of the aeration tank corresponding to the aeration volume G2* of the measuring device obtained in (1-3) is obtained from the correlation (G2*=F(Gr2*));
- and
- (1-5) controlling to maintain the appropriate aeration volume during the ultralow DO treatment operation, wherein, over one or more calculation periods, setting of the aeration volume of the aeration tank to the appropriate aeration volume Gr2* obtained in (1-4) is carried out for each calculation period.