Análisis de la comunidad

The mechanistic models available for biological phosphorus removal have been developed substantially in the last years, as a result of intensive investigations in several parts of Europe, North America and South Africa. However, their degree of complexity is very high in view of the great number of variables and parameters involved, some of which are not directly measurable. The IWA models are an example of widely accepted models for the activated sludge process, including BNR. However, their degree of complexity is outside the scope of this book.

For this reason, the following simplified approach is presented for the estimation of the effluent phosphorus concentration, based mainly on the research by Professor Marais and co-workers, in South Africa (WRC, 1984).

(a) Determination of the fraction of P in the suspended solids

The main phosphorus removal route from the system is through its incorporation, in excessive amounts, into the biological excess sludge. With the removal of the excess sludge from the system, phosphorus removal is also achieved. Therefore, it is important to quantify the phosphorus fraction in the excess sludge solids (mgP/mgSS). Usually, this fraction is from 2% to 7% in systems with biological phosphorus removal. However, this value can be estimated using the methodology described below.

The propensity factor of excess phosphorus removal (Pf) is a parameter that reflects the system’s ability to remove phosphorus. The value of Pfcan be estimated using the following equation (WRC, 1984):

Pf= (frb× COD − 25)·fan (35.30) where:

frb= fraction of rapidly biodegradable COD in the influent COD= total COD of the influent wastewater (mg/L)

fan= mass fraction of the anaerobic sludge

The rapidly biodegradable fraction frbusually represents 15 to 30% of the total COD of the raw sewage, and 20 to 35% of the total COD of the sewage after primary settling (Orhon and Artan, 1994).

994 Activated sludge

Influent BOD is converted into COD by simply multiplying it by a factor (COD/BOD5ratio) between 1.7 and 2.4.

With respect to the anaerobic sludge fraction fan, if the concentration of solids is the same in all zones of the reactor, fancan be considered equal to the ratio between the volume of the anaerobic zone and the total volume of the reactor (Vanaer/Vtot).

Values of this anaerobic fraction vary between 0.10 and 0.25 (Vanaervaries between 10% and 25% of the total volume of the reactor).

The phosphorus fraction in the active biomass (mgP/mgXa) can be expressed using the following relation (WRC, 1984):

P/Xa= 0.35 − 0.29·e^−0.242·Pf (35.31)

As seen in Section 9.5.8, the active fraction of the mixed liquor volatile sus-pended solids (Xa/Xv) is given by:

fa= 1

1+ 0.2·Kd·θc

(35.32)

where:

Kd= coefficient of endogenous respiration (0.08 to 0.09 d⁻¹) θc= total sludge age (d)

The ratio between the volatile suspended solids and the total suspended solids in the reactor (Xv/X) can be calculated, as shown in the example in Chapter 34, or be obtained from Table 31.8. Typical values are: (a) conventional activated sludge:

0.70 to 0.85, (b) extended aeration: 0.60 to 0.75. A quick way of calculating the ratio for the treatment of domestic sewage is to use the regression equations with the sludge age contained in Table 31.9, namely:

• system with primary sedimentation:

Xv/X = 0.817·θc−0.043 (35.33)

• system without primary sedimentation:

Xv/X = 0.774·θc−0.038 (35.34) Thus, the phosphorus fraction in the suspended solids can be calculated through the following equations, whose terms can be obtained from Equations 35.31 to 35.34:

• Fraction of P in the volatile suspended solids in the excess sludge (mgP/mgVSS):

P/Xv= fa·(P/Xa) (35.35)

• Fraction of P in the total suspended solids in the excess sludge (mgP/mgSS):

P/X =

VSS SS



·fa·(P/Xa) (35.36)

Depending on the values of the influent COD and the rates and coefficients adopted, it is possible to obtain P/X values much higher than the value of 7%

mentioned by EPA (1987b) and Orhon and Artan (1994). For safety reasons, it is suggested that, for design purposes, a maximum value of 7% is assigned for this relation.

(b) Removal of P with the excess sludge

The ratio of the phosphorus removed per unit of BOD removed (mgP/mgBOD) can be expressed as follows (EPA, 1987b):

P/BOD = Yobs·(P/Xv) (35.37) or

P/BOD = Y

1+ fb·Kd·θc

·(P/Xv) (35.38)

where:

P/Xv = fraction of P in VSS (calculated from Equation 35.35) (mgP/mgVSS) Y= yield coefficient (0.4 to 0.8 mgVSS/mgBOD)

fb = biodegradable fraction of the VSS (mgSSb/mgVSS)

The fbvalue can be calculated from Equation 9.68 (Section 9.5.8), as follows:

fb= 0.8 1+ 0.2·Kd·θc

(35.39)

Typical values of fb are: (a) conventional activated sludge: 0.55 to 0.70 and (b) extended aeration: 0.40 to 0.65.

The amount of phosphorus removed in the excess sludge, taking into consid-eration the amount of BOD removed, can be determined by multiplying the result of Equation 35.38 by the removed BOD concentration (So− S):

Prem= Y

1+ fb·Kd·θc

·(P/Xv)·(So− S) (35.40)

where:

Prem = concentration of P removed in the excess sludge (mg/L) So = total influent BOD concentration to the biological stage (mg/L)

S= soluble effluent BOD concentration from the biological stage (mg/L)

996 Activated sludge

(c) Effluent P concentration

The concentration of the effluent soluble phosphorus is given by the difference between the total effluent concentration of P and the removed concentration of P (given by Equation 35.40):

Psol eff= Ptot inf− Prem (35.41)

The concentration of the effluent particulate phosphorus (present in the effluent SS) is determined by multiplying the SS concentration in the effluent from the system by the fraction of P in the suspended solids (P/X). P/X is given in Equa-tion 35.36.

Ppart eff= SS·(P/X) (35.42)

The total effluent phosphorus concentration is the sum of the concentrations of soluble P and particulate P in the effluent:

Ptot eff= Psol eff+ Ppart eff (35.43) The example in Section 36.2 illustrates this calculation method for biological phosphorus removal.

36

Design of continuous-flow systems