Handbook of Civil Engineering Calculations

The BOD entering the digester 25,000 Ib/d (11,350 kg/d). Therefore, the volumetric load-

ing to the digester is

Volumetric Loading = 25?°^ ^ B°D/d = 0.21 Ib/ftM (3.37 kg/m^3 -d)

For high rate digesters, loadings range from 0.10 to 0.35 Ib/ft^3 -d (1.6 to 5.61 kg/m^3 -d).
Assuming 60 percent waste utilization, the BODL exiting the digester is

(25,000 lb/d)(l - 0.6) = 10,000 Ib/d (4,540 kg/d)

2. Compute the daily quantity of volatile solids produced
   The quantity of volatile solids produced each day is computed using

= y[(BODin?lb/d)-(BODout,lb/d)]

i-kdec

where Px = Volatile solids produced, Ib/d (kg/d)
Y= Yield coefficient (Ib VSS/lb BODL)
kd = Endogenous coefficient (d"^1 )

(^6) C = Mean cell residence time (d)
Using values obtained above, the volatile solids produced each day are
_ (0.06 Ib VSS/lb BODL)[25,000 Ib/d) - (10,000 Ib/d)]
PX
~ 1+(0.03^
1
XlSd)
= 6211b/d(281.9kg/d)

3. Determine the volume of methane produced
   The volume of methane gas produced at standard conditions (32^0 F and 1 arm) (O^0 C and
   101.3 kPa) is calculated using

FCH 4 = 5.62 ft

3

/lb[(BODin, Ib/d) - (BODout, Ib/d) - 1.42 PJ

where FCH4 = Volume of methane gas produced at standard conditions (ft
3
/d) (m
3
/d)

Using values obtained above:

FCH 4 = 5.62 ft^3 /lb[(25,000 Ib/d) - (10,000 Ib/d) - 1.42(621 Ib/d)]

= 79,344 fVVd (2,245 m^3 /d)

Since digester gas is approximately 2/3 methane, the volume of digester gas produced is

(79,344 ft

3

/d)10.67 = 118,424 ft

3

/d (3,351.4 m

3

/d)

4. Calculate the percent stabilization
   Percent stabilization is calculated using:

0/c,,.r ,. [(BOD1n, Ib/d) - (BODout, Ib/d) - 1.42 Px]

% Stabilization = —• x 100

BOD1n, Ib/d