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♦ Aerobic Digestion
Design criteria for aerobic digesters a
Parameter Value
Hydraulic retention time, 20°C, d b
Waste activated sludge only 10–15
Activated sludge from plant without primary settling 12–18
Primary plus waste activated or trickling-filter sludge c 15–20
Solids loading, lb volatile solids/ft 3 •d
Oxygen requirements, lb O2/lb solids destroyed
0.1–0.3
Cell tissue d ~2.3
BOD5 in primary sludge
Energy requirements for mixing
1.6–1.9
Mechanical aerators, hp/10 3 ft 3 0.7–1.50
Diffused-air mixing, ft 3 /10 3 ft 3 •min 20–40
Dissolved-oxygen residual in liquid, mg/L 1–2
Reduction in volatile suspended solids, % 40–50
a Adapted in part from Ref. 59.
b Detention times should be increased for operating temperatures below 20°C.
c Similar detention times are used for primary sludge alone.
d Ammonia produced during carbonaceous oxidation oxidized to nitrate
(see Eq.12–11).
Note: lb/ft 3 •d × 16.0185 = kg/m 3 •d
hp/10 3 ft 3 × 26.3342 = kW/10 3 m 3
ft 3 /10 3 ft 3 •min × 0.001 = m 3 /m 3 •min
0.556(°F – 32) = °C
Tank Volume
i ( i + i)
( + 1 θ )
Q X
Vol =
Xd KdPv FS
c
, where
Vol = volume of aerobic digester (ft 3 ),
Qi = influent average flowrate to digester (ft 3 /d),
Xi = influent suspended solids (mg/L),
F = fraction of the influent BOD5 consisting of raw
primary sludge (expressed as a decimal),
Si = influent BOD5 (mg/L),
Xd = digester suspended solids (mg/L),
Kd = reaction-rate constant (d –1 ),
Pv = volatile fraction of digester suspended solids
(expressed as a decimal), and
θc = solids residence time (sludge age) (d).
♦
VOLATILE SOLIDS REDUCTION, %
60
50
40
30
20
10
0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
TEMPERATURE ˚C × SLUDGE AGE, DAYS
VOLATILE SOLIDS REDUCTION IN AN AEROBIC
DIGESTER AS A FUNCTION OF DIGESTER LIQUID
TEMPERATURE AND DIGESTER SLUDGE AGE [59].
160
Anaerobic Digester
ENVIRONMENTAL ENGINEERING (continued)
Design parameters for anaerobic digesters
Parameter Standard-rate High-rate
Solids residence time, d 30–90 10–20
Volatile solids loading, kg/m 3 /d 0.5–1.6 1.6–6.4
Digested solids concentration, % 4–6 4–6
Volatile solids reduction, % 35–50 45–55
Gas production (m 3 /kg VSS added) 0.5–0.55 0.6–0.65
Methane content, % 65 65
Standard Rate
Vol1 + Vol2
Reactor Volume = tr + Vol2ts 2
High Rate
First stage
Reactor Volume = Vol1tr
Second Stage
Vol1 + Vol2
Reactor Volume = tt + Vol2ts, where
2
Vol1 = raw sludge input (volume/day),
Vol2 = digested sludge accumulation (volume/day),
tr = time to react in a high-rate digester = time to react
and thicken in a standard-rate digester,
tt = time to thicken in a high-rate digester, and
ts = storage time.
Biotower
Fixed-Film Equation without Recycle
Se −kD
n
q
= e
So
Fixed-Film Equation with Recycle
S
S
e
a
=
e
n
−kD
q
( 1+
R)
− R(
e )
n
−kD
q
Se =
So
+ RSe
S a = , where
1 + R
effluent BOD5 (mg/L),
So = influent BOD5 (mg/L),
D = depth of biotower media (m),
q = hydraulic loading (m 3 /m 2 • min),
= (Qo + RQo )/Aplan (with recycle),
k = treatability constant; functions of wastewater and
medium (min −1 ); range 0.01−0.1; for municipal
wastewater and modular plastic media 0.06 min −1
@ 20°C,
kT = k2o(1.035) T−20 ,
n = coefficient relating to media characteristics;
R =
modular plastic, n = 0.5,
recycle ratio = Qo / QR, and
QR = recycle flow rate.
♦ Tchobanoglous, G. and Metcalf and Eddy, Wastewater Engineering: Treatment, Disposal, and
Reuse, 3rd ed., McGraw-Hill, 1991.