The document provides design criteria and an example design for an anaerobic sludge digestion system. Key points: - It selects anaerobic digestion with two completely mixed digesters operated at 35°C for sludge stabilization. - Design parameters include volatile solids loading, retention time, mixing method, gas production estimates, and characteristics of influent and digested sludge. - The example design calculates digester sizing based on flow and loading, dimensions two 13.7m diameter digesters, and verifies loading rates and retention times meet requirements.

1. Information Checklists for Design
of Sludge Stabilization Facility
1. Select the ultimate sludge disposal method as the degree of
sludge stabilization will depend on the requirements of the
disposal practice.
2. Develop the characteristics of thickened sludge that will reach
the sludge stabilization facility.
3. Select the sludge stabilization method that is compatible with the
influent sludge characteristics, dewatering, and ultimate disposal
method.
4. Develop design parameters (organic loading, hydraulic loading,
chemical dosage, reaction period, etc.) for the selected sludge
stabilization facility.
5. Obtain the design criteria from the concerned regulatory agency.
6. Obtain necessary manufacturers’ catalogs and equipment
selection guides.
1

2. Design Criteria for
Anaerobic Digester
Design
1. Select anaerobic sludge digestion for stabilization of organic solids.
2. Provide two completely-mixed, high-rate anaerobic heated digesters
with digestion temperature of 35°C.
3. The design flow to the sludge digester shall be equal to thickened
sludge under the daily design flow condition.
4. Total volatile solids loading to the digester shall not exceed 3.6
kg/m3·day under extreme high loading condition.
5. The solids retention time at extreme high-flow condition shall not be
less than 10 days.
6. The digester mixing shall be achieved by internal gas mixing.
7. The solids content in digested sludge is 5% and S.P. is 1.03.
8. The TSS content in the supernatant is 4,000 mg/L.
9. The ratio TVS/TS = 0.71, Y = 0.05 g VSS produced/g BOD5 utilized,
E = 0.8, and kd = 0.03 1/day.
2

3. Design Criteria for Anaerobic
Digester Design - continued
10. The digester heating shall be achieved by recirculation of sludge through
external heat exchanger. The sludge recirculation system shall also be
designed to provide digester mixing.
11. Provide floating digester cover for gas collection.
12. The heat loss from the digester cover, side walls, and floor shall be
calculated using the standard heat transfer coefficients for the digester
construction material.
13. Provide gas-fired hot water boiler for external heat exchanger.
14. Explosion prevention devices shall be provided to minimize the possibility
of an explosive mixture being developed inside the floating covers. Proper
flame traps shall be provided to assure protection against the passage of
flame into the digester, gas storage sphere, and supply lines.
15. The digester design shall include supernatant withdrawal system, sight
glass, sampler, manhole, etc.
16. Arrangement shall be provided to break the scum that may form on the
sludge surface.
3

4. Characteristics of Sludge
Reaching Anaerobic
DigesterAverage Extreme Extreme
Factors flow low flow high flow
Sludge production, kg/day 8,180 6,952a 8,681b
Solids concentration, % dry wt 6 8 4
Specific gravity 1.03 1.04 1.02
Average daily flow rate, m3/day 132 84 213c
Pumping rate into each digester 0.85d 0.85 0.85
during the pumping cycle
Influent temperature, °C 21 30 12
Volatile solids fraction before digestion 0.71 0.71 0.71
a
Extreme low solids to the digester = 85% of the average solids loading
b
Extreme high solids to the digester = quantity of thickened sludge withdrawn
under sustained loading = 10,213 kg/day (p. 664 Step A.3) × 0.85 (solids
capture) = 8,681 kg/day
c
8,681 kg/day×103 g/kg÷(0.04 g/g×1.02×1 g/cm3×103 L/m3) = 213 m3/day
d
The pumping rate of 0.85 m3/min gives a velocity of 0.8 m/sec in the 15-cm
diameter pipe.
4

5. Design Example
A. Digester Capacity and Dimensions
1. Compute digester capacity at average flow condition using 15
days digestion period
Assume average flow to the digester = 132 m3/day
Digester volume = 132 m3/day × 15 days = 1,980 m3
2. Compute digester capacity using volatile solids-loading factor
Assume VS loading at ave. flow condition = 2.5 kg/m3·day
Total VS reaching the digester = 8,180 × 0.71 = 5,808 kg/day
Digester volume = 5,808 kg/day × 2.5 kg/ m3·days = 2,323 m3
3. Compute digester capacity using volume per capita allowance
Assume 0.03 m3 digester capacity per capita
Population served = 80,000
Digester capacity = 80,000 × 0.03 m3 = 2,400 m3
4. Compute digester capacity using volume reduction method
Volume of the digested sludge = 97 m3/day (Table 13.13)
Volume of raw sludge to the digester = 132 m3/day
Digester capacity = [132 - 2/3 (132 – 97)] × 15 = 1,630 m3
5

6. Design Example - continued
5. Select digester capacity
Select active digester capacity of 2,500 m3.
B. Digester Dimensions and Geometry
1. Correct for volume displaced by grit and scum accumulations,
and floating cover level
Provide 1-m depth for grit accumulation
Provide 0.6-m depth for scum blanket
Provide 0.6-m min. space between floating cover and max.
digester level
Total displaced height = 1 + 0.6 + 0.6 = 2.2 m
Assume that the active side water depth is 8 m (26.3 ft).
additional volume will be available in the cone.
Volume of each digester = 1,250 m3
6

7. Design Example - continued
Area of each digester = 1,250 m3 ÷ 8 m = 156.3 m2
Diameter of each digester = (4/π × 156.3 m2)0.5 = 14.1 m
Because floating covers come in 1.5-m (5-ft) diameter
increments, provide digesters with 13.7-m (45-ft) diameter.
Revised side water depth = 1,250 m3 ÷ [4/π × (13.7 m)2]
= 8.5 m (27.9 ft)
Provide two digesters each 13.7 m (45 ft) diameter and 8.5 m
(28 ft) side water depth.
2. Check the active vol. of the digesters, including vol. of cone
The floor of the digester is sloped at 1 vertical to 3 horizontal.
The bottom cone depth of 2.3 m adds additional volume.
Active digester volume = (Vol. of active cylindrical portion)
+ (Total vol. of the cone) - (Allowance for grit
accumulation) = π/4 (13.7 m)2 × 7.3 m† + 1/3 × π/4 ×
(13.7 m)2 × 2.3 m - 1/3 × π/4 × (6 m)2 × 1 m
= 1076.1 m3 + 113 m3 - 9.4 m3 = 1,179.7 m3
†
8.5 m – Scum blanket (0.6 m) – Space below floating cover (0.6 m) = 7.3 m 7

8. 8

9. 9

10. Design Example - continued
Active vol. of two digesters = 2 × 1,179.7 m3 = 2,359.4 m3
Total vol. of two digesters = 2 × (π/4 × 13.7 m2 × 8.5 m +
113 m3) = 2,732 m3
Active vol. ratio including cone = 2,359.4 m3 ÷ 2,732 m3
= 0.86
C. Actual Solids Retention Time and Solids Loading
1. Compute actual digestion period at average, extremely low, and
extremely high flows
Digestion period at average flow = 2,359.4 m3 ÷ 132 m3/day
= 17.9 day
Digestion period at extreme high flow = 2,359.4 m3 ÷ 213 (#4)
m3/day = 11.1 day
Digestion period at extreme low flow = 2,359.4 m3 ÷ 84 (#4)
m3/day = 28.1 day
2. Compute actual solids loading at average, extreme low, and
extreme high conditions
10

11. Design Example - continued
Solids loading at ave. loading condition = 8,180 kg/day × 0.71
VS ÷ 2,359.4 m3 = 2.5 kg VS/m3·day
Solids loading at ave. loading condition = 6,952 kg/day × 0.71
VS ÷ 2,359.4 m3 = 2.1 kg VS/m3·day
Solids loading at ave. loading condition = 8,681 kg/day × 0.71
VS ÷ 2,359.4 m3 = 2.6 kg VS/m3·day
D. Gas Production
1. Calculate gas production
BOD5 in the thickened sludge (Stream 10) = 4,253 kg/d
BODL in sludge = 4,253 kg/d × BOD5/0.68 BODL = 6,254 kg/d
Assume 65% solids are biodegradable and 1 g of biodegradable
solids = 1.42 g BODL, Y = 0.05, kd = 0.03 1/day, and E = 0.8.
11

12. Design Example - continued
YQ 0 ES0 (10 −3 kg/g)
Px =
1 + k dθc
0.05 ×112 m 3 /day × 0.8 × 6,254 g/m 3 × (10 −3 kg/g)
= = 163 kg/day
1 + 0.03 1/day ×17.9 day
V = 0.35 m3/kg × (EQ0S0 × 10-3 kg/g - 1.42 Px) = 0.35 m3/kg
× (0.8 × 132 m3/day × 6,254 g/m3 × 10-3 kg/g – 1.42 ×
163 kg/day = 1,670 m3/day
If methane is 66% in the digester gas,
Digester gas production = 1,840 m3/day ÷ 0.66 = 2,531 m3/day
2. Estimate gas production from other rules of thumb
a. Based on VS loading using VS = 0.75 of TS and gas production
rate of 0.5 m3/kg VS
Gas produced = 8,180 kg/day × 0.71 × 0.5 m3/kg
= 2,904 m3/day
b. Based on VS reduction
12

13. Design Example - continued
Assume average VS reduction of 52% and gas production of
0.9 m3/kg VS reduced
Total VS reduced = 8,180 × 0.71 × 0.52 = 3,020 kg/day
Gas produced = 3,020 kg/day × 0.94 m3/kg = 2,839 m3/day
c. Based on per capita
Total population served = 80,000
Used gas production rate of 0.032 m3/capita
Gas produced = 80,000 persons × 0.032 m3/person·day
= 2,560 m3/day
Based on the above analysis, assume a conservative gas
production rate of 2,550 m3/day at standard conditions (0°C
and 1 atm).
E. Digested Sludge Production
1. Compute the quantity of solids in digested sludge
TVS = 8,180 kg/day × 0.71 = 5,807 kg/day
TVS destroyed = 5,807 kg/day × 0.52 = 3,020 kg/day
13

14. Design Example - continued
TS remaining after digestion = Nonvolatile solids + VS
remaining = (8,180 - 5,807) kg/day + 0.48 × 5,807 kg/day
= 5,159 kg/day
2. Compute total mass reaching the digester
Total solids reaching the digester = 8,180 kg/day
Total solids in thickened sludge = 6% by wt
Total mass reaching digester = 8,180 kg/day ÷ 0.06 kg/kg
= 136,317 kg/day
3. Compute volume and TSS in digested sludge and the digester
supernatant
Assume that no liquid volume change occurs in the digester
Vol. of influent thickened sludge (Vinf) = Vol. of digested
sludge removed from digester (Vsludge) + Vol. of digester
supernatant (Vsupernatant)
Vsludge = 132 m3/d; Wremaining = 5,139 kg/d
Vsludge = Wsludge/(0.05 g/L × 10-6 kg/mg × 10-3 L/m3
Vsupernatant = Wsupernatant/(4,000 mg/L × 10-6 kg/mg × 10-3 L/m3)
132 m3/d = Wsludge/(0.05 × 1,030) + Wsupernatant/(0.004 × 1,000)
14

15. Design Example - continued
Wsupernatant = Total solids remaining after digestion in digested
sludge – Wsludge
Wsludge = 5,021 kg/d; Wsupernatant = 138 kg/d; Vsludge = 98 m3/d;
Vsupernatant = 35 m3/d (similar to mass balance)
4. Determine the mass and concentration of the components in digested
sludge and supernatant
Parameter Digested sludge, kg/d (Stream 12) Supernatant (Stream 13)
kg/d mg/L
Flow, m3/d 97 (98a) a 35 (35a)a -
TSS 5,008 (5,021 ) 140 (138 ) 4,000 (3,942)
BOD5 1,596 105 3,000
Org.-N 320 19 533
NH4+-N 44 16 453
NO3—N 0 9 0
TN 364 35 986
NPP 67 7.4 211
PP 126 - -
TP 193 7.4 211
TVSS/TSS ratio 0.54
Biodeg. solids/TSS 0.33
Org.-N/TVSS 0.12
NPP/TVSS 0.025
a
computed 15

16. Design Example - continued
5. Select a supernatant selector system
To withdraw liquid from the top.
a. Allow direct visual inspection of sludge
b. Allow removal of clear liquid from the top
c. Permit operation by one person
d. Be extremely reliable
e. Minimize the danger of allowing air
to enter the digester
f. Be easy to serve in
case of blockage
grease, scum or
by sludge
16

17. Design Example - continued
F. Influent Sludge Line to the Digester
Intermittent pump operation at 0.85 m3/min for each
thickener controlled by a timer.
15-cm (6-in) diameter
17

18. Design Example - continued
G. Digester Heating Requirements
1. Compute heating required for raw sludge
HR = Q0 × Cp (T2 – T1)
where HR = heat required, J/day; Cp = specific heat of sludge
(same as for water = 4,200 J/kg·°C or 1 BTU/lb·°C); T2 =
digestion temperature, °C; and T1 = temperature of the
thickened sludge, °C.
The critical heat requirement for raw sludge is reached when
sludge flow is maximum and influent temperature is lowest:
Heat req. = 8,681 kg/day × 4,200 J/kg ·°C × (35 – 12)°C ÷
0.035 kg/kg = 2.39 × 1010 J/day
2. Compute heat loss from the digester
HL = UA × (T2 – T1)
where HL = heat loss, J/hr; U = overall coefficient of heat
transfer, J/sec·m2 ·°C (BTU/hr·ft2·°F); A = area through which
heat loss occurs, m2 (ft2); T2 = digester operating temperature,
°C (°F); and T1 = outside air temperature, °C (°F)†.
Critical average air and ground temperatures are 0 and 5°C, respectively.
†
18

19. Design Example - continued
Heat losses from the digester occur from the roof, bottom, and
side walls
a. Compute area of roof
Roof slope = 15:1 = (13.7/2) m:0.46 m
Roof area = πD(slant length/2)
2
D
Slant height =   + (Vertical rise of cover)2
2
2
 13.7 m 
=   + (0.46 m) = 6.87 m
2
 2 
Roof area = (π × 13.7 m × 6.87 m)÷2 = 147.9 m2
b. Compute area of side walls
Area of side wall above ground level = πD × Exposed height
Assume 50% side wall is exposed
Side wall area above ground = π×13.7 m×8.5 m/2=182.9 m2
19

20. Design Example - continued
Area of side wall below ground = 182.9 m2
c. Computed bottom area
Digester bottom is sloped at 1 vertical to 3 horizontal.
Total drop of the bottom slope at the center = D ÷ (2 × 3)
= 13.7 m ÷ (2 × 3) = 2.3 m
Bottom area = π × 13.7 m × ½ × √(13.7 m/2)2 + (2.3 m)2
= 155.5 m2
d. Select overall coefficients of heat transfer for different areas
Digester floating covers and roofing consist of 6.5-mm (1/4-
in.) plate steel, 76-mm (3-in.) rigid foam insulation*, inside air
space, and buildup roofing - 1,236 kg/m2 (70 lb/ft2) – U† = 0.9
J/sec·m2 ·°C (BTU/hr·ft2·°F)
*
Common insulating materials are glass wool, insulation
board, urethane foam, lightweight insulating concrete, dead
air space, etc.
†
J/sec·m2 ·°C × 0.1763 = BTU/hr·ft2·°F
20

21. Design Example - continued
Exposed digester side 300-mm (12-in.) concrete, 76-mm (3-
in.) urethane foam insulation, 100-mm (4-in.) brick siding – U
= 0.68 J/sec·m2 ·°C
Buried digester side 300-mm (12-in.) concrete surrounded by
moist soil – U = 0.8 J/sec·m2 ·°C
Digester bottom surrounded by moist soil – U = 0.62 J/sec·m2
·°C
e. Computed heat loss from the digester
Heat loss from the cover and roofing = 147.9 m2 × 0.9
J/sec·m2 ·°C × (35 – 0)°C × 86,400 sec/day
= 4.03 × 108 J/day
Heat loss from exposed wall = 182.9 m2 × 0.68 J/sec·m2 ·°C
× (35 – 0)°C × 86,400 sec/day
= 3.76 × 108 J/day
Heat loss from buried wall = 182.9 m2 × 0. 8 J/sec·m2 ·°C
× (35 – 0)°C × 86,400 sec/day
= 4.43 × 108 J/day
21

22. Design Example - continued
Heat loss from bottom = 155.5 m2 × 0.62 J/sec·m2 ·°C
× (35 – 5)°C × 86,400 sec/day
= 2.50 × 108 J/day
Total heat loss from each digester = 14.72 × 108 J/day
Total heat loss from both digesters, including 20% minor
losses, and 25% emergency condition = 14.72× 108 J/day
× 2 × 1.45 = 5.09 × 109 J/day
f. Compute the heating requirements for the digester
Heat requirements for raw sludge
under critical condition = 2.39 × 1010 J/day
Heat loss from the digester = 42.69 × 108 J/day
Total heating requirement = 2.82 × 1010 J/day
= 1.175 × 109 J/hr
= 1.175 × 106 kJ/hr
22

23. Design Example -
continued
H. Selection of Heating Units and Energy Balance
1. Select external heat exchanger
Provide two heating units each rated as 1.25 × 106 kJ/hr
(1.19 × 106 BTU/hr) with natural gas. The digester gas has
approx. 65% of the heating value of the natural gas (37,300
kJ/m3). Therefore, each unit will be derated at 0.813 × 106
kJ/hr (0.77 × 106 BTU/hr). Total heat provided by two units
= 2 × 0.813 × 106 = 1.626 × 106 kJ/hr.
% extra capacity (1.626 ×106 − 1.175 ×106 ) ×100
= = 38%
available 1.175 ×10 6
The actual average heat requirements are substantially less.
2. Compute digester gas requirements
At 75% efficiency of heating units
Digester gas 1.626 ×106 kJ/hr
= = 89.22 m 3 /hr = 2,141 m 3 /day
needed 0.75 × 0.65 × 37,300 kJ/m 3
23

24. Design Example - continued
Total quantity of digester gas produced = 2,550 m3/day
This gives approx. 20% excess gas under the most critical
condition when the digester heating demand is greatest.
Excess gas will be used to produce heated water for other
plant uses.
3. Design makeup heat exchangers for external sludge heating
a. Compute average temperature rise of the sludge through the
external exchangers
Provide 23-cm (9-in) diameter sludge recirculation pipe, and
a constant flow recirculation pump for each digester. A
common external jacketed type heat exchanger will be used
to heat the recirculated sludge. If velocity of 1 m/sec is
maintained in the pipe.
Sludge pumping rate π
= × (0.23 m) 2 × 1 m/sec × 86,400 sec/day = 3,590 m 3 /day
from each digester 4
= 3,590 m 3 /day × 1.02 × 1,000 kg/m 3 = 3.662 × 10 6 kg/day
24

25. Design Example - continued
Average sludge temperature entering the external heat
exchanger = 35°C
Assume average sludge temperature increase after passing
through the heat exchanger = ∆T°C
Assume specific heat of sludge is 4,200 J/kg°C (same as for
water)
Total heat supplied J
= 4,200 × Δ T °C × 3.662 × 10 6 kg/day
to the sludge kg ⋅ °C
= 1.538 × 1010 × Δ T J/day
Total heat required from each digester = 2.82 × 1010 J/d (#22)
= 1.41 × 1010 J/d.
If the efficiency of the heat exchanger is 80%.
1.538 × 1010 × ∆T J/day × 0.8 = 1.41 × 1010 J/day
1.41 × 1010 J/day
ΔT = 10
= 1.15°C
1.538 × 10 J/day × 0.8
25

26. Design Example - continued
Average temp. of the sludge entering heat exchanger = 35°C
Ave. temp. of the sludge leaving heat exchanger = 36.15°C
Sludge recirculation of 3,590 m3/day (660 gpm) in each digester will
also provide digester mixing.
b. Compute hot water recirculation rate through the external heat
exchanger
Provide one jacketed pipe heat exchanger for both digesters.
Assume that the water enters the jacket pipe at 95°C and leaves at
60°C.
Drop in heating water temperature = 95 - 60 = 35°C
Total heating required for each digester = 1.41 × 1010 J/day
If 25%additional heating is provided to account for heat losses,
Total heat required per digester = 1.41 × 1010 J/day × 1.25
= 1.76 × 1010 J/day
Total heat required for both digesters = 3.52 × 1010 J/day
Total heat available in digester gas = 23,000 kJ/m3 × 1.162 kg/m3
× 2,550 m3/d × 1,000 J/kJ = 6.82
Using specific heat of water = 4,200 J/kg·°C
Heating value of digester gas = 23,000 kJ/m3 26

27. Design Example - continued
Total heat supplied by water = 4,200 J/kg·°C × 35°C
= 147,000 J/kg
Hot water recirculation rate through the common heat
exchanger = 3.52 × 1010 J/day ÷147,000 J/kg
= 2.40 × 105 kg/day
Volume of water recirculated = 2.4 × 105 kg/day × 1,000
g/kg ÷ (1 g/cm3 × 106 cm3/m3) = 240 m3/day
c. Compute the length of sludge pipe in heat exchanger jacket
Average temp. of the sludge in the heat exchanger
= (35 + 36.15)°C ÷ 2 = 35.58°C
Average temp. of the heating water in the heat exchanger
= (95 + 60)°C ÷ 2 = 77.5°C
Assume heat transfer coefficient of external water jacketed
heat exchanger = 4,000 kJ/hr·m2 ·°C (196 BTU/hr·ft2·°F)
Total heat radiated from the heating water
= (77.5 - 35.58)°C × 4,000 kJ/hr·m2 ·°C × 24 hr/day
= 4.02 × 106 kJ/day·m2
27

28. Design Example - continued
Total area of the sludge pipe for each heat exchanger
= 1.76 × 1010 J/day ÷ (4.02 × 106 kJ/day·m2 × 1,000 J/kJ)
= 4.38 m2
Length of 23-cm (9-in) diameter jacketed pipe
= 4.38 m2 ÷ (π × 0.23 m) = 6 m
Provide 6-m long, 23-cm diameter heat exchanger sludge pipe
per digester into a common hot water jacket.
I. Gas Storage and Compressor Requirements
1. Compute the diameter of the gas storage sphere
Provide a total of 3-day gas storage to serve the digester
heating requirements and other plant uses
Total gas stored = 3 day × 2,550 m3/day
= 7,650 m3 (standard condition, 0°C and 1 atm)
Storage pressure = 5.1 atm (assume)
Storage temperature = 50°C summer
Storage volume, V2 = P1V1T2/P2T1
Subscript 1 stands for gas produced and 2 for gas stored.
28

29. Design Example - continued
V2 = 1 atm × 7,650 m3 (273 + 50)K ÷ [5.1 atm × (273 +
0)]K = 1,774.7 m3
Provide a high volume gas storage sphere
Volume of sphere = π/6 (diameter)3
Diameter of sphere = (1,774.7 m3 × 6/ π)1/3 = 15 m (49 ft)
Provide 15 m (49 ft) diameter sphere for gas storage
2. Compute size of high-pressure gas compressors
Compressors are used to compress the digester gas into the
gas storage sphere.
Total weight of digester gas produced under standard
conditions = 2,548 kg/day (Step E.3)
Assuming weight of gas compressed is 200% of production
rate, then
w = 2 × 2,548 kg/day × 1/864,000 day/sec = 0.059 kg/sec
R = 8.314 kJ/kmol ·K
e = compressor efficiency of 75%
T0 = inlet temperature = (273 + 35)°C
29

30. Design Example - continued
wRT0  P  0.283 
Pw =  
P  − 1
8.41⋅ e  0  
 
0.059 × 8.314 × (273 + 35)  5.1  
0.283
=   − 1 = 13.7kW(17.5hp)
8.41 ⋅ 0.75  1.03 
 

30

31. Design Example - continued
The volume of each digester, V = 1179.7 m3
µ = 2 times the viscosity of water at 35°C
= 2 × 0.73 × 10-3 N·sec/m2 = 1.46 × 10-3 N·sec/m2
Velocity gradient for sludge above 5% solids is over 75 1/sec.
Use G = 85 1/sec.
P = 852 × 1,46 × 10-3 N·sec/m2 × 1,179.7
= 12.444 N·m/sec (9,178 ft·lb/sec) = 12.4 kW = 16.6 hp
Total power required for two digesters = 2 × 12.4 kW
= 24.8 kW = 33.2 hp
Provide three compressors each driven by 15-kw (20-hp)
motor.
Total power provided for mixing = 45 kw (60 hp)
Two compressors will deliver the required power, while the
third compressor will be a stand-by, serving both digesters.
2. Compute gas flow
The digester gas flow rate for mixing, w, can be calculated
using the equation in the previous slide.
31

32. Design Example - continued
The volume of each digester, V = 1,179.7 m3
µ = 2 times the viscosity of water at 35°C
= 2 × 0.73 × 10-3 N·sec/m2 = 1.46 × 10-3 N·sec/m2
15 kW × 8.41 kg/kmole × 0.75
w= = 0.14 kg/sec
 2.4  0.283

8.314 kJ/kmole ⋅ K × 308 K   −1
 1.03 
 

Gas flow per digester = 0.14 kg/s ÷ (1.162 kg/m3 × 0.86) = 0.14 m3/s
3. Select digester-mixing arrangement
The digester mixing will be achieved by flow recirculation, raw sludge,
and internal gas mixing.
The sludge recirculation of 3,590 m3/day in each digester was calculated
in step H3. The sludge will be withdrawn from the mid-depth and
discharged above the scum blanket level to assist in scum mixing.
A multi-port mixing system is provided for effective use of the gas. The
gas is withdrawn from the top and recirculated by means of nine ports
for gas injection.
Density of digester gas = 86% of air (1.162 kg/m3) 32

33. Digester Startup
1. Haul approx. 250 m3 seed and transfer it into one digester.
2. Fill the digester with raw wastewater.
3. Start heating and mixing, and bring to operating
temperature.
4. Begin feeding raw sludge at a uniform rate approx. 25% of
daily feed per digester. Increase the loading gradually.
5. Maintain the following records
a. Quantity of TVS fed daily
b. TVS, VS/Alk ratio, and pH
c. Temperature, gas production, and CO2 content in gas
6. At low feeing rate, it is possible to bring normal operation
without adding chemicals for pH control. If VA/Alk ratio
rises to ≥ 0.8, and pH is below 6.5, addition of chemicals
such as lime or soda ash may be considered.
7. Fairly stable conditions should be reached in 30~40 days
if loading is kept below 1 kg VS/m3·day.
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34. Common Operating Problems
1. A rise in VA/Alk ratio (> 0.3), increase in CO2 content,
decrease in pH, rancid or H2S odors are indication of
hydraulic or organic loading, excessive withdrawal of
digested sludge, or incoming toxic materials.
2. Poor supernatant quality may be due to excess mixing,
insufficient settling time before sludge withdrawal, too low
supernatant drawoff point, and insufficient sludge
withdrawal rate.
3. Foam in supernatant may be use to scum blanket breaking
up, excessive gas recirculation, and organic loading.
4. Thin digested sludge may be due to short circuiting,
excessive mixing, or too high sludge-pumping rate.
5. Tilting floating cover may be due to uneven distribution of
load, thick scum accumulation around the edges, rollers or
guide broken or rollers out of adjustment.
6. Binding cover (even when rollers and guides are free) may
be due to damaged internal guides or guy wires.
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35. Operation and Maintenance
Key Operational Goals
• Minimize excess water
• Control organic loading
• Control temperature
• Control mixing
• Reduce accumulation of scum
• Withdraw supernatant that is low in solids
Routine Digester Operation and Maintenance Checklist
• Check digester gas pressure.
• Drain daily the condensate traps.
• Drain daily sediment traps.
• Check daily gas burner for proper flame.
• Record daily floating cover position, check cover guides, and
check for gas leaks.
• Record daily digester and natural gas meter readings.
• Check daily and record fuel oil.
• Chemical daily gas-mixing equipment for flow and gas.
• Check daily pressure relief and vacuum breaker valves.
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