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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.

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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.

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Design Criteria forAnaerobic Digester

Design1. 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.

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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.

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Characteristics of Sludge Reaching Anaerobic

Digester Average Extreme Extreme Factors flow low flow high

flow

Sludge production, kg/day 8,180 6,952a 8,681b

Solids concentration, % dry wt 6 8 4Specific gravity 1.03 1.04 1.02Average daily flow rate, m3/day 132 84 213c

Pumping rate into each digester 0.85d 0.85 0.85 during the pumping cycleInfluent temperature, °C 21 30 12Volatile solids fraction before digestion 0.71 0.71 0.71 aExtreme low solids to the digester = 85% of the average solids loadingbExtreme 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

c8,681 kg/day×103 g/kg(0.04 g/g×1.02×1 g/cm3×103 L/m3) = 213 m3/daydThe pumping rate of 0.85 m3/min gives a velocity of 0.8 m/sec in the 15-cm diameter pipe.

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Design ExampleA. Digester Capacity and Dimensions1. Compute digester capacity at average flow condition using 15

days digestion periodAssume 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 factorAssume 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 allowanceAssume 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 methodVolume 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

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5. Select digester capacitySelect active digester capacity of 2,500 m3.

B. Digester Dimensions and Geometry1. Correct for volume displaced by grit and scum accumulations,

and floating cover levelProvide 1-m depth for grit accumulationProvide 0.6-m depth for scum blanketProvide 0.6-m min. space between floating cover and max. digester level Total displaced height = 1 + 0.6 + 0.6 = 2.2 mAssume 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

Design Example - continued

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Area of each digester = 1,250 m3 8 m = 156.3 m2

Diameter of each digester = (4/ × 156.3 m2)0.5 = 14.1 mBecause 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/ × (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 coneThe 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) = /4 (13.7 m)2 × 7.3 m† + 1/3 × /4 ×(13.7 m)2 × 2.3 m - 1/3 × /4 × (6 m)2 × 1 m

= 1076.1 m3 + 113 m3 - 9.4 m3 = 1,179.7 m3

Design Example - continued

†8.5 m – Scum blanket (0.6 m) – Space below floating cover (0.6 m) = 7.3 m

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Active vol. of two digesters = 2 × 1,179.7 m3 = 2,359.4 m3

Total vol. of two digesters = 2 × (/4 × 13.7 m2 × 8.5 m +113 m3) = 2,732 m3

Active vol. ratio including cone = 2,359.4 m3 2,732 m3

= 0.86C. Actual Solids Retention Time and Solids Loading1. 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 day2. Compute actual solids loading at average, extreme low, and

extreme high conditions

Design Example - continued

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Solids loading at ave. loading condition = 8,180 kg/day × 0.71VS 2,359.4 m3 = 2.5 kg VS/m3·day

Solids loading at ave. loading condition = 6,952 kg/day × 0.71VS 2,359.4 m3 = 2.1 kg VS/m3·day

Solids loading at ave. loading condition = 8,681 kg/day × 0.71VS 2,359.4 m3 = 2.6 kg VS/m3·day

D. Gas Production1. Calculate gas production

BOD5 in the thickened sludge (Stream 10) = 4,253 kg/dBODL in sludge = 4,253 kg/d × BOD5/0.68 BODL = 6,254 kg/dAssume 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.

Design Example - continued

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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 thumba. 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

Design Example - continued

kg/day 163day 9.711/day 0.031

kg/g) (10g/m 254,60.8/daym 1120.05

θk1

kg/g) (10ESYQP

333

cd

300

x

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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 = 80,000Used gas production rate of 0.032 m3/capita Gas produced = 80,000 persons × 0.032 m3/person·day = 2,560 m3/dayBased 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 Production1. 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

Design Example - continued

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Design Example - continued

TS remaining after digestion = Nonvolatile solids + VSremaining = (8,180 - 5,807) kg/day + 0.48 × 5,807 kg/day

= 5,159 kg/day2. 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/day3. Compute volume and TSS in digested sludge and the digester

supernatantAssume 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 digestersupernatant (Vsupernatant)

Vsludge = 132 m3/d; Wremaining = 5,139 kg/dVsludge = 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)

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Design Example - continued

Wsupernatant = Total solids remaining after digestion in digested sludge – WsludgeWsludge = 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/LFlow, m3/dTSSBOD5Org.-NNH4

+-NNO3

—NTNNPPPPTPTVSS/TSS ratioBiodeg. solids/TSSOrg.-N/TVSSNPP/TVSS

97 (98a)5,008 (5,021a)

1,596320440

364671261930.540.330.120.025

35 (35a)140 (138a)

10519169

357.4-

7.4

-4,000 (3,942)

3,0005334530

986211

-211

a computed

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Design Example - continued

5. Select a supernatant selector systemTo withdraw liquid from the top.a. Allow direct visual inspection of sludgeb. Allow removal of clear liquid from the topc. Permit operation by one persond. Be extremely reliablee. Minimize the danger of allowing air to enter the digesterf. Be easy to serve in case of blockage grease, scum or by sludge

16

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Design Example - continued

F. Influent Sludge Line to the DigesterIntermittent pump operation at 0.85 m3/min for each thickener controlled by a timer.

15-cm (6-in) diameter

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Design Example - continued

G. Digester Heating Requirements1. 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/day2. 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.

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Design Example - continued

Heat losses from the digester occur from the roof, bottom, and side walls

a. Compute area of roofRoof slope = 15:1 = (13.7/2) m:0.46 mRoof area = D(slant length/2)

Roof area = ( × 13.7 m × 6.87 m)2 = 147.9 m2

b. Compute area of side wallsArea of side wall above ground level = D × Exposed heightAssume 50% side wall is exposedSide wall area above ground = ×13.7 m×8.5 m/2=182.9 m2

m 6.87m) (0.462

m 13.7

cover) of rise (Vertical2

DheightSlant

22

22

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Design Example - continued

Area of side wall below ground = 182.9 m2

c. Computed bottom areaDigester 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 = × 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 areasDigester 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

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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 ·°CBuried digester side 300-mm (12-in.) concrete surrounded by moist soil – U = 0.8 J/sec·m2 ·°CDigester 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

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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/dayf. 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

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Design Example - continued

H. Selection of Heating Units and Energy Balance1. 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.

The actual average heat requirements are substantially less.2. Compute digester gas requirements

At 75% efficiency of heating units

38%10 1.175

100)101.17510 (1.626

available

capacity extra %6

66

/daym 2,141/hrm 89.22kJ/m 300,3765.0 0.75

kJ/hr 10 1.626

needed

gasDigester 333

6

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Design Example - continued

Total quantity of digester gas produced = 2,550 m3/dayThis 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 heatinga. Compute average temperature rise of the sludge through the

external exchangersProvide 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.

kg/day 103.662kg/m 1,000 1.02/day m 3,590

/daym 3,590sec/day 86,400m/sec 1m) (0.234

πdigestereach from

rate pumping Sludge

63

32

3

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Design Example - continued

Average sludge temperature entering the external heat exchanger = 35°CAssume average sludge temperature increase after passing through the heat exchanger = T°CAssume specific heat of sludge is 4,200 J/kg°C (same as for water)

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

J/day TΔ 101.538

kg/day103.662C TΔCkg

J4,200

sludge thetosuppliedheat Total

10

6

C1.150.8J/day 101.538

J/day 101.41ΔT

10

10

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Design Example - continued

Average temp. of the sludge entering heat exchanger = 35°CAve. temp. of the sludge leaving heat exchanger = 36.15°CSludge 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 exchangerProvide 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/dayIf 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

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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/dayc. 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

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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 ( × 0.23 m) = 6 mProvide 6-m long, 23-cm diameter heat exchanger sludge pipe per digester into a common hot water jacket.

I. Gas Storage and Compressor Requirements1. 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/P2T1Subscript 1 stands for gas produced and 2 for gas stored.

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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 = /6 (diameter)3 Diameter of sphere = (1,774.7 m3 × 6/ )1/3 = 15 m (49 ft)Provide 15 m (49 ft) diameter sphere for gas storage

2. Compute size of high-pressure gas compressorsCompressors 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

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Design Example - continued

P0 = 1.03 atm (gas pressure inside the floating cover is normally less than 0.4 m of water)

P = 5.1 atm

Provide two constant-speed compressors each driven by 7.5 kW (10-hp) electric motor.

J. Digester Gas Mixing1. Compute power requirements for gas mixing

Power requirements for gas mixing of the digesterP = G2µV

where G = velocity gradient, 1/sec.

5hp)13.7kW(17.11.03

5.1

0.758.41

35)(2738.3140.059

1P

P

e8.41

wRTP

0.283

0.283

0

0w

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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 hpProvide 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 flowThe digester gas flow rate for mixing, w, can be calculated using the equation in the previous slide.

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32

kg/sec 0.14

11.032.4

K 308KkJ/kmole 8.314

0.75kg/kmole 8.41kW 15w

0.283

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

Gas flow per digester = 0.14 kg/s (1.162 kg/m3 × 0.86) = 0.14 m3/s3. 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)

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Digester Startup1. 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 dailyb. TVS, VS/Alk ratio, and pHc. 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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Common Operating Problems1. 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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Operation and MaintenanceKey Operational Goals• Minimize excess water• Control organic loading• Control temperature• Control mixing• Reduce accumulation of scum• Withdraw supernatant that is low in solidsRoutine 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.