2) solids
b) biosolids from WAS - 30-40% of BOD ---> biomass
so if So = 200 mg/l BOD5, 90% removal in system, 52-57% of So is removed in form of sludge
\ Sludge treatment accounts for 3 50% of capital & operationg costs of wwtps
Five processes for sludge treatment
2. Stabilization - anaerobic digestion
3. Conditioning
4. Dewatering
5. Heat drying or combustion
Traditional sludge treatment = anaerobic digestion
- reduces volatile content 50-60%
- compaction & concentration of solids
- sludge becomes more granular
- coliform reduction 98-99% - but not disinfected
- byproduct formation of burnable gas (methane)
- cogeneration facility
Advantages of anaerobic digestion
- requires less nutrients, N & P
- produces CH4
- hard to dewater sludge from digester (by mechanical means)
- subject to upsets "sour digester"
- poor quality side streams (supernatant high in organics, nutrients)
- slow methane production
Process Description
1. Standard rate
sludge allowed to stratify
batch process
usually unheated
td = 30-60 days
loading = 0.03 - 0.1 lb VS/ft3/d
1/3 tank for digestion
other 2/3 = scum layer, stabilized solids, supernatant
\ large tanks; usually for wwtps 2 1 MGD
2. High rate
1st stage
1. Hydrolysis - high molecular wt. compounds converted to lower molecular wt. ones suitable for cellular use
polysaccharides ---> monosaccharides
protein ---> amino acids
nucleic acids ---> purines, pyrimidines
C6H12O6 ------> 3CH3COOH
3CH3COOH + 3NH4HCO3 ----> 3CH3COONH4 + 3H2O + 3CO2
3. Methanogenesis
3CH3COONH4 + 3H2O ----> 3CH4 + 3NH4HCO3
Notice utilization of buffer in eqn in #2 and reformation in #3
- methanogens very sensitive here
operating = 6.5 - 7.8 (see handout)
\ Must maintain bicarb alkalinity Å 1000 mg/l as CaCO3
SRT and temp key to design
1st-stage high rate design based on:
YT = 0.04
kd = 0.015/d
Look at kinetics based on methane formation being the rate limiting step:
Kc = S Ks for all fatty acids
applicable from 20-35° C, complex waste with high lipid content
low lipid content - use values in Table 4-14, handout
HL = heat loss, BTU/hr
U = overall coeff. heat transfer, BTU/h-ft2-°F
A = area normal to heat flow, ft2
T2 = fermentation temp., °F
T1 = critical winter temp., °F
Overall Heat Transfer Coefficients for Anaerobic Digesters
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Concrete roof |
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Floating cover |
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Concrete wall air space |
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Concrete wall in wet earth |
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Concrete wall in dry earth |
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Floor |
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Southern US - 1300 BTU/h-1000 ft3
HR = heat necessary to bring raw sludge to ferm temp., BTU/d
W = ave mass flow of sludge to digester, lb/d
C = mean specific heat of sludge = 1 BTU/lb-°F
T2 = fermentation temp., °F
T1 = critical winter temp., °F
If no other info on sludge feed temps:
Central US 45° F
Northern US 40° F
G = total methane produced, ft3/d
Go = ft3 of CH4/lb degradable COD or BODu oxidized, ft3/lb
DS = degradable COD or BODu removed, lb/d
DX = biomass produced, lb/d
T in °K
CH4 + 2CO2 ---> CO2 + 2H2O
1 mole CH4 --> 64 g COD
Realize that total gas production = G/0.67 (methane only 2/3 of total)
Liptak: total gas production 15 ft3/lb VSS destroyed; BTU value of gas = 640-703 BTU/ft3
Also 1 ft3 CH4 ---> 960 BTU
net heating value
Vs = lb dry solids/Ssgwfs
1. Compute daily vol. of sludge removed in primary clarifier.
Specific gravity of sludge:
Daily volume of primary sludge:
BODu in primary sludge:
Specific gravity of sludge:
Daily volume of sludge:
5. Calc sludge flow in MGD for primary and WAS
6. Estimate & VS of mixed sludge stream
7. Compute BODu of mixed sludge stream
8. Correct K for temp.
9. Correct Kc for temp.
10. Compute qcm.
11. Calc. design qc using safety factor.
12. Determine 1st-stage digester volume.
13. Determine x-sect. and diameter of 1st-stage digester.
14. Calculate 1st-stage digester effluent.
15. Compute biomass concentration in 1st-stage digester.
16. Calc. cf of methane per lb of BODu removed.
17. Determine total methane production.
18. Compute total available heat content of digester gas.
19. Determine digester heat losses.
Sidewall:
Floor:
Heat necessary to raise sludge temp.
Calc % of volatile matter after digestion:
Calc sp. gr. of sludge assuming 7% solids:
Calc. vol. of digested sludge
24. Determine x-sect. and dia. of 2nd-stage digester.
25. Determine horsepower req’d for given power level.