Anaerobic Sludge Digestion
 
Introduction Secondary biological treatment of ww produces two sidestreams:
1) effluent - what was once soluble organic material

2) solids

a) primary solids - from primary clarifier; organic in nature; Å 35% BOD influent removed here

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

1. Concentration

2. Stabilization - anaerobic digestion

3. Conditioning

4. Dewatering

5. Heat drying or combustion

Traditional sludge treatment = anaerobic digestion

- breaks down odorous components

- 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)

- heat digester

- cogeneration facility

Advantages of anaerobic digestion

- less biomass production than any other sludge treatment

- requires less nutrients, N & P

- produces CH4

Disadvantages - relatively high cost

- 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

Two types of digesters in use (in US)

1. Standard rate

no mixing

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

2 digesters in series; separates functions of fermentation and solids/liquids separation

1st stage

complete mix and heated
td = 10 -15 days
organic loading = 0.1 - 0.4 lb VS/ft3/d
fixed cover
2nd stage solids-liquids separation
gas extraction
unheated
floating covers
 
Process Microbiology Three-step process

1. Hydrolysis - high molecular wt. compounds converted to lower molecular wt. ones suitable for cellular use

lipids ---> fatty acids

polysaccharides ---> monosaccharides

protein ---> amino acids

nucleic acids ---> purines, pyrimidines

2. Acidogenesis organic matter + bugs + combined oxygen ----> organic acids + reduced compounds + bugs
  organic matter = fatty acids, monosaccharides, amino acids, etc.
bugs = anaerobic, facultative
combined oxygen = NO3-, SO4-2, CO3-2
organic acids = acetic, propionic, butyric, etc.
reduced compounds = NH3, H2S, CO2

C6H12O6 ------> 3CH3COOH

3CH3COOH + 3NH4HCO3 ----> 3CH3COONH4 + 3H2O + 3CO2

3. Methanogenesis

organic acids + bugs + H2 + CO2 ----> CH4 + H20 + bugs

3CH3COONH4 + 3H2O ----> 3CH4 + 3NH4HCO3

Notice utilization of buffer in eqn in #2 and reformation in #3

- if not enough buffer present, pH drops ---> "sour digester"

- methanogens very sensitive here

opt. pH = 6.8 - 7.2

operating = 6.5 - 7.8 (see handout)

- methanogens are rate limiters in overall digestion process due to their slow growth

\ Must maintain bicarb alkalinity Å 1000 mg/l as CaCO3

  Bicarb Alk = (Total Alk - 0.8 Volatile Acids) 0.8 for conversion of organic acids to CaCO3 Volatile Acids < 0.5 for good operation
Total Alk

 

Kinetics Similar to complete mix WAS without solids recycle:

SRT and temp key to design

1st-stage high rate design based on:

(2 - 10) x qcm (safety factor)

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

 

Gas Production and Heat Requirements Digester Heat Loss HL = UA(T2 - T1)

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

 
Digester Component
Overall heat transfer coeff. (BTU/h-ft2-ûF)
Concrete roof
0.5
Floating cover
0.24
Concrete wall air space
0.35
Concrete wall in wet earth
0.25
Concrete wall in dry earth
0.18
Floor
0.12
  Typical heat losses: Northern US - 2600 BTU/h-1000 ft3

Southern US - 1300 BTU/h-1000 ft3

 

Sludge Heat Requirements HR = WC (T2 - T1)

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:

Southern US 50 F

Central US 45 F

Northern US 40 F

Gas Production G = Go[DS - 1.42DX]

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

or Go = 0.0206 T2 (T2 = fermentation temp., K)

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
 

Sludge Characteristics Sludge volume:

Vs = lb dry solids/Ssgwfs

Vs = sludge produced, ft3/d
S = sp. gr. sludge
f = wt. fraction sludge solids
gw = sp. wt. water
 
Sw = sp. gr. water = 1.0
Sf = sp. gr. fixed solids = 2.5
Sv = sp. gr. volatile solids = 1.0
fw, ff, fv = wt. fract. water, fixed solids, volatile solids
to find weight fraction of solids that are volatile after digestions from % VS destroyed: Standard Rate Vd = 30 + t/2
Vd = VS destroyed during digestion, %
t = time of digestion, d  
First Stage, High Rate Vd =13.7 ln qc + 18.94  
Anaerobic Digestion Design   Design a high-rate anaerobic digestion system to process the sludge from a 10-MGD complete mix WAS plant with a biomass production of 4879 lb/d. Assume following conditions: 1. After grit removal, total SS are 235 mg/l (which are 65% volatile) and ultimate BOD of 476 mg/l.
2. The surface loading rate for primary clarifier is 600 gpd/ft2.
3. Sludge concentrates to 5% solids in primary clarifier which produces a 60% reduction of total SS and 40% BOD reduction.
4. WAS is 70% volatile fraction and thickened to 3% solids by DAF prior to mixing with primary sludge.
5. First-stage digester temp. = 35 C.
6. Use safety factor of 3 for qcm.
7. Due to winter, provide storage volume in second-stage digester of 100 days.
8. Power level in first-stage digester = 150 hp/MG.
9. Critical design temp. = 10 F. Digester sidewalls are exposed to air. Digester floor is in dry earth with a winter temp. of 20 F.
10. Digester depth is 25 ft.
11. Digester sludge concentrates to 7% in second-stage.
Solution:

1. Compute daily vol. of sludge removed in primary clarifier.

Total suspended solids removal:

 

Specific gravity of sludge:

 

Daily volume of primary sludge:

 

 

2. Estimate BODu of primary sludge. Daily BODu reomoval:

 

BODu in primary sludge:

 

3. Compute daily vol of WAS Total solids produced each day:

 

Specific gravity of sludge:

 

Daily volume of sludge:

 

4. Estimate BODu of WAS
 
 

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.

Concrete cover:

 
 

Sidewall:

 

 

Floor:
 
 
 
 

20. Calc. heat reqd to raise feed sludge temp to 95û F - assume feed sludge temp = 50 F. Total mass of sludge flow to 1st-stage digester:

 

Heat necessary to raise sludge temp.

 

 

21. Estimate vol of sludge in 2nd-stage digester Volatile solids destroyed during digestion:

 

 

Calc % of volatile matter after digestion:

 

 

Calc sp. gr. of sludge assuming 7% solids:

 

 

 

Calc. vol. of digested sludge

 

 

 

23. Compute vol. of 2nd-stage digester.

 

 

24. Determine x-sect. and dia. of 2nd-stage digester.

 

 

25. Determine horsepower reqd for given power level.