Design and development of an anaerobic bio-digester for application in sewage sludge digestion for biogas and bio-solids generation using Acti-zyme as bio-catalyst

Anaerobic digestion for biogas production is vital in sewage sludge management. This paper focuses on the design and development of an anaerobic bio-digester that can be utilized for biogas production utilizing Acti-zyme as the digestion bio-catalyst from sewage sludge. A stainless steel lab scale bio-digester with a capacity of 390 L/day and an operating efficiency of 75% was considered. The bio-digester allowed for addition of both the Acti-zyme and sewage sludge, the removal of the biogas and bio-solids generated during the digestion process. Process and equipment safety was ensured by maintaining the temperature at 35°C, pressure at 1 atmosphere as well as ensuring the flow of the substrate was maintained at 75% to minimize as operational hazards. Agitation was maintained at 60 rpm for uniform mixing whilst pH was maintained at 7 for enhancing Acti-zyme activity. The anaerobic bio-digester can either be up scaled or downscaled for application in sewage sludge management using Acti-zyme.

1. Design and development of an anaerobic bio-digester for
application in sewage sludge digestion for biogas and bio-
solids generation using Acti-zyme as bio-catalyst
2
nd
ZIMBABWE RENEWABLE ENERGY CONFERENCE & EXHIBITION
Harare, Zimbabwe, 2015


By

M. M. Manyuchi, R. Marisa, D.I.O Ikhu-Omoregbe and O. O. Oyekola

2. Introduction
•Sewage sludge problems

•Energy Issues

•Environmental issues with sewage

•Biogas production- green solution to sewage management

•Acti-zyme properties

•Development of digester

3. Acti-zyme
•A bio-catalyst that promotes digestion of
sewage to produce biogas
•Exists in pellets forms
•Has been successfully used in sewage
treatment
2
•Hinders production of H
2S and
ammonia gases
2
•Hence improves biogas quality

•Low quantities of bio-solids produced at
the end

4. Sewage properties
Parameter High Medium Low
COD 1200 750 500
BOD 560 350 230
TKN 100 60 30
NH
3 75 45 20
TP 25 15 6
TSS 600 400 250
pH 8.0 7.5 7.0
Alkalinity eqv/m
3
7 4 1
COD/BOD 2.5–3.5 2.0–2.5 1.5–2.0
COD/TP 45–60 35–45 20–35
Typical municipal wastewater characterization adopted from (Metcalf and Eddy Inc., 2003).

5. Materials and methods
•Retention time varied up to 60 days

•Acti-zyme loading: 35-70 g/m
3

•Temperature: 35
o
C

•Conditions: Anaerobic in 190 mL
digesters

•Agitation: 60 rpm
•Lab experiments
•Fabrication of Prototype
•Biogas Generation
•Bio-solids collection
(digestate for use as bio-fertiliser) Sampling valve
Biodigester
Bio-solids outlet
Biogas
collector
Biogas outlet
Biogas quantity measuring jar
Acti-zyme

6. Biogas and bio-solids composition
Gas % (Acti-zyme) % (without Acti-zyme)
CH
4 72-78 53-65
CO
2 16-20 22-27
Traces (H
2S, N
2, H
2) 5-9 8-12
Biogas

Parameter % Composition
Nitrogen
8.17±0.15
Phosphorous
5.84±0.03
Potassium
1.32±0.02
Copper
0.0073±0.0002
Iron
0.0087±0.0003
Calcium
0.0079±0.002
Magnesium
0.016±0.0021
Bio-solids

7. Mass balance over the bio-digester

8. Bio-digester design considerations
Components of Bio Digester

Body Construction

Material of construction

Sealing

Safety Valve

Capacity

9. Bio-digester design considerations
cont.….
•Baffles

•Shape of Bio-Digester

•Batch Time


Designed bio-digester depended on the Chitungwiza plant capacity

10. Batch kinetics in the bio-digester
0
10
20
30
40
50
60
70
80
90
0 10 20 30 40 50 60
CH4 (A1) CH4 (Ao)
Biogas composition (%)

Retention time (days)
Exponential phase
Stationery phase
Deceralation phase
Lag phase
Bio-digester with Acti-zyme
generated the most biogas >15%

11. Bio-digester geometric considerations

12. Bio-digester fabricated prototype

13. Bio-digester process control
•Control and monitoring bio-digester system

•Temperature

•Sewage sludge flow rate

•Pressure

•pH

14. Bio-digester design parameters
summary
Parameter Value
Volume 0.39 m
3
(390 L)
Height 2 m
Diameter 0.5 m
Temperature 35
o
C
Working pressure 1 atmos
Retention time 40 days
Height to diameter ratio 4
Type of head Hemispherical

15. Bio-digester design parameters summary
cont…..
Depth of dished bottom 0.125 m
Wall thickness 0.019 m
Head thickness 0.021 m
No. of baffles 4
Width of Baffle 0.042 m
Height of Baffle 1.5 m
Power per unit volume 1.3
Sewage sludge loading 290 L/day
Acti-zyme loading 0.0195 L/day

16. Bio-digester HAZOP Analysis
Guide word Parameter Deviation Possible cause Consequences Corrective measures
Less Temperature Temperature < 35°C Heating steam inlet valve
fails it is closed

Low digestion rate hence low biogas yield

Inefficient reaction of Acti-zyme

Production of high quantities of bio-solids
Install temperature sensors (thermocouples, thermometers) to
detect changes in temperatures
Repair of the valve
Valves should be checked regularly
More Temperature Temperature > 35°C Heating steam inlet valve
fails, it is fails to close
Excess of temperature have an effect on product quality

Acti-zyme activity are deactivated
Mesophillic conditions deactivated, lower biogas yield
Install temperature sensors such as thermocouple, thermometer
NO Flow No flow of sewage
sludge and Acti-zyme
into the bio-digester
Pipe blockage

Valve fails to open
(control valve failure)
Pump failure
Pipe leakages

Less biogas production

No sewage sludge and Acti-zyme in the bio-digester
Low biogas yield
Install flow alarms

Maintenance (repairing the control valve and pipes)
Check valves regularly

Install sensors that detect detects any changes and gives feedback

17. Bio-digester HAZOP Analysis cont……
Less Flow Low flow of
sewage sludge and
Acti-zyme
Clogging of the
sewage sludge and
Acti-zyme inside the
pipes

Leakage
Low yield of biogas
Acti-zyme does not have substrate to act on
Install flow meters
Alarms

Constant repairs of valves and pipes
More


Pressure Pressure > 1 atmos Build-up of gases
during the digestion
process
Bursting of the bio-digester Install pressure relief valve to purge the excess gases
inside the bio-digester

Less Pressure Pressure < 1 atmos Causes mechanical stress on the bio-digester Install pressure sensors to detect changes in the pressure

18. Bio-digester HAZOP Analysis cont.…
NO Agitation No agitation
inside the bio-
digester
Stirrer motor not
functioning
No mixing
Possible accumulation of sewage
sludge at bottom of bio-digester
Acti-zyme cells tend to clump
together
Rate of digestion is slower
Low retention time
Maintenance of the stirrer motor
More Agitation More agitation
inside the bio-
digester
Stirrer motor
control fails (high
motor speed)
More collision of Acti-zyme and may
result in breakage of the cells
Repair of the stirrer motor

19. Conclusion
•The designed bio-digester can be applied in sewage sludge management
•The bio-digester can be used for biogas generation from any organic waste

20. Acknowledgements

•Harare Institute of Technology

•Cape Peninsula University of Technology

•HIT Chemical Engineering Department

21. Thank You!!!!!!!