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
2nd 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
treatment2
• Hinders production of H2S and
ammonia gases2
• 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
NH3 75 45 20
TP 25 15 6
TSS 600 400 250
pH 8.0 7.5 7.0
Alkalinity eqv/m3 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/m3
• Temperature: 35 oC
• 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)
CH4 72-78 53-65
CO2 16-20 22-27
Traces (H2S, N2, H2) 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)
Biogascomposition(%)
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
summaryParameter Value
Volume 0.39 m3 (390 L)
Height 2 m
Diameter 0.5 m
Temperature 35 oC
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!!!!!!!