design and development of an anaerobic bio-digester for application in sewage sludge digestion for...
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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
Introduction
• Sewage sludge problems
• Energy Issues
• Environmental issues with sewage
• Biogas production- green solution to sewage management
• Acti-zyme properties
• Development of digester
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
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).
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
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
Mass balance over the bio-digester
Bio-digester design considerations
Components of Bio Digester
Body Construction
Material of construction
Sealing
Safety Valve
Capacity
Bio-digester design considerations cont.….
• Baffles
• Shape of Bio-Digester
• Batch Time
Designed bio-digester depended on the Chitungwiza plant capacity
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)
Bio
gas
com
posi
tion
(%
)
Retention time (days)
Exponential phase
Stationery phase Deceralation phase
Lag phase
Bio-digester with Acti-zyme
generated the most biogas >15%
Bio-digester geometric considerations
Bio-digester fabricated prototype
Bio-digester process control
• Control and monitoring bio-digester system
• Temperature
• Sewage sludge flow rate
• Pressure
• pH
Bio-digester design parameters summary
Parameter 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
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
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
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
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
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
Acknowledgements
• Harare Institute of Technology
• Cape Peninsula University of Technology
• HIT Chemical Engineering Department
Thank You!!!!!!!
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