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!!!!!!!