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Reduction of pathogenic bacteria in organic waste
through production of biogas
Lauren Harroffa, Vianney Tumwesigeb, Andrew Apsleyb, Lisa Averyc, Norval Strachanb, Jo Smithb
a Makerere University; b University of Aberdeen;
cJames Hutton Institute
a b c
Lauren Haroff,
Makerere University
Reduction of pathogenic bacteria in organic waste
through production of biogas
Lauren Harroffa, Vianney Tumwesigeb, Andrew Apsleyb, Lisa Averyc, Jo Smithb
a Makerere University; b University of Aberdeen;
cJames Hutton Institute
a b c
As few as 15% of household in SSA
have access to improved water
supply (WHO, UNIEF,
2009)
In 2004, only 37% of the population of SSA had access to sanitation (WHO,
UNIEF, 2006)
~5M people die from water related disease each year Momba et
al., 2008)
Widely accepted that small sale
biogas digesters can reduce pathogen
burden (eg. Clarsen et al., 2007)
Is this true in Sub-Saharan
Africa?
9 biogas digesters installed in Tiribogo, Uganda
Approach
Measure faecal coliforms and indicator organisms (Escherichia coli, total aerobic coliforms, Clostridium perfringens and Enterococcus faecalis)
• Samples refrigerated overnight
• Serial dilutions with buffered peptone water
• Plated on Chromocult and Chromoselect agar
• Incubated ~24 hours at 37° C and 44° C
• Colonies counted
Approach
Measure pathogens in feed and digestate
Feed
Digestate
Around animals
Inside home
Approach Sample in local environment
before and after installation
On hands
Around home
Door handles
Dishes
Pathogens in Digester
0
2
4
6
8
10
12
E. coli Total coliforms C. perfringens E. faecalis
Log
CFU/
g sa
mple
Feed mean
Digestate Mean
Significant reduction in
aerobic bateria
Significant increase in anaerobic bacteria
Change not significant
Environmental Results: In home
Small significant increase in E.coli with digesters
No significant change in E.coli without digesters
No significant change in C.perfringens
Environmental Results: Outside home
Significant increase with digesters in both E. coli and
C. perfringens
No significant change without digesters
Environmental Results: Animals
Significant increase with digesters in both E. coli and C. perfringens
Significant reduction in C. perfringens without digesters
Environmental Results: Dishes
No significance due to high variability
Environmental Results: Hands
Results below detection limits
Cumulative environmental results in and around the home
0
1
2
3
4
5
6
Total Coliforms E. coli C. perfringens
Mean
Log
CFU/
sample
BeforeDuringAfter
Significant increase in
aerobic bateria
Change not significant
Increase in aerobic bacteria in the local
environment following
installation of the digester
Contamination during handling
Importance of design of layout
Limitations • Need better replication to make robust comparisons
between paired households with and without digesters
• Need longer sampling period to allow users to adapt to the digester before sampling
• Need to consider the impact of retention time
• Not a strictly controlled experiment
Strengths • Not a strictly controlled experiment so more
representative of what actually happens in the field
Conclusions • Aerobic pathogens in bioslurry are
significantly reduced by anaerobic digestion
• Anaerobic pathogens in bioslurry are not significantly reduced during anaerobic digestion
• Aerobic pathogens are increased in the local environment
• Spillage and excessive handling of manures should be avoided when feeding the digester
• Users are washing hands sufficiently
Acknowledgements
Thanks to the Department for International Development (DfID) for supporting this work