Upload
lisa
View
216
Download
4
Embed Size (px)
Citation preview
ww.sciencedirect.com
b i om a s s a n d b i o e n e r g y 7 0 ( 2 0 1 4 ) 1e3
Available online at w
ScienceDirect
http: / /www.elsevier .com/locate/biombioe
Editorial
The potential for small-scale biogas digesters in Sub-Saharan Africa to improve sustainable rural livelihoods
This Special Issue of Biomass and Bioenergy on “The Potential
for Small-Scale Biogas Digesters in Sub-Saharan Africa to
Improve Sustainable Rural Livelihoods” came about as a result
of funding from the UK Government Department for Inter-
national Development (DfID). Biogas digesters have great po-
tential to improve the lives of rural people in Sub-Saharan
Africa (SSA), and DfID are interested in this as an option for
international development.
Biogas digesters provide a clean source of energy, and so
could improve indoor air quality and reduce the need to
collect wood or buy charcoal. They provide a treatment for
animal faeces and other organic wastes, so potentially
reducing the spread of pathogens and the incidence of dis-
ease. The residue from the digestion process is an organic
slurry, bioslurry, that is rich in available nutrients and could be
used to improve crop production and increase the organic
matter content of the soil. Investing in a small-scale biogas
digester could reduce household expenditure on energy,
health and fertilisers, and so improve the long term economic
status of rural households in SSA.
Whether these benefits are realised depends on the options
available to farmers. What are the alternative sources of en-
ergy available to the household? If organicwastes are not used
to produce biogas, how could they be used? Where financial
resources are limited, does investing in biogas provide more
benefits to the household than other options? The papers in
this Special Issue consider the factors that determine the
impact of small-scale biogas digesters on rural livelihoods in
SSA.
Orskov et al. [1] discuss how a biogas digester can form a
central component of a holistic farming system, allowing ef-
ficiency to be optimised by providing energy for household
use, by cleaning and recycling of waste water and by pro-
ducing an organic fertiliser that can be used in aquaculture or
can return carbon and nutrients to the soil to improve crop
production. They discuss how environmental, socioeconomic
and cultural constraints determine best use and design of the
biogas digester; low temperatures require an insulated design
of digester, limited water supply demands installation of
rainwater harvesting equipment, a shortage of labour requires
the farm layout to minimise time needed to maintain the
digester.
The socioeconomic constraints to adoption of biogas in
SSA are further examined by Mwirigi et al. [2]. These con-
straints have resulted in lower uptake of the technology in
SSA compared to uptake in Asia, and Mwirigi et al. recom-
mendways to overcome the constraints of initial construction
costs and to increase the value of the products from the
digester. Approaches include mobilisation of domestic and
external funding, credit funding policies, standardisation of
proven technologies, publicity to increase public awareness
and promotion of digesters as an integrated system for both
biogas and bioslurry production.
The economic viability of small-scale biogas digesters is
assessed in more detail for Uganda by Walekhwa et al. [3].
For household digesters of volume 8 m3, 12 m3 and 16 m3,
they calculate positive net present values, suggesting small-
scale biogas systems are economically viable, with payback
times of just over a year. They calculate that borrowing
money to finance the digester will only be economically
viable at annual interest rates below 36%, 37% and 39%
respectively. The viability of biogas is also highly dependent
on discount rates, capital and operating and maintenance
costs.
Economic viability relies on effective use of the biogas
produced. The most commonly used biogas appliance in SSA
is the biogas stove, used for cooking, but Tumwesige et al. [4]
also review biogas appliances for different purposes,
including biogas lamps, biogas fuelled engines, refrigerators,
radiant heaters and incubators. Tests on locally available
biogas burners show them to be of poor quality with very low
efficiency. They assert that in order to have an impact on rural
livelihoods in SSA, appliances need to be made locally to meet
well-defined design standards while remaining affordable to
local people.
In order to remain economically viable, the biogas digester
must also be productive over the long term. Naik et al. [5]
consider the factors that govern the productivity and stabil-
ity of small-scale systems and suggest that the key parame-
ters that should be considered when looking at widespread
deployment of digesters are feedstock variability, retention
time, temperature and acidity of the system.
The potential of bioslurry to improve farmers' incomes
by improving crop production in SSA is reviewed by Smith
b i om a s s a n d b i o e n e r g y 7 0 ( 2 0 1 4 ) 1e32
et al. [6]. Whether applying bioslurry will improve crop
yields depends on how organic wastes would otherwise
have been used, so the supply of nutrients from bioslurry is
compared to the supply from fresh wastes, composts and
biochars (produced by pyrolysis). The composition of bio-
slurry is more homogeneous than that of untreated wastes,
meaning that the risks of locking up or losing nutrients by
immobilisation, leaching or volatilisation are much reduced.
Only 5e10% of the nitrogen in the organic waste is lost
during anaerobic digestion, compared to 26e51% during
composting and 70e90% during pyrolysis. This suggests that
a greater amount of nutrients are potentially available to
crops from applied bioslurry than from fresh wastes, com-
posts or biochars. Smith et al. [7] also consider the impact of
applying bioslurry on soil carbon. In contrast to nutrient
losses, a larger proportion of the carbon is lost during
anaerobic digestion (69e80%) than during composting
(52e74%) or pyrolysis (50e80%). A modelling study reveals
that the stability of the organic residue is higher in biochar
than in compost or bioslurry. As a result, applying bioslurry
sequesters no more carbon than applying fresh waste,
whereas applying biochar or compost sequesters signifi-
cantly more. This will have an impact on soil structure and
water holding capacity, so despite the higher availability of
nutrients in bioslurry, where soil organic matter is limiting
crop growth, crop yields may be further improved by
applying compost or biochar.
Using wood fuel or charcoal for household energy places a
daily economic burden on rural households. Deforestation
means that the average time spent collecting wood and the
cost of charcoal are increasing. The impact on deforestation of
replacing wood fuel or charcoal by biogas is discussed by
Subedi et al. [8]. They attribute 70(±42)% of the deforestation
observed in 2010 to wood fuel demand, increasing to 83(±50)%in 2030, and estimate that biogas production could reduce
energy demand from wood fuel by 6e36% in 2010 and 4e26%
in 2030, equivalent to 10e40% of total deforestation in 2010
and 9e35% in 2030.
Biogas digesters could further improve the livelihoods of
rural households by reducing the occurrence of diseases that
reduce productivity through decreased human and livestock
resources. Biogas digesters have potential to improve human
health, by reducing the pathogen load in the environment, so
reducing the diseases contracted by farmers, their families
and the wider population. Yongabi et al. [9] review the in-
fectious diseases associated with agricultural practices in
SSA, concluding that 80% of cases with high morbidity and
mortality are from helminths, protozoa and bacterial in-
fections. Avery et al. [10] consider the potential of biogas
digesters to reduce the pathogen loadings in rural SSA
through treatment of livestock manures and effluent from
pit latrines. Using data from the world literature, they
anticipate significant reductions in the burden of enteric
pathogens from human and animal wastes. However, some
organisms, particularly spore formers, anaerobes and micro-
aerophiles, are likely to withstand mesophilic treatment
temperatures. This includes pathogenic species of Clostridia,
which can cause gastrointestinal illness, wound infections
and neurological illness. Avery et al. [10] recommend that
post-processing of bioslurry, such as by composting, might
further reduce pathogen numbers, but emphasise the need
for further research.
Exposure to high levels of fine particulate matter and gases
such as carbon monoxide has been shown to be linked to
increased risk of respiratory and cardiovascular illness.
Household biogas digesters have potential to further improve
human health by significantly improving household air qual-
ity. Using comparisons with studies on liquified petroleum
gas, Semple et al. [11] suggest that biogas digesters could
reduce the risks of contracting diseases associated with poor
indoor air quality by 20e25%.
The supply of food to urban areas has resulted in the export
of nutrients from farmland to the cities. Failure to treat the
organic wastes so that they can be returned to the areaswhere
primary crop production occurs creates a resource mine,
which results in a long term decline in the productivity of
these rural areas. Gebreegziabher et al. [12] review opportu-
nities and challenges for urban application of biogas in SSA,
suggesting that biogas digesters play an essential role in
adding value to organic residues so as to facilitate urbanwaste
management. They also provide the opportunity for sustain-
able and reliable domestic energy provision through elec-
tricity generation.
Small-scale biogas digesters have great potential to
improve the sustainability of rural livelihoods in SSA. Di-
gesters can be economically viable if interest rates on any
money borrowed to purchase equipment are low, but the
quality and cost of locally produced equipment is a key issue
requiring further governmental standardisation and support.
Biogas could make a significant contribution to household
energy requirements, reducing the rates of deforestation,
labour and costs associated with energy provision, and
improving indoor air quality and human health. However, if
water is limiting, additional equipment will be needed for
rainwater harvesting, and so very dry regions might be better
suited to other methods of energy provision. Bioslurry should
be valued as a second product from the digester. It is a source
of readily available nutrients for the crop, although it returns
less carbon to the soil than composting or pyrolysis. If carbon
is the factor limiting crop growth, better yields might be
achieved by incorporating compost or biochar. An organic
fertiliser rich in both carbon and nutrients could be produced
by combining bioslurry from anaerobic digestion of wet,
nutrient rich materials such as manure, with biochar from
pyrolysis of dry, carbon rich materials such as straw. Since
post-processing of bioslurry by composting is also recom-
mended to reduce the anaerobic pathogen content, sequen-
tial treatment could be used to provide a pathogen-safe
organic fertiliser that is rich in both carbon and nutrients by
anaerobic digestion of wet wastes to provide energy, followed
by composting with dry wastes. However, a higher propor-
tion of the nutrients would be lost by this approach and the
likely reduction in pathogens has not yet been quantified.
Further work is needed to determine the efficiency of energy
provision and the quantity and quality of organic fertilisers
produced by such sequential treatment methods.
b i om a s s a n d b i o e n e r g y 7 0 ( 2 0 1 4 ) 1e3 3
r e f e r e n c e s
[1] Orskov ER, Yongabi K, Subedi M, Smith J. Overview of holisticapplication of biogas for small scale farmers in Sub-SaharanAfrica. Biomass Bioenergy 2014;70:4e16.
[2] Mwirigi J, Balana B, Mugisha J, Walekhwa P, Melamu R,Nakami S, et al. Socio-economic hurdles to widespreadadoption of small-scale biogas digesters in Sub-SaharanAfrica: a review. Biomass Bioenergy 2014;70:17e25.
[3] Walekhwa PN, Drake L, Mugisha J. Economic viability ofbiogas energy production from family-sized digesters inUganda. Biomass Bioenergy 2014;70:26e39.
[4] Tumwesige V, Fulford D, Davidson GC. Biogas appliances inSub-Sahara Africa. Biomass Bioenergy 2014;70:40e50.
[5] Naik L, Gebreegziabher Z, Tumwesige V, Balana B, Mwirigi J,Austin G. Factors determining the stability and productivity ofsmall scale anaerobic digesters. Biomass Bioenergy2014;70:51e7.
[6] Smith J, Abegaz A, Matthews R, Subedi M, Orskov RE,Tumwesige V, et al. What is the potential for biogas digestersto improve soil fertility and crop production in Sub-SaharanAfrica? Biomass Bioenergy 2014;70:58e72.
[7] Smith J, Abegaz A, Matthews R, Subedi M, Orskov RE,Tumwesige V, et al. What is the potential for biogas digestersto improve soil carbon sequestration in Sub-Saharan Africa?Comparison with other uses of organic residues. BiomassBioenergy 2014;70:73e86.
[8] Subedi M, Matthews R, Pogson M, Abegaz A, Balana B,Oyesiku-Blakemore J, et al. Can biogas digesters help toreduce deforestation in Africa? Biomass Bioenergy2014;70:87e98.
[9] Yongabi K, Avery LM, Pertiwiningrum A. A commentary onoccupational infectious diseases due to agricultural practicesin Sub-Saharan Africa. Biomass Bioenergy 2014;70:99e111.
[10] Avery LM, Yongabi K, Tumwesige V, Strachan N, Goude PJ.Potential for pathogen reduction in anaerobic digestion andbiogas generation in Sub-Saharan Africa. Biomass Bioenergy2014;70:112e24.
[11] Semple S, Apsley A, Wushishu A, Smith J. Commentary:switching to biogas e What effect could it have on indoorair quality and human health? Biomass Bioenergy2014;70:125e9.
[12] Gebreegziabher Z, Naik L, Melamu R, Balana BB. Prospectsand challenges for urban application of biogas installationsin Sub-Saharan Africa. Biomass Bioenergy 2014;70:130e40.
Jo Smith*
Institute of Biological & Environmental Science, University of
Aberdeen, 23 St Machar Drive, Aberdeen, AB24 3UU, UK
Lisa Avery
The James Hutton Institute, Craigiebuckler, Aberdeen, AB15 8QH,
UK
*Corresponding author.
E-mail address: [email protected]
Available online 29 September 2014
http://dx.doi.org/10.1016/j.biombioe.2014.09.0010961-9534/© 2014 Elsevier Ltd. All rights reserved.