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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 Livelihoodscame 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 organic wastes 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- mend ways 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 m 3 , 12 m 3 and 16 m 3 , 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 Available online at www.sciencedirect.com ScienceDirect http://www.elsevier.com/locate/biombioe biomass and bioenergy 70 (2014) 1 e3

The potential for small-scale biogas digesters in Sub-Saharan Africa to improve sustainable rural livelihoods

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Page 1: The potential for small-scale biogas digesters in Sub-Saharan Africa to improve sustainable rural livelihoods

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

Page 2: The potential for small-scale biogas digesters in Sub-Saharan Africa to improve sustainable rural livelihoods

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.

Page 3: The potential for small-scale biogas digesters in Sub-Saharan Africa to improve sustainable rural livelihoods

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.