The potential of cassava biomass and applicable technologies for sustainable biogas production in South Africa: A review
Introduction
The key issues faced by many developed and developing countries of the world today are mainly future energy security and better use of natural resources. A look at the African continent in terms of energy production and consumption shows the inequality in the distribution of fossil fuels. About 70% of the countries in Africa rely on imported energy. This situation is aggravated by huge unemployment and low gross domestic product (GDP). According to a recent report by the Food and Agriculture Organization (FAO) in 2013, Africa has the lowest overall GDP ($1629.5 billion) in the world, eight times less than Asia, five times less than Latin America and 26 times less than developed economies. GDP per capita in Africa was $1623.6 in 2010, two times less than Asia, five times less than Latin America and 21 times less than the per capita GDP of developed economies. GDP growth in 2010 was exceeding 6% only for Nigeria, Botswana and some East and Central African countries but was less than 3% for South Africa and Angola. A huge chunk of African countries’ national budget which could have gone into development is spent on energy imports. Moreover, limited availability and lack of access to energy remains one of the most important factors affecting industrial development (i.e. agriculture, mining and tourism) in the continent. This leads to in-fighting (wars), violent service delivery protests, poor infrastructural development and spread of lethal diseases.
The above scenario supports the call for urgent development of renewable energy (RE) resources and in particular bioenergy. The bioenergy potential by 2050 on unutilized land for sub-Saharan Africa is estimated to be 317 Exajoules (EJ) per year [1]. This figure is higher than most other regions of the world. For example, only about 20% of South Africa’s total land mass of 120 million hectares (Mha) is currently used for biomass production [2]. Conversion of biomass to energy will help reduce the dependence on fossil fuels as well as mitigate the negative social and environmental impacts such as rural unemployment and global warming [3].
Of all the different biofuels, biogas currently presents the most opportunities to the rural population in Africa while it is a major low-carbon fuel source. In addition, to the national governments in Africa, it offers multiple-benefits such as:
- •
Foreign exchange savings for non-oil producing countries.
- •
Boost for rural farming and economy through job creation and income gains.
- •
Beneficial use of organic agricultural and municipal solid waste (MSW) for energy production.
- •
Improved environmental quality through CO2 emission reduction [4].
The use of cassava provides huge potential for bioenergy production and in particular biogas with several advantages. According to a report by the Forum for Agricultural Research in Africa (FARA) in 2010, there are thousands of acres of degraded and unutilized land in Africa where crops like cassava can be produced for biofuels on a large scale without damage to food production or natural habitats [5]. The advantage that cassava has over many other crops is that it can thrive in areas where the land has been degraded and has the highest yield of carbohydrates (4.742 kg/carb) per hectare with the exception of sugarcane and sugar beet [6]. It thrives very well on soils of relatively low fertility where the cultivation of other crops will be uneconomical [7]. It also has the ability to thrive in drought conditions and requires low input of agro-chemicals [8]. Cassava contains large amounts of starch (20–35% fresh and 80.6% dry weight) [9], [10] and total dry matter (38.6%) [11], and has been reported to have the smallest water-footprint (21 m3/GJ) compared to all other crops [12]. Based on the above reasons, cassava has recently gained considerable attention for the production of bioenergy [10] and in particular for the production of biogas [13], [14], [15], [16], [17].
Biogas from biomass is one of the best sources of renewable energy because it can be used for heating, as a fuel or natural gas equivalent and can be converted to electricity. In Germany, for example, the number of biogas plants has exceeded 7000 units in 2011 with electrical capacity already exceeding 2.8 GW [18], [19]. The production of biogas from cassava biomass is a biochemical process that takes place through the anaerobic food chain involving mainly prokaryotes [20], [21]. The major constituents of biogas are methane (CH4) and carbon dioxide (CO2). Trace amounts of H2S, NH3, H2, N2 are also present [22]. Methane is the most valuable component of biogas and typically accounts for more than 60%. Biogas is considered to be a valuable fuel [23], [24] with the calorific value ranging from 5000 to 7000 kcal/m3 depending on the concentration of CH4 in it. For comparison, one cubic meter of biogas containing about 60% methane is equal to 0.7 m3 of natural gas, 0.7 kg of fuel oil, 0.6 kg of kerosene, 0.4 kg of petrol, 3.5 kg of wood, 12 kg of manure briquettes, 4 kWh of electrical energy, 0.5 kg of carbon and 0.43 kg of butane [25]. Biogas production employs various types of feedstock. However, the quality and yield of the biogas produced depends on the composition of the feedstock used [26], [27].
Traditionally, biogas studies were biased towards soluble substrates such as present in industrial wastewaters [28] and only recently increasing numbers of studies focused on energy crops such as fodder beet or cassava. High yielding and disease resistant cassava varieties are now developed for both food and non-food applications [29], [30]. Whilst cassava has had a long history in Africa, it is not a well-known crop in South Africa [31]. Very few studies were carried out in the 1980s for the purpose of starch production [32], [33] and since then not much has improved.
Therefore, the objective of this review is to renew interest in cassava and argue for its adoption for bioenergy production in Africa, and especially so in South Africa. The first section will deal with the nature of cassava and its importance while the second section will scope and evaluate the potential of cassava biomass for bioenergy production. The third section will look at existing biogas production technologies and critically evaluate factors that influence its production and approaches to optimize cassava biogas yields. The discussion and conclusions attempt to argue for cassava biomass adoption for sustainable bioenergy in Africa, especially in a developing economy like South Africa.
Section snippets
What is cassava?
Cassava (Manihot esculenta Crantz), synonyms: manioc, yucca, tapioca, is a tubercle 5 to10 cm in diameter and 15 to 35 cm in length (Fig. 1). It is produced in almost all mild and tropical countries and grows in degraded soils where almost nothing else can grow [7]. It does not need fertilizers, insecticides, or additional water. Furthermore, cassava can be harvested any time between 8 to 24 months after planting [34]. Native to South America cassava has historically been a human and animal feed
Cassava bioenergy potential
There are three potential biofuels that can be generated at an industrial scale from cassava biomass viz., bioethanol, biodiesel and biogas.
Biogas production from cassava
Biogas is a product of a microbial process known as anaerobic digestion (AD). Anaerobic digestion involves microorganisms decomposing organic matter in the absence of oxygen to produce mainly methane and carbon dioxide. The residues left after AD adds value to the process as they can be used for agricultural purposes as fertilizer. Other important applications of AD include the reduction of sludge volume generated from wastewater treatment processes, sanitation of industrial organic waste, and
Optimization of cassava biogas production
Most of the techniques used to improve biogas production and methane yields at landfills and wastewater treatment plants also apply to cassava biomass because of its organic material content. The rate of biogas production depends on three main factors; the degree of hydrolysis, the rate of production or consumption of intermediates, and the rate of methanogenesis. Therefore, optimization of cassava biogas production is based on the need to improve material digestibility and biological activity
Discussion
Some people have criticized the biogas-from-energy crops development in Africa based on the food versus fuel debates highlighting the following: (a) potential increase in food prices due to competition with biofuel industries; (b) possible dispossession/acquisition of land from rural dwellers and (c) possible diversion of resources from food to fuel thus threatening food security. However, the current biogas-from-energy crops developments in Africa are planned in such a manner that they take
Conclusions
In conclusion, this review highlights that there is a potential to use cassava biomass as a bioenergy crop, in particular for biogas. Biogas production is an old and mature technology; however, there is still room for improvement in order to overcome the existing shortcomings. Some of the shortcomings include: (a) limitations posed by the current bioreactor designs; (b) process control still relies on in- and output data; (c) the role of microbial population dynamics is still not sufficiently
Future research recommendations
For future research on cassava biogas we recommend a multidisciplinary approach involving industry, Agriculture, Engineering and Science departments of our universities. This will include:
- •
Biogas research that utilizes waste and non-food energy crops.
- •
Flexible feedstock requirements.
- •
Combinations of different technologies.
- •
Use of improved feedstock varieties that will not diminish but complement food production.
Finally, biogas production from an energy crop such as cassava can really add value to
Acknowledgement
Financial support from the University of KwaZulu Natal and the Cape Peninsula University of Technology during this research project are gratefully acknowledged.
References (166)
- et al.
A bottom-up assessment and review of global bio-energy potentials to 2050
Prog Energy Combust Sci
(2007) - et al.
Cassava, a potential biofuel crop in (the) People’s Republic of China
Appl Energy
(2009) - et al.
Production of methane by co-digestion of cassava pulp with various concentrations of pig manure
Biomass Bioenergy
(2010) - et al.
Ultrasound improved ethanol fermentation from cassava chips in cassava-based ethanol plants
Bioresour Technol
(2010) - et al.
Enhancement of methane production from cassava residues by biological pretreatment using a constructed microbial consortium
Bioresour Technol
(2011) - et al.
Biogasification of rice straw with anaerobic phased solid disgester system
Bioresour Technol
(1999) - et al.
Biotechnological potential of agro-industrial residues II: Cassava bagasses
Bioresour Technol
(2000) - et al.
Biofuel development in China: technology options and policies needed to meet the 2020 target
Energy Policy
(2012) - et al.
Simultaneous non-thermal saccharification of cassava pulp by multi-enzyme activity and ethanol fermentation by Candida tropicalis
J Biosci Bioeng
(2009) - et al.
Methane production in low-cost, unheated, plug-flow digesters treating swine manure and used cooking grease
Bioresour Technol
(2010)
History and future of domestic biogas plants in the developing world
Energy Sustainable Dev
Biohydrogen and methane production by co-digestion of cassava stillage and excess sludge under thermophilic condition
Bioresour Technol
Methanogenic ethanogenic fermentation of cassava peel using a pilot flow digester
Bioresour Technol
Codigestion of cow and guinea pig manure in low-cost tubular digesters at high altitude
Ecol Eng
Improving biodegradability and biogas production of wheat straw substrates using sodium hydroxide and hydrothermal pretreatments
Energy
Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives
Bioresour Technol
Methano-compost, a booster and restoring agent for thermophilic anaerobic digestion of energy crops
Biomass Bioenergy
Integrated biogas technology in the tropics 1. Performance of small-scale digesters
Waste Manage Res
Biohydrogen and methane production by co-digestion of cassava stillage and excess sludge under thermophilic condition
Bioresour Technol
Diffusion and innovation in the Chinese biogas program
World Dev
Biogas production with horse dung in solid-phase digestion systems
Bioresour Technol
Pilot project of biogas production from pig manure and urine mixture at ambient temperature in Ventanilla (Lima, Peru)
Waste Manage
A preliminary study on biogas production from cowdung using fixed-bed digesters
Biol Wastes
Evaluating variable organic waste to produce methane
Energy Convers Manage
Horse dung as a partial substitute for cattle dung for operating family-size biogas plants in a hilly region
Bioresour Technol
Biogas as a sustainable energy source in Nepal: present status and future challenges
Renewable Sustainable Energy Rev
Solid-state anaerobic digestion of cattle dung and agro-residues in small-capacity field digesters
Bioresour Technol
Preparation and characteristics of bio-oil from the marine brown alga Sargassum patens C. Agardh
Bioresour Technol
Measures of appropriateness: the resource requirements of anaerobic digestion (biogas) systems
World Dev
A preliminary analysis of the biomass energy production potential in Africa in 2025 considering projected land needs for food production
Biomass Bioenergy
Monitoring of anaerobic digestion processes: a review perspective
Renewable Sustainable Energy Rev
Enhanced methane production in a two-phase anaerobic digestion plant, after CO2 capture and addition to organic wastes
Bioresour Technol
Next-generation cellulosic ethanol technologies and their contribution to a sustainable Africa
Interface Focus
Nutrition per hectare for staple crops
Bioethanol from cassava. Ethanol from cassava
Cassava prodcution for industrial utilization in China—present and future perspectives
Bio-ethanol production from non-food parts of cassava (Manihot esculenta Crantz)
AMBIO
The water footprint of bioenergy
Proc Nat Acad Sci USA
Anaerobic digestion of cassava peels in batch-operated plastic biodigester and the use of stored biogas for heating
J Home Econ Res
Comparison of biogas productivity of cassava peels mixed in selected ratios with major livestock waste types
Afr J Agric Res
Editorial: progress in biogas—state of the art and future perspectives
Eng Life Sci
Biogas production: current state and perspectives
Appl Microbiol Biotechnol
Biomass digestion in agriculture: a successful pathway for the energy production and waste treatment in Germany
Eng Life Sci
Major pathway of methane formation from energy crops in agricultural biogas digesters
Crit Rev Environ Sci Technol
Biogas as a renewable energy source—a review
Energy Sources Part A
Rev Environ Sci Biotechnol
Cited by (110)
Effects of three different polysaccharides on the sol gel-behavior, rheological, and structural properties of tapioca starch
2024, International Journal of Biological MacromoleculesValorization of waste cassava peel into biochar: An alternative to electrically-powered process
2023, Total Environment Research ThemesA Review on the Production of Thermo-Plastic Starch From the Wastes of Starchy Fruits and Vegetables
2022, Encyclopedia of Materials: Plastics and PolymersThe influence of longitudinal dispersion on the capacity and stability of UASB operation with substrate inhibition
2022, South African Journal of Chemical Engineering