The potential of cassava biomass and applicable technologies for sustainable biogas production in South Africa: A review

Abstract

Bioenergy production from agricultural crop biomass or residues is gaining interest due to the escalating cost of fossil fuels and the need to mitigate global warming caused by increasing GHG emissions. Of all the different feed stocks used for bioenergy production in Africa, cassava biomass potentially offers multiple benefits for producing biofuels such as biogas. However, its potential for food and non-food applications has so far been grossly underestimated by farmers and energy operators in Africa. This critical review on cassava intends to highlight the bioenergy potential of the crop especially for biogas production in Africa. Initially, the basic agricultural properties of cassava will be reviewed. Cassava contains large amounts of fermentable sugars. Its starch content is in the range of 20-35% based on fresh and at about 80.6% based on dry weight with a total dry matter content of 38.6%. It has the highest yield of carbohydrates per hectare with the exception of sugarcane and sugar beet. It thrives well in all ecological zones with one of the best water-footprints especially on soils of relatively low fertility, in drought conditions and requires low agrochemical input. High yielding and disease resistant cassava varieties have been developed for both food and nonfood applications with China adopting the crop to meet its 2020 biofuel target. Based on the available literature, various pretreatment techniques including mechanical, chemical, thermal, ultrasonic and wet explosion strategies were considered. In addition, the use of co-digestion and stimulation of microbial activity will be highlighted. The advantages and disadvantages of each technology as well as adoptable technologies for cassava biogas production and its optimization in Africa and in particular in South Africa will be critically discussed. This review highlights the highly politicized food vs energy debate as the most relevant bottleneck for using “potential” food (like cassava and other energy crops) for energy production. It suggests a paradigm shift on both sides of the debate and a more holistic view of food and biomass energy production which can be very complementary rather than exclusive. In conclusion, this review recommend considering cassava and its biomass as the next energy crop for biogas production in Africa and especially South Africa while providing guidance to future researchers and government policy makers.