SA Graan November 2015

39 / 92

generated from sugar cane was calculated from the total sugar cane
crushed per annum. The potential biogas production per annum was
calculated using the potential biogas yield for each substrate.
For solid waste, the biogas volume per kilogram of volatile solid
(m3/kg VS) was used. For liquid waste, the biogas volume per kilo-
gram of chemical oxygen demand removed (m3/kg COD) was used.
A calorific value of 6 kWh per cubic meter of biogas and an electric-
ity generation efficiency of 30% were assumed for calculating the
potential electricity generation.
The assumptions above were the basis for calculating the biogas
production potential.
Biogas produced from the solid and liquid
wastes
The main solid waste residues arising from sugar processing is ba-
gasse and sugar filter mud/pressed cake. Bagasse residues were
excluded from the calculations as they are already used as an en-
ergy source at mills. The amount of filter mud generated was calcu-
lated as a percentage of the total sugar cane crushed which is 5%
(GIZ, 2014a), therefore filter mud generated for the 2014/2015 period
amounted to 0,89 million tons.
Biogas yield for filter mud ranges between 20 m3 - 80 m3/ton. A
conservative value of 25 m3 /ton was used (GIZ, 2014b), giving a
potential annual biogas production of 22,19 million m3. The amount
of biogas that could potentially be produced from the solid waste
(filter mud) generated from the 14 mills is shown in Table 1.
Wastewater generation averages 18 m3 per 100 tons of sugar cane
crushed (Biogas Forum, 1999). This means that for the 2014/2015
production season, 3,2 million m3 of wastewater was generated.
A COD value of 2 kg/m3, 80% COD removal efficiency and a biogas
yield of 500 m3/ton of COD (Biogas Forum, 1999) were used for cal-
culating the biogas production potential. This gave a potential of
2,56 million m 3 of biogas that could be produced from the sugar
cane wastewater generated in the 2014/2015 production cycle. The
solid and liquid waste generated form the main sugar mills in South
Africa for the 2014/2015 season has a total biogas production
potential of 24,75 million m 3. Taking the calorific value of biogas to be
6 kWh/m3, the potential energy that could be recoverd from
24,75 million m 3 of biogas is 148,50 GWh.
Assuming an electricity generation of 30%, this translates to
44,55 GWh of electricity. The potential biogas and potential energy
recovery and electricity generation capacity for the sugar cane indus-
try is summarised in Table 1.
Conclusion
The analysis in this article points to the fact that there is consider-
able potential for the South African sugar cane industry to further
produce renewable energy from currently untapped
solid and liquid waste by-products. This is waste
material that could have otherwise polluted the
environment.
References
Biogas Forum, 1999. Evaluation of biomethanation potential of selected indus-
trial organic effluents in India. Biogas users survey 1998/199 in Nepal. Avail-
able from:
066%2525Biogas %Forum%25201999%2520II2520No%252077.pdf. Accessed
3 July 2015.
Bureau for Food and Agricultural Policy (2015). BFAP Baseline Agricultural Outlook
2015 - 2024.
GIZ, 2014a Project Opportunity Fact Sheet: Biogas potential from filter mud. Avail-
able on:
potential-filter-mud-sugar-mills.pdf. Accessed 3 July 2015.
GIZ, 2014b. Project opportunity fact sheet: The Central Azucarera Don Pedro Use of
bagasse and filter mud from sugar mills for energy. Available on: .
de/fachexpertise/downloads/giz-en-philippinen-energy-biomass.
try-2009-07-03.
Cane%20Wastewater_0.pdf.
20(1246-1253)IPACT.pdf.
37
November 2015
The sugar cane industry makes a significant contribution to the South African economy.