Organic Waste Recycling

210 Organic waste recycling: technology and management

• Nutrient requirements, such as N, P, K, essential for yeast growth,
  have to be provided.


• The optimum temperatures for Saccharomyces cerevisae are 27-
  35 oC. Fermentation rates will decrease at temperatures above 35oc
  and will stop at temperature above 43oC. Temperatures below 27oC
  will also decrease the fermentation rates. In general, the
  fermentation period is 2-3 days to produce an ethanol concentration
  of 8-10%.


• Contamination of the fermenters with other microorganisms (such
  as bacteria) should be avoided because bacteria will also utilize
  sugar for their growth, resulting in less ethanol production.


• SinceSaccharomyces cerevisae grow best at pH range 4-6, this pH
  level (about 5) should be maintained in the fermenters.

Table 4.25 Environmental conditions for ethanol fermentation

Design parameters Optimum values

Sugar concentration 20%

Temperature 27-35°C

Fermentation period 2-3 days

pH range 4-6

Distillation and drying

In general, the fermented ethanol will contain about 10 % ethanol and 90 %
water and other by-products. Distillation process is usually employed to produce
about 96% pure ethanol which is the highest ethanol solution that can be
recovered by simple distillation (called azeotropic condition where the relative
volatilities of ethanol and water are the same). The 96% pure ethanol solution
can be used in various applications such as fuel in engines and disinfectants, etc.
Ethanol drying is done to further remove the remaining 4% water content
before its use as a blend with gasoline or as fuel for engines and automobiles.
Three proven methods of ethanol drying include: azeotropic distillation (in
which some compounds such as benzene are added to change the relative
volatilities of the ethanol/water mixture); adsorption of water by the reactions of
dessicants, such as CaO; and molecular sieving of the water molecules by some
zeolites.
Figure 4.24 is a schematic diagram showing the process of ethanol
production from cassava. In addition to ethanol, other valuable by-products are
also produced which are applicable for agricultural and aquacultural reuses.