Introduction 71.2.2 Biofuels production
Organic wastes can be biochemically converted into biofuels such as biogas and
ethanol which can be used for heating or as fuel for combustion engines and co-
generators for electricity and heat generation.
Biogas, a by-product of anaerobic decomposition of organic matter, has been
considered as an alternative source of energy. The process of anaerobic
decomposition takes place in the absence of oxygen. The biogas consists mainly of
methane (about 65 percent), carbon dioxide (about 30 percent), and trace amounts
of ammonia, hydrogen sulfide, and other gases. The energy of biogas originates
mainly from methane (CH 4 ) whose calorific value is 1,012 BTU/ft^3 (or 9,005
kcal/m^3 ) at 15.5 °C and 1 atmospheric pressure or 211 kcal/g molecular weight,
equivalent to 13 kcal/g. The approximate calorific value of biogas is 500-700
BTU/ft^3 (4,450-6,230 kcal/m^3 ).
For small-scale biogas digesters (1-5 m^3 in size) situated at individual
households or farm lands, the biogas produced is used basically for household
cooking, heating and lighting. In large wastewater treatment plants, the biogas
produced from anaerobic digestion of sludge is frequently used as fuel for boiler
and internal combustion engines. Hot water from heating boilers may be used for
digester heating and/or building heating. The combustion engines, fuelled by the
biogas, can be used for wastewater pumping, and have other miscellaneous uses in
the treatment plants or in the vicinity.
The slurry or effluent from biogas digesters, though still polluted, is rich in
nutrients and is a valuable fertilizer. The normal practice is to dry the slurry, and
subsequently spread it on land. It can be used as fertilizer to fish ponds, although
little work has been conducted to date. There are potential health problems with
biogas digesters in the handling and reuse of the slurry. It should be treated further;
such as through prolonged drying or composting prior to being reused.
Ethanol or ethyl alcohol (C 2 H 5 OH) can be produced from three main types of
organic materials such as: sugarcane and molasses (containing sugar); cassava,
corn and potato (containing carbohydrates) and wood or agricultural residues
(containing cellulose). Except those containing sugar, these organic materials need
to be converted firstly into sugar, then fermented by yeast into ethanol, and finally
distilled to remove water and other fermentation products from ethanol. The
calorific value of ethanol is 7.13 kcal/g or 29.26 kJoule/g.
1.2.3 Aquacultural reuses
Three main types of aquacultural reuses of organic wastes in hot climates involve
the production of micro-algae (single-cell protein), aquatic macrophytes, and fish.
Micro-algal production normally utilizes wastewater in high-rate photosynthetic