sludges, and animal slurries can also be digested in this process (Valorga) developed by
Valorga SA (Vendargues, France). This is termed a dry process.
Wet processes handle wastestreams with only 10 to 15 percent solids content. Featur-
ing more than one digestion stage, it is easier to control parameters such as pH and solids
concentration than dry fermentation. The first plant to use wet digestion to process MSW
is 20,000-mt/yr installation in Denmark. About two-thirds of the annual operating cost of
$2 million is recovered through the sale of biogas. In a 14,000-mt/yr plant in Finland the
biogas produced is used to fire a gas turbine. Multiple stages are said to make wet fer-
mentation 65 percent faster than single-stage processes, with a 50 percent higher gas
yield.
These developments show that sanitary engineers will be more concerned than ever
with the environmental aspects of their designs. With the world population growing
steadily every year and the longer lifespan of older individuals, biogas and similar recov-
ery-conversion processes will become standard practice in every major country.
The data on biogas given above was reported in Chemical Engineering.
singke
(singke)
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