Bioenergy By-Products

There are three primary wastes resulting from bioenergy production:  fly ash (combustion), waste water (anaerobic digestion, fermentation), and gaseous effluent (combustion, gasification, pyrolysis, anaerobic digestion).  Fly ash and waste water are discussed here.  See our course on Anaerobic Digestion (ANDIG 1-7) for a discussion of digestion processes

Fly ash is fine particulate matter resulting from combustion of solid fuel, which is either carried into the air or may fall to the bottom of certain furnaces/boilers.  It is composed of substantial amounts of silica.  It is an air pollutant and known health hazard regulated by U.S. Environmental Protection Agency who issues specifications for its collection and removal.  Less than half of the fly ash produced in the U.S. is recycled, most often as an amendment to Portland cement.

Waste water is a major challenge in the production of sugar/starch-based ethanol, and production of biodiesel.  Grains are crushed and then mixed with water for fermentation to produce bioethanol.  After fermentation, water is removed from the alcohol through distillation.  The leftover water contains proteins, residual sugars, enzymes and dead yeast cells.  For every gallon of bioethanol produced, 4-6 gallons of water is used.   Water is used in the biodiesel process to remove methanol and glycerol contaminants.  Bioethanol and biodiesel effluents must be treated before being reused or being discharged to nearby surface waters (which is an activity subject to regulation).  The primary treatment option for biofuel effluents is biodigestion (i.e., anaerobic digestion for which biogas is a co-product).