The demand for external carbon sources to achieve enhanced nitrogen removal is expected to grow significantly in the Chesapeake Bay watershed area. Although methanol remains the least cost and most frequently applied alternative, the price of methanol has increased substantially in
recent years, methanol is flammable (burns without a visible flame) and toxic, and cold weather denitrification kinetics using methanol are problematic. Biodiesel fuel is an alternative to conventional petroleum diesel. It is produced by combining virgin or waste vegetable oil, or animal fat,
with 10–20% methanol and sodium or potassium hydroxide. The reaction results in a product that is 80–90% biodiesel fuel and 10–20% waste glycerin by volume. Glycerin is fully miscible in water, biodegradable by ordinary heterotrophic organisms, nonflammable
and nontoxic. Biodiesel waste glycerin is typically contaminated with <5% methanol, significant residual sodium or potassium salts, long chain fatty acids, and other contaminants. In many cases, biodiesel plants are paying to dispose of waste glycerin as if it were a waste oil source.
The objective of this project was to conduct a full-scale pilot test of biodiesel glycerin waste as at an alternative carbon source at the Parkway WWTP, and to characterize a wide range of glycerin waste materials for physical and chemical properties. Samples of waste glycerin were collected
from approximately ten different biodiesel plants. Pilot testing demonstrated that the waste glycerin was immediately and effectively used as a carbon source for denitrification, though at dosages considerably higher than predicted by stoichiometry (though this might be due somewhat to Parkway
WWTP process configuration). To maximize the benefit of the added glycerin, it was necessary to implement effective upstream DO control (even during low flow periods) and to control carbon addition based on mass load pacing using flow and online nitrate measurement. Evaluation of waste glycerin
samples suggested a wide range of product characteristics in terms of water content, COD, temperature/viscosity relationship, gelling/freezing point, and phase separation. It is clear that the use of any waste carbon source for enhanced denitrification results in concerns associated
with product consistency, quality and reliability of supply not typical of methanol. Although it is expected that some of these concerns can be managed, it is important to note that the use of waste materials has the potential to dramatically decrease the cost per pound of nitrate denitrified
and is a more sustainable practice to meet enhanced nitrogen removal limits.
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