BNR with Carbon Augmentation
Performance of a BNR system is strongly affected by the characteristics of the wastewater influent to each zone of the processes.
Neither biological nitrogen removal nor EBPR can be accomplished without sufficient biodegradable organic substrate.
Carbon augmentation is needed when there is insufficient carbon available to achieve complete de-nitrification.
This is normally the case when low levels of Total Nitrogen (TN), e.g., < 5 mg/L, are required in the treated effluent.
For typical medium-strength municipal wastewater, readily biodegradable COD (rbCOD, which is typically 1.6 times BOD),
TN and Total Phosphorus (TP) are 300, 40, and 7 mg/L, respectively.
Based on rbCOD/N of 5.8 and rbCOD/P of 25 required for de-nitrification and EBPR, theoretically 365 mg rbCOD/L is required to achieve TN
of 5 mg/L and TP of 0.5 mg/L, respectively. This results in a shortfall of rbCOD, so addition of an external carbon source is required.
In a wastewater treatment facility, a considerable portion of influent rbCOD is utilized through aerobic respiration.
This means just a portion of the 300 mg rbCOD/L is practically available for de-nitrification and EBPR processes.
The choice of a carbon source can have profound implications, not just on the efficacy of nutrient removal but also on plant
and personnel safety, sludge yields, aeration adequacy, environmental sustainability, overall effluent quality and other factors.
Recent studies also indicate that different carbon sources could have differing effects on nitrogen and phosphorus removal, even in the same treatment process.
Soluble and readily degradable substrates support the highest rate of de-nitrification.
Methanol has been the most widely used external carbon source. But it often requires an adaption period
of up to seven months before de-nitrification rates significantly increase, due to low growth rates of methylotrophs.
The flammability, safety concerns and price fluctuations for methanol have limited its use for wastewater treatment.
Agriculturally derived carbon sources such as molasses, glycerol, corn syrup, and sucrose tend to have more predictable and less volatile price profiles.
Recently, glycerin has drawn significant attention as an alternative to alcohols (methanol and ethanol) for de-nitrification
application and acetate for enhanced biological phosphorus removal. It is safer, noncorrosive and nonflammable.
Its price, biodegradability, high COD value and ability to promote nutrient removal behaviour
are all advantages that make this supplemental carbon source a viable alternative.
In addition, glycerin˘®?s abundance in nature has led to microbial adaptations for its uptake and use as a source of carbon and energy.