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Successful biological nutrient removal often requires a supplementary carbon source

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overhead view of municipal wastewater treatment plant
Enhanced biological phosphorus removal at this municipal wastewater plant resulted in significant savings.

By Sara Arabi and Mehran Andalib

Due to environmental concerns caused by excessive nutrient discharges, regulatory agencies are implementing stringent limitations on both point source and non-point source nutrient discharges. Upgrades to some wastewater treatment facilities to incorporate biological nutrient removal (BNR) processes are required to achieve the necessary nutrient load reductions.

In addition to controlling eutrophication in receiving water bodies and environmental benefits, BNR facilities have demonstrated economic and operational benefits. Their utilization is potentially more economic than conventional activated sludge treatment or physical/chemical processes. Incorporation of an un-aerated zone ahead of the aerobic zone in BNR processes, results in a substantial reduction of aeration energy costs. Also, the aeration energy needs are reduced because most of the substrate removal and some stabilization of organics occur in the un-aerated zone.

Enhanced biological phosphorus removal (EBPR) provides an economic benefit through the reduction or elimination of chemical addition for phosphorus removal. The BNR processes are also known to produce less waste activated sludge due to lower sludge yield. An additional benefit of BNR processes is that the denitrification process recovers approximately one-half of the alkalinity consumed during autotrophic nitrification.

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It has been shown that anaerobic and/or anoxic zones placed ahead of aerobic zones in BNR processes act as biological selectors that discourage the growth of filamentous organisms, generally improve sludge settling properties and enhance process stability.

Biological nutrient removal 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 denitrification. 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 denitrification 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 denitrification 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 denitrification. Methanol has been the most widely used external carbon source. But it often requires an adaption period of up to seven months before denitrification 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, sucrose and MicroC™, 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 denitrification 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.

Two case examples of the application of MicroC™ products for biological nutrient removal follow:

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1 COMMENT

  1. My dad’s client is requesting the removal of excessive nutrients in one of his resorts’ lake. It’s great to know that Enhanced biological phosphorus removal helps in reducing chemical addition for phosphorus removal. Right now, my dad is focused on looking for a biological removal company that can start the job immediately.

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