Advantages and Disadvantages of Glycerin Based on Experiences at Five Full-Scale BNR Plants


  • K. Bilyk, J. Rohrbacher, R. Latimer, W. Khunjar, T. Bruton, P. Pitt - Hazen and Sawyer
  • C. Bott, W. Balzer - Hampton Roads Sanitation District

A generic five-stage BNR process schematic with downstream denitrification filters is shown here to illustrate the supplemental carbon addition points.

Denitrification batch tests were conducted on-site by Hazen and Sawyer to determine carbon utilization (COD:NO3-N ratio) and specific denitrification rates. The tests were carried out in a seven liter reactor and filled with undiluted mixed liquor from the end of the aerobic zone.

A full-scale pilot at one of the facilities in this project confirmed glycerin products were effective carbon sources for post-anoxic zone denitrification, although cold weather storage and pumping issues related to product viscosity need to be addressed. No acclimation period is required, as robust denitrification was observed within one day of adding these products.

In another pilot test, TN removal by the Brenntag (glycerin) basin was typically greater than or equal to TN removal in the control basin even during the early phase of the pilot.

The results of a third pilot test demonstrated that the mass of nitrate denitrified with the Brenntag glycerin product was comparable to the mass of nitrate removed with methanol at an equivalent feed rate (1 gallon methanol = 1 gallon of Brenntag). The Brenntag product was effective immediately (see 6/13/08, which represents Day 1 of the Brenntag feed) and there was no acclimation period required for the biomass to fully utilize this product.

One of the full-scale test set ups utilized in this project.

In January 2011, many treatment plants in the Chesapeake Bay watershed began meeting stringent total nitrogen (TN) and total phosphorus (TP) standards. In order to comply with the TN limits, many of these facilities rely on purchased supplemental carbon to aid denitrification. Several of these facilities opted to start their process with glycerin instead of methanol because glycerin is a safer, albeit more costly (based on current pricing), alternative.

Recently, it has been found that the nitrate to nitrite reduction rate using glycerin as the electron donor, is much faster than the rate at which nitrite is converted to nitrogen gas (Uprety, 2012). As a result of this preferential reduction of nitrate, nitrite accumulation has been consistently observed in lab-scale system mimicking 5-stage biological nutrient removal facilities (Uprety, 2012). In these lab-scale studies, nitrite accumulation could be further exaggerated if the glycerin dose is not sufficient to completely denitrify (NOx-N < 1 mg/L) the nitrate load entering the second anoxic zone (Uprety, 2012) and the effect was magnified as the biomass acclimated to glycerin. Further, it was found that even when the carbon was dosed at the stoichiometric rate required to fully denitrify the nitrate load, preferential nitrate reduction and subsequent nitrite accumulation were observed. In this system, the only strategy that resulted in complete denitrification was an overdose of glycerin (> 6:1 ratio), which would result in several issues for full-scale facilities including but not limited to increased operating costs.

This paper will present observations and lessons learned from using glycerin for denitrification at five biological nutrient removal (BNR) plants in Virginia; and document findings that indicate:
1) Glycerin addition improves biological phosphorus removal (BPR) in full-scale facilities.
2) Extended glycerin use in full-scale facilities results in increased carbon demand per pound of nitrate removed.

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