Secondary Impacts of Supplemental Carbon Addition to BNR/ENR Treatment Processes


  • Ron Latimer, P.E., Paul Pitt, P.E., Ph.D., Andre van Niekerk, P.E., Ph.D.,
  • Theresa Bruton, P.E., Sarah Dailey, P.E., Hazen and Sawyer, P.C.
  • James Grandstaff, Henrico WRF, Kevin Selock, Parkway WWTP

Nitrogen removal to increasingly strict discharge standards requires, in many cases the use of supplemental carbon (methanol, acetate, glycerol, sugar water, etc). The supplemental carbon provides the driving force for further biological denitrification and is typically applied as a polishing treatment such as to a post anoxic zone or a tertiary denitrification filter.

The practical use of supplemental carbons has attracted substantial attention from both process optimization and cost minimization perspectives. This paper presents the operational experiences gained with the secondary and indirect impacts of supplemental carbon addition to BNR/ENR treatment facilities at the Parkway WWTP, Maryland (Figure 1) and at Henrico County WRF, Virginia (Figure 2). The focus of the paper deals with the sometimes unexpected beneficial secondary effects of supplemental carbon addition to post anoxic zones in the BNR/ENR treatment processes. These indirect benefits will be demonstrated and discussed to include:

• Improved performance of the primary anoxic process due reduced recycle of the nitrate (NOx-N) mass to the upstream end of the BNR/ENR process (Figure 3). It has been demonstrated at the Henrico WRF that carbon addition to the post anoxic cells resulted in improved performance of the pre anoxic cells in terms of better denitrification.

• Better utilization of the influent wastewater readily biodegradable carbon, due to the reduced NOx-N concentration in the upstream anaerobic and pre anoxic cells.

• Stimulation and improvement in biological phosphate removal, due to the increased presence of RBCOD (Figure 4). Under routine operating conditions at Henrico WRF, no significant Bio-P population appears to be active. This situation changed significantly with carbon addition to the post anoxic cells. The bio-P organisms established immediately following carbon addition to the secondary anoxic cells, with very low residual PO4-P concentrations during times of supplemental carbon addition.

• Supplemental carbon storage in the activated sludge cells and utilization of the stored substrate in the upstream anaerobic bio-P and pre anoxic denitrification prospects.

• Increased Nitrogen removal due to the increased biosynthesis requirements. Additional carbon stimulated some additional biomass growth with associated uptake of Nitrogen and Phosphorus.

Another secondary benefit of supplemental carbon addition is associated with improved DO control to optimize the benefits of carbon addition to the BNR aeration tanks. This typically involves the tight DO control in the last aerated zone upstream of the post anoxic cells. This has the benefit of reducing the residual DO concentration in the NRCY flow back to the upstream pre anoxic cells.

Another interesting aspect which will be presented is the concept of the CODadded/NO3-N removed ratio and the different ways of calculating this often quoted ratio. The different process aspects which influence the calculation of the COD/N ratio will also be presented, as follows:

COD/N ratio utilizing the overall BNR treatment process performance data.

COD/N ratio using the post anoxic cell mass balance information.

• Correction of the COD/N ratio based on the endogenous decay removal of NOx-N.

• Importance of recognizing both the NO3-N and the NO2-N concentrations, when calculating the COD/N ratio.

Figure 5 details various interpretations of how this ratio can be calculated.

A detailed evaluation of the COD/N ratio calculation issue was investigated at the Parkway WWTP in Laurel, MD. Full scale testing of multiple alternative carbon sources was performed in 2007 and 2008. Hazen and Sawyer conducted full scale basin profiles and bench scale testing during several of the full scale trials. Parkway staff calculated relatively high COD/N ratios in the 10 to 20 range for the supplemental carbon using plant effluent TN data before and after supplemental carbon addition. Hazen and Sawyer calculated much lower ratios at Parkway, in the more typical 5 to 7 range, using a mass balance approach across the post anoxic zone where the carbon was added based on basin profiles. However, a detailed BioWin modeling investigation revealed that calculating the COD/N ratio based on effluent TN without carbon addition versus with carbon addition showed that significantly higher ratios (12.7) can occur even in an ideal modeling situation (Figure 6 – note similar results were obtained using a total N balance). This was determined to be due to the fact that at Parkway the pre-anoxic zone was completely removing all NOx both before and after carbon addition. When carbon was added to the post anoxic zone, less NOx was being returned/removed in the RAS (in the final clarifier underflow and in the pre anoxic zone) resulting in additional NOx load being discharged to the post anoxic zone. This demonstrated the potential inefficiency in supplemental carbon addition to the activated sludge process if the internal recycle to the pre anoxic zone is not increased to fully utilize the denitrification capacity of that zone.

The paper and presentation will include full-scale plant operational observations, bench scale testing results and process modelling outcomes for the Parkway WWTP and the Henrico County WRF.

For a copy of the full paper, please contact the author at

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Horizons Fall 2017 (pdf)

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