Process Control to Achieve Simultaneous Low-Level Effluent Nitrogen and Phosphorus Concentrations with Post-Denitrification Moving Bed Biofilm Reactor (MBBR) and Biologically Active Filter (BAF) Systems
Post-denitrification is a ‘popular’ application of biofilm reactors. Biofilm based unit processes such as the moving bed biofilm reactor (MBBR) and biologically active filter (BAF) retain temperature sensitive bacteria (e.g., methylotrophs) in a biofilm rather than relying
on their accumulation in a mixed liquor, and have a small physical footprint when compared with activated sludge processes. In addition, effluent water-quality standards requiring very low nitrogen and phosphorus concentrations are increasingly common. There is a general lack of information
describing system mechanics and their impact on the design and operation of these processes. This paper describes a simple mathematical approach for identifying potentially ratelimiting conditions, provides evidence that ortho-phosphate (PO4-P) is a reasonable indicator of biochemical
transformation rate-limitation by (the macronutrient) phosphorus, and uses the approach to evaluate two post-denitrification biofilm reactors types: MBBR and BAF. Operational data recorded from the pilot-scale MBBR reported by Stinson et al. (2009), pilotscale continuous backwash BAF reported
by deBarbadillo et al. (2006), and full-scale up flow (polystyrene bead) BAF at the Viikinmäki WWTP, Helsinki, Finland, was analyzed. The analysis assumed fixed stoichiometry for the electron donor and acceptor, and that the fixed stoichiometry is directly linked to the macronutrient
phosphorus required for cell synthesis. Equating soluble substrate penetration depths into the biofilm derived from analytical solutions, assuming zero-order kinetics, to the diffusion-reaction equations that describe a one-dimensional biofilm were linked with the soluble substrate penetration
depths into the biofilm derived from analytical solutions, assuming zero-order kinetics, to the diffusion-reaction equations that describe a one-dimensional biofilm were linked with the soluble substrates' stoichiometric coefficients (vED,EA). Then, by reducing terms
a basis for identifying the rate-limiting soluble substrate was created for methanol (MeOH), PO4-P, and nitrate/nitrite nitrogen (NOX-N). PO4-P rate-limiting regions are separated by SPO4-P:SNO3-N = 0.00865 and SPO4-P:SMeOH
= 0.00129. Further analysis demonstrated that JNOXavg = 5.25 g m −2 d−1 when SPO4:SNO3 > 0.00865, and JNOXavg = 2.45 g m−2 d−1 when
SPO4:SNO3 < 0.00865. A 2.80 g m −2 d −1 difference existed between the average NOX-N flux rates under reported operating conditions. The effluent NOX-N concentration begins marked increase when SPO4:SNO3
> 0.02 (2 × 0.00865 = 0.0173). Effluent stream SNOx-N from the pilot-scale post-denitrification biofilm reactors evaluated began to increase when SPO4-P:SNOx-N was in the range 0.01 to 0.05. Post-denitrification biofilm reactors evaluated may be P-limited
when SPO4-P:SNOx-N is < 0.00865. Remedial actions to avoid P-limitations include de-tuning CEC, and phosphoric acid storage and dosing.
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