Method to identify potential phosphorus rate-limiting conditions in post-denitrification biofilm reactors within systems designed for simultaneous low-level effluent nitrogen and phosphorus concentrations

Water-quality standards requiring simultaneous low level effluent N and P concentrations are increasingly common in Europe and the United States of America. Moving bed biofilm reactors (MBBRs) and biologically active filters (BAFs) have been used as post-denitrification biofilm reactors in processes designed and operated for this purpose (Boltz et al., 2010a). There is a paucity of information describing systematic design and operational protocols that will minimize the potential for phosphorus rate-limited conditions as well as a lack of information describing the interaction between these post-denitrification biofilm reactors and unit processes that substantially alter phosphorus speciation (e.g., chemically enhanced clarification). In this paper, a simple mathematical model for estimating the threshold below which P becomes rate-limiting, and the model is presented and evaluated by comparing its predictions with operational data from post-denitrification MBBRs and BAFs. Ortho-phosphorus (PO4–P), which is the dissolved reactive component of total phosphorus, was a primary indicator of P rate-limiting conditions in the evaluated post-denitrification biofilm reactors. The threshold below which PO4–P becomes the rate-limiting substrate is defined: SPO4–P:SNOx–NSPO4–P:SNOx–N = 0.0086 g P/g N and SPO4–P:SMSPO4–P:SM = 0.0013 g P/g COD. Additional analyses indicate JNOx−Navg=0.48g/m2/d when SPO4–P:SNOx–NSPO4–P:SNOx–N > 0.0086, and JNOx−Navg=0.06g/m2/d when SPO4–P:SNOx–NSPO4–P:SNOx–N < 0.0086. Effluent nitrate–nitrogen plus nitrite–nitrogen concentration (SNOx–N)(SNOx–N) from the evaluated post-denitrification biofilm reactors began to rapidly increase when SPO4–P:SNOx–NSPO4–P:SNOx–N was 0.01, approximately (consistent with the rate-limitation threshold of SPO4–P:SNOx–NSPO4–P:SNOx–N < 0.0086 predicted by the mathematical model described in this paper). Depending on the processes used at a given WWTP, optimizing chemically enhanced clarification to increase the amount of PO4–P that remains in the clarifiers effluent stream, dosing phosphoric acid in the MBBR or BAF influent stream, and/or optimizing secondary process EBPR may overcome phosphorus rate-limitations in the biofilm-based post-denitrification process.

► PO4–P is an indicator of P availability in post-denitrification biofilm reactors.
► SPO4–P:SNOx–NSPO4–P:SNOx–N < 0.01 may be characteristic of P rate-limited operations in systems analyzed.
► P as PO4–P is rate-limiting when SPO4–P:SNOx–NSPO4–P:SNOx–N < 0.0086 according to a model developed in this paper.
► To avoid P rate limitations – optimize CECs, dose H3PO4, or optimize EBPR.