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-N = 0.0086 g P/g N and SPO4-P:SM = 0.0013 g P/g COD. Additional analyses indicate JNOx-Navg=0.48g/m2/d when SPO4-P:SNOx-N > 0.0086, and JNOx-Navg=0.06g/m2/d when SPO4-P:SNOx-N < 0.0086. Effluent nitrate-nitrogen plus nitrite-nitrogen concentration (SNOx-N) from the evaluated post-denitrification biofilm reactors began to rapidly increase when SPO4-P:SNOx-N was 0.01, approximately (consistent with the rate-limitation threshold of SPO4-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.