Upgrading the efficiency of an external nitrification BNR system – The modified Dephanox process

Two types of external nitrification biological nutrient removal (BNR) plants were successively operated for municipal wastewater treatment. The first plant configuration was based on the conventional Dephanox design. Basic operational parameters were chosen as variables in order to assess their impact on the plant efficiency. The experimental results indicated that minimization of the aerobic mass fraction to values as low as 11% was practically feasible, resulting to high nutrient removal percentages, of 81% and 83%, respectively for nitrogen and phosphorus. However, the activated sludge characteristics with pin flocs formation and the absence of protozoa, led to increased total suspended solids concentration in the plant effluent (42 mg TSS/L in average). By increasing the aerated mass fraction to 16–17%, the sludge quality characteristics were improved and the pin flocs phenomenon was moderated. The increase in the aerated mass fraction was achieved “indirectly”, because of the lower biomass concentration in the anaerobic reactor, by establishing an additional mixed liquor recirculation flow in the conventional Dephanox design. This new configuration, referred as the modified Dephanox process, allowed for maintaining high anoxic P uptake extend with simultaneous enhancement of the plant denitrification potential. Fluorescence in situ hybridization (FISH) demonstrated that the well-known Phosphorus Accumulating Organism (PAO), Candidatus Accumulibacter phosphatis, was present in both the conventional and modified Dephanox process. The new configuration was proven to be more efficient regarding nutrient removal, being an interesting modification of the typical Dephanox design.

► Basic operational parameters of the Dephanox system were investigated.
► Split ratio of 0.10 was identified as the most suitable for the plant performance.
► Minimization of the aerobic mass fraction negatively affected the activated sludge.
► The modified Dephanox process was found to be more efficient in nutrient removal.