Influence of growth media and internal recycle rate on nitrogen removal from high-strength wastewater in a low footprint anoxic-upflow/aerobic process

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This research investigated the use of integrated fixed-film media in comparison to suspended-growth in a low-footprint upflow biological nitrogen removal (BNR) process to find a robust solution for treatment of high-strength dairy farm wastewater (DFW). Using real DFW and synthetic wastewater (SWW), two juxtaposed anoxic upflow bioreactor/aerobic systems (UBR-A) were operated under different internal recycle ratios (R) of 3 and 4 in both modes. Performance of the UBR-A system was sensitive to R, particularly in suspended-growth. The denitrification efficiency (ηD) and nitrate removal rates in this mode were significantly lower (p < 0.05) at R= 4 compared to R= 3 with both real DFW and SWW. With DFW, this was observed despite a higher COD/NO3-N of 20–50. In integrated fixed-film, ηD at R= 4 significantly improved to ∼95 % with both feeds. Whereas a COD/NO3-N > 9 was required to offset the imbalance caused by increased NO3-N loading at R= 4. Modeling of the denitrification potential controlled by R (PD-IR) and COD availability (PD-COD) showed that denitrification reciprocated with PD-IR. From the multiple regression analysis, nitrate removal was linearly correlated with growth medium, R, and wastewater type (p < 0.00001) accounting for 43 % of the variation. Nitrification efficiency was consistently higher > 90 % under all conditions but specific nitrification rates nearly doubled in integrated fixed-film mode. Correspondingly, the total nitrogen removal increased from 67 % to 80 % in DFW-system and 59–84 % in SWW-system at R= 4 compared to suspended growth at the same R. The study results add to the understanding of integrated media in BNR processes for high-strength wastewater treatment.