KINETCIS AND NITRIFYING POPULATIONS IN NITROGEN REMOVAL PROCESSES AT A FULL-SCALE INTEGRATED FIXED-FILM ACTIVATED SLUDGE (IFAS) PLANT

This study evaluated the nitrification rates and kinetics in an Integrated Fixed-Film Activated Sludge (IFAS) plant with two-stage nitrification process at Broomfield, CO. Batch tests were conducted at site with both fixed-film media and suspended mixed liquor from the two nitrification stages. The nitrification rates for mixed liquor only was 2.64 for stage 1 and 2.0 for stage 2 in unit of mgN/gTS/h. The rate for media was 5.89 mgN/gTS-hr for stage 1 and 2.36 mgN/gTS-hr for stage 2. Overall nitrification rate tests with both suspended mixed liquor and media at 40% fill yielded rates of 4.77 mgN/gTS-hr for stage 1 and 2.3 mgN/gTS-hr for stage 2, which agree with the values calculated based the rates obtained with mixed liquor or media alone. The results showed that the nitrification mainly occurs on the media-biofilm and incorporation of the media in the hybrid system increased the nitrification capacity by about 155% for Stage #1 and by 25% for Stage #2. The overall nitrification rate for the stage #1 was 1.12 gNOX-N/m2/d at 20°C, which agrees with values previously reported. The nitrification rate in stage 2 was 0.28 gNOX-N/m2/d at 20°C and the very low residual ammonia concentration in stage 2 seems to limit the nitrification rates and yield lower nitrifying biomass. This indicates that the benefits of maintaining higher nitrification activity on the fixed-film starts to diminish with increasing stages of reactor. The observed denitrification rates for raw wastewater ranged from 3.4–5.46 mgNOX-N/gVSS*h at COD of 10 to 50mg/l. When fed acetate, value up to 9.3 mgNOX-N/gVSS*h were found at COD of 90 mg/L. The in situdenitrification rate calculated based on full-scale plant data was 7.28 mgNOX-N/gVSS*h and this value is higher in comparison to similar study conducted with mixed liquor from conventional AS plant as a result of the much shorter SRT (4 days on average) in the suspended mixed liquor in the IFAS system where most of the denitrifiers likely reside. Identification and quantification the nitrifying populations and their distribution on the fixed-film and in the suspended activated sludge were also assessed using quantitative fluorescence in situ hybridization (FISH). The results showed that most AOB nitrifiers are attached to the media and they belong to the Nitrosomonas europaea/eutropha/halophila and the Nitrosomonas oligotrophalineages. NOB were rather abundant in the nitrifying part of the biofilm (10–20% of the total population), and identified as belonging to the Nitrospirasp. NOB of the genus Nitrobactercould not be detected. Only few of both AOB and NOB were detected in the mixed liquor, mostly as very small aggregates, indicating, as the results of the batch test suggested, that nitrification mainly occurs on the media and that the nitrifiers in the mixed liquor are originated from the biofilm by detachment.