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Nitrification Resilience Enhanced Under Dynamic Loading Using MABR
Dwight Houweling, Amit Kaldate
SUEZ Water Technologies & Solutions, Canada;
The membrane aerated biofilm reactor (MABR) is an innovative technology for activated sludge plant upgrades that employs a gas permeable media to deliver oxygen to a biofilm. Experience shows the process can achieve oxygen transfer rates of 8 to 12 g/m2/d at field oxygen transfer efficiencies of 30 to 60 percent. Due to the unique “counter-diffusional” properties of the MABR biofilm, oxygen transfer is primarily directed to nitrification when the MABR is located at the upstream end of an activated sludge bioreactor. In contrast, conventional "co-diffusional" biofilms typically require removal of BOD before a nitrifying biofilm can be established.
One of the interesting aspects of MABR is that exhaust gas is captured and can be measured using an on-line O2 sensor. When plotted against online measurements of ammonia, the trends in “exhaust O2” show kinetics that are primarly ammonia-limited: oxygen transfer is highest when ammonia is high and lowest when ammonia is low. What this means is that the MABR biofilm has a built-in form of ammonia based aeration control (ABAC). This provides a natural degree of load balancing: the biofilm removes less ammonia when the process is underloaded and more ammonia when the downstream mixed liquor would otherwise be overloaded.
To illustrate this, field data from a full-scale activated sludge plant retrofit with ZeeLung cassettes (MABR/AS) will be presented to demonstrate a ±30% variation in oxygen transfer and nitrification in the biofilm in response to a ±40% loading variation. The full paper will explore the benefits of this natural load balancing to limit diurnal breakthrough of ammonia in the final effluent of a MABR/AS process. Combined with field data, process model simulation results will be presented to show performance under different stress conditions of the activated sludge process with and without MABR cassettes. Optimal balancing of nitrification in the biofilm and mixed liquor will be discussed in the context of intensifying process loading while maintaining resilience to dynamic loading events.
11:15am - 12:00pm
The Role of Solids Retention Time on the Fate of Trace Organic Compounds and Antibiotic Resistance
Majid Neyestani1, Daniel Gerrity2
1Carollo Engineers, Inc.; 2University of Nevada, Las Vegas;
Solids retention time (SRT) is one of the most important factors in designing and operating activated sludge systems for biological wastewater treatment. In particular, longer SRTs have shown to alter the structure and function of the microbial community, thereby more efficiently treating bulk and trace organics, completing nutrient removal, and preventing membrane fouling. However, longer SRTs also contribute to the proliferation of antibiotic resistance (AR), which is now considered a contaminant of emerging concern (CEC) that poses a threat to public health. This study’s main goal was to characterize the effect of varying SRTs on the fate of trace organic compounds (TOrCs) and AR in the biological wastewater treatment. To monitor TOrCs concentrations and AR as a function of SRT, four parallel, laboratory-scale sequencing batch reactors (SBRs) were designed to mimic activated sludge systems. The acrylic SBRs were initially seeded with return activated sludge (RAS) and fed with primary effluent from a full-scale municipal wastewater treatment plant. In the experiment, SRTs of 2 days, 7 days (in duplicate), and 20 days were targeted, while hydraulic retention time was held constant at 6.5 hours. A suite of indicator TOrCs was then selected and quantified by liquid chromatography tandem mass spectrometry (LC-MS/MS). Finally, bacteria resistant to ampicillin, tetracycline, trimethoprim, sulfamethoxazole, and vancomycin were isolated using the spread plate technique and monitored before and after each SBR. Results demonstrated that longer SRTs significantly improved effluent water quality and TOrC removal, but often increased the relative prevalence of antibiotic resistance.