Process Densification and Emerging Contaminant, PFAS and EE2, Management in Wastewater Treatment using Reactive Migrating Carriers

Abstract
In addressing stringent environmental regulations, aging wastewater treatment infrastructure, and heightened sustainability goals, advanced biological nutrient removal (BNR) systems have emerged as essential components for modern water reclamation facilities (WRFs). This abstract delves into the innovative application of reactive migrating biocarriers for process intensification to combat increased flows. The objective of this study was to utilize granular activated carbon as migrating carrier to simultaneous achieve nitrogen management and micropollutants removal. The model micropollutants used were short and long chain PFAS compounds, and one synthetic estrogen EE2. The findings outline operational strategy, key findings, challenges, and valuable lessons derived from this effort.

Background
There is an urgent need to increase the capacity of existing WWTPs and achieve process intensification through densification and efficient nitrogen management. Much of the infrastructure associated with WWTPs was built between the 1940s and 1970s. Most Wastewater Treatment Plants (WWTPs) in the U.S. need significant upgrades in terms of the treatment of wastewater to accommodate population growth (e.g., increased flows), industrialization, and more stringent effluent regulations for nitrogen management. There is an increase in concern for managing micropollutants, especially for treatment plants discharging into sensitive waterbodies. Expanding existing WWTPs through new construction is often not feasible due to space and budget limitations related to high infrastructure costs. We use granular activated carbon (GAC) as migrating reactive carriers to accomplish densification-intensification. The reactive or absorptive media such as cellulosic plant-based such as hemp or kenaf and granular activated carbon add to the removal mechanisms and are less expensive, and environmentally friendly compared to the use of plastic carriers. The use of migrating carrier allowed for SRT decoupling between slow (such as nitrifiers) and fast-growing organisms (thus enabling shorter floc-based SRT), as well as enabled the sorption of micropollutants to increase their localized concentrations.

Approach
Two identical lab-scale plexiglass SBRs (2H/D ratio, 3L working volume) were operated at room temperature (22±1oC) in Dr. Goel's lab. 2 g/L of GAC was added in RAC. Both RAC and RC were operated in an anaerobic-anoxic-oxic (A2O) process. Each cycle was comprised of 6-h, involving 9-min of anaerobic synthetic wastewater feeding, 51-min of anaerobic, 50-min anoxic (by dosing nitrate to mimic internal recycling), 229-min of aerobic, 20-min of settling, and 1-min of effluent discharge. The DO levels were maintained in the 0.4 to 0.6 mg/L range in oxic phase to achieve simultaneous nitrification and denitrification (SND) with P removal. The targeted feed concentrations of EE2 and each PFAS were 5 µg/L and 50 ng/L, respectively. The hydraulic retention time (HRT) was 10-h, and the solids retention time (SRT) was maintained at 10-d by wasting sludge through a 500 µm sieve.

Important findings and discussion
The migrating reactive carrier concept is fundamentally different from moving bed biofilm reactors (MBBR). Unlike inert plastic carriers, the reactive migrating carrier concept is a novel concept. In reactive migrating carriers, the media supporting biofilm is reactive (i.e., adsorptive). Reactive or absorptive carriers can interact with various dissolved contaminants, especially micropollutants.
- Both RC and RAC showed nearly similar removal efficiencies for organic carbon (represented by COD), ammonium, ortho-P, and total inorganic nitrogen (TIN). TIN and ortho-P removals signify the presence of active denitrification and enhanced bio-P removal (EBPR) activities in both reactors. A typical complete cycle analysis for both reactors in terms of nutrients, organic carbon and EE2 is shown in figure 1. GAC supplemented migrating carrier reactor had much higher granulation >500µm (nearly 87 % as compared to 52 % in the control based on volatile suspended solids) than in the conventional reactor.
- The short-chain PFAS compounds passed through both reactors without showing any removal (figure 2). However, nearly 40 to 50 % removal of long-chain PFAS compounds was recorded consistently in both reactors, with the GAC-supplemented reactor (red dots) showing consistently higher removal than the other reactor (blue dots) over more than 100 days of feeding. Similar observation for EE2. The primary removal mechanism of both types of micropollutants from the bulk liquid was adsorption on GAC and granules followed by potential biodegradation revealed by batch tests. The long-chain PFOA and PFOS concentrations in the biomass (including GAC biofilm) were 413.58±12.3 and 474.39±11.7 ng/g TSS in the RAC- and 31.2±8.8 and 36.3±3.9 ng/g TSS in the RC control SBR respectively. We also conducted a mass balance on long-chain PFAS in RAC based on concentrations in the influent, effluent and measured concentrations of both long-chain PFAS in the biomass. Based on this estimate, we concluded that although the immediate mechanism of removal is sorption to the biomass/biofilm, other processes, such as biodegradation, are also contributing to the disappearance of PFAS parent compounds.

Conclusion: The use of GAC intensify the process and micropollutant management is creative and presents the next generation of innovation in wastewater.