Biotransformation of Trace Organic Compounds Via Microbial Ammonia Oxidation in Biofiltration Systems for Water Reuse

Background
The use of biofiltration for water reuse has recently been viewed as an attractive option due its low cost and ability to achieve high quality effluent (Gerrity et al., 2013). However, one challenge that is particularly problematic for biofiltration is the occurrence of trace organic compounds (TOrCs), such as pharmaceuticals and personal care products, in municipal wastewater (Lee et al., 2012). These TOrCs may be degraded by three ammonia oxidizing microorganisms (AOMs) including ammonia oxidizing bacteria (AOBs), ammonia oxidizing archaea (AOAs), and comammox that are found in biofiltration systems (Brown et al., 2015, 2018; Greenstein et al., 2018). Cometabolism, the fortuitous biodegradation of compounds by enzymes used for primary metabolism, allow AOMs to biodegrade some TOrCs, but the extent of degradation is dependent on chemical structure and composition of the microbial community (Zearley and Summers, 2012). This study focuses on removal of TOrCs via microbial ammonia oxidation by three types of biofiltration columns 1) a control column, fed only secondary effluent, 2) a column fed secondary effluent with additional ammonia, and 3) a column fed secondary effluent with a low dose of monochloramine.
Objectives 1. Investigate the impact of additional ammonia dosing on the degradation of TOrCs. 2. Examine the impact of chloramine dosing on AOMs and the degradation of TOrCs. 3. Investigate the abundance of AOB, AOA, and comammox and their contribution to TOrC degradation. Status This research project was completed in July 2021. A full manuscript with all the details will be submitted upon acceptance of the abstract. Methodology Experimental setup and operation Three parallel continuous downflow columns (PVC, 1' ID x 37' height) containing 10+ year-old exhausted GAC (Filtrasorb F400) that is biological activated carbon (BAC) were operated for five months. Each column was operated with a target empty bed contact time (EBCT) of 10 minutes (flow rate = 45 mL/min). Secondary effluent from a water reclamation facility in Las Vegas, NV was used as the feed water for the BAC pilot. The treatment train for this facility is depicted in Figure 1 and the pilot is shown in Figure 2. Impact of additional ammonia on TOrC attenuation While a control column was fed only secondary effluent, another column was fed secondary effluent and continuously dosed with an ammonia solution inline to achieve a dose of 2 mg N/L of additional ammonia. Over the course of the study the feed water was analyzed weekly for ammonia, nitrate, and nitrite while biofilter effluents were analyzed weekly for ammonia, nitrate, nitrite, and adenosine triphosphate (ATP). Monthly sampling included pH and temperature for the feed water as well as a suite of 17 TOrCs for both feed water and BAC effluent. Impact of monochloramine dosing on TOrC attenuation The final BAC column was fed secondary effluent and continuously sequentially dosed with chlorine and ammonia solutions inline to achieve a dose of 0.3 mg Cl2/L of monochloramine. Over the five-month study, this biofilter effluent was analyzed for the same constituents as the control and ammonia dosed columns but with free/total chlorine. Abundance of AOB, AOA, and comammox and their impacts on TOrC attenuation Samples of BAC media were collected monthly from two depths of all three columns so that AOB, AOA, and comammox DNA could be extracted, visualized, and finally quantified with quantitative polymerase chain reaction (qPCR). Using this method, relative abundances of each AOM was compared between the three columns. Results Impact of additional ammonia on TOrC attenuation The ammonia dosed column experienced nitrification with an average of 67% removal of ammonia. The ammonia dosed column heavily favored relative AOB abundance over comammox that dominated the control column. The addition of ammonia negatively impacted TOrC attenuation as the control column exhibited higher removals of all TOrCs. Nitrogen concentrations are shown in Table 1, while TOrC and microbial results are presented in Figures 3 and 4, respectively. Impact of monochloramine dosing on TOrC attenuation The chloraminated column was operated with essentially no free ammonia (<0.1 mg N/L) and therefore no significant nitrification or denitrification occurred. The microbial analysis revealed a comammox dominated environment similar to that of the control column with the main difference being that the control favored AOB abundance more as the depth increased while the chloramine column was more consistent. The chloraminated column did outperform the control column in the attenuation of some highly biodegradable TOrCs, but not significantly. Additionally, the removal of moderate and low biodegradability TOrCs was not improved. Abundance of AOB, AOA, and comammox and their impacts on TOrC attenuation The results of the PCR on the media sampling show that the control column was heavily dominated by comammox in the top sampling depth, but relative AOB abundance increased in the bottom sampling depth. The ammonia dosed column on the other hand was dominated by AOBs in both depths while the chloraminated column was heavily dominated by comammox in both depths.
Discussion The results of this study found that additional ammonia increases relative abundance of AOBs compared to comammox but performs worse than the control and chloramine dosed columns in TOrC reduction. Additionally, although the relative AOB abundance increased, the combined relative AOM abundance was actually less than the control and chloramine dosed BAFs which both had far higher comammox abundances. The addition of monochloramine did increase highly biodegradable TOrC removal but not moderate and low biodegradability TOrC removal. Comammox were more abundant in the bottom depth than the control; however, all AOMs were actually relatively more abundant or unchanged in the top depth of the media compared to the control and ammonia dosed columns. The comammox dominated the control and chloraminated BAFs, but not the ammonia BAF which was AOB dominated. The nature of this study makes it difficult to draw concrete conclusions on which AOM provides the best TOrC removal; however, the comammox dominated the control and chloraminated columns which performed the best TOrC removal indicating that comammox likely provide the best TOrC attenuation.
Significance of the Study Biofiltration is an important process for water reuse treatment trains and therefore optimal operation conditions should be realized. The finding of this study indicate that the presence of ammonia in the biofiltration process leads to decreased TOrC attenuation whereas the addition of monochloramine may provide some benefit.