Trace organic contaminant degradation by isolated bacteria bioaugmented into lab-scale reactors and identification of associated degradation genes

Abstract

Discharge of trace organic contaminants (TOrCs) with wastewater treatment plant (WWTP) effluents is a surface water quality concern due to their potentially negative effects on aquatic life. TOrCs are currently partially removed during wastewater treatment, though new technologies will be needed if increasingly lower discharge levels are to be achieved. Additionally, removal of TOrCs by sorption to solids is not desirable due to societal concerns about their presence in biosolids has increased substantially in recent years. TOrCs are biologically removed during wastewater treatment and complete mineralization of TOrCs is possible. Therefore, this study hypothesized that Enhanced Biological Trace Organic Contaminant Removal (EBTCR) can be achieved through continuous bioaugmentation with TOrC-degrading bacteria. Eleven bacteria capable of degrading various TOrCs were isolated from activated sludge, including the first isolated bacteria known to degrade gemfibrozil. The bacteria were characterized for their ability to function under conditions that might be encountered during bioaugmentation in a WWTP. Eight of the isolated bacteria were capable of degrading the TOrCs to below concentrations thought to cause adverse environmental impacts (i.e. low ng/L concentrations). The bacteria grew on a variety different carbon sources, with most bacteria growing best on protein-rich substrates. This suggests that commonly available materials could be used to grow the bacteria on-site at a WWTP. The bacteria maintained their ability to degrade the contaminant after being grown in the absence of the TOrC, as would be necessary when implementing continuous bioaugmentation at full-scale WWTPs. Eight of bacteria also degraded their TOrC before measurable growth, or during early growth of the bacteria in a nutrient-rich media. This was important because during continuous bioaugmentation the bacteria will bioaugmented in the activated sludge portion of a wastewater treatment, which has many carbon sources at much higher concentrations than the TOrC. Finally, seven of the isolated bacteria were determined to degrade the TOrCs at a rate that predicted potential for improved contaminant removal without increasing the activated sludge biomass by more than 10%. Continuous bioaugmentation of five bacteria was modeled in two full-scale activated sludge processes, a completely mixed activated sludge (CMAS) process and a 4-stage activated sludge process. The model found that continuous bioaugmentation for EBTCR was a potential solution for all but one of the modeled bacteria in the 4-stage activated sludge process, and was not feasible for a CMAS except with bioaugmentation of one of the isolated bacteria. Next, continuous bioaugmentation for EBTCR was tested in lab-scale sequencing batch reactors with an isolated BPA-degrading bacterium, Sphingobium sp. BiD32. This study found that daily bioaugmentation improved the BPA degradation rates, concentrations in the effluent, and concentrations in the waste solids. This study also found that the enhanced BPA removal was lost with time from bioaugmentation, demonstrating that continuous bioaugmentation can be used to overcome bacterial losses as a way to maintain predictably low TOrCs effluent concentrations. As a part of this study, proteins were identified that are likely involved in the degradation of TOrCs by isolated bacteria. Identification of these proteins and genes can assist in bioaugmentation monitoring by allowing for the production of biomarkers to monitor degradation genes in WWTPs rather than the augmented bacteria specifically. Also, if the gene is known, it can be searched for in other bacteria as a way to identify other similar bacteria capable of degrading TOrCs more rapidly than through the enrichment process. Using genomics, proteomics, and metabolomics, a novel p-hydroxybenzoate hydroxylase enzyme and genes previously identified to be involved in protocatechuate degradation were hypothesized to be involved in BPA degradation by Sphingobium sp. BiD32. These methods were also used to identify proteins involved in 17β-estradiol (E2) degradation by Rhodococcus sp. EsD8, confirming the involvement of proteins related to 3-beta-hydroxysteroid dehydrogenase and estradiol 17beta-dehydrogenase. These findings have identified genes involved in BPA and E2 degradation, which will allow for the creation of qPCR primers to monitor these genes in future studies. This study demonstrated the potential of continuous bioaugmentation to achieve EBTCR. Bioaugmentation may also be applicable for a variety of other waste streams, including hospital waste, pharmaceutical manufacturing waste, landfill leachate, and prior to reverse osmosis for direct potable reuse. Future work should focus improving the bioaugmentation process. This includes continuous bioaugmentation studies at the pilot-scale with automated bioaugmentation and a continuously stirred tank reactor design instead of a sequencing batch reactor. This will help demonstrate its applicability to full-scale systems. TOrC degradation genes should also be confirmed and used to monitor bacteria augmented into the pilot-scale system. This will allow for monitoring of the TOrC degradation activity, instead of just the bacterial signature. This study identified seven bacteria meeting the bioaugmentation criteria and demonstrated improved TOrCs removal with bioaugmentation.