Evaluation of Ceramic-GAC Dual Media Biofiltration for Enhanced Removal of Selected Micropollutants for Water Reuse
Open AccessIncreasing worldwide water cycle contamination with thousands of micropollutants is one of the current critical environmental problems. Bioactive and persistent in the water systems, these micropollutants can cause various health problems even at very low concentrations. Hence, this study focused on identifying strategies to remove selected micropollutants in ceramic-GAC dual media biofilters. As a proof-of-concept study, biodegradation rate and behavior of three selected compounds – salicylic acid, acetaminophen, and ibuprofen -- was examined by the bacteria from backwash seed collected from a full-scale biofiltration plant in Northern Virginia at different seasons. These compounds were chosen due to their predominant occurrence in water bodies, their consideration as micropollutants, and their differing biodegradability. Based on the results of the biodegradation study, further research was conducted to investigate the role of biofilm thickness on the biodegradation of the selected micropollutants, considering their slow and fast biodegradation nature. The next phase of the study was conducted at pilot scale, where the objective was to manage and control the growth of thick and thin biofilms in the ceramic-GAC dual media biofilter to improve the biodegradation of selected slow and fast biodegrading micropollutants. At the final stage, the study aimed to evaluate several enhancement strategies to improve the biodegradation and removal of micropollutants in ceramic-GAC dual-media biofilters.Biodegradation experiments were conducted using backwash water seed collected at four different seasons – summer, early fall, late fall, and winter with the ambient temperature of 32-33 ºC, 27-28 ºC, 21-22 ºC, and 7-8 ºC, respectively. The backwash water temperature during summer, early fall, late fall and winter was 32.4 ºC, 27.1 ºC, 21.2 ºC and 8.1 ºC, respectively. The results showed that salicylic acid was biodegraded entirely in 27 to 66 h, depending on the season, with the fastest degradation using the summer (32-33 ºC) backwash water microbial seed. Unlike salicylic acid, acetaminophen degradation showed a slower biodegradation rate requiring 225 h for 85-92% removal, while Ibuprofen degradation was the slowest among the three, requiring 230 h for only approximately 50% removal. For all three micropollutants studied, the biodegradation rate slowed as the seasons changed from early fall (27-28 ºC) to late fall (21-22 ºC), and finally was the slowest with bacteria collected in the winter (7-8 ºC). The study also found that biodegradation is related to the corresponding adenosine triphosphate (ATP), oxygen uptake rate (OUR), and extracellular polymeric substances (EPS). Further investigation of the role of biofilm thickness on the biodegradation of selected micropollutants were conducted using three different biofilm thicknesses for each selected micropollutant. Based on optical coherence tomography (OCT) analysis, biofilm thicknesses were 23.1 ± 8.3 µm, 87.0 ± 14.1 µm, and 154.8 ± 22.1 µm for salicylic acid biodegradation; 19.2 ± 5.1 µm, 71.1 ± 9.8 µm, and 158.9 ± 26.2 µm for acetaminophen biodegradation; and 29.6 ± 4.7 µm, 69.1 ± 25.8 µm, and 187.9 ± 53.1 µm for ibuprofen biodegradation. Studies found that the thinnest (23.1 ± 8.3 µm) biofilms achieved 100% salicylic acid removal from the initial concentration of 10 µg/L in 8 h, whereas the thickest (154.8 ± 22.1 µm) one required 13.25 h. Acetaminophen and ibuprofen were biodegraded faster by the thinnest (19.2 ± 5.1 µm and 29.6 ± 4.7 µm, respectively) biofilms than the thickest (158.9 ± 26.2 µm and 187.9 ± 53.1 µm, respectively) ones. For acetaminophen, complete biodegradation required 14 h and 9 h by the thickest (158.9 ± 26.2 µm) and thinnest (19.2 ± 5.1 µm) biofilms from the initial concentration of 1 µg/L, respectively. Moreover, the thickest (187.9 ± 53.1 µm) and thinnest (29.6 ± 4.7 µm) biofilms completely degraded ibuprofen at 17.25 h and 10.75 h, respectively, from 1 µg/L initial concentration. Overall, the results suggested that micropollutants need thin biofilms to be biodegraded faster since they are not biomass limited. The study also found that biodegradation by thin and thick biofilms is correlated with the half-saturation coefficient (Ks), implying that the Ks can be lowered by making the biofilm thinner. The 16S rRNA gene sequencing results showed that microbial communities in the biofilms with specific micropollutants biodegrading bacteria were quite diverse. Regarding observed taxonomic units (OTUs), 47 OTUs were observed in the backwash water while in the salicylic acid, acetaminophen, and ibuprofen degrading biofilms, observed OTUs were 144, 135, and 137, respectively. Bacillus had the most abundance in the backwash water seed, with an absolute dominance of 98.2%. Salicylic acid degrading biofilms had three major dominant genera, including Rhodoferax (21.7%), Polaromonas (21.4%), and Undibacterium (16.6%). Pseudomonas was present in high proportions (62.9%) in acetaminophen degrading biofilm samples. However, ibuprofen degrading biofilms showed Sediminibacterium as the most abundant genus comprising 28.5% of total species, which was not found in salicylic acid and acetaminophen degrading biofilms. The pilot ceramic-GAC dual media biofiltration column study showed that it was possible to grow thin biofilms in the top ceramic media and thick biofilms in the bottom GAC media, using more frequent air scouring on ceramic and less frequent backwashing on GAC. The study found significant differences in the percent removal of salicylic acid, acetaminophen, and ibuprofen at different empty bed contact times (EBCTs), while the optimum was 15 minutes. The results also confirmed that increasing ozone dose provided increased removal of salicylic acid, acetaminophen, ibuprofen, and DOC with the optimum O3 to DOC ratio of 0.5 and 0.67. The microbial communities in GAC and ceramic media were characterized by the species diversity (Shannon diversity index) of 4.0 and 2.3, respectively, where 227 OTUs were observed in GAC and 150 OTUs in ceramic media. GAC has an array of different bacterial genera including Corynebacterium (13.1%), Acidovorax (10.2%), Staphylococcus (4.5%), Serratia (3.8%), Arthrobacter (2.8%), Dietzia (2.9%), Lactobacillus (2.9%), Paracoccus (2.4%), and Turicibacter (2.6%). However, in the ceramic media, the dominant genera found were Cupriavidus (29.6%), Arthrobacter (25.4%), Aquabacter (14.4%), Burkholderia (9.5%), Novosphingobium (4.3%), and Xanthobacter (3.4%). Enhancement strategies applied in this study included substrate, nutrient, and oxidant enhancement. Among all the strategies used, nutrient and oxidant enhancement was the most promising for ceramic-GAC dual media biofiltration. H2O2 addition improved salicylic acid removal by 11%, however, no significant differences were observed for acetaminophen and ibuprofen removal. Nutrient and oxidant enhancement decreased biofilter EPS production by 18.5% to 19.6%, and 19.9% to 21.4%, respectively that was reflected by less headloss buildup in the ceramic-GAC biofilter. Substrate enhancement with ethanol and acetic acid was found ineffective in this study regarding either improved micropollutants removal or the biofilter hydraulic properties.
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