Biodegradation of dihydroxybenzenes (hydroquinone, catechol and resorcinol) by granules enriched with phenol in an aerobic granular sequencing batch reactor

During the last 20 years, intensive research in the field of wastewater treatment has been conducted to focus on aerobic granular sludge because of its efficiency for water purification. Previous researchers have studied the characterization of the granules and the granular sequencing batch reactor's (GSBR) applications. However, there is little research done on how enrichment with one compound influences the biodegradation of structurally similar compounds. In this research, phenol-enrichment was investigated as a potential method to improve the biodegradation of the three dihydroxybenzenes (hydroquinone, catechol and resorcinol), which are structurally similar to phenol and even share biodegradation pathway reactions. The objectives of this research were: (1) Cultivate stable aerobic phenol-degrading granular sludge using acetate-fed granules as microbial seed; (2) Compare the removal over one anaerobic/aerobic cycle of the three dihydroxybenzene compounds (hydroquinone, catechol and resorcinol) added individually by the phenol-enriched acetate-fed granules (PAFG) reactor and the non-phenol-enriched acetate-fed granules (NPAFG) reactor (control); (3) Compare the removal of phenol and the three dihydroxybenzene compounds (hydroquinone, catechol and resorcinol) added individually during the anaerobic phases and the aerobic phases; (4) Compare the removal of the dihydroxybenzenes when they are provided as a mixture to the PAFG and the NPAFG reactors; (5) Discuss implications for GSBR operation to treat aromatic chemicals. Two reactors, the PAFG reactor and the NPAFG (control) reactor, were created and used in this project. GSBRs are thought to require both anaerobic and aerobic phases (and phosphorous-accumulating organisms) in order to form granules and there are anaerobic pathways for degradation of phenolics, so to determine the possible significance of the anaerobic degradation one GSBR cycle included both anaerobic and aerobic phases in this study. Stable aerobic acetate-degrading granules were cultivated first using acetate-containing synthetic wastewater of 600 mg COD/L (chemical oxygen demand), resulting in two identical reactors, both with more than 90% acetate-removal and a 6-hour hydraulic retention time (HRT). Then the PAFG reactor was fed synthetic wastewater containing acetate and phenol at a total of 600 mg COD/L, while the NPAFG reactor was still fed with acetate-containing synthetic wastewater at 600 mg COD/L. In the PAFG reactor, phenol concentrations were gradually increased from 10 mg/L to 100 mg/L (246 mg COD/L). Removal tests of three dihydroxybenzenes were conducted individually at 25 mg/L, 50 mg/L, 75 mg/L and 100 mg/L and the removal test of a mixture of the four compounds stated above was conducted at 50 mg/L each. Samples from degradation experiments with phenol and dihydroxybenzenes were analyzed and quantified with high-performance liquid chromatography using a UV detector (HPLC-UV) to analyze phenol and the three dihydroxybenzenes and NanoDrop to analyze COD concentrations via colorimetric COD assays. After phenol-enrichment, in the PAFG reactor phenol removal percentages were more than 90% in a single cycle. Overall, of the three dihydroxybenzenes tested, hydroquinone was removed most effectively (80% -- 90% in a single cycle) in both the PAFG and the NPAFG reactors at all the four concentrations. There was 20% increase of the catechol removal in the PAFG reactor (80% removal on average) compared with the NPAFG reactor (60% removal on average). Resorcinol removal percentages were lowest overall but they showed the greatest improvement after phenol-enrichment. In the NPAFG reactor, only 20% of resorcinol was removed during one GSBR cycle, while in the PAFG reactor, around 60% of resorcinol was removed. During one cycle, most of the aromatic compounds were degraded in the aerobic phase (most rapidly during the first 40 minutes of the aerobic phase). The observed aerobic degradations of phenol, hydroquinone and catechol were consistent with several kinetics models (first-order, Monod or Haldane kinetics) and the best model cannot be chosen without follow-up studies with many more data points and a wider range of concentrations tested. Together, data from these experiments demonstrated that phenol-enrichment of GSBR granular sludge can improve the biodegradation of the three dihydroxybenzenes (hydroquinone, catechol and resorcinol), which can be potentially used in wastewater treatment.