메틸디에탄올아민 이용 미생물 분리 및 메틸디에탄올아민 함유 폐수의 생물학적 처리: 생장역학 및 실규모 적용 연구

Abstract

Methyldiethanolamine (MDEA) is commonly used in refineries and gas plants to eliminate acid gas impurities from natural gas and liquid hydrocarbon streams However, the process inevitably generates MDEA-rich wastewater; it wastewater flows into existing wastewater treatment plants and is treated together with other wastewater. However, MDEA is difficult to degrade and is known as a toxicant, and can affect the process efficiency of the plants. Previous studies have reported the biodegradability of MDEA, but few studies combine information and biokinetic modeling on microorganisms that degrade them. Thus, in this study, isolation of MDEA-utilizing bacteria and biological treatment of MDEA-containing wastewater by using biokinetics and field application were examined. In the first chapter, two species that can use MDEA as a carbon source were isolated from a full-scale wastewater treatment plant that treats petroleum-refining wastewater (PRW). One (M1) of the two isolated species had a 16S rRNA sequence that was 99.6% identical to Paenarthrobacter ureafaciens, and the other (M2) had 100% sequence identity to Achromobacter ruhlandii; the isolates were assigned to these species. The abilities of these species to degrade MDEA were validated by confirming their population growth, and by an MDEA-degradation test. Response surface analysis (RSA) was conducted to investigate optimal growth conditions of these bacteria. P. ureafaciens had an estimated maximum substrate-consumption rate kmax = 0.323/d at pH = 7.73, T = 25.0 ℃, and maximum specific growth rate µmax = 0.456 OD600/d at pH = 7.48, T = 23.6 ℃. A. ruhlandii had estimated kmax = 0.333/d at pH = 7.48, T = 23.8 ℃, and µmax = 0.437 OD600/d at pH = 7.50, T = 25.0 ℃. For quantification of target species, specific primers and probe sets were also designed for the detection of these species. In the second chapter, the biokinetic parameters of the two species were estimated using the Monod model. P. ureafaciens had μmax = 0.497/d and half-saturation coefficients KS = 3,007 MDEA/L, and A. ruhlandii had max = 0.451/d and KS = 2359 mg MDEA/L. P. ureafaciens had microbial yield coefficient Y = 0.426 mg VSS·(mg MDEA)-1 and biomass decay rate coefficients kd = 0.011/d. A. ruhlandii had Y = 0.439 mg VSS/(mg MDEA)-1 and kd = 0.008/d. These coefficients were used to predict the residual MDEA concentration ([MDEA]) and microbial populations at various hydraulic retention times (HRTs) under continuous conditions. P. ureafaciens and A. ruhlandii were predicted to wash out at HRT = 3.3 and 3.4 d, respectively, such that residual [MDEA] would increase continuously to an influent concentration of 5,000 mg/L. Effects of various environmental factors such as influent concentration, flow rate, and effluent concentration were also predicted from simulation results and compared with actual experimental values in continuous system. As a result of model validation, the MDEA-removal efficiency after 30 d in two reactors treated with 5,000 mg MDEA/L was 71.4% and 73.4%, respectively, which was about 10% different from the predicted value but showed a similar pattern. The third chapter mainly considered full-scale plants. First, the temporal variation in microbial communities of a full-scale activated sludge process (ASP) treating PRW was investigated for six months, and the microbial dynamics were linked to the influent characteristics of PRW that includes toxic organic compounds. The PRW contained benzene and phenol as major toxic pollutants (~70% of SCOD); 100% of the benzene and phenol were removed during the ASP, despite the large change in the composition of influent. More than 30 microbial genera were identified by metagenomic analysis using ion-semiconductor sequencing. Among them, genera Sulfuritalea (11.9 ± 3.5%), Ottowia (4.3 ± 2.2%), Thauera (3.1 ± 7.2%), and Hyphomicrobium (1.3 ± 0.7%) were predominant and important bacterial genera that may be responsible for the degradation of aromatic compounds such as benzene and phenol in PRW. Considering the results of monitoring the microbial community structure, activated sludge (AS) was used in the sequential batch reactions for MDEA treatment using mixed culture for five cycles sequential batch reaction to treat 1 g MDEA/L ≤ [MDEA] ≤ 5 g MDEA/L. MDEA was completely degraded in all five-cycle reactions. P. ureafaciens became the most dominant species by the 2nd sequential batch: its relative abundance increased from 0.7% in the seed source to 27.6%; this result supports our isolation results that this is one of the species that degrades MDEA. Starting with the 3rd batch, the populations of various other bacterial species increased; this result suggests that several microorganisms can treat MDEA in a full-scale WWTP, and that high concentration MDEA can be treated in full-scale plants if these microorganisms are used as mixed culture for sequential batch reaction. Lastly, research was conducted to apply anaerobic digestion (AD) to sludge generated after water treatment and to improve degradation by microwave (MW) pretreatment (0 to 16 min, 4-min interval). The SCOD increased from 1.3 to 50.4% in the non-microwaved samples (MW0). When the duration DMW of MW pretreatment was 16 min, methane yield was 4.6 times higher than in the MW0. MW pretreatment caused considerable changes observed in the microbial community. In the bacterial community, Ruminococcaceae and Lentimicrobiaceae were enriched in MW-pretreated samples whereas Enterobacteriaceae, Porticoccaceae, and Shewanellaceae dramatically decreased. In the archaeal community, genus Methanosaeta (70.5 ± 3.4%) was the dominant methanogen in all samples. Results obtained from this study on the application of microbial isolation and biokinetic modeling can provide useful information the help understand MDEA biodegradation and to predict and improve the efficiency of biological treatment of MDEA on an industrial scale. In addition, this confirmation of the feasibility of the anaerobic digestion of activated sludge generated from a full-scale WWTP and the improvement of the sludge hydrolysis efficiency by MW pretreatment, suggest that the final energy conversion of organic waste is possible in the future.