The fate of methanol in thermophilic-anaerobic environments

Methanol is a simple C1-compound, which sustains a complex web of possible degradation routes under anaerobic conditions. Methanol can be the main pollutant in some specific wastewaters, but it is also a compound that may be formed under natural conditions, as intermediate in the decomposition of organic matter. The information available in literature enables one to design a satisfactory application of a stable high-rate mesophilic-methanogenic reactor system, but the same does not apply for thermophilic conditions. The main objective of this thesis was to assess the feasibility of treating methanol-containing wastewater under thermophilic (55 ° C) conditions in a single-step UASB-reactor. The research was focused on the stability of the reactor performance and on the environmental factors that may play a role on the anaerobic conversion of methanol including pH, inorganic carbon and trace metals. Good reactor performance was achieved at organic loading rates (OLR) up to 47.3 gCOD.L -1 . d -1 in a bicarbonate buffered medium, with 93% of methanol removal where the major end product was methane. Moreover, the accumulation of volatile fatty acids (VFA), often reported as a drawback, was not detected. The assessed physical characteristics of the cultivated sludge showed that a good quality, well settleable granular sludge, was cultivated and retained in the reactor. Further, the stability of the system was studied. When the bicarbonate buffered-reactor was exposed to specific environmental stress situations (temperature drop, overloading and no feeding), the performance was temporarily affected during the shocks but the system promptly recovered, after the normal conditions were restored. On the other hand, when the methanol conversion was studied in a bicarbonate-deprived medium (either phosphate buffered or with automatic addition of NaOH, neutral pH range), the reactor performance was poor, and the system was quite sensitive to disturbances, even at low OLR. When phosphate was present in the medium, acetate accumulation manifested, indicating that phosphate inhibited the acetotrophic microorganisms present in the consortium. The cultivated thermophilic consortium showed to be sensitive to pH shocks, both acidic (pH 4) and alkaline (pH 9.5). A recovery of methanogenesis was not possible by simply restoring the reactor pH, besides, the addition of bicarbonate at this stage, stimulated the formation of acetate. A proposed reactor-recovery strategy, based on the stepwise addition of bicarbonate, however, was found to be very effective to recover methanogenesis from methanol from complete failure or reactor upset caused by pH shock, even in case where (homo)acetogens were outcompeting methanogens. To obtain an insight in the degradation pathway of methanol and better understanding of the influence of the parameters mentioned above, a detailed study using specific inhibitors, and nuclear magnetic resonance (NMR) technique was conducted. Results showed that about 50% of methanol was directly converted to methane by the methylotrophic methanogens and 50% via the intermediates H 2 /CO 2 and acetate. The results also indicated that inorganic carbon ( S ([HCO 3 - ] + [CO 2 ]) ) is required as "electron" (H 2 ) sink as well as cosubstrate for efficient and complete chemical oxygen demand (COD) removal. Furthermore, we studied the importance of cobalt to the thermophilic cultivated consortium in continuous experiments and in a cobalt-deprived enrichment culture. The cobalt requirement of our cultivated consortium was lower as compared to that of a mesophilic-methylotrophic consortium. For the cobalt-deprived enriched culture, 0.1 µM of cobalt was found to be the most appropriate concentration, leading to the highest methanol conversion rate with methane as sole end product from methanol.The information contained in this thesis enables a successful application of the UASB reactor for methanol-containing wastewaters under thermophilic conditions. For that purpose, it is also recommended the use of bicarbonate for the treatment of methanol-containing wastewater where the syntrophic conversion via H 2 /CO 2 is involved. Another important recommendation is that, to develop a balanced consortium with methane as the target end product, cobalt and bicarbonate should always be stepwise introduced to the system.