Anaerobic Biodegradation of Long Chain Fatty Acids Biomethanisation of biomass-associated LCFA as a challenge for the anaerobic treatment of effluents with high lipid/LCFA content
Year: 2003
Type: dissertation
Abstract: This work was focused on the anaerobic biodegradation of Long Chain Fatty Acids, especially those that are associated to anaerobic sludge by mechanisms of adsorption, precipitation or entrapment. When continuously fed with oleic acid (EGSB reactors, influent concentrations between 2 and 8 g COD/l and HRT=1 day), suspended and granular anaerobic sludge accumulated palmitic acid. This LCFA was efficiently biomethanised in batch assays, but the addition of oleic acid inhibited this conversion. The application of stirring conditions (150 rpm) enhanced the biomethanisation rate, when compared to static conditions. The maximum plateau achieved in the methane production curve was considered an indirect measurement of the amount of accumulated LCFA and attained a value as high as 3272±877 mg COD-CH4/gVSS, for the suspended sludge, which was mineralised at a rate of 243±9 mg COD-CH4/(gVSS.day). The amount of accumulated palmitate and the rate of methane production were highly dependent on biomass structure, suspended sludge being more efficient than granular sludge for that purpose. The application of image analysis techniques gave important insights on relative importance of fragmentation and filament release during granular sludge deterioration, induced by the contact with oleic acid. The equivalent diameter of the aggregates larger than 1 mm increased with the increase on the amount of accumulated LCFA. After a threshold value of about 200 mg COD-CH4/gVSS the aggregates were mainly located in a top floating layer. In the granular sludge the amount of accumulated LCFA exponentially increased with the total free filament length, but exponentially decreased with the increase on the percentage of aggregates smaller than 1 mm, which evidenced the importance of filamentous bacteria on the process of LCFA accumulation/degradation and the relative irrelevance of the surface area for the phenomenon. In granular and suspended sludge, the composition of the bacterial community, based on 16S rDNA sequence diversity, was more affected during the oleate loading process than the archaeal consortium. Archaeal consortium remained rather stable in the granular sludge, whereas in the suspended sludge the relative abundance of Methanosaeta-like organisms became gradually weaker. The combination of molecular-based sludge characterisation with physiological and morphological characterisation provided complementary information on the mechanisms of sludge disintegration, flotation and washout, in relation with the contact/accumulation of LCFA. The specific methanogenic activity in the presence of individual substrates (acetate, propionate, butyrate, ethanol and H2/CO2) measured before and after the depletion of the accumulated LCFA (mainly palmitic) strongly contradicted the accepted theory of bactericidal effect of LCFA towards anaerobic consortia as well as their permanent toxic effect. Although the encapsulated sludge had only detectable methanogenic activity in H2/CO2 and ethanol (24±6 and 437±12 mg COD-CH4/(gVSS.day), respectively), after allowing the mineralisation of the accumulated LCFA, all the measured activities increased significantly, attaining values of 533±95, 16±4, 222±71, 67±1 and 2709±38 mg COD-CH4/(gVSS.day), for acetate, propionate, butyrate, ethanol and H2/CO2, respectively. Aceticlastic tolerance to LCFA toxicity and the capacity of LCFA biodegradation exhibited by the consortia were also improved after allowing the mineralisation of the biomass-associated LCFA. These results put in evidence the possible strong effect of diffusion limitations imposed by the LCFA layer that could hamper the access of the added substrates to the cells as well as the subsequent biogas release. This physical effect could easily mislead the data obtained in previous works that suggest severe conclusions about LCFA inhibitory effects. The methanisation kinetics of biomass-associated LCFA (mainly palmitic) was established in batch assays, according to an inhibition model based on Haldane’s enzymatic inhibition kinetics. An optimal value of 1080 mg COD-CH4/gVSS was determined for the amount of accumulated LCFA that could be mineralised at the maximal rate of 250 mg COD-CH4/(gVSS.day). Near this optimal specific concentration of LCFA, the effect of adding VFA to the medium was studied, in order to evaluate their interactions with the methanisation of the biomass-associated LCFA. Different patterns were obtained for each individual substrate. Although acetate and butyrate were preferentially consumed by the consortium, in the case of propionate no evidence of a sequential consumption pattern could be withdrawn. From a practical viewpoint, the results presented in this work suggest that, in order to enhance methane production, it should be advantageous to sequence cycles of adsorption/accumulation and degradation, when treating effluents with high lipid/LCFA content.
Author: M. A. Pereira
Language: English
Field: Energy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W92084111