Thermal pre-treatment of primary and secondary sludge at 70ºC prior to anaerobic digestion

In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared tothermophilic digestion, mainly because of the lower energy requirements and higher stability of the process. However, the thermophilic anaerobic digestion process is usually characterised by accelerated biochemicalreactions and higher growth rate of microorganisms resulting in an increased methanogenic potential atlower hydraulic retention times. Furthermore, thermal pre-treatment is suitable for the improvement ofstabilization and could be realized at relatively low cost especially at low temperatures. The present studyinvestigates the effect of the pre-treatment at 70 °C on thermophilic (55 °C) anaerobic digestion of primaryand secondary sludge in continuously operated digesters. Thermal pre-treatment of primary and secondarysludge at 70 °C enhanced the removal of organic matter and the methane production during the subsequentanaerobic digestion step at 55 °C. It also greatly contributed to the destruction of pathogens present inprimary sludge. Finally it results in enhanced microbial activities of the subsequent anaerobic stepsuggesting that the same efficiencies in organic matter removal and methane recovery could be obtained atlower HRTs

Controlling the microbial competition between hydrogenotrophic methanogens and homoacetogens using mass transfer and thermodynamic constraints

International audienceThe reduction of CO2 allows the synthesis of platform molecules for the chemical and energy industry. Anaerobicmicrobial consortia contain homoacetogenic microorganisms (HAC) capable of reducing CO2 to acetate. How-ever, one of the obstacles to their use is the understanding and management of their functional diversity. Inparticular, managing the competition between HAC and hydrogenotrophic methanogens (HM) that convert CO2into methane is crucial to selectively produce acetate.This study contributes to bring new knowledge on the competition between HAC and HM. This microbialcompetition is encountered in numerous anaerobic systems, and it is necessary to know how to manage it. In thissense, mass transfer between the gas phase where the substrates are located, and the liquid phase which containsthe microbial catalysts, as well as kinetic and thermodynamic aspects of biological reactions have been inte-grated in this work. The microbial competition between HM and HAC was studied in successive batches. Theeffect of temperature between 25 ◦C and 35 ◦C was investigated, as well as different states of mass transferlimitation in the system.A clear effect of temperature between 25 ◦C and 35 ◦C on the outcome of the competition between HM andHAC was highlighted, as well as the effect of mass transfer limitation. Enrichment of Acetobacterium homoace-togens and elimination of hydrogenotrophic methanogens was possible at 25 ◦C without mass transfer limitation.Acetate product selectivity was of 100% by the end of the enrichment period in successive batches. On thecontrary, under mass transfer limitation, and/or at 35 ◦C, Methanobacterium hydrogenotrophic methanogenswere promoted with 100% of methane selectivity by the end of the enrichment. This study contributes to identifyspecific process parameters influencing the selection of HAC over HM and should help in the design of experi-ments depending on the target product from H2/CO2. Furthermore, the results obtained could be applied tocontinuous acetate producing systems, at a larger scale, and they can also be useful in other gas fermentationsystems and processes

Análise comparativa dos efeitos da carga orgânica e do tempo de detenção hidráulica na digestão anaeróbia mesofílica de lodo adensado de estação de tratamento de esgoto

RESUMO Esta pesquisa comparou o desempenho de um digestor anaeróbio de lodo sob diferentes estratégias operacionais. Foi avaliada a influência do aumento da carga orgânica volumétrica (COV) (OP I) e o efeito da redução do tempo de detenção hidráulica (TDH) (OP II e OP III) no processo anaeróbio. As cargas aplicadas variaram entre 0,5 e 4,5 kgSV.m-3.d-1 e o TDH foi reduzido de 15 a 5 dias. Produção de gás metano, degradação do material orgânico e a diversidade microbiana foram utilizadas para medição e comparação do desempenho do processo. Foram necessários períodos de aclimatação a cada nova COV aplicada o que levou às instabilidades na remoção de SV e DQO do lodo. A operação com TDH entre 7 e 5 dias apresentou as maiores eficiências de remoção de SV, superiores a 70%, o que influenciou positivamente na estabilidade do processo. As COV aplicadas de 2,5 e 3,5 kgSV.m-3.d-1 resultaram nas maiores produções de metano durante a OP I. Para TDH inferiores a sete dias a produção de CH4 foi prejudicada apesar da existência de microorganismos metanogênicos atuantes no digestor. Comparativamente, a estratégia de redução do TDH resultou em um melhor desempenho do sistema que a fixação da COV. Quanto menor o TDH aplicado, melhor os resultados obtidos na operação do digestor, sugerindo que a eficiência do processo é otimizada em sistemas de alta carga com operação em baixos tempos de detenção hidráulica