Optimization and Control of Intermittent UASB Reactors and Microbial Population Dynamics

Principal Investigator of the IST team: Jorge H. Leitão

Contract: PTDC/AMB/65025/2006

Start date: 01.01.2008

Duration: 36 months

 Sludge bed reactors (UASB reactors and more recently EGSB reactors) are an anaerobic technology with high potential for the treatment of various kinds of wastewaters. Yet their application for the treatment of complex fat containing effluents (e.g. dairy effluents) has some drawbacks as for instance scum forming, sludge degranulation, biomass wash-out and loss of biological activity. It is well known that this difficulties can be ascribed to the presence of complex substrates, proteins and especially fats, that accumulate in the biomass forming a film of organic matter which, besides imposing mass transfer limitations on the process kinetics, also causes sludge flotation and biomass wash-out. It is also known that the fats and long chain fatty acids (LCFA) present in dairy effluents may inhibit the anaerobic process resulting in the need of a highly adapted population for the degradation of these substrates. The solution to overcome these problems may be an intermittent operation mode that consists in interrupting the feed to the reactor for a certain period of time (stabilisation period) allowing for the biological degradation of the accumulated organic matter having slower degrading rates. It is also known that the granular sludge is predominantly methanogenic whilst in flocculent biomass the prevailing species have higher hydrolytic and acidogenic activities. Since it is possible and quite successful to operate UASB reactors with flocculent sludge another strategy adopted in this investigation will be the use of flocculent biomass because it is more adequate for the first steps, the most problematic steps, in the degradation of complex substrates. Finally, some published results indicate that fats and some complex substrates are more readily degraded in the termophilic temperature as compared to the rates in mesophilic temperature. This project aims to the optimisation of the performance of UASB reactors with intermittent feed used for the treatment of complex wastewater. Therefore, several time lengths of the feed and the stabilisation periods will be studied, and also different temperature ranges in the stabilisation period as well as different recirculation ratios during the feed and/or the stabilisation periods, in order to establish the optimum operating conditions. The most important parameters to study will be the duration of the feed and the stabilisation periods in relation with the whole operating cycle (feed + stabilisation), the temperature range during the stabilisation, several recirculation flows and the applicable loads. The effect of these parameters on the biological removal, in the acidification and the conversion to methane of the removed organic matter will be used as optimisation criteria. Simultaneously, the microbial populations developed in the intermittent UASB reactors will be studied in batch reactors to assess their capacity for the degradation of several substrates. The microbial populations will be characterized by molecular techniques based on the determination of the nucleotide sequences of genes encoding 16S RNA. Their metabolic profiles will be determined based on biochemical experiments. These approaches will allow the identification and quantification of the microbial species of relevance for the degradation of complex substrates. Results from 16S RNA-encoding genes sequencing will also be used to develop new probes which allow the application of FISH (Fluorescence in situ hybridization) techniques for controlling of this type of reactors. The obtained results will provide the establishment of correlations between the populations dynamics and reactor performance thus permitting their better optimisation and control both in start-up and in long tem operation. The results of this project will also enable the analysis of whether the improved performance of intermittent reactors as compared to continuous reactors, observed in previous studies, is due to an adaptation of the anaerobic biomass to the conditions prevailing in the stabilisation period, which if confirmed will establish intermittent operation as a way to optimise biomass adaptation.