The effect of hygienic treatment on the microbial flora of biowaste at biogas plants

In Sweden, full-scale, commercial biogas plants (BGP), which process low-risk animal waste, operate a separate pre-pasteurisation at 70 °C for 60 min as required by EEC regulation 1774/2002. The purpose of this study was to establish if, during pasteurisation and further processing and handling in full-scale BGPs, pathogens in biowaste could be sufficiently reduced to allow its use on arable land.

Four BGPs were sampled on six occasions during 1 year. Sampling was performed from six locations during biogas production. The samples being analysed quantitatively to detect indicator bacteria (Escherichia coli, Enterococcus spp. and coliforms) and spore-forming bacteria (Clostridium spp. and Bacillus spp.) and qualitatively for bacterial pathogens (salmonella, listeria, campylobacter and VTEC O157).

Salmonella was the most frequently isolated pathogen before pasteurisation In general, the treatment adequatly reduced both indicator and pathogenic bacteria. Spore-forming bacteria were not reduced. However, recontamination and regrowth of bacteria in biowaste was frequently noted after pasteurisation and digestion.     

Antifoaming effect of chemical compounds in manure biogas reactors

A precise and efficient antifoaming control strategy in bioprocesses is a challenging task as foaming is a very complex phenomenon. Nevertheless, foam control is necessary, as foam is a major operational problem in biogas reactors. In the present study, the effect of 14 chemical compounds on foam reduction was evaluated at concentration of 0.05%, 0.1% and 0.5% v/v_sample, in raw and digested manure. Moreover, two antifoam injection methods were compared for foam reduction efficiency. Natural oils (rapeseed and sunflower oil), fatty acids (oleic, octanoic and derivative of natural fatty acids), siloxanes (polydimethylsiloxane) and ester (tributylphosphate) were found to be the most efficient compounds to suppress foam. The efficiency of antifoamers was dependant on their physicochemical properties and greatly correlated to their chemical characteristics for dissolving foam. The antifoamers were more efficient in reducing foam when added directly into the liquid phase rather than added in the headspace of the reactor.     

Evaluation of molecular methods used for establishing the interactions and functions of microorganisms in anaerobic bioreactors

Molecular techniques have unveiled the complexity of the microbial consortium in anaerobic bioreactors and revealed the presence of several uncultivated species. This paper presents a review of the panoply of classical and recent molecular approaches and multivariate analyses that have been, or might be used to establish the interactions and functions of these anaerobic microorganisms. Most of the molecular approaches used so far are based on the analysis of small subunit ribosomal RNA but recent studies also use quantification of functional gene expressions. There are now several studies that have developed quantitative real-time PCR assays to investigate methanogens. With a view to improving the stability and performance of bioreactors, monitoring with molecular methods is also discussed. Advances in metagenomics and proteomics will lead to the development of promising lab-on chip technologies for cost-effective monitoring.