Optimisation of propionibacterial ECP production and the influence of propionibacteria on the UASB granulation process

[摘要] ENGLISH ABSTRACT:The classical propionibacteria are used in a variety of natural dairy fermentationswhere they produce natural preservatives (propionic and acetic acids andbacteriocins) and large amounts of vitamin B12. The extracellular polysaccharide(ECP) producing ability of these bacteria also make them of special interest to thefood and waste water management industries as the ECP has been illustrated toplaya role in the initial granule formation in upflow anaerobic bioreactor systems.There is little known on the ECP production by propionibacteria and in thisstudy different environmental conditions that influence ECP production were studied.Nineteen different Propionibacterium strains were examined in terms of ECPproduction and Propionibacterium strain 278 was identified as the best ECPproducer. Further studies were only done on this strain because of its high ECPproduction and because it was originally isolated from an anaerobic digester. Theinfluence of temperature, pH and sucrose concentration was determined through themeasurement of ECP production and medium viscosity. It was found that more ECPwas produced at temperatures lower than the optimum for growth with the optimumbeing between 22° and 25°C. Lower initial pH conditions of the growth medium(below pH 7.0) were found to inhibit ECP production and the influence when theinitial pH values were between 7.0 and 8.5, was not significant. A higher carbon:nitrogen ratio, when 8% sucrose was added, was also found to enhance the ECPproduction.The upflow anaerobic sludge bed (UASB) bioreactor process depends on theupward movement of soluble matter through a blanket of active methanogenicgranular sludge. The long start-up times as a result of the slow granulation process,as well as the need for a speedy replacement of granules once they have beenwashed out of the system, are limitations that restrict the general application of thisexcellent waste water treatment technology. Full exploitation of this biomassimmobilisation technique can thus not be realised until the granule formationconditions are defined and optimised. The precise nature of the mechanismsinvolved in the formation of granules and the reason for their stability, is still not fullyunderstood. It was hypothised by Britz et al. in 1999 that, through theimplementation of environmental 'stress' conditions, a shift in the populationdynamics of the anaerobic community can be obtained. This results in a concurrentincrease in ECP formation that appears to enhance aggregate formation.In the second study it was found that, when 'stress' conditions were applied toalready formed granules, the Gram-positive lactate-utilising acidogenic populationgained an advantage and more propionic acid producing bacteria were present. Thepropionic and acetic acid concentrations were also found to increase, andconcurrently, a decrease in the growth medium pH occurred. This confirms part ofthe granulation hypothesis that, when granules are 'stressed', the acidogenicpopulation dynamics change and the lactate-utilising population responds to thegradual decrease in pH and the more acid-tolerant propionic acid producing bacteriagain a competitive advantage resulting in the increase in the propionic acidconcentration.When propionibacteria were added to raw sludge during the granuleproduction process, the granules were found to be more active than when nopropionibacteriahad been added. This was probably due to the ECP formation bythe propionibacteria that enhances the aggregation of the granules. Enhancedgranulation was thus found in the batch systems with the fatty acids formed incorrelation with the model for granulation. A good correlation was evident betweenthe hypothesis and the experimental data and the hypothesis was partially verified inthis study.

[发布机构] Stellenbosch University