Effect of extra cellular polymeric substances (EPS) on change in viscosity on completely stirred tank reactors for biogas production, related to substrate mix and micro-flora composition

Time: 2009-2011

Financed by: Formas-Vinnova, Scandinavian Biogas Fuels AB

Project led by: Bo Svensson and Jörgen Ejlertsson

Researcher involved at WES: Annika Björn, Anna Karlsson

Abstract

In Sweden there is a potential to increase the biogas production by a more efficient utilization of existing biogas facilities (e.g. at waste water treatment plants). This means addition of new substrate materials to the plants. However, it may result in shifts in the viscosity of the process liquid and, hence, problems with inadequate mixing, break down of stirrers and foaming. The disturbances may seriously impact both the treatment of the organic material and gas-production contracted. In this context we analyze differences and shifts in the rheological characteristics of fluids for biogas production depending on substrates used and the micro-flora availble. Also, the research project implements an investigation on the reasons for the changing viscosity by addressing the occurrence and formation of EPS (extracellular polymeric substances). EPS are compounds excreted by microorganisms and the basis for biofilm formation. They have been shown to affect the rheological properties of activated sludge. By comparing the EPS-patterns (characterization and quantification) of processes that in a preliminary test revealed different viscosities and then do the same mapping on potential substrate mixes we will gain knowledge for prediction of rheological properties in relation to the substrate composition. This tool will be important for future dimensioning of biogas plants and as a means to guide the use of new substrate mixes in existing reactors.

The overall research questions are:

• Is there a change in the composition of EPS in biogas reactors, when a shift in rheology occurs as a result of e.g. a change in substrate mix?
• What in a substrate mix triggers the shift in rheology of the reactor liquid and how may this be abated?
• Is there a change in the micro-flora linked to the shifts in EPS spectra?