Effect of acidogenic biomass on sludge filterability

Abstract

Anaerobic wastewater treatment is energy favorable procedure because since useful energy in the form of biogass is produced from the conversion of the organic substances. Anaerobic membrane reactors is a new innovative anaerobic technology, that is has the advantage over other anaerobic methods, that is producing very clean effluent and also can treat high strength wastewater because it achieves almost complete sludge retention therefore high treatment efficiencies. Problem definition The biggest drawback that this technology is presenting, is that the efficiency of the whole procedure is immediately related with the achieved membrane fluxes. These fluxes are strongly affected by the characteristics of the sludge. The influent of the reactors (the kind of substrate that is used) is affecting intensively the sludge characteristics, and the therefore, the cake layer (formed on the membrane surface) and subsequently the attainable fluxes and the system’s efficiency and cost. One of the parameters that is expected to affect the filterability characteristics of the sludge is the microbial population. Different microbial species are expected to have different properties (e.g excreted concentration of extracellular polymeric substances, and soluble microbial products). Particularly, acidogenic microorganisms are expected to decrease the filterability of the sludge due to their smaller particle sizes. Research In order to be able to verify that the presence of acidogens is indeed affecting the sludge filterability characteristics, 4 lab-scale batch reactors were installed. The sludge of these reactors was checked on a weekly basis (both biological characteristics analysis and filterability parameters) Results 1) Fully acidified substrate, will cause the development of a purely methanogenic bacterial population in the reactor. This methanogenic sludge showed indeed a higher particle size than the other reactors, and better filterability. 2) The single stage methanogenesis reactor that contained non acidified whey and therefore acidogenic populations were also present showed lower filterabilities than the 2-stage rectors (pre-acidification tank combined with pure-methanogenic reactor). 3) The increase of Food to Mass ratio (F/M) resulted in an accumulation of soluble proteins in the Reactors. This accumulation led to an impairment of sludge filterability and this effect was much more intense compared to the presence of acidogenic biomass. Conclusions and recommendations It was observed that indeed the acidogenic biomass will affect the sludge characteristics with a negative manner. Therefore, it can be proposed that a two step reactor that will be composed of a pre-acidification tank followed by a methanogenic membrane reactor, can increase the filterability of the sludge, increase the fluxes, and maybe decrease the operational cost. A further research proposal will be to examine, with the same wastewater applied here, the effect of the different acidification degree on an AnMBR to check the real effect on the membrane surface.