Rheological characterization of biomaterial recovered from Anammox granular sludge

Autotrophic biological processes based on the metabolism of anaerobic ammonium oxidizing bacteria (anammox) allow the removal of nitrogen from wastewater with important savings in terms of energy for oxygen supply (-60%), organic carbon supply (-100%) and reduced excess sludge production compared to conventional nitrification/denitrification process (Hu et al., 2013). Anammox-based technologies are becoming the new standard for the treatment of nitrogen-rich wastewaters of municipal and industrial origins (Lackner et al., 2014). Furthermore, the application of anammox-based bioprocess to the treatment of municipal sewage would allow a complete re-design of the wastewater treatment scheme with the potential to implement an energy autarkic or even energy generating system (Kartal et al., 2010). Anammox bacteria easily form granules (self-aggregated biofilms without the presence of inert carriers). Compared to conventional activated sludge, granules have better settleability resulting in efficient biomass retention systems characterized by higher kinetics and smaller volumes and in fact granular systems treat more than 50% of the N-load treated in total by anammox technologies worldwide (Lackner et al., 2014). In anammox granules, like conventional biofilms, microorganisms are embedded in a matrix of hydrated extracellular polymeric substances (EPS) (Flemming and Wingender, 2010; Seviour et al., 2012). EPS are a complex mixture, consisting of polysaccharides, proteins (structural proteins or exoenzymes), nucleic acids, (phospho)lipids, humic substances and some intercellular polymers and account for up to 90% w/w of the organics in biofilm (Flemming et al., 2007). In biological wastewater treatment processes microorganisms use pollutants as substrates and gain energy for growth from their transformation into harmless products. The biomass hence produced, the so called excess sludge, is considered as a waste product and the related costs of handling/disposal represents up to 50% of the wastewater treatment operative costs (Kroiss, 2004). The recovery of biomaterial from excess sludge to be applied in other industrial sectors would therefore substantially increase the sustainability and economics of wastewater treatment and would promote the development of a circular economy. In this research EPS were extracted from anammox granular sludge and used to form a viscoplastic biomaterial. A physico-chemical characterization of the recovered biomaterial was then performed investigating its rheological properties as function of the EPS concentration, which is considered a prerequisite for the exploration of its potential application in other industrial sectors.