Extraction of extracellular polymeric substances from activated sludge using sodium oxalate

In this study, sodium oxalate was employed to extract extracellular polymeric substances (EPS) from activated sludge samples. The optimum dose of sodium oxalate was 1.34 g/g VSS (volatile suspended solids), and the extraction time was 60 min at pH 7. The total EPS contents obtained under the optimized conditions were approximately 191 mg/g VSS, and they predominantly consisted of proteins, polysaccharides and humic substances. The extraction efficiencies of the chosen method for the major biopolymers were 39, 43 and 410 % higher compared with the ethylenediaminetetraacetic acid (EDTA), cation exchange resin and control methods. Cell lysis measured in terms of deoxyribonucleic acid, and unidentified EPS were approximately 50 % lower in the sodium oxalate method than in the NaOH and EDTA methods. The EPS composition and the ratio of proteins/ polysaccharides (1.38–2.21) were extremely dependent on the operating conditions maintained during the extraction. The inductively coupled plasma spectroscopic results demonstrated that the release of divalent metals, such as Ca²⁺ and Mg²⁺, from the sludge matrix was directly correlated with higher EPS extraction efficiencies in the sodium oxalate protocol. Moreover, the precipitation of Ca²⁺ and Mg²⁺ as metallic oxalates disrupted the floc structure and subsequently enhanced the EPS recovery. Fourier transform infrared studies revealed the presence of functional groups due to EPS molecules in all extracted samples.

Keywords
Cation bridging; Fourier transform infrared characterization; Ion exchange; Process optimization; Quantification