2.5. Physico-Chemical Analytical Methods. Characterization of sludge samples was performed by determining the following parameters: pH, soluble chemical oxygen demand (soluble COD), total chemical oxygen demand (total COD), Biological oxygen demand (BOD5), total solids (TS), volatile solids (VS), total suspended solids (TSS), volatile suspended solids (VSS), total kjeldahl nitrogen (TKN), proteins (PN), and polysaccharides (PS). Details of methods used for the determination of these parameters were mentioned in previous study [21].