Enzymatic treatment of wastewater sludge. Sludge solubilisation, improvement of anaerobic digestion and extraction of extracellular polymeric substances

Abstract: The processing and disposal of sludge from wastewater treatment plants is an important topic of increasing environmental, ecological, technological, economic and social concern. The large amounts of sludge produced represent an urgent problem today, new treatment methods being very much needed. Reducing the volume of sludge can be achieved either by decreasing the amounts of it produced during the wastewater treatment process or by reducing the volume of sludge once it has been produced. The thesis considers different methods of the latter sort involving biological and chemical disintegration of municipal wastewater sludge prior to or during anaerobic digestion of it. Municipal wastewater sludge contains such valuable and renewable resources as (i) carbon, which can be converted to biogas, and (ii) phosphorus and nitrogen, which can be utilized as fertilizers. These valuable components accumulate in sludge in polymeric form. The biological disintegration of sludge by hydrolytic enzymes was studied here. The effects of enzymatic hydrolysis were tested, the following factors being varied: total solids (TS), type of enzymes, enzyme dose, dosing point, reaction time, temperature, pH, the ratio of TS to enzyme concentration, the volume of sludge treated and the operating conditions. In batch laboratory tests, 20% to 40% mass solubilisation of sludge was found after treatment. The reduction obtained depended upon the enzyme dose and the operational conditions during treatment. Enzyme treatment was found to increase methane production by 60%. This was confirmed in both pilot- and full-scale continuous digestion experiments, the addition of the enzyme mixture resulting in an improvement in both biogas production and the dewatering properties of the digested sludge. Furthermore, the enzymes added were found to be partially entrapped by, bound or adsorbed to the sludge structure, reducing the activity of the enzymes that were added. To counteract adsorption and reduce the enzyme dose, the sludge was pre-treated by several organic and inorganic cation-binding agents. The cation-binding agents used either alone or in combination with the enzymes disrupted the formation of sludge flocs and improved solubilisation of the sludge. Citric acid was the most effective agent tested. Additionally, the level of activity of the residual enzymes increased by up to 20% in the presence of the cation-binding agents, indicating the stabilizing effect these additives had on the enzymes that were added. Hydrolytic enzymes were also used for the extraction and characterization of extracellular polymeric substances (EPS) from sludge, in efforts to predict the bio-digestibility of the sludge. EPS have been found to be partly responsible for the formation, the structure and the settling of sludge flocs in particular. Wastewater-activated sludges were also treated by selected enzymes, the polysaccharides and glycoconjugates released being characterized by use of a lectin panel.