Improvement of Wastewater Sludge Digestion

Sammanfattning: The reduction of sludge volumes for disposal is a topic of high interest for wastewater treatment plants (WWTP) all over the world. Steadily increasing volumes for disposal create an economic, environmental, legislative and technological challenge. Researchers and process developers try to minimise the sludge during its genesis or in the downstream treatment. Due to rising costs and scarcity of fossil raw materials, the view on sludge streams are about to change. Efforts are made to reuse the valuable components and to put them back into a new lifecycle. Wastewater sludge contains a number of interesting compounds like carbon that can be converted to biogas as an energy carrier, as well as nitrogen and phosphorus that can be recycled as fertilisers. Anaerobic digestion is a widespread method to stabilise the sludge by decreasing the carbon content in a downstream treatment step at WWTPs. Our studies showed that certain additives can increase the biogas potential. Wastewater sludge contains to a large extent extracellular polymeric substances (EPS) that are formed during the biological treatment steps and that consist of a polymeric network structure of mainly carbohydrates, proteins and humic substances that are interlinked with multivalent metal cations. The addition of a cation-binding agent like sodium citrate extracts the metal ions from this structure and solubilises the EPS. The former inertial compounds that passed the anaerobic digestion almost unchanged become accessible to microbial activity. A combination of cation-binding agents and hydrolytic enzymes showed that the enzymes were protected from entrapment and deactivation in the sludge matrix. In lab-scale trials, the following factors were varied for the better understanding of the reaction mechanisms: Species and dosage of enzymes and cation-binding agents, total solids, reaction time, temperature and pH. Both the sludge solubilisation and enzymatic hydrolysis were found to increase by 40 % and over 80 % respectively depending on the chosen settings. In a subsequent development step, this process was tested in industrial scale on two municipal WWTP in different setups. The settings had to be adapted to the industrial surroundings, which meant mainly going from batch to continuous setups, extended experimental times and decreased chemical dosages. The performance of the digesters was observed during the experimental time of several months up to two years. In an initial industrial trial over six months, two glycosodic enzymes were added to one digester in a setup of two. The results were then compared between the treated and the reference digester. In an advanced industrial trial over two years of chemical and enzymatic addition, a set of four hydrolytic enzymes together with sodium citrate as cation-binding agent was added to a complete digestion system in reduced dosages. For the evaluation of the results, the collected data from the year preceding the two-year trial period were selected as references. In both cases, the biogas production and the sludge dry solids for disposal after the centrifuge were considerably improved. The lab model and the results from the lab-scale trials were confirmed by the experiments in industrial scale.