Proceedings

Abstract Large masses of sludge will inevitably be generated as by-product in the wastewater treatment plants of pulp and paper mills. Previously it has been shown that anaerobic digestion of this sludge is feasible and its low degradability will be compensated by the large quantities available for treatment. It has become evident though that the future of such a production system might not lie in the hands of the sludge producers, but rather a new player might be more suited to own and run it. A pre-study of an integrated production system that generates biogas from pulp and paper mill sludge will be done in this study with the aim to present all data that is required for making decisions on whether such an investment is sound. The process will be visualised and an investment estimate within ±30% will be provided in order to facilitate implementation.

Keywords: Biogas, pulp and paper industry, sludge, bioenergy, sludge management, waste management.

Introduction Biofuels made from wet organic residues are a very important component of the bioenergy spectrum, see figure 1. The inherent energy in these materials is challenging to convert into fuels that can be used in the tranportation sector. Anaerobic digestion offers a simple process to do this, however, the economics of this path have not been extensivelly studied before. This is an attempt to do that within the Swedish context. This report is part of a larger study that has studied improved energy efficiency within the wastewater and sludge management at four Swedish mills.

Large masses of sludge are continuously being generated as by-product in the wastewater treatment plants of pulp and paper mills. Previous research has shown that anaerobic digestion of this sludge is feasible and its low degradability will be compensated by the large quantities available for treatment (Hagelqvist 2013). Sludge from pulp and paper mills has several positive properties. It is produced continuously, it is difficult to dewater, high in carbon content and low in nitrogen content which makes it an excellent candidate for anaerobic digestion, especially as co-substrate for co-digestion with nitrogen-rich material. The process must be viewed from a societal perspective because the sludges value as biogas substrate will be much more obvious in comparison with other available substrates such as municipal sewage sludge and food waste. This is partly because the scale within pulp and paper mill production is much greater than that of municipal bioenergy production. Furthermore, anaerobic digestion of pulp and paper mill sludge is a fairly simple process, as stable thermophilic conditions are expected and the quality of substrate varies insignificantly with season. The substrate does not require to be fractioned, as it will consist of a constant mix of very small solid particles. The wastewater generated by the process can be treated in the existing wastewater treatment plant already present onsite. Hot water for heating is also available at low cost. This project aims to increase the likelihood for implementation of this technology by presenting its economy and by visualizing the process. A pre-study of an integrated production system that generates biogas from pulp and paper mill sludge has been done in this study with the aim to present all data that is required for making decisions on whether such an investment is sound. The investment evaluation includes a cost estimate, within ± 30%, of a full-scale plant for biogas production from pulp and paper mill sludge. Design of the facility is based on the results of sludge and biogas production experiments, analyzes and evaluations that are undertaken within the study. The investment estimate is based on the need for equipment, engineering and installation of the conversion process from sludge to biogas. Operational and maintenance costs are presented and revenues are presented and analysed.