Forest industries produce large amounts of carbon rich sludges as by-products in their processes. Presently sludge is treated as a poor quality biofuel for co-incineration, some mills treat it solely as a disposal problem. This thesis provides an introduction to production, composition and disposal issues of sludge. It also includes a presentation of strategies for sludge handling.

The main concern with energy recovery from sludge is connected to high content of water (50-80%). Mechanical dewatering is an energy efficient method of decreasing the water content. However, there are limitations to how far sludge can be dewatered mechanically. Thermal dewatering is sometimes required to dewater the sludge beyond these limits, in order to obtain a high quality biofuel for incineration and/or thermal gasification. It is often inefficient, from an energy point of view, to incorporate thermal dewatering in the sludge handling strategy.

An interesting alternative to thermal processes is anaerobic digestion, which is a biological process used for energy recovery. Advantages with anaerobic digestion include biogas production, efficient treatment of sludge with high content of water and potential for nutrients recovery. The process and the kinetics of anaerobic digestion are presented.

The aim of this thesis is to present a method for evaluating different sludge handling strategies from an energy perspective, and to further develop anaerobic digestion as a process for energy recovery from sludge. The thesis is based on two papers. Paper I presents an inclusive approach with focus on energy use and energy recovery in wastewater management, including wastewater treatment and sludge handling. Paper II explores the possibility to enhance biogas production by anaerobic co-digestion of pulp mill sludge with municipal sewage sludge.

The production of pulp and paper is associated with the generation of large quantities of wastewater that has to be purified to avoid severe pollution of the environment. Wastewater purification in pulp and paper mills combines sedimentation, biological treatment, chemical precipitation, flotation and anaerobic treatment, and the specific combination of techniques is determined by the local conditions. Wastewater treatment generates large volumes of sludge that after dewatering can be incinerated and thus used for bio-energy production. Sludge is currently viewed as biofuel of poor quality due to its high water content, and some mills treat it solely as a disposal problem.

Two strategies have been identified as feasible options to improve the energy efficiency of sludge management. One is drying using multi-effect evaporation followed by incineration. The other is anaerobic digestion of the wet sludge to produce methane.

This thesis explores the energy balances of sludge management strategies in pulp and paper mills with special focus on anaerobic digestion. The first part consists of a system analysis, used to evaluate some wastewater treatment processes and sludge management, and the second part of empirical studies of anaerobic digestion of pulp and paper mill sludge. It was shown that the use of energy for aeration in aerobic biological treatment should be kept to the minimum required for acceptable quality of the processed water. Additional aeration for reduction of the generated sludge will only result in reduced energy generation in a subsequent methane generation stage. In the second part of the thesis, it is shown that anaerobic digestion is a feasible option for sludge management as it leads to production of high value biogas. Co-digestion with grass silage, cow/pig manure or municipal sewage sludge should then be used to counteract the low nitrogen content of pulp and paper mill sludge.