Handling excess sludge produced by wastewater treatment is a common problem worldwide. Due to limited space available in landfills, as well as difficulties involved in using excess sludge in agriculture, there is a need for alternative disposal methods. Although anaerobic digestion (AD) is widely used in processing sludge, only partial energy recovery from methane and sludge volume reduction can be achieved, resulting in a substantial amount of sludge remaining, which needs to be disposed of. Direct incineration after sludge drying is one possible option, a practice that is already in place in some cities in China. A comparison between direct incineration and conventional AD (with or without pretreatment by thermal hydrolysis) has to be made with respect to the energy balance and investment & operational (I & O) costs. This comparison reveals direct incineration to have the lowest energy deficit and I & O costs. Therefore, it is expected that direct incineration without AD will become the preferred sustainable approach to handling sludge.

In dealing with wastewater, chemical energy has traditionally been perceived as the only source of recoverable energy in moving towards the carbon-neutral operation of wastewater treatment plants. Based on an estimation of practically recoverable energy embedded in municipal wastewater, however, the potential for thermal energy (90% recovery from wastewater) is much higher than for chemical energy (COD, 10% recovery). The carrier of chemical energy (COD) has a high exergy value which should, from a sustainability point of view, be utilized to the greatest extent possible. Rather than being converted into methane (and subsequently into carbon dioxide), carbon (COD) contained in wastewater should be converted into highly valuable organic products. Thermal energy could be utilized for district heating/cooling, agricultural greenhouses, and even for drying dewatered sludge. In this way, thermal energy can indirectly offset the energy demand for wastewater treatment. The limitations in utilizing thermal energy are not generally based on technical difficulties; in fact, they can be mainly attributed to supply distances and governmental policies. It would, therefore, be greatly beneficial if municipal authorities would work together to jointly plan utilization of this thermal energy.

Conventional wastewater treatment plants (WWTPs) clean wastewater and minimize water pollution; but, while doing so, they also contribute to air pollution and need energy/material input with associated emissions. However, energy recovery (e.g. anaerobic digestion) and resource recovery (e.g. water reuse) allow us to offset the adverse environmental impacts of wastewater treatment. Life cycle assessments (LCA) have been used more and more to evaluate the environmental impacts of WWTPs and to suggest improvement options. There is a need to search for resource recovery applications that genuinely realize a net-zero impact on the total environment of WWTPs. In this work, a scheme with highly efficient energy and resource recovery (especially for thermal energy) is proposed and evaluated. The environmental impact of a conventional WWTP in comparison with the scheme proposed here, with energy/resource recovery included, was calculated, and discussed with reference to LCA methodology. In the process of using LCA, it was necessary to choose a regional situation to focus on. In this case, a Chinese situation was focused as a reference, but the qualitative information gained is of worldwide relevance. The results clearly revealed that conventional WWTP does not benefit the total environment as a whole while the new scheme benefited the total environment via resource/energy recovery-based processes. Among others, thermal energy recovery played a significant role towards a net-zero LCA analysis (contributing around 40%) which suggests that more attention and research should be focused on it.