Life-Cycle Assesment of different technologies to process sewage sludge

Abstract

Wastewater production has been steadily increasing globally. Treatment of wastewater and the resulting sludge is crucial for human health and ecosystem conservation. Furthermore, water supply and treatment is an energy intensive sector where the electricity consumption is projected to increase by more than 80% in the next 25 years.

Sludge treatment is one of the few processes in the water sector where energy can be recovered. And with the environmental issues faced by the sludge treatment sector, it becomes essential to not only optimize the energy recovery but also to holistically analyze environmental impacts of sludge treatment technologies. Thus helping to reduce the human and environmental impact to a minimum. In this report, the current sludge treatment chain in the Netherlands was environmentally analyzed along with two promising technologies, plasma gasification and supercritical water gasification for sewage sludge treatment using lifecycle assessment methodology. The end use of the produced syngas in a gas engine and solid oxide fuel cell was also modeled to find their maximum energy generation potential which is also technologically feasible. Furthermore, impact of anaerobic digestion of sludge in combination with the mentioned technologies was also studied resulting in a total of 9 systems. Literature review and macro modeling of the mentioned systems are used to determine the inputs and outputs over their entire lifecycle. Results from this study show that under the modeled conditions, supercritical water gasification has the potential to considerably reduce environmental impacts since it performed better than other thermochemical conversion technologies in all impact categories. This was primarily due to high energy recovery and relatively clean syngas produced. Furthermore, anaerobic digestion still produced the highest net energy per ton of sludge processed without requiring secondary fuel input. Due to large amount of energy required for drying sewage sludge and material intensive fluegas
cleaning, plasma gasification did not offer promising results for sewage sludge treatment. It can still however offer substantial benefits over incineration especially for waste treatment of feedstock with lower moisture content. Solid oxide fuel cell was the environmentally preferred option over gas engine for electricity generation from syngas due to low nitrous oxide emissions combined with higher energy recovery. Results in literature have shown safe short
term combination of gasifier-solid oxide fuel cell. However, uncertainties with the respect to its long term reliability still remain. Energy generation volatility with respect to the scale of anaerobic digestion and feedstock properties for supercritical water gasification performance present two possible sources of
uncertainties to the above results. The electricity generation from anaerobic digestion is only economically feasible for capacities of 30,000 population equivalents or higher. Whereas, energy generation and consequently the environmental performance of supercritical water gasification is highly dependent on dry solids content. However, with considerable effort going into research and process optimization, this uncertainty can be eliminated soon.