Comparative Life Cycle Assessment of Sludge Treatment Systems : Is recycling aluminium based coagulant from chemical sludge the way of the future?

Sammanfattning: Chemical coagulation is a widely used wastewater treatment method around the world to reduce impurities from the process water in various industries. However, the large amounts of coagulation chemicals that are required for the removal of dissolved particles create a chemical sludge which poses a great environmental problem. Purac AB, a Swedish wastewater treatment company attempts to solve this problem with a new technology called the ReAl process. The ReAl process can recycle the aluminium ions from the commonly used coagulant aluminium sulfate, which reduces the amount of chemical sludge and the amount of aluminium sulfate needed in the coagulation process. In this study, a comparative life cycle assessment was conducted with a cradle-to-grave approach and mostly in accordance with the ISO-14040 series with the only deviation of not including resource-based impact categories. The goal was to evaluate the environmental impact of two sludge treatment systems – a conventional system (system 1) and a system which includes the ReAl process (system 2). Furthermore, the environmental performance of two dewatering equipment’s, a decanter centrifuge and a filter press, were examined in system 1, while in system 2, the exclusion of sludge drying was investigated. The scope of the study did not include the infrastructure of the sludge treatment systems and the ReAl process since previous studies have shown that, the environmental impact from the infrastructure in the wastewater treatment industry is relatively small compared to other factors, such as the energy and coagulation chemical used in these systems. The characterization results showed that system 2 had the lowest environmental impact on all the evaluated impact categories. The results also revealed that system 1 would have a slightly lower environmental impact if the chemical sludge was dewatered with a decanter centrifuge instead of a filter press. Similarly, system 2 would have a slightly lower environmental impact if sludge drying was excluded from the system. However, the environmental performance gain from selecting the best dewatering and drying equipment is limited and considered within the margin of error. Thus, this thesis suggests selecting the sludge treatment equipment based on their economic and technical factors before their environmental performance. The largest environmental impact in system 1 derived from the use of the coagulation chemical aluminium sulfate, while in system 2, sulfuric acid used in the ReAl process contributed the most to its environmental impact. The sensitivity analysis showed that a “clean” electricity mix is essential for system 2 and the ReAl process overall impact on the environment compared to system 1.