Assessment of sewage sludge for revalorisation in agriculture via pyrolysis : a case study at Lake Atitlán, Guatemala

In countries like Guatemala, access to clean sanitation is not a given, and the operation and maintenance of wastewater treatment plants often creates new problems for municipalities already operating a sewerage system. Inadequate wastewater treatment leads to the pollution of rivers and lakes, which in turn poses a health risk to people. In addition, large quantities of sewage sludge are produced and there is still no satisfactory management system for its use. Mosan, an organisation working in the field of ecological sanitation at Lake Atitlán, Guatemala, converts the sewage sludge into biochar using pyrolysis. The objective of this thesis is to do a sewage sludge characterisation with sludges accessible around Lake Atitlán in order to understand the variability between different wastewater treatment plants and the influence of different treatment processes on the sludge composition in order to determine the optimal conditions for a feedstock material for pyrolysis treatment in the Mosan transformation process. Experience has shown that sludge from aerobic treatment works in Mosan’s current pyrolysis reactor, but sludge from anaerobic treatment does not. A proximate analysis will be run on both the sludge and biochar derived from it. The pyrolysis process will also be monitored using known process parameters. The data will be compared with that of the analysed biochar in order to make suggestions to optimise and standardize the process. Drying coconut shells takes a lot of time and space and could become a limiting factor in upscaling. Therefore, trials to reduce the amount of coconut shells used as a co-substrate will be conducted. Both proximate analyses of the sewage sludge (incl. volatile solids, ash and fixed carbon content) as well as calorific value, show a largely uniform picture with only marginal differences between sludges from different treatment processes. A reduction in the calorific value is observed after anaerobic digestion and long storage times. Nevertheless, the analysis indicates that all sludges are potential feedstocks for pyrolysis. The reason why empirical evidence shows different behaviours of sludges with Mosan’s TLUD pyrolysis reactor needs to be further investigated and could be connected to granulometry or other physical characteristics. Results from the pyrolysis process show that controlled pyrolysis retention time significantly increases the VS content of biochar, suggesting potential for process optimisation. A significant achievement was the 75% reduction in the use of coconut shell co-substrate per process, improving efficiency and sustainability.