Municipal Sewage Sludge Variability: Biodegradation through Composting with Bulking Agent

Municipal sewage sludge is a waste with high organic load generated in large quantities that can be treated by biodegradation techniques such as composting to reduce its risk to the environment. This research studies the physicochemical variability of sewage sludge from treatment plants in the south of Galicia (Spain) and determines if it is possible to establish a protocol for the use of bulking agent depending on the composition of the sludge and the development of the composting process. Therefore, physicochemical analyses of 35 sewage sludge from different municipalities and 10 samples from the same treatment plant are discussed. Three different mixtures bulking agent:sewage sludge (3:1, 2:1, 1:1, v:v) were carried out in 30 L reactors in triplicate. Finally, proportion 2:1 was replicated six times in a 600 L reactor. High inter-sludge variability was observed specially in key parameters such as moisture and C/N ratio. Intra-variability was lower, and 2:1 proportion was the most suitable mixture since extending the thermophilic phase of the composting process at a greater degree. However, repeatability of the process at a higher scale showed different responses in the temperature evolution. Variability of sewage sludge makes difficult to establish treatment protocols although minimum requirements are necessary for proper composting.