Anaerobic co-digestion of sewage sludge and fruit wastes: Evaluation of the transitory states when the co-substrate is changed

•Transitory stage assessment when co-substrate is changed.•A brief peak of volatile fatty acids was recorded when co-digestion started.•Changing the co-substrate, while keeping the OLR, did not affect the VFA level.•Digester alkalinity was affected by both sewage sludge and co-substrate.•The process reached mono-digestion values 10days after co-digestion was stopped. Some existing anaerobic digesters treating sewage sludge have a non-used capacity. The use of this extra capacity by introducing additional wastes to conduct the co-digestion could enhance biogas production and plant economic feasibility. Fruit wastes from the food industry could be proper co-substrates due to their high biodegradability, but the harvesting seasons require the use of different kind of fruits causing many transitory conditions throughout the year. Two lab-scale continuous anaerobic digesters treating sewage sludge were operated, one as a reference reactor and the other one as a co-digester. The transitory state was evaluated when fruit waste supply was started, when the co-substrate was changed (peach, banana and apple waste) and when fruit waste supply was stopped. In the transition from mono- to co-digestion, volatile fatty acids concentration rose from 0.07 to 1.70gL−1 due to the OLR increase, but this situation was recovered in only 5days. The introduction of different kind of fruit wastes resulted in an alteration of alkalinity, without affecting volatile fatty acids concentration, and in an increase of methane production between 110% and 180% depending on the characteristics of the co-substrate. Finally, when co-digestion was stopped, the parameters converged, at different rates, to the values recorded in the reference digester. It could be concluded that the change of one co-substrate by another one of the same type did not lead to system instability.