Feasibility of coupling hydrogen and methane production in WWTP: Simulation of sludge and food wastes co-digestion

Wastewater treatment plants are essential in improving life quality by degrading organic matter, reducing contamination, and therefore greatly impacting human activities. The role of these critical treatment units can be further promoted by integrating new biological processes that currently are still under experimental scale. The co-digestion of sewage sludge and food waste has been proposed as an efficient way to increase plant treatment capacity and energy recovery. The assessment of hydrogen production along with food waste co-digestion is carried out in the present manuscript. Assessing several parameters is necessary to implement a new biological process in an operating plant, and quantifying its effects on the plant's overall performance is crucial. The implications associated with the extra equipment needed to handle additional waste material were evaluated. Results indicated that a conventional unit may treat a 10 % addition of food waste (expressed as VS) without experiencing severe modifications in process parameters, thus obtaining 16 % extra energy. However, the increase in food waste by over 10 % translates into substantial plant modifications requiring the installation of digesters with higher volumes and handling an additional amount of sludge. Another relevant factor is the lower energetic content of biogas when mixed with hydrogen. The increase in food waste until 50 % VS in the mixture reduced the biogas lower heating value to 15.5 MJ/m3. Future research will deal with an economic analysis of the approach and the effect on engine performance when dealing with a fuel mixture with different combustion properties.