Co-digestion of activated sludge with periurban organic wastes

Anaerobic digestion of sludge from wastewater treatment is largely applied in France for the plants with an important capacity (50 000 eq. hab.) but remains little used for the smaller plants. Indeed, such a process is generally not interesting economically for the smaller plants compared to other sludge treatment techniques. However, the advantages of anaerobic digestion of sludge are well-know: production of renewable and local energy, reduction of greenhouse gas emissions, odour abatement and sludge reduction. In order to promote anaerobic digestion of sludge even for the small wastewater treatment plants, one of the possibilities for increase the rentability of the process is to add co-substrates available locally with the sludge in order to increase methane production. Such an approach was studied considering the Mordelles town (about 7000 hab.). Mordelles is located close to a larger city (Rennes, France) and is equipped with an activated sludge wastewater treatment plants for about 15000 eq. hab. So, activated sludge from the wastewater treatment plants and various co-substrates available locally were characterized for their ability to be anaerobically co-digested. Subsequently, co-digestion experiments were performed with activated sludge and lipid-rich waste from meat industry using batch and continuous experiments. Additionally to the specific results obtained, this work shows the difficulty to extrapolate from the results obtained in batch reactor to the prediction of continuous process.