Dry Anaerobic Digestion of Solid Organic Waste in a Biocel Reactor at Pilot-Plant Scale

Dry anaerobic digestion of the source separated organic fraction of Municipal Solid Waste (Vegetable, Fruit Yard waste) was studied in pilot-plant-scale reactors (5 m³, 450 m³). The maximum loading rate that could be applied was 7 kg VS/(m³.day) which is similar to the loading rate reported for experiments on laboratory scale. Per ton organic waste 70 m³ (STP) biogas could be recovered. A reactor temperature at start-up of 20 °C which was gradually increased to 35 resulted in a prolonged digestion time, while a temperature of 43 °C at start-up with a gradual decrease to 30 °C gave a similar digestion time as start-up at 35 °C. Particle size reduction of the Vegetable, Fruit and Yard waste resulted in a longer solids retention time which was related to the reduced leachate recycle flow that could be applied. The optimum value for the inoculum factor I, i.e. the ratio of inoculum solids and the initial total solids at start-up, was 0.5 - 0.6. A solids retention time of 30 days was observed. At higher values of I longer retention times were observed due to a suboptimal leachate recycle flow rate. At lower values of the inoculum factor (0.40 or lower) the solids retention time was 50 days or longer due to the relatively long period of suboptimal conditions (low pH, high organic acid concentrations).