The ORWARE simulation model: anaerobic digestion and sewage plant sub-models

The ORWARE model is intended as a tool for simulating different systems for handling organic liquid and solid waste in a densely populated area. The model includes truck transport, incineration, landfill, composting, anaerobic digestion, sewer transport, sewage plant, residue transport and spreading of residues on arable land. All physical flows in the model are described by the same variable vector including 43 substances. The output of the model consists of energy flows, emissions to air and water and the amount of residues returned to arable land, including pollutants and nutrients. The development of an anaerobic digestion sub-model and sewage plant sub-model, included in the ORWARE-model, is described in detail. The anaerobic digestion sub-model included in ORWARE is based on a continuous, single-stage, mixed-tank reactor operating under meso-philic conditions. Degradation dynamics are calculated based on the substrate’s composition of fat, protein and carbohydrates and the retention time. The sewage plant sub-model includes mechanical, biological and chemical purification of the wastewater, as well as anaerobic digestion and dewatering of the sewage sludge. Evaluation of waste handling scenarios indicate, e.g., that source separation systems combined with biological treatment of the organic solid fraction generate lower environmental impacts and a higher recirculation of nutrients, but a lower energy turnover, compared with the incineration or landfilling.