Comparative assessment of technological systems for recycling sludge and food waste aimed at greenhouse gas emissions reduction and phosphorus recovery

We used life cycle assessment to compare alternative technologies for the disposal of sewage sludge and food waste with the aim of identifying the preferred methods for reducing greenhouse gas (GHG) emissions, recovering phosphorus (P), and minimizing risks to public health. We examined two systems: (i) traditional incineration of food waste and separate treatment of sewage sludge, and (ii) a proposed new system in which domestic food waste is mixed with sewage sludge in an anaerobic digestion tank at a sewage treatment plant. We examined variations to each of these systems as alternative technologies for the processing of sludge: low-temperature incineration, high-temperature incineration, composting, production of cement feedstock, low-temperature carbonization, dry granulation, and pyrolysis gasification. We also evaluated the effect of different P recovery technologies: the calcium hydroxyapatite (HAP) method, the magnesium ammonium phosphate (MAP) method, alkaline extraction, and partial-reduction melting. A functional unit of 100,000 people receiving disposal services was used as the basis for life cycle assessment calculations. We found that for each of the technologies compared, the combined sludge and food waste digestion system was superior to separate treatment and disposal. Among the various technology configurations of the combined system, two were judged to be superior to the others with respect to both GHG emissions and P recovery: (i) the pyrolysis gasification of sludge with the MAP method and alkaline extraction from ash for P recovery (Y-5); and (ii) composting of sludge and the MAP method for P recovery (Y-1). When a detailed consideration of health risks was added to the analysis, configuration Y-5 was identified as the best option to target for technology renewal because of its superior disability adjusted life year (DALY) to that of configuration Y-1, which caused greater heavy metal emissions.