Keynote Paper 2 Xiamen International Forum on Urban Environment ZeroWasteWater: Short-cycling of Wastewater Resources for Sustainable Cities of the Future

Actual sewage treatment relies on conventional activated sludge (CAS), which reaches sufficiently low carbon, nitrogen and phosphorus effluent levels, but is not cost-effective, hardly achieves recovery, requires electricity equivalent to a fossil fuel consumption of 85 kWh per inhabitant equivalent (IE) per year and has an operational CO2 footprint of 80 kg CO2 IE year. Projected water and phosphorus shortages and the need to lower greenhouse gas emissions force us to rethink wastewater treatment for sustainable cities of the future. ZeroWasteWater offers an approach to short-cycle water, energy and valuable materials while adequately abating pathogens, heavy metals and trace organics. A less diluted waste stream will be obtained from sewerage improvements, a more rational use of potable water and the addition of ground kitchen waste, complemented by an advanced physicochemical or/and biological concentration step at the entry of the sewage treatment plant. Anaerobic digestion will recover electricity from the concentrated stream and further treatment will render a value of 6.2 EUR IE year under the form of the nutrients nitrogen and phosphorus, and the carbon-sequestrating biochar. In the water stream, residual nitrogen will be removed through partial nitritation and anammox, followed by heat recovery and add-on membrane technologies yielding potable water (> 65 m IE year). Overall, compared to a CAS system without sludge digestion, the recovery of energy and nutrient in ZeroWasteWater avoids a fossil fuel use of 439 kWh IE year and an operational emission of 88 kg CO2 IE year. Interestingly, such approach is expected to be economically viable. The key challenges remain to incorporate water chain management in holistic urban planning and to render a cradle to cradle approach which society will find acceptable.