ZeroWasteWater: short-cycling of wastewater resources for sustainable cities of the future

Sewage treatment relies mainly on conventional activated sludge (CAS) systems, reaching sufficiently low pollutant effluent levels. Yet, CAS has a low cost-effectiveness and recovery potential and a high electricity demand and environmental footprint. By 2050, globally we have to solve severe water and phosphorus shortages while significantly decreasing greenhouse gas emissions. In this review and opinion paper, the ZeroWasteWater concept is proposed as a sustainable centralised technology train to short-cycle water, energy and valuable materials from sewage, while adequately abating pathogens, heavy metals and trace organics. Electrical energy recovery from anaerobic digestion of the organics present in sewage and kitchen waste (KW) has a value of 4.0 per inhabitant equivalent (IE) per year. In addition to sewerage improvements and water conservation, prerequisites include an advanced physico-chemical and/or biological concentration step at the entry of the sewage treatment plant. In the side stream, the recovery of phosphorus and carbon-sequestrating biochar from the digested sludge and of nitrogen from the digestate has a value of 6.3IE-1 year-1. Alternatively, recovery of biogas and materials can occur directly on source-separated black water. In the main stream, partial nitritation and anammox oxidise residual nitrogen. Moreover, two serial heat pumps recover thermal energy, valued at 6.9IE-1 year-1, cooling the water by 5 degrees C, and membrane technologies recover potable water at 65IE-1 year-1. Interestingly, ZeroWasteWater is expected to be economically viable. Key steps are to incorporate water chain management into holistic urban planning and thus produce a cradle-to-cradle approach that society will find acceptable.