Recovery of freshwater from wastewater: upgrading process configurations to maximize energy recovery and minimize residuals.
Analysis of conventional and novel wastewater treatment configurations reveals large differences in energy consumed or produced and solids generated per cubic meter of domestic wastewater treated. Complete aerobic BOD removal consumes 0.45 kWh and produces 153 g of solids, whereas complete anaerobic treatment produces 0.25 kWh and 80 g of solids. Emerging technologies, that include short-circuit nitrogen removal (SHARON, CANON with Anammox, CANDO) and mainstream anaerobic digestion, can potentially remove both BOD and nitrogen with an energy surplus of 0.17 kWh and production of 95 g of solids. Heat from biogas combustion can completely dry the solids, and these solids can be converted to syngas without imported energy. Syngas combustion can produce ∼ 0.1 kWh with an inorganic residue of just 10 g. If salt is removed, freshwater can be recovered with net production of electrical energy from methane (0.03-0.13 kWh) and syngas (∼ 0.1 kWh) and an inorganic residue of ∼ 0.1-0.3 kg as brine. Current seawater desalination requires 3-4 kWh (thermodynamic limit of 1 kWh) and results in an inorganic residue of ∼ 35 kg as brine.