Feasibility of ammonium sulfate recovery from wastewater sludges: Hydrothermal liquefaction pathway vs. anaerobic digestion pathway

This study evaluated two pathways to recover the nitrogen-content of wastewater sludges as ammonium sulfate (AmS) for use as fertilizer. The first pathway entails sludge stabilization by hydrothermal liquefaction (HTL) followed by recovery of AmS from the resulting aqueous product by gas permeable membrane (GPM) separation. The second one entails stabilization of the sludges by anaerobic digestion (AD) followed by recovery of AmS from the resulting centrate by GPM separation. A bench-scale GPM reactor is shown to be capable of recovering >90% of N in the feed. Recoveries of NH3–N in the HTL-pathway ranged 96–100% in 5.5–7.5 h at mass removal rates of 0.2–0.3 g N/day, yielding 3.3–6.0 g AmS/L of feed. Recoveries of 98% were noted in the AD-pathway in 4 h at mass removal rates of 0.06–0.97 g N/day and a yield of 1.7–2.1 g AmS/L of feed. Inductively coupled plasma optical emission spectrometer analysis confirmed that both pathways yielded AmS meeting the US EPA and European region guidelines for land application. The GPM reactor enabled higher nitrogen-recoveries in the HTL-pathway than those reported for current practice of AD followed by ammonia stripping, ion exchange, reverse osmosis, and/or struvite precipitation (96–100% vs. 50–90%). A process model for the GPM reactor is validated using performance data on three different feedstocks.