Nitrogen-fertilizer recovery from urban sewage via gas permeable membrane: Process analysis, modeling, and intensification

Abstract

Feasibility of recovering nitrogen (N)-fertilizers from sewage has gained attention recently due to concerns about the sustainability of the Haber-Bosch process in producing N-fertilizers and of the nitrification–denitrification processes in removing N from wastewaters. This study proposes a non-pressurized gas permeable membrane reactor (GPMR) with the potential to recover high-purity N-fertilizer at lower energy demand than current technologies. Performance of the GPMR in recovering N-fertilizer from the following two waste streams of sewage-origin is demonstrated: 1) centrate generated by anaerobic digestion of primary and secondary sludge; and 2) aqueous phase generated by hydrothermal liquefaction of sewage-grown algal biomass. A process model developed for the GPMR was calibrated using test results on a synthetic feed, and validated using results from the tests on the above two waste streams. Temporal ammoniacal-N concentrations predicted by the model agreed well with the measured values (r2 = 0.82; n = 70). Tests conducted on the two wastes at 24–25 °C indicated that feed-side pH of 10 and mild mixing of the feed maximized N-fertilizer recovery. The GPMR was able to recover 80–100% of ammoniacal-nitrogen from both waste streams, yielding 2–86 g of ammonium sulfate from 1 L of the feed. Energy dispersive X-ray analysis and heavy metal analysis of ammonium sulfate recovered from both feeds confirmed compliance with the US Environmental Protection Agency limits for use as fertilizer. Confocal microscopic images of virgin and used membrane surfaces were examined to assess membrane fouling. A process intensification analysis was performed to relate the physical parameters to the performance of the GPMR and to identify areas for further studies.