Nitrogen-Fertilizer Recovery from the Centrate of Anaerobically Digested Sludge
S.P. Munasinghe-Arachchige, Nagamany Nirmalakhandan
Abstract
Provision of nitrogenous nutrient needs for plants, animals, and humans is heavily dependent on manufactured N-fertilizers. While the Haber–Bosch process has served the global demand for manufactured N-fertilizers, concerns have been raised about its energy footprint (12.1 kWh/kg of NH3-N) and the challenges in meeting increasing demands (1.2 × 1011 kg of NH3-N/year). Recognizing that ∼30% of the manufactured N-fertilizers ends up in sewage and 60–65% of that is dissipated into the atmosphere at an additional energy expenditure of 3–4 kWh/kg of N, it is acknowledged that recovering N from sewage can be a sustainable approach for meeting a fraction of the future demand. Currently, anaerobically digested municipal sewage and livestock manure offer the greatest potential for the recovery of N. Here, we review and rank the following technologies that have been recommended for recovering N-fertilizers from these streams: air stripping, ultrafiltration/ion exchange, struvite precipitation, ultrafiltration/reverse osmosis, and gas permeable membrane separation. Because these technologies differ widely in terms of pretreatment, operating conditions, recovery performance, chemical and energy demands, and post-treatment, we have adopted the PROMETHEE method of multicriterion analysis to rank them considering 10 performance criteria. Results of this analysis indicated the gas permeable membrane technology as the preferred option followed by struvite precipitation.