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.

Biofertilizer recovery from organic solid wastes via hydrothermal liquefaction

Hydrothermal liquefaction (HTL) has emerged as a viable pathway for processing wet organic solid wastes (OSW) to yield biocrude oil which could be upgraded to transportation fuels and specialty chemicals. The HTL process results in two byproducts laden with high levels of carbon, nitrogen, and phosphorous. Recovery of phosphates in the byproducts as struvite and ammoniacal-nitrogen (NH₄-N) as ammonium sulfate is proposed here as a promising pathway to utilize the HTL byproducts. A case study of this pathway using algal biomass as a model OSW yielded 8.2 g struvite/100 g OSW and 10.7 g ammonium sulfate/100 g OSW. Heavy metal levels in both struvite and ammonium sulfate crystals were below EPA regulations for land application. This biofertilizer recovery pathway could render OSW processing by HTL a greener alternative to anaerobic digestion, offering feedstock versatility, substantially smaller footprint, and a higher degree of OSW valorization.

Techno-economic optimization of phosphorous recovery in an algal-based sewage treatment system

Previous reports have documented the technical viability of an algal pathway for treating primary effluent and recovering its phosphorus-content (P) via hydrothermal liquefaction (HTL) of the resulting biomass. In this pathway, leaching P from HTL-derived biochar was found as the critical step impacting the economics of P-recovery. As such, a process model was developed in the current study to optimize P leaching from biochar as a function of five parameters. Model predictions under various conditions agreed well with measured data (r² = 0.93; n = 184). The validated process model was then integrated with a cost model to establish the following conditions as optimal for leaching P from biochar: batch leaching time of 72 h; eluent NaOH concentration of 0.5 M; eluent-to-biochar ratio of 20; temperature of 60 °C; and provision of mixing. Under these conditions, 73.5% of P from biochar could be recovered at $5.98/kg P.

Removal and recovery of nutrients from municipal sewage: Algal vs. conventional approaches

This paper presents a pilot scale study of an algal-based sewage treatment and resource recovery (STaRR) system capable of treating municipal sewage and recovering its nitrogen- and phosphorous-content as fertilizer. Core components of the STaRR system include i) mixotrophic cultivation of algal biomass in settled sewage; ii) hydrothermal liquefaction (HTL) of the resulting algal biomass, and iii) processing of the products of HTL to recover energy in the form of biocrude and nutrients in the form of struvite. Performance of a pilot-scale STaRR system in recovering nitrogen (N) and phosphorus (P) from settled sewage as struvite is documented and compared with that of existing and emerging technologies. Nutrient removal per unit energy input in the STaRR system is estimated as 257.1 g N/kWh and 36.6 g P/kWh while that in eight full-scale sewage treatment plants (STPs) averaged 74.3 g N/kWh and 135.1 g P/kWh. Energy required to treat primary effluent in the STaRR system (531.5 kWh/MG) is estimated to be lower than the average in the 8 STPs (1,037.9 ± 503.3 kWh/MG). While existing technologies had been originally designed for removal of nutrients rather than any recovery, a review of the literature revealed 12 emerging technologies for nutrient recovery. Nutrient recovery performance of the STaRR system (5.9% N and 71.6% P) is shown to be superior to that of those 12 emerging technologies. Recoveries recorded in the STaRR system translate to a yield of 2.4 kg struvite per 100 m³ of primary effluent. Results of this study imply that the STaRR system deserves due consideration as a greener and sustainable pathway for nutrient removal and recovery from sewage.