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.