Strategies for recovery and recycling of nutrients from municipal sewage treatment effluent and hydrothermal liquefaction wastewaters for the growth of the microalga Scenedesmus sp. AMDD

Abstract Developing reliable processes for recycling nutrients obtained from hydrothermal liquefaction of algae could improve the sustainability and scalability of algae based biofuels. In this study, hydrothermal liquefaction (HTL) wastewater was obtained from continuous liquefaction experiments in plug flow reactor and two strategies were evaluated to recycle both phosphorous and nitrogen for the growth of Scenedesmus sp. AMDD. The first strategy involved using HTL wastewater as a source of nitrogen while minimizing the algae growth inhibitors present in wastewater using hydrothermal gasification and activated carbon adsorption. The second strategy focused on recovering nitrogen as ammonia and phosphorous as struvite from HTL wastewater and recycling only the nutrients thereby decoupling the HTL wastewater and most inhibitors from nutrient recycle process. Water samples obtained from these two strategies were analyzed using various techniques (elemental analysis, gas chromatography, and hydrogen NMR) and potential growth inhibitors were identified as nitrogen containing heteroaromatics. The second approach relied solely on activated carbon treatment to remove these heteroaromatics. Although comparable growth rates were obtained using the first approach after strong dilution, algae growth and biomass yields obtained using the latter approach was much more robust as indicated by less variability, similar rates and biomass yields as compared to the synthetic medium. Apparent nitrogen and phosphorous removal rates for this case were > 99% and 68 ± 5.7%, respectively. This approach of decoupling nitrogen and phosphorous nutrients from HTL water offers a flexible, reliable, and scalable process for recycling nutrients without the inhibitors in HTL water and an important step towards the commercial production of algae for biofuels.