Nutrients (N & P) Recovery from Algae Hydrolyzate By Hydrothermal Mineralization

Nutrients (N & P) Recovery from Algae Hydrolyzate by Hydrothermal Mineralization

Ali Teymouri and Sandeep Kumar

Department of Civil and Environmental Engineering, Old Dominion University, Norfolk, VA 23529

Abstract

Microalgae are being considered a sustainable feedstock to produce advanced biofuels and biocommodities. However, there is a considerable cost associated with nutrients (N & P) use during algae cultivation particularly phosphorous which is a finite natural resource. It is important to note the need to seek comprehensive recovery systems for biomass and all of its components in order to exploit the microalgae potential. In this study, majority of algae nutrients were extracted in hydrolyzate (aqueous phase) using Flash Hydrolysis (FH), a process which fractionates nutrients and lipids under subcritical water conditions in a very short residence time. The FH process was proved to be an environmentally benign method which extract more than 60 wt% of the total nitrogen and more than 80 wt% of phosphorous content (dry basis) from Scenedesmus sp. within 9 s of residence time above 280ºC in the hydrolyzate while preserving the lipids in solids(biofuel intermediates).

Followed by the FH process, the algae hydrolyzate was subjected to hydrothermal mineralization (HTM) in the temperature range of 100-280ºC in a batch reactor. The HTM is a technique to recover and store nutrients as solids (precipitates) that could be used as slow-released fertilizers. Solubilized N and P precipitates as hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) and struvite (MgNH₄PO₄.6H₂O) by the addition of Ca(OH)₂/Mg(OH)₂ mineralizer under hydrothermal condition. Algae hydrolyzate and model compounds were studied for HTM to measure the nitrogen and phosphorus recovery. More than 90% of the nutrients were recovered in solid forms which could be stored and recycled as a nutrient source.

A comprehensive characterization of effluent liquid and solids were conducted using ion chromatography, XRD, SEM, FT-IR in order to determine the HTM process parameters and mineralizers use. The integration of HTM with FH or Hydrothermal Liquefaction (HTL) of algae is a novel concept considering that no additional heating or pressurization will be required since the algae hydrolyzate is a hot-compressed liquid which is directly suitable for HTM. The results of this study suggested that it will provide an opportunity to produce biofuels and recover nutrients simultaneously.