Transformation of phosphorus during sub- and supercritical water gasification of dewatered cyanobacteria and one-step phosphorus recovery

An important approach for resource utilization is to use supercritical water gasification to recover phosphorus from dewatered cyanobacteria. Most phosphorus elements are converted to solid residues after supercritical water gasification and must be transferred to liquid residues by extraction before recovery. The present study explored an environmentally efficient method to convert phosphorus directly to liquid residues via a one-step recovery process, by studying the distribution and transformation of phosphorus during supercritical water gasification of dewatered cyanobacteria. Basic additives (KOH, K2CO3, NaOH, and Na2CO3) were used to promote the transfer of phosphorus originally concentrated in the solid form to liquid residues. A total of 74% of phosphorus was recovered as liquid residues by adding 6 wt% KOH, and total phosphorus reached 604 mg/L.Besides, pH and NH4+-N concentration satisfied the criteria of the struvite precipitation method. Therefore, a one-step method was established to recover nitrogen and phosphorus from liquid residues.