Nutrients recovery from liquid anaerobic digestate by combining nanofiltration and struvite precipitation - The case of dairy effluents valorization

Results are reported herein of a study aiming to develop an integrated process for valorizing liquid digestate (LD) from an industrial anaerobic digestion (AD) plant treating dairy-processing effluents. The investigated approach involves LD treatment by nanofiltration (NF) membranes, at pilot-scale on-site, followed by precipitation/recovery of struvite in the retentate stream. The obtained NF permeate, for a typical 60% recovery, meets the required standards for restricted irrigation. The significant concentration of nutrients in the NF-retentate (~500 mg/L NH4-N, ~230 mg/L PO4-P) allowes the investigation of struvite precipitation and the determination of near optimum process conditions, through lab- and pilot-scale testing. The results show that the maximum removal of nutrients and production of struvite-rich precipitate is obtained at molar ratio of N:Mg:P = 1:1.5:1.5 and pH=10 in the retentate stream. Additionally, almost complete struvite precipitation can be achieved within ~30 min, whereas solids drying at modest/ambient temperature is desirable to avoid struvite degradation. Under the aforementioned conditions, a significant amount of dry precipitate (~11 g dry mass/L of treated retentate) including crystalline struvite is obtained. The results of this study warrant further development of this integrated process as well as its overall sustainability assessment in comparison to alternative LD valorization approaches.