Testing new strategies to improve the recovery of phosphorus from anaerobically digested organic fraction of municipal solid waste

BACKGROUND: This work is focused on phosphorus (P) recovery from the anaerobically digested organic fraction of municipal solid waste (referred to as ‘digestate’) as a fertilizer. The main purpose was to propose and test modifications to the electrodialytic process that increase P extraction, improve the quality of the fertilizer by removing contaminants, and reduce hydraulic retention time to allow for smaller system footprints.

Strategies tested were: (i) lowering the pH of the digestate suspension to <4.5 using the electrochemical reactions and enhance P solubilization from the waste; (ii) changing the configuration of the electrodialytic cell from three to two chambers; and (iii) stirring the sample to shorten the duration of the extraction.

RESULTS: Results showthat the acidification of digestate by the electrochemical reactionswas effective to enhance P extraction yield. Three-chamber electrodialytic experiments enabled the removal of heavy metals from the digestate, producing P-rich solutions with low metal concentrations. This resulted in the production of high-quality fertilizer which can be used for agricultural applications.

The modification of the electrodialytic cell set-up from three- to two-chamber neither resulted in an increase of the P extraction yields, nor contributed to the removal of metals fromthe liquid phase of digestate. Reduction of the hydraulic retention time of electrodialytic extraction of P from 16 days to 9 days was attained by the use of stirring and by electrodialytic acidification.

CONCLUSIONS: An increase of P recovery was accomplished by modification of electrodialytic extraction, resulting in 90% of P being successfully extracted fromthe digestate and transformed into struvite.