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

Abstract

BACKGROUND: This work is focused on phosphorus recovery from anaerobically digested organic fraction of municipal solid waste (referred to as "digestate") as a fertiliser. The main purpose was to propose and test modifications to the electrodialytic process that increase phosphorus extraction, improve the quality of the fertiliser by removing contaminants, and reduce hydraulic retention time to This article is protected by copyright. All rights reserved. allow for smaller system footprints. Strategies tested were: i) lowering the pH of the digestate suspension to below 4.5 using the electrochemical reactions and enhance phosphorus solubilisation from the waste; ii) change the configuration of the electrodialytic cell from 3-to 2-chambers; and iii) stir the sample to shorten the duration of the extraction.RESULTS: Results show that the acidification of digestate by the electrochemical reactions was effective to enhance phosphorus extraction yield. Three-chamber electrodialytic experiments enabled the removal of heavy metals from digestate, producing phosphorus-rich solutions with a low level of metals. This resulted in the production of high quality fertiliser which can be used for agricultural applications. The modification of electrodialytic cell set-up from 3-chamber to 2-chamber did not result in an increase of the phosphorus extraction yields nor did it contribute to the removal of metals from the liquid phase of digestate. The reduction of the hydraulic retention time of electrodialytic extraction of phosphorus from 16d to 9d was attained by the use of stirring and by the electrodialytic acidification.CONCLUSIONS: The increase of phosphorus recovery was accomplished. 90% of phosphorus was successfully extracted from digestate and transformed into struvite.

Improving the energy efficiency of an electrodialytic process to extract phosphorus from municipal solid waste digestate through different strategies

“…In previous work, the use of a stirred ED cell achieved higher phosphorus extraction values, when compared to the stationary system [5]. Looking at Fig.…”

“…The total phosphorus content was 7.53 ± 0.48 g kg -1 (dry weight), the electrical conductivity was 5.72 ± 0.39 mS cm -1 and the pH ranged between 7.67 -7.71. The complete chemical characterisation can be found in Oliveira et al [5].…”

“…The latter option is the major contributor affecting the mobility of charged species dissolved from the waste. Based on previous work [5], the extraction of phosphorus from MSW digestate requires a pH range of 2.5 -3.0 during the experiments, in order to promote the continuous dissolution of phosphorus contained in the MSW digestate suspension. Figure 11 shows that in 100% current ON and mixed current experiments, the decrease of pH of suspension to about 3.0 was faster than in the other four experiments of the series CURRENT.…”

“…Recently, Oliveira et al [5] proposed a process to extract phosphorus from the municipal solid waste (MSW) digestate, followed by its transformation into struvite, a phosphorus-based fertiliser used in agriculture. The electrodialytic (ED) process used for the extraction, still at laboratory scale, can extract 90 % of the phosphorus present in the MSW digestate, thus being highly promising in terms of phosphorus recovery.…”

“…Another improvement is related to the energy consumption upon stirring the waste during the extraction. This operation strongly enhances the phosphorus extraction yields [5] in comparison with a stationary cell [23], emphasizing the need of using a stirrer. Previously, Ottosen et al [24] referred the possibility of saving energy by applying a pulse stirring of 4 times 1 h a day on ED extraction of phosphorus from sewage sludge ash.…”