Phosphorus recycling potential assessment by a biological test applied to wastewater sludge

Abstract

Phosphorus (P) recycling as mineral fertilizer from wastewater activated sludge (WAS) depends on the amount that can be dissolved and separated from the organic matter before the final crystallization step. The aim of the biological phosphorus dissolution potential (BPDP) test developed here was to assess the maximum amount of P that could be biologically released from WAS prior that the liquid phase enters the recovery process. It was first developed for sludge combining enhanced biological phosphorus removal and iron chloride. Because carbohydrates are known to induce acidification during the first stage of anaerobic digestion, sucrose was used as a co-substrate. Best results were obtained after 24-48 h, without inoculum, with a sugar/sludge ratio of 0.5 gCOD/gVS and under strict anaerobic conditions. Up to 75% of the total phosphorus in sludge from a wastewater treatment plant combining enhanced biological phosphorus removal and iron chloride phosphorus removal could be dissolved. Finally, the test was applied to assess BPDP from different sludge using alum compounds for P removal. No dissolution was observed when alum polychloride was used and less than 20% when alum sulphate was used. In all the cases, comparison to chemical acidification showed that the biological process was a major contributor to P dissolution. The possibility to crystallize struvite was discussed from the composition of the liquids obtained. The BPDP will be used not only to assess the potential for phosphorus recycling from sludge, but also to study the influence of the co-substrates available for anaerobic digestion of sludge.

Sequencing biological acidification of waste-activated sludge aiming to optimize phosphorus dissolution and recovery

“…Soluble P presented a second order polynomial tendency with a maximum value of 11,2% of soluble P at pH 3.4. This result was consistent if compared with previous publications [19] in which around 15% of soluble P was found at pH 2 with non-stored WAS from the same WWTP.Total soluble iron also presented a second order polynomial tendency with a maximum measured value of 105 mg.L -1 at pH 2.…”

“…Tests with higher charges of sugar (>0.5 gCOD.VS -1 ) could be performed in order to achieve pH 4 on biological acidification and to compare results. Regardless of the applied method, soluble P did not exceed 55%, what is significantly lower than the 75% value previously obtained by directly adding white sugar on 0.5 gCOD.gVS -1 organic charge at the beginning of the pre-fermentation with WAS from the same WWTP [19]. In the present tests, the smaller charges intentionally applied to limit pH decrease during Prelease step has perhaps not provided the needed amount of VFA to properly enhance P-release by PAO.…”

“…Our previous works showed that depending on the charge and the biochemical composition of the co-substrates used for the pre-fermentation, it was possible to biologically decrease the pH down to 3 under controlled conditions. However, in sludge from WWTP combining EBPR and chemical process for P removal, P is mainly intracellular in PAO and we observed that P dissolution was low because P release by the PAO was inhibited when pH was lower than 5 [19]. Excessively low pH increases the pH gradient between the internal and external parts of cell membranes and thus decreases the efficiency in substrate transporting inside the cell, resulting in a smaller quantity of released P by absorbed substrate, as described by Deronzier and Choubert [20].…”

“…dissolution was found to be described by a polynomial function, which was used to estimate the contribution of chemical dissolution during the biological process as described by Braak, Auby [19]. The iron dissolution curve was described by a polynomial function.…”

“…So, the parameters to optimize P-release from PAO, among which neutral pH, tend to readily precipitate the P with available cations as iron, aluminum or calcium depending on the process used for phosphorus removal in the WWTP. We have recently presented a fermentation batch test that can be easily applied to assess and predict biologically releasable P [19]. However, we have noticed that biological P solubilization could be theoretically optimized by separating intracellular P-release and P dissolution in a twostep process.…”