Extraction and Purification of Phosphorus from the Ashes of Incinerated Biological Sewage Sludge

Abstract

Phosphorus depletion represents a significant problem. Ash of incinerated biological sewage sludge (BSS) contains P, but the presence of heavy metals (e.g., Fe and Al) is the main issue. Based on chemical characterization by SEM-EDS, ED-XRF and ICP-OES techniques, the characteristics and P content of bottom ash (BA) and fly ash (FA) of incinerated BSS were very similar. On BA, P extraction carried out in counter- current with an S:L ratio of 1:10 and H2SO4 0.5 M led to better extraction yields than those of a similar test with H2SO4 1 M and an S:L ratio of 1:5 (93% vs. 86%). Comparing yields with H2SO4 0.5 M (S:L ratio of 1:10), the counter-current method gave better results than those of the crossflow method (93% vs. 83.9%), also improving the performance obtained with HCl in crossflow (93% vs. 89.3%). The results suggest that the purification of the acid extract from heavy metals with pH variation was impractical due to metal precipitation as phosphates. Extraction with H2SO4 and subsequent treatment with isoamyl alcohol represented the best option to extract and purify P, leading to 81% extraction yields of P with low amounts of metals.

The assessment of phosphorus recovery potential in sewage sludge incineration ashes — a case study

“…The results of the sequential extraction showed that the highest amounts of phosphorus were released due to sulfuric acid leaching (on average 1470 mg L −1 ) causing the release of phosphorus combined with organic matter in F4 at the level of about 64%. The efficiency was, however, low, compared to other studies where 80-99% of phosphorus was released, e.g., Baldi et al (2021); Biswas et al (2009); Cohen (2009); Franz (2008); Wang et al (2018). In addition, Fang et al (2018) noted that sulfuric acid is the most efficient acid in phosphorus recovery due to higher concentration of hydrogen ions.…”

“…A dissolution of heavy metals and potentially toxic elements occurred, therefore purification process needs to be applied. The acid extract can be subjected to organic solvent treatment, as suggested by Baldi et al ( 2021 ) or purified using chelating solutions, as suggested by Fang et al ( 2018 ).…”

Exploiting the Nutrient Potential of Anaerobically Digested Sewage Sludge: A Review

“…Organic acids induce chelating effects that greatly increase the leaching of metals/metalloids from ash and soil [126]. Inorganic acids leach alkali metal oxides and release all phases containing P, of which sulfuric acid and nitric acid proved to have a high P releasing capacity [128,129]. In order to ameliorate these drawbacks, Petzet et al [127] developed a combined acid-base extraction procedure: the investigated process requires an acidic pre-treatment in which the P fraction of the raw SS ashes that is bound as calcium phosphate is converted into aluminum phosphate.…”

Phosphorus recovery from a pilot-scale grate furnace: influencing factors beyond wet chemical leaching conditions

“…Then, the first attempts at leachate purification were made by ion exchange resins (Franz 2008;Xu et al 2012;Wang et al 2018), adsorbent materials (Biswas et al 2009;Yu et al 2021), nanofiltration (Niewersch et al 2014) and liquid-liquid extraction (Hong et al 2005;Baldi et al 2021). Considering the recent literature, Yu et al (2021) proposed the use of a phosphorus selective adsorbent (based on zirconium) to recover 70% of phosphorus in ash as hydroxyapatite, while Baldi et al (2021) found to be effective keeping in contact the acid leachate with an organic solvent solution (isoamyl alcohol) with a ratio of 1:1. They indeed found a phosphorus extraction rate of 81% with low amount of iron and aluminium (extraction rate of 25% and 10%, respectively).…”

“…They indeed found a phosphorus extraction rate of 81% with low amount of iron and aluminium (extraction rate of 25% and 10%, respectively). Looking at industrial scale, ion-exchange and selective nanofiltration are the actual techniques adopted (e.g., Remondi Tetraphos and EcoPhos), but alternatives to lower the costs are required (Baldi et al 2021).…”