Struvite precipitation in wastewater treatment plants anaerobic digestion supernatants using a magnesium oxide by-product

• Published in: The Science of the total environment 2023-09, Vol.890, p.164084-164084, Article 164084
• Main Authors: Aguilar-Pozo, V.B., Chimenos, J.M., Elduayen-Echave, B., Olaciregui-Arizmendi, K., López, A., Gómez, J., Guembe, M., García, I., Ayesa, E., Astals, S.
• Format: Article
• Language: English
• Subjects: Anaerobiosis; Chemical Precipitation; Circular economy; Low-grade MgO; Magnesium - chemistry; Magnesium ammonium phosphate; Magnesium by-product; Magnesium Compounds - chemistry; Magnesium Oxide - chemistry; Phosphates - chemistry; Phosphorus - chemistry; Phosphorus recovery; Struvite - chemistry; Waste Disposal, Fluid - methods; Wastewater treatment; Water Purification
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2023.164084

Struvite precipitation is a well-known technology to recover and upcycle phosphorus from municipal wastewater as a slow-release fertiliser. However, the economic and environmental costs of struvite precipitation are constrained by using technical-grade reagents as a magnesium source. This research evaluates the feasibility of using a low-grade magnesium oxide (LG-MgO) by-product from the calcination of magnesite as a magnesium source to precipitate struvite from anaerobic digestion supernatants in wastewater treatment plants. Three distinct LG-MgOs were used in this research to capture the inherent variability of this by-product. The MgO content of the LG-MgOs varied from 42 % to 56 %, which governed the reactivity of the by-product. Experimental results showed that dosing LG-MgO at P:Mg molar ratio close to stoichiometry (i.e. 1:1 and 1:2) favoured struvite precipitation, whereas higher molar ratios (i.e. 1:4, 1:6 and 1:8) favoured calcium phosphate precipitation due to the higher calcium concentration and pH. At a P:Mg molar ratio of 1:1 and 1:2, the percentage of phosphate precipitated was 53–72 % and 89–97 %, respectively, depending on the LG-MgO reactivity. A final experiment was performed to examine the composition and morphology of the precipitate obtained under the most favourable conditions, which showed that (i) struvite was the mineral phase with the highest peaks intensity and (ii) struvite was present in two different shapes: hopper and polyhedral. Overall, this research has demonstrated that LG-MgO is an efficient source of magnesium for struvite precipitation, which fits the circular economy principles by valorising an industrial by-product, reducing the pressure on natural resources, and developing a more sustainable technology for phosphorus recovery. [Display omitted] •LG-MgO by-product is an efficient reagent for struvite precipitation.•The addition of LG-MgO released both magnesium and hydroxide ions.•The P:Mg molar ratio controls the mineral phases of the precipitate.•P:Mg molar ratios of 1:1 and 1:2 favour struvite precipitation.