Optimization of Struvite Recovery Utilizing Magnesium Oxide

Abstract

Magnesium oxide (MgO) is a cost-effective and environmentally sustainable alternative to magnesium chloride (MgCl2) and sodium hydroxide (NaOH) used for sidestream struvite recovery from anaerobically digested supernatant (centrate) through the Pearl® process. MgO is produced from magnesite (MgCO3) calcination, and different calcination conditions can alter the quality and characteristics of the MgO product. It was hypothesized that the insolubility of MgO could provide a "slowly available" form of Mg2+ in the reactor and consequently allow the reactor to be operated beyond design phosphorus (P) reactor loading. MgO has been utilized in other P recovery technologies, e.g. the Phospaq™ Process, but operation and performance of MgO using a full-scale Pearl® 500 fluidized bed reactor was investigated. Performance at rated reactor loading utilizing MgO was initially comparable to baseline conventional MgCl2 reactor operation, ≥50% struvite yield (P recovered/theoretical P recovery) and ≥70% total phosphorus (TP) removal. However, the pilot reactor operated at 2X reactor loading showed comparable results to baseline performance at 1.5X reactor loading. During the full-scale pilot, optimization of the reactor utilizing MgO was limited by the struvite product size that the struvite post-processing equipment could effectively harvest. Additionally, the MgO characteristics due to calcination conditions were hypothesized to affect struvite precipitation kinetics. In struvite precipitation jar testing, MgO products were used to analyze the saturation index, measure precipitation kinetics, and understand the effect that MgO hydration and reactivity had on struvite precipitation. Jar testing showed that initial P removal increased with increasing MgO product reactivity. The most reactive MgO used, Timab AK98, showed 1-40% P removal and substantial decrease in solution saturation index immediately after dosing MgO to centrate. The slower P removal and decrease in saturation index observed with the less reactive material suggests that MgO can provide a "slowly available" Mg2+ reserve throughout the struvite precipitation reaction.