Use of side stream-based MgSO4 as chemical precipitant in the simultaneous removal of nitrogen and phosphorus from wastewaters

Abstract

Wastewaters and agricultural sludges can contain high concentrations of phosphate and ammoniumor nitrate-nitrogen, which run off easily to waterways causing eutrophication in water bodies. However, it is possible to precipitate phosphate and ammonium as struvite and use it as recycled fertilizer. In this research, MgSO 4 solution obtained by treating fly ash (FA) or dolomite (DOL) with sulfuric acid was used as a precipitant for simultaneous phosphorus and nitrogen removal from synthetic (NH 4) 2 HPO 4 solution. Precipitation experiments were performed at room temperature (20°C ± 2°C) using different molar ratios Mg:P:N (1.1-2:1-2:1-2) and precipitation times (4-24 h). The pH was adjusted to 9.0 and kept constant. In all cases, there were only minor changes in the removal of both ammonium and phosphate after 4 h of reaction time. Highest ammonium removal percentages after the 4 h test were 79.

Precipitation of struvite using MgSO4 solution prepared from sidestream dolomite or fly ash

“…In both cases, the reaction was slower than with commercial MgSO 4 . Similar findings were made in a previous study ( Pesonen et al., 2020 ), where precipitation chemicals were prepared from DOL and FA. Commercial MgSO 4 reacted during the first 30 min, whereas sidestream-based MgSO 4 solutions required a 4-h reaction time.…”

“…The procedure used in this study originated from Pesonen et al. (2020) and used the same batch of DOL and FA.…”

“…Because Mg is also included in the EU's critical material list ( “Critical Raw Materials – CRM Alliance,” 2022 ), side/waste stream-based Mg sources could be a cheaper and more preferred option for struvite precipitation. Research on these alternative Mg sources has focused on seawater ( Crutchik et al., 2013 ; Liu et al., 2013 ; Shaddel et al., 2020 ), brucite ( Hövelmann and Putnis, 2016 ; Huang et al., 2011 ), dolomite (DOL) ( Chen et al., 2017 ; Pesonen et al., 2019 , 2020 ), and fly ash (FA) ( Myllymäki et al., 2019 ; Pesonen et al., 2020 ).…”

“…Both DOL (CaMg(CO 3 ) 2 ) and FA contain high concentrations of calcium, which can prevent struvite formation because calcium precipitates phosphate quickly as calcium phosphates ( Hu et al., 2020 ; Pesonen et al., 2020 ; Shah et al., 2004 ). Treatment with sulfuric acid precipitates calcium as insoluble gypsum while transforming magnesium oxides/carbonates to soluble MgSO 4 ( Pesonen et al., 2020 ; Tolonen et al., 2015 ; Xiao et al., 2018 ). Pesonen et al.…”

“…Treatment with sulfuric acid precipitates calcium as insoluble gypsum while transforming magnesium oxides/carbonates to soluble MgSO 4 ( Pesonen et al., 2020 ; Tolonen et al., 2015 ; Xiao et al., 2018 ). Pesonen et al. (2020) used this type of treatment for both FA and DOL, with good results, as they precipitated struvite from synthetic waters.…”