Use of Calcined Dolomite as Chemical Precipitant in the Simultaneous Removal of Ammonium and Phosphate from Synthetic Wastewater and from Agricultural Sludge

Abstract

Phosphorus as phosphate and nitrogen as ammonium or nitrate are the main nutrients in wastewaters and agricultural sludges. They runoff easily to waterways and cause eutrophication in water bodies. However, ammonium and phosphate could be precipitated simultaneously and used as recycled nutrients. In this research, dolomite calcined at 650 °C, 750 °C, or 950 °C and commercial MgO were used as precipitants in simultaneous phosphate and ammonium removal from synthetic (NH4)2HPO4 solution and agricultural sludge. Calcination at 750 °C was the preferred option as dolomite was decomposed to MgO and CaCO3 for optimal struvite precipitation. Molar ratios of 1.1–1.6:1–2:2 (Mg:P:N) were employed in the experiments. Very robust ammonium removal was obtained with MgO (57%), dolomite 650 °C and dolomite 750 °C (75%). MgO removed almost all phosphate, while dolomite 650 °C removed 65%, and dolomite 750 °C removed 60% (70% from agricultural sludge). Some part of the phosphate was adsorbed, most likely by CaCO3, during dolomite precipitation. Struvite was the only identified reaction product in all samples after 24 h of precipitation. Calcined dolomite had great potential in ammonium and phosphate precipitation from both synthetic waters and agricultural sludges and the precipitates could be used as recycled fertilizers.

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

“…With FA (Mg:P:N = 1.6:1:1) and DOL based MgSO 4 solutions (Mg:P:N = 1.6:1:1 and 2:1:1), the removal percentages were 59%, 62%, and 63.5% respectively. In all cases, there was only a minor change in the removal percentages after 4 h. Pesonen et al [19] reported similar removal percentages calcined DOL. Myllymäki et al [17] reported nearly 100% ammonium removal but some of the ammonium was evaporated as ammonia gas due to high pH.…”

“…However, there was a clear drop in the removal percentage at the end of the experiments. Therefore, the lengths of the experiments were reduced to 4-6 h. Others found that when calcined, powdered DOL [19] was used as a precipitation agent after being dissolved to water, the contact time needed was at least 24 h. The same finding was made with FA in another study [17]. This is due to the low solubility of MgO to water.…”

“…The procedure used for the precipitation is based on previous studies [17,19]. 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).…”

“…Due to the relatively high price of these commercial salts, and since magnesium is listed in the European Union's critical raw material list, alternative magnesium sources should be preferred instead of commercial salts [13]. For these reasons, several authors have studied various inexpensive Mg-sources for struvite precipitation, including brucite [14], seawater [12,15], magnesite [16], bottom and fly ash [17,18], dolomite (DOL) [19,20], and MgO-saponification wastewater [21].…”

“…Depending on the fuel used, it mostly contains calcium, magnesium, aluminum, and silicon oxides in varying proportions [23]. Even though both fly ash and DOL are potential precipitants for struvite precipitation [17,19,20,24] the high Ca concentration they contain can cause the phosphate ions to precipitate as hydroxyapatite preventing struvite formation [17,19,25], therefore it would be best to first precipitate Ca as insoluble CaSO 4 and transform the Mg ions to soluble MgSO 4 [26].…”