The Use of Ca- and Mg-Rich Fly Ash as a Chemical Precipitant in the Simultaneous Removal of Nitrogen and Phosphorus—Recycling and Reuse

Abstract

The European Union’s circular economy strategy aims to increase the recycling and re-use of products and waste materials. According to the strategy, the use of industry waste material should be more effective. A chemical precipitation method to simultaneously remove phosphorus and nitrogen from synthetic (NH4)2HPO4 solution and the liquid phase of anaerobic digestate using fly ash as a precipitant was tested. Fly ash is a waste material formed in the power plant process. It mainly contains calcium oxide (CaO) and magnesium oxide (MgO). Saturated precipitant solution was prepared from fly ash, which was added in small proportions to (NH4)2HPO4 solution during the experiment. Fly ash’s effectiveness as a precipitant was compared with that of commercial CaO and MgO salts, and it can be observed that fly ash removed as much ammonium and phosphate as commercial salts. Fly ash sufficiently removed ammonium nitrogen and phosphate from the liquid phase of anaerobic digestate, which led to the formation of ammonium magnesium hydrogen phosphate hydrate, struvite (NH4MgPO4·6H2O), and calcium hydroxide phosphate, monetite, CaPO3(OH). In this study, we have shown for the first time that fly ash can be used to manufacture recycled, slow-release fertilizers from anaerobic digestate.

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

“…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).…”

“…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. Also, shorter contact times (2-4 h) have been used in other studies [18,20].…”

“…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].…”