Wastewater treatment valorisation by simultaneously removing and recovering phosphate and ammonia from municipal effluents using a mechano-thermo activated magnesite technology

Abstract

Phosphate and nitrate enrichment largely impair aquatic ecosystem functions and services, thus comprising an emerging problem of environmental concern. The problem pertains to developing countries where phosphate and nitrate discharge to surface water is on the rise due to a rapid growth in population. Herein, these pollutants (phosphate and ammonia) were removed from real municipal wastewater using a simple, fast, and cost-effective process. Raw cryptocrystalline magnesite, a mineral abundant in South Africa, was simply milled and calcined (mechano-thermo processing) in order to produce the activated magnesite (feed). The feed was then used in batch processing for pollutants adsorption and precipitation from real wastewater. The process was optimised by varying the treatment or contact time, feed dosage, concentration, pH, and temperature. The feed and product mineral (produced sludge) were characterised using X-ray Diffraction (XRD), field emission scanning electron microscopy (FESEM) compatible with energy dispersive spectroscopy (EDS), and Fourier Transform Infrared Spectrometer (FTIR). It was identified that the optimal conditions differed for each pollutant, highlighting the importance of tailoring the process to fit the local wastewater characteristics and as part of a treatment train system. Specifically, maximum P removal was achieved after 5 min of mixing, using 1 g L-1 of feed, 123 mg L-1 initial phosphate concentration, pH 8-10, and was not affected by temperature variations; whereas, for ammonia removal, optimal conditions were 180 min, 16 g L-1 feed dosage, 80 mg L-1 initial concentration, pH 10 and temperature > 45 °C. The optimal conditions for the removal of both pollutants from real wastewater were 30 min, 8 g L-1 dosage, 7.5 pH, 35 o C. Furthermore, Mg and Ca concentration was found to influence the process. Reduction in total dissolved solids and electrical conductivity suggest an attenuation of chemical species. Characterisation revealed that the product mineral obtained under the optimal conditions for pollutants removal is rich in quartz, periclase, brucite, calcite, magnesite, and struvite. This was further supported by the FTIR results, which indicated the presence of Mg-O, PO 4 3− , N-H and-OH stretches. In addition, the EDS verified the presence of Mg, Ca and P in product mineral. Results are suggestive of the high efficiency of the mechano-thermo activated magnesite treatment process for P and N removal and struvite crystallization. Thus, this technology could valorise municipal wastewater effluents and open new horizons for the

Optimization and reaction kinetics analysis for phosphorus removal in struvite precipitation process

“…Specifically, it has been reported that the growth of Mg 3 (PO 4 ) 2 was observed when pH level was higher than 9.5 based on the results of XRD (Hu et al, 2019; Yan & Shih, 2016). Moreover, the increase in pH level would result in the precipitation of Mg(OH) 2 according to XRD spectra (Mavhungu et al, 2019). Therefore, the optimal pH level for phosphorus removal from struvite precipitation is 9.5 in the current study, which were in agreement with the results of previous studies (Li et al, 2019; Shalaby & El‐Rafie, 2015).…”

Advocating circular economy in wastewater treatment: Struvite formation and drinking water reclamation from real municipal effluents

“…The first stage of the treatment process is the synthesis of struvite by means of thermally activated cryptocrystalline magnesite (calcined magnesite thereafter). The raw cryptocrystalline magnesite activation process, along with the main characteristics of the activated cryptocrystalline magnesite can be found in [1]. The conditions that optimize struvite synthesis, both in terms of pollutants removal and environmental sustainability, were adopted using the results of previous works of our group [1,20].…”

“…The raw cryptocrystalline magnesite activation process, along with the main characteristics of the activated cryptocrystalline magnesite can be found in [1]. The conditions that optimize struvite synthesis, both in terms of pollutants removal and environmental sustainability, were adopted using the results of previous works of our group [1,20]. Specifically, struvite was synthesized at 30 minutes of equilibration time, using a 0.5 g calcined magnesite to 500 mL wastewater ratio (i.e.…”

“…Municipal wastewater influents and poorly treated effluents comprise a heterogeneous mixture of microbial, organic, and inorganic contents [1][2][3][4][5]. Of prime concern is the ubiquitous presence of ammonia, nitrate, and phosphate, which can pose a major environmental threat to the receiving ecosystem if not managed properly.…”

“…This pilot study examines the performance of an actual struvite recovery system, scaled up at village-level and operating under the South African setting. Noteworthy, previous works focused on optimization of the operational parameters [1,20], which were used for the sustainable and effective operation of this pilot unit. Furthermore, it was identified that ammonia removal was dependent on phosphate and magnesium concentration, with decreasing phosphate concertation largely affecting the system's efficacy in terms of ammonia removal [1].In light of that, a fixed system under optimised conditions, which will yield high efficacy for ammonia removal, was examined as will be demonstrated.…”