Phosphorus as phosphate and nitrogen as ammonium or nitrate are the main nutrients in wastewaters and agricultural sludges. They runoff easily to waterways, especially when agricultural sludges are used as fertilizers. This promotes the growth of organic matter and algae, which causes eutrophication in water bodies. In addition, nitrogen is typically present in the form of ammonium, which volatilizes easily as ammonia gas under alkaline conditions [1
]. These factors reduce the effect of fertilization.
In the optimum case, both ammonium and phosphate would be precipitated simultaneously and employed as recycled fertilizer. Especially seeing that the European Commission has released a proposal for the revision of the EU’s fertilizer legislation, the usage of recycled fertilizers will rise [2
]. The proposal is still under discussion, but the use of bio-based and recycled fertilizers will be encouraged in the revised directive. In Finland alone, the economic potential of nutrient circulation is approximated to be €0.5 billion annually [3
]. Moreover, traditional nitrogen fertilizers are produced mainly through use of energy-intensive Haber–Bosch reactions, which cause large amounts of greenhouse gas emissions as production consumes high amounts of natural gas [4
]. It has been estimated that, in the following decades, exploitable phosphate resources will be significantly depleted [5
]. The geographic distribution of exploitable reserves is very inhomogeneous with 77% of global reserves being located in Morocco [6
]. In addition, the remaining rock phosphate reserves are characterized by decreasing quality (higher Cd and U concentrations) and more expensive mining technology is necessary [7
]. Therefore, the need for the recovery of phosphorus is very significant.
Ammonium and phosphate can be simultaneously precipitated as a phosphate mineral, struvite (NH4
O), which could be used as a slow-release fertilizer, reducing the nutrient supply to the waterways and diminishing ammonium losses as ammonia gas. Typical precipitants for struvite precipitation are commercial MgCl2
, MgO, and Mg(OH)2
]. Owing to the relatively high price of these commercial salts, there is a need to find other suitable cost-efficient precipitants for struvite precipitation. Inexpensive magnesium sources, such as magnesite [9
], brucite [10
], a byproduct of marine salt manufacturing and the thermal treatment of meat waste and bone meal [11
], MgO-saponification wastewater [12
], seawater [13
], and wood ash from a residential fireplace [15
], to produce struvite have been tested previously.
In this study, dolomite was utilized as a precipitant. Dolomite is a carbonite mineral composed of calcium magnesium carbonate, CaMg(CO3
, and it is used, e.g., as a soil improver to increase the pH of acidic soils [16
]. However, it could also be applied for struvite precipitation but there exists a very limited amount of research on this subject. Xiao et al. [17
] used dolomite to neutralize waste sulfuric acid from the mining industry and employed the magnesium sulfate-containing supernatant to precipitate struvite from swine wastewater. However, as waste sulfuric acid can contain large amounts of toxic compounds [18
] that can possibly precipitate together with nutrients, this procedure cannot necessarily be recommended. Chen et al. [19
] used commercial dolomite calcined at 750 °C to precipitate struvite from model wastewater but did not perform any tests with authentic wastewaters. Waste dolomite has not been previously tested for struvite precipitation, and there has been no research on authentic wastewaters.
In this research, calcined dolomite (a waste fraction) was used to simultaneously precipitate phosphate and ammonium from agricultural sludge in addition to synthetic (NH4)2HPO4 solution. Effects of calcination temperature, precipitation time and precipitant dosing on the removal efficiency of phosphate and ammonium were studied. Comparisons were made to commercial MgO.
2. Materials and Methods
A Finnish lime quarry provided dolomite for this study. It was of a small-sized (<0.05 mm) fraction that is leftover as the dolomite is sieved to desired-size fractions. The demand for these small particle sizes is very limited and is currently mostly considered a waste fraction for the lime quarry. Before the experiments, dolomite was calcined at 650 °C, 750 °C or 950 °C. As a comparison, pure MgO was utilized as a precipitant. Molar ratios for the precipitation experiments were chosen using the MineQL program. Molar ratios, Mg:P:N of 1.1–1.6:2:2 (dolomite) or 1.1–1.6:1:2 (MgO), were employed in the experiments. Precipitant solutions were prepared by dissolving 0.5–2.3 g of precipitant in 10 mL of de-ionized water. The formed suspension was hence saturated. Synthetic (NH4
solution (200 mg·L−1
and 1050–2100 mg·L−1
) was prepared from ammonium chloride (NH4
Cl) and potassium hydrogen phosphate (KH2
) salts. Precipitation experiments were conducted at room temperature (20 °C) and the schematic diagram of the batch-mode reactor used in the experiments is presented in Figure 1
In the experiments, 10 mL of the saturated precipitant solution was added to 1.6 L of the ammonium phosphate solution while stirring the solution at a constant speed of 450 rpm for 1 min to mix up the two solutions properly. Afterwards, the rotor speed was reduced to 50 rpm for the duration of the experiment (4 h, 24 h, or 48 h) and the pH was adjusted to either 8.5 (MgO and dolomite 650 °C and 750 °C) or 9.0 (dolomite 950 °C) and kept constant. Water samples (25 mL) were taken in the beginning, and then after every half hour until the end of the experiments. They were filtered through 4–12 µm filter paper before analysis.
Further, one experiment with agricultural sludge was performed. Sludge was first filtered through 13–15 µm filter paper before the experiment. Filtrate had a pH of 8.95, a phosphate concentration of 25 mg·L−1, and an ammonium concentration of 137 mg·L−1, therefore KH2PO4 was added to adjust the N:P ratio to 1:1. Dolomite 750 °C was used as a precipitation agent (pH 9; reaction time, 24 h).
Ammonium concentration was measured from the water samples with the use of an NH4+ selective electrode and phosphate concentration was measured by ion chromatography (IC; Methrom 761 Compact IC). Precipitate was collected after the experiments and dried at 105 ˚C (4 h experiments) or room temperature (24 h experiments). It was analyzed using a CHNS analyzer, x-ray diffractometer (XRD), and scanning electron microscope (SEM). Dolomite was characterized with SEM, an X-ray fluorescence spectrometer (XRF) and a thermogravimetry differential scanning calorimeter (TG-DSC). The microstructure shown in the FESEM images were obtained with a Zeiss Sigma field-emission scanning-electron microscope (FESEM) at the Centre of Microscopy and Nanotechnology at the University of Oulu operated at 5 kV. A Bruker AXS S4 Pioneer was used for XRF measurement. The sample powders were added 6% C-wax as binder and pressed into pellet specimens with a diameter of 37 mm in a steel ring. The detectable element concentration was 0–5 mg·kg−1 for XRF. A Rigaku SmartLab 9 kW XRD was applied for the phase and structure evaluation. Elemental analysis was performed with a Flash 2000 CHNS-O Organic elemental analyzer produced by Thermo Scientific. The TG-DSC was performed at the research unit of Process Metallurgy at the University of Oulu using a Netzsch STA 449F3 thermal analyzer.