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Phosphorus removal in wastewater

University of Queensland, Australia

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ACTI-Mag removes phosphate through struvite crystallization – University of Queensland, Australia.

case study summary

Challenges
  • High Phosphorous (P) content in wastewater leading to struvite scale built up in wastewater management systems.
  • Usage of chemicals for Phosphorous removal have issues such as:
  • – High operational costs;
  • – The requirement of additional chemicals for pH maintenance;
  • – Low solubility in alkaline conditions;
  • – Longer disassociation times.
Objective
  • Reduce struvite formation by decreasing phosphorous levels in wastewater
  • Reduce operational and chemical resource costs
  • Lessen disassociation times
  • Improve operational efficiency
Solution

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ACTI-Mag removes phosphate through struvite crystallization – a research study by the University of Queensland.

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Benefits of ACTI-Mag

ACTI-Mag provide to be an effective solution for struvite crystallization.

pH was naturally maintained above 8.5 without the use of Sodium Hydroxide.

Phosphate recovery using ACTI-Mag was similar to MgCl2.

ACTI-Mag was relatively highly soluble in real and synthetic wastewater.

ACTI-Mag reduced additional cost of chemicals such as caustic.

Non hazardous to humans and the environment, non-toxic and safe to handle.

INTRODUCTION

Phosphorus (P) removal from wastewater via struvite crystallization is an established technology, and its uptake is recently gaining momentum, both in domestic and industrial wastewater in Australia, primarily to meet discharge regulation (Mehta, Khunjar et al. 2015). Magnesium (Mg) and caustic are typically added during struvite crystallization and are the major operating cost of the technology ($0.6-0.8/kgP).

Magnesium salts (chlorides and sulphates) are highly water soluble, but they are expensive and additional chemical is required to maintain the alkaline pH condition required for struvite crystallization. Magnesium oxide (MgO) and magnesium hydroxide are comparatively less

expensive and raise solution pH, but they are sparingly soluble in alkaline conditions and require longer disassociation times. Hence, an excess amount of MgO/Mg(OH)2 is often required to achieve high P removal.

Calix ACTI-Mag is a magnesium hydroxide slurry with high surface area (350 m2/g), produced through the patented calcination process. Preliminary test at UQ showed quicker kinetics and higher P removal (P < 10 mg/L) at pH 8.5 (Mg:P = 1:1). The exact mechanism of high product reactivity and its performance as suitable material for P removal is unknown, and this needs to be tested.

Experimental plan

Struvite crystallization was performed using a 1L jacketed stirred tank reactor operated at 25oC in batch mode. Calix ACTI-Mag was comlab-grade MgCl2 using synthetic and real wastewater at different pH and dosing conditions. The synthetic wastewater was prepared using NaNO3 (0.58 mN), NaCl (10 mM), Na2SO4 (1.2 mM), NaHCO3 (17.86 mM), (NH4)2HPO4 (5 mM) and NH4Cl (25 mM). This equates to 154.5 mg/L of P-PO4 and 490 mg/L of N-NH4, with N:P molar ratio of 7. High N:P molar ratio (>5) is typical for domestic and agriculture wastewater.

Centrate was collected from a local sewage treatment plant operated by Queensland Urban Utilities (Luggage Point). The centrate was stored in cold room overnight and 1L supernatant was used for the study. The soluble P, N (NH4) and Mg concentrations of the centrate were 92.7, 715.6 and 4.5 mg/L respectively. The amount of ACTI-Mag used in this experiment was estimated using the specific gravity of 1.594, a total solid content of 1028 g/L, and 78% of total solids is Mg(OH)2 in active and available (data supplied by Calix).

The experiments were performed in absence of struvite crystals, as spontaneous primary nucleation was expected at P-PO4 = 0.5 mM (Mehta and Batstone 2013). A list of experiments is shown in Table 1 (Appendix A). The solution pH was continuously measured using conventional bench-scale glass pH probes, allowing automatic 1.0 M NaOH and 0.5M HCl addition using a PLC unit and LabView software. The dosing of acid/base and pH of the crystallizer solution was recorded for each experiment using a data logger. During each test samples were collected at t = 0, 0.5, 30, 60, 120, 480 and 1440 min. For soluble analyses, the collected sample was filtered through a 0.45-micron filter immediately. The elements (P, Mg and Ca) were analyzed in an Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Ionic concentrations P-PO4 and N-NH4 were measured using Flow Injection Analyzer using the QuikChem method.

Fig. 1: Change in phosphate concentration during struvite crystallization at different pH conditions using MgCl2 and ACTI-Mag.

In synthetic solutions, struvite crystallization kinetics using MgCl2 was rapid due to high supersaturation and the reaction was completed within a few minutes (Fig. 1). Sodium hydroxide was continuously dosed to maintain alkaline conditions using MgCl2. The kinetics using ACTI-Mag were relatively slower, but achieve around 90% of final equilibrium within 120 min (Fig. 1). The kinetics using ACTI-Mag were influenced by pH, the dissolution of ACTI-Mag and struvite formation was slower at pH 8.5 compared to 7.5. Sodium hydroxide was not required to maintain the pH, as pH was naturally buffered above 8.5 using ACTI-Mag. The pH was controlled by acid using ACTI-Mag, particularly at low alkaline pH conditions.

Fig. 2: Phosphate recovery during struvite crystallization at different pH conditions using MgCl2 and ACTI-Mag.

The phosphate recovery was measured as a percentage change in phosphate concentration from initial to the final concentration. The final concentration was measured from the samples collected at 120 min and 1440 min using MgCl2 and ACTI-Mag respectively. Longer dissolution time was expected using ACTI-Mag due to poor solubility of Mg(OH)2 in alkaline conditions. Influence of pH was observed on the equilibrium concentration, the phosphate recovery increased with pH (Fig. 2). The phosphate recovery was 5-10% lower using ACTI-Mag compared to MgCl2, but this could be due to lower available Mg in ACTI-Mag or error in estimating total Mg concentration. Based on mass balance, nearly 10% of Mg remaining unreacted from the product supplied. Phosphate recovery using ACTI-Mag was similar to MgCl2 at higher Mg dosing (1.5x or 2.0x).

The batch test was performed using centrate at pH 8.5 using ACTI-Mag at Mg:P ratio 1.3:1. The reaction kinetic for struvite crystallization was similar to the synthetic solutions. The equilibrium concentration of P-PO4 was 6.2 mg/L, similar values were observed using MgCl2. Excess Mg was completely dissolved during the experiment (24 h), as shown in Fig. 3. The results confirmed relative high solubility of ACTI-Mag for both synthetic and real wastewater. Mass balance of the elements confirmed the formation of struvite during this test.

PRODUCT CHARACTERISTICS

The struvite crystals produced at pH 8.5 using MgCl2 and ACTI-Mag were harvested from the crystallization reactor. Pure struvite contains 12.6% P, 5.7% N and 9.9% Mg, while the cleaned and dried product in this study contained 12.5-12.9% phosphorus and 3.2-4.5% nitrogen. Nitrogen could have lost during the drying process. No substantial difference was observed in struvite elemental composition recovered using the two forms of Mg tested. Calcium was the main impurities and is a challenge for struvite crystallization processes that leads to reduced struvite formation.

Scanning electron microscopy (SEM) for secondary electron and backscattered electron imaging as well as energy dispersive x-ray analysis (EDS) was performed to provide further information about the variation in scale size, structure and elemental composition of the struvite samples collected at pH 8.5 and a dried ACTI-Mag sample. Precipitants (<10 micron) and agglomerates (>200 microns) were observed in the ACTI-Mag sample (Fig. 3, Appendix B), Mg was dominating element in these particles. Presence of Si and Ca was also observed in the ACTI-Mag sample.

Orthorhombic crystals were observed in both ACTI-Mag and MgCl2 samples (Fig. 4, Appendix B), with Mg and P as key elements of the crystals suggesting the formation of struvite. Struvite crystals were relatively smaller size using ACTI-Mag (30-60 micron) compared to MgCl2 (70-100 micron). Narrow size distribution of struvite crystals was observed using ACTI-Mag, which could assist with recovery design.

Conclusion

Struvite crystallization seems to be the key driver for phosphate removal using ACTI-Mag. Mg dissolution from ACTI-Mag was relatively slower compared to MgCl2, with 90% Mg dissolution in 2h.

Smaller and similar size struvite crystals were observed using ACTI-Mag compared to MgCl2. The study confirms that ACTI-Mag is an effective product as Mg source for struvite crystallization and would substantially reduce caustic dosing cost required for struvite recovery.

APPENDIX A (LIST OF EXPERIMENTS)

Table 1: List of experiments to reduce soluble P concentration in the digested sludge.

APPENDIX B (SEM RESULTS)

Fig.3: SEM image of ACTI-Mag with EDX data

Fig.4: SEM image of Struvite using (a) MgCl2 and (b) ACTI-Mag.

ACTI-Mag for phosphate removal

ACTI-Mag, a very high surface area magnesium hydroxide liquid, targets phosphate while improving pH and organics removal within the wastewater treatment plant.

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