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Evaluating the effects of operational parameters and conditions on struvite crystallization and precipitation with the focus on temperature.

Background:

Duo to the rapid population growth during the past century the aim of the wastewater treatment plants has shifted from sanitation and environment protection to environmental sustainability. At the same time, the idea of more ideal sustainability systems has obtained attraction during the past two decades. As the new treatment problems arise new approaches are felt to be needed. Instead of linear treatment systems new integrated treatment systems are to be used in order to better consider water cycle and in broader meaning the cyclic character of nature and of course their better and higher efficiency.

In order to prevent the oxygen depletion in water streams organic matters are supposed to be removed prior to any other further treatment processes. In water and specially wastewater streams, organic matter is mostly known and measured as chemical oxygen demand (COD) and removed through some biological processes like biological oxidation. This is the same for nutrients--phosphorous and nitrogen--removal as they are also known as pollutant for wastewater streams. The following are some of the main reasons that nutrients need to be removed in wastewater streams.

Both, ammonium and phosphate are considered to be contaminants for water streams. In most wastewater streams regarding the total nitrogen content, Ammonium (NH4+) is considered to be the main constituent which needs to be removed as the free ammonia (NH3) is considered to be dangerous to most aquatic animals, duo to its high toxicity. At the same time, ammonium in receiving waters will be easily oxidized to nitrate (NO3) which leads to further depletion of oxygen consideration and increase in COD.

INTRODUCTION

Struvite is a well-known salt which consists of [Mg.sup.2+], [NH4.sup.+] and P[O.sub.4.sup.3+] and in specific conditions crystallizes in the form of MgN[H.sub.4]P[O.sub.4].6[H.sub.2]O. Struvite or MAP (Magnesium Ammonium Phosphate) is a white or sometimes yellowish crystalline substance which is consisting of equimolecular ratios of magnesium, Ammonium and phosphate. There have been some researches on the possibility of wastewater treatment through this substance (struvite) precipitation (Taylor, A.W., 1963; Abbona, F., 1982; Ohlinger, K.N., 1998; Burke, S., 2004).

Struvite (MgNH4P[O.sub.4].6[H.sub.2]O) precipitation is one of the major concerns in wastewater treatment plants (Doyle, J.D., 2003). This scale deposits were first observed as crust of crystalline materials in some stages of sludge digestion systems in wastewater treatment plants (Rawn, A.M., 1931). It was observed that the presence of this precipitates highly affect the treatment process efficiency and at the same time causes lots of maintenance problems.

Struvite and the possible ways of controlling its precipitation have been investigated like mixing and diluting the crystals with water effluent (Snoeyink, D. and V.L. Jenkins, 1980) or even the addition of chemical inhibitors was also tried (Doyle, J.D., 2003; Snoeyink, D. and V.L. Jenkins, 1980). Nowadays, this process is considered as a complementary step in water treatment processes which led to its integration in the majority of wastewater treatment plants due to its efficiency in recovering phosphate and nitrogen from the wastewater stream (in the form of struvite).

The application of the struvite is so vast and this process could be applied on any type of the wastewater streams like, industrial wastewater, landfill leachate and any type of wastewater which is proved to maintain a minimum trace of phosphate and nitrogen contents. Combined with anaerobic digester system has been shown to have the maximum efficiency in nutrients removal and recovery.

There are several parameters which their effects on struvite crystallization have been experimentally assessed. Among them, the following parameters have been proved to have the most influences:

* pH

* Temperature

* Molar Ratios'

* Foreign Ions

* Super Saturation

* Aeration Rate

For an effective removal an optimum combination of these mentioned parameters is required then. The struvite solubility is very low in the water and is highly dependent on pH. So, if the conditions are favourable almost all the nutrients (phosphate, nitrogen) could be removed from the wastewater.

Literature of the past studies on struvite:

Within the past decade, interests have been raised again in struvite precipitation as a mean of phosphorous and nitrogen removal and recovery process. Its composition with equal molar concentrations of nitrogen [N], P, and magnesium [Mg] ions and as its nucleation and the quality of recovered crystals are controllable is going to make this scale deposits a good and marketable product for the industry of fertilizer production (Booker, N., 1999). Research in struvite precipitation and formation is now spread out all over the world since its first recognition in 1937 and now includes different aspects from alternative phosphorous and nitrogen removal and recovery process to prevention of scaling process and even in the fertilizers industry as a slow release fertilizer.

There have been so many studies carried out to evaluate the potential methods of phosphorous and nitrogen recovery and removal from wastewater streams at different scales and even few processes integrated in treatment plants which were found effective in Japan (Ueno, Y. and M. Fujii, 2001), The Netherlands (Giesen, A., 1999) and Italy (Battistoni, P., 2005). Based on their findings, struvite precipitation is an efficient approach of wastewater treatment as it reduces both Phosphorous and Nitrogen contents from the effluent stream to fulfill the legislation and regulations set by the region.

There are lots of researches being conducted on struvite precipitation in order to explore its possible applications on wastewater streams for recovering reusable phosphorous content (Burns, R. and L. Moody, 2002). As the results of kinetics and settling properties studies carried out by Li et al. (2003), Shin and Lee (1998) and Durrant et al. (1999) the crystal nature and the struvite ease of settling were discovered. Struvite reuse and recovery studies still are not adequately studied yet. The characteristics of struvite as fertilizer were studied by Edge (2013) and Schuiling and Andrade (1998).

The conversion of struvite to magnesium phosphate ([Mg.sub.3][(P[O.sub.4]).sub.2]) as a suitable product for recycling and removal of ammonium has been studied by Stefanowicz et al. (1992). Besides all of these studies which helped and improved the understanding of struvite crystallization and precipitation, additional studies have also been conducted on the applications of struvite. Schulze-Rettmer (1991) studied and discussed the potential of struvite application in domestic wastewater treatment.

Animal farm wastewater was known to be rich in phosphorous and ammonia contents for a long time. The struvite precipitation of piggery effluent wastewater was studied by Webb and Ho (1991). The effect of struvite precipitation has been experimentally investigated on domestic wastewater, the effluent of nitrogen work and coking plant by Zdybiewska and Kula (1991). Struvite precipitation and its effect on ammonia removal and its efficiency with the initial concentration of 1000 [mgl.sup.-1] and above have been studied and investigated by Shin and Lee (1998) which led to a very high percentage of removal.

Kabdasali et al. (2000) reported the phosphorous removal of 95% by applying struvite precipitation to the wastewater of textile printing. Struvite precipitation on anaerobically treated leachate with the concentration of 2000 [mgl.sup.-1] has been experimentally studied by them (2000). They achieved 90% removal of ammonia which is a quite considerable amount for ammonia removal. Again for leachate, containing ammonia with the concentration of 5618 [mgl.sup.-1], Li et al. (2003) conducted a study in which based on their results, struvite precipitation provided more than 90% ammonia removals.

These studies plus the ongoing studies proved a fact that struvite precipitation can be considered as a reliable alternative for phosphorous, nitrogen and ammonia removal. Further studies need to be done on struvite precipitation in order to extend the data to potential applications. This is the same for evaluating the results and the data to derive application criteria which is theoretically meaningful.

The effects of temperature on struvite crystallization:

Aage HK, et al, (1997) declares that the effect of temperature on solubility demonstrates a steady upward trend at the beginning which is followed then by a decrease. They found out that 50 degree Celsius is the temperature which has the highest solubility and after that the downward trend begins. It was also shown that temperatures above 64 degree Celsius could be influencial as the structure of struvite is changed.

Nebergall, W.H., et al, (1980) discussed the effects of temperature on the nature of precipitation as absorption or release of heat. They imply that as the temperature of a solute is being increased which means endorthermic reaction the solubility will also be increased. They actually believe that temperature and solubility has a direct relation with each other as one increases the other on increase and vice versa.

There are actually lots of researchers who have investigated the effect of temperature on solubility of struvite. In a study (Bhuiyan, M.I.H., 2007) conducted on anaerobic digestion system sludge it was concluded that as the temperature increases from 0 to 20 degrees Celsius the struvite solubility reaches its ultimate amount. They measured the struvite solubility over a temperature range of 10 to 60 degrees Celsius with 5 degrees Celsius intervals. To attain the equilibrium all the mixtures were kept for about three days. Table 1 better shows how struvite solubility changes by different temperatures.

In another research about the struvite crystallization it was concluded that 30 degrees Celsius is the temperature of maximum struvite solubility. The higher temperatures might affect the struvite structure and this transition which is accompanied by phase change would affect the struvite solubility to high extent (Burns, J.R. and B. Finlayson, 1982; Babic-Ivancic, V., 2002). They also mentioned that the higher temperatures means over 70 degrees Celsius caused the ammonium to evaporate. Figure 1 better illustrates the solubility of struvite at different temperatures.

The effects of temperature on ammonia removal have been studied by I. Celen & M. Turker. They studied the effect of temperature changes on ammonia removal as struvite between 25 to 40 degrees Celsius at Mg:N:P ratio of 1.2:1:1.2 and came to this conclusion that the effects of temperature in the mentioned conditions can be ignored as they are not influential. This is what M. I. Ali also indicated in his research as well (Ali, M.I., 2007). Figure 2 gives picture of the ammonia removal percentage versus temperature.

They summarized their findings as follows:

1. The equilibrium concentration of ammonia between 25 to 40 degrees Celsius is not influenced by the reaction temperature.

2. They also found out the process could only be viable if a reliable market would be on hand, otherwise the costs of the experiments would exceed the income of the project in any scale.

On the other hand, Borgerding (Borgerding, J., 1972) believes that as temperature affects the solubility of struvite so it cannot be ignored. He submitted the graph below means figure 3, which better illustrates the effects of temperature changes on struvite precipitation.

Based on the graph, as the temperature increases the solubility also shows an upward trend which reaches 500 mg/L around 20 degrees Celsius and after that the solubility decreases as temperature increases. Based on their findings the effect of temperature on struvite precipitation could not be easily ignored as the process is sensitive to temperature and the solubility of the struvites changes by temperature.

Conclusion:

Going through the literature the concepts of struvite precipitation became known. Based on the mentioned concepts some major parameters affecting the process of struvite precipitation was mentioned. Among the affecting parameters some researchers believe that the effects of temperature were ignorable and some had different opinion. Both opinions were discussed.

ARTICLE INFO

Article history:

Received 25 January 2014

Received in revised form 12 March 2014

Accepted 14 April 2014

Available online 5 May 2014

REFERENCES

Aage, H. and B. Andersen, 1997. "The solubility of struvite," J. Radioanal., 223: 213-215.

Abbona, F., H.E. Lundager Madsen and R. Boistelle, 1982. "Crystallization of two magnesium phosphates, struvite and newberyite: Effect of pH and concentration," J. Cryst. Growth, 57(1): 6-14.

Ali, M.I., 2007. "Struvite Crystallization in Fed-Batch Pilot Scale and Description of Solution Chemistry of Struvite," Chem. Eng. Res. Des., 85(3): 344-356.

Babic-Ivancic, V., J. Kontrec, D. Kralj and L. Brecevic, 2002. "Precipitation diagrams of struvite and dissolution kinetics of different struvite morphologies," Croat. Chem. acta, 75(1): 89-106.

Battistoni, P., R. Boccadoro, F. Fatone and P. Pavan, 2005. "Auto-nucleation and crystal growth of struvite in a demonstrative fluidized bed reactor (FBR)," Environ. Technol., 26(9): 975-982.

Bhuiyan, M.I.H., D.S. Mavinic and R.D. Beckie, 2007. "A solubility and thermodynamic study of struvite.," Environ. Technol., 28(9): 1015-26.

Booker, N., A. Priestley and I. Fraser, 1999. "Struvite formation in wastewater treatment plants: opportunities for nutrient recovery," Environ. Technol., no. February 2013, pp: 37-41.

Borgerding, J., 1972. Phosphate Deposits in Digestion Systems., 44(5): 813-819.

Burke, S., L. Heathwaite and N. Preedy, 2004. "Transfer of phosphorus to surface waters; eutrophication," Phosphorus Environ. Technol. Princ. Appl., pp: 120-140.

Burns, J.R. and B. Finlayson, 1982. "Solubility product of magnesium ammonium phosphate hexahydrate at various temperatures," J. Urol., 128(2): 426-428.

Burns, R. and L. Moody, 2002. "Phosphorus recovery from animal manures using optimized struvite precipitation," Proc. Coagulants ..., no. 2001: 1-7.

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(1) A. Salsabili, (2) Mohamad Amran Mohd. Salleh, (2) Mahmood Zohoori

(1) Department of Chemical and Environmental Engineering, University Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia.

(2) Institute of Advanced Technology, University Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia.

Corresponding Author: A. Salsabili, Department of Chemical and Environmental Engineering, University Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia.

Table 1: Struvite solubility changes by temperature.

Temperature (0C)    pKsp       Ksp(X10-14)

10                  14.36      0.436
15                  14.04      0.916
20                  13.69      2.050
25                  13.36      4.330
30                  13.17      6.840
35                  13.23      5.920
40                  13.40      4.000
45                  13.60      2.530
50                  13.68      2.110
55                  03.84      1.460
60                  14.01      0.973
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Author:Salsabili, A.; Salleh, Mohamad Amran Mohd.; Zohoori, Mahmood
Publication:Advances in Natural and Applied Sciences
Date:Apr 1, 2014
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