Elsevier

Chemosphere

Volume 136, October 2015, Pages 289-296
Chemosphere

Chlorination decomposition of struvite and recycling of its product for the removal of ammonium-nitrogen from landfill leachate

https://doi.org/10.1016/j.chemosphere.2014.10.078Get rights and content

Highlights

  • The chlorination decomposition of struvite was performed in this study.

  • 98% of NH4-N in struvite was removed at pH 6 and Cl/NH4-N wt ratio of 8.2:1.

  • 92% of NH4-N in landfill leachate could be removed at a precipitation pH of 9.

  • Struvite chlorination decomposition product could be effectively recycled.

Abstract

Struvite (MgNH4PO4  6H2O) precipitation is a promising method for ammonium (NH4+) removal from the wastewater. However, the high cost incurred with the use of magnesium and phosphate sources hinders the successful application of this method. This paper presents a novel recycling technology of struvite that is based on the chlorination decomposition of struvite. The study results indicated that struvite can be effectively decomposed by sodium hypochlorite and that the solid/liquid ratio of struvite in solution did not affect the ammonium-nitrogen (NH4-N) decomposition efficiency of struvite. Through the analysis of the reaction process, the mechanism of struvite decomposition was proposed to be simultaneous dissolution and oxidation of struvite, and the main component of the decomposition product generated was determined to be newberyite, dissolved HPO42− and Mg2+, and magnesium phosphate. When the decomposition product was recycled, its pH had to be adjusted for high NH4-N removal. NH4-N of 92% could be removed from landfill leachate when the decomposition product solution pH before recycling was adjusted to 3 and the precipitation pH was maintained at 9. A five-cycle recycling process showed that recycling of struvite chlorination decomposition product was a highly efficient and sustainable method for the removal of NH4-N. An economic evaluation showed that the use of recycled struvite for the five-process cycles by the proposed process could save the cost of chemicals by approximately 34% as compared to the use of pure chemicals.

Introduction

Nitrogen is an important nutritional element required by all life forms, but the presence of substantial quantities of NH4+ can lead to water eutrophication in the receiving water bodies such as lakes and rivers (Cooperband and Good, 2002, Stone, 2011). NH4+ pollution mainly occurs due to the discharge of industrial and municipal wastewater into water bodies. The most direct and effective way to prevent NH4+ pollution is to treat the wastewater before discharging it. However, the existing treatment methods of NH4+ used in industries, such as ammonia stripping (Quan et al., 2010), breakpoint chlorination (Charrois and Hrudey, 2007), and biological treatment processes (Ge et al., 2014), have various limitations. For example, problems such as the fouling of stripping tower and the secondary pollution of ammonia often occur during ammonia stripping. When breakpoint chlorination is used in the treatment of wastewaters containing high concentrations of organic matters, the consumption of chlorine is significantly increased.

In recent years, struvite precipitation has received much research interest (Di Iaconi et al., 2010, Zhang et al., 2012, Mehta and Batstone, 2013, Song et al., 2014). Struvite is a white, insoluble crystal mineral, with the solubility of 23 mg/100 mL (He et al., 2007). It can naturally crystallize when the combined ion concentration of magnesium, ammonium, and phosphate exceeds its solubility (Li and Zhao, 2003, Song et al., 2011). Thus, NH4+ in the wastewater can be removed by struvite crystallization as given in Eq. (1) (Mijangos et al., 2004).Mg2+(aq)+NH4+(aq)+HPO42(aq)+6H2OMgNH4PO4·6H2O+H+

However, because NH4+ wastewater commonly lacks magnesium and phosphate ions, numerous quantities of both magnesium and phosphate are required to be added externally for successful struvite precipitation. This requirement incurs an increase in the wastewater treatment cost, which seriously restricts its practical application. Presently, alternative ways to decrease the cost of struvite precipitation mainly involves the use of low-cost magnesium source (Chimenos et al., 2003, Chen et al., 2009, Lahav et al., 2013a) and struvite recycling (Türker and Çelen, 2007, Liu et al., 2011). Struvite recycle has received much attention since the past decade because it can greatly reduce the treatment cost of NH4+ wastewater. The conventional approaches to the recycling of struvite include direct pyrolysis (Sugiyama et al., 2005, Huang et al., 2011a) and sodium hydroxide pyrogenation (Zhang et al., 2009, Huang et al., 2011b).

The chemical reaction involved in struvite direct pyrolysis is given in Eq. (2). MgNH4PO4·6H2O(s)MgHPO4(s)+NH3(g)+6H2O(g)

Sugiyama et al. (2005) revealed that magnesium hydrogen phosphate (MgHPO4) with high NH4+ removal performance is favorably obtained by struvite pyrolysis at a pyrolysis temperature of >353 K, and magnesium phosphate [Mg3(PO4)2] and magnesium pyrophosphate (Mg2P2O7) are formed as derivatives of the reaction. Moreover, the amount of these derivatives increases with increasing pyrolysis temperature and time. However, Mg2P2O7 and Mg3(PO4)2 has only a weak effect on NH4+ removal, which results in a steady decline in the NH4+ removal rate with repeated recycling (Yu et al., 2012).

The sodium hydroxide pyrogenation reaction of struvite is expressed in Eq. (3). MgNH4PO4·6H2O(s)+NaOH(s)MgNaPO4(s)+NH3(g)+7H2O(g)

The content of the active ingredient (MgNaPO4) in struvite NaOH pyrogenation is greatly enhanced as compared to that in struvite direct pyrolysis. However, similar byproducts are obtained in both struvite direct pyrolysis and NaOH pyrogenation, except that the amount of byproducts produced is comparatively lesser in NaOH pyrogenation reaction. Nevertheless, the amount of Mg2P2O7 and Mg3(PO4)2 in the pyrolysate has been reported to increase with increase in the number of times of recycling (He et al., 2007, Türker and Çelen, 2007), resulting in a rapid decrease in the NH4+ removal rate.

The abovementioned analysis suggested that the production of byproducts is the major restraining factor for the optimization of the conventional struvite recycling techniques. To address this problem, Yu et al. (2012) used a mixture of NaOH and Mg(OH)2 to decompose struvite and found that the valid components in the struvite pyrolysate increased by this method. Zhang et al., 2009, Yu et al., 2012 attempted to eliminate the effect of byproducts on NH4+ removal by adding fresh phosphate and magnesium salts and by dissolving pyrolysate prior to reuse. They found that a steady NH4+ removal rate could be maintained with an increase in the number of times of recycling. Although these methods are feasible, they do not address issues such as the formation of byproducts, complexity of the operation, and difficulty in controlling the dosage of reagents. Therefore, to fundamentally resolve these problems, a breakthrough is required in the conventional struvite decomposition principle.

It is well-known that the breakpoint chlorination can quickly and effectively remove NH4+ from aqueous solution. This technique has been widely applied for the treatment of industrial wastewater and for the disinfection of drinking water. To the best of our knowledge, no literature is available about struvite decomposition by breakpoint chlorination for the purpose of recycling struvite. Decomposing struvite by breakpoint chlorination may present the following three advantages as compared to the conventional struvite pyrogenation methods: (1) the ammonium in struvite can be rapidly (within few minutes) oxidized to nitrogen gas by breakpoint chlorination to generate active phosphate and magnesium; (2) the reaction conditions of breakpoint chlorination is mild, which does not support the formation of other phosphate and magnesium complexes (such as Mg2P2O7); furthermore, due to almost no adsorption of organic matters in the struvite solid–liquid (S/L) separation, no harmful chloric compounds is produced in the chlorination decomposition process; and (3) the NH4+ in the conventional struvite pyrogenation method is not eliminated, rather only transformed to ammonia gas that has to be recovered. However, this problem does not occur in the chlorination decomposition process.

In this study, we adopted a different approach and proposed the oxidation–reduction decomposition method of struvite. It is expected that the production of invalid ingredients can be avoided by modifying the conventional struvite pyrogenation theory. The main objectives of the current study were to investigate the efficiency and the mechanism of decomposing struvite by breakpoint chlorination, the performance of NH4+ removal by reusing the decomposition product, and the efficiency of NH4-N removal by multiple recycling.

Section snippets

Experiment materials

The raw wastewater used in the experiments was obtained from a municipal sanitation landfill site located at the suburban area of Beijing, China. The collected wastewater was filtered through filter paper prior to use. Table 1 shows the characteristics of the filtered wastewater sample. The struvite used in the experiments was prepared as followed: magnesium chloride (MgCl2  6H2O) and disodium hydrogen phosphate (Na2HPO4  12H2O) of analytical grades were added to 2000 mL of aqueous solution with

NH4-N-decomposition efficiency of struvite

The effects of S/L ratio and pH on the NH4-N-decomposition efficiency of struvite are depicted in Fig. 1a. This figure reveals that the NH4-N-decomposition efficiency of struvite increased at the pH range of 5–6 and was maximum at pH 6, followed by a slight decrease at pH range of 7–9. The changing profile of the NH4-N-decomposition efficiency of struvite was similar to that reported previously in the treatment of NH4+ wastewater by breakpoint chlorination at different pHs (Pressley et al., 1972

Conclusions

The chlorination decomposition of struvite is very rapid and can be completed within a few minutes. When the decomposition condition of struvite was maintained at pH 6 and Cl/NH4-N weight ratio of 8.2:1, 98% of NH4-N in struvite was removed. To obtain high removal ratio of NH4-N, the solution pH before recycling the struvite decomposition product as the magnesium and phosphate sources was required to be adjusted. Here, >92% of NH4-N could be removed from landfill leachate at a precipitation pH

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51408529 and 21207112), the Natural Science Foundation of Hebei Province (Grant No. E2014203080) and the Free Research Foundation for Young Teachers of Yanshan University (13LGB023).

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