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Effects of ionizing radiation on struvite crystallization of livestock wastewater
Radiation Physics and Chemistry 97 (2014) 332–336
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effects of ionizing radiation on struvite crystallization of livestock wastewater Tak-Hyun Kim n, Yun-Ku Nam, Seung Joo Lim Research Division for Industry and Environment, Korea Atomic Energy Research Institute, Jeongeup 580-185, Republic of Korea
H I G H L I G H T S
Gamma ray was applied prior to struvite crystallization of livestock wastewater. Gamma ray resulted in an enhancement of struvite crystallization efficiency. This is due to the decrease of COD concentration by gamma ray irradiation.
art ic l e i nf o
a b s t r a c t
Article history: Received 6 August 2013 Accepted 26 December 2013 Available online 4 January 2014
Livestock wastewater is generally very difficult to be treated by conventional wastewater treatment techniques because it contains high-strength organics (COD), ammonium (NH4 þ ), phosphate (PO43 ) and suspended solids. Struvite crystallization has been recently studied for the simultaneous removal of NH4 þ and PO43 . In this study, gamma ray irradiation was carried out prior to struvite crystallization of the anaerobically digested livestock wastewater. The effects of gamma ray irradiation on the struvite crystallization of livestock wastewater were investigated. As a result, gamma ray irradiation can decrease the concentration of COD, NH4 þ and PO43 contained in the livestock wastewater. This results in not only an enhancement of the struvite crystallization efficiency but also a decrease in the chemical demands for the struvite crystallization of livestock wastewater. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Livestock wastewater Struvite crystallization Gamma ray irradiation NH4 þ PO43
1. Introduction Livestock wastewater contains high-strength COD, BOD5, nitrogen, phosphorus, and suspended solids. In particular, livestock wastewater has high concentrations of ammonium and phosphate, which can reach values of around 8000 mg NH4 þ /l and 1000 mg PO43 /l. The presence of nitrogen in wastewater discharge is undesirable for several reasons. Free ammonia is toxic to many aquatic organisms, and moreover, ammonium ions are oxygenconsuming compounds that deplete the dissolved oxygen in the receiving water. In addition, all forms of nitrogen and phosphorus can be made available to aquatic plants and can consequently contribute to eutrophication. Furthermore, the presence of nitrite and nitrate ions, i.e., the oxidized forms of ammonium ions through a biological nitrification reaction, in drinking water is a potential public health hazard. However, it is very difficult to treat livestock wastewater using a conventional wastewater treatment technique such as a biological nitrification/denitrification process for nitrogen removal and chemical coagulation for phosphorus removal.
n
Corresponding author. Tel.: þ 82 63 570 3343; fax: þ82 63 570 3348. E-mail addresses: [email protected], [email protected] (T.-H. Kim).
0969-806X/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.12.033
As an alternative to treating the high concentration of NH4 þ and PO43 in livestock wastewater, the precipitation of NH4 þ and PO43 by forming magnesium ammonium phosphate (struvite, MgNH4PO4 6H2O) has been recently studied (Lee et al., 2003; Ryu et al., 2008). Struvite, magnesium ammonium phosphate (MAP), is a naturally occurring crystal, that is formed when the combined concentrations of Mg2 þ , NH4 þ and PO43 exceed its solubility limit. Struvite precipitation is controlled by pH, supersaturation, temperature, and impurities such as calcium ions. As struvite has a low solubility constant, pKs, between 12.60 (Stumm and Morgan, 1970) and 13.26 (Ohlinger et al., 1998), its insoluble form can be easily formed and simply separated from the water phase. The struvite crystallization technique has been applied to various types of wastewater, such as human urine (Lind et al., 2000; Ganrot et al., 2007), semiconductor wastewater (Ryu et al., 2008), swine wastewater (Suzuki et al., 2007; Uludag-Demirer et al., 2005, and landfill leachate (Kabdash et al., 2008; He et al., 2007; Li and Zhao, 2003; Kim et al., 2007a, 2007b). The recovered struvite can be considered a useful slow-release fertilizer (Kim et al., 2007a, 2007b). However, the successful application of the struvite crystallization process depends on two main factors: the molar ratio of Mg: NH4 þ :PO43 and the pH value in the reactor. In most cases,
T.-H. Kim et al. / Radiation Physics and Chemistry 97 (2014) 332–336
magnesium deficiency in wastewater allows it to be added to remove all available nutrients such as phosphorus and/or ammonium–nitrogen in the water phase. If the struvite crystallization process is applied to remove only ammonium–nitrogen, the phosphate forms of H3PO4 or NaHPO4 are usually needed. The required alkaline condition of an elevated pH in the struvite crystallization system can be achieved through alkaline addition or pre-aeration (Suzuki et al., 2007). Struvite solubility decreases with an increasing pH, while above a pH of 9, its solubility begins to increase. In general, crystal formation primarily proceeds by the nucleation from crystal embryos followed by crystal growth (Kim et al., 2007a, 2007b). Since magnesium tends to be low relative to the concentrations of ammonia and phosphate in many wastewaters, the cost of adding magnesium salts is a major economic constraint to the application of struvite crystallization for nutrient recovery. The main obstacle to apply the struvite crystallization process is the high consumption of magnesium and phosphate salts, which leads to a high operation cost. Kumashiro et al. (2001) have demonstrated the use of seawater as a source of Mg2 þ for struvite crystallization at the pilot scale. In addition, Lee et al. (2003) used bittern as a source of Mg2 þ . Bittern is produced through the evaporation of seawater. It contains mostly magnesium chloride with smaller amounts of other inorganic compounds. Its Mg2 þ content is approximately 32 g/l, which is 27-times that of seawater (Lee et al., 2003). The ionizing radiation such as a gamma ray or electron beam has been considered a promising method for wastewater treatment since the secondary polluted materials are not produced and the process is relatively simple and compact. Radiation technology has been applied to oxidize the organic compounds and enhance the biodegradability of wastewaters containing various biologically refractory organic compounds such as landfill leachate, paper mill wastewater, effluent from a petroleum production, methyl tert-butyl ether (MTBE), cork wastewater, and pharmaceuticals (Auslender et al., 2002; Bae et al., 1999; Cooper et al., 2002; Duarte et al., 2004; Yamazaki et al., 1983; Madureira et al., 2013; Kimura et al., 2012) In this study, the gamma ray was applied as the pretreatment of struvite crystallization of livestock wastewater, and the effects of gamma ray irradiation on the removal of organics, ammonium and phosphate ions were investigated.
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Table 1 Characteristics of supernatant of anaerobically digested livestock wastewater. Items
Concentration, mg/l Range
TSS TCODcr T–N NH4 þ NO2 NO3 T–P PO43 pH ( )
470–650 1446–2840 445–1279 426–1132 0.1–0.7 10–26 78–229 69–116 8.2–8.8
Mean value 520 1557 724 653 0.4 15 94 80 8.7
(Nordion, Inc., Canada) at the Korea Atomic Energy Research Institute. The samples were irradiated in sealed Pyrex glass vessels by 60Co. The applied doses were in the range of 10–100 kGy. The absorbed doses were measured using an alanine-EPR dosimetry system (ISO/ASTM 51607:2003) (ASTM, 2004). Samples were taken and filtered through a 0.45 μm glass filter to obtain a filtrate. The changes in soluble chemical oxygen demand (SCOD), ammonium (NH4 þ ), and phosphate (PO43 ) were determined based on the standard methods (APHA, 1998).
3. Results and discussion 3.1. Composition of anaerobically digested livestock wastewater Table 1 shows the characteristics of the anaerobically digested livestock wastewater. The anaerobically digested livestock wastewater contains approximately 520 mg/l of total suspended solids (TSS) and 1557 mg/l of total COD (TCOD). The pH of the wastewater was about 8.7. It also contains approximately 653 mg/l of NH4 þ and 80 mg/l of PO43 . The molar ratio of NH4 þ :PO43 is just 1:0.06. Struvite is magnesium ammonium phosphate and forms a crystalline deposit when the molar ratio of NH4 þ :Mg2 þ :PO43 is greater than 1:1:1. Therefore, not only the external magnesium sources but also the external phosphate sources should be added for the struvite crystallization. The livestock wastewater contains a high level of insoluble P (Table 1), a major part of which was thought to settle along with suspended solids.
2. Experimental The livestock wastewater was collected from public livestock wastewater treatment plants located in Gongju, Korea. The characteristics of the raw and anaerobically digested livestock wastewater are summarized in Table 1. For struvite crystallization, a magnesium chloride (MgCl2 6H2O) solution and potassium phosphate (KH2PO4) solution were used as alternate sources of magnesium ions and orthophosphate ions, respectively. 10 N NaOH was used to control the pH of the solutions. Experiments were carried out at ambient laboratory temperature using a jar test apparatus. The high purity of struvite deposits can be achieved through the addition of magnesium and orthophosphate prior to the pH control of 9. When the pH was initially controlled at 9, after feeding the magnesium and orthophosphate, the final pH dropped to as low as 6. Straful et al. (2001) demonstrated that hydrogen is released into the solution when struvite is formed, resulting in a decrease in pH. The solutions in the jar were mixed at a mixing speed of 200 rpm and a mixing duration of 3 min. The mixed solution was then allowed to be settled for a duration of 30 min. Gamma ray radiolysis experiments were carried out using a 60Co source
3.2. Optimization of struvite crystallization conditions of anaerobically digested livestock wastewater In this study, struvite crystallization was carried out to treat livestock wastewater. To determine the optimal dose of MgCl2 6H2O, the mol of MgCl2 6H2O varied from 0.8 to 1.2 at NH4 þ 1 mol and PO43 1.2 mol. As shown in Fig. 1, the removals of ammonium and phosphate were increased from 89.4% and 71.2% to 98.9% and 74.8%, respectively, as the mol of MgCl2 6H2O was increased. However, the removal efficiency of COD was not significantly changed in spite of the increase of Mg2 þ dose. It fluctuated in the range of 68.3–72.4%. In addition, NH4 þ was more effectively removed than COD or PO43 by struvite crystallization. Generally, the concentration of NH4 þ is higher than that of PO43 in the livestock wastewater. Therefore, phosphate was added by varying from 0.8 to 1.2 mol, and the optimal dose was determined for the struvite crystallization. As the dose PO43 of was increased, the removal efficiency of NH4 þ increased from 87.8 to 98.1%. However, those of COD and PO43 were not affected by the increase of PO43 dose. As a result, 1.0:1.2:1.2 was determined
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T.-H. Kim et al. / Radiation Physics and Chemistry 97 (2014) 332–336
Fig. 2. Effects of gamma ray irradiation of livestock wastewater, COD, PO43 , .
Fig. 1. Determination of optimal molar ratio of NH4 þ :Mg2 þ :PO43 for struvite crystallization of livestock wastewater: variation of (a) Mg2 þ mole and (b) PO43 mole; COD, ; NH4 þ , ; PO43 , .
as the mol ratio of NH4 þ :Mg2 þ :PO43 . At the optimal condition of NH4 þ :Mg2 þ :PO43 mol, 1.0:1.2:1.2 for the struvite crystallization of anaerobically digested livestock wastewater, the removal efficiencies were 72.4, 98.9, and 75.4%, respectively. The use of Mg2 þ and PO43 at a concentration greater than the stoichiometric amount needed to achieve the desired effluent NH4 þ concentration is mainly due to the composition of livestock wastewater, especially by other ions and suspended solids. The presence of complexing agents forming complexes with Mg2 þ and the ionic strength are the major factors affecting the amount of Mg2 þ required (Uludag-Demirer et al., 2005). Similarly, Fujimoto et al. (1991) obtained the highest phosphorous removal efficiency by struvite crystallization at a molar ratio of 1.05:1.0 Ma2 þ :PO43 , and Siegrist et al. (1992) obtained the highest removal efficiency at 1.3:1. 3.3. Effects of gamma-ray irradiation on the struvite crystallization of livestock wastewater The property changes after gamma ray irradiation for anaerobically digested livestock wastewater were monitored. As the absorbed dose of the gamma ray was increased from 0 to 100 kGy, the concentrations of COD, NH4 þ and PO43 decreased (Fig. 2). The value of COD, NH4 þ , and PO43 decreased from 2840 mg/l, 1455 mg/l, and 355 mg/l to 2335 mg/l, 1110 mg/l, and 322 mg/l, respectively, at a 100 kGy dose. The removal efficiencies were 17.8, 23.7, and 9.6%, respectively. COD and NH4 þ were more effectively removed than
; NH4 þ ,
;
PO43 by gamma ray radiation. When aqueous solutions were irradiated by gamma rays, the following chemical species were formed. The reactive primary species were hydroxyl radicals, hydrated electrons, and hydrogen atoms. It was reported that gamma ray radiation is very effective for decomposing refractory organic compounds such as phenols (Shim et al., 2009), pesticide diazinon and atrazine (Mohamed et al., 2009), and herbicide 2,4-D (Campos et al., 2003). The effects of gamma ray irradiation for anaerobically digested livestock wastewater on the removal efficiency enhancement of COD, NH4 þ , and PO43 removal were investigated. For pretreated livestock wastewater using a gamma ray, MgCl2 6H2O and KH2PO4 were added at a mol ratio of 1.0:1.2:1.2 (NH4 þ :Mg2 þ :PO43 ), the pH of the solution was adjusted to 9 using a 10 N NaOH solution, and the solutions were mixed for 3 min at 200 rpm and settled for 30 min. The changes in the COD, NH4 þ , and PO43 concentrations were investigated. For COD (Fig. 3), as the absorbed dose of the gamma ray was increased from 0 to 10, 20, 50, and 100 kGy, the COD concentration of the anaerobically digested livestock wastewater decreased from 2840 to 2750, 2530, 2440, and 2335 mg/l, respectively (Fig. 3(a)). It can be inferred that this resulted from the oxidation of hydroxyl radicals produced during the gamma ray radiation. The COD concentration of the solutions gamma ray pretreated at 0, 10, 20, 50, 100 kGy, were decreased to 965, 690, 635, 540, 500 mg/l, respectively, through struvite crystallization. The COD removal efficiency was increased from 66.0 to 75.7, 77.6, 81.0, and 82.4%, respectively. Therefore, the COD removal efficiency of struvite crystallization was enhanced by 16.4% at 100 kGy of gamma ray irradiation (Fig. 3(b)). The effects of the combination of gamma ray pre-irradiation and struvite crystallization of anaerobically digested livestock wastewater on the NH4 þ removal were studied (Fig. 4). The initial concentration of NH4 þ was 1455 mg/l, and was decreased to 1296, 1214, 1198, and 1110 mg/l by the gamma ray oxidation reaction at 10, 20, 50, and 100 kGy, respectively (Fig. 4(a)). After the struvite crystallization at the optimal mol ratio of NH4 þ : Mg2 þ :PO43 , they decreased to 262, 28, 10, 4, and 2 mg/l, respectively. The NH4 þ removal efficiency by only struvite crystallization was 82.0%, and it was enhanced to 98.0% by gamma ray irradiation at 100 kGy. As a result, 17.8% of the NH4 þ removal efficiency was increased by the gamma ray preirradiation of struvite crystallization (Fig. 4(b)). According to Chu et al.’s (2011) study, ammonium is oxidized by hydrated radicals produced during the water radiolysis at a high irradiation dose, which resulted in a decrease in the ammonium concentration. Fig. 5 shows the effects of gamma ray pre-irradiation and struvite crystallization on the PO43 removal. The initial concentration of PO43 in anaerobically digested livestock wastewater
T.-H. Kim et al. / Radiation Physics and Chemistry 97 (2014) 332–336
Fig. 3. Enhancement of COD removal efficiency by gamma ray irradiation prior to struvite crystallization of livestock wastewater: (a) COD removal by gamma ray irradiation, COD concentration (mg/l), ; removal efficiency (%), ; (b) COD removal by struvite crystallization after gamma ray irradiation, COD concentration (mg/l), ; removal efficiency (%), ; total COD removal efficiency by combined process (%), .
335
Fig. 4. Enhancement of NH4 þ removal efficiency by gamma ray irradiation prior to struvite crystallization of livestock wastewater: (a) NH4 þ removal by gamma ray irradiation, NH4 þ concentration (mg/l), ; removal efficiency (%), ; (b) NH4 þ removal by struvite crystallization after gamma ray irradiation, NH4 þ concentration (mg/l), ; removal efficiency (%), ; total NH4 þ removal efficiency by combined process (%), .
3.4. Chemical demand for struvite crystallization was 355 mg/l, and was decreased to 340, 337, 331, and 322 mg/l, respectively, as the absorbed dose of the gamma ray was increased to 10, 20, 50 and 100 kGy (Fig. 5(a)). From this result, we can see that the gamma ray irradiation was less effective for the removal of PO43 than those of COD and NH4 þ in anaerobically digested livestock wastewater. After the struvite crystallization at the optimal mol ratio of NH4 þ :Mg2 þ :PO43 (1.0:1.2:1.2) and pH 9, the concentration of PO43 decreased to 94, 68, 63, 58, and 47 mg/l, respectively. Therefore, the PO43 removal efficiency by only struvite crystallization was 73.4%, and was enhanced to 86.9% by gamma ray irradiation at 100 kGy (Fig. 5(b)). As a result, the PO43 removal efficiency was enhanced 13.5% by gamma ray pretreatment of struvite crystallization. As shown in Figs. 3–5, the struvite crystallization efficiency of COD, NH4 þ , and PO43 of anaerobically digested livestock wastewater was enhanced to 82.4, 99.8, and 86.9% by a pre-irradiation gamma ray of 100 kGy. The organic compound (COD) can inhibit the struvite crystallization and result in a decrease of struvite crystalline purity. The high-energy ionizing radiation can treat COD as well as NH4 þ , and thus the efficiency of struvite crystallization can be improved through gamma ray pretreatment. In particular, the organic compound which exists in the form of a suspended solid can be settled and removed through struvite crystallization (Suzuki et al., 2007). Furthermore, the enhancement of struvite crystallization efficiency can be obtained by gamma ray pre-irradiation owing to the decrease of COD, NH4 þ , and PO43 concentrations of livestock wastewater.
The gamma ray pre-irradiation and struvite crystallization of anaerobically digested livestock wastewater might result in not only an enhancement of the removal efficiencies but also a decrease in the demand for chemicals (MgCl2 6H2O and KH2PO4) for struvite crystallization of livestock wastewater. Fig. 6 shows the chemical doses for MgCl2 6H2O and KH2PO4 and a decrease in the need for chemicals before and after gamma ray pre-irradiation. As the absorbed dose of the gamma ray was increased from 0 to 10, 20, 50, and 100 kGy, the dose of MgCl2 6H2O for struvite crystallization was decreased from 19.7 to 17.6, 16.4, 16.2, and 15.1 g/l, and that of KH2PO4 was also decreased from 12.7 to 11.8, 10.4, 10.3, and 9.5 g/l. Therefore, 23.4% of MgCl2 6H2O and 25.2% of KH2PO4 can be saved through the pretreatment of 100 kGy of gamma ray irradiation. In particular, the effects of gamma ray pre-irradiation on the chemical demands were significant between 0 and 20 kGy. 4. Conclusions Struvite crystallization was applied to treat livestock wastewater, and gamma ray irradiation was carried out prior to struvite crystallization. For the anaerobically digested livestock wastewater, 1.0:1.2:1.2 was determined as the optimal NH4 þ :Mg2 þ : PO43 mol ratio for struvite crystallization. The removal efficiency of COD, NH4 þ , and PO43 by struvite crystallization was 72.4, 98.9, and 75.4%, respectively. An enhancement of the struvite crystallization efficiency can be obtained through gamma ray pre-irradiation owing to the decrease of COD, NH4 þ , and PO43 concentrations.
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References
Fig. 5. Enhancement of PO43 removal efficiency by gamma ray irradiation prior to struvite crystallization of livestock wastewater, (a) PO43 removal by gamma ray irradiation, PO43 concentration (mg/l), ; removal efficiency (%), ; (b) PO43 removal by struvite crystallization after gamma ray irradiation, PO43 concentration (mg/l), ; removal efficiency (%), ; total PO43 removal efficiency by combined process (%), .
Fig. 6. Effects of gamma ray pretreatment on the chemical demands for the struvite crystallization of livestock wastewater: MgCl2 dose, ; KH2PO4 dose, ; decrement of MgCl2 dose demand, ; decrement of KH2PO4 dose demand, .
The decrease in the concentration of COD and NH4 þ by gamma ray irradiation caused not only an enhancement of the removal efficiencies but also a decrease in the need for chemicals (MgCl2 6H2O, KH2PO4) for struvite crystallization of livestock wastewater. In this study, 23.4% of MgCl2 6H2O and 25.2% of KH2PO4 were saved using the pretreatment of gamma ray irradiation at 100 kGy. Acknowledgement This research was supported by the Nuclear R&D program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.
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Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effects of ionizing radiation on struvite crystallization of livestock wastewater Tak-Hyun Kim n, Yun-Ku Nam, Seung Joo Lim Research Division for Industry and Environment, Korea Atomic Energy Research Institute, Jeongeup 580-185, Republic of Korea
H I G H L I G H T S
Gamma ray was applied prior to struvite crystallization of livestock wastewater. Gamma ray resulted in an enhancement of struvite crystallization efficiency. This is due to the decrease of COD concentration by gamma ray irradiation.
art ic l e i nf o
a b s t r a c t
Article history: Received 6 August 2013 Accepted 26 December 2013 Available online 4 January 2014
Livestock wastewater is generally very difficult to be treated by conventional wastewater treatment techniques because it contains high-strength organics (COD), ammonium (NH4 þ ), phosphate (PO43 ) and suspended solids. Struvite crystallization has been recently studied for the simultaneous removal of NH4 þ and PO43 . In this study, gamma ray irradiation was carried out prior to struvite crystallization of the anaerobically digested livestock wastewater. The effects of gamma ray irradiation on the struvite crystallization of livestock wastewater were investigated. As a result, gamma ray irradiation can decrease the concentration of COD, NH4 þ and PO43 contained in the livestock wastewater. This results in not only an enhancement of the struvite crystallization efficiency but also a decrease in the chemical demands for the struvite crystallization of livestock wastewater. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Livestock wastewater Struvite crystallization Gamma ray irradiation NH4 þ PO43
1. Introduction Livestock wastewater contains high-strength COD, BOD5, nitrogen, phosphorus, and suspended solids. In particular, livestock wastewater has high concentrations of ammonium and phosphate, which can reach values of around 8000 mg NH4 þ /l and 1000 mg PO43 /l. The presence of nitrogen in wastewater discharge is undesirable for several reasons. Free ammonia is toxic to many aquatic organisms, and moreover, ammonium ions are oxygenconsuming compounds that deplete the dissolved oxygen in the receiving water. In addition, all forms of nitrogen and phosphorus can be made available to aquatic plants and can consequently contribute to eutrophication. Furthermore, the presence of nitrite and nitrate ions, i.e., the oxidized forms of ammonium ions through a biological nitrification reaction, in drinking water is a potential public health hazard. However, it is very difficult to treat livestock wastewater using a conventional wastewater treatment technique such as a biological nitrification/denitrification process for nitrogen removal and chemical coagulation for phosphorus removal.
n
Corresponding author. Tel.: þ 82 63 570 3343; fax: þ82 63 570 3348. E-mail addresses: [email protected], [email protected] (T.-H. Kim).
0969-806X/$ - see front matter & 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.12.033
As an alternative to treating the high concentration of NH4 þ and PO43 in livestock wastewater, the precipitation of NH4 þ and PO43 by forming magnesium ammonium phosphate (struvite, MgNH4PO4 6H2O) has been recently studied (Lee et al., 2003; Ryu et al., 2008). Struvite, magnesium ammonium phosphate (MAP), is a naturally occurring crystal, that is formed when the combined concentrations of Mg2 þ , NH4 þ and PO43 exceed its solubility limit. Struvite precipitation is controlled by pH, supersaturation, temperature, and impurities such as calcium ions. As struvite has a low solubility constant, pKs, between 12.60 (Stumm and Morgan, 1970) and 13.26 (Ohlinger et al., 1998), its insoluble form can be easily formed and simply separated from the water phase. The struvite crystallization technique has been applied to various types of wastewater, such as human urine (Lind et al., 2000; Ganrot et al., 2007), semiconductor wastewater (Ryu et al., 2008), swine wastewater (Suzuki et al., 2007; Uludag-Demirer et al., 2005, and landfill leachate (Kabdash et al., 2008; He et al., 2007; Li and Zhao, 2003; Kim et al., 2007a, 2007b). The recovered struvite can be considered a useful slow-release fertilizer (Kim et al., 2007a, 2007b). However, the successful application of the struvite crystallization process depends on two main factors: the molar ratio of Mg: NH4 þ :PO43 and the pH value in the reactor. In most cases,
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magnesium deficiency in wastewater allows it to be added to remove all available nutrients such as phosphorus and/or ammonium–nitrogen in the water phase. If the struvite crystallization process is applied to remove only ammonium–nitrogen, the phosphate forms of H3PO4 or NaHPO4 are usually needed. The required alkaline condition of an elevated pH in the struvite crystallization system can be achieved through alkaline addition or pre-aeration (Suzuki et al., 2007). Struvite solubility decreases with an increasing pH, while above a pH of 9, its solubility begins to increase. In general, crystal formation primarily proceeds by the nucleation from crystal embryos followed by crystal growth (Kim et al., 2007a, 2007b). Since magnesium tends to be low relative to the concentrations of ammonia and phosphate in many wastewaters, the cost of adding magnesium salts is a major economic constraint to the application of struvite crystallization for nutrient recovery. The main obstacle to apply the struvite crystallization process is the high consumption of magnesium and phosphate salts, which leads to a high operation cost. Kumashiro et al. (2001) have demonstrated the use of seawater as a source of Mg2 þ for struvite crystallization at the pilot scale. In addition, Lee et al. (2003) used bittern as a source of Mg2 þ . Bittern is produced through the evaporation of seawater. It contains mostly magnesium chloride with smaller amounts of other inorganic compounds. Its Mg2 þ content is approximately 32 g/l, which is 27-times that of seawater (Lee et al., 2003). The ionizing radiation such as a gamma ray or electron beam has been considered a promising method for wastewater treatment since the secondary polluted materials are not produced and the process is relatively simple and compact. Radiation technology has been applied to oxidize the organic compounds and enhance the biodegradability of wastewaters containing various biologically refractory organic compounds such as landfill leachate, paper mill wastewater, effluent from a petroleum production, methyl tert-butyl ether (MTBE), cork wastewater, and pharmaceuticals (Auslender et al., 2002; Bae et al., 1999; Cooper et al., 2002; Duarte et al., 2004; Yamazaki et al., 1983; Madureira et al., 2013; Kimura et al., 2012) In this study, the gamma ray was applied as the pretreatment of struvite crystallization of livestock wastewater, and the effects of gamma ray irradiation on the removal of organics, ammonium and phosphate ions were investigated.
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Table 1 Characteristics of supernatant of anaerobically digested livestock wastewater. Items
Concentration, mg/l Range
TSS TCODcr T–N NH4 þ NO2 NO3 T–P PO43 pH ( )
470–650 1446–2840 445–1279 426–1132 0.1–0.7 10–26 78–229 69–116 8.2–8.8
Mean value 520 1557 724 653 0.4 15 94 80 8.7
(Nordion, Inc., Canada) at the Korea Atomic Energy Research Institute. The samples were irradiated in sealed Pyrex glass vessels by 60Co. The applied doses were in the range of 10–100 kGy. The absorbed doses were measured using an alanine-EPR dosimetry system (ISO/ASTM 51607:2003) (ASTM, 2004). Samples were taken and filtered through a 0.45 μm glass filter to obtain a filtrate. The changes in soluble chemical oxygen demand (SCOD), ammonium (NH4 þ ), and phosphate (PO43 ) were determined based on the standard methods (APHA, 1998).
3. Results and discussion 3.1. Composition of anaerobically digested livestock wastewater Table 1 shows the characteristics of the anaerobically digested livestock wastewater. The anaerobically digested livestock wastewater contains approximately 520 mg/l of total suspended solids (TSS) and 1557 mg/l of total COD (TCOD). The pH of the wastewater was about 8.7. It also contains approximately 653 mg/l of NH4 þ and 80 mg/l of PO43 . The molar ratio of NH4 þ :PO43 is just 1:0.06. Struvite is magnesium ammonium phosphate and forms a crystalline deposit when the molar ratio of NH4 þ :Mg2 þ :PO43 is greater than 1:1:1. Therefore, not only the external magnesium sources but also the external phosphate sources should be added for the struvite crystallization. The livestock wastewater contains a high level of insoluble P (Table 1), a major part of which was thought to settle along with suspended solids.
2. Experimental The livestock wastewater was collected from public livestock wastewater treatment plants located in Gongju, Korea. The characteristics of the raw and anaerobically digested livestock wastewater are summarized in Table 1. For struvite crystallization, a magnesium chloride (MgCl2 6H2O) solution and potassium phosphate (KH2PO4) solution were used as alternate sources of magnesium ions and orthophosphate ions, respectively. 10 N NaOH was used to control the pH of the solutions. Experiments were carried out at ambient laboratory temperature using a jar test apparatus. The high purity of struvite deposits can be achieved through the addition of magnesium and orthophosphate prior to the pH control of 9. When the pH was initially controlled at 9, after feeding the magnesium and orthophosphate, the final pH dropped to as low as 6. Straful et al. (2001) demonstrated that hydrogen is released into the solution when struvite is formed, resulting in a decrease in pH. The solutions in the jar were mixed at a mixing speed of 200 rpm and a mixing duration of 3 min. The mixed solution was then allowed to be settled for a duration of 30 min. Gamma ray radiolysis experiments were carried out using a 60Co source
3.2. Optimization of struvite crystallization conditions of anaerobically digested livestock wastewater In this study, struvite crystallization was carried out to treat livestock wastewater. To determine the optimal dose of MgCl2 6H2O, the mol of MgCl2 6H2O varied from 0.8 to 1.2 at NH4 þ 1 mol and PO43 1.2 mol. As shown in Fig. 1, the removals of ammonium and phosphate were increased from 89.4% and 71.2% to 98.9% and 74.8%, respectively, as the mol of MgCl2 6H2O was increased. However, the removal efficiency of COD was not significantly changed in spite of the increase of Mg2 þ dose. It fluctuated in the range of 68.3–72.4%. In addition, NH4 þ was more effectively removed than COD or PO43 by struvite crystallization. Generally, the concentration of NH4 þ is higher than that of PO43 in the livestock wastewater. Therefore, phosphate was added by varying from 0.8 to 1.2 mol, and the optimal dose was determined for the struvite crystallization. As the dose PO43 of was increased, the removal efficiency of NH4 þ increased from 87.8 to 98.1%. However, those of COD and PO43 were not affected by the increase of PO43 dose. As a result, 1.0:1.2:1.2 was determined
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Fig. 2. Effects of gamma ray irradiation of livestock wastewater, COD, PO43 , .
Fig. 1. Determination of optimal molar ratio of NH4 þ :Mg2 þ :PO43 for struvite crystallization of livestock wastewater: variation of (a) Mg2 þ mole and (b) PO43 mole; COD, ; NH4 þ , ; PO43 , .
as the mol ratio of NH4 þ :Mg2 þ :PO43 . At the optimal condition of NH4 þ :Mg2 þ :PO43 mol, 1.0:1.2:1.2 for the struvite crystallization of anaerobically digested livestock wastewater, the removal efficiencies were 72.4, 98.9, and 75.4%, respectively. The use of Mg2 þ and PO43 at a concentration greater than the stoichiometric amount needed to achieve the desired effluent NH4 þ concentration is mainly due to the composition of livestock wastewater, especially by other ions and suspended solids. The presence of complexing agents forming complexes with Mg2 þ and the ionic strength are the major factors affecting the amount of Mg2 þ required (Uludag-Demirer et al., 2005). Similarly, Fujimoto et al. (1991) obtained the highest phosphorous removal efficiency by struvite crystallization at a molar ratio of 1.05:1.0 Ma2 þ :PO43 , and Siegrist et al. (1992) obtained the highest removal efficiency at 1.3:1. 3.3. Effects of gamma-ray irradiation on the struvite crystallization of livestock wastewater The property changes after gamma ray irradiation for anaerobically digested livestock wastewater were monitored. As the absorbed dose of the gamma ray was increased from 0 to 100 kGy, the concentrations of COD, NH4 þ and PO43 decreased (Fig. 2). The value of COD, NH4 þ , and PO43 decreased from 2840 mg/l, 1455 mg/l, and 355 mg/l to 2335 mg/l, 1110 mg/l, and 322 mg/l, respectively, at a 100 kGy dose. The removal efficiencies were 17.8, 23.7, and 9.6%, respectively. COD and NH4 þ were more effectively removed than
; NH4 þ ,
;
PO43 by gamma ray radiation. When aqueous solutions were irradiated by gamma rays, the following chemical species were formed. The reactive primary species were hydroxyl radicals, hydrated electrons, and hydrogen atoms. It was reported that gamma ray radiation is very effective for decomposing refractory organic compounds such as phenols (Shim et al., 2009), pesticide diazinon and atrazine (Mohamed et al., 2009), and herbicide 2,4-D (Campos et al., 2003). The effects of gamma ray irradiation for anaerobically digested livestock wastewater on the removal efficiency enhancement of COD, NH4 þ , and PO43 removal were investigated. For pretreated livestock wastewater using a gamma ray, MgCl2 6H2O and KH2PO4 were added at a mol ratio of 1.0:1.2:1.2 (NH4 þ :Mg2 þ :PO43 ), the pH of the solution was adjusted to 9 using a 10 N NaOH solution, and the solutions were mixed for 3 min at 200 rpm and settled for 30 min. The changes in the COD, NH4 þ , and PO43 concentrations were investigated. For COD (Fig. 3), as the absorbed dose of the gamma ray was increased from 0 to 10, 20, 50, and 100 kGy, the COD concentration of the anaerobically digested livestock wastewater decreased from 2840 to 2750, 2530, 2440, and 2335 mg/l, respectively (Fig. 3(a)). It can be inferred that this resulted from the oxidation of hydroxyl radicals produced during the gamma ray radiation. The COD concentration of the solutions gamma ray pretreated at 0, 10, 20, 50, 100 kGy, were decreased to 965, 690, 635, 540, 500 mg/l, respectively, through struvite crystallization. The COD removal efficiency was increased from 66.0 to 75.7, 77.6, 81.0, and 82.4%, respectively. Therefore, the COD removal efficiency of struvite crystallization was enhanced by 16.4% at 100 kGy of gamma ray irradiation (Fig. 3(b)). The effects of the combination of gamma ray pre-irradiation and struvite crystallization of anaerobically digested livestock wastewater on the NH4 þ removal were studied (Fig. 4). The initial concentration of NH4 þ was 1455 mg/l, and was decreased to 1296, 1214, 1198, and 1110 mg/l by the gamma ray oxidation reaction at 10, 20, 50, and 100 kGy, respectively (Fig. 4(a)). After the struvite crystallization at the optimal mol ratio of NH4 þ : Mg2 þ :PO43 , they decreased to 262, 28, 10, 4, and 2 mg/l, respectively. The NH4 þ removal efficiency by only struvite crystallization was 82.0%, and it was enhanced to 98.0% by gamma ray irradiation at 100 kGy. As a result, 17.8% of the NH4 þ removal efficiency was increased by the gamma ray preirradiation of struvite crystallization (Fig. 4(b)). According to Chu et al.’s (2011) study, ammonium is oxidized by hydrated radicals produced during the water radiolysis at a high irradiation dose, which resulted in a decrease in the ammonium concentration. Fig. 5 shows the effects of gamma ray pre-irradiation and struvite crystallization on the PO43 removal. The initial concentration of PO43 in anaerobically digested livestock wastewater
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Fig. 3. Enhancement of COD removal efficiency by gamma ray irradiation prior to struvite crystallization of livestock wastewater: (a) COD removal by gamma ray irradiation, COD concentration (mg/l), ; removal efficiency (%), ; (b) COD removal by struvite crystallization after gamma ray irradiation, COD concentration (mg/l), ; removal efficiency (%), ; total COD removal efficiency by combined process (%), .
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Fig. 4. Enhancement of NH4 þ removal efficiency by gamma ray irradiation prior to struvite crystallization of livestock wastewater: (a) NH4 þ removal by gamma ray irradiation, NH4 þ concentration (mg/l), ; removal efficiency (%), ; (b) NH4 þ removal by struvite crystallization after gamma ray irradiation, NH4 þ concentration (mg/l), ; removal efficiency (%), ; total NH4 þ removal efficiency by combined process (%), .
3.4. Chemical demand for struvite crystallization was 355 mg/l, and was decreased to 340, 337, 331, and 322 mg/l, respectively, as the absorbed dose of the gamma ray was increased to 10, 20, 50 and 100 kGy (Fig. 5(a)). From this result, we can see that the gamma ray irradiation was less effective for the removal of PO43 than those of COD and NH4 þ in anaerobically digested livestock wastewater. After the struvite crystallization at the optimal mol ratio of NH4 þ :Mg2 þ :PO43 (1.0:1.2:1.2) and pH 9, the concentration of PO43 decreased to 94, 68, 63, 58, and 47 mg/l, respectively. Therefore, the PO43 removal efficiency by only struvite crystallization was 73.4%, and was enhanced to 86.9% by gamma ray irradiation at 100 kGy (Fig. 5(b)). As a result, the PO43 removal efficiency was enhanced 13.5% by gamma ray pretreatment of struvite crystallization. As shown in Figs. 3–5, the struvite crystallization efficiency of COD, NH4 þ , and PO43 of anaerobically digested livestock wastewater was enhanced to 82.4, 99.8, and 86.9% by a pre-irradiation gamma ray of 100 kGy. The organic compound (COD) can inhibit the struvite crystallization and result in a decrease of struvite crystalline purity. The high-energy ionizing radiation can treat COD as well as NH4 þ , and thus the efficiency of struvite crystallization can be improved through gamma ray pretreatment. In particular, the organic compound which exists in the form of a suspended solid can be settled and removed through struvite crystallization (Suzuki et al., 2007). Furthermore, the enhancement of struvite crystallization efficiency can be obtained by gamma ray pre-irradiation owing to the decrease of COD, NH4 þ , and PO43 concentrations of livestock wastewater.
The gamma ray pre-irradiation and struvite crystallization of anaerobically digested livestock wastewater might result in not only an enhancement of the removal efficiencies but also a decrease in the demand for chemicals (MgCl2 6H2O and KH2PO4) for struvite crystallization of livestock wastewater. Fig. 6 shows the chemical doses for MgCl2 6H2O and KH2PO4 and a decrease in the need for chemicals before and after gamma ray pre-irradiation. As the absorbed dose of the gamma ray was increased from 0 to 10, 20, 50, and 100 kGy, the dose of MgCl2 6H2O for struvite crystallization was decreased from 19.7 to 17.6, 16.4, 16.2, and 15.1 g/l, and that of KH2PO4 was also decreased from 12.7 to 11.8, 10.4, 10.3, and 9.5 g/l. Therefore, 23.4% of MgCl2 6H2O and 25.2% of KH2PO4 can be saved through the pretreatment of 100 kGy of gamma ray irradiation. In particular, the effects of gamma ray pre-irradiation on the chemical demands were significant between 0 and 20 kGy. 4. Conclusions Struvite crystallization was applied to treat livestock wastewater, and gamma ray irradiation was carried out prior to struvite crystallization. For the anaerobically digested livestock wastewater, 1.0:1.2:1.2 was determined as the optimal NH4 þ :Mg2 þ : PO43 mol ratio for struvite crystallization. The removal efficiency of COD, NH4 þ , and PO43 by struvite crystallization was 72.4, 98.9, and 75.4%, respectively. An enhancement of the struvite crystallization efficiency can be obtained through gamma ray pre-irradiation owing to the decrease of COD, NH4 þ , and PO43 concentrations.
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References
Fig. 5. Enhancement of PO43 removal efficiency by gamma ray irradiation prior to struvite crystallization of livestock wastewater, (a) PO43 removal by gamma ray irradiation, PO43 concentration (mg/l), ; removal efficiency (%), ; (b) PO43 removal by struvite crystallization after gamma ray irradiation, PO43 concentration (mg/l), ; removal efficiency (%), ; total PO43 removal efficiency by combined process (%), .
Fig. 6. Effects of gamma ray pretreatment on the chemical demands for the struvite crystallization of livestock wastewater: MgCl2 dose, ; KH2PO4 dose, ; decrement of MgCl2 dose demand, ; decrement of KH2PO4 dose demand, .
The decrease in the concentration of COD and NH4 þ by gamma ray irradiation caused not only an enhancement of the removal efficiencies but also a decrease in the need for chemicals (MgCl2 6H2O, KH2PO4) for struvite crystallization of livestock wastewater. In this study, 23.4% of MgCl2 6H2O and 25.2% of KH2PO4 were saved using the pretreatment of gamma ray irradiation at 100 kGy. Acknowledgement This research was supported by the Nuclear R&D program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.
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