The aim of the current study is to evaluate the effectiveness of combined persulphate with hydrogen peroxide (S2O82−/H2O2) oxidation as a post-treatment of biologically treated palm oil mill effluent (POME) for the first time in the literature. The removal efficiencies of chemical oxygen demand (COD), ammoniacal nitrogen (NH3-N), and suspended solids (SS) were 36.8%, 47.6%, and 90.6%, respectively, by S2O82− oxidation alone under certain operation conditions (i.e., S2O82− = 0.82 g, pH 11, and contact time 20 min). Nevertheless, the combined process (S2O82−/H2O2) achieved 75.8% and 87.1% removals of NH3-N and SS, respectively, under 2.45/1.63 g/g H2O2/S2O82−, pH 11, and 20 min oxidation. Moreover, 56.9% of COD was removed at pH 8.4.

INTRODUCTION

Oil palm plantations have been well developed in Malaysia since the 1870s. Malaysia so far has the second largest palm oil production globally (Green Palm 2015). In 2014, Malaysia generated around 20 million tonnes of crude palm oil, while for Indonesia around 30 million tonnes of crude palm oil was produced (SGS 2015). The oil palm species planted in Malaysia is called Elaeis guineensis Jacq. or African oil palm; each hectare of oil palm results in 10–35 tonnes of fresh fruit annually, and the economic life of an oil palm tree is around 20–25 years (Singh et al. 2010). During palm oil processing, biomass wastes such as palm press fibres, palm kernel shells, empty fruit bunches, and palm oil mill effluent (POME) are generated (Hassan et al. 2013). POME is the largest portion of waste generated from the palm oil industry: 28% by weight in relation to fresh fruit bunches (FFB) (Abdullah & Sulaiman 2013). POME is an intractable wastewater which is thick brown in appearance and has an unpleasant odour from palm oil extraction. POME discharge is normally as hot as 80–90 °C an consists of water, oil and fine suspended solids (SS) (Liew et al. 2015). It is a form of colloidal suspension which is mainly yielded from hydrocyclone wastewater (4%), sterilizer wastewater (36%) and clarification wastewater (60%), accordingly (DOE 1999; Wu et al. 2010; Liew et al. 2015). POME is a non-toxic wastewater (Rupani et al. 2010) but it comprises nutrients such as nitrogen and phosphorous as well as other insignificant mineral constituents (Mohammed & Chong 2014) which can cause environmental problems and endanger aquatic life. Thus, around 85% of effluent treatment facilities in the oil palm industry have installed the ponding system (Ahmad & Krimly 2014). A series arrangement of anaerobic, facultative and aerobic open ponding is one of the conventional biological treatment systems for POME in Malaysia (Aris et al. 2008). However, the biologically treated POME (BIOTPOME) requires a post-treatment due to insufficient removal of colour and chemical oxygen demand (COD) prior to discharge into watercourses (Fadzil et al. 2013; Bashir et al. 2016). Successive amounts of organic and nutrient contents in term of COD will introduce eutrophication problems and further increase the amount of oxygen depleted in water (Fadzil et al. 2013). Advanced oxidation processes have recently been used to treat recalcitrant pollutant contained in water and soil. It involves the generation of highly reactive radicals and degradation of contaminants by the oxidants. Persulphate oxidation is an advanced chemical oxidation which utilizes anions as an oxidant. The oxidation-reduction potential for oxidation is shown as Equation (1) (Block et al. 2004). 
formula
1
can also produce highly reactive sulphate radical with 2.6 V redox potential through activation (Deng et al. 2014). can be activated by different agents such as heat, UV light and metal. Hydrogen peroxide (H2O2) is an oxidant which potentially functions as activator of . In this dual oxidant system, the reactivity of peroxide in reduction of targeted organic substances collaborates with the improved stability of persulphate (Block et al. 2004). In the mechanism of activation, and hydroxyl radical are released and consumed through the initiation, propagation and termination reactions as shown in Equations (2)–(5) (Cronk 2008). The end product of oxidation, sulphate ion , is an inert salt and not harmful to the environment as it is not considered as a pollutant (Ahmadi et al. in press). However, the residual of sulphate ions can be removed following further aerobic and anaerobic treatment processes. Thabet et al. (2009) removed 91% of sulphate from leachate using anaerobic treatment reactor processes.

NH3-N would be oxidized into nitrogen compounds as shown in Equations (6) and (7) under presence of silver ions (Ali 2005).

Initiation: 
formula
2
Propagation: 
formula
3
Termination I: 
formula
4
Termination II: 
formula
5
where RH and represent an organic matter and oxidized organic matter. 
formula
6
 
formula
7

In this study, oxidation alone and H2O2 activated persulphate oxidation were investigated for the removals of COD, NH3-N and SS from BIOTPOME for the first time. The pH, contact time and dosage of and H2O2 were the operational parameters used to achieve better removal efficiency of BIOTPOME.

MATERIALS AND METHODS

BIOTPOME sampling and sample characterization

The BIOTPOME sample was collected from Tian Siang Oil Mill (Air Kuning) Sdn. Bhd., Perak, Malaysia. The mill has been in commission since 2000 and the daily capacity of processing FFB has increased to 120 tons/hr. The samples were collected using 5.5 L clean and empty polypropylene containers and transported instantly to the environmental laboratory. The samples were preserved at 4 °C in order to minimize the possibility of biodegradation of sample and chemical reaction occurrence (APHA 2005). The initial characteristics of BIOTPOME were analysed and are presented in Table 1.

Table 1

Characteristics of BIOTPOME sample

Parameters Unit Range Average 
BOD5 mg/L 110–191 151 
COD mg/L 2,990–3,590 3,290 
Total dissolved solids mg/L 5,880–5,980 5,930 
SS mg/L 340–361 351 
NH3-N mg/L 116–130 123 
Turbidity NTU 640–689 665 
Colour PtCo 5,410–6,250 5,830 
pH – 8.0–8.7 8.4 
Temperature °C 25 25 
Conductivity mS/cm 11.75–11.96 11.86 
Parameters Unit Range Average 
BOD5 mg/L 110–191 151 
COD mg/L 2,990–3,590 3,290 
Total dissolved solids mg/L 5,880–5,980 5,930 
SS mg/L 340–361 351 
NH3-N mg/L 116–130 123 
Turbidity NTU 640–689 665 
Colour PtCo 5,410–6,250 5,830 
pH – 8.0–8.7 8.4 
Temperature °C 25 25 
Conductivity mS/cm 11.75–11.96 11.86 

Experimental setup of S2O82− and S2O82−/H2O2 oxidation system

Sample preserved under 4 °C was warmed to ambient temperature (27 °C) before starting the experiment. A 250 mL conical flask filled with 100 mL of sample was prepared for each experiment. The sample was placed on an orbital shaker (NB-101M) at shaking speed of 200 rpm under ambient temperature. The crystallized form of sodium persulphate (Na2S2O8, ≥98%, Sigma-Aldrich) was utilized for and oxidation. A 30% H2O2 solution was employed after adding into sample in oxidation. For oxidation, operational conditions involved were dosage, pH and contact time. For oxidation, the operational conditions were dosage ratios of , dosage concentration of and pH.

dosage was pre-determined in term of :COD0 ratio (g/g). The :COD0 ratio was set to 4:1 and 1.32 g was required for 100 mL sample per 0.329 g initial COD (COD0). The desired actual amount of powdered (Na2S2O8) was 1.67 g with considering its purity by using Equation (8). 
formula
8

In oxidation, the optimum dosage of found from oxidation, 0.82 g, was applied constantly to find out the optimum ratio of among 0.5:1.0–3.0:1.0. Based on the ratios, dosages of H2O2 used ranged between 0.41 g and 2.45 g. For the experiment of varying dosages, the optimum dosage ratio of 1.5:1.0 was fixed for varying dosages of H2O2 and , and the dosages were adjusted in terms of (0.61 g/0.41 g to 6.12 g/4.08 g).

For pH adjustment 5 M sodium hydroxide and 5 M hydrochloric acid were used. Different pH values (3–12) were examined in both (60 min; 200 rpm) and oxidations (20 min; 200 rpm). For oxidation, 2.45 g/1.63 g of dosage obtained as optimum from the previous oxidation was fixed for the pH variation of oxidation. The filtered samples were analysed after each run of experiment. All experiments were carried out in duplicate. Effectiveness of and oxidation of BIOTPOME were studied in term of removal efficiencies of pH, COD, NH3-N, colour and SS.

Analytical method

COD, NH3-N, colour, SS and pH parameters were analysed before and after the oxidation treatment using Standard Methods for the Examination of Water and Wastewater (APHA 2005) and Hach Water Analysis Book (Hach 2005). NH3-N concentration was determined by the Nessler method. Colour was examined by the spectrophotometric method – single wavelength method. The pH was measured using a digital pH meter. COD measurement of samples was carried out using the dichromate reactor digestion method (Hach Method 8000) while SS was tested by photometric method. Removal efficiencies of COD, NH3-N, colour and SS were calculated as Equation (9): 
formula
9
where Ci and Cf represent the initial and final COD, NH3-N or SS concentrations.

RESULTS AND DISCUSSION

BIOTPOME treatment: S2O82− oxidation

In oxidation, the operational conditions of pH, contact time and dosage were investigated in removing the contaminants of BIOTPOME such as COD, NH3-N, colour and SS. All oxidation processes were carried out at shaking speed 200 rpm under ambient temperature.

Effect of pH adjustment

Different initial pHs (3, 5, 7, 9, 10, 11 and 12) were adjusted accordingly while concentration was fixed at 1.32 g (:COD0 = 4.0:1.0) before initiating the oxidation. Figure 1 shows the removal efficiencies of COD, NH3-N, colour and SS in BIOTPOME under pH influence. The highest removal efficiency of oxidation was achieved under alkali condition, pH 11, with 21.9% COD, 59.0% NH3-N, and 87.8% SS. In terms of COD and colour, the removal percentages were decreased from pH 3 to pH 7 and rose again after pH 7 until pH 11; however, removal percentages of parameters at pH 12 were lowered compared to pH 11. For the parameters NH3-N and SS, the removals dropped slightly from pH 3 and started rising after pH 5, and the removal efficiencies at pH 12 eventually were also reduced compared to pH 11. It seems that alkali and acidic condition led to high removals on the parameters but lower removal efficiencies under neutral pH.
Figure 1

Effect of Initial pH on removals of SS, NH3-N and COD by S2O82− oxidation (1 h contact time; S2O82−:COD0 = 4.0:1.0; shaking speed, 200 rpm).

Figure 1

Effect of Initial pH on removals of SS, NH3-N and COD by S2O82− oxidation (1 h contact time; S2O82−:COD0 = 4.0:1.0; shaking speed, 200 rpm).

In oxidation, releasing of free radicals through breakdown of persulphate (acid-catalysation) in acidic condition are shown in Equations (10) and (11) (Epold & Dulova 2015). Proton acts as catalyst to react with anions and leads to the generation of sulphate radical . Under alkali condition, hydroxide ions could be decomposed when is activated under alkaline condition in order to generate as Equation (12), resulting in a slight decrease in pH. This shows pH has significant influence on oxidation. Another possible mechanism of formation of according to Liang et al. (2007) is shown in Equation (13) but this reaction has a smaller reaction rate constant of < 60 M−1s−1 compared to Equation (12), 7 × 107 M−1s−1. Among the radical formations, is the predominant radical at acidic pH while is the main radical at alkali pH (Zhao et al. 2013). 
formula
10
 
formula
11
 
formula
12
 
formula
13

Effect of contact time

The concentration removals of COD, NH3-N, SS and colour were studied in different contact times (5, 10, 20, 40, 60, 120, 180 and 240 min) in oxidation. The dosage (1.32 g) was fixed by the ratio of :COD0 = 4.0:1.0 as in the oxidation with previous pH adjustment. As shown in Figure 2, the optimum removal for SS, NH3-N and COD were 91.4%, 50.7% and 30.2% at 20 min. This may represent the free radicals attacking rapidly on BIOTPOME due to fast kinetic of those radicals (Cronk 2008). The COD removed was higher compared with the 221.9% of COD eliminated from previous operational condition, pH adjustment at 60 min. However, the removal of NH3-N at 20 min, 50.7%, was lower than NH3-N removed at 60 min, 59.0%, in the previous study of pH adjustment. In these conditions, or radicals possibly attacked the organic compound rather than NH3-N. SS was also greatly removed with a range of 89.9% to 94.3%.
Figure 2

Effect of contact time on removals of SS, NH3-N and COD by S2O82−oxidation (pH 11; S2O82−:COD0 = 4.0:1.0; shaking speed, 200 rpm).

Figure 2

Effect of contact time on removals of SS, NH3-N and COD by S2O82−oxidation (pH 11; S2O82−:COD0 = 4.0:1.0; shaking speed, 200 rpm).

Effect of dosages of S2O82−

Figure 3 demonstrates the removal percentage of parameters of BIOTPOME with varied dosages of (0.20–2.45 g) in relation to ratio of :COD0 (0.5:1.0, 1.0:1.0, 2.0:1.0, 3.0:1.0, 4.0:1.0, 5.0:1.0 and 6.0:1.0). The optimum removal of parameters like SS, NH3-N and COD was 57.7%, 47.6% and 36.8% when ratio of :COD0 = 2.0:1.0. By increasing dosage higher than 2.0:1.0 of :COD0 ratio, there was significant decrease on COD removal. COD removal ranged from 24.0% to 31.8% for :COD0 = 3.0:1.0 to 6.0:1.0. Excessive amount of may affect the treatment efficiency. Sulphate ion, , formed from can act as scavenger to attack oxidant and intermittently reduce the decomposition rate of COD as in the study of degradation of trichloroethylene by oxidation (Liang et al. 2007).
Figure 3

Effect of dosages of S2O82− on removal of SS, NH3-N and COD by S2O82− oxidation (20 min contact time; pH 11; shaking speed, 200 rpm).

Figure 3

Effect of dosages of S2O82− on removal of SS, NH3-N and COD by S2O82− oxidation (20 min contact time; pH 11; shaking speed, 200 rpm).

Table 2 shows the removal efficiencies for oxidation of BIOTPOME. The removal efficiencies ranged from 21.9% to 36.8% for COD, 47.6% to 59.0% for NH3-N, and 87.8% to 91.4% for SS. The pH values of samples rose to 10.5–10.6 after oxidation treatment. By adjusting the operational conditions to optimum, as :COD0 = 2, pH 11 and 20 min contact time, 36.8% COD, 47.6% NH3-N, 57.7% colour and 90.6% SS removal efficiencies were achieved.

Table 2

Removals of COD, NH3-N and SS of BIOTPOME in different operational conditions using S2O82− oxidation

 Operational conditions Final pH COD removal (%) NH3-N removal (%) SS
removal (%) 
pH 11
(Contact time = 1 hr;
S2O82−:COD0 = 4.0:1.0) 
10.5 21.9 59.0 87.8 
Contact time = 20 min
(pH 11; S2O82−:COD0 = 4.0:1.0) 
10.5 30.2 50.7 91.4 
S2O82−:COD0 = 2.0:1.0
(pH 11; Contact time = 20 min) 
10.6 36.8 47.6 90.6 
 Operational conditions Final pH COD removal (%) NH3-N removal (%) SS
removal (%) 
pH 11
(Contact time = 1 hr;
S2O82−:COD0 = 4.0:1.0) 
10.5 21.9 59.0 87.8 
Contact time = 20 min
(pH 11; S2O82−:COD0 = 4.0:1.0) 
10.5 30.2 50.7 91.4 
S2O82−:COD0 = 2.0:1.0
(pH 11; Contact time = 20 min) 
10.6 36.8 47.6 90.6 
Table 3

Removals of COD, NH3-N and SS of BIOTPOME in different operational conditions using S2O82−/H2O2 oxidation

Optimum operational conditions Final pH COD removal (%) NH3−-N removal (%) SS
removal (%) 
Dosage ratio of H2O2:S2O82−
=1.5:1.0
(Contact time = 20 min; S2O82−:COD0 = 2.0:1.0; pH 8.4) 
8.2 52.5 26.9 61.4 
Dosage of H2O2/S2O82− = 2.45 g/1.63 g
(Contact time = 20 min; S2O82−:COD0 = 2.0:1.0; H2O2:S2O82− dosage ratio =1.5:1.0; pH 8.4) 
8.2 56.9 26 63.7 
pH = 11
(Contact time = 20 min; S2O82−:COD0 = 2.0:1.0; H2O2/S2O82− = 2.45 g/1.63 g) 
10.1 35.8 75.8 87.1 
Optimum operational conditions Final pH COD removal (%) NH3−-N removal (%) SS
removal (%) 
Dosage ratio of H2O2:S2O82−
=1.5:1.0
(Contact time = 20 min; S2O82−:COD0 = 2.0:1.0; pH 8.4) 
8.2 52.5 26.9 61.4 
Dosage of H2O2/S2O82− = 2.45 g/1.63 g
(Contact time = 20 min; S2O82−:COD0 = 2.0:1.0; H2O2:S2O82− dosage ratio =1.5:1.0; pH 8.4) 
8.2 56.9 26 63.7 
pH = 11
(Contact time = 20 min; S2O82−:COD0 = 2.0:1.0; H2O2/S2O82− = 2.45 g/1.63 g) 
10.1 35.8 75.8 87.1 

S2O82−/H2O2 oxidation

Hydrogen peroxide (H2O2) was investigated and played an enhancement role in oxidation for treating BIOTPOME. In the study, COD, NH3-N and SS parameters were analysed before and after each experiment accordingly. The operational conditions of pH and dosages of and H2O2 were also adjusted in the oxidation process under ambient temperature.

Effect of dosage ratio of H2O2:S2O82−

By fixing the optimum ratio of :COD0 = 2.0:1.0, the dosage ratios of were varied (0.5:1.0, 1.0:1.0, 1.5:1.0, 2.0:1.0, 2.5:1.0 and 3.0:1.0) for the oxidation of BIOTPOME under pH 8.4 within 20 minutes and the result is shown in Figure 4. The optimum removals of SS, NH3-N and COD were 61.4%, 26.9% and 52.5% by using dosage ratio of 1.5:1.0. When the lowest dosage ratio (0.5:1.0) was applied 53.4% SS, 11.1% NH3-N and 32% COD removals were achieved. As dosage ratio increased, removal percentage of parameters were also increased. However, the removals of COD, NH3-N and colour were decreased as the dosage ratio was increased over 1.5:1.0.
Figure 4

Effect of dosage ratios of H2O2:S2O82− on removals of SS, NH3-N and COD by S2O82−/H2O2 oxidation (20 min contact time; S2O82−:COD0 = 2.0:1.0; pH 8.4 at 200 rpm).

Figure 4

Effect of dosage ratios of H2O2:S2O82− on removals of SS, NH3-N and COD by S2O82−/H2O2 oxidation (20 min contact time; S2O82−:COD0 = 2.0:1.0; pH 8.4 at 200 rpm).

Figure 5

Effect of dosage of S2O82−:H2O2 on removals of SS, NH3-N and COD by S2O82−/H2O2 oxidation (20 min contact time; S2O82−:COD0 = 2.0:1.0; H2O2:S2O82− ratio = 1.5:1.0; pH 8.4 at 200 rpm).

Figure 5

Effect of dosage of S2O82−:H2O2 on removals of SS, NH3-N and COD by S2O82−/H2O2 oxidation (20 min contact time; S2O82−:COD0 = 2.0:1.0; H2O2:S2O82− ratio = 1.5:1.0; pH 8.4 at 200 rpm).

Effect of dosage of H2O2/S2O82−

The dosages of (g/g) were varied based on the optimum ratio of (1.5:1.0), as shown in Figure 5. Different dosages of (g/g: 0.61/0.41, 1.22/0.82, 2.45/1.63, 3.67/2.45, 4.89/3.26 and 6.12/4.08) in order to enhance the performance of oxidation. The optimum parameter was obtained based on overall removals of parameters and the optimum dosage of determined was 2.45 g/1.63 g. SS, NH3-N and COD removals were as high as 63.7%, 56.9% and 26%, respectively. By fixing the dosage ratio and varying the dosages of and H2O2, the SS removal was increased gradually overall. When the dosage of was increased from 0.61 g/0.41 g to 2.45 g/1.63 g, COD removal was between 58.8% and 56.9%. The removal was decreased afterward when additional dosage of (g/g) was added. Removal efficiencies of SS and NH3-N were increased gradually as dosages of and H2O2 were increased.

As dosage of H2O2 increases, H2O2 and may non-productively degrade and scavenge, respectively, and result in a quicker termination reaction because the free radicals will scavenge oxidants and oxidants may scavenge radicals too (Ko et al. 2012) as in Equations (14) and (15). Ko et al. (2012) have also mentioned and may react with each other although the reaction rate is slow. 
formula
14
 
formula
15

Effect of pH adjustment

Different initial pHs (3, 5, 7, 8.4, 9, 10, 11 and 12) were also studied in oxidation of BIOTPOME. Fixed variables of :COD0 = 2.0:1.0, dosage of of 2.45 g/1.63 g and 20 minutes contact time were employed in the oxidation system. In Figure 6, the optimum pH for overall removals of parameters was 11; 87.1% SS, 75.8% NH3-N and 35.8% COD were eliminated under pH 11. SS removal increased steeply as pH increased. The removal efficiencies of SS were 64.5% under acidic condition, pH 3. Colour removal of 90.3% and SS removal of 93.6% were also gained with initial pH of 12. The removal of NH3-N was improved from 21% to 77.8% as pH increased, while the maximum removal of COD (53.2%) was achieved at pH 8.4.
Figure 6

Effect of initial pH on removals of SS, NH3-N and COD by S2O82−/H2O2 oxidation (20 min contact time; S2O82−:COD0 = 2.0:1.0; H2O2/S2O82− = 2.45 g/1.63 g at 200 rpm).

Figure 6

Effect of initial pH on removals of SS, NH3-N and COD by S2O82−/H2O2 oxidation (20 min contact time; S2O82−:COD0 = 2.0:1.0; H2O2/S2O82− = 2.45 g/1.63 g at 200 rpm).

Table 3 shows a summary of oxidation. Through adjustment of the dosage ratios and dosages of (g/g), removal efficiencies of COD, NH3-N and SS were 52.5%–56.9%, 26%–26.9%, and 61.4%–63.7%, correspondingly, with the final pH values of 8.2. However, under adjustment of pH, COD removal was decreased and NH3-N removal was increased sharply compared with COD and NH3-N removals under varied dosages of and H2O2. In the studied conditions (:COD0 = 2.0:1.0, pH 11, 20 min contact time and dosage of 2.45 g/1.63 g), the highest removal efficiencies of COD, NH3-N and SS were 35.8%, 75.8% and 87.1%, respectively, at final pH of 10.1.

Although the optimum removal for ammonia and SS was achieved at pH 11, significant removal for COD was also achieved at pH 8.4. This findings is in agreement with Hilles et al. (2016a, 2016b) using for concentrated landfill leachate treatments.

In the light of the above, operational cost for using was calculated for kg COD removal. As the major cost of the treatment process is related to chemicals used, the operational costs were USD0.4/L 37% H2O2 and USD0.8/kg sodium persulfate , and about 4.98 kg of persulfate and 5.2 L of 37% H2O2 are required to remove 1 kg COD (in 303 L of POME). Around USD6 is required to treat 303 L of POME using oxidation process. Hilles et al. (2016a) reported USD10.7/kg COD removal using the same treatment process for landfill leachate.

CONCLUSIONS

In this study, and oxidation were used for removing SS, NH3-N and COD from BIOTPOME. To the best of our knowledge, this is the first experimental study conducted to treat BIOTPOME via and oxidation. For oxidation, 36.8% COD, 47.6% NH3-N and 90.6% SS were removed at the optimum operation conditions ( = 0.82 g, pH 11, and 20 min contact time). In oxidation, 35.8% COD, 75.8% NH3-N and 87.1% SS were achieved when operation conditions were :COD0 = 2.0:1.0, = 2.45 g/1.63 g, pH 11 and 20 min contact time. The highest removal of COD was 56.9% when :COD0 = 2.0:1.0 and 2.45 g/1.63 g of at pH 8.4 and 20 min contact time. High removal of COD could be obtained without adjusting pH while NH3-N was effectively removed at alkaline condition. Values of final COD and pH of and oxidation above the standard discharge limit and not suitable for release into the environment according to the industrial discharge standard for POME (DOE 1999). This study concluded that and oxidation were efficient to reduce certain parameters of BIOTPOME under certain operational conditions.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the financial support of Universiti Tunku Abdul Rahman, Malaysia, by allowing the completion of this study. A special thanks to Tian Siang Oil Mill (Air Kuning) Sdn. Bhd., Perak, Malaysia for on-site collection of BIOTPOME sample.

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