Treatment of Composting Leachate by Wet Air Oxidation: Assessment of the Role of Operating Parameters

By Behroz Karimi,¹ * Mohammad Hassan Ehrampoush,² Asghar Ebrahimi³ and Mehdi Mokhtari²
April 2013

  1. Department of Environmental Health Engineering, Arak University of Medical Sciences, Sardasht, Arak, 0098861, Markazi, Iran
    *Corresponding Author
  2. Environmental Science and Technology Research Center, Shahid Sadoughi University of Medical Sciences and Health Services, Yazd, 0098351, Iran
  3. Environmental Research Center, School of Health, Isfahan University of Medical Sciences, Isfahan, 0098311, Iran
Abstract
Wet air oxidation is regarded as one of the advanced oxidation methods for reducing the concentration of organic compounds in industrial wastewaters, biologically non-biodegradable and toxic compounds, garbage leachate, etc. The purpose of this study is to determine the efficiency of wet air oxidation method in treating garbage leachate of Isfahan compost plant. The leachate sample with a volume of 1.5 L was put in a steel reactor with a volume of 3 L under 10 bars pressure, and temperatures of 100, 200, and 300 °C, as well as three residence times of 30, 60 and 90 min. The sample was taken from leachate storage ponds with the volume of 20 L in Isfahan compost plant, and wet air oxidation method was used for leachate treatment. The removal efficiency of COD, BOD, NH4-N, NO3, and TSS was taken into account. The results showed that the removal efficiency was obtained more than 35% for COD, 38% for BOD5, and 85% for TSS in an hour of residence time, and the maximum removal efficiency of NH4-N (53.3%) and NO3-N (73.9%) was obtained during the process. The results also indicated that the reaction temperature has the most effect on the decomposition of organic matter. In addition, BOD5 and COD removal efficiency was added by increasing the residence time in the Wet air oxidation process. The ratio of BOD5/COD was also considered as the important indicator of biodegradation capabilities from the leachate sample and the improvement of this indicator to 84% was obtained in the process. It was shown in the study that this process is very effective in reducing COD, BOD5, and nitrate.

Keywords: Wet air oxidation, compost, leachate, advanced oxidation methods

Introduction

Garbage leachate is one of the highly polluted and toxic liquids that results in adverse effects on the environment and health (1). A special care should be taken to control, collect, dispose and treat this pollutant since lack of its proper treatment, collection, and disposal leads to severe contamination of surface water, groundwater and soil with toxic organic compounds and resistant to biodegradation, nitrogen, aromatic and phenolic compounds and also results in the threat of human life and aquatic organisms (1, 2). High values of COD between 20000 to 200000 mg L-1 and low values of BOD will lead to the reduction of the ratio of BOD5/COD > 0.1; in addition, the high values of ammonia nitrogen (2000 to 5000 mg L-1) and the existence of heavy metals as well as xenobiotic compounds indicate high intensity of pollution of this thick liquid and its long-term effects (3).

Leachate treatment methods can be referred to as 1- aerobic treatment (including sticking and non-sticking growth processes) such as a widespread aeration method and synthetic wetland, 2- anaerobic treatment (4), 3- the use of coagulants (5), 4- chemical methods, like Fenton and Electro-Fenton method (6), 5- membrane methods like reverse osmosis (RO) (7), 6- physical and nanofiltration methods (2, 3), 7-adsorption (8, 9), 8- different methods of advanced oxidation process (AOPs) such as ozonation (9, 10), or combination of O3/H2O2 and H2O2/UV and in combination with Fenton process (11, 12). In AOPs a combination of strong oxidizers like ozone, ultraviolet radiation (9), temperature, pressure (13). and H2O2 are applied (14). Among different methods of physical and chemical treatment, AOPs process is known as strong methods in stabilizing leachate pollutants.

Wet air oxidation (WAO) is one of the advanced oxidation methods. WAO process is one of the severe pollution recovery processes of aquatic environments to toxic materials and organic matter like industrial wastewaters (15, 16) and can be applied for sludge pretreatment (17). This method is conducted by setting a proper temperature and pressure and injecting certain values of oxidants (air, oxygen, hydrogen peroxide or ozone, etc.) (18). The process is operated in high concentrations of COD between 10000 to 150000 mg L-1. In this method, organic matter is placed under a temperature of 100 to 350 °C in a liquid phase, and special oxidation like air, oxygen or hydrogen peroxide is injected (8). The method has a high capacity to change resistant and complex compounds in leachate to simpler ones with a biodegradation capability. Advantages of this method are small facilities, easy operation, and high efficiency around 80% (19). The aim of the study was to determine the removal efficiency of COD, BOD, NH4, and NO3 from garbage leachate of Isfahan composting plant, and also to determine operational parameters.

Materials and Methods

1. Sample

The leachate sample was taken from storage lagoons from Isfahan compost plant, weekly in six months. At each stage, 20 L of the sample was taken in plastic containers and to avoid a change in physicochemical properties, the raw leachate was kept in a cool place. All tests at each stage were repeated five times and the average removal efficiency of each parameter was obtained. Properties of the raw leachate are shown in Table 1.

Table 1 · Physical and Chemical Properties of the Row Leachate Samples
Components Concentration (g L-1)
Mean maximum-minimum standard deviation
COD 118.2 97.9 – 146.98 14.4
BOD5 75.19 54.5 – 99 15.3
Ammonium 180.2 105.8 – 360 99
Nitrate (mg L-1) 578 500 – 680 67.2
TSS(mg L-1) 3990 600 – 5800 2156
EC (µs/cm) 979 900 – 1100 62.8
pH 7.08 6.3 – 7.8 0.57
TSS (mgL-1) 5200 3300 – 6600 985
Figure 1 · Wet peroxide oxidation reactor system
Figure 1

2. Reactor

A reactor with a volume of 3 L that is able to withstand a 100-bar pressure and a temperature of 500 °C and equipped with a pressure drain valve, manometer, an injection site, sample output, etc. was used. A schematic of the WAO process is in Fig. 1. The prepared sample with a volume of 500 mL was entered a steel (stainless) reactor. Prior to the entry of the prepared sample into the reactor, the reactor preheat was conducted to avoid a change in input leachate properties before entering the reactor. Preheating the reactor was performed at a temperature of 80 °C for 2 h. After initial setting of the reactor temperature, a sample with a volume of 1500 mL was entered to the reactor, and adjusted by setting the intake air amount of an internal pressure on 10 bars. The work was done at three temperatures of 100, 200, and 300 °C and at three residence times of 30, 60, and 90 min. A heater of HACH model and also to set pressure, and providing required O2, a pure oxygen cylinder were applied (20).

3. Chemical

The iron sulfate (FeSO4.7H2O), H2O2 (30 % W/V), H2SO4, NaOH, acetic acid (CH3COOH), potassium dichromate (K2Cr2O7), HgSO4, Ag2SO4, manganese oxide and powder and granular activated carbon were purchased from Merck, Germany.

4. Analytical

After the reactions, the reactor was cooled and its internal pressure reduced, and the output sample was taken for COD, BOD5, ammonia, nitrate and TSS experiments. Other operational conditions are available in Table 2.

Table 2 · Wet Oxidation Reactions Operating Conditions
Oxidizing Agent Pure Oxygen
Partial Pressure of O2 10 bar
Temperature 100, 200 and 300 °C
Volume of reactor and sample 3L and sample Volume1500 CC
Duration of the reactions 1.5 h after preheating period
Reaction time 30, 60 and 90 min
Cooling of reactor 2-3 h
Mixing inside reactor 50 S-1

After doing the experiments and separating a supernatant, a centrifuge was used with a speed of 6000 rpm per 15 min. Ten percent of lime water was applied for setting pH to about 8-9. COD measurement was conducted based on Dichromate method (closed reflux, 5220C, colorimetric method), and BOD5 in accordance to Winkler's method (5210 B) (21). In addition, to measure ammonia and nitrate concentrations, a spectrophotometer DR/2000 was used in accordance to EPA method (960 and 351). Other parameters like pH, temperature, and EC were also measured before and after the reaction using a pH device (with a model of 520-HACH) based on APHA Standard Methods.

All data were subjected to two-way analysis of variance (ANOVA), paired and independent sample T-test and Pearson correlation using SPSS Version 14. Statistical significance was tested using Confidence Interval 95%. The results are shown as mean ± standard deviation with excel Version 2010.

Figure 2 · Effect of reaction temperature (0, 100, 200 and 300 °C),
residence time (0, 30, 60 and 90 min) and pressure 10 bars on COD
removal efficiency in the WAO process
Figure 2
Figure 3 · Effect of reaction temperature (0, 100, 200 and 300 °C),
residence time (0, 30, 60 and 90 min) and pressure 10 bars on the BOD
removal efficiency in the WAO process
Figure 3
Figure 4 · Effect of reaction temperature (0, 100, 200 and 300 °C),
residence time (0, 30, 60 and 90 min) and pressure 10 bars on
improvement of BOD5/COD
Figure 4
Figure 5 · Effect of reaction temperature (0, 100, 200 and 300 °C),
residence time (0, 30, 60 and 90 min) and pressure 10 bars on the NH3
removal efficiency in the WAO process
Figure 5
Figure 6 · Effect of reaction temperature (0, 100, 200 and 300 °C),
residence time (0, 30, 60 and 90 min) and pressure 10 bars on the NO3
removal efficiency in the WAO process
Figure 6

Results and Discussion

1. COD Removal

In Fig. 2, the effect of temperature changes on removing COD at different residence times is provided. The concentration of COD was reduced from 118.25 g L-1 to 80.1 g L-1 at a temperature of 300 °C and the residence time of 90 min indicating 32% of the removal efficiency. At this temperature, the maximum efficiency was obtained. At temperatures of 100 and 200 °C, and the residence time of 30 min, COD removal 6.7% and 16.54%, respectively, were obtained. To study the effect of operational conditions on BOD5 removal value, the experiments were conducted 10 times considering a constant pressure. BOD5 reached from 75.2 g L-1 to 45.8 g L-1 at a temperature of 300 °C in raw leachate indicating 39% removal efficiency (Fig. 3). These results show the maximum removal value of this parameter in the process.

2. BOD5/COD Ratio

The ratio of BOD5/COD is called biodegradation capability. 78% of the biodegradation capability was improved at a temperature of 200 °C. With an increase of residence time, the biodegradation capability was also improved. This ratio was 64.6% in the raw leachate, reaching 84% at a temperature of 300 °C (Fig. 4). The value of biodegradation capability generally improves from 42.2% to 82%. In this study, the maximum value of biodegradation capability was obtained at the temperature of 300 °C and residence time of 90 min.

3. NH4-N Removal

The average value of NH4-N changed from 180.2 g L-1 to 84-300 g L-1 in the raw leachate; in addition, NO3-N was reduced from 578 mg L-1 to 150.6-252 mg L-1 (Fig. 5 and Fig. 6). The maximum removal percentage (efficiency) NH4-N 53.3% was obtained 56.4%-73.9 for NO3. Generally, the average removal percentage obtain for NO3-N is 65%.

Since WAO method can result in the reduction of leachate toxicity and the increase of biodegradation capability, the method can have plenty of applications in thick wastewater pretreatment (with very high pollution potential). Results of the present study indicate the effect of using the method on breaking large-ring compounds in the leachate. It was determined that WAO process has the average efficiency 6-35% in COD removal (Fig. 2). Temperature as a variable has great importance in decomposition of the organic matter in this process (22). During the process, intermediate compounds may be created; in other words, fatty acids are possible to be formed with a short chain (VFAs) that finally, following the oxidation process, carbon dioxide and water are produced (14). During a study on alkyl benzene detergents of linear sulfonate from wastewater by wet air oxidation, (23) have found out the formation of intermediate compounds like short fatty acids (VFAs) and organic acids like formic, acetic, and propionic acids during the oxidation of organic pollutants.

Laboratory results indicated (Fig. 3) that the reduction value of BOD5 is between 6-40% at the residence time of 1-5 h; the operational temperature is also between 100-300 °C under a pressure of 10 bars. It can be concluded that the reduction of residence time and the increase of temperature have direct effects on BOD5 removal efficiency, and by reducing the reaction time from 90 to 30 min at a constant temperature, the removal efficiency is improved which is indicative of reverse relationship of the residence time with BOD5 removal (24). There is a significant relationship between the organic matter removal (COD, BOD5) at different temperatures especially at higher ones, and in general, a complete removal of BOD5 and COD does not happen. For instance, the maximum removal percentage of COD is 39.3% at 100 °C and 60 min; COD is increased due to the degradation of heavy organic matter to smaller molecules (25).

Due to resistant organic compounds in leachate, biodegradation is usually low; so, it is required to improve degradation capability and pretreatment (Fig. 4). Verenich et.al found out the increase of biodegradation after WAO process (13). In addition, using a catalyst in this process helps the removal efficiency increase. Other researchers proved biodegradation improvement after this process. Fox and Noike studied the increase of biodegradation capability of waste newspaper by wet air oxidation; after WAO process, and the anaerobic process, lignin removal efficiency in the newspaper reached 84-95% (20).

Increasing the temperature and residence time, ammonium values was increased so that at a temperature of 300 °C and residence time of 90 min, the average value reached 300 g L-1. Nitrogenous organic compounds, including proteins in raw leachate, decomposition and extraction of amine groups at a high temperature and pressure result in the formation of NH4-N and concentration increase of this compound along with the increase of operational parameters (14). However, the formation of NH4-N is much less at lower temperatures (26). It is observed more the increase of ammonium production and more removal of COD at reaction temperatures and times. In general, the average percentage of NO3 removal is 65% in this process. Huang et al observed that with the increase of temperature to more than 300 °C, ammonia production is increased and pH decreased; more ammonia is also converted into NO3. Using the catalyst, in addition to the temperature reduction to 130 °C, conversion efficiency of ammonia to NO3 will be increased (90%) (27). Similar to these results were obtained by Kaewpuang et al., (28). They observed that with increasing the residence time, and creating alkaline conditions, the amount of nitrate ion formation and ammonia removal was added (28). Changes in pH values can disturb a balance between NH4-NH3 in the reactor so that more produced ammonia is converted again to ammonium by the decomposition of nitrogenous organic compounds (including proteins) and entered the solution (29). This will give rise to sharp increase of ammonium values by increasing reaction temperature and time in the reactor (30, 31). Hung et al applied a catalytic process of wet oxidation for removing ammonia. According to the study, in higher pHs, the ratio of NH3/NH4 in the solution is increased. In non-catalytic processes, the maximum ammonia removal was 20%. Increasing the residence time and temperature, the ammonia value is added in non-catalytic processes, but it is reduced in catalytic processes (32). In this study, TSS removal efficiency is about 80% after the process with residence time of 60 min and a temperature of 200 °C after the process which is indicative of relatively favorable results for this process.

An analysis comparison of (ANOVA) among different parameters with temperature and residence time represents more effect of the temperature compared to the residence time on BOD and COD removal so that statistical significance was not seen between different residence times and COD removals. In addition, in NH4-N removal, the residence time had more effect (0.000) compared to the temperature (0.039) (Fig. 5). As it is observed in Table 3, there is a relationship between COD and temperature (-0.522) in accordance to Pearson correlation test, but no connection was seen between the BOD5 removal with temperature.

Table 3 · Pearson Correlation Analysis to Significant Data (Residence Time and Temperature) with COD, NH3 and NO3
Parameter COD NH4-N NO3
Time --- --- 0.452(*)
Temperature -0.522(**) 0.665(**) 0.507(**)
COD --- -0.430(*) 0.546(**)
NH4-N -0.430(*) --- 0.558(*)
NO3-N -0.546(**) 0.558(*) ---
* Correlation is significant at the 0.05 level (2-tailed)
** Correlation is significant at the 0.01 level (2-tailed)

Conclusions

The study considers the wet air oxidation process to reduce organic load from the composting factory leachate. The WAO process is very effective in oxidizing high concentrations of organic matter to obtain more than 35% and 38% removal efficiency of COD and BOD5. It is shown that in wet air oxidation process, the reaction time increase will be increased due to doing the oxidation degradation process of organic matter. The limitations of this method include pH reduction, corrosion of the reactor wall, the increase of electrical conductivity (EC) value, the ammonium value increase with temperature increase, energy supply expenses. High values of ammonium, electrical conductivity and alkalinity can increase the toxicity in wastewater, and this will be effective on BOD5 values and biodegradation capability. Results of this operation in a laboratory scale indicated that due to increased breakdown of the organic compounds to simpler ones at higher temperatures, the use of this method along with a biological treatment (aerobic and anaerobic) can be a promising option for the leachate treatment of the compost plant.

Acknowledgement

The authors acknowledge financial and scientific support provided by the Faculty of Environment Health, Shahid Sadoughi University of Medical Sciences, Yazd, Iran.

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