A combination process has developed as a pretreatment of non biodegradable leachate. The processes consist of air stripping coupled with agitation as a modified of ammonia stripping followed by coagulation-flocculation processes. The main aims of these processes are reducing a concentration of NH3-N and organic matter as well as enhancing the biodegradability of landfill leachate. Ammonia stripped by the airflow rate of 10 L min-1 at pH 11 for 3 h, while the agitation process applied to air stripping effluent for 2 h at the pH of 11.5 in 150 sec-1 gradient velocity. NH3-N was removed at 96% as removal ratio by the modified ammonia stripping in 5 h stripping time. Ferric sulfate, poly ferric sulfate (PFS) and aluminum poly chloride (PAC) was tested as a coagulant material in the coagulation process. Chemical oxygen demand (COD), suspended solids (SS), turbidity as well as the sludge ratios were discovered for each material operated under optimum condition of pH and dosage.
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The overall removal of NH3-N, COD, biochemical oxygen demand (BOD5), total organic carbon (TOC), and SS obtained by these processes were 96.5%, 71.5%, 56.5%, 48.5% and 96.5%, respectively at the corresponding biodegradable ratio was modified from 0.20 to 0.31.
Keywords: landfill leachate; Pretreatment; Ammonia stripping; Coagulation; Flocculation
The sanitary landfill method has been widely accepted for waste treatment and disposal due to its lower cost of operation and maintenance when compared to other technologies. However, the major problem resulting from this method of waste management is the generation of large quantities of liquid leachate, which is a kind of wastewater with a high content of dissolved organic matter (DOM) (Calace et al., 2001). Leachate is a high-strength wastewater formed as a result of percolation of rainwater and moisture through waste in landfills (Hasar et al., 2009). For instance, it may contain a large amount of organic matter (biodegradable, but also refractory to biodegradation), where constituents of humic-type consist of an important group (Kang et al., 2002). The characteristics of landfill leachate depending on the type of MSW being dumped, the degree of solid waste stabilization, site hydrology, moisture content, seasonal weather variations, landfill age, and the stage of decomposition in the landfill (Wang et al., 2003). Leachate has become the most important environmental focus of landfill because the discharging of it may cause serious pollution to groundwater and surface waters (Pi et al., 2009). In addition, Leachate generated the early stages, termed young leachate, is characterized by elevated concentration of ammonia and organic matter with a high biodegradable content. As it ages, the concentration of ammonia increases, while the biodegradable fraction declines due to the stabilization process. Furthermore, Leachate with these characteristics is termed mature or old (Wang et al., 2006). 'Young leachates', usually contains high amounts of volatile fatty acids (i.e., a high BOD/COD ratio), are easier to be treated than 'old and medium-age leachates' since the latter contains a fraction of organic compounds recalcitrant to biological treatments.
Numerous methods of physicochemical can be used as pretreatment wastewaters containing toxic contaminants. These treatment methods are selected based on the characterization of wastewater, investment and operating cost, and some local regulations (Yilmaz et al., 2010). Pi et al. (Pi et al., 2009) applied air stripping followed by a coagulation/ultrafiltration (UF) process as a combined process in pretreatment of municipal landfill leachate. While Maranon et al. (Maranon et al., 2008) studied application of coagulation-flocculation as a pretreatment process for young landfill leachate. Tatsi et al. (Tatsi et al., 2003) examined the application of coagulation-flocculation for the treatment of raw and partially stabilized leachates. Zhang et al. (Zhang et al., 2009) used chemical precipitation as pretreatment of ammonium removal from landfill leachate. Uygur and KargÄ± (Uygur and Kargi, 2004) pre-treated landfill leachate by coagulation-flocculation with lime followed by air stripping of ammonia. Neczaj et al. (Neczaj et al., 2005) applied ultrasound pretreatment for enhancement of biological treatability of leachates generated in a typical sanitary landfill. Many researches were adopted advanced oxygen processes (AOPs) as pre-treatment in a mature landfill leachate, the most AOPs processes used are Fenton's reagent (Zgajnar Gotvajn et al., 2011; Zhang et al., 2006a), electro-Fenton (Atmaca, 2009; Zhang et al., 2006b), ozone (Wu et al., 2004), photo-Fenton processes (Hermosilla et al., 2009), combined Fenton and ozone (Cortez et al., 2011).
Ammonium stripping is the most widely employed treatment for the removal of NH3-N from landfill leachate (Collivignarelli et al., 1998; Marttinen et al., 2002). NH3-N is transferred from the waste stream into the air and then absorbed from the air into a strong acid such as sulfuric acid or directly fluxed into the ambient air. Air stripping gives an NH3-N treatment performance in the range of 85-95% with the concentrations ranging between 220 to 3260 mg Lâˆ’1 on the contact time of 18-24 h (Bonmati and Flotats, 2003). Coagulation-flocculation has been employed for the removal of suspended solids (SS), colloid particles, non-biodegradable organic compounds, and heavy metals from landfill leachate (Amokrane et al., 1997). Colloidal particles can be destabilized by add the coagulant to the coagulation process, and usually coagulation is followed by the flocculation process to increase the particle size and unstable particles into bulky floccules, so they can settle more easily (Cheng et al., 1994).
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In the present study, the leachate generated from Chang Shankou landfill in Wuhan city was collected and treated by combined processes. The processes consist of air stripping coupled with agitation as ammonium stripping followed by coagulation-flocculation as a pretreatment process to reduce the high concentration of NH3-N and COD and also to improve the ratio of BOD/COD of leachate in order to prepare it for further treatment.
2. Materials and methods
2.1. Landfill leachate
The leachate used in the experiments was collected from Chang Shankou landfill site in Wuhan city, it has been operated since 2007. The total amount of municipal solid waste (MSW) received about 2800 tons disposed daily. Leachate generation was about 400-500 m3 d-1. A 50 L leachate sample was obtained from a wastewater pond in the landfill site. Then, the sample was transported to the laboratory in sealed plastic barrels, and stored at 4 o C before being used and analyzed. The collected leachate was filtered through a glass-fiber filter to remove suspended solids.
2.2. Chemicals and analytical methods
All used chemicals were of analytical grade. The values of pH measured with a Sartorius pH meter PB-11. COD, BOD5, NH3-N, TOC and SS measured according to the Standard Methods for the Examination of Water and Wastewater (APHA, 2005). The adjustment of pH was done by using 1 M H2SO4 and 1 M NaOH solutions. All the experiments were carried out at room temperature 25 Â± 2 o C.
2.3. Experimental setup and procedures
In this process, each technology was used to treat leachate separately to discover the efficiency of each technology indicating parameter's condition of each one. Ammonia stripping experiments were performed in two processes. Air stripping is the first process of ammonia stripping carried out by passage quantities of air over the exposed surface of the leachate, thus causing the partial pressure of the ammonia gas within the water to drive the ammonia from the liquid to the gas phase. This process was carried out in the following sequential steps:) i) 1 L leachate sample was put in 10 L plastic barrels, (ii) its pH was adjusted to a certain value, (iii) the mixture was aerated for a specific period of air stripping time through diffusers at a certain air flow rate, and (iv) the mixtures was let to settle for a half hour. The NH3-N of the supernatant was measured. The optimum pH value and aeration rate as well as time were investigated.
The second process employed in ammonia stripping is agitation. It is a process employed for NH3-N removal by applying a high gradient velocity. The agitation process was carried out in the following sequential steps: (i) leachate sample (500 mL) was put in a (1 L) beaker), (ii) its pH was adjusted to a fixed value, (iii) The mixture was agitated for a specific time with a certain speed or gradient velocity using a jar-test device ZR-6, and (iv) the mixtures was allowed to settle for half an hour. The NH3-N of the supernatant was measured. The optimum pH, gradient velocity (G) and agitation time were investigated as the main operating conditions of this process.
The coagulation-flocculation process was performed in a conventional jar test apparatus ZR-6. Experimental process consisted of the following stages: (i) 1 L treated leachate by stripping was put in a 1L beaker, (ii) pH was adjusted to a fixed value, (iii) a desired dosage of coagulant material was added to the leachate, (iv) experiments were carried out at a rapid mixing of 250 rpm for 150 sec, and at a slow mixing of 60 rpm for 25 min, (v) 0.1% polyacrylamide was added to the sample as flocculent in slow mixing stage to increase the flocculation settling rate, and (vi) mixture was let to settle 1 h.
The supernatant was withdrawn from a point located about 2 cm below the top of the liquid level in the beaker. The COD of the supernatant measured. The sludge ratio in addition to the optimum pH value and dosage was investigated for each coagulant material.
In this way sequential treatment test run; the landfill leachate was first fed to the air stripping, followed with agitation process to remove ammonia. The effluent from ammonia stripping unit was treated by the coagulation-flocculation process to reduce the concentration of organic pollutant and suspended solids in addition to modifying BOD/COD ratio. COD, BOD5, and NH3-N of the effluents were measured at the end of each process.
3. Results and Discussion
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The leachate samples collected from the landfill site were analyzed. The characteristics of it were explained in table 1. The BOD/COD ratio of leachate was 0.16-0.2. The landfill leachate was considered to have low BOD/COD ratio and high content of NH3-N. Thus, it was as a 'mature' or 'old' and non-biodegradable leachate (Guo et al., 2010).
[Insert "Table 1" about here]
3.2. Ammonia stripping
3.2.1 Air Stripping
Air stripping is the most common method for eliminating a high concentration of NH3-N involved in wastewater treatment technologies. For leachate, it is usually used to eliminate this pollutant, which can increase wastewater toxicity (Ozturk et al., 2003). This method can provides removal efficiency of NH3-N higher than 90% in time more than 20 h (Cheung et al., 1997; Guo et al., 2010; Marttinen et al., 2002; Silva et al., 2004). In order to determine optimum pH, a required amount of NaOH and H2SO4 have been used to change pH values in leachate from 8 to 12 as shown in Fig. 1(a). The ammonia removal ratios have significant linear increases at pHâ‰¤11; beyond this, the increase in ammonia removal was not so significant. Therefore, the optimum pH was 11. Ammonium hydroxide was formed as an intermediate product in the reaction at pH more than 10. The bubbling of air through ammonium hydroxide solutions results in the freeing of ammonia gas.
Since, ammonia stripping is mass transfer controlled; the surface area of the liquid exposed must be maximized. This can be achieved by creating fine droplets with the help of diffuser or sprayers. Fig. 1(b) shows the variation of NH3-N removal efficiency and its effluent concentration with air flow rate. After adjusting the optimal pH by the adding required dosage of NaOH to 1 L raw leachate, the mixtures were aerated by using air compressors and diffusers at different rates 5, 7.5, 10, 15 and 25 L min-1, respectively, for 3 h stripping time. It is evidence that the ammonia removal ratios have significant increase at air flow rate 10 L min-1 L-1; while the increase in ammonia removal was not significant beyond an air flow rate of 10 L min-1 L-1. However, an air flow rate of 10 L min-1 L-1 should be optimal because the significantly more expensive method of using an air flow rate of 15 L min-1 L-1 increases the ammonia removal efficiency less than 2% compared with that of 10 L min-1 L-1. This result is belong to an increase of the air flow rate increases the gas-liquid surface area, which in turn controls the amount of NH3 diffused from water (Bonmati and Flotats, 2003).
[Insert "Fig. 1" about here]
3.2.2 Agitation process
The value of pH and gradient velocity are the main parameters effected on the NH3-N removal ratio in this process. Optimum pH was determined by two steps, firstly, leachate agitated without any pH adjustment and secondly by using required dosages of NaOH for raising pH to 10, 10.5, 11, 11.5 and 12, respectively. After a specific time of agitation, ammonium removal was 48% for leachate without pH changing while the ammonium removals were around 90% and 94% at pH of 11 and 11.5, respectively. Therefore, the optimum pH was 11.5 (Fig. 2(a)).
Fig. 2(b) shows the effect of gradient velocity on the NH3-N removal. The removal ratio increased significantly with increases in the agitation rate. NH3-N removal was about 92%, 95% and 96% at 120, 150 and 240 sec-1, respectively. As a result, the gradient velocity agitation 150 sec-1 should be the optimum because the removal efficiency is not significantly more than it. The effect of agitation time on the NH3-N removal was shown in Fig. 2(c). NH3-N removal increased significantly with increases in the agitation time in the 4 and 5 h were 92% and 96%, respectively. Thereafter, the increasing in NH3-N removal was not significant. The optimum agitation time was 5 h. After that the removal ratio will be slow more and more because of decreasing pH due to the re carbonation of lime in leachate by the absorption of CO2 from the ambient air (Bonmati and Flotats, 2003; Cheung et al., 1997; Collivignarelli et al., 1998; Kilic et al., 2007).
[Insert "Fig. 2" about here]
Agitation process can be carried out by applying a high rotating speed to the exposed surface of leachate, thus causing the partial pressure of the ammonia gas within the water to drive the ammonia from the liquid to the gas phase. The process is further subject to careful pH control and involves the mass transfer of volatile contaminants from water into air. Free ammonia begins to form when the pH is above 7. Over 85% of the present ammonia may be liberated as a gas through agitation in the absence of air at pH greater than 11 (Wichitsathian et al., 2005).
This result is mainly due to the fact that the reaction of NH3 with water can be represented by eq. 1. From this equation, raising the pH (as represented by the OHâˆ’ will drive the reaction to the left, increasing the concentration of NH3. This makes ammonia more easily removed by stripping.
NH3 +H2O NH4OH (1)
Ammonium hydroxide is formed as an intermediate product in the reaction at pH between 10 and 12. Solubility increases at low ambient temperatures since ammonia is highly soluble in water. In this study, the ammonia removal was 96% at contact times of 5 h, at a gradient velocity of 150 sec-1 and pH 11.5, in the absence of air.
3.2.3. Air coupled with agitation as a combined process of ammonia stripping
A combined process of ammonia stripping consists of air stripping coupled with agitation process. Leachate was treated by air stripping at 3 h; this contact time can achieve about 70% in NH3-N removal. Then the effluent mixture agitated, both methods were operated under optimum conditions, which investigated in individual processes. This process was adopted to reduce operation cost and time. Fig. 3 shows the removal percentage of NH3-N from leachate. Removal ratio reaches to 96% when leachate has been treated by a combined process (3 h air stripping and 2 h agitation).
[Insert "Fig. 3" about here]
The air stripping coupled with agitation process as a novel stripping method of ammonia compared with respect to other previous work to evaluate its performances as shown in table 2.
[Insert "Table 2" about here]
As seem from table 2, NH3 -N removal was in the range 60-96.6 %. Among the stripping treatments reviewed above, it is observed that the air stripping coupled with agitation process is appears to be an effective option for pre-treatment of landfill leachate to remove ammonia.
3.3. Coagulation-flocculation processes
The initial pH and coagulant dosage are the important operating conditions in the chemical coagulation. Ferric sulfate, poly ferric sulfate and poly aluminum chloride were used as a coagulant material in this study. The pH and dosage of each coagulant were determined as a function of COD removal. The pH values of samples were adjusted to the desired level by the addition appropriate amounts of NaOH and H2SO4 solutions. This addition generated some problems due to the formation of foams, which made it difficult to reduce the pH below 7. The coagulation process runs by the addition of 1000 mg Lâˆ’1 of each coagulant material in leachate samples. Fig. 4(a) presents the effect of pH values on COD removal in the coagulation process. It shows that the highest COD removal percentage, 60%, 56% and 34% for ferric sulfate, PFS and PAC, respectively, were achieved at the optimum pH of 5 for all of them. The results clearly indicate that the removal efficiency has been increased in acidic condition rather than alkalinity and very low on neutral condition and also indicate that the removal efficiency bas been increased with increases in pH up to pH 5. Then, the removal efficiency has been decreased for pH below 5.
In general, chemical coagulation is a process which is highly pH dependent. The pH influences the nature of producing polymeric metal species that will be formed as soon as the metal coagulants are dissolved in water. The influence of pH on chemical coagulation may be considered as a balance of two competitive forces: (1) between hydrogen ion's H+ and metal hydrolysis products for interaction with organic ligands and (2) between OHâˆ’ and organic anions for interaction with metal hydrolysis products (Stephenson and Duff, 1996). At low pH values (pHâ‰¤5), H+ out-competes metal hydrolysis products for organic ligands, and hence poor removal rates occur, and some of the generated organic acids will not precipitate. At higher pH values (pH > 5), OHâˆ’ competes with organic compounds for metal adsorption sites and the precipitation of metal-hydroxides occur mainly by co-precipitation (Stephenson and Duff, 1996).
The effect of coagulant material dosage on the efficiency of COD removal was also investigated. Fig. 4(b) shows the removal of COD by different dosages of them at pH 5. Coagulant dosage varied from 300 to 2400 mg Lâˆ’1. As shown in fig. 4(b), the optimum dosage of coagulant to attain a better COD removal percentage of 64% was 1200 mg Lâˆ’1 when using ferric sulfate and 68% COD removal obtained at 1500 mg Lâˆ’1 of for PFS while PAC gives low removal ratio 36%. The results indicated that COD removal increased with increasing coagulant dosages up to the optimum dosage. Then, the COD removal decreased. This result is mainly due to the fact that the optimum coagulant dosage produced flocs having a good structure and consistency. But in lower doses than optimum, the produced flocs are small and influence the settling velocity of the sludge. In higher doses than the optimum, in addition to the small size of floc, rest ability of floc can happen.
In addition to pollutant removal, sludge production was considered in this work, as it may effect the economic feasibility of the proposed method. The sludge produced during the physicochemical treatment of wastewaters, is composed by the amount of originally suspended organic matter and solids, as well as by the compounds formed due to the possible addition of chemical reagents. The amount and the characteristics of the sludge produced during the coagulation-flocculation processes are highly depended on the specific coagulant used and on the operating conditions. Sludge production was estimated in this work from the wet sludge volumes remained on the bottom of the jar-test beakers and the volume of it after separate water by a centrifuge process. The combined organic polyelectrolyte was used of inorganic salts and found very efficient, resulting in the production of relatively low sludge volumes, i.e. less than 10% of initial treated leachate volume, similar to the corresponding findings of literature studies (Aguilar et al., 2002; Tatsi et al., 2003). The volume of sludge generated for each coagulant was calculated after settling of 30, 60 min and 24 h and after centrifugation for 10 min at 4000 rpm of the total volume of sample used in the experiment, the results listed in Table 3.
[Insert "Table 3" about here]
3.3.1. Comparison of results for the different coagulants
Table 4 shows the best results obtained for each of the tested coagulants. As can be seen, in function of COD removal PFS can give the best results when used at pH 5 and dosage of 1500 mg Lâˆ’1, (after treatments pH were 4.22).This implies that it is necessary to raise the pH of the effluent obtained after this treatment before subjecting it to the biological process, which is a disadvantage from the point of view of the industrial application. Furthermore, comparing the results of coagulation experiments, it can be observed that iron salts were more efficient than alum salts for the removal of organic matter. As proved in previous researchers (Li et al., 2010; Ntampou et al., 2006; Tatsi et al., 2003; Wang et al., 2009),especially at pH of 5, hydrous iron hydroxides are precipitating in greater degree than the corresponding aluminum flocs, resulting in more efficient removal of pollutants, than that of obtained at lower pH. As shown in Table 4, both of iron salt coagulants appear to be close in COD removal because both of them are depending on reaction of Fe+3. As a result of coagulation processes in this work PFS can be considered the best material which can be used as a coagulant in landfill leachate as a function of COD removal in a dosage of 1200 mg L-1 at pH 5 with settling time of 1 h.
[Insert "Table 4" about here]
[Insert "Fig. 4" about here]
3.4. Combined processes
The overall performances for combined pre-treatment of landfill leachate by air stripping coupled with agitation and coagulation-flocculation process under the optimum conditions are listed in Table 5. It is seen that COD, NH3-N, and BOD5 removal were 71.5%, 96%, and 56.5%, respectively. The BOD/COD ratio is also improved from 0.20 to 0.31.
[Insert "Table 5" about here]
The landfill leachate collected was pretreated by air coupled with agitation as a modified process of ammonia stripping and a coagulation-flocculation. Leachate was characterized as low BOD/COD and high content of NH3-N, showing that it can be classified as old and non-biodegradable. Air stripping coupled with agitation as ammonia stripping is simple and low-cost than other available physicochemical methods. It is appeared to be a cost-effective pre-treatment option for landfill leachate to remove ammonia in time less than conventional air stripping. The ammonia removal achieved was 96% at time of 5 h.
Coagulation-flocculation process appears that iron salts were more efficiency than aluminum salts in organic removal from landfill leachate, the removal of COD reached to 68% in poly ferric sulfate and 64% for ferric sulfate while for Poly aluminum chloride 32% was the highest COD removal. In combined treatment PFS used as coagulant material, the effluent of ammonia stripping was used as the influent of a coagulation process. After the process of coagulation, the biodegradation ratio was improved from 0.20 to 0.31 with the dosage of PFS 1200 mg Lâˆ’1 at pH 5.0. The overall all removal ratios of NH3-N, COD, BOD5, TOC, SS and turbidity were 96.5% ,71.5% ,56.5% , 48.5% , 96% and 92%, respectively.
This work was financially supported by the Hubei Provincial Science and Technology Department with grant No.2006AA305A05. Mr Alkhafaji R. Abood thanks the Ministry of Higher Education and Thi-Qar University in Iraq for enabling him to purse a higher degree. He is also grateful to the China Scholarship Council (CSC) and China University of Geosciences (CUG) for the financial support of this research.
Figure and table captions
Figure 1. Optimum conditions effected on NH3-N removal by air stripping. (a) pH effect and (b) airflow rate effect. Error bars represent standard deviations.
Figure 2. Optimum conditions effected on NH3-N removal by agitation process. (a) pH effect, (b) gradient velocity effect and (c) time effect. Error bars represent standard deviations.
Figure 3. NH3-N ratio removal in a combined ammonia stripping process. Error bars represent standard deviations.
Figure 4. Parameters effected on the COD removal by the coagulation process. (a) pH effect and (b) coagulant dosage effect. Error bars represent standard deviations.
Table 1 Characteristics of leachate
Table 2 Comparison of the current ammonia stripping process with other previous work for landfill leachate treatment.
Table 3 Ratio of sludge volume of the total volume of sample
Table 4 Results obtained for each of the tested coagulants
Table 5 Concentration and removal of pollutants in effluent by each treatment process
Table 1Characteristics of leachate
Table 2 Comparison of the current ammonia stripping process with other previous work
for landfill leachate treatment.