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Ammonia removal from landfill leachate by air stripping and absorption a
a
b
Fernanda M. Ferraz , Jurandyr Povinelli & Eny Maria Vieira a
Department of Hydraulics and Sanitation, Escola de Engenharia de São Carlos, Universidade São Paulo, São Carlos, Brazil b
Sao Carlos Chemistry Institute, Universidade de São Paulo, São Carlos, Brazil Accepted author version posted online: 17 Jan 2013.Published online: 11 Feb 2013.
To cite this article: Fernanda M. Ferraz, Jurandyr Povinelli & Eny Maria Vieira (2013) Ammonia removal from landfill leachate by air stripping and absorption, Environmental Technology, 34:15, 2317-2326, DOI: 10.1080/09593330.2013.767283 To link to this article: http://dx.doi.org/10.1080/09593330.2013.767283
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Environmental Technology, 2013 Vol. 34, No. 15, 2317–2326, http://dx.doi.org/10.1080/09593330.2013.767283
Ammonia removal from landfill leachate by air stripping and absorption Fernanda M. Ferraza∗ , Jurandyr Povinellia and Eny Maria Vieirab a Department
of Hydraulics and Sanitation, Escola de Engenharia de São Carlos, Universidade São Paulo, São Carlos, Brazil; b Sao Carlos Chemistry Institute, Universidade de São Paulo, São Carlos, Brazil
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(Received 26 May 2012; final version received 11 January 2013 ) An old landfill leachate was pre-treated in a pilot-scale aerated packed tower operated in batch mode for total ammoniacal nitrogen (TAN) removal. The stripped ammonia was recovered with a 0.4 mol L−1 H2 SO4 solution, deionized water and tap water. Ca(OH)2 (95% purity) or commercial hydrated lime was added to the raw leachate to adjust its pH to 11, causing removal of colour (82%) and heavy metals (70–90% for Zn, Fe and Mn). The 0.4 mol L−1 H2 SO4 solution was able to neutralize 80% of the stripped ammonia removed from 12 L of leachate. The effectiveness of the neutralization of ammonia with deionized water was 75%. Treating 100 L of leachate, the air stripping tower removed 88% of TAN after 72 h of aeration, and 87% of the stripped ammonia was recovered in two 31 L pilot-scale absorption units filled with 20 L of tap water. Keywords: absorption; aerated packed tower; ammonium sulphate; aquammonia; heavy metals removal
1. Introduction Many studies have established that sanitary landfills generate leachate, a high-strength wastewater that contains biodegradable organic matter, recalcitrant organic matter (such as humic substances), inorganic salts, heavy metals, and has a high ammoniacal nitrogen concentration [1–3]. Among the leachate constituents, ammoniacal nitrogen is one of particular concern because its concentration remains high (800–5210 mg L−1 ), even in leachates from old landfills [1,2]. A total ammoniacal nitrogen (TAN) concentration as high as 1000 mg L−1 may inhibit the activity of microorganisms, which decreases the efficiency of leachate treatments that depend on biological processes [4,5]. Several studies have reported that air stripping is successful in removing ammonia from landfill leachate [6–15] and many other wastewaters, such as those from the fertilizer industry [16], pig slurry [17,18], anaerobic digestion effluent [19,20] or source-segregated food waste [21]. The effectiveness of ammonia removal obtained in these studies was in the range of 90–99%. The air stripping process is based on mass transfer, and according to Reaction 1, ammonia removal is favoured by raising the pH, which would cause the chemical equilibrium to shift towards the direction of gaseous ammonia (NH3 ) [22]: NH4+ ↔ NH3 + H +
(Reaction 1)
For leachate, a pH of 11 is recommended to favour ammonia removal by air stripping, and calcium hydroxide is ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
commonly used as the added alkali [13,14]. The advantages of pH adjustment using Ca(OH)2 include the removal of heavy metals and colour caused by the co-precipitation of organic macromolecules, such as the humic acids present in landfill leachate [23]. An alternative method of shifting the chemical equilibrium of Reaction 1 to the right is the use of high temperatures, because the solubility of gases decreases with increasing temperature [24]. However, this alternative may be limited because of the need for an economically feasible energy source [17]. To prevent air pollution caused by the mass transfer from the liquid to gaseous phase, treatment of the stripped ammonia by absorption is recommended [25]. In contrast to air stripping, this process consists of the transfer of the gaseous pollutant to a liquid phase [26]. In general, acidic solutions are used for ammonia neutralization, and the ammonia is recovered as an ammonium salt [17]. However, due to its high solubility, ammonia can be neutralized with water [16] instead of with acidic solutions. Nonetheless, only a few studies have reported the application of air stripping followed by ammonia recovery (absorption) in detail [16,17,27]. In some cases, no information was provided concerning the effectiveness of the ammonia recovery [18,20,21] or which absorbent was used [11]. In addition, the majority of studies on air stripping relied on small stripping units in which air was bubbled at low rates (from 1.2–300 L h−1 ) and only a small volume of leachate (0.8–4 L) was treated [6–10,13,15,17,18,20].
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Although the ammonia concentration may be high, the total mass to be removed is proportional to the small volume of the treated wastewaters, which may explain why most of the previous studies reported 90–99% efficiency for short aeration times (6–24 h). Furthermore, in the cited studies, the stripped ammonia was typically released into the atmosphere instead of being treated. The objective of this paper was to investigate the pre-treatment of landfill leachate using air stripping and ammonia recovery by absorption. Two steps of the pretreatment process were evaluated: (1) ammonia removal with an aerated packed tower under different operational conditions, and (2) ammonia absorption using a 0.4 mol L−1 sulphuric acid solution (to produce ammonium sulphate), deionized water or tap water (to produce aquammonia). Both ammonium sulphate and aquammonia can be recycled as liquid fertilizers. 2. Materials and methods 2.1. Landfill leachate This study used leachate from the municipal sanitary landfill of Sao Carlos, a medium-sized city located in Sao Paulo, Brazil that has approximately 220,463 inhabitants and generates 160 tons of municipal solid waste per day. This municipal landfill has been in operation since 1990; however, its closure is already in progress, and a new landfill is currently being built. The major constituent of the Sao Carlos municipal solid waste is organic matter (60% by mass); the solid waste also includes glass, paper, plastic and metals, even though the city has a recycling programme. The sampling point was located at the end of the landfill drainage system that carries the leachate from the landfill to the treatment ponds. According to Table 1, the leachate used in this study has the characteristics of old leachate because of its low biochemical oxygen demand (BOD)/chemical oxygen demand (COD) ratio (0.2), average pH of 8 and high TAN concentration (approximately 1200 mg L−1 ). Table 1.
Physico-chemical characterization of raw leachate. Mean
Standard deviation
Total alkalinity (mgCaCO3 L−1 ) 7770 pH 8 1,876 Total carbon (mg L−1 ) 718 Total organic carbon (mg L−1 ) Biochemical oxygen demand (mgO2 L−1 ) 427 2393 Chemical oxygen demand (mgO2 L−1 ) 19,338 Conductivity (μS cm−1 ) 1421 Total Kjeldahl nitrogen (mg L−1 ) 208 Organic nitrogen (mg L−1 ) 1213 Total ammoniacal nitrogen (mg L−1 ) 11,430 Total solids (mg L−1 ) 8542 Fixed solids (mg L−1 ) 2888 Volatile solids (mg L−1 )
1528 0.4 259 67 34 532 3904 573 64 547 2794 2019 1010
Parameter
2.2.
Air stripping unit and the ammonia absorption system
The air stripping tower consisted of a 2.24 m-high PVC tube with a 0.15 m diameter and a total volume of 39 L. The packing material in the tower was composed of corrugated polyethylene Raschig rings, which were 1.5 cm in diameter and 5 cm long, with a specific surface area of 300 m2 m−3 . The packed bed was 1.80 m high, comprising a working volume of 30 L. The air stripping tower was operated in batch mode at room temperature (approximately 25◦ C) and, as shown in Figure 1, the leachate was carried to the top of the tower using a dosing pump. Inside the upper part of the tower, a perforated PVC board was used as the leachate distribution device. The top of the tower was closed to prevent the release of gaseous effluent into the atmosphere and was connected to an absorption system. Initially, two 6 L glass flasks were tested as the benchscale absorption units (Figure 1). In this case, 12 L of leachate was treated by the air stripping tower and recirculated at a flow rate of 30 L h−1 . Compressed air was injected into the bottom of this tower at rates of 1600 or 4500 L h−1 . As shown in Figure 1, the bench-scale absorption units each had one opening for sampling and charge/discharge of the absorbents. The plastic pipe of the air stripping tower carrying the gaseous effluent was directly connected to the first flask, which was then connected to the second flask, to focus on enhancing the ammonia absorption efficiency. Both of the bench-scale absorption units were filled with 4 L of either a 0.4 mol L−1 sulphuric acid solution or deionized water. The sulphuric acid used was of analytical grade, and the deionized water presented a maximum conductivity of 0.5 μS cm−1 . The acid-base indicator phenolphthalein was added to the absorbents to indicate the level of neutralization, which was used to avoid unnecessary opening of the flasks and the consequent ammonia losses in their headspace. Only once the colour of the flasks’ content was dark pink (pH ≥ 8) were samples obtained to determine the ammoniacal nitrogen content; the content of the absorption units was replaced at this time. To treat a large volume of leachate (100 L) by the air stripping tower, a large quantity of ammonia would have to be recovered; therefore, two 31 L absorption towers were tested as the pilot-scale absorption units. These towers were filled with 20 L of tap water (pH 6.5) and operated as the bench-scale absorption units: when their content was saturated, the towers were discharged and refilled with 20 L of tap water. In the experiments with the pilot-scale absorption towers, the air stripping tower was operated at a leachate flow rate of 90 L h−1 , whereas the air flow rate was 4500 L h−1 . The ratio of air/liquid (50) was similar to that of the previous batch runs.
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Figure 1. Schematic of the air stripping tower and the absorption units (adapted from Bonmatí and Flotats [17]): (1) air stripping tower; (2–3) absorption units; (4) dosing pump; (5) air compressor; (a–c) sampling points; (b,c) sampling and charge/discharge points.
The TAN removal and absorption (abs) efficiencies were calculated by Equations 1 and 2, respectively: TANinitial − TANfinal ∗ 100 (1) TANinitial (TANabs unit1 (g) + TANabs unit2 (g)) TANabsorption (%) = TANstripped (g) (2) TANremoval (%) =
The mass of TAN used in Equation (2) is obtained by multiplying the TAN concentration measured in the leachate or absorption unit samples by the volume of Table 2.
leachate that is being treated (12 or 100 L) or the volume of absorption units content (4 or 20 L). The operational conditions that were evaluated in this research are presented in Table 2. 2.3.
pH adjustment Powdered Ca(OH)2 (95% purity) and commercial hydrated lime (with non-volatile total oxides greater than 88%) were used to raise the pH of the leachate to 11. This pH value was selected because it was previously reported to be in the optimum range for ammonia stripping [13,14,17].
Operational conditions evaluated for the air stripping and ammonia absorption processes. Leachate
Qleachate
Batch run
(L)
pH
Batch 1 Batch 2 Batch 3 Batch 4 Batch 5 Batch 6 Batch 7
12 12 12 12 100 100 100
11∗ 11∗ 11∗∗ 11∗ 11∗∗ 11∗∗ 11∗∗
Qair
(L h−1 ) 30 30 30 30 90 90 90
4500 1600 1600 1600 4500 4500 4500
Initial TAN (mg L−1 )
Operational time (days)
Absorption media
830 1980 1953 1310 850 850 850
1 12 12 12 3 3 3
H2 SO4 (0.4-mol L−1 )a H2 SO4 (0.4-mol L−1 )a H2 SO4 (0.4-mol L−1 )a Deionized watera Tap waterb Tap waterb Tap waterb
Notes: Qair : air flow rate; Qleachate : liquid flow rate; (∗ ) pH adjusted with standard grade Ca(OH)2 ; (∗∗ ) pH adjusted with commercial hydrated lime; (a) batch runs with the bench scale absorption units; (b) batch runs with the pilot-scale absorption units.
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Analytical grade Ca(OH)2 and commercial hydrated lime were used to evaluate whether the metallic concentration in the generated sludge was in accordance with Brazilian environmental law. Therefore, the concentration of metals in the raw leachate, supernatant leachate and generated sludge were measured. Because the leachate was stirred during the addition of alkali, the removal of ammoniacal nitrogen by volatilization was also evaluated. From the raw and supernatant leachate samples, the removal of organic matter by the precipitation of calcium carbonate was also determined. 2.4. Physico-chemical analyses The following parameters were measured according to the Standard Methods for the Examination of Water and Wastewater [28]: the BOD was determined by the 5-day test method 5210 B (Hach BODTrakTM II respirometric BOD apparatus); the COD was determined by colorimetric method 5220 D (Hach COD reactor 45600-00/Hach DR 2010 spectrophotometer); the conductivity was determined by method 2510 B; the colour was determined by method 2120 C (Hach DR 2010 spectrophotometer operated at 455 nm); the heavy metals (total Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) were determined by method 3111 B and D (Varian AA240 FS atomic absorption spectrophotometer); the pH was determined by method 4500 – H+ ; the total alkalinity was determined by method 2320 B; the total carbon and total organic carbon (TOC) was determined by method 5310 B (Shimadzu TOC 5000 A Analyser); the total Kjeldahl nitrogen (TKN) was determined by method 4500 C – Norg Nitrogen (Büchi digestion unit B-426); the TAN was determined by method 4500 C – NH3 Nitrogen (Büchi distillation unit B-339); the total solids were determined by method 2540 B and the fixed and volatile solids were determined by method 2540 D. 3. Results and discussion The results are presented in four main subsections, in accordance with the objectives of this research. All of the data were obtained from the operation of the stripping tower in batch runs. Each batch run ended when the concentration of the TAN in the treated leachate was equal to or below 20 mg L−1 , which is the discharge limit established by Brazilian environmental law [29]. First, we evaluated the effect of pH adjustment on leachate quality, and then the effect of the use of different air flow rates on the ammonia removal by air stripping. The ammonia recovery was initially evaluated with a 0.4 mol L−1 sulphuric acid solution. Next, the use of Ca(OH)2 and commercial hydrated lime to adjust the pH of the raw leachate was compared by evaluating the volume of sludge generated and its heavy metal concentration. Ammonia recovery using deionized water or tap water was subsequently examined as an alternative to the use of the acidic solution.
3.1.
The influence of pH adjustment on landfill leachate quality
In general, the consumption of analytical grade Ca(OH)2 in the batch runs varied from 10–20 g per litre of leachate. Because this alkali was used in excess, after 30 min of sedimentation, the generated sludge varied from 80–140 mL per litre of leachate. The consumption of commercial hydrated lime was 24 g per litre of leachate on average, generating approximately 180 mL of lime sludge per litre of leachate. The pH adjustment effectively removed the colour (82%), as the leachate changed from dark brown (true colour, 8050 PtCo) to yellow (true colour, 1440 PtCo). This result was also observed by Renou et al. [23], who associated the clarification of the leachate with the removal of humic substances. Because the leachate was kept under agitation while Ca(OH)2 was added, 13% of the initial ammonia concentration (1213 mg L−1 on average) was removed by volatilization, whereas 20% of the initial TOC (718 mg L−1 on average) was most likely removed by precipitation. Another advantage of using Ca(OH)2 or commercial lime to adjust the pH of the raw leachate was the high removal efficiency for metals: in Batches 2 and 3, their removal varied from 70–90%, particularly for iron, zinc and manganese (Table 3). This high heavy metal removal efficiency was also observed for the other batches (data not shown). It is important to note that CONAMA Resolution 430/2011 is only valid for wastewaters; thus, its limits cannot be applied to the sludge generated during the adjustment of pH with the tested alkalis. The appropriate legislation for sludge quality that focuses on its reuse in agriculture is CONAMA Resolution 380/2006 [30]. The concentration of heavy metals in the sludge was also much lower than the limits established by CONAMA Resolution 380/2006, even for pH adjustment using commercial hydrated lime (Table 3). 3.2.
The influence of air flow rate on ammonia removal and recovery To more accurately examine the influence of air injection on the performance of the air stripping tower and absorption units, only the air flow rate was varied in the following experiments. Using air flow rates of 4500 L h−1 (Batch 1) and 1600 L h−1 (Batch 2), the TAN concentration in the treated leachates of Batches 1 and 2 was 10 mg L−1 and 15 mg L−1 , respectively (Figure 2). In both cases, the ammonia removal behaviour followed first-order kinetics, as observed by Zhang et al. [18]. The treated leachates of both batches were in accordance with the discharge limit of 20 mg L−1 for TAN, which was established by the Brazilian National Council of the Environment (CONAMA) in Resolution 430/2011 [29]. In Batch 1, only 24 h were required to substantially remove the TAN from the leachate. By plotting the natural
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Table 3. The removal of heavy metals from the raw leachate resulting from the pH adjustment with 95% purity Ca(OH)2 (Batch 2) and commercial hydrated lime (CHL) (Batch 3). Batch 2
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pH adjustment with 95% purity Ca(OH)2 Metals
RL
Total Zn (mg L−1 ) Total Pb (mg L−1 ) Total Cd (mg L−1 ) Total Ni (mg L−1 ) Total Fe (mg L−1 ) Total Mn (mg L−1 ) Total Cu (mg L−1 ) Total Cr (mg L−1 ) Total Zn (mg kg−1 ) Total Pb (mg kg−1 ) Total Cd (mg kg−1 ) Total Ni (mg kg−1 ) Total Fe (mg kg−1 ) Total Mn (mg kg−1 ) Total Cu (mg kg−1 ) Total Cr (mg kg−1 )
0.50 0.20 0.02 0.33 3.81 0.12 0.06 0.31 -
pH 11 L GSa 0.07 0.13 0.01 0.24 0.40 0.02 0.02 0.07 -
17 66 7 18 240 22 8 18
Batch 3
Brazilian environmental law
pH adjustment with commercial hydrated lime
Discharge Standards for limits for sludge reuse in wastewaterc agricultured
Ca(OH)2
RL
pH 11 L
GSb
CHL
-
0.32 0.21 0.03 0.44 4.23 0.17 0.05 0.39 -
0.10 0.16 0.00 0.23 0.30 0.05 0.03 0.11 -
26 76 6 33 2177 314 10 32
-
5 64 6 14 142 20 8 11
4 71 3 12 1784 303 7 4
5 0.5 0.2 2 15 1 1 0.5 -
2800 300 39 420 NAe NAe 1500 1000
RL: raw leachate; L: leachate; GS: generated sludge; (a) sludge generated by pH adjustment with Ca(OH)2 ; (b) sludge generated by pH adjustment with commercial hydrated lime; (c) CONAMA 430/2011; (d) CONAMA 380/2006; (e) N.A: not available.
logarithm of the ratio TAN at a time ‘t’/initial TAN (TAN/TAN0 ) versus the operational time and fitting the curve to a line, a TAN removal rate of 0.18 h−1 was obtained (Figure 2(a)). The results of treating landfill leachate by air stripping in this work were compared with those reported by several authors and are summarized in Table 4. The TAN removal rate of Batch 1 was the greatest among the previously reported values [6–15]. Based on the cited literature data, the times required to reach the Brazilian environmental law discharge limit for TAN were estimated. In Batch 1, the air stripping tower could be operated for 20 h, saving energy costs, whereas in the other references with similar TAN removal rates, the discharge limit of 20 mg TAN L−1 would be reached after 35 h [8,14]. Despite the high TAN removal rate obtained in Batch 1, the air flow rate of 4500 L h−1 was not appropriate for the absorption units used. This air flow rate resulted in strong turbulence in the absorption flasks, and the air bubbles containing ammonia ascended too quickly. These conditions did not favour the transfer of ammonia from the gaseous to liquid phase (i.e. the 0.4 mol L−1 H2 SO4 solution) because sufficient contact was not provided between the sulphuric acid and ammonia molecules. In the second absorption flask, this strong turbulence resulted in the discharge of the sulphuric acid solution, and therefore, the ammonia recovery could not be evaluated. However, when an air flow rate of 1600 L h−1 was applied to the tower (Batch 2), the ammonia recovery efficiency was 80% (Figure 2(c)). Although Batch 2 offered better conditions for ammonia recovery in the bench-scale absorption units, decreasing the air flow rate to 1600 L h−1
resulted in a low TAN removal rate for achieving the Brazilian discharge limit: 0.0154 h−1 (Table 4) or 0.37 d−1 (Figure 2(b)). Nevertheless, this TAN removal rate was greater than the rate of 0.0131 h−1 obtained by Nurisepehr et al. [15], whose air stripping unit would have to be operated for 14 days to meet the Brazilian discharge limit for TAN. The data in Table 4 are only related to the treatment of landfill leachate by air stripping, so the work of De La Rubia et al. [21] was not included. However, these authors evaluated the treatment of source-segregated food waste by air stripping, focusing on the removal of its high TAN content. The waste pH was 8, and the experiments were carried out at 35◦ C. After 78 h of aeration at a rate of 45 L h−1 , the TAN decreased from approximately 7000 to 5200 mg L−1 . The TAN removal rate (0.0040 h−1 ) was almost four times lower than that of Batch 2 (0.0154 h−1 ), and if the Brazilian discharge limit for the TAN must be met under the operational conditions evaluated by these authors, the aeration time would be 61 days. By analysing the results of this research in conjunction with those of other published papers [11,19], it can be inferred that the air flow rate is an important factor that may influence the TAN removal rate. Guštin et al. [19] evaluated the treatment of anaerobic digestion effluent by air stripping using an air flow rate of 90,000 L h−1 , which is almost 60 times higher than that of Batch 2. The reported TAN removal rate was 1.3 h−1 , and at this rate it would take only 4 h to produce a treated effluent with 20 mg TAN L−1 . The long operational time of Batch 2 may not be considered practical in terms of energy costs, but it could be significantly decreased by implementing further
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absorption units. Therefore, as Batches 3 and 4 still used the bench-scale absorption units, the air flow rate of 1600 L h−1 was maintained. Because the bench-scale absorption units only had one opening to charge/discharge the absorbent solution, it was not possible to prevent the loss of their head space content to the atmosphere as they were opened. Despite the simplicity of the bench-scale absorption units, the ammonia recovery with the sulphuric acid solution in Batch 2 was close to the range reported by Bonmatí and Flotats [17], who obtained 84–94% recovery efficiency in the treatment of pig slurry by air stripping with pH 11.5 at 80◦ C. It is worth mentioning that besides ammonia, volatile organics may also be stripped. Bonmatí and Flotats [17] reported that less than 5% of the COD stripped from pig slurry could be recovered with sulphuric acid by absorption, and recommended other treatments for stripping organics. Traps of sulphuric acid and sodium hydroxide have been tested to recover ammonia and other volatile compounds from stripped gaseous effluent [21]. However, the objective of this current research was only ammonia removal and recovery and did not include other substances that could be stripped from leachate. 3.3.
Figure 2. The effect of different air flow rates on the ammonia removal in (a) Batch 1 and (b) Batch 2 and (c) on the ammonia recovery in Batch 2.
post-treatments for leachate. For biological co-treatment, it has been reported that leachate pre-treated by air stripping, containing 100 mg TAN L−1 , was added to domestic wastewater at 2% (volume/volume), and the resultant mixture contained approximately 30 mg TAN L−1 , which was almost the same concentration found in domestic water [31]. When evaluating the results from Batch 2, one should understand that the focus of this experiment was to use conditions that could enable the evaluation of ammonia removal in conjunction with ammonia recovery in the bench-scale
The effect of pH adjustment with 95% purity Ca(OH)2 or commercial hydrated lime on the air stripping efficiency This experiment was focused on reducing the operational costs of using an alkali. Commercial hydrated lime can be obtained at a low cost, particularly in the Brazilian market, where it is approximately 70% cheaper than standard grade Ca(OH)2 . Comparing Figure 2(b) and Figure 3(a), the ammonia removal was identical for Batch 2 (kL a = 0.37 d −1 ) and Batch 3 (kL a = 0.37 d −1 ). In both cases, after 12 days, the TAN concentration in the treated leachate was below 20 mg L−1 . Thus, the use of commercial hydrated lime did not negatively affect the air stripping performance. Figures 2(c) and 3(b) demonstrate that the use of a 0.4 mol L−1 sulphuric acid solution resulted in the effective recovery of ammonia: in Batches 2 and 3, 80% of the stripped ammonia was neutralized, generating an ammonium sulphate solution. As shown in Table 5, in Batches 2 and 3, ammonia absorption was rapidly observed in the first unit; however, as the acidic solution became saturated (the pH increased above 8), the second unit began to absorb the ammonia that was not recovered in the first unit. To maximize the effectiveness, the content of the absorption units was replaced whenever saturation was observed. For both batches, the acidic solution was replaced in the first unit after 2 days and in the second unit after 6 days. The objective of the final experiment was to evaluate ammonia absorption in deionized water with a focus on aquammonia production. In Batch 4, the air stripping tower
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Table 4.
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Comparison of the TAN removal rates (kL a) obtained in this research and those reported previously.
TL
T
Qar (L h−1 ) 4500 1600 4500 300 2 456 17 120 70,000 3600 120 900 1500
Reference
pH
(L)
(◦ C)
Batch 1 Batch 2 Batch 5 Cheung et al. [6] Marttinen et al. [7] Ozturk et al. [8] Silva et al. [9] Gotvanj et al. [10] Pi et al. [11] Souto et al. [12] Cotman and Gotvajn [13] Guo et al. [14] Nurisepehr et al. [15]
11 11 11 11 11 12 11 11 11 12 11 11 10
12 12 100 3.0 1.0 1.0 0.8 0.8 10 12 1.0 10 2
25 25 25 N.A 20 N.A N.A N.A 50 25 N.A N.A NA
TAN0
TANf
(mg L−1 ) 830 1493 850 705 150 1025 800 2300 1243 1900 480 1,350 1480
Figure 3. The effects of using commercial hydrated lime for pH adjustment in Batch 3: (a) ammonia removal and (b) ammonia recovery.
was operated in the same manner as for Batch 3 such that the same TAN removal rate was obtained (data not shown). As shown in Table 6, 75% of the stripped ammonia could be recovered as aquammonia, which was the same
10 20 100 200 89 800 5 460 198 0 50 150 1000
TANREM
kL a
Time
Estimated time to reach TAN 20 mg L−1
(%)
(h−1 )
(h)
(h)
99 99 88 72 41 22 99 80 89 100 90 89 32
0.1796 0.0154 0.0247 0.0525 0.0217 0.1239 0.0529 0.0322 0.0765 0.0684 0.0942 0.1221 0.0131
24 288 72 24 24 2 96 50 24 144 24 18 30
20 280 152 68 93 32 70 147 54 67 34 34 329
range of efficiency obtained in Batches 2 and 3, in which the 0.4 mol L−1 sulphuric acid solution was used. Considering that the standard molar Gibbs energy of formation (Gfo ) for ammonium sulphate, −903.1 kJ mol−1 , is more negative than the Gfo of aquammonia, −236.5 kJ mol−1 [32], one would expect that ammonia neutralization using the sulphuric acid solution would be more effective. This research obtained similar results for ammonia recovery using either the sulphuric acid solution or deionized water, mostly because of losses that occurred during the sampling and opening of the bench-scale absorption units when charging/discharging their contents. However, the ammonia recovery in Batches 2 and 3 was greater (80%) than that observed in Batch 4, confirming the aforementioned thermodynamic data. Regarding ammonia recovery, Batch 4 differed from the previous batches because ammonia neutralization occurred much faster in the deionized water than in the sulphuric acid solution, making it necessary to replace the content of the absorption units more frequently. This conclusion can be observed by analysing the pH behaviour over 33 h (Figure 4) and the frequency of deionized water replacements over the 12-day aeration period (Table 6). The concentration of H+ ions in the absorption units filled with the 0.4 mol L−1 sulphuric acid solution was significantly greater (pH 0.4) than the concentration in deionized water (pH 6). Because of this difference, the pH of the deionized water in the first absorption unit increased from 6 to approximately 11 in the first hour, and this value was roughly maintained for the subsequent 33 h. The saturation point could be associated with a pH of 11 in the absorption units because after this value was reached, no additional ammonia recovery was observed in the two absorption units. After only 3–4 h, the first absorption unit became saturated (the measured pH was 11), and the unneutralized ammonia was partially recovered in the second unit,
2324 Table 5.
F.M. Ferraz et al. pH behaviour during ammonia recovery using bench-scale absorption units in Batches 2 and 3. Batch 2
Batch 3
Replacement of the 0.4-mol L−1 Replacement of the Time pH range 0.4-mol L−1 H2 SO4 solution Time pH range H2 SO4 solution intervals intervals (days) Abs unit 1 Abs unit 2 Abs unit 1 Abs unit 2 (days) Abs unit 1 Abs unit 2 Abs unit 1 Abs unit 2 0–2 2–6 6–12
0.4–10.0 0.4–2.2 2.2–9.0
0.4–1.5 1.5–7.1 0.4–1.3
– Yes No
– No Yes
0–2 2–6 6–12
0.4–8.8 0.4–5.4 5.4–8.2
0.4–3.7 3.7– 7.8 0.4–1.6
– Yes No
– No Yes
Note: Abs: absorption.
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Table 6.
The pH behaviour during ammonia recovery in bench-scale absorption units with deionized water (Batch 4). pH range
Time intervals (days) 0–1.4 2–5 5–10 10– 12
Replacement of the deionized water
Ammonia absorption (%)
Abs unit 1
Abs unit 2
Abs unit 1
Abs unit 2
Abs unit 1
Abs unit 2
Total Abs
6.0–11.0 6.0–9.7 6.0–9.3 6.0–9.0
6.0–11.0 6.0–9.7 6.0–9.0 6.0–9.0
– Yes Yes Yes
– Yes Yes Yes
28 42 47 50
16 20 25 25
44 62 70 75
Note: Abs: absorption.
Figure 4. pH variations in the first 33 h of ammonia absorption in Batch 4.
in which the content became saturated after 9 h (when the pH increased from 6 to 11). To maintain the absorption effectiveness, the content of each unit was replaced with 4 L of deionized water. 3.4.
Ammonia removal by air stripping to pre-treat 100 L of leachate and ammonia recovery with tap water using two 31 L pilot-scale absorption units To verify whether the high ammonia removal and recovery efficiencies obtained in Batches 1–4 would also be observed in the treatment of a major volume of leachate, 100 L was
tested in triplicate batch runs, and the operating conditions are presented in Table 2 as Batch 5. Although these runs used higher air (4500 L h−1 ) and liquid (90 L h−1 ) flow rates than in the previous batches, the air/liquid ratio was maintained in the range of 50. The data presented in Table 4 represent the average values of the triplicate batches. After 72 h of aeration, the air stripping tower could remove, on average, 88% of the TAN concentration initially present in the pH 11 leachate. The duration of Batch 5 was four times shorter than that of Batches 2–4, and it is worth mentioning that the volume of leachate to be treated was considerably greater than the 12 L initially studied or the volumes evaluated by the authors cited in Table 4. In contrast to the previous batch runs, the final TAN was 100 mg L−1 because the leachate was intended for biological treatment processes [31]. As discussed, ammonia recovery could not be evaluated in Batch 1 because of the strong turbulence in the bench-scale absorption units. Nonetheless, the ammonia was recovered in Batch 5 by the use of the two 31 L pilot-scale absorption towers. To focus on the better neutralization of the stripped ammonia, the two pilot-scale absorption units were discharged and refilled with 20 L of tap water in 24 h time intervals. This procedure resulted in an average ammonia recovery effectiveness of 87%. The minimization of ammonia losses during sampling also contributed to this efficiency, which was possible because of the better structure of the pilot-scale absorption towers compared with the simple bench-scale absorption units.
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4. Conclusions From the results, the following conclusions can be drawn: • pH adjustment: This procedure improved the leachate quality by the removal of colour (82%), metals (70–90% for Zn, Fe and Mn), TOC (19%) and ammonia (13%). The ammonia removal was not affected by the quality of the tested alkalis. The results also show that the concentration of heavy metals in the sludge generated by the two tested alkalis was in accordance with Brazilian environmental law. Thus, the use of commercial hydrated lime is an interesting alternative to lower the operational costs of air stripping. The possibility of reusing the generated sludge to adjust the pH of the raw leachate should be tested in future studies; • Ammonia removal by air stripping: the high performance (99%) of the aerated packed tower in the treatment of 12 L of leachate was also observed in the treatment of a higher volume of leachate. After only 72 h of aeration, the ammonia removal from 100 L of leachate was 88%; • Ammonia recovery by absorption: approximately 80% of the stripped ammonia could be recovered in the bench-scale absorption units filled with a 0.4 mol L−1 H2 SO4 solution or deionized water. The feasibility of this process was confirmed in the experiments using the two 31 L pilot-scale absorption towers. On average, 87% of the TAN stripped from 100 L of the leachate could be neutralized in the two pilot-scale absorption units filled with tap water. Acknowledgements The authors would like to thank CAPES (Coordenação de Aperfeiçoamento Pessoal de Ensino Superior), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo, processos no 2010/51955-2; no 2011/50627-4) and the University of Sao Paulo for their financial support.
References [1] Kjeldsen P, Barlaz MA, Rooker AP, Baun A, Ledin A, Christensen TH. Present and long-term composition of MSW landfill leachate: a review. Crit Rev Environ Sci Technol. 2002;32:297–336. [2] Lopez A, Pagano M, Volpe A, Di Pinto A. Fenton’s pre-treatment of mature landfill leachate. Chemosphere 2004;54:1005–1010. [3] Renou S, Givaudan JG, Poulain S, Dirassouya NF, Moulin P. Landfill leachate treatment: Review and opportunity. J Hazard Mater. 2008;150:468–493. [4] Shiskoswski DM, Mavinic DS. Biological treatment of a high ammonia leachate: influence of external carbon during initial startup. Water Res. 1998;32:2533–2541. [5] Wiszniowski J, Surmacz-Górska J, Robert D, Weber JV. The effect of landfill leachate composition on organics and nitrogen removal in an activated sludge system with bentonite additive. J Environ Manage. 2007;85:59–68.
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[6] Cheung KC, Chu LM, Wong MH. Ammonia stripping as a pre-treatment for landfill leachate. Water Air Soil Pollut. 1997;94:209–221. [7] Marttinen SK, Kettunen RH, Sormunen KM, Soimasuo RM, Rintala JA. Screening of physical-chemical methods for removal of organic material, nitrogen and toxicity from low strength leachates. Chemosphere 2002;46: 851–858. [8] Ozturk I, Altinbas M, Koyuncu I, Arikan O, GomecYangin C. Advanced physico-chemical treatment experiences on young municipal landfill leachates. Waste Manage. 2003;23:441–446. [9] Silva AC, Dezotti M, Sant’Anna Jr. GL. Treatment and detoxication of a sanitary landfill leachate. Chemosphere 2004;55:207–214. [10] Gotvanj AŽ, Tišler T. Zagorc-Konˇcan J. Comparison of different treatment strategies for industrial landfill leachate. J Hazard Mater. 2009;162:1446–1456. [11] Pi KW, Li Z, Wan DJ, Gao LX. Pretreatment of municipal landfill leachate by a combined process. Process Saf Environ Protect. 2009;87:191–196. [12] Souto GDB, Povinelli SCS, Povinelli J. Ammonia stripping from landfill leachate using packed towers. Paper presented at: 8th International Symposium on Sanitary and Environmental; 2008 June 24–27; Florence, Italy. [13] Cotman M, Gotvajn AZ. Comparison of different physicochemical methods for the removal of toxicants from landfill leachate. J Hazard Mater. 2010;178:298–305. [14] Guo JS, Abbas AA, Chen YP, Liu ZP, Fang F, Chen P. Treatment of landfill leachate using a combined stripping, Fenton, SBR, and coagulation process. J Hazard Mater. 2010;178:699–705. [15] Nurisepehr M, Jorfi S, Kalantary RR, Akbrari H, Soltani RDC, Samaei M. Sequencing treatment of landfill leachate using ammonia stripping, Fenton oxidation and biological treatment. Waste Manage Res. 2012;30: 883–887. [16] Minocha VK, Rao AVS. Ammonia removal and recovery from urea fertilizer plant waste. Environ Technol Lett. 1998;9:655–664. [17] Bonmatí A, Flotats X. Air stripping of ammonia from pig slurry: characterisation and feasibility as a pre- or posttreatment to mesophilic anaerobic digestion. Waste Manage. 2003;23:261–272. [18] Zhang L, Lee Y, Jahng D. Ammonia stripping for enhanced biomethanization of piggery wastewater. J Hazard Mater. 2012;199–200:36–42. [19] Guštin S, Marinšek-Logar R. Effect of pH, temperature and air flow rate on the continuous ammonia stripping of the anaerobic digestion effluent. Process Saf Environ Protect. 2011;89:61–66. [20] Lei X, Sugiura N, Feng C, Maekawa T. Pretreatment of anaerobic digestion effluent with ammonia stripping and biogas purification. J Hazard Mater. 2007;145:391–397. [21] De la Rubia MA, Walker M, Heaven S, Banks CJ, Borja R. Preliminary trials of in situ ammonia stripping from source segregated domestic food waste digestate using biogas: effect of temperature and flow rate. J Hazard Mater. 2007;145:391– 397. [22] Metcalf and Eddy. Wastewater engineering: treatment and reuse. New York: McGraw-Hill; 2003. [23] Renou S, Poulain S, Givaudan JG, Moulin P. Amelioration of ultrafiltration process by lime treatment: case of landfill leachate. Desalination 2009;249:72–82. [24] Saracco G, Genon G. High temperature ammonia stripping and recovery from process liquid wastes. J Hazard Mater. 1994;37:191–206.
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[25] USEPA – United States Environmental Protection Agency. Control and pollution prevention options for ammonia emissions. Research Triangle Park (NC): Environmental Protection Agency; 1995. [26] Perry R, Green DW, Maloney JO. Perry’s chemical engineer’s handbook. New York: McGraw-Hill; 1997. [27] Basakçılardan-Kabakcı S, ˙Ipeko˘glu AN, Talınlı ˙I. Recovery of ammonia from human urine by stripping and absorption. Environ Eng Sci. 2007;24:615–624. [28] APHA, AWWA and WEF. Standard methods for examination of water and wastewater. New York: American Public Health Association; 2005.
[29] Brazilian Ministry of the Environment [Internet]. 2011. [cited 31 Jan 2013]. Available from: http://www.mma.gov.br/ port/conama/legiabre.cfm?codlegi=646. [30] Brazilian Ministry of the Environment [Internet]. 2006. [cited 31 Jan 2013]. Available from: http://www.mma.gov.br/ port/conama/res/res06/res38006.pdf. [31] Ferraz FM, Povinelli J, Vieira EM, Tognon AL. Performance of a submerged aerobic biofiltrer in treatment of landfill leachate and municipal sanitary sewage. 9th Edition of the International Symposium of Sanitary and Environmental Engineering; 2012 June 26–29; Milan, Italy. [32] Lide D. CRC handbook of chemistry and physics. Boca Raton (FL): Taylor & Francis; 2009.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Ammonia removal from landfill leachate by air stripping and absorption a
a
b
Fernanda M. Ferraz , Jurandyr Povinelli & Eny Maria Vieira a
Department of Hydraulics and Sanitation, Escola de Engenharia de São Carlos, Universidade São Paulo, São Carlos, Brazil b
Sao Carlos Chemistry Institute, Universidade de São Paulo, São Carlos, Brazil Accepted author version posted online: 17 Jan 2013.Published online: 11 Feb 2013.
To cite this article: Fernanda M. Ferraz, Jurandyr Povinelli & Eny Maria Vieira (2013) Ammonia removal from landfill leachate by air stripping and absorption, Environmental Technology, 34:15, 2317-2326, DOI: 10.1080/09593330.2013.767283 To link to this article: http://dx.doi.org/10.1080/09593330.2013.767283
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Environmental Technology, 2013 Vol. 34, No. 15, 2317–2326, http://dx.doi.org/10.1080/09593330.2013.767283
Ammonia removal from landfill leachate by air stripping and absorption Fernanda M. Ferraza∗ , Jurandyr Povinellia and Eny Maria Vieirab a Department
of Hydraulics and Sanitation, Escola de Engenharia de São Carlos, Universidade São Paulo, São Carlos, Brazil; b Sao Carlos Chemistry Institute, Universidade de São Paulo, São Carlos, Brazil
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(Received 26 May 2012; final version received 11 January 2013 ) An old landfill leachate was pre-treated in a pilot-scale aerated packed tower operated in batch mode for total ammoniacal nitrogen (TAN) removal. The stripped ammonia was recovered with a 0.4 mol L−1 H2 SO4 solution, deionized water and tap water. Ca(OH)2 (95% purity) or commercial hydrated lime was added to the raw leachate to adjust its pH to 11, causing removal of colour (82%) and heavy metals (70–90% for Zn, Fe and Mn). The 0.4 mol L−1 H2 SO4 solution was able to neutralize 80% of the stripped ammonia removed from 12 L of leachate. The effectiveness of the neutralization of ammonia with deionized water was 75%. Treating 100 L of leachate, the air stripping tower removed 88% of TAN after 72 h of aeration, and 87% of the stripped ammonia was recovered in two 31 L pilot-scale absorption units filled with 20 L of tap water. Keywords: absorption; aerated packed tower; ammonium sulphate; aquammonia; heavy metals removal
1. Introduction Many studies have established that sanitary landfills generate leachate, a high-strength wastewater that contains biodegradable organic matter, recalcitrant organic matter (such as humic substances), inorganic salts, heavy metals, and has a high ammoniacal nitrogen concentration [1–3]. Among the leachate constituents, ammoniacal nitrogen is one of particular concern because its concentration remains high (800–5210 mg L−1 ), even in leachates from old landfills [1,2]. A total ammoniacal nitrogen (TAN) concentration as high as 1000 mg L−1 may inhibit the activity of microorganisms, which decreases the efficiency of leachate treatments that depend on biological processes [4,5]. Several studies have reported that air stripping is successful in removing ammonia from landfill leachate [6–15] and many other wastewaters, such as those from the fertilizer industry [16], pig slurry [17,18], anaerobic digestion effluent [19,20] or source-segregated food waste [21]. The effectiveness of ammonia removal obtained in these studies was in the range of 90–99%. The air stripping process is based on mass transfer, and according to Reaction 1, ammonia removal is favoured by raising the pH, which would cause the chemical equilibrium to shift towards the direction of gaseous ammonia (NH3 ) [22]: NH4+ ↔ NH3 + H +
(Reaction 1)
For leachate, a pH of 11 is recommended to favour ammonia removal by air stripping, and calcium hydroxide is ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
commonly used as the added alkali [13,14]. The advantages of pH adjustment using Ca(OH)2 include the removal of heavy metals and colour caused by the co-precipitation of organic macromolecules, such as the humic acids present in landfill leachate [23]. An alternative method of shifting the chemical equilibrium of Reaction 1 to the right is the use of high temperatures, because the solubility of gases decreases with increasing temperature [24]. However, this alternative may be limited because of the need for an economically feasible energy source [17]. To prevent air pollution caused by the mass transfer from the liquid to gaseous phase, treatment of the stripped ammonia by absorption is recommended [25]. In contrast to air stripping, this process consists of the transfer of the gaseous pollutant to a liquid phase [26]. In general, acidic solutions are used for ammonia neutralization, and the ammonia is recovered as an ammonium salt [17]. However, due to its high solubility, ammonia can be neutralized with water [16] instead of with acidic solutions. Nonetheless, only a few studies have reported the application of air stripping followed by ammonia recovery (absorption) in detail [16,17,27]. In some cases, no information was provided concerning the effectiveness of the ammonia recovery [18,20,21] or which absorbent was used [11]. In addition, the majority of studies on air stripping relied on small stripping units in which air was bubbled at low rates (from 1.2–300 L h−1 ) and only a small volume of leachate (0.8–4 L) was treated [6–10,13,15,17,18,20].
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Although the ammonia concentration may be high, the total mass to be removed is proportional to the small volume of the treated wastewaters, which may explain why most of the previous studies reported 90–99% efficiency for short aeration times (6–24 h). Furthermore, in the cited studies, the stripped ammonia was typically released into the atmosphere instead of being treated. The objective of this paper was to investigate the pre-treatment of landfill leachate using air stripping and ammonia recovery by absorption. Two steps of the pretreatment process were evaluated: (1) ammonia removal with an aerated packed tower under different operational conditions, and (2) ammonia absorption using a 0.4 mol L−1 sulphuric acid solution (to produce ammonium sulphate), deionized water or tap water (to produce aquammonia). Both ammonium sulphate and aquammonia can be recycled as liquid fertilizers. 2. Materials and methods 2.1. Landfill leachate This study used leachate from the municipal sanitary landfill of Sao Carlos, a medium-sized city located in Sao Paulo, Brazil that has approximately 220,463 inhabitants and generates 160 tons of municipal solid waste per day. This municipal landfill has been in operation since 1990; however, its closure is already in progress, and a new landfill is currently being built. The major constituent of the Sao Carlos municipal solid waste is organic matter (60% by mass); the solid waste also includes glass, paper, plastic and metals, even though the city has a recycling programme. The sampling point was located at the end of the landfill drainage system that carries the leachate from the landfill to the treatment ponds. According to Table 1, the leachate used in this study has the characteristics of old leachate because of its low biochemical oxygen demand (BOD)/chemical oxygen demand (COD) ratio (0.2), average pH of 8 and high TAN concentration (approximately 1200 mg L−1 ). Table 1.
Physico-chemical characterization of raw leachate. Mean
Standard deviation
Total alkalinity (mgCaCO3 L−1 ) 7770 pH 8 1,876 Total carbon (mg L−1 ) 718 Total organic carbon (mg L−1 ) Biochemical oxygen demand (mgO2 L−1 ) 427 2393 Chemical oxygen demand (mgO2 L−1 ) 19,338 Conductivity (μS cm−1 ) 1421 Total Kjeldahl nitrogen (mg L−1 ) 208 Organic nitrogen (mg L−1 ) 1213 Total ammoniacal nitrogen (mg L−1 ) 11,430 Total solids (mg L−1 ) 8542 Fixed solids (mg L−1 ) 2888 Volatile solids (mg L−1 )
1528 0.4 259 67 34 532 3904 573 64 547 2794 2019 1010
Parameter
2.2.
Air stripping unit and the ammonia absorption system
The air stripping tower consisted of a 2.24 m-high PVC tube with a 0.15 m diameter and a total volume of 39 L. The packing material in the tower was composed of corrugated polyethylene Raschig rings, which were 1.5 cm in diameter and 5 cm long, with a specific surface area of 300 m2 m−3 . The packed bed was 1.80 m high, comprising a working volume of 30 L. The air stripping tower was operated in batch mode at room temperature (approximately 25◦ C) and, as shown in Figure 1, the leachate was carried to the top of the tower using a dosing pump. Inside the upper part of the tower, a perforated PVC board was used as the leachate distribution device. The top of the tower was closed to prevent the release of gaseous effluent into the atmosphere and was connected to an absorption system. Initially, two 6 L glass flasks were tested as the benchscale absorption units (Figure 1). In this case, 12 L of leachate was treated by the air stripping tower and recirculated at a flow rate of 30 L h−1 . Compressed air was injected into the bottom of this tower at rates of 1600 or 4500 L h−1 . As shown in Figure 1, the bench-scale absorption units each had one opening for sampling and charge/discharge of the absorbents. The plastic pipe of the air stripping tower carrying the gaseous effluent was directly connected to the first flask, which was then connected to the second flask, to focus on enhancing the ammonia absorption efficiency. Both of the bench-scale absorption units were filled with 4 L of either a 0.4 mol L−1 sulphuric acid solution or deionized water. The sulphuric acid used was of analytical grade, and the deionized water presented a maximum conductivity of 0.5 μS cm−1 . The acid-base indicator phenolphthalein was added to the absorbents to indicate the level of neutralization, which was used to avoid unnecessary opening of the flasks and the consequent ammonia losses in their headspace. Only once the colour of the flasks’ content was dark pink (pH ≥ 8) were samples obtained to determine the ammoniacal nitrogen content; the content of the absorption units was replaced at this time. To treat a large volume of leachate (100 L) by the air stripping tower, a large quantity of ammonia would have to be recovered; therefore, two 31 L absorption towers were tested as the pilot-scale absorption units. These towers were filled with 20 L of tap water (pH 6.5) and operated as the bench-scale absorption units: when their content was saturated, the towers were discharged and refilled with 20 L of tap water. In the experiments with the pilot-scale absorption towers, the air stripping tower was operated at a leachate flow rate of 90 L h−1 , whereas the air flow rate was 4500 L h−1 . The ratio of air/liquid (50) was similar to that of the previous batch runs.
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Figure 1. Schematic of the air stripping tower and the absorption units (adapted from Bonmatí and Flotats [17]): (1) air stripping tower; (2–3) absorption units; (4) dosing pump; (5) air compressor; (a–c) sampling points; (b,c) sampling and charge/discharge points.
The TAN removal and absorption (abs) efficiencies were calculated by Equations 1 and 2, respectively: TANinitial − TANfinal ∗ 100 (1) TANinitial (TANabs unit1 (g) + TANabs unit2 (g)) TANabsorption (%) = TANstripped (g) (2) TANremoval (%) =
The mass of TAN used in Equation (2) is obtained by multiplying the TAN concentration measured in the leachate or absorption unit samples by the volume of Table 2.
leachate that is being treated (12 or 100 L) or the volume of absorption units content (4 or 20 L). The operational conditions that were evaluated in this research are presented in Table 2. 2.3.
pH adjustment Powdered Ca(OH)2 (95% purity) and commercial hydrated lime (with non-volatile total oxides greater than 88%) were used to raise the pH of the leachate to 11. This pH value was selected because it was previously reported to be in the optimum range for ammonia stripping [13,14,17].
Operational conditions evaluated for the air stripping and ammonia absorption processes. Leachate
Qleachate
Batch run
(L)
pH
Batch 1 Batch 2 Batch 3 Batch 4 Batch 5 Batch 6 Batch 7
12 12 12 12 100 100 100
11∗ 11∗ 11∗∗ 11∗ 11∗∗ 11∗∗ 11∗∗
Qair
(L h−1 ) 30 30 30 30 90 90 90
4500 1600 1600 1600 4500 4500 4500
Initial TAN (mg L−1 )
Operational time (days)
Absorption media
830 1980 1953 1310 850 850 850
1 12 12 12 3 3 3
H2 SO4 (0.4-mol L−1 )a H2 SO4 (0.4-mol L−1 )a H2 SO4 (0.4-mol L−1 )a Deionized watera Tap waterb Tap waterb Tap waterb
Notes: Qair : air flow rate; Qleachate : liquid flow rate; (∗ ) pH adjusted with standard grade Ca(OH)2 ; (∗∗ ) pH adjusted with commercial hydrated lime; (a) batch runs with the bench scale absorption units; (b) batch runs with the pilot-scale absorption units.
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Analytical grade Ca(OH)2 and commercial hydrated lime were used to evaluate whether the metallic concentration in the generated sludge was in accordance with Brazilian environmental law. Therefore, the concentration of metals in the raw leachate, supernatant leachate and generated sludge were measured. Because the leachate was stirred during the addition of alkali, the removal of ammoniacal nitrogen by volatilization was also evaluated. From the raw and supernatant leachate samples, the removal of organic matter by the precipitation of calcium carbonate was also determined. 2.4. Physico-chemical analyses The following parameters were measured according to the Standard Methods for the Examination of Water and Wastewater [28]: the BOD was determined by the 5-day test method 5210 B (Hach BODTrakTM II respirometric BOD apparatus); the COD was determined by colorimetric method 5220 D (Hach COD reactor 45600-00/Hach DR 2010 spectrophotometer); the conductivity was determined by method 2510 B; the colour was determined by method 2120 C (Hach DR 2010 spectrophotometer operated at 455 nm); the heavy metals (total Cd, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) were determined by method 3111 B and D (Varian AA240 FS atomic absorption spectrophotometer); the pH was determined by method 4500 – H+ ; the total alkalinity was determined by method 2320 B; the total carbon and total organic carbon (TOC) was determined by method 5310 B (Shimadzu TOC 5000 A Analyser); the total Kjeldahl nitrogen (TKN) was determined by method 4500 C – Norg Nitrogen (Büchi digestion unit B-426); the TAN was determined by method 4500 C – NH3 Nitrogen (Büchi distillation unit B-339); the total solids were determined by method 2540 B and the fixed and volatile solids were determined by method 2540 D. 3. Results and discussion The results are presented in four main subsections, in accordance with the objectives of this research. All of the data were obtained from the operation of the stripping tower in batch runs. Each batch run ended when the concentration of the TAN in the treated leachate was equal to or below 20 mg L−1 , which is the discharge limit established by Brazilian environmental law [29]. First, we evaluated the effect of pH adjustment on leachate quality, and then the effect of the use of different air flow rates on the ammonia removal by air stripping. The ammonia recovery was initially evaluated with a 0.4 mol L−1 sulphuric acid solution. Next, the use of Ca(OH)2 and commercial hydrated lime to adjust the pH of the raw leachate was compared by evaluating the volume of sludge generated and its heavy metal concentration. Ammonia recovery using deionized water or tap water was subsequently examined as an alternative to the use of the acidic solution.
3.1.
The influence of pH adjustment on landfill leachate quality
In general, the consumption of analytical grade Ca(OH)2 in the batch runs varied from 10–20 g per litre of leachate. Because this alkali was used in excess, after 30 min of sedimentation, the generated sludge varied from 80–140 mL per litre of leachate. The consumption of commercial hydrated lime was 24 g per litre of leachate on average, generating approximately 180 mL of lime sludge per litre of leachate. The pH adjustment effectively removed the colour (82%), as the leachate changed from dark brown (true colour, 8050 PtCo) to yellow (true colour, 1440 PtCo). This result was also observed by Renou et al. [23], who associated the clarification of the leachate with the removal of humic substances. Because the leachate was kept under agitation while Ca(OH)2 was added, 13% of the initial ammonia concentration (1213 mg L−1 on average) was removed by volatilization, whereas 20% of the initial TOC (718 mg L−1 on average) was most likely removed by precipitation. Another advantage of using Ca(OH)2 or commercial lime to adjust the pH of the raw leachate was the high removal efficiency for metals: in Batches 2 and 3, their removal varied from 70–90%, particularly for iron, zinc and manganese (Table 3). This high heavy metal removal efficiency was also observed for the other batches (data not shown). It is important to note that CONAMA Resolution 430/2011 is only valid for wastewaters; thus, its limits cannot be applied to the sludge generated during the adjustment of pH with the tested alkalis. The appropriate legislation for sludge quality that focuses on its reuse in agriculture is CONAMA Resolution 380/2006 [30]. The concentration of heavy metals in the sludge was also much lower than the limits established by CONAMA Resolution 380/2006, even for pH adjustment using commercial hydrated lime (Table 3). 3.2.
The influence of air flow rate on ammonia removal and recovery To more accurately examine the influence of air injection on the performance of the air stripping tower and absorption units, only the air flow rate was varied in the following experiments. Using air flow rates of 4500 L h−1 (Batch 1) and 1600 L h−1 (Batch 2), the TAN concentration in the treated leachates of Batches 1 and 2 was 10 mg L−1 and 15 mg L−1 , respectively (Figure 2). In both cases, the ammonia removal behaviour followed first-order kinetics, as observed by Zhang et al. [18]. The treated leachates of both batches were in accordance with the discharge limit of 20 mg L−1 for TAN, which was established by the Brazilian National Council of the Environment (CONAMA) in Resolution 430/2011 [29]. In Batch 1, only 24 h were required to substantially remove the TAN from the leachate. By plotting the natural
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Table 3. The removal of heavy metals from the raw leachate resulting from the pH adjustment with 95% purity Ca(OH)2 (Batch 2) and commercial hydrated lime (CHL) (Batch 3). Batch 2
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pH adjustment with 95% purity Ca(OH)2 Metals
RL
Total Zn (mg L−1 ) Total Pb (mg L−1 ) Total Cd (mg L−1 ) Total Ni (mg L−1 ) Total Fe (mg L−1 ) Total Mn (mg L−1 ) Total Cu (mg L−1 ) Total Cr (mg L−1 ) Total Zn (mg kg−1 ) Total Pb (mg kg−1 ) Total Cd (mg kg−1 ) Total Ni (mg kg−1 ) Total Fe (mg kg−1 ) Total Mn (mg kg−1 ) Total Cu (mg kg−1 ) Total Cr (mg kg−1 )
0.50 0.20 0.02 0.33 3.81 0.12 0.06 0.31 -
pH 11 L GSa 0.07 0.13 0.01 0.24 0.40 0.02 0.02 0.07 -
17 66 7 18 240 22 8 18
Batch 3
Brazilian environmental law
pH adjustment with commercial hydrated lime
Discharge Standards for limits for sludge reuse in wastewaterc agricultured
Ca(OH)2
RL
pH 11 L
GSb
CHL
-
0.32 0.21 0.03 0.44 4.23 0.17 0.05 0.39 -
0.10 0.16 0.00 0.23 0.30 0.05 0.03 0.11 -
26 76 6 33 2177 314 10 32
-
5 64 6 14 142 20 8 11
4 71 3 12 1784 303 7 4
5 0.5 0.2 2 15 1 1 0.5 -
2800 300 39 420 NAe NAe 1500 1000
RL: raw leachate; L: leachate; GS: generated sludge; (a) sludge generated by pH adjustment with Ca(OH)2 ; (b) sludge generated by pH adjustment with commercial hydrated lime; (c) CONAMA 430/2011; (d) CONAMA 380/2006; (e) N.A: not available.
logarithm of the ratio TAN at a time ‘t’/initial TAN (TAN/TAN0 ) versus the operational time and fitting the curve to a line, a TAN removal rate of 0.18 h−1 was obtained (Figure 2(a)). The results of treating landfill leachate by air stripping in this work were compared with those reported by several authors and are summarized in Table 4. The TAN removal rate of Batch 1 was the greatest among the previously reported values [6–15]. Based on the cited literature data, the times required to reach the Brazilian environmental law discharge limit for TAN were estimated. In Batch 1, the air stripping tower could be operated for 20 h, saving energy costs, whereas in the other references with similar TAN removal rates, the discharge limit of 20 mg TAN L−1 would be reached after 35 h [8,14]. Despite the high TAN removal rate obtained in Batch 1, the air flow rate of 4500 L h−1 was not appropriate for the absorption units used. This air flow rate resulted in strong turbulence in the absorption flasks, and the air bubbles containing ammonia ascended too quickly. These conditions did not favour the transfer of ammonia from the gaseous to liquid phase (i.e. the 0.4 mol L−1 H2 SO4 solution) because sufficient contact was not provided between the sulphuric acid and ammonia molecules. In the second absorption flask, this strong turbulence resulted in the discharge of the sulphuric acid solution, and therefore, the ammonia recovery could not be evaluated. However, when an air flow rate of 1600 L h−1 was applied to the tower (Batch 2), the ammonia recovery efficiency was 80% (Figure 2(c)). Although Batch 2 offered better conditions for ammonia recovery in the bench-scale absorption units, decreasing the air flow rate to 1600 L h−1
resulted in a low TAN removal rate for achieving the Brazilian discharge limit: 0.0154 h−1 (Table 4) or 0.37 d−1 (Figure 2(b)). Nevertheless, this TAN removal rate was greater than the rate of 0.0131 h−1 obtained by Nurisepehr et al. [15], whose air stripping unit would have to be operated for 14 days to meet the Brazilian discharge limit for TAN. The data in Table 4 are only related to the treatment of landfill leachate by air stripping, so the work of De La Rubia et al. [21] was not included. However, these authors evaluated the treatment of source-segregated food waste by air stripping, focusing on the removal of its high TAN content. The waste pH was 8, and the experiments were carried out at 35◦ C. After 78 h of aeration at a rate of 45 L h−1 , the TAN decreased from approximately 7000 to 5200 mg L−1 . The TAN removal rate (0.0040 h−1 ) was almost four times lower than that of Batch 2 (0.0154 h−1 ), and if the Brazilian discharge limit for the TAN must be met under the operational conditions evaluated by these authors, the aeration time would be 61 days. By analysing the results of this research in conjunction with those of other published papers [11,19], it can be inferred that the air flow rate is an important factor that may influence the TAN removal rate. Guštin et al. [19] evaluated the treatment of anaerobic digestion effluent by air stripping using an air flow rate of 90,000 L h−1 , which is almost 60 times higher than that of Batch 2. The reported TAN removal rate was 1.3 h−1 , and at this rate it would take only 4 h to produce a treated effluent with 20 mg TAN L−1 . The long operational time of Batch 2 may not be considered practical in terms of energy costs, but it could be significantly decreased by implementing further
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absorption units. Therefore, as Batches 3 and 4 still used the bench-scale absorption units, the air flow rate of 1600 L h−1 was maintained. Because the bench-scale absorption units only had one opening to charge/discharge the absorbent solution, it was not possible to prevent the loss of their head space content to the atmosphere as they were opened. Despite the simplicity of the bench-scale absorption units, the ammonia recovery with the sulphuric acid solution in Batch 2 was close to the range reported by Bonmatí and Flotats [17], who obtained 84–94% recovery efficiency in the treatment of pig slurry by air stripping with pH 11.5 at 80◦ C. It is worth mentioning that besides ammonia, volatile organics may also be stripped. Bonmatí and Flotats [17] reported that less than 5% of the COD stripped from pig slurry could be recovered with sulphuric acid by absorption, and recommended other treatments for stripping organics. Traps of sulphuric acid and sodium hydroxide have been tested to recover ammonia and other volatile compounds from stripped gaseous effluent [21]. However, the objective of this current research was only ammonia removal and recovery and did not include other substances that could be stripped from leachate. 3.3.
Figure 2. The effect of different air flow rates on the ammonia removal in (a) Batch 1 and (b) Batch 2 and (c) on the ammonia recovery in Batch 2.
post-treatments for leachate. For biological co-treatment, it has been reported that leachate pre-treated by air stripping, containing 100 mg TAN L−1 , was added to domestic wastewater at 2% (volume/volume), and the resultant mixture contained approximately 30 mg TAN L−1 , which was almost the same concentration found in domestic water [31]. When evaluating the results from Batch 2, one should understand that the focus of this experiment was to use conditions that could enable the evaluation of ammonia removal in conjunction with ammonia recovery in the bench-scale
The effect of pH adjustment with 95% purity Ca(OH)2 or commercial hydrated lime on the air stripping efficiency This experiment was focused on reducing the operational costs of using an alkali. Commercial hydrated lime can be obtained at a low cost, particularly in the Brazilian market, where it is approximately 70% cheaper than standard grade Ca(OH)2 . Comparing Figure 2(b) and Figure 3(a), the ammonia removal was identical for Batch 2 (kL a = 0.37 d −1 ) and Batch 3 (kL a = 0.37 d −1 ). In both cases, after 12 days, the TAN concentration in the treated leachate was below 20 mg L−1 . Thus, the use of commercial hydrated lime did not negatively affect the air stripping performance. Figures 2(c) and 3(b) demonstrate that the use of a 0.4 mol L−1 sulphuric acid solution resulted in the effective recovery of ammonia: in Batches 2 and 3, 80% of the stripped ammonia was neutralized, generating an ammonium sulphate solution. As shown in Table 5, in Batches 2 and 3, ammonia absorption was rapidly observed in the first unit; however, as the acidic solution became saturated (the pH increased above 8), the second unit began to absorb the ammonia that was not recovered in the first unit. To maximize the effectiveness, the content of the absorption units was replaced whenever saturation was observed. For both batches, the acidic solution was replaced in the first unit after 2 days and in the second unit after 6 days. The objective of the final experiment was to evaluate ammonia absorption in deionized water with a focus on aquammonia production. In Batch 4, the air stripping tower
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Table 4.
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Comparison of the TAN removal rates (kL a) obtained in this research and those reported previously.
TL
T
Qar (L h−1 ) 4500 1600 4500 300 2 456 17 120 70,000 3600 120 900 1500
Reference
pH
(L)
(◦ C)
Batch 1 Batch 2 Batch 5 Cheung et al. [6] Marttinen et al. [7] Ozturk et al. [8] Silva et al. [9] Gotvanj et al. [10] Pi et al. [11] Souto et al. [12] Cotman and Gotvajn [13] Guo et al. [14] Nurisepehr et al. [15]
11 11 11 11 11 12 11 11 11 12 11 11 10
12 12 100 3.0 1.0 1.0 0.8 0.8 10 12 1.0 10 2
25 25 25 N.A 20 N.A N.A N.A 50 25 N.A N.A NA
TAN0
TANf
(mg L−1 ) 830 1493 850 705 150 1025 800 2300 1243 1900 480 1,350 1480
Figure 3. The effects of using commercial hydrated lime for pH adjustment in Batch 3: (a) ammonia removal and (b) ammonia recovery.
was operated in the same manner as for Batch 3 such that the same TAN removal rate was obtained (data not shown). As shown in Table 6, 75% of the stripped ammonia could be recovered as aquammonia, which was the same
10 20 100 200 89 800 5 460 198 0 50 150 1000
TANREM
kL a
Time
Estimated time to reach TAN 20 mg L−1
(%)
(h−1 )
(h)
(h)
99 99 88 72 41 22 99 80 89 100 90 89 32
0.1796 0.0154 0.0247 0.0525 0.0217 0.1239 0.0529 0.0322 0.0765 0.0684 0.0942 0.1221 0.0131
24 288 72 24 24 2 96 50 24 144 24 18 30
20 280 152 68 93 32 70 147 54 67 34 34 329
range of efficiency obtained in Batches 2 and 3, in which the 0.4 mol L−1 sulphuric acid solution was used. Considering that the standard molar Gibbs energy of formation (Gfo ) for ammonium sulphate, −903.1 kJ mol−1 , is more negative than the Gfo of aquammonia, −236.5 kJ mol−1 [32], one would expect that ammonia neutralization using the sulphuric acid solution would be more effective. This research obtained similar results for ammonia recovery using either the sulphuric acid solution or deionized water, mostly because of losses that occurred during the sampling and opening of the bench-scale absorption units when charging/discharging their contents. However, the ammonia recovery in Batches 2 and 3 was greater (80%) than that observed in Batch 4, confirming the aforementioned thermodynamic data. Regarding ammonia recovery, Batch 4 differed from the previous batches because ammonia neutralization occurred much faster in the deionized water than in the sulphuric acid solution, making it necessary to replace the content of the absorption units more frequently. This conclusion can be observed by analysing the pH behaviour over 33 h (Figure 4) and the frequency of deionized water replacements over the 12-day aeration period (Table 6). The concentration of H+ ions in the absorption units filled with the 0.4 mol L−1 sulphuric acid solution was significantly greater (pH 0.4) than the concentration in deionized water (pH 6). Because of this difference, the pH of the deionized water in the first absorption unit increased from 6 to approximately 11 in the first hour, and this value was roughly maintained for the subsequent 33 h. The saturation point could be associated with a pH of 11 in the absorption units because after this value was reached, no additional ammonia recovery was observed in the two absorption units. After only 3–4 h, the first absorption unit became saturated (the measured pH was 11), and the unneutralized ammonia was partially recovered in the second unit,
2324 Table 5.
F.M. Ferraz et al. pH behaviour during ammonia recovery using bench-scale absorption units in Batches 2 and 3. Batch 2
Batch 3
Replacement of the 0.4-mol L−1 Replacement of the Time pH range 0.4-mol L−1 H2 SO4 solution Time pH range H2 SO4 solution intervals intervals (days) Abs unit 1 Abs unit 2 Abs unit 1 Abs unit 2 (days) Abs unit 1 Abs unit 2 Abs unit 1 Abs unit 2 0–2 2–6 6–12
0.4–10.0 0.4–2.2 2.2–9.0
0.4–1.5 1.5–7.1 0.4–1.3
– Yes No
– No Yes
0–2 2–6 6–12
0.4–8.8 0.4–5.4 5.4–8.2
0.4–3.7 3.7– 7.8 0.4–1.6
– Yes No
– No Yes
Note: Abs: absorption.
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Table 6.
The pH behaviour during ammonia recovery in bench-scale absorption units with deionized water (Batch 4). pH range
Time intervals (days) 0–1.4 2–5 5–10 10– 12
Replacement of the deionized water
Ammonia absorption (%)
Abs unit 1
Abs unit 2
Abs unit 1
Abs unit 2
Abs unit 1
Abs unit 2
Total Abs
6.0–11.0 6.0–9.7 6.0–9.3 6.0–9.0
6.0–11.0 6.0–9.7 6.0–9.0 6.0–9.0
– Yes Yes Yes
– Yes Yes Yes
28 42 47 50
16 20 25 25
44 62 70 75
Note: Abs: absorption.
Figure 4. pH variations in the first 33 h of ammonia absorption in Batch 4.
in which the content became saturated after 9 h (when the pH increased from 6 to 11). To maintain the absorption effectiveness, the content of each unit was replaced with 4 L of deionized water. 3.4.
Ammonia removal by air stripping to pre-treat 100 L of leachate and ammonia recovery with tap water using two 31 L pilot-scale absorption units To verify whether the high ammonia removal and recovery efficiencies obtained in Batches 1–4 would also be observed in the treatment of a major volume of leachate, 100 L was
tested in triplicate batch runs, and the operating conditions are presented in Table 2 as Batch 5. Although these runs used higher air (4500 L h−1 ) and liquid (90 L h−1 ) flow rates than in the previous batches, the air/liquid ratio was maintained in the range of 50. The data presented in Table 4 represent the average values of the triplicate batches. After 72 h of aeration, the air stripping tower could remove, on average, 88% of the TAN concentration initially present in the pH 11 leachate. The duration of Batch 5 was four times shorter than that of Batches 2–4, and it is worth mentioning that the volume of leachate to be treated was considerably greater than the 12 L initially studied or the volumes evaluated by the authors cited in Table 4. In contrast to the previous batch runs, the final TAN was 100 mg L−1 because the leachate was intended for biological treatment processes [31]. As discussed, ammonia recovery could not be evaluated in Batch 1 because of the strong turbulence in the bench-scale absorption units. Nonetheless, the ammonia was recovered in Batch 5 by the use of the two 31 L pilot-scale absorption towers. To focus on the better neutralization of the stripped ammonia, the two pilot-scale absorption units were discharged and refilled with 20 L of tap water in 24 h time intervals. This procedure resulted in an average ammonia recovery effectiveness of 87%. The minimization of ammonia losses during sampling also contributed to this efficiency, which was possible because of the better structure of the pilot-scale absorption towers compared with the simple bench-scale absorption units.
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4. Conclusions From the results, the following conclusions can be drawn: • pH adjustment: This procedure improved the leachate quality by the removal of colour (82%), metals (70–90% for Zn, Fe and Mn), TOC (19%) and ammonia (13%). The ammonia removal was not affected by the quality of the tested alkalis. The results also show that the concentration of heavy metals in the sludge generated by the two tested alkalis was in accordance with Brazilian environmental law. Thus, the use of commercial hydrated lime is an interesting alternative to lower the operational costs of air stripping. The possibility of reusing the generated sludge to adjust the pH of the raw leachate should be tested in future studies; • Ammonia removal by air stripping: the high performance (99%) of the aerated packed tower in the treatment of 12 L of leachate was also observed in the treatment of a higher volume of leachate. After only 72 h of aeration, the ammonia removal from 100 L of leachate was 88%; • Ammonia recovery by absorption: approximately 80% of the stripped ammonia could be recovered in the bench-scale absorption units filled with a 0.4 mol L−1 H2 SO4 solution or deionized water. The feasibility of this process was confirmed in the experiments using the two 31 L pilot-scale absorption towers. On average, 87% of the TAN stripped from 100 L of the leachate could be neutralized in the two pilot-scale absorption units filled with tap water. Acknowledgements The authors would like to thank CAPES (Coordenação de Aperfeiçoamento Pessoal de Ensino Superior), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo, processos no 2010/51955-2; no 2011/50627-4) and the University of Sao Paulo for their financial support.
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