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Sustainable landfill leachate treatment using refuse and pine bark as a carbon source for bio-denitrification ab
a
ab
R.R. Frank , C. Trois & F. Coulon a
Centre for Research in Environmental, Coastal and Hydrological Engineering (CRECHE), School of Engineering, University of KwaZulu-Natal, Howard College Campus, Durban 4041, South Africa b
School of Energy, Environment and Agrifood, Cranfield University, Bedfordshire MK43 0AL, UK Accepted author version posted online: 09 Dec 2014.Published online: 17 Dec 2014.
Click for updates To cite this article: R.R. Frank, C. Trois & F. Coulon (2015) Sustainable landfill leachate treatment using refuse and pine bark as a carbon source for bio-denitrification, Environmental Technology, 36:11, 1347-1358, DOI: 10.1080/09593330.2014.989279 To link to this article: http://dx.doi.org/10.1080/09593330.2014.989279
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2015 Vol. 36, No. 11, 1347–1358, http://dx.doi.org/10.1080/09593330.2014.989279
Sustainable landfill leachate treatment using refuse and pine bark as a carbon source for bio-denitrification R.R. Franka,b , C. Troisa∗ and F. Coulona,b a Centre
for Research in Environmental, Coastal and Hydrological Engineering (CRECHE), School of Engineering, University of KwaZulu-Natal, Howard College Campus, Durban 4041, South Africa; b School of Energy, Environment and Agrifood, Cranfield University, Bedfordshire MK43 0AL, UK
Downloaded by [The Aga Khan University] at 01:29 10 March 2015
(Received 20 March 2014; accepted 13 November 2014 ) Raw and 10-week composted commercial garden refuse (CGR) materials and pine bark (PB) mulch were evaluated for their potential use as alternative and sustainable sources of carbon for landfill leachate bio-denitrification. Dynamic batch tests using synthetic nitrate solutions of 100, 500 and 2000 mg NO3 L−1 were used to investigate the substrate performance at increasing nitrate concentrations under optimal conditions. Further to this, sequential batch tests using genuine nitrified landfill leachate with a concentration of 2000 mg NO3 L−1 were carried out to evaluate substrates behaviour in the presence of a complex mixture of chemicals present in leachate. Results showed that complete denitrification occurred in all conditions, indicating that raw and composted CGR and PB can be used as sustainable and efficient media for landfill leachate bio-denitrification. Of the three substrates, raw garden refuse yields the fastest denitrification rate followed by 10-week composted CGR and PB. However, the efficiency of the raw CGR was lower when using genuine leachate, indicating the inhibitory effect of components of the leachate on the denitrification process. Ten-week composted CGR performed optimally at low nitrate concentrations, while poor nitrate removal ability was found at higher nitrate concentrations (2000 mg L−1 ). In contrast, the PB performance was 3.5 times faster than that of the composted garden refuse at higher nitrate concentrations. Further to this, multi-criteria analysis of the process variables provided an easily implementable framework for the use of waste materials as an alternative and sustainable source of carbon for denitrification. Keywords: denitrification; leachate treatment; carbon source; pine bark; commercial garden refuse
1. Introduction South Africa produces 108 million tonnes of waste per annum, of which 98 million tonnes are sent to landfill sites.[1] This significant amount of waste contains a large proportion of bioreactive wastes, which produce mainly gas and wastewater known as leachate.[2,3] Leachate treatment and disposal is one of the biggest issues during solid waste management practices. Leachate has very high strength regarding pH, chemical oxygen demand (COD), biochemical oxygen demand (BOD), ammonia, chloride, colour, odour and heavy metals. If it is not collected carefully and not discharged safely, leachate has the ability to cause major environmental impacts as well as to affect human health due to its high toxicity. The major concern associated with leachate is its ammonia content, which can reach levels of up to 1000 mg L−1 .[3–6] Leachate can contaminate ground and surface water resources, which can consequently affect potable water supply. Furthermore, it can affect biological systems and ecological communities of many fauna and flora exposed to contaminated water.[4,7–9] There are typically few wastewater treatment
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
facilities in developing countries due to the high cost of treatment and lack of environmental pollution control laws and enforcement.[8] The ammonia in leachate can be treated by biological nitrification.[10–12] Such a process approach has been adopted at the Mariannhill landfill site (LFS) (Durban, South Africa), which receives between 550 and 700 tonnes of municipal solid waste per day [12] and produces and nitrifies approximately 30 m3 of leachate per day. The leachate treatment plant operates by aerobically converting ammonia to nitrites and then to nitrates. Nitrates can still have significant environmental and human health implications. Nitrates can lead to adverse eutrophication in aquatic environments.[13] Nitrate levels > 45 mg L−1 can also affect human health [14] through ingestion of nitrate-containing water or vegetables, causing among others abdominal pains, diarrhoea, vomiting, diabetes, birth defects, infant mortality, hypertension and respiratory tract infections.[15] Therefore, a denitrification step is required to reduce nitrate levels to acceptable discharge limits. In South Africa, the nitrate discharge limit set by the Department of Water Affairs and Forestry is
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15 mg NO3 L−1 .[16] The nitrate concentration of the effluent from the nitrification sequencing batch reactor (SBR) installed at the Mariannhill LFS ranged between 285 and 1425 mg L−1 in 2011/2012. The SBR effluent is currently recirculated into the landfill through use as a dust suppressant. Closure of the landfill site is expected in 2022, at which point recirculation will no longer be a viable treatment option.[17] The use of biological denitrification, in the form of a biological anaerobic filter bed operated in a flowing system, will be adopted at the landfill site as this is believed to be one of the most promising methods of nitrate removal.[3] Biological denitrification is the process by which oxidized nitrogenous compounds such as nitrates or nitrites are reduced to nitrogen gas under anoxic conditions through the assistance of a diverse group of bacteria.[3,18– 20] The biological denitrification process typically follows a nitrification step whereby ammonia and much of the organics are removed.[21] There is thus a deficiency of carbon essential for denitrification. As a result, an external carbon source is required as an electron donor in order for microorganisms to survive.[19,21] Typically methanol, glucose, ethanol, propionic and acetic acid are commonly used as they are easily biodegradable.[22–24] These carbon sources, however, are expensive, which consequently restricts their viability in full-scale application.[24,25] The use of waste materials as a carbon source in denitrification has been researched for over 20 years.[3,23,26– 30] It has the dual benefit of removing wastes from the waste stream, diverting it from landfill sites and use as an alternative carbon source. It is therefore an economically and environmentally sustainable carbon source alternative. Alternative carbon sources from waste materials that have been found to successfully denitrify leachate include tree barks, sawdust, corncobs, wood chips, newspapers, yeast, whey and compost.[23,28] Many of these alternative carbon sources have shown denitrification rates and chemical oxygen demand:nitrogen ratios (COD:N) comparable to traditional chemicals such as methanol and acetic acid.[31] There is, however, still a need to continue identifying feasible alternative carbon sources in terms of cost, availability and denitrification efficiency in order to continue the development of sustainable nitrate removal solutions.[31] Trois et al. [3,24] previously investigated the use of composted garden waste and pine bark (PB) as an alternative and low-cost carbon source for supporting bio-denitrification as they are found in high quantities in South African landfills. They demonstrated that complete nitrate removal was achievable and provided insights into the key microbes involved in the bio-denitrification process. However, little information on the chemical characterization during the denitrification process was provided. Therefore, the main objectives of this study were to (1) investigate the feasibility of using raw commercial garden refuse (raw CGR), 10-week composted commercial garden refuse (CGR 10)
and PB as sustainable alternative carbon sources for the denitrification of treated landfill leachate at an initial nitrate concentration of up to 2000 mg NO3 L−1 ; (2) characterize the substrates’ performance against the nitrate concentrations load using synthetic and genuine leachate and (3) provide a decision support tool to inform future treatment strategies.
2.
Materials and methods
2.1. Substrate selection Substrates tested were raw CGR, CGR composted for 10 weeks and PB. The CGR substrate was sourced from the waste stream of the Bisasar Road Landfill site in Durban, South Africa. The CGR substrate, which was made up of mainly thin twigs and leaves, went through an onsite chipper which reduced the chip size to smaller than 5 cm in length. It was subsequently stored in an onsite pile and collection of sample happened within days of the chipping process. To obtain the CGR 10, composting of the raw CGR was conducted on site for 10 weeks through a turned windrow technology. The PB was obtained from the MONDI paper company in South Africa. They were prepared as wood chips with a length of approximately 3–5 cm.
2.2.
Substrate and leachate characterization
Substrate and leachate characterization methodology was conducted according to standard analytical methods as published by the American Public Health Association.[32] Characterization was conducted on both the solid and eluate fractions of the substrate. The eluate was attained by immersing the substrate in distilled water for 24 h at a liquid:solid (L/S) ratio of 10:1 by weight. This enabled optimal liquid to solid contact. Characterization tests which were conducted on the solid substrates included moisture content (w), total solids (TS), volatile solids (VS), respiration index (RI7 ), total carbon (TC), total nitrogen (TN) and carbon to nitrogen ratio (C/N). Characterization tests which were conducted on the eluate samples and leachate included TS, VS, pH, soluble chemical oxygen demand (sCOD), BOD5 , TC, TN, C/N, ammoniacal nitrogen (NH3 -N) and total oxidized nitrogen (NOx -N). TC and TN were determined through total com® bustion using a Leco Truspec CN analyser. RI7 was tested by adding 5 drops of allylthiourea (ATH) to 25 g of substrate in a 1.5 L glass bottle. Five drops of 45% potassium hydroxide (KOH) was placed into a rubber cylinder situated below a pressure sensor lid. Samples were incubation at 20°C for 7 days and measurements were recorded using ® an OxiTop respirometric apparatus. NH3 -N and NOx -N was measured using UV–VIS spectroscopy according to − standard methods (4500−NO− 2 and 4500−NO3 ).
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Environmental Technology NOx-N, NH3-N and pH values for raw CGR, CGR 10 and PB during 100 mg NO 3 L-1 batch tests 8 40 RAW NH3
35
7
30
6
25
5
20
4
15
3
PB NOX
10
2
RAW pH
5
1
10 pH
0
0
PB pH
0
2
4
5
8 9 10.5 Time (Hours)
24
36
10 NH3 pH value
mg/L
(a)
PB NH3 RAW NOX 10 NOX
47
NOx-N, NH3-N and pH values for raw CGR, CGR 10 and PB during 500 mg NO 3 L-1 batch tests 120 9 100
mg/L
80
8
RAW NH3
7
10 NH3
6
PB NH3
5
60
4 3
40 20 0 0
0.04
0.17
0.33
1 2 3 Time (days)
3.5
3.9
4
pH value
Downloaded by [The Aga Khan University] at 01:29 10 March 2015
(b)
RAW NOX 10 NOX PB NOX
2
RAW pH
1
10 pH
0
PB pH
5.1
NOx-N, NH3 -N and pH values for raw CGR, CGR 10 and PB during 2000 mg NO 3 L-1 batch tests 450 10 9 400 8 350 7 300 6 250 5 200 4 150 3 100 2 50 1 0 0 0 0.58 0.92 1.08 1.38 2 4 6 7 7.2 12 18 25 Time (days) pH value
mg/L
(c)
RAW NH3 10 NH3 PB NH3 RAW NOX 10 NOX PB NOX RAW pH 10 pH PB pH
Figure 1. NH3 -N, NOx -N and pH at intermittent points throughout the 100 (a), 500 (b) and 2000 (c) mg NO3 L−1 batch tests.
2.3.
Batch tests set-up and analysis
Denitrification of both synthetic leachate containing 100, 500 and 2000 mg NO3 L−1 respectively and treated (nitrified) leachate sourced from the Mariannhill LFS containing 2000 mg NO3 L−1 was carried out in triplicate batch reactors. Batch tests were conducted using raw CGR, CGR 10 and PB as a carbon source. All batch tests were conducted in 1 L anaerobic bottles equipped with two airtight silicone septa that allow for continuous sampling while avoiding air ingress. Each substrate
(S) was mixed with the leachate solution (L) at L/S = 10/1, by weight, to ensure full saturation and optimal liquid– solid contact in the batch reactors.[3] A control test replacing nitrate solution/leachate with distilled water was also carried out for each batch test. Optimal environmental conditions and full liquid to solid transfer were obtained by performing the experiments at a controlled temperature of 25°C and shaking speed of 150 rpm. The batch systems were flushed with N2 to set anoxic conditions. Nitrate concentration was measured at regular intervals daily using
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nitrate test sticks (type Merckoquant). pH, NH3 -N, NOx N and C/N ratio were measured in triplicate at regular time intervals. The intervals between sampling were dependent on the substrate and initial nitrate load of the batch test, which are presented in Figure 1. pH, NH3 -N and NOx -N were tested on the eluate fraction of the batch test while C/N ratio was tested on the solid fraction. sCOD was measured at the start and end of the 2000 mg NO3 L−1 batch tests. Variability in results was less than 5%. Batch tests incorporating the use of treated leachate were conducted subsequent to the synthetic leachate batch tests, and were designed to assess the effect of genuine leachate on the denitrification process. The genuine leachate batch tests were further used to determine the substrate longevity in terms of denitrification efficiency. This was done by replacing denitrified leachate, after denitrification was complete, with fresh pre-treated (nitrified) leachate at a concentration of 2000 mg NO3 L−1 , while keeping the same substrate. Pre-treated leachate was obtained from the Mariannhill LFS, South Africa. Leachate was collected after nitrification was conducted in an on-site SBR.
3. Results and discussion 3.1. Substrate and leachate characterization Substrate characterization shows that the RI7 value of raw CGR, an indication of the extent to which readily biodegradable organic matter has been decomposed,[33] was the highest (Table 1). This indicated that raw CGR was the least decomposed and consequently the most readily biodegradable. The C/N, sCOD and BOD5 values of raw CGR were high, displaying a high level of organic strength. Due to these chemical characteristics, raw CGR was identified as the substrate that would best promote the action of denitrifying bacteria by providing the highest amount of biodegradable carbon in the shortest time while depositing little nitrogenous compounds back into the system (Table 1). PB showed a high C/N and a high RI7 as it is a relatively fresh material having not undergone any stabilization. The C/N in this study was approximately three times higher than that found in Trois et al. [3] This finding suggests that the heterogeneous character of the PB composition influences directly the amount of carbon available to sustain the denitrification process. Furthermore, the eluate characterization showed that TS and sCOD were low, suggesting that carbon from PB was not immediately released into the system, and therefore had a poor leaching ability. The poor leaching ability of PB, particularly in the leaching of organic compounds, was also reported by Ribe et al.[34] The use of CGR 10 was evaluated, as it is a theoretically more biologically stable substrate, possessing a
lower organic strength than raw CGR, due to the composting process. This would ideally result in low COD effluent, requiring less COD management of the effluent. Results confirmed that CGR 10 possessed a lower sCOD than raw CGR and the lowest RI7 of the three substrates. The substrate showed good potential as a carbon source as it still possessed a sufficient organic load to promote biodenitrification. The optimum C/N for stabilized compost to promote denitrification ranges between 13 and 16.[23] The C/N of the CGR 10 was 21, which was 2.4 and 9 times lower than those of the raw CGR and PB, respectively, similar to the findings of Trois et al.[3] CGR 10 showed good potential to desorb its available carbon adequately as indicated by the significant TS and sCOD in the eluate fraction. The pH for all substrates was below the optimum pH, which ranges normally between 6 and 8. The pH was expected to increase to optimal levels at the onset of denitrification as carbonate alkalinity increases during nitrate reduction.[23] 3.2. Simulated leachate batch tests An initial lag phase was observed in most batch tests, where no denitrification took place (Table 2). Microbial populations require time to acclimate to environmental conditions, and reach sufficient densities to initiate the denitrification process. The occurrence of microbial acclimatization is a welldocumented process.[35,36] CGR 10 showed the shortest lag phase of the substrates at 1, 3 and 2 h for the 100, 500 and 2000 mg NO3 L−1 batch tests, respectively (Table 2). It is possible that composting promoted the rapid establishment of a specialized microbial consortium.[37] Lag phase for raw CGR was 5, 2 and 22 h for the 100, 500 and 2000 mg NO3 L−1 batch tests, respectively, and the lag phase for PB was between 23 and 24 h in all batch tests. A possible reason for the short lag phases observed is due to the low pH variation during acclimatization (Figure 1). Findings from Trois et al. [3] confirmed a short lag phase when using CGR substrates; however they found a longer lag phase when using PB (20–80 h), particularly at higher nitrate concentrations. They accounted this to pH buffering and microbial competition. Since results from this study showed less pH variation during the lag phase when compared to Trois et al.,[3] it is likely that this contributed to the shorter lag phases observed. Complete denitrification was achieved in all batch tests using synthetic leachate solution, indicating that raw CGR, CGR 10 and PB can all adequately be used as alternative carbon sources to facilitate denitrification at a nitrate concentration of up to 2000 mg L−1 under optimal conditions. Raw CGR was the most efficient substrate in facilitating biological denitrification in batch tests at all nitrate concentrations. Raw CGR and CGR 10 showed similar rates of denitrification at lower nitrate concentrations
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Environmental Technology Table 1. Characterization tests conducted on solid and eluate phases of raw CGR, CGR 10 and PB and on nitrified leachate obtained from the Mariannhill LFS in Durban, South Africa. Raw CGR
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Characterization on solid material Moisture content (w) (%) 32 ± 1 TS (%) 68 ± 1 VS (%) 93 ± 1 RI7 (mg O2 /g DM) 23 TC (%) 47 TN (%) 0.9 C/N 51 Characterization on eluate TS (%) VS (%) pH COD (mg L−1 ) BOD5 (mg L−1 ) TC (%) TN (%) C/N NH3 -N (mg L−1 ) NOx -N (mg L−1 )
17 ± 0.05 14 ± 0.03 4.25 21899 ± 478 2678 0.40 0.07 5.71 3.1 1.1
CGR 10
PB
Genuine leachate
54 ± 0.2 46 ± 0.2 77 ± 3 13 36 1.8 21
25 ± 2 75 ± 2 98 ± 0.5 17 43 0.2 189
– – – – – – –
16 ± 0.02 12 ± 0.02 5.08 6460 ± 40 1651 0.59 0.05 11.80 3.3
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Sustainable landfill leachate treatment using refuse and pine bark as a carbon source for bio-denitrification ab
a
ab
R.R. Frank , C. Trois & F. Coulon a
Centre for Research in Environmental, Coastal and Hydrological Engineering (CRECHE), School of Engineering, University of KwaZulu-Natal, Howard College Campus, Durban 4041, South Africa b
School of Energy, Environment and Agrifood, Cranfield University, Bedfordshire MK43 0AL, UK Accepted author version posted online: 09 Dec 2014.Published online: 17 Dec 2014.
Click for updates To cite this article: R.R. Frank, C. Trois & F. Coulon (2015) Sustainable landfill leachate treatment using refuse and pine bark as a carbon source for bio-denitrification, Environmental Technology, 36:11, 1347-1358, DOI: 10.1080/09593330.2014.989279 To link to this article: http://dx.doi.org/10.1080/09593330.2014.989279
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2015 Vol. 36, No. 11, 1347–1358, http://dx.doi.org/10.1080/09593330.2014.989279
Sustainable landfill leachate treatment using refuse and pine bark as a carbon source for bio-denitrification R.R. Franka,b , C. Troisa∗ and F. Coulona,b a Centre
for Research in Environmental, Coastal and Hydrological Engineering (CRECHE), School of Engineering, University of KwaZulu-Natal, Howard College Campus, Durban 4041, South Africa; b School of Energy, Environment and Agrifood, Cranfield University, Bedfordshire MK43 0AL, UK
Downloaded by [The Aga Khan University] at 01:29 10 March 2015
(Received 20 March 2014; accepted 13 November 2014 ) Raw and 10-week composted commercial garden refuse (CGR) materials and pine bark (PB) mulch were evaluated for their potential use as alternative and sustainable sources of carbon for landfill leachate bio-denitrification. Dynamic batch tests using synthetic nitrate solutions of 100, 500 and 2000 mg NO3 L−1 were used to investigate the substrate performance at increasing nitrate concentrations under optimal conditions. Further to this, sequential batch tests using genuine nitrified landfill leachate with a concentration of 2000 mg NO3 L−1 were carried out to evaluate substrates behaviour in the presence of a complex mixture of chemicals present in leachate. Results showed that complete denitrification occurred in all conditions, indicating that raw and composted CGR and PB can be used as sustainable and efficient media for landfill leachate bio-denitrification. Of the three substrates, raw garden refuse yields the fastest denitrification rate followed by 10-week composted CGR and PB. However, the efficiency of the raw CGR was lower when using genuine leachate, indicating the inhibitory effect of components of the leachate on the denitrification process. Ten-week composted CGR performed optimally at low nitrate concentrations, while poor nitrate removal ability was found at higher nitrate concentrations (2000 mg L−1 ). In contrast, the PB performance was 3.5 times faster than that of the composted garden refuse at higher nitrate concentrations. Further to this, multi-criteria analysis of the process variables provided an easily implementable framework for the use of waste materials as an alternative and sustainable source of carbon for denitrification. Keywords: denitrification; leachate treatment; carbon source; pine bark; commercial garden refuse
1. Introduction South Africa produces 108 million tonnes of waste per annum, of which 98 million tonnes are sent to landfill sites.[1] This significant amount of waste contains a large proportion of bioreactive wastes, which produce mainly gas and wastewater known as leachate.[2,3] Leachate treatment and disposal is one of the biggest issues during solid waste management practices. Leachate has very high strength regarding pH, chemical oxygen demand (COD), biochemical oxygen demand (BOD), ammonia, chloride, colour, odour and heavy metals. If it is not collected carefully and not discharged safely, leachate has the ability to cause major environmental impacts as well as to affect human health due to its high toxicity. The major concern associated with leachate is its ammonia content, which can reach levels of up to 1000 mg L−1 .[3–6] Leachate can contaminate ground and surface water resources, which can consequently affect potable water supply. Furthermore, it can affect biological systems and ecological communities of many fauna and flora exposed to contaminated water.[4,7–9] There are typically few wastewater treatment
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
facilities in developing countries due to the high cost of treatment and lack of environmental pollution control laws and enforcement.[8] The ammonia in leachate can be treated by biological nitrification.[10–12] Such a process approach has been adopted at the Mariannhill landfill site (LFS) (Durban, South Africa), which receives between 550 and 700 tonnes of municipal solid waste per day [12] and produces and nitrifies approximately 30 m3 of leachate per day. The leachate treatment plant operates by aerobically converting ammonia to nitrites and then to nitrates. Nitrates can still have significant environmental and human health implications. Nitrates can lead to adverse eutrophication in aquatic environments.[13] Nitrate levels > 45 mg L−1 can also affect human health [14] through ingestion of nitrate-containing water or vegetables, causing among others abdominal pains, diarrhoea, vomiting, diabetes, birth defects, infant mortality, hypertension and respiratory tract infections.[15] Therefore, a denitrification step is required to reduce nitrate levels to acceptable discharge limits. In South Africa, the nitrate discharge limit set by the Department of Water Affairs and Forestry is
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R.R. Frank et al.
15 mg NO3 L−1 .[16] The nitrate concentration of the effluent from the nitrification sequencing batch reactor (SBR) installed at the Mariannhill LFS ranged between 285 and 1425 mg L−1 in 2011/2012. The SBR effluent is currently recirculated into the landfill through use as a dust suppressant. Closure of the landfill site is expected in 2022, at which point recirculation will no longer be a viable treatment option.[17] The use of biological denitrification, in the form of a biological anaerobic filter bed operated in a flowing system, will be adopted at the landfill site as this is believed to be one of the most promising methods of nitrate removal.[3] Biological denitrification is the process by which oxidized nitrogenous compounds such as nitrates or nitrites are reduced to nitrogen gas under anoxic conditions through the assistance of a diverse group of bacteria.[3,18– 20] The biological denitrification process typically follows a nitrification step whereby ammonia and much of the organics are removed.[21] There is thus a deficiency of carbon essential for denitrification. As a result, an external carbon source is required as an electron donor in order for microorganisms to survive.[19,21] Typically methanol, glucose, ethanol, propionic and acetic acid are commonly used as they are easily biodegradable.[22–24] These carbon sources, however, are expensive, which consequently restricts their viability in full-scale application.[24,25] The use of waste materials as a carbon source in denitrification has been researched for over 20 years.[3,23,26– 30] It has the dual benefit of removing wastes from the waste stream, diverting it from landfill sites and use as an alternative carbon source. It is therefore an economically and environmentally sustainable carbon source alternative. Alternative carbon sources from waste materials that have been found to successfully denitrify leachate include tree barks, sawdust, corncobs, wood chips, newspapers, yeast, whey and compost.[23,28] Many of these alternative carbon sources have shown denitrification rates and chemical oxygen demand:nitrogen ratios (COD:N) comparable to traditional chemicals such as methanol and acetic acid.[31] There is, however, still a need to continue identifying feasible alternative carbon sources in terms of cost, availability and denitrification efficiency in order to continue the development of sustainable nitrate removal solutions.[31] Trois et al. [3,24] previously investigated the use of composted garden waste and pine bark (PB) as an alternative and low-cost carbon source for supporting bio-denitrification as they are found in high quantities in South African landfills. They demonstrated that complete nitrate removal was achievable and provided insights into the key microbes involved in the bio-denitrification process. However, little information on the chemical characterization during the denitrification process was provided. Therefore, the main objectives of this study were to (1) investigate the feasibility of using raw commercial garden refuse (raw CGR), 10-week composted commercial garden refuse (CGR 10)
and PB as sustainable alternative carbon sources for the denitrification of treated landfill leachate at an initial nitrate concentration of up to 2000 mg NO3 L−1 ; (2) characterize the substrates’ performance against the nitrate concentrations load using synthetic and genuine leachate and (3) provide a decision support tool to inform future treatment strategies.
2.
Materials and methods
2.1. Substrate selection Substrates tested were raw CGR, CGR composted for 10 weeks and PB. The CGR substrate was sourced from the waste stream of the Bisasar Road Landfill site in Durban, South Africa. The CGR substrate, which was made up of mainly thin twigs and leaves, went through an onsite chipper which reduced the chip size to smaller than 5 cm in length. It was subsequently stored in an onsite pile and collection of sample happened within days of the chipping process. To obtain the CGR 10, composting of the raw CGR was conducted on site for 10 weeks through a turned windrow technology. The PB was obtained from the MONDI paper company in South Africa. They were prepared as wood chips with a length of approximately 3–5 cm.
2.2.
Substrate and leachate characterization
Substrate and leachate characterization methodology was conducted according to standard analytical methods as published by the American Public Health Association.[32] Characterization was conducted on both the solid and eluate fractions of the substrate. The eluate was attained by immersing the substrate in distilled water for 24 h at a liquid:solid (L/S) ratio of 10:1 by weight. This enabled optimal liquid to solid contact. Characterization tests which were conducted on the solid substrates included moisture content (w), total solids (TS), volatile solids (VS), respiration index (RI7 ), total carbon (TC), total nitrogen (TN) and carbon to nitrogen ratio (C/N). Characterization tests which were conducted on the eluate samples and leachate included TS, VS, pH, soluble chemical oxygen demand (sCOD), BOD5 , TC, TN, C/N, ammoniacal nitrogen (NH3 -N) and total oxidized nitrogen (NOx -N). TC and TN were determined through total com® bustion using a Leco Truspec CN analyser. RI7 was tested by adding 5 drops of allylthiourea (ATH) to 25 g of substrate in a 1.5 L glass bottle. Five drops of 45% potassium hydroxide (KOH) was placed into a rubber cylinder situated below a pressure sensor lid. Samples were incubation at 20°C for 7 days and measurements were recorded using ® an OxiTop respirometric apparatus. NH3 -N and NOx -N was measured using UV–VIS spectroscopy according to − standard methods (4500−NO− 2 and 4500−NO3 ).
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Environmental Technology NOx-N, NH3-N and pH values for raw CGR, CGR 10 and PB during 100 mg NO 3 L-1 batch tests 8 40 RAW NH3
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NOx-N, NH3-N and pH values for raw CGR, CGR 10 and PB during 500 mg NO 3 L-1 batch tests 120 9 100
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NOx-N, NH3 -N and pH values for raw CGR, CGR 10 and PB during 2000 mg NO 3 L-1 batch tests 450 10 9 400 8 350 7 300 6 250 5 200 4 150 3 100 2 50 1 0 0 0 0.58 0.92 1.08 1.38 2 4 6 7 7.2 12 18 25 Time (days) pH value
mg/L
(c)
RAW NH3 10 NH3 PB NH3 RAW NOX 10 NOX PB NOX RAW pH 10 pH PB pH
Figure 1. NH3 -N, NOx -N and pH at intermittent points throughout the 100 (a), 500 (b) and 2000 (c) mg NO3 L−1 batch tests.
2.3.
Batch tests set-up and analysis
Denitrification of both synthetic leachate containing 100, 500 and 2000 mg NO3 L−1 respectively and treated (nitrified) leachate sourced from the Mariannhill LFS containing 2000 mg NO3 L−1 was carried out in triplicate batch reactors. Batch tests were conducted using raw CGR, CGR 10 and PB as a carbon source. All batch tests were conducted in 1 L anaerobic bottles equipped with two airtight silicone septa that allow for continuous sampling while avoiding air ingress. Each substrate
(S) was mixed with the leachate solution (L) at L/S = 10/1, by weight, to ensure full saturation and optimal liquid– solid contact in the batch reactors.[3] A control test replacing nitrate solution/leachate with distilled water was also carried out for each batch test. Optimal environmental conditions and full liquid to solid transfer were obtained by performing the experiments at a controlled temperature of 25°C and shaking speed of 150 rpm. The batch systems were flushed with N2 to set anoxic conditions. Nitrate concentration was measured at regular intervals daily using
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R.R. Frank et al.
nitrate test sticks (type Merckoquant). pH, NH3 -N, NOx N and C/N ratio were measured in triplicate at regular time intervals. The intervals between sampling were dependent on the substrate and initial nitrate load of the batch test, which are presented in Figure 1. pH, NH3 -N and NOx -N were tested on the eluate fraction of the batch test while C/N ratio was tested on the solid fraction. sCOD was measured at the start and end of the 2000 mg NO3 L−1 batch tests. Variability in results was less than 5%. Batch tests incorporating the use of treated leachate were conducted subsequent to the synthetic leachate batch tests, and were designed to assess the effect of genuine leachate on the denitrification process. The genuine leachate batch tests were further used to determine the substrate longevity in terms of denitrification efficiency. This was done by replacing denitrified leachate, after denitrification was complete, with fresh pre-treated (nitrified) leachate at a concentration of 2000 mg NO3 L−1 , while keeping the same substrate. Pre-treated leachate was obtained from the Mariannhill LFS, South Africa. Leachate was collected after nitrification was conducted in an on-site SBR.
3. Results and discussion 3.1. Substrate and leachate characterization Substrate characterization shows that the RI7 value of raw CGR, an indication of the extent to which readily biodegradable organic matter has been decomposed,[33] was the highest (Table 1). This indicated that raw CGR was the least decomposed and consequently the most readily biodegradable. The C/N, sCOD and BOD5 values of raw CGR were high, displaying a high level of organic strength. Due to these chemical characteristics, raw CGR was identified as the substrate that would best promote the action of denitrifying bacteria by providing the highest amount of biodegradable carbon in the shortest time while depositing little nitrogenous compounds back into the system (Table 1). PB showed a high C/N and a high RI7 as it is a relatively fresh material having not undergone any stabilization. The C/N in this study was approximately three times higher than that found in Trois et al. [3] This finding suggests that the heterogeneous character of the PB composition influences directly the amount of carbon available to sustain the denitrification process. Furthermore, the eluate characterization showed that TS and sCOD were low, suggesting that carbon from PB was not immediately released into the system, and therefore had a poor leaching ability. The poor leaching ability of PB, particularly in the leaching of organic compounds, was also reported by Ribe et al.[34] The use of CGR 10 was evaluated, as it is a theoretically more biologically stable substrate, possessing a
lower organic strength than raw CGR, due to the composting process. This would ideally result in low COD effluent, requiring less COD management of the effluent. Results confirmed that CGR 10 possessed a lower sCOD than raw CGR and the lowest RI7 of the three substrates. The substrate showed good potential as a carbon source as it still possessed a sufficient organic load to promote biodenitrification. The optimum C/N for stabilized compost to promote denitrification ranges between 13 and 16.[23] The C/N of the CGR 10 was 21, which was 2.4 and 9 times lower than those of the raw CGR and PB, respectively, similar to the findings of Trois et al.[3] CGR 10 showed good potential to desorb its available carbon adequately as indicated by the significant TS and sCOD in the eluate fraction. The pH for all substrates was below the optimum pH, which ranges normally between 6 and 8. The pH was expected to increase to optimal levels at the onset of denitrification as carbonate alkalinity increases during nitrate reduction.[23] 3.2. Simulated leachate batch tests An initial lag phase was observed in most batch tests, where no denitrification took place (Table 2). Microbial populations require time to acclimate to environmental conditions, and reach sufficient densities to initiate the denitrification process. The occurrence of microbial acclimatization is a welldocumented process.[35,36] CGR 10 showed the shortest lag phase of the substrates at 1, 3 and 2 h for the 100, 500 and 2000 mg NO3 L−1 batch tests, respectively (Table 2). It is possible that composting promoted the rapid establishment of a specialized microbial consortium.[37] Lag phase for raw CGR was 5, 2 and 22 h for the 100, 500 and 2000 mg NO3 L−1 batch tests, respectively, and the lag phase for PB was between 23 and 24 h in all batch tests. A possible reason for the short lag phases observed is due to the low pH variation during acclimatization (Figure 1). Findings from Trois et al. [3] confirmed a short lag phase when using CGR substrates; however they found a longer lag phase when using PB (20–80 h), particularly at higher nitrate concentrations. They accounted this to pH buffering and microbial competition. Since results from this study showed less pH variation during the lag phase when compared to Trois et al.,[3] it is likely that this contributed to the shorter lag phases observed. Complete denitrification was achieved in all batch tests using synthetic leachate solution, indicating that raw CGR, CGR 10 and PB can all adequately be used as alternative carbon sources to facilitate denitrification at a nitrate concentration of up to 2000 mg L−1 under optimal conditions. Raw CGR was the most efficient substrate in facilitating biological denitrification in batch tests at all nitrate concentrations. Raw CGR and CGR 10 showed similar rates of denitrification at lower nitrate concentrations
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Environmental Technology Table 1. Characterization tests conducted on solid and eluate phases of raw CGR, CGR 10 and PB and on nitrified leachate obtained from the Mariannhill LFS in Durban, South Africa. Raw CGR
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Characterization on solid material Moisture content (w) (%) 32 ± 1 TS (%) 68 ± 1 VS (%) 93 ± 1 RI7 (mg O2 /g DM) 23 TC (%) 47 TN (%) 0.9 C/N 51 Characterization on eluate TS (%) VS (%) pH COD (mg L−1 ) BOD5 (mg L−1 ) TC (%) TN (%) C/N NH3 -N (mg L−1 ) NOx -N (mg L−1 )
17 ± 0.05 14 ± 0.03 4.25 21899 ± 478 2678 0.40 0.07 5.71 3.1 1.1
CGR 10
PB
Genuine leachate
54 ± 0.2 46 ± 0.2 77 ± 3 13 36 1.8 21
25 ± 2 75 ± 2 98 ± 0.5 17 43 0.2 189
– – – – – – –
16 ± 0.02 12 ± 0.02 5.08 6460 ± 40 1651 0.59 0.05 11.80 3.3
