Leachate treatment before injection into a bioreactor landfill: Clogging potential reduction and benefits of using methanogenesis

Waste Management 30 (2010) 2030–2036

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Leachate treatment before injection into a bioreactor landfill: Clogging potential reduction and benefits of using methanogenesis Stanislaw Lozecznik a,*, Richard Sparling b, Jan A. Oleszkiewicz a, Shawn Clark a, Jamie F. VanGulck c a

Department of Civil Engineering, University of Manitoba, Winnipeg, Canada R3T 5V6 Department of Microbiology, University of Manitoba, Winnipeg, Canada R3T 2N2 c ARKTIS Solutions Inc., 117 Loutitt St., Yellowknife, Canada X1A 3M2 b

a r t i c l e

i n f o

Article history: Received 5 October 2009 Accepted 21 April 2010 Available online 18 May 2010

a b s t r a c t In this study, an anaerobic sequencing batch reactor (ASBR) was operated with leachate from Brady Road Municipal Landfill in Winnipeg, Manitoba, Canada. Leachate was collected twice from the same cell at the landfill, during the first and 70th day of the study, and then fed into the ASBR. The ASBR was seeded at the start-up with biosolids from the anaerobic digester from Winnipeg’s North End Water Pollution Control Center (NEWPCC). Due to the higher COD and VFA removal rates measured with the second batch of leachate, an increase of approximately 0.3 pH units was observed during each cycle (from pH 7.2 to 7.5). In addition, CO2 was produced between cycles at constant temperature where a fraction of the CO2 became dissolved, shifting the CO2/bicarbonate/carbonate equilibrium. Concurrent with the increase in pH and carbonate, an accumulation of fixed suspend solids (FSS) was observed within the ASBR, indicating a buildup of inorganic material over time. From it, Ca2+ and Mg2+ were measured within the reactor on day 140, indicating that most of the dissolved Ca2+ was removed within cycles. There is precedence from past researches of clogging in leachate-collection systems (Rowe et al., 2004) that changes in pH and carbonate content combined with high concentrations of metals such as Ca2+ and Mg2+ result in carbonate mineral precipitants. A parallel study investigated this observation, indicating that leachate with high concentration of Ca2+ under CO2 saturation conditions can precipitate out CaCO3 at the pH values obtained between digestion cycles. These studies presented show that methanogenesis of leachate impacts the removal of organic (COD, VFA) as well as inorganic (FSS, Ca2+) clog constituents from the leachate, that otherwise will accumulate inside of the recirculation pipe in bioreactor landfills. In addition, a robust methanogenesis of leachate was achieved, averaging rates of 0.35 L CH4 produced/g COD removed which is similar to the theoretical removal of 0.4 L CH4/g COD. Therefore, using methanogenesis of leachate prior to recirculation in bioreactor landfills will help to (1) control clog formation within leachate pipes and (2) produce an important additional source of energy on-site. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction A common method to dispose of municipal and industrial solid waste around the world is landfilling. Organic and inorganic waste material located within waste layers in the landfill is typically anaerobically degraded through biological, chemical and physical interactions with emission of biogas and, depending on the amount of moisture present, leachate production (Kjeldsen et al., 2002). Leachate is primarily formed as water percolates through the waste layers, where organic and inorganic components from the waste are solubilized and incorporated within it. To avoid subsurface and surface water contamination, a large amount of infrastructure can be integrated in the landfill design to safeguard the environment against leachate release into the surrounding * Corresponding author. Tel.: +1 204 4749320; fax: +1 204 4747513. E-mail address: [email protected] (S. Lozecznik). 0956-053X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2010.04.024

environment. An engineered leachate-collection system and a liner(s) below the waste have two main purposes: (1) to minimize the transport of leachate into any aquifer(s) and surrounding subsurface soil, and (2) to transport the leachate outside of the waste cell for storage and treatment purposes (Rowe et al., 2004). The leachate-collection consists typically of a granular material with embedded perforated pipes. Collected leachate can be managed and treated on-site or off-site before discharge into the environment. In bioreactor landfills, the collected leachate is injected back into the landfill through a network of perforated pipes buried in the refuse. Liquid injection into waste can increase the rate of solubilization into the passing leachate. This enhances refuse degradation and settlement, as well as increases biogas production and on-site treatment of leachate (Reinhart and Townsend, 1997). Failure of a leachate-collection system to perform can potentially result in increased migration of leachate into the surface and subsurface, slope instability, surface and groundwater

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contamination, and ineffective landfill gas generation and collection. Failure of a leachate injection system can result in ineffective leachate management and limit the benefits of the bioreactor landfill technology. Thus, the long-term efficiency of the leachatecollection and injection system is therefore essential to the management and operation of the landfill. Studies from around the world (Canada, USA, Germany, France, Central America, Europe, Japan) have reported clogging (sometimes called bio-rock) in leachate-collection and injection pipes that convey leachate (VanGulck et al., 2009; Lozecznik and VanGulck, 2009; VanGulck and Rowe, 2004a,b; Lozecznik, 2006; Rittmann et al., 2003; Rowe et al., 2004). Clogging results in the eventual failure of these engineered systems to perform as designed. In a review of research needs for bioreactor landfills, SCS Engineers (SCS Engineers, 2000) stated that bioreactor landfills have greater potential to clog compared to conventional landfills, and that there is a need to evaluate the performance of leachate-collection system components in bioreactor landfills. Clogging consists of a hard mineral encrustation (mainly Ca2+ and Mg2+ carbonate minerals) inorganic material from the leachate termed fixed suspend solids (FSS), biological material from the leachate (COD, VFA and VSS) and in situ biofilm development. This can occur on the transmission pipe wall and perforations, as well on the surface of drainage material and soil particles. A clogged collection system can no longer effectively remove leachate from the base of the landfill, resulting in increased potential for contamination of subsurface aquifers and/or surface water. A clogged injection pipe will experience a reduction in hydraulic performance for transmitting leachate, and result in a non-uniform infiltration of leachate into the waste, thereby reducing the efficiency of methane gas production (VanGulck et al., 2009; Lozecznik and VanGulck, 2009). Among other things, clogged leachate transmission systems seriously limit the long-term viability of the landfill design, operation, and maintenance. The objective of this laboratory study is to assess the use of leachate methanogenesis on the reduction of leachate components that are known to contribute to clogging in leachate transmission pipes. Reduction of organic and inorganic leachate constituents may reduce the operational challenges resulting from leachate injection pipe clogging, and extend the service life of the engineered components of the bioreactor landfill, thereby maximizing its benefits over a longer period of time. In addition, treating leachate under anaerobic conditions may generate an important source of methane gas outside of the waste cell, which can provide a value-added to the landfill owner.

2. Experimental methods Brady Road Landfill in Winnipeg, Manitoba, Canada is an active landfill and the source of the leachate used in this study. Leachate was collected from the same cell and leachate well on day 1 and 70 of the 186 day operation. Select leachate characteristics from each day collected are shown in Table 1. Leachate was transported to the laboratory in 25 and 45 L carboys, which were then stored at 4 °C to limit biological processes from occurring. Despite the storage temperature, leachate COD removal was observed between carboys. VanGulck (2003) observed a similar effect while feeding real leachate (KVL Landfill in Ontario) into columns representing leachate-collection systems. He explained it as the development of biofilm and activity of suspend bacteria within the carboys and within the leachate distribution lines while feeding the columns. The leachate composition of Brady Road Landfill was representative of leachate generated from waste that is between about 5 and 15 years in age. The differences in leachate composition are

Table 1 Composition of leachate collected from Brady Road Landfill at day 1 and 70 of the laboratory study (Sampling was completed on July 10 and September 24 of 2009). Parameter

COD Alkalinity pH TSS VSS FSS Acetate [mg/L] Propionate [mg/L] Butyrate [mg/L]

Units

[mg/L] [mgCaCO3/L] – [mg/L] [mg/L] [mg/L] [mg/L] [mg/L] [mg/L]

Brady Leachate day 1

day 70

2518 4325 7.2 240 140 100 510 178 154

7695 6225 6.7 720 350 370 2137 171 622

inherent to the variability of the conditions within the landfill waste cell. An anaerobic sequencing batch reactor (ASBR) with a working volume of 3.4 L was operated under anaerobic conditions with leachate. The reactor was set-up to have a constant 1.4 L of biomass and 2 L of leachate (called feed). The reactor was designed to include a sample port for biogas analysis (CO2 and CH4) and analysis of leachate composition, as well a port for feeding and measurement of gas production, as shown in Fig. 1. The ASBR was seeded at the start-up with biosolids from the anaerobic digester from the North End Water Pollution Control Center (NEWPCC) in Winnipeg, Canada. The ASBR was operated inside of a temperature controlled chamber at mesophilic temperature (35 °C). Timur and Özturuk (1999) operated an ASBR for 2 years to treat leachate from 3.5 year old waste obtaining 0.29 L CH4/g COD. The solids retention time (SRT) and hydraulic retention time (HRT) values used for Timur and Özturuk (1999) ranged from 9 to 40 days and from 1.5 to 10 days. This work guided the selection of SRT’s and one HRT (48 h), the other HRT was selected to assess the daily treatment (24 h) of leachate, as shown in Table 2. After the first two weeks of operation, the sampling and feeding protocol was performed with industrial N2(g) flushed into the reactor during each procedure to ensure anaerobic conditions. Waste activated sludge or digester samples were collected from the reactor before and after each anaerobic digestion cycle and tested on a daily basis for the following characteristics: total chemical oxygen demand (tCOD), soluble chemical oxygen demand (sCOD), oxidation, reduction potential (ORP), pH, total alkalinity, total suspend solids (TSS), volatile suspend solids (VSS), fixed suspend solids (FSS), and the volatile fatty acids (VFA) acetate (AA), propionate (PA) and butyrate (BA). Biogas volume and content (CO2 and CH4) were measured weekly. The leachate solids and supernatant extracted from the reactor were controlled daily. Ca2+and Mg2+ concentrations were analyzed during one digestion cycle, on day 140. COD from sCOD and tCOD samples was measured using the HATCHÓ method except that the sCOD samples were filtered through a 0.45 lm filter prior to analysis. The pH and ORP were measured using an ORION 5STAR MULTI WPHH equipped with the appropriate electrical probes. Total suspended solids (TSS) and fixed suspended solids (FSS) were determined using a gravimetric measurement of the residue retained on a 0.45 lm glass fiber filter dried at 105 and 550 °C using the ASTM D854 method. Volatile suspended solids (VSS) concentrations were calculated as the difference between TSS and FSS concentrations. VFA concentrations in the samples were analyzed by a Varian CP 3800 gas chromatographer (GC) equipped with a flame ionization detector, CP-8400 autosampler and WCOT fused silica 25 m  0.32 mm internal diameter (ID) coating FFAP-CB capillary column. The optimized GC operating conditions were: 270 °C in the injector and 300 °C in the detector. Temperature in the oven was initially set at 70 °C and then ramped up to 140 °C at the rate of 10 °C/min, from 140 to 200 °C at the rate of 25 °C/min, and then

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Biogas line

Solution line Atmospheric pressure

Liquid sampling

Feeding port Biogas sample port

6L

3.4 L

Saturated solution

Saturated solution

ASBR

Mixer

Fig. 1. Schematic of SBR and liquid displacement set-up (not to scale).

Table 2 Batch reactor SRT and HRT operation. Days

SRT [days]

HRT [hours]

0–16 16–24 24–120 120–184

20 40 40 40

24 24 48 24

from 200 to 240 °C at the rate of 30 °C/min. The column was maintained at 240 °C for 3.97 min to let the residual contaminates flush out. The total running time was 15 min. The gas flow rates were: helium in the column at 6.5 mL/min, hydrogen at 30 mL/min, and air at 300 mL/min. Crotonic acid was added as internal standard to improve the analytical reproducibility and accuracy. Under the above conditions, excellent resolution and quantitative accuracy were obtained for all VFAs. Following ‘‘Standard Methods for the Examination of Water and Wastewater” (APHA, 21st Edition, 2005), samples were acidified to pH 2 using 85% O-phosphoric acid and then filtered by 0.22 lm syringe filter prior to injection. CH4 and CO2 in the reactor headspace samples were analyzed by a Varian CP 3800 gas chromatographer (GC) equipped with a thermal conductivity detector. CO2 was separated from other components using CP-Porabond Q fused silica column (25 m  0.53 mm ID) and CH4 was resolved using fused silica 25 m  0.53 mm ID coating molecular sieve 5A column together with the second CPPorabond Q fused silica column. The optimized GC operating conditions were 250 °C in the injector and 180 °C in the detector. Temperature in the oven was initially set at 40 °C for 1 min and then ramped up to 100 °C at the rate of 20 °C/min for a total running time of 15 min. The flow rate of carrier gas helium in column is constant 3 mL/min. Samples were taken directly from the reactor headspace and injected into GC, the volume of sample loop is 250 lL. The biogas was collected and the volume was measured using a liquid displacement method in air-tight calibrated vessels of 10 L (see Fig. 1). The liquid inside of each vessel contained 6 L of deionized water saturated with 2.1 kg of NaCl, 300 mL of H2SO4 and 0.18 g of methyl orange to prevent gas from dissolving (Puchajda, 2006; Wohlgemut, 2008). Finally, Ca2+, Mg2+ and Na+ were measured using a Varian ICP, Model VISTA-MPX, CCD with simultaneous ICP-OES. The CH4 and CO2 percentage within the biogas were compared from day 17 and the CH4 produced and COD removed were compared at different elapsed times from day 70. To illustrate the precipitability of Ca2+ within the digester, a batch test was carried out using synthetic leachate saturated with 20% of CO2

(in N2) at different pH’s values. The synthetic leachate was prepared following the formula used by VanGulck and Rowe (2004b) that emulated the leachate characteristics of the Keele Valley Landfill in Ontario, Canada, collected between June and August 1993. This solution consisted mainly of three volatile fatty acids (acetate, propionate and butyrate) with various salts and a trace metal solution, as shown in Table 3. A volume of 500 mL of this synthetic leachate was poured into a 1 L Pyrex reagent bottle together with a magnetic stirrer, pH meter ORION Model 420 A with a probe ORION 911600 THERMO semimicro pH, and a gas diffuser attached to a 20% CO2 and N2 gas cylinder. Ca2+ was measured using a HACH universal digital titrator kit with the proper reagents to measure hardness for Ca2+. Initially, the gas was bubbled via the gas diffuser for 30 min without taking any other measurement to achieve CO2(aq) saturation level at ambient temperature. After this 20% CO2(aq) saturation was achieved, sodium hydroxide (NaOH) of 1 N was dosed into the reactor to increase the pH at the values chosen while bubbling. As the pH values chosen were achieved (6.5, 7, 7.21, 7.35, 7.4, 7.46 and

Table 3 Composition of synthetic leachate. Component

Per litre

Acetate Propionate Butyrate K2HPO4 KHCO3 K2CO3 NaCl NaNO3 NaHCO3 CaCl2 MgCl2
6H20 MgSO4 NH4HCO3 CO(NH2)2 Na2S
9H2O NaOHa Trace metal solutions (TMS)b Distilled Water

7 ml 5 ml 1 ml 30 mg 312 mg 324 mg 1440 mg 50 mg 3012 mg 2882 mg 3114 mg 156 mg 2439 mg 695 mg Titrate to an Eh 120 mV:0180 mV Titrate to a pH 5.8–6.0 1 ml To make 1 l

a The addition of sodium from this titration increases the total sodium concentration in the solution to about 2800 mg/L. b TMS is composed of: FeSO4, H3BO4, ZnSO4
7H2O, CuSO4
5H2O, MnSO4
7H2O, (NH4)6Mo7O24
4H2O, Al2(SO)3
16H2O, CoSO4
7H2O, NiSO4
6H2O, 96% concentration H2SO4 (AnalR) in different amounts to make 1 L of distilled water. For the exact amounts, see VanGulck and Rowe (2004b).

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8.04), total calcium concentrations were sampled from the reactor. The sampling involved taking two supernatant samples of 20 mL from the reactor after settling any precipitate for 20 min. Each sample was acidified with hydrochloric acid (HCl) between pH 4 and 5 to ensure that most of the CO2 dissolved was gasified prior to titration with the HACH method. Since a volume of 40 mL was sampled from the reactor at different pH values and NaOH was added to achieve the higher pH values, the reactor volume ranged between 450 and 560 mL during this study. Thus, dilution of the total calcium sampled was small compared with the effects observed at different pH’s, therefore the original measurements were not corrected for dilution effects.

3. Results and discussion The performance of the digester using leachate from Brady Road Landfill changed over time as measured using tCOD and sCOD percentage removals, as shown in Fig. 2. Cycle start represents the composition of the reactor mix liquor at the start-up of the digestion at each cycle (t = 0) after biomass and leachate were mixed. Cycle end represents the composition of the reactor mix liquor at the end of the digestion cycle (t = HRT). Samples of the mix liquor from the digester were taken during each cycle to assess the performance of the digester, as well as to maintain the SRT’s adopted. To assess the effects of different SRT and HRT on leachate treatability, a criterion based on a minimum percentage removal of COD (>20%) between cycles was used as a control to modify these variables at different time intervals. A settling time period of 1 h was employed between anaerobic cycles to replace the treated leachate with fresh leachate. For the first 16 days, there was no substantial change in COD (<20% removal) so it was decided to increase the SRT to 40 days. A week later, little change was observed within the digester so the HRT was increased to 48 h. After this time, the digester started COD removal over 20% and averaged 34% of tCOD removed and 43% of sCOD removed before the second batch of leachate was fed into the digester on day 72 (solid arrow shown in Fig. 2a and b). After this second batch of leachate with higher COD concentration (Table 1) was fed into the reactor on day 72, the digester had higher removal rates of tCOD and sCOD that averaged approximately 42% and 48%. At day 120, it was decided to decrease the HRT to 24 h to assess the performance of the reactor at shorter feeding times. After this time, the influent tCOD and sCOD stabilized and the aver-

age removal rates during the last 66 days of the study were around 27% and 51%. The changes on HRT at different elapsed times are indicated with the dash dot arrows in Fig. 2. These changes in COD removal indicate an increase in biological activity, which would also suggest an increase in pH effluent, as observed in past studies of leachate degradation (see Rowe et al., 2004). After the 72nd day, there was an increase in pH of approximately 0.2 units during the course of each sequence batch, as shown in Fig. 3, which is consistent with the fact that the second batch of leachate had a higher VFA content (see Table 1) and higher removal of VFA’s was observed within the reactor. The solid arrows in Fig. 3 indicate the elapsed time that the ASBR was fed with the second batch of leachate between days 72 and 184. The pH shift was associated with the removal of tCOD and sCOD within the reactors (Fig. 2). As shown in Fig. 4a–c, the change in tCOD and sCOD is associated with the consumption of acetate and butyrate, but not propionate. This link between VFA removal and pH increase was observed in VanGulck et al. (2003), following the digestion of real leachate within columns that represent plug flow reactors. A relationship was made between the change in pH coupled with the fermentation of VFA and the biological production of carbonate and the measured removal of Ca2+ as CaCO3 through the columns. The increase of VFAs measured in this study (Fig. 4) from day 70 was due to the striking differences in leachate composition of the two batches used. The second batch of leachate had approximately four times higher acetate and butyrate concentrations than the first batch (Table 1). The greater amount of fermentation of these VFA after day 72 had an impact on the pH between cycles of the digester, which resulted in an increase in pH between 0.2 and 0.3 units during the course of each sequence batch (shown in Fig. 3). Rittmann et al. (2003) showed that the main mechanism for CaCO3 precipitation from leachate was acetate fermentation producing methane (CH4) and carbonic acid (H2CO3). As the pH in the digester effluent was higher than the influent and acetate was consumed within the reactor, a change on FSS was expected. Fig. 5 shows FSS accumulation as the difference in TSS and VSS at the start of each cycle from about day 80, and increased at about a constant rate after the second batch of leachate was added. The accumulation of TSS, with a relatively constant concentration of VSS as shown in Fig. 5, indicates a buildup of inorganic material within the digester over time. There is precedence from past researches of clogging in leachate columns (see Rowe et al., 2004) that changes in pH and carbonate content combined with high concentration of metals such as Ca2+ or Mg2+, results in carbonate

100 90

New leachate

80

HRT 24/48

8.0

HRT 48/24

60

7.5

50 40

pH

COD removed [%]

70

30 7.0

20 tCOD 10

Cycle start Cycle end

sCOD

0 0

20

40

60

80 100 120 140 Elapsed Time [days]

160

180

200 6.5 0

Fig. 2. Percentage of total COD and soluble COD removed over time within the digester (48/24 and 24/48 correspond to the change in HRT from 48 to 24 h or vice versa).

20

40

60

80

100

120

140

160

180

Elapsed Time [days] Fig. 3. Variation in pH within the digester versus time.

200

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(a)

(b) 2000

2000 Cycle start Cycle end

1800

1600

1600

1400

1400 Butyrate [mg/L]

Acetate [mg/L]

1800

1200 1000 800 600

1200 1000 800 600

400

400

200

200 0

0 0

(c)

Cycle start Cycle end

20

40

60

80

100

120

140

160

180

200

80 100 120 140 Elapsed Time [days]

160

180

200

Elapsed time [days]

2000 1800

0

20

40

60

80 100 120 140 Elapse Time [days]

160

180

200

Cycle start Cycle end

Propionionate [mg/L]

1600 1400 1200 1000 800 600 400 200 0 0

20

40

60

Fig. 4. Variation in (a) acetate, (b) propionate and (c) butyrate concentrations within the digester versus time.

TSS and VSS concentration [mg/L]

22000

(a)

20000 18000

TSS cycle start VSS cycle start

HRT 48/24

16000 14000

HRT 24/48 New leachate

12000 10000 8000 6000 4000 2000 0

0

20

40

60

80 100 120 140 160 180 200 Time [Days]

Fig. 5. Variation in total suspend solids (TSS) and volatile suspend solids (VSS) within the digester versus time (48/24 and 24/48 correspond to the change in HRT from 48 to 24 h or vice versa).

mineral precipitants. To verify whether Ca2+ or Mg2+ is lost from solution within the reactor, a sample of leachate and supernatant was analyzed before and after the digestion cycle on day 140 for Ca2+ and Mg2+. These results show that leachate at cycle start contained 354 and 561 mg/L of Ca2+ and Mg2+, where the supernatant after the cycle end contained 24 and 480 mg/L Ca2+ and Mg2+,

respectively. Therefore, a significant amount of Ca2+ was precipitated from the leachate and accumulated inside the reactor, while much less Mg2+ was precipitated at the operational pH of the reactor. Concurrent with VFA removal, biogas was produced during each digestion cycle. Biogas composition (CH4 and CO2) and production were measured weekly starting from day 17. The CH4 and CO2 content within the biogas and the CH4 produced per COD removed at different elapsed times are shown in Fig. 6a and b. From Fig. 6a, it can be seen that the biogas formed between digestion cycles with the first batch of leachate did not add up to 100%, which may be due to incomplete methanogenesis (with H2 production) or an insufficient amount of biogas produced to dilute out the N2(gas) gas used for liquid sampling and feeding the reactor. An enrichment of CH4 and a decrease in CO2 content was observed within the biogas starting on day 80. From Fig. 6b, an average of approximately 0.35 L CH4/gCOD removed was achieved during the course of the study, similar to the 0.4 L CH4/gCOD removed theoretically by oxidizing methane at 35 °C (Metcalf and Eddy, 2004). This shows that a high methane (CH4) production and COD removal can be achieved when treating leachate using well-controlled anaerobic conditions. The presence of a 75–25% CH4–CO2 ratio in the digester may indicate a shifting of the carbonate equilibrium within the ASBR, where there is an increase in carbonate availability within the ASBR as shown in Fig. 6a. This is consistent with the observed increase in pH in the digester. The increase in carbonate content may be the cause for the increase in precipitation of carbonate minerals, as indicated by the observed FSS accumulation (Fig. 5), within the reactor.

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0.7

(b) L CH4/g COD

0.6

90 80

0.5 70

LCH 4 /g COD

CH4 (%) and CO 2 (%) in the biogas

100

60 50 40 30 20

0.4 0.3 0.2

(a)

0.1

% CH 4 % CO 2 Total % biogas

10 0 0

20

40

60

80

100

120

140

160

180

0.0 200

60

80

100

120

140

160

180

200

Time [days]

Time [days]

Fig. 6. Variation of (a) percentage of CH4 and CO2 within the biogas produced and (b) CH4 produced per gram of COD removed within the digester versus time.

1000

Total soluble Ca 2+ [mg/L]

800

600

400

200

0 6.6

6.8

7.0

7.2

7.4

7.6

7.8

8.0

8.2

pH Fig. 7. Variation of pH and total soluble calcium within the reactor (solid arrows indicate pH range at which total soluble calcium is removed from solution).

These results suggest that since CO2(gas) is produced between cycles due to biological activity and stripped out from solution depending on the mixing conditions, an increment on the partial pressure of CO2(gas) within the headspace of the reactor is expected. In addition, as the pH rises due to the removal of VFA’s, the carbonate equilibrium between the headspace and medium is shifted towards the medium, as shown in Fig. 6a. This increase in CO2(aq) at higher pH will increase the amount of carbonate available within the reactor that can be coupled with the excess metals (Ca2+ and Mg2+) and precipitate out, as observed with the increment on FSS within the reactor. In order to illustrate the effect of pH and CO2(aq) on the concentration of FSS within the digester, a parallel batch test study was carried out. Synthetic leachate was prepared and saturated with 20% of CO2 at ambient temperature. Soluble Ca2+, at different pH values, was measured. These results are shown in Fig. 7. From Fig. 7, it can be observed that between pH 7.2 and 7.46 (solid arrows) at CO2(aq) saturation conditions (20%), the concentration of total calcium decreased between 65% and 80%, precipitating out from solution. As shown in Fig. 3 (see solid arrows), the average pH within the ASBR before and after each digestion cycle was 7.18 and 7.46, and the CO2 saturated conditions averaged approximately 25% (see Fig. 6a) for the period between days 72 and 184. This provides an explanation of why calcium was precipitated

and there was an accumulation of FSS within the reactor at the pH range and CO2 saturation conditions were observed. Furthermore, VanGulck et al. (2003) permeated synthetic leachate through a leachate column, and after 300 days the leachate pH increased from the initial pH of 6 to a value of approximately 7.4. At pH 7.4, the removal of calcium from solution within the column was approximately 80%, which is similar to the result shown in Fig. 7. These results may have been predicted through geochemical modeling, but the intent of this experiment was to demonstrate it using the synthetic leachate used from past leachate studies. Previous research on clogging within well-controlled leachate column experiments (VanGulck et al., 2003; Rittman et al., 2003) have concluded that the main source of the carbonate forming calcium carbonate precipitants was the fermentation of VFA, primarily acetate. Although VFAs represented approximately 40% of the tCOD in the second batch of leachate in this laboratory study, a considerable increase in FSS within the digester was observed over time. One of the main differences between the column studies and the ASBR that can explain this phenomenon was the analysis of CO2(gas) in the headspace, thus its effect on the carbonate concentration. The CO2(gas) formed within the columns was degassed from the top of the columns before reacting with the leachate, so its impact was not considered in the clog analysis. 4. Conclusions This study shows that performing leachate fermentation prior to recirculation reduces organic (COD, VFA) and inorganic (Ca2+, FSS) clog constituents within the leachate, which otherwise may impair the operation and the service life of the recirculation pipes. The removal of COD and VFA increases both the pH as well as the carbonate content within the reactor, which precipitates with metals such as soluble Ca2+ (e.g. as CaCO3), accumulating as FSS. This indicates that dissolved Ca2+ removal is a simultaneous effect of the reduction of organic components from the leachate, such as COD and VFA. A decrease of the CO2 gas content was observed within the reactor after the second batch of leachate was added, evolving to the aqueous phase, increasing the carbonate content within the reactor and enriching the CH4 content within the headspace. From the changes of leachate tested and organic removal rates observed, it can be deduced that loss of dissolved Ca2+ is primary affected by the variability of the organic content within the leachate and rate of methanogenesis within the ASBR. Finally, solid waste engineers may be able to use fermentation of leachate prior

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to leachate recirculation as a way of controlling clogging in recirculation pipes. When doing so, consideration should be given to (a) an equalization tank to control the influent to the ASBR to insure consistent concentrations of leachate constituents and (b) a pH control to achieve the pH values at which methanogenesis is performed and Ca2+ concentration is reduced. By causing the precipitation of mineral carbonates, methanogenic treatment of leachate prior to recycling has the potential to reduce pipe clogging during leachate injection, while the methane generated can be collected for energy production. Acknowledgments This research was funded by a grant from the Environmental Research and Education Foundation (EREF), Natural Sciences and Engineering Research Council of Canada (NSERC), Canadian Foundation for Innovation (CFI) and the City of Winnipeg. The writers acknowledge the help of Mr. Chris Kozak and Mr. Mark Kinsley from the City of Winnipeg. Mr. Joshua Pawluk, Mr. Zacharie Durand and Mr. Victor Wei’s help with laboratory analyses is also acknowledged. References ASTM D854. Standard Test Methods for Specific Gravity of Soil Solids by Water Pycnometer. The American Society for Testing and Materials, West Conshohocken, PA, USA. Kjeldsen, P., Barlaz, M.A., Rooker, A.P., Baun, A., Ledin, A., Christensen, T.H., 2002. Present and long term composition of MSW landfill leachate: a review. Environmental Science and Technology 32 (4), 297–336.

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