Stabilization of solid digestate and nitrogen removal from mature leachate in landfill simulation bioreactors packed with aged refuse

Journal of Environmental Management 232 (2019) 957–963

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Research article

Stabilization of solid digestate and nitrogen removal from mature leachate in landfill simulation bioreactors packed with aged refuse

T

Wei Penga, Alberto Pivatob,∗, Francesco Garboa, Tianfeng Wangb,c a

DII - Department of Industrial Engineering, University of Padova, Via Marzolo 9, 35131, Padova, Italy ICEA - Department of Civil, Environmental and Architectural Engineering, University of Padova, Via Marzolo 9, 35131, Padova, Italy c School of Chemistry and Environmental Engineering, Jiujiang University, 332005, Jiujiang, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Solid digestate Aged refuse Organic fraction of municipal solid waste Landfill leachate Nitrogen removal Circular economy

Digestate from biogas plants managing municipal solid waste needs to be stabilized prior to final utilization or disposal. Based on the concept of urban mining, aged refuse from a closed landfill was used to treat landfill leachate, but nitrogen removal by biological denitrification was limited. The aim of this study was to use a digestate layer in bioreactors containing aged refuse to enhance the biological denitrification capacity of the aged refuse, stabilize digestate, and mitigate the ammonia emissions from digestate leaching with leachate recirculation. Six identical landfill columns filled with 0% (R0), 5% (R5), and 15% (R15) of solid digestate above aged refuse (ratios based on Total Solids) were setup and nitrified leachate was periodically fed and recirculated to the columns. The nitrate removal rate in R5 and R15 was 3.4 and 10 times higher relative to the control (no digestate added). A 31.5–35.9% increase of solid digestate biostability was confirmed by tests performed under both aerobic and anaerobic conditions. The results showed that instead of land use, the solid fraction of digestate could be utilized as an inexpensive functional layer embedded in an old landfill site to enhance the denitrification capacity and achieve digestate stabilization with minimal ammonia leaching from digestate.

1. Introduction The Organic Fraction of Municipal Solid Waste (OFMSW) can be valorized by the production of renewable energy (e.g., biogas) and biofertilizer (e.g., digestate), thereby closing the waste-energy-food loop (Sisto et al., 2017; Fuldauer et al., 2018). Digestate obtained from biogas plants managing OFMSW is an unstable material consisting of undigested organic carbon and pollutants (Tampio et al., 2016; Peng and Pivato, 2017). According to Italian regulations (D.Lgs. 152/2006), digestate from agricultural biomasses (e.g., energy crops or animal manures) can be directly applied as a biofertilizer while the digestate from OFMSW is classified as organic wastes which must be aerobically treated prior to the final use (Saveyn and Eder, 2014). However, aerobic post-treatment of digestate should be operated carefully because of high nitrogen emissions, especially nitrous oxide (Tremier et al., 2013). Furthermore, anaerobically digested OFMSW mechanically separated from unsorted waste needs to be disposed of in landfills or incinerators (Bolzonella et al., 2006; Li et al., 2011). Based on the concept of circular economy, those digestates which fail to meet the agricultural requirements, might need to consider alternative options to completely utilize the digestate (Peng and Pivato, 2017).

∗

After the end of anaerobic digestion, the digestate will undergo a solid-liquid separation. The solid fraction of the digestates from OFMSW could be innovatively applied for nitrogen removal from mature landfill leachate (Peng et al., 2018a). Both nitrogen removal from mature landfill leachate and stabilization of municipal solid waste (MSW) can be achieved in facultative landfill bioreactors with recirculating nitrified landfill leachate (Chen et al., 2009; He et al., 2006; Price et al., 2003; Sun et al., 2017; Xie et al., 2013; Zhong et al., 2009). Aged refuse is excavated from closed landfills and is typically characterized by a Respiration Index (RI4) below 7 mg-O2/g-total solids (TS) (Binner et al., 2012). As a result of the absence of easily biodegradable organic carbon in aged refuse, 11-year-old refuse showed significantly lower denitrification capacity compared to 1-year-old refuse (Chen et al., 2009). Although autotrophic metabolic pathways (e.g. Anammox) might occur in waste landfill bioreactors, its contribution to nitrogen removal was low (Valencia et al., 2011). Thus, biodegradable organic matters, working as electron donors, still play essential roles in nitrate denitrification. A carbon source derived from food waste was successfully applied to mature landfill leachate treatment (Yan et al., 2018). Waste ethanol was successfully applied to enhance the denitrification capacity in wastewater treatment (Peng et al., 2007).

Corresponding author. E-mail address: [email protected] (A. Pivato).

https://doi.org/10.1016/j.jenvman.2018.12.007 Received 3 August 2018; Received in revised form 19 November 2018; Accepted 2 December 2018 0301-4797/ © 2018 Elsevier Ltd. All rights reserved.

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2.2. Experimental design

Digestate with an undigested food fraction from OFMSW could be used in old landfill sites to enhance the in-situ denitrification capacity for treating oxidized mature landfill leachate. Mature landfill leachate is generated from closed landfill sites and is characterized by low biodegradability (BOD/COD < 0.1) and ammonia concentrations between 12 mg-N/L to 1571 mg-N/L (Kjeldsen et al., 2002). Despite the high denitrification potential, ammonia leaching from the digestate or MSW might be considered as a drawback when digestate is applied in bioreactor landfills (Peng et al., 2018a; Lubberding et al., 2012). To solve this potential problem, aged refuse with an adsorption capacity can be used as a biofilter to relieve the ammonia emissions (Zhao et al., 2002). Furthermore, soluble ammonia nitrogen in aged refuse bioreactors could be removed through biological nitrification. After leachate recirculation and drainage, “dry condition” will be created, which restores the high porosity of aged refuse and allows air or oxygen to invade the reactor for nitrification (He et al., 2017). Although previous studies showed high denitrification potential of digestate application to leachate remediation, ammonia leaching from the digestate was regarded as a disadvantage (Peng et al., 2018a; 2018b). Additionally, digestate stabilization has not been adequately evaluated. Therefore, it is necessary to optimize the application techniques for landfilling digestate. In this study, digestate stabilization and denitrification of mature landfill leachate was investigated in landfill simulation columns with dual layers consisting of digestate and aged refuse.

The experiment design consisted of a nitrification reactor and six landfill simulation columns. Fig. 1 illustrates the experimental design. The landfill simulation bioreactors were operated using six identical columns with a volume of 7.85 L and height of 100 cm. The columns were named R0A, R0B, R5A, R5B, R15A, and R15B, where R indicates the reactor, the number indicates the solid digestate percentages (%) based on TS, and A or B represents the duplicates. Accordingly, each column was filled with solid digestates and aged refuses at a temperature 25 ± 2 °C and initial Nitrogen loading rate of 0.76 g-N/(kg-VS day). The initial nitrate loading rate was set as 0.76 g-N/(kg-VS day) for each column to ensure that stable denitrification occurred (Peng et al., 2018a). Effluents (denitrified leachate) obtained from the bottom of each column were characterized weekly and then discharged. The following parameters were analyzed, ammonia-nitrogen (N-NH4+), nitrite-nitrogen (N-NO2-), nitrate-nitrogen (N-NO3-), TKN, TOC, TC, chemical oxygen demand (COD) and five-day biochemical oxygen demand (BOD5). Nitrified leachates were replenished weekly to each column. Effluents from the landfill columns were drained from the bottom of the reactor and recirculated to the top of the landfill simulation columns twice a day. The experiment lasted a total of 100 days after 14 feeding cycles (14 weeks). The N-NO3- removal efficiencies (%) and Average Nitrate Removal Rate (ANRR, mg N/(kg-TS day)) were calculated using the following equation. n

2. Materials and methods

Nitrate removal efficiency =

∑1 (Cin, i·Vin, i − Cout , i·Vout , i ) n

∑1 Cin, i·Vinf , i

× 100% (1)

2.1. Mature landfill leachate, aged refuse and solid digestate n

ANRR =

Mature landfill leachate was obtained from a landfill site in Northern Italy. In 1981, the landfill was built to manage the dispose of unsorted MSW. However, it was not well design as there were no impermeable bottom liners or leachate collection system and was finally closed in 1990. The raw leachate was nitrified in an aerobic reactor. The characteristics of the raw and aerobically nitrified landfill leachate are summarized in Table 1. The aged refuse was excavated from the same landfill. The aged refuse was regarded as 40-year-old refuse. The aged refuse samples were characterized as TS = 94.7 ± 0.7%, Volatile Solids (VS) = 3.5 ± 0.2% TS, Total Kjeldahl Nitrogen (TKN) = 3.3 ± 0.2 g-N/kg-TS and Total Organic Carbon (TOC) = 27.1 ± 11.7 g-C/kg-TS. The solid fraction of the digestate was collected from a biogas plant in Camposampiero (Italy) treating source-segregated OFMSW and sewage sludge. The digestate was centrifuged for solid-liquid separation. The solid digestate was characterized as: TS = 19.4 ± 0.1%, VS = 62.0 ± 0.3% TS, TKN = 51.0 ± 0.2 g-N/kg-TS and TOC = 285.0 ± 1.2 g-C/kg-TS.

pH TOC (mg-C/L) TC (mg-C/L) BOD5 (mg-O2/L) COD Cr (mg-O2/L) N-NH4+ (mg-N/L) N-NO2- (mg-N/L) N-NO3- (mg-N/L)

Raw leachate Range

Mean

Range

8.87 907 2570 60 1360 1250 1.9 0

8.68–9.05 478–1336 2110–3030 58–62 722–1998 758–1741 1.6–2.2 0

7.86 834 1221 20.6 1325 34.5 16.2 1131

7.28–8.56 351–1030 705–1500 6.5–32.9 444–1590 16.4–38.8 0.9–56.5 517–1332

(2)

Cin,i, Cout,i = input and output nitrate concentration in each column at feeding cycle i; Vin,i, Vout,i = input and output volumes in each column at feeding cycle i; n = the sum of feeding cycles; n is 14 in this study; M = the dry mass in each column, kg; t = operation time, days. 2.3. Adsorption test Adsorption of ammonium ions onto the aged refuse were studied using batch tests (OECD, 2000). The aged refuses samples were airdried at ambient temperatures (20–25 °C) before being analyzed. With L/S fixed as 25 ml/g, batch experiments were carried out by adding 10 g of aged refuse sample and 250 mL of an ammonium solution (100 mgNH4+/L in 0.01 M CaCl2) to a plastic bottle (270 ml) and shaken at 20 rpm. The difference in the volume space (20 ml) between the bottle and the solution was enough for mixing because rotation was applied. Therefore, the headspace did not limit the mixing efficiency. Triplicate experiments were performed at laboratory ambient temperatures (20 ± 2 °C). After 3 h, 6 h, 24 h, 72 h, and 96 h, ammonium concentrations in the liquid supernatant were analyzed. Two control samples (only the ammonium in 0.01 M CaCl2 solution without the aged refuse sample) and two blank samples (with the aged refuse and 0.01 M CaCl2 solution without ammonium) were treated using identical test procedures. Adsorption equilibrium was determined by the achievement of the equilibrium plateau, which was estimated by the plots of the concentration of ammonium in the aqueous phase versus time. After full adsorption equilibrium, the mixture was centrifuged, and 150 ml of the aqueous phase was removed. The removed solution was replaced with 150 ml of 0.01 M CaCl2. The new mixture was rotated until

Nitrified leachate

Mean

M·t

where:

Table 1 Characteristics of raw mature leachate and nitrified landfill leachatea. Parameters

∑1 (Cin, i·Vin, i − Cout , i·Vout , i )

a Analytical procedures are described in Section 2.4 “Analytical Methodology”.

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Fig. 1. Schematic of the experimental design.

was carried out in 500 ml serum bottles. Granular sludge was used as an inoculum and the initial ratio of inoculum and substrate was set as 2.0 on the basis of the VS content. All analyses on the solid-state samples were conducted in triplicate. The standard method, EN-12457-2 (BSI, 2002), was used for all leachcing test. All eluates were measured for TOC, TC, CODCr, BOD5, NNH4+, TKN, N-NO2-, and N-NO3-. All of the results of the duplicate columns are reported as arithmetic means.

complete desorption equilibrium as reached. ads Adsorption capacity (Qeq , mg-NH4+/g-TS) and desorption capacity des , mg-NH4+/g-TS) of the aged refuse were calculated using the (Qeq following equations: ads ads Qeq = [C0 − (Ceq − Ce, blank )] ×

V0 M

des des ads Qeq = [Ceq V0 − Ceq (V0 − VR)]/ M

(3) (4)

where:

3. Results and discussion

C0 = initial concentrations of ammonium in contact with the aged refuse (mg-NH4+/L), ads = equilibrium concentrations of ammonium in contact with the Ceq aged refuse after the adsorption test (mg-NH4+/L), Ce,blank = equilibrium concentrations of ammonium in the blank (mg-NH4+/L), des = equilibrium concentrations of ammonium in contact with the Ceq aged refuse after the desorption test (mg-NH4+/L), V0 = initial concentration of ammonium in contact with the aged refuse (L), VR = volume of the liquid supernatant extracted from the bottle after adsorption equilibrium and substituted by the identical volume of 0.01 M CaCl2 solution (L), M = TS of aged refuse (g-TS).

3.1. Concentrations of nitrogen species in the effluents 3.1.1. N-NO3- and N-NO2 The variations in N-NO3- and N-NO2- for the reactors are presented in Fig. 2. As illustrated, all of the columns achieved a low nitrate concentration in the effluents at the first sampling which can be attributed to the adsorption onto the digestate and aged refuse (Fu et al., 2009; Peng et al., 2018a). N-NO3- concentrations in samples from the

2.4. Analytical methodology Liquid samples were analyzed following IRSA-CNR methods (IRSACNR, 2003). Effluent samples from the landfill simulation columns were periodically analyzed for TOC, TC, CODCr, BOD5, N-NH4+, TKN, N-NO2and N-NO3-. TOC and TC were determined through a TOC analysis meter (Shimadzu TOC-V CSN). CODCr was measured by digestion with potassium dichromate (K2Cr2O7) in an acid solution (Spectroquant® COD Cell Test and Spectroquant® 320) and determined by a COD analyzer (Spectroquant® NOVA 60). BOD5 was determined by measuring the oxygen consumption over five days using a dissolved oxygen probe. TKN was determined by a distillation (VELP® Scientifica UDK 127 Distillation Unit) and titration (HACH® Crison TitroMatic 2S) procedure after acid digestion. N-NH4+, N-NO2-, and N-NO3- were analyzed using a spectrophotometer (Shimadzu UV-1601). Solid digestate and aged refuse were characterized at the beginning and end of the experimental period. TS, VS, TOC, Respiration Index (RI4 and RI7), and TKN were measured. TS and VS were analyzed using Standard Analytical Methods (IRSA-CNR, 2003). TOC of the dried solid samples were determined using a TOC analyzer (Shimadzu TOC-V CSN). RI4 was determined through the cumulative oxygen consumption over four days in a Sapromat apparatus (Labortechnik, Germany). The Kjeldahl method was used to determine the TKN. To evaluate the anaerobic biostability of the original solid digestate and treated digestate, the biochemical methane potential (BMP) test for all solid samples

Fig. 2. Changes of N-NO3-, N-NO2-, N-NH4+, and N-NH4+/TKN of influent and effluent concentrations in landfill simulation bioreactors under 25 ± 2 °C and 0.76 g-N/(kg-VS day) of initial nitrate loading rate. 959

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columns without digestate addition (R0) fluctuated around the influent throughout the entire duration of the experiment, which suggested that the aged refuse has limited denitrification capacity. Zhao et al. (2007) also found that the aged refuse had a weak denitrification ability. On the other hand, the nitrate removal efficiency of both R5 and R15 were on average 43.8% and 59.6% (calculated by eq. (1)) higher than 23.2% of R0, respectively, which suggests that the digestate addition enhanced the denitrification capacity of the aged refuse in the landfill simulation columns. The optimal nitrate removal efficiency was achieved in R15, which suggests that the increase of the digestate addition can enhance the nitrate removal under the same nitrate loading rates. The nitrite concentration in the influent drastically dropped after 16 days because of the full nitrification of landfill leachate. Effluent nitrite concentrations in all columns were much lower relative to nitrate because of the low nitrite input. The significant removal of nitrite in all columns was observed at the first 16 days, which could be due to adsorption and denitrification. However, the nitrite concentrations in the effluents were comparable to the influent after day 16. Nitrite accumulation in R15 on day 37 indicated that the over-loading of nitrate could occur as nitrite might increase because of the carbon sources were limited (Oh and Silverstein, 1999).

3.1.2. N-NH4+ and N-NH4+/TKN Ammonia concentrations in the effluents of R0, R5, and R15 fluctuated around 40 mg/L which was similar to the ammonia concentrations in the influent, as shown in Fig. 2. In a previous study, the ammonia concentration in the effluent was approximately 2000 mg/L in a simulated landfill bioreactor only packed with solid digestate (Peng et al., 2018a). These results suggested that the aged refuse used in this study can decrease the ammonia release originating from the digestate since this type of waste has a high ammonia adsorption capacity (Chen et al., 2009; He et al., 2017). As shown in Fig. 3, the ammonia adsorption capacity was 0.49 ± 0.02 mg/g lower than the 0.83 mg/g reported by He et al. (2017), which could be explained by the differences in the characteristics of aged refuse. The ammonia adsorption by aged refuse was reversible as the desorption capacity was 0.12 ± 0.06 mg/g. The ammonia adsorption by aged refuse occurs when the aged refuse layer in the columns undergoes a “water distribution period” after feeding with nitrified leachate or recirculating the effluent leachate. After leachate drainage, the aged refuse layer will be porous and permeable, which could allow air to enter for ammonia nitrification. The average N-NH4+/TKN ratio for R0 was 0.459, which is comparable with 0.475 in the influents. The average N-NH4+/TKN ratios for R5 and R15 were 0.773 and 0.501, respectively. The higher N-

Fig. 4. Changes of TC, TOC, CODCr, and BOD5/CODCr of influent and effluent concentrations in landfill simulation bioreactors under 25 ± 2 °C and 0.76 gN/(kg-VS day) of the initial nitrate loading rate.

NH4+/TKN ratios for R5 and R15 could be contributed to the ammonia release from digestate solubilization. Italian environmental legislation (D. Lgs. 152/2006) regulates the final discharge of leachate into the surface waterbodies. Therefore, the solution suggested in this study can be regarded as a pretreatment to decrease the nitrogen loading up to 75.8% of the initial content. 3.2. Biological parameters of the effluents The variations of TOC were similar to those of TC among R0, R5, and R15 (Fig. 4). The low level of TOC in the effluents of R0 could be due to the TOC removal capacity of the aged refuse (Lei et al., 2007). As aged refuse works as a filter in the bottom layer, TOC in the effluents of both R5 and R15 were relatively lower than those of a previous study by Peng et al. (2018a), which suggests that the addition of the aged refuse layer impeded the TOC emission from the digestate to the effluent. Additionally, it was observed that the original dark brown nitrified leachate became a pale-yellow effluent. The change in color was due to the filtration effect of the aged refuse in the bottom layer. The TOC removal by aged refuse reactors with or without digestate resulted from the combined effect of biological removal and physico-chemical adsorption. Due to the TOC leaching from the digestate and depletion of the TOC removal capacity of aged refuse, the TOC reduction in all the six columns gradually decreased over time, as shown in Fig. 4. As shown in Fig. 4, all of the CODCr in the effluents were lower than those in the influent at the beginning, which could be attributed to the adsorption of aged refuse. The following CODCr increase could not be a result of the emissions from solid digestate but originated from the aged refuse since the CODCr of the control (R0) also increased. The effluent BOD5 in all treatments were lower than 20 mg-O2/L. The low BOD5 values, were due to the low BOD5 content in the influent. BOD5 from the leaching of solid digestate were presumably consumed in the aged refuse layers. The average influent BOD5/CODCr was 0.016 reflecting a leachate with low biodegradability (Sekman et al., 2011). Nonetheless,

Fig. 3. Ammonium adsorption/desorption kinetics of the aged refuse over 96 h. 960

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(e.g. CH4) emissions, which is a growing concern for digestate management (Baldé et al., 2016). The mass balances (Table 2) revealed that on average, 50.7%, 17.6%, and 4.8% of the carbon in R0, R5, and R15, respectively, was unaccounted for. However, the unaccounted TC or lose in R5 and R15 was mainly due to the TC reduction of solid digestate and influent leachate, as shown in Table 2 and Fig. 5. The TC reduction coincided with the degradation of organic matter in the solid digestate. On average, 58.5%, 51.6%, and 50.6% of the nitrogen in R0, R5, and R15 was lost, as reported in Table 2. Similar to TC in R0, the TN reduction in R0 might be due to the TN loss from aged refuse (Fig. 5). Conversely, the TN reduction of liquid leachate and solid digestate in R5 and R15 contributed to the TN losses through denitrification.

Table 2 Mass balance distribution in landfill simulation bioreactors. R0

Carbon (C) Cinput.l (C input from leachate) Cinitial.s-AR (Initial C in aged refuse) Cinitial.s-SD (Initial C in solid digestate) Coutput.l (C output from leachate) Cfinal.s-AR (Final C in aged refuse) Cfinal.s-SD (Final C in solid digestate) Cunaccounted (Unaccounted) Nitrogen (N) Ninput.l (N input from leachate) Ninitial.s-AR (Initial N in aged refuse) Ninitial.s-SD (Initial N in solid digestate) Noutput.l (N output from leachate) Nfinal.s-AR (Final N in aged refuse) Nfinal.s-SD (Final N in solid digestate) Nunaccounted (Unaccounted)

R5

R15

gC

%

gC

%

gC

%

8.5 63.9 0 5.7 30.0 0 36.7

11.7 88.3 0 7.8 41.5 0 50.7

12.8 51.1 56.8 9.4 51.3 38.8 21.25

10.6 42.3 47.1 7.7 42.5 32.2 17.6

19.5 32 119.1 14.7 38.3 109.4 8.2

11.4 18.7 69.8 8.6 22.4 64.1 4.8

8.5 15.6 0 6.95 3.05 0 14.1

35.3 64.7 0 28.85 12.7 0 58.5

12.7 12.5 10.2 7.8 3.6 5.75 18.25

35.9 35.3 28.8 22.1 10.1 16.2 51.6

19.5 7.8 21.3 8.7 2 13.3 24.6

40.1 16 43.8 17.9 4.1 27.4 50.6

3.4. Degradation of solid digestate and aged refuse The initial and final characteristics of the digestate and aged refuse from the landfill columns are presented in Table 3. Compared to the initial solid digestate, VS of the solid digestate at the end of the experimental period of R5 and R15 were on average reduced by 13.7% and 9.8%, respectively. These reductions in VS can be partly attributed to the organic matter transferring from the solid digestate to the aged refuse as the VS of the aged refuse in R5 and R15 slightly increased relative to the initial ones. Additionally, the eluate of BOD5 from the leaching tests of treated digestate in R5 and R15 decreased by 81.8% and 87.1%, respectively, compared to that of the initial digestate. The anaerobic degradation by methanation and anoxic activities of denitrifiers could contribute to the reduction of VS and BOD5. RI is generally used as a descriptor of the biostability of organic wastes. RI4 and RI7 clearly supported the degradation of the solid digestate as the RI of both the solid digestates and aged refuse in R5 and R15 was reduced. Compared to the initial RI7, the final RI7 of the solid digestate in R5 and R15 decreased by 33.9% and 32.6%, respectively. Meanwhile, the final RI7 of the aged refuse in R5 and R15 was reduced by 58.3% and 52.1%, respectively, relative to the initial conditions. As the ratio of the BOD5/ CODCr in all effluents remained relatively low, the reduction of RI was not a result of leaching biodegradable matter of aged refuse to the effluents through leachate recirculation. Therefore, deteriorating the biostability of the effluent leachate. Except for the aerobic stability, the anaerobic stability of the solid digestate was also achieved through the decrease of the BMP. Compared to the BMP of the initial solid digestate, a 31.5% and 35.9% reduction of the BMP reduction in R5 and R15 was achieved. This decrease in the BMP was proportional to the decrease in IR7, which suggests that both aerobic and anaerobic stability of solid digestate was obtained in the aged refuse reactors packed with solid digestate. The partial BMP reduction could be a result of the biodegradable organic carbon transferring from the digestate to the aged refuse as there was a slight increase in the BMP of the aged refuse samples (Table 3). According to the Italian landfill regulations (D. Lgs. 36/2003), the mixture of solid digestate and aged refuse can be used as a temporary top cover before the final closure of the landfill. An alternative option is to use the solid digestate in the portions of final top covers of closed landfills that use leachate recirculation to facilitate in-situ leachate pretreatment.

the average BOD5/CODCr of the effluent in R0, R5, and R15 were 0.008, 0.005, 0.003, respectively. The lower BOD5/CODCr in the effluents suggested that the organics in the nitrified leachate was slightly degraded in the columns. 3.3. Carbon and nitrogen mass balance Table 2 and Fig. 5 illustrates the mass balance of carbon and nitrogen. Within the first week, biogas production was observed in R15 (data not shown) but after continued operations there was no significant biogas was generated due to the inhibitory effect of high nitrate concentrations. Because of the possible diffusion of atmospheric nitrogen gas into the gas collecting bags and the minimal amount of biogas production, nitrogen gas collection for denitrification quantification was difficult in this study (Berge et al., 2006). The low methane emissions from the solid digestate due to the inhibition from high nitrate concentrations might be a benefit for avoiding greenhouse gas

3.5. Enhancement of denitrification capacity of aged refuse without ammonia emission Table 4 represented the average nitrate removal rate (ANRR) (Calculated by eq. (2)) of the different studies. Although partially degraded aged refuse has a strong denitrification capacity (Chen et al., 2009), the aged refuse in this study (40-year old) has only a 4.2 mg N/(kg-TS day) capacity for denitrification due to the lack of biodegradable organic matter (Table 3). Nonetheless, the aged refuse excavated from a 40-year old landfill can be used as the biofilter in the column to reduce ammonia concentrations (Fig. 2) in the effluent and improve the hydraulic

Fig. 5. Carbon and nitrogen partitions in landfill simulation bioreactors. 961

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Table 3 Characteristics of solid digestate, aged refuse, and their eluates from leaching tests at the start and the end of the experiment. Solid Digestate Start

Solid samples TS (%) VS (%TS) RI4 (mg O2/g-TS) RI7 (mg O2/g-TS) TKN (g N/kg-TS) TOC (g C/kg-TS) BMP (L-CH4/kg-VS) Eluates from leaching test pH TC (mg C/L) TOC (mg C/L) CODCr (mg O2/L) BOD5 (mg O2/L) N-NH4+ (mg N/L) N-NO3- (mg N/L) N-NO2- (mg N/L)

Aged Refuse R5

R15

Start

End

End

19.4 ± 0.1 62.0 ± 0.3 13.8 ± 0.4 22.4 ± 1.0 51.0 ± 0.2 285 ± 1.2 27 ± 0.3

22.2 53.5 9.7 14.8 26.4 180 18.5

19.9 55.9 10.1 15.1 31.2 258 17.3

8.73 2670 725 964 452 801 <5 < 0.2

8.77 1144 211 432 82 < 30 11 0.8

8.41 1595 359 732 58.4 < 30 16 0.5

R0

R5

R15

End

End

End

94.7 ± 0.7 3.5 ± 0.2 2.4 ± 0.2 4.8 ± 1.9 3.3 ± 0.2 14 ± 0 0.015

89.9 2.4 2.0 3.0 0.6 6 2.9

86.3 3.4 1.6 2.0 1.0 14 3.9

82.3 4.1 1.6 2.3 0.9 18 5.1

8.06 31 18 37 <5 < 30 12 0.5

8.34 114 55 112 <5 < 30 10 0.2

8.29 139 50 101 <5 < 30 11 0.2

7.58 154 41 85 <5 < 30 12 0.2

solid digestate integrated with the aged refuse could be superior to the columns with only aged refuse.

conditions of leachate recirculation since no clogging was observed in the R0 columns. Based on the TS in the landfill simulation columns, the ANRR of R5 and R15 were 3.4 and 10.0 times higher than that of R0, respectively. After deducting the contribution of aged refuse on the nitrate removal, the ANRR of the solid digestate in R5 and R15 were 203 mg-N/(kg-TS day) and 257 mg-N/(kg-TS day), which were comparable to the 256 mg-N/(kg-TS day) achieved in reactors packed with a single solid digestate layer (Peng et al., 2018a). Despite of the high denitrification capacity, the ammonia leaching from the solid digestate may be a drawback that could impede the application of landfilling digestate. Compared to the ammonium concentrations in the influents, no significant increase of ammonium concentrations in the effluents was observed during the operation (Fig. 2). However, ammonium concentrations in the eluate of the treated digestates in R5 and R15 were lower than 30 mg-N/L while ammonium concentrations in the eluate of the original solid digestate was as high as 801 mg-N/L. Unlike the high ammonia leaching in a column only containing digestate (Peng et al., 2018a), the ammonia emissions were dramatically mitigated by applying the excavated aged refuse in this study. The significant enhancement in the denitrification capacity through the addition of digestate to aged refuse will benefit the in-situ nitrogen removal from mature landfill leachate. It can be concluded that the

4. Conclusions After 100 days of recirculating nitrified leachate in the aged refuse bioreactors packed with solid digestate, approximately 4.2 mg N/(kg-TS day), 14.1 mg N/(kg- TS day), and 42.1 mg N/(kg-TS day) of the nitrate removal rates were separately achieved in R0, R5, and R15, respectively. Additionally, the aerobic biostability and anaerobic biostability of the treated solid digestate in R5 and R15 increased by 31.5–35.9%. These results suggest that the solid digestate with a high denitrification capacity could be applied as a denitrification layer in old landfill sites to facilitate in-situ nitrogen removal from mature landfill leachate which also results in a stabilized digestate. Further studies are needed to develop the kinetics and isotherm models of the adsorption/desorption with aged refuse mixed with or without digestate. Acknowledgement Wei Peng gratefully acknowledges the financial support of the China Scholarship Council (CSC) (No. 201506260166). The authors would also like to thank Miss. Valentina Menapace and Mrs. Annalisa Sandon

Table 4 Nitrate removal rates of landfill simulation columns with different types of wastes. Waste Types

Nitrate Content (mg-N/L)

ANRRa

Reference

140 28.6 163.2b 72.0b 26.4b 256 199c

– – – – – 256 199c

Price et al. (2003) Zhong et al. (2009) Chen et al. (2009) Chen et al. (2009) Chen et al. (2009) Peng et al. (2018a) Peng et al. (2018b)

4.2 14.1 42.1

– 203 257

This study

ANRR mg N/(kg-TS day)

Composed MSW MSW 1-year-aged MSW 6-year-aged MSW 11-year-aged MSW Solid Digestate (SD) Solid Digestate 100% Aged Refuse (AR) 95% AR+5% SD 85% AR+15% SD a b c

400 200–2200 1000 1000 1000 1438 1004 (N-NO2-) +428 (N-NO3-) 517–1332 517–1332 517–1332

The ANRR was calculated based on the TS of the solid digestate. The results reflect the deduction of the contribution by aged refuse. The data were calculated by the authors. The ANRR here can be considered as Average Nitrogen Removal Rate. 962

Journal of Environmental Management 232 (2019) 957–963

W. Peng et al.

for the help with sample analysis and Razieh Rafieenia for improving the manuscript. Special thanks to Mrs. Qiao Li for designing the graphical abstract.

OECD, 2000. OECD Guideline for the Testing of Chemicals: Adsorption - Desorption Using a Batch Equilibrium Method, OECD Guidelines for the Testing of Chemicals Guideline for the Testing of Chemicals. http://doi.org/10.1787/9789264069602-en. Oh, J., Silverstein, J., 1999. Acetate limitation and nitrite accumulation during denitrification. J. Environ. Eng https://doi.org/10.1061/(ASCE)0733-9372(1999) 125:3(234). Peng, W., Pivato, A., 2017. Sustainable management of digestate from the organic fraction of municipal solid waste and food waste under the concepts of back to earth alternatives and circular economy. Waste Biomass Valori (in press). https://doi.org/ 10.1007/s12649-017-0071-2. Peng, W., Pivato, A., Lavagnolo, M.C., Raga, R., 2018a. Digestate application in landfill bioreactors to remove nitrogen of old landfill leachate. Waste Manag. 74, 335–346. https://doi.org/10.1016/j.wasman.2018.01.010. Peng, W., Pivato, A., Cerminara, G., Garbo, F., Raga, R., 2018b. Denitrification of mature landfill leachate with high nitrite in simulated landfill columns packed with solid digestate from organic fraction of municipal solid waste. Waste Biomass Valoriz. https://doi.org/10.1007/s12649-018-0422-7. (in press). Peng, Y.Z., Ma, Y., Wang, S.Y., 2007. Denitrification potential enhancement by addition of external carbon sources in a pre-denitrification process. J. Environ. Sci. 19 (3), 284–289. https://doi.org/10.1016/S1001-0742(07)60046-1. Price, G.A., Barlaz, M.A., Hater, G.R., 2003. Nitrogen management in bioreactor landfills. Waste Manag. 23, 675–688. https://doi.org/10.1016/S0956-053X(03)00104-1. Saveyn, H., Eder, P., 2014. End-of-waste Criteria for Biodegradable Waste Subjected to Biological Treatment (Compost & Digestate): Technical Proposals. Publ. Off. Eur. Union, Luxemb. http://publications.jrc.ec.europa.eu/repository/bitstream/ JRC87124/eow%20biodegradable%20waste%20final%20report.pdf. Sekman, E., Top, S., Varank, G., Bilgili, M.S., 2011. Pilot-scale investigation of aeration rate effect on leachate characteristics in landfills. Fresenius Environ. Bull. 20, 1841–1852. Sisto, R., Sica, E., Lombardi, M., Prosperi, M., 2017. Organic fraction of municipal solid waste valorisation in southern Italy: the stakeholders' contribution to a long-term strategy definition. J. Clean. Prod. 168, 302–310. https://doi.org/10.1016/J. JCLEPRO.2017.08.186. Sun, X., Zhang, H., Cheng, Z., 2017. Use of bioreactor landfill for nitrogen removal to enhance methane production through ex situ simultaneous nitrification -denitrification and in situ denitrification. Waste Manag. 66, 97–102. https://doi.org/10.1016/j. wasman.2017.04.020. Tampio, E., Salo, T., Rintala, J., 2016. Agronomic characteristics of five different urban waste digestates. J. Environ. Manag. 169, 293–302. https://doi.org/0.1016/J. JENVMAN.2016.01.001. Tremier, A., Buffet, J., Daumoin, M., Corrand, V., 2013. Composting as digestate posttreatment: composting behaviour and gaseous emissions of three types of digestate compared to non-digested waste. In: 15th RAMIRAN International Conference. Recycling of Organic Residues for Agriculture: from Waste Management to Ecosystem Services, Jun 2013, Versailles, France, pp. 4. Valencia, R., van der Zon, W., Woelders, H., Lubberding, H.J., Gijzen, H.J., 2011. Anammox: an option for ammonium removal in bioreactor landfills. Waste Manag. 31, 2287–2293. https://doi.org/10.1016/j.wasman.2011.06.012. Xie, B., Lv, Z., Hu, C., Yang, X., Li, X., 2013. Nitrogen removal through different pathways in an aged refuse bioreactor treating mature landfill leachate. Appl. Microbiol. Biotechnol. 97, 9225–9234. https://doi.org/10.1007/s00253-012-4623-x. Yan, F., Jiang, J., Zhang, H., Liu, N., Zou, Q., 2018. Biological denitrification from mature landfill leachate using a food-waste-derived carbon source. J. Environ. Manag. 214, 184–191. https://doi.org/10.1016/J.JENVMAN.2018.03.003. Zhao, Y., Li, H., Wu, J., Gu, G., 2002. Treatment of leachate by aged-refuse-based biofilter. J. Environ. Eng. 128, 662–668 https://doi.org/10.1061/(ASCE)07339372(2002)128:7(662). Zhong, Q., Li, D., Tao, Y., Wang, X., He, X., Zhang, J., Zhang, J., Guo, W., Wang, L., 2009. Nitrogen removal from landfill leachate via ex situ nitrification and sequential in situ denitrification. Waste Manag. 29, 1347–1353. https://doi.org/10.1016/j.wasman. 2008.10.014.

References Baldé, H., VanderZaag, A.C., Burtt, S.D., Wagner-Riddle, C., Crolla, A., Desjardins, R.L., MacDonald, D.J., 2016. Methane emissions from digestate at an agricultural biogas plant. Bioresour. Technol. 216, 914–922. https://doi.org/10.1016/J.BIORTECH. 2016.06.031. Berge, N.D., Reinhart, D.R., Dietz, J., Townsend, T., 2006. In situ ammonia removal in bioreactor landfill leachate. Waste Manag. 26, 334–343. https://doi.org/10.1016/j. wasman.2005.11.003. Binner, E., Böhm, K., Lechner, P., 2012. Large scale study on measurement of respiration activity (AT4) by Sapromat and OxiTop. Waste Manag. 32 (10), 1752–1759. https:// doi:10.1016/j.wasman.2012.05.024. Bolzonella, D., Pavan, P., Mace, S., Cecchi, F., 2006. Dry anaerobic digestion of differently sorted organic municipal solid waste: a full-scale experience. Water Sci. Technol. 53, 23–32. https://doi.org/10.2166/wst.2006.232. British Standards Institution (BSI), 2002. BS EN 12457 Characterisation of Wasteleaching-compliance Test for Leaching of Granular Waste Materials and Sludges. British Standards Institution (BSI), London, UK. Chen, Y., Wu, S., wei, Wu, xiang, W., Sun, H., Ding, Y., 2009. Denitrification capacity of bioreactors filled with refuse at different landfill ages. J. Hazard. Mater. 172, 159–165. https://doi.org/10.1016/j.jhazmat.2009.06.150. D. Lgs. 36/2003 D. Lgs. 36/2003 (Italian Legislative Decree 36/2003). Attuazione della direttiva 1999/31/CE relativa alle discariche di rifiuti. http://www.camera.it/ parlam/leggi/deleghe/03036dl.htm (accessed 26 October 2018). D. Lgs. 152/2006 D. Lgs. 152/2006 (Italian Legislative Decree 152/2006). Norme in Materia Ambientale. http://www.camera.it/parlam/leggi/deleghe/06152dl.htm (accessed 26 October 2018). Fu, Z., Yang, F., An, Y., Xue, Y., 2009. Characteristics of nitrite and nitrate in situ denitrification in landfill bioreactors. Bioresour. Technol. 100, 3015–3021. https://doi. org/10.1016/j.biortech.2008.12.034. Fuldauer, L.I., Parker, B.M., Yaman, R., Borrion, A., 2018. Managing anaerobic digestate from food waste in the urban environment: evaluating the feasibility from an interdisciplinary perspective. J. Clean. Prod. 185, 929–940. https://doi.org/10.1016/J. JCLEPRO.2018.03.045. He, P.J., Shao, L.M., Guo, H.D., Li, G.J., Lee, D.J., 2006. Nitrogen removal from recycled landfill leachate by ex situ nitrification and in situ denitrification. Waste Manag. 26, 838–845. https://doi.org/10.1016/j.wasman.2005.11.014. He, Y., Li, D., Zhao, Y., Huang, M., Zhou, G., 2017. Assessment and analysis of aged refuse as ammonium-removal media for the treatment of landfill leachate. Waste Manag. Res. 35, 1168–1174. https://doi.org/10.1177/0734242X17730136. IRSA-CNR, 2003. Metodi analitici per le acque. Manuali e Linee Guida. http://www. isprambiente.gov.it/it/pubblicazioni/manuali-e-linee-guida/metodi-analitici-per-leacque, Accessed date: 26 July 2018. 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. Crit. Rev. Environ. Sci. Technol. 32 (4), 297–336. https://doi.org/10.1080/ 10643380290813462. Lei, Y., Shen, Z., Huang, R., Wang, W., 2007. Treatment of landfill leachate by combined aged-refuse bioreactor and electro-oxidation. Water Res. 41, 2417–2426. https://doi. org/10.1016/j.watres.2007.02.044. Li, Y., Park, S.Y., Zhu, J., 2011. Solid-state anaerobic digestion for methane production from organic waste. Renew. Sustain. Energy Rev. 15, 821–826. https://doi.org/10. 1016/j.rser.2010.07.042. Lubberding, H.J., Valencia, R., Salazar, R.S., Lens, P.N.L., 2012. Release and conversion of ammonia in bioreactor landfill simulators. J. Environ. Manag. 95, S144–S148. https://doi.org/10.1016/J.JENVMAN.2010.08.030.

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