Landfill leachate phytotreatment with sunflowers grown in a waste-derived substrate

Objectives of the research
Sunflowers, irrigated with old landfill leachate, were cultivated in a waste-derived substrate: a mixture of sand from sweeping of streets and compost containing sewage sludge. Vertical and horizontal flow units were connected in series to verify whether this could enhance the removal efficiencies, especially nitrogen removal, and volume reduction. The influence of leachate irrigation on oil production, in view of its conversion to biodiesel, was also assessed. Sand from sweeping of streets and compost containing sewage sludge were used to close the cycle of waste management: they can be valorized as new resources for the production of renewable energy, in the framework of the circular economy concept.

1. MATERIAL AND METHODS
1.1. Research program
Six reactors, filled with the growing substrate, were placed in a controlled climatic chamber. Three reactors were operated as Vertical (V) flow systems; three as Horizontal (H) sub-superficial flow systems. Vertical flow units were named V1, V2, and VC; horizontal flow units were named H1, H2, and HC. V and H units were connected in series to promote the occurrence of nitrification and denitrification (Wang et al., 2017; Vymazal, 2013). V1 and V2 were irrigated with diluted leachate and the effluents were used to feed H1 and H2, respectively; VC was fed with tap water throughout the entire duration of the experiment and the effluent was used to feed HC: they were used as plants and substrate control units (Table 1). The vertical flow units were fed twice per day and drained once per week; the horizontal flow units were fed and drained once per week (irrigation provided after the total weekly drainage).
After an initial acclimation period, lasting 7 days, in which all the plants were irrigated with tap water, V1 and V2 were irrigated with increasing leachate dosages (up to a maximum nitrogen influent concentration of 370 mgN/L) (Table 5.2). Dilution rates were set taking into account that the maximum tolerable nitrogen concentration (N was used as reference parameter) for sunflowers is 400 mgN/L (Garbo et al., 2017; Lavagnolo et al., 2016).
In H units, the initial acclimation period with tap water lasted for 14 days as during week 1 the effluents of vertical flow units were not available.
After drainage, effluent samples were stored at -20°C and subsequently analyzed for the following parameters: Total Kjeldhal Nitrogen (TKN), ammonia, nitrate, nitrite, phosphorous and COD. Once clear senescence was reached, plants were harvested, oven dried at 60 °C, and weighed. Nitrogen and heavy metals content in leaves, roots, stems and seeds was determined. At the end of the experiment, the substrate was sampled from each experimental unit: samples were air-dried and both TKN and nitrate contents were determined to complete the final nitrogen mass balance.

Table 1. Set up of experimental reactors

Experimental unitIrrigationColumn descriptionIrrigation frequencyDrainage frequency
V1

Diluted leachateExperimental unitDailyOnce per week
V2Diluted leachateExperimental unitDailyOnce per week
VCTap waterPlants and substrate control unitDailyOnce per week
H1V1 effluentExperimental unitOnce per weekOnce per week
H2V2 effluentExperimental unitOnce per weekOnce per week
HCVC effluentPlants and substrate control unitOnce per weekOnce per week

Table 2. Leachate dosages and contaminants concentrations in the feeding of V1 and V2

WeekFeeding quality (V1 and V2)TKN influent concentration
(mgN/L)
COD influent concentration
(mgO2/L)
P influent concentration (mgP/L)
190% tap water + 10% leachate1851432.5
2-1280% tap water + 20% leachate3702865

1.2. Equipment

300 L HDPE tanks, sized 95 x 50 x 65 cm, were used for the experiment.
In V units, the growing medium was made up of a 35 cm substrate layer, laying above a gravel drainage layer, 10 cm thick, composed by medium-size gravels (d=20-30mm) (Fig. 1a). Six plants, organized in two lines of three sunflowers each, were grown in each reactor (Fig.1b). Irrigation was provided over the entire surface to simulate the vertical flux from the top the bottom. A step-feeding strategy (irrigation every 12 hours) was applied to allow the oxygen intrusion, essential for the nitrification process (Pellissari et al., 2016).
In H units, two vertical drainage layers were placed upstream and downstream the substrate, composed by coarse (40-60 mm) and medium (20-30 mm) gravels, respectively (Fig. 2a). Four plants were grown in each reactor to maintain the same plant density of vertical flow reactors (Fig. 2b) The effluent of V units was distributed homogeneously in the upstream zone of H reactors (Fig. 3) to allow a plug-flow movement of water, driven by the hydraulic gradient, and favour the occurrence of denitrification (Pellissari et al., 2016).
The V-H connection in series allowed to maintain an Hydraulic Retention Time (HRT) of 11 days, significantly longer than the suggested minimum of 7 days (Garbo et al., 2017; Sawaittayothin and Polprasert 2007).
The units were placed into a climatic chamber in which a 14-hour photoperiod with 300 µmol·m-2·s-1 light intensity was imposed. Average air temperature was maintained at 24 °C (MIN=17 °C, MAX=35 °C).

Fig. 1. Cross section (a) and plan view (b) of vertical flow units (green dots represent the position of sunflowers)

Fig. 2. Cross section (a) and plan view (b) of horizontal sub-superficial flow units (green dots represent the position of sunflowers)

Fig. 2. Cross section (a) and plan view (b) of horizontal sub-superficial flow units (green dots represent the position of sunflowers)

Fig. 3. Sketch of the reactors connected in series

1.3. Substrate

A mixture of compost containing sewage sludge (25% on volume basis) and sand from sweeping of streets (75% on volume basis) was used as medium to fill the experimental units. The use of a nutrients rich material (compost) and a material with high porosity (sand) provides the optimal living conditions for plants and microorganisms involved in the phytotreatment process (Stottmeister et al., 2003; Weerakoon et al., 2013; Lavagnolo et al., 2016; Garbo et al., 2017).
Sand from sweeping of streets was collected from a plant in which it is mechanically separated from other wastes and then washed to remove residual hydrocarbons and heavy metals. Leaching tests were performed according to the standard UNI EN 12457-2 (L/S was brought to 10 L/kgTS, mixed for 24 h and filtered at 45 μm) to check wheter such contaminants were still present, but the concentrations in the eluate were always below the detection limits.
Compost containing sewage sludge was collected from a composting plant located in the Veneto Region (Italy) and complied with the regional requirements for high quality compost, defined in DGR 568/2005. It was produced from dried sewage slude (1/8 by volume) and shredded green waste (7/8 by volume).
The texture of the mixture (Fig. 4) was determined with the Bouyoucos method (Bouyoucos, 1962) and, according to the soil taxonomy proposed by the USDA (USDA-NRCS, 1999), was classified as sand (94% sand, 2% silt, 4% clay). The substrate soil chemical characterization is reported in Table 3. Heavy metals concentration in the substrate was similar to that of the experiment reported by Lavagnolo et al. (2016), in which sandy and clayey soils were used.

Table 3. Chemical characterization of the mixture (DM = Dry Matter)

ParameterUnitValue
VS

% on DM

15
TOC
% on DM
10.3
TKN
mgN/kgDM
1439
NO3-mgNO3-N/kgDM
24
Crmg/kgDM
41.3
Cumg/kgDM
33
Nimg/kgDM
13
Znmg/kgDM
82
Femg/kgDM
8144
Mnmg/kgDM307

Fig. 4. Substrate texture, classified according to USDA standards (USDA-NRCS, 1999).

1.4. Landfill leachate
The leachate used for the experiment was collected in a closed landfill located in the North of Italy, in which the residual waste fraction of Municipal Solid Waste (MSW) was disposed. The chemical characteristics of leachate are reported in Table 4. NO2- was absent, while NO3- was 5 mgNO3-N/L: TKN was representative of almost all the total influent nitrogen.

Table 4. Landfill leachate characteristics

Parameter Unit Value
TS mg/L7771
VS mg/L 2525
COD mgO2/L1430
TOC mgC/L 1145
BOD5 mgO2/L495
TKN mgN/L 1849
NH4+-N mgNH4-N/L 1714
NO2- mgNO2-N/L 0
NO3- mgNO3-N/L 5
PTOT mgP/L 25.74
pH - 8.5
Alkalinity mgCaCO3/L 14610
Conductivity mS/cm 15.5
Cdμg/L<10
Cr μg/L 751
Cu μg/L 52
Fe μg/L3850
Mn μg/L176
Ni μg/L152
Pb μg/L <10
Zn

μg/L115

1.5. Analytical methods
Leachate and all the liquid samples were analyzed according to the CNR-IRSA standard Italian analytical methods (CNR-IRSA, 29/2003). BOD5 was measured with a respirometer apparatus (Sapromat E); ammonia was evaluated by means of a distillation-titration procedure; TKN was measured through a distillation-titration procedure after an acid digestion phase; dissolved components (nitrite and nitrate) were determined using a UV-VIS spectrophotometer (Shimadzu UV-1601). The same spectrophotometer was used to detect total phosphorus after sample digestion. Nitrogen content in soil and plants at the end of the trial was analyzed according to CNR-IRSA standard Italian analytical guidelines for solid specimens (CNR-IRSA, 64/1986). Oil seeds were analyzed in oil content and Free Fatty Acids (FFA) quality according to the European standards (Reg. CEE 2568/2011, G.U. CEE L248/91 All. II, Reg. CE 702/2007, G.U. CE L161/2007).

2. RESULTS AND DISCUSSION
2.1. Sunflowers growth
Plants grew vigorously and uniformly throughout all experimental units. Blooming occurred during week 7, followed by a sudden senescence. Sunflowers grown in leachate irrigated reactors showed no symptoms of toxicity and were not affected by the presence of leachate in the irrigation water.
Maximum average height of plants ranged between 1.4 and 1.6 m in both vertical and horizontal flow units, with the only exception of plants grown in VC which reached heights close to 2.00 m (Fig. 5). Results reported in Table 5 indicate that sunflowers grown in vertical flow reactors developed larger biomasses compared to sunflower cultivated in horizontal flow reactors, as already observed by Garbo et al. (2017), with a peak of 123.02 gTS/plant observed in VC.
Final distribution among the plants components (Fig. 6), however, did not reveal any difference among plants grown in V and H units, as well as between leachate irrigated reactors and controls.

a)

b)

Fig. 5. Average height of sunflower grown vertical (a) and horizontal (b) flow units

Table 5. Final dry weights among the different plants components. Results expressed as gDM/plant (DM = Dry Matter)

 V1  V2 VC 
Average RangeAverageRangeAverageRange
Stems 41.2219.04-69.7438.9015.31-63.19 59.4847.08-99.71
Leaves 37.37 18.69-57.67 28.61 16.69-40.5240.7324.11-73.69
Roots 5.28 1.93-13.56 6.28 1.75-9.65 17.919.67-30.66
Seeds 7.554.25-13.16 8.104.43-14.05 4.90 2.53-8.09
Total biomass 91.42 81.89 123.02
H1 H2 HC
Average RangeAverage RangeAverage Range
Stems 32.009.48-58.9034.70 27.47-47.33 33.41 18.86-58.55
Leaves 19.56 5.23-34.9720.78 18.24-27.65 16.789.60-29.12
Roots 2.251.22-3.43 3.36 2.21-3.92 3.62 1.22-3.95
Seeds 7.47 2.57-13.16 10.325.17-16.66 8.18 2.34-15.92
Total biomass 61.2869.1961.99

Fig. 6. Final dry weight distribution among the different plant components in vertical (a) and horizontal (b) flow units

where:
ET = evapo-transpiration (%)
VIN = inlet volume (L/week)
VOUT = outlet volume (L/week)

The feeding volumes and the volumes exiting each experimental reactor are reported in Fig. 7.

Fig. 7. Influent and effluent volumes in systems V1-H1, V2-H2, and VC-HC

The inlet volumes were increased gradually to adapt the plants to the presence of contaminants, starting from 25 L/week (approximately 8.3 mm/d) during the first and second weeks, up to 50 L/week (16.7 mm/d) from week 5: the maximum value was maintained till the end of the experiment. Similar hydraulic loading rates have been applied by Garbo et al. (2017) and Ogata et al. (2015). The effluent volumes were extremely low (0-20 L in V units, 0-5 L in H units) until week 7, when plants blooming occurred. Similar evapo-transpiration rates were obtained by Bialoweic et al. (2014, 2007), but they used reed which are known to have a marked transpiration ability. After blooming, the outlet volumes increased especially in V reactors (effluent volume up to 40 L/week), as a result of a reduced plants water requirement during the senescence phase, as already reported by Lavagnolo et al. (2016). Anyhow, considering the cumulative performances of the V-H systems, evapo-transpiration was always above 80%: similar water losses were observed by Albuquerque et al. (2009) and Bialowiec et al. (2006) in experiments in which vertical and horizontal flow units with hydrophytes were connected in series. No differences were detected between leachate irrigated units and controls, indicating that leachate addition did not influence evaporation from the soil and transpiration from plants.

2.3. Nitrogen removal
Due to a strong evapo-transpiration effect, evaluation of contaminants removal should take into account the reduction of the liquid volumes, therefore Removal Efficiencies (RE) of the whole systems (V+H) were based on weekly loads:

where
VIN = influent volume in vertical flow units (L/week)
VOUT = effluent volume in horizontal flow units (L/week)
CIN = influent concentration in the irrigation of vertical flow units (mg/L)
COUT = effluent concentration in the horizontal flow units (mg/L)

The influent and effluent concentrations of the monitored nitrogen compounds are reported in Fig. 8. NO2- is not shown because always below the detection limits.

Fig. 8. Influent and effluent concentrations of nitrogen compounds in the experimental reactors

In V1 and V2, TKN and ammonia effluent concentrations decreased rapidly from week 1 to week 3 and then remained both below 50 mgN/L, even when the maximum leachate dose was applied (370 mgN/L in the influent). An anomalous increase of NO3- concentrations (up to approximately 1000 mgNO3-N/L) was observed from week 1 to week 3 in the effluents of these units: this phenomenon occurred also in VC and could be related to the leaching of nitrates from the substrate, due to the frequent irrigation procedure. After the peak detected in week 3, nitrate concentrations decreased rapidly reaching values close to 0 mgN/L in VC, while in V1 and V2 the concentrations were almost equal or slightly exceeded the influent nitrogen concentration, suggesting that leachate irrigation likely stimulated the growth of nitrifying bacteria which oxidized not only the influent nitrogen but also nitrogen compounds which were already contained in the substrate. Focusing on horizontal flow units, the influent was rich in nitrates produced by the vertical flow reactors. Denitrification was observed, as NO3- was always below the corresponding influent concentrations. Effluent nitrate concentrations remained below 100 mgN/L in both H1 and H2 till the flowering point (which occurred during week 7), then increased up to 300 mgNO3-N/L but still below the influent concentrations, probably due to a reduced nitrate uptake capacity of plants during the senescence phase. Nevertheless, the reactors proved to be extremely efficient in removing the total nitrogen (TN) supplied with the landfill leachate, even after the flowering point, as shown by Fig. 9 in which the performances of the whole leachate irrigated systems (V+H), calculated according to Eq. (2), are reported. Removal efficiencies based on weekly loads were always above 80%, aligned with the best performances reported in literature (Cheng and Chu, 2011, Garbo et al., 2017; Lavagnolo et al., 2016).

Fig. 9. Influent and effluent total nitrogen weekly loads and removal efficiency (RE) in the leachate irrigated systems V1-H1 and V2-H2

2.4. Sunflower seeds characterization
At the end of the experimental trial, sunflowers seeds were analyzed to verify the effect of leachate irrigation on their composition, thus on biodiesel potential production. In particular, oil content and Free Fatty Acid (FFA) composition have been analyzed and compared with values detected in sunflowers grown in the controls and with values found in plants cultivated in traditional ways (Table 6).
Plants grown in controls VC and HC showed the highest oil content, but the values of the plants grown in the leachate irrigated reactors were found in the upper part of the range suggested by Karmakar et al. (2010): leachate irrigation and the presence of a waste-derived substrate did not inhibit the oil production.
Biodiesel is produced through a transesterefication process that does not alter the oil fatty acids composition, which affects many critical parameters of the biodiesel. The most important parameter is the Cetane number. High Cetane numbers have been associated to less highly polyunsaturated components (Cx:2,3) and more saturated fatty acids (Cx:0; Cx:1) in the oil (Karmakar et al., 2010; Ramos et al., 2009). Results have been plotted in Fig. 10: according to Ramos et al. (2009), vegetable oils falling in the green area satisfy the technical requirements of standard EN 14214:2008+A1:2009 for biodiesel production and utilization in engines. Oil extracted from seeds of sunflowers grown in conventional ways are far from the optimal area, while the oil extracted from the seeds obtained in this study (red area) is closer to the green part of the graph: the cultivation in a waste-derived substrate, even if combined with landfill leachate irrigation, registered a particular positive composition in view of renewable energy production. In the experiment reported by Lavagnolo et al. (2016), in which sunflowers, grown in sandy and clayey soils, were irrigated with old landfill leachate, FFA analysis revealed a higher amount of monounsaturated acids: this is probably due to the use of different cultivars, which may result in significant differences in the oil composition.

Table 6. Oil content in seeds and Free Fatty Acids content in the oil

 V1 V2 VC H1H2HCLiterature data (Karmakar et al., 2010)
Oil content in seeds (%) 33.7 30.835.4731.9 34.9 40.0 25-35
Free fatty acids composition (% on oil)
Saturated (Cx:0) 11.29 11.6810.819 11.6611.4711.079.00-17.00
Monounsaturated (Cx:1)36.47 35.75 39.080 46.96 39.67 42.92 19.00-34.00
Polyunsaturated (Cx:2,3) 52.23 52.56 50.102 41.37 48.8546.00 48.00-67.00

Fig. 10. Oil (or biodiesel) characterization by monounsaturated, polyunsaturated and saturated fatty acids. Green area: biodiesel that satisfied EN 14214:2008+A1:2009. Red area: oil from seeds obtained in this experiment (adapted from Barros et al., 2009).

3. CONCLUSION
Landfill leachate phytotreatment using sunflowers grown in a waste-derived substrate proved to be feasible under lab-scale conditions, as no inhibition was detected, while seeds oil characterization revealed a favorable composition in view of the biodiesel production. The use of a vertical flow unit, followed by an horizontal one, resulted to be effective in removing nitrogen due to nitrification and denitrification.

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