Leachate phytotreatment with Pennisetum Purpureum (elephant grass) in view of its cultivation on the top of closed landfills

Objectives of the research

Despite the gradual decreasing of waste landfilling in most of industrialized countries, leachate treatment is still considered one of the main issues in landfills management. Economically and environmentally sustainable solutions are in growing demand and leachate phytotreatment with energy crops seems to be a suited solution. This study evaluated the phytotreatment capacity of Pennisetum Purpureum (elephant grass). The plants were grown on reactors which were designed to assure a vertical sub-superficial irrigation flow. Reactors were irrigated with landfill leachate produced by a landfill in operation; the pollutants loads were increased over time. The removal efficiencies of TKN, ammonia, and COD were investigated. Effects of leachate irrigation on growth and accumulation of heavy metals in the biological tissues were also investigated.

1. MATERIALS AND METHODS
1.1. Research program
Five columnar reactors were placed in a controlled greenhouse. All the experimental units were characterized by vertical sub-superficial irrigation. Pennisetum Purpureum plants were grown in three units called P1, P2, and PC (three plants in each unit). P1 and P2 were irrigated with diluted landfill leachate while PC was irrigated with tap water and synthetic nitrogen. Two additional reactors without plants were irrigated with landfill leachate (CL) and tap water (CW). CL was used as control to investigate the role of the substrate in the leachate treatment, CW to detect potential leaching of substances from the substrate itself (Table 1)

Table 1. Set up of experimental units

Experimental unit IrrigationColumn description Plant species
P1Leachate
Experimental column
Pennisetum P.
P2Leachate
Experimental columnPennisetum P.
PCTap water + synthetic nitrogenPlant control column
Pennisetum P.
CLLeachateSoil control column
-
CWTap waterSoil control column
-

All the plants were irrigated with tap water during the initial 15 days. Then, the reactors P1, P2 and CL were fed with increasing diluted leachate dosages up to 300 mgN/L (nitrogen was the reference parameter) (Table 2). This value was set in order to avoid phyto-toxicity phenomena, which may occur on plants exposed to excess nitrogen (Garbo et al., 2017).
The control column with plants (PC) was irrigated with tap water with the addition of synthetic nitrogen, provided as NH4Cl: the nitrogen supplied was equal to the amount provided by the leachate in the irrigation of reactors P1 and P2.
The feeding volume of 5 L/week was maintained constant for the duration of the experiment in each unit.

Table 2. Leachate dosages and contaminants concentration in the feeding of P1, P2, and CL

Phase
Leachate dose (%)
TKN conc.
(mgN/L)*
TKN load (mgN/week)

COD conc. (mgO2/L)
COD load (mgO2/week)

Acclimation
0
-
-
-
-
Week 1
1050
25049245
Week 2
20
100
500
98
490
Week 330150750147735
Week 4402001500196
980
Week 55025010002451225
Weeks 6-86030012502941470

* the same concentrations of input nitrogen (as NH4Cl) were applied to PC control unit

Each column was drained once per week and the liquid samples were stored at -20°C and subsequently analyzed for the following parameters: Total Kjeldhal Nitrogen (TKN), ammonia, nitrate, nitrite, and COD. Plants of Pennisetum Purpureum were harvested at their maximum growth (about 10 weeks after planting). Plant biomass, after roots separation from stems, was dried at 60 °C in the oven and weighed. Dry mass, total nitrogen and heavy metals contents in the biological tissues were determined.

1.2. Equipment
Columnar PVC pipes, with a diameter of 25 cm and a height of 100 cm, were used. The reactors, arranged in vertical position to assure a vertical flow, were sealed at the bottom. A drainage layer, 15 cm thick, made up of gravels with a diameter of 20-30 mm, was placed at the bottom. The columns were then filled with 75 cm of growing substrate to simulate the layout of a final top cover of a landfill. A fine plastic net was installed horizontally, between the two layers, to avoid the occurrence of intermixing phenomena. A flexible tube was installed at the bottom of the columns to allow the collection of the effluents. The column’s scheme is reported in Fig.1. The experimental units were placed in a controlled climatic chamber in which the temperature was maintained in the range 16-36 °C (26 °C on average). A 10-hour photoperiod with 300 μmol·m-2·s-1 light intensity was imposed.

Fig. 1. Reactors used for the experiment (a: reactors in which Pennisetum Purpureum was grown – P1, P2, and PC; b: reactors without plants – CL and CW). Measures expressed in cm.

1.3. Landfill leachate
The leachate used in this experiment was sampled from a currently operated MSW landfill, located in the North of Italy, in which stabilized residual municipal solid waste is disposed of. Its composition is reported in Table 3 and the results are consistent with the kind of waste landfilled.

Table 3.  Landfill leachate chemical characterization

Parameter
Unit

Values
pH

7.8±0.3
TKN
(mgN/L)
500±25
NH4+
(mgN/L)453±22
PTOT
(mgP/L)
2.3±0.2
TS

(mg/L)2451±113
VS(mg/L)495±45

COD(mgO2/L)490±32
BOD(mgO2/L)<50
Cl-(mg/L)685±36
NO3-
(mgN/L)-
NO2-(mgN/L)-
SO42-
(mgSO4/L)
-
Ca
(mg/L)
142±11
K
(mg/L)225±21
Mg(mg/L)88±10
Na
(mg/L)
353±18
Cd(μg/L)< 10
Cr
(μg/L)
64±3
Cu
(μg/L)
91±5
Fe
(μg/L)5733±56
Mn
(μg/L)
313±20
Ni
(μg/L)168±8
Pb
(μg/L)46±3
Zn(μg/L)269±6

1.4. Substrate soil properties
A mixture made up of 50% of quartz sand and 50% of locally available soil (on volume basis) was used as substrate. Studies on artificially constructed wetlands recommend mixtures of natural soil and sand to provide the best conditions in terms of hydraulic conductivity and contaminant removal. Sand guarantees enough macro-porosity for air recirculation and roots development, avoiding water stagnation, while the agricultural soil is a substrate rich in micro-nutrients, fundamental for the plants growth (Garbo et al., 2017; Jones et al., 2006).
Substrate texture was determined with the Bouyoucos method (Bouyoucos, 1962) and, according to the soil taxonomy proposed by the USDA (USDA-NRCS, 1999), the substrate soil used was classified as sandy loam (14% clay, 10% silt, and 76% sand). Its chemical characterization is reported in Table 4.

Parameter
Unit
Value
Volatile Solids
mg/kgTS
2.2
Total CarbonmgC/kgTS56900
Total Nitrogen
mgN/kgTS
150
Cdmg/kgTS0.38
Cr
mg/kgTS20
Pb
mg/kgTS19
Cu
mg/kgTS
27
Ni
mg/kgTS18
Znmg/kgTS65
Fe
mg/kgTS4924
Mnmg/kgTS308

1.5. Hydraulic retention time
The Hydraulic Retention Time (HRT) is defined as the average time that a fluid remains inside a reactor:

where:
Vreactor = volume of the reactor
Q = flow rate

This formula is based on the hypothesis that the volume remains constant during the process, that is not the case under investigation because of the presence of a strong evapotranspiration factor which reduces the volume of the liquid phase. This is typical of constructed wetlands, as already reported by Bialowiec et al. (2014). Anyhow, in order to provide an average value, an indicative HRT* was estimated with the following empirical formula:

where:
V = volume of water provided in the i day
t = number of days until the following total drainage
i = day of the week
This formula takes into account the residence time of every liter poured in the column until the weekly total drainage, corresponding to the day zero for the HRT assessment.
Therefore, being all the reactors fed with the same amount of water and drained at the same time, the retention time was the same in each reactor and resulted to be 3.4 d.

1.6. Analytical methods
Analysis on liquid samples were performed according to the IRSA-CNR methods for water quality analysis (CNR-IRSA, 29/2003). Analysis on plants and substrate samples were carried out according to the IRSA-CNR guidelines for solid samples (CNR-IRSA, 64/1986).

1.7. Nitrogen mass balance
At the end of the experimental trial, the nitrogen mass balance of vegetated experimental units was performed to assess the role of the different systems components (plants, substrate) in removing nitrogen. It was based on the following equation:

where:
Nin = total amount of nitrogen entering each unit (mgN)
Nout = total amount of nitrogen in the outflow (mgN)
NP = amount of nitrogen accumulated in the plant tissue (mgN)
NS = nitrogen accumulated in the substrate (mgN)
NL = nitrogen gaseous losses due to nitrification and denitrification phenomena (mgN)

2. RESULTS AND DISCUSSION
2.1. Pennisetum Purpureum growth
Elephant grass grew uniformly in all the units, as shown in Fig. 2. Plants grown in reactors P1 and P2 reached maximum heights ranging between 120 and 140 cm. Comparable heights were reached by the plants in the control unit PC, suggesting that leachate irrigation did not inhibit the growth in P1 and P2. The maximum height was reached during week 7.
Plants dry weights confirmed that leachate did not limit plant growth, on the contrary it seemed to stimulate the biomass growth: average weight of plants cultivated in leachate irrigated reactors (12.87 gTS/plant in P1 and 11.41 gTS/plant in P2) was even higher than the average weight in the control column PC (9.95 gTS/plant) (Fig. 3).

Fig. 2. Above-ground height of Purpureum Purpureum in reactors P1, P2, and PC over the whole experimental period.

Fig. 3. Final dry weights of Pennisetum Purpureum in units P1, P2, and PC

2.2 Contaminant removal
Due to a strong evapotranspiration effect, evaluation of contaminant removal should take into account the reduction of the effluent volumes therefore Removal Efficiency (RE) should be based on weekly loads:

where
VIN = influent volume (L/week)
VOUT = effluent volume (L/week)
CIN = influent concentration of the considered contaminant (mg/L)
COUT = effluent concentration of the considered contaminant (mg/L)

Input and output weekly loads, and average removal efficiencies of the investigated contaminants in leachate irrigated units are reported in Table 5. NO2- are not included because they were always below the detection limits. Columns were subjected to increasing contaminants loads over time, with maximum values reached in week 6 and kept constant till the end of the experimental trial (Table 2). RE of TKN and ammonia were always in the range 95-99%, even in the control column without plants (CL). Nitrate was not present in the influent water, but was detected in the effluent of each column, suggesting the occurrence of nitrification process. Its formation was significant: the output loads were much higher than output loads of TKN and ammonia in each leachate irrigated reactor. COD removal was always above 92%, with the best performances observed in the experimental units with the essences (P1 and P2). Summarizing, P1 and P2 showed excellent performances with no differences among the replicas but RE were excellent also in the control column CL, highlighting the primary role of the substrate in contributing in the removal of the investigated contaminants.

Table 5. Input and output weekly loads, and average Removal Efficiency (RE) in leachate irrigated units

   P1  P2  CL 
Parameter Week
Load in (mg/week) Load out (mg/week) RE (%)Load out (mg/week)RE (%) Load out (mgN/week) RE (%)
TKN1
250

20
92
15

94

34

86

2
500
22
95
14
97
55
89
3
750
23

97
16
98
50
93
4

1000
22
98
15
98
68
93
5
1250
18
98
12
99
55
95
6
1500
18
99
12
99
61
96
7
1500
20
99

12
99
56
96
8
1500
20
99
11
99
65
96
Average1031219814985595
NH4+ (as N)1226

8
96

5

98

13
94
2453
9
98
5
99
22

95

3679
9
99
5
99
20
97
4906
9
99
5
99
27
97
51132
7
99
4
99
22
98
61359
7
99
4
99
24
98
71359
8
99
4
99
22
98
81359
8
99
4
99
26
98
Average9348
995992298
NO3- (as N)1-168

-186

-55

-
2-162
-145
-133
-
3-145
-135
-149
-
4-179
-179
-266
-
5-174
-192
-274
-
6-237
-248
-417
-
7-339
-280
-578
-
8-464
-380
-816
-
Average-234-218-336-
COD1245

35

86
51
79

110
55
2490
35
93
45
91
185
62
3735
29
96

31
96
78

89

4980
49
95
18
98
82
92
51225
27
98
21
98
48
96
61470
39
98
24

98
32
98
71470
32
98
33
98
34
98
81470
22
97
24
97
26
98
Average937329632967592

2.3. Nitrogen effluent concentrations
Nitrogen in the irrigation reached values up to 300 mgN/L, well above the limit set by the Italian legislation (D. Lgs. 152/2006) for the discharge in water bodies: 15 mgN/L for TKN, 20 mgN/L for NO3-. Therefore analysis of effluent concentrations was useful to evaluate whether the phytotreatment process allowed the compliance with legal requirements.
The analysis of TKN and NO3 concentrations in the effluents (Fig. 4) showed an almost total conversion of TKN into NO3, as TKN concentrations in the effluent were negligible in all the experimental units. This was expected, as vertical flow reactors are engineered to promote the nitrification process. The replicas with leachate irrigated plants behaved in a very similar way: trends in P1 were similar to P2. Nitrate concentrations never exceeded 150 mgNO3-N/L in both P1 and P2 reactors.
The gap between the influent TKN and the effluent NO3- concentrations might be due to a higher nitrate accumulation in the plants or in the soil, but the nitrogen mass balance (Table 6) showed that it could not be just related to the role of plants and substrate. Therefore, the occurrence of denitrification could not be excluded. Although vertical flow enhanced the presence of aerobic conditions inside the columns, proved by the occurrence of an almost total nitrification, probably some parts of clay-rich soil, along the columns, limited the air intrusion, thus promoting the denitrification. In similar experiments with vertical flow reactors fed with synthetic wastewater, Fan et al. (2013a, c) observed the occurrence of both nitrification and denitrification, but only if forced aeration was applied.
On the contrary, nitrate effluent concentrations in control column with plants (PC) were higher than P1 and P2; in some cases even close to the influent concentration, suggesting the occurrence of a complete nitrification but the lack of denitrification.
The lack of denitrification in control columns PC is confirmed looking at the TKN and nitrate effluent concentrations in CL: in the latter, denitrification was observed as the NO3- concentration did not exceed 200 mgNO3-N/L. Denitrification is performed by heterotrophic bacteria naturally present in the soil in presence of bio-available organic substances (Pajares and Bohannan, 2016). Therefore, denitrification occurred only in the experimental units irrigated with leachate, which provided the organic matter needed for the process. Excellent removal performances were achieved even if the COD/N ratio in the influent was 0.98: Fan et al. (2013b) reported that for similar efficiencies the COD/N should be higher than 10, and that forced aeration should be applied. Nitrogen leaching from the substrate was negligible, as shown by the unit CW.
Considering the leachate irrigated reactors, only TKN concentrations in the outflow always met the Italian discharge limit of 15 mg/L (D. Lgs. 152/2006) during the experiment. On the contrary, nitrate concentrations in the outflow did not accomplish the Italian regulation, which sets a discharge limit of 20 mgN/L. In order to improve the nitrogen removal, an horizontal sub-surface flow could be provided after the vertical one (Garbo et al., 2017; Lavagnolo et al., 2016; Cheng and Chu, 2011).

Fig. 4. TKN influent and effluent concentrations, and nitrate effluent concentrations in the experimental units

2.4. COD effluent concentrations
As for nitrogen, COD in the influent was higher than the legal limit value of 160 mgO2/L for the discharge in water bodies (D. Lgs. 152/2006). Outlet COD concentrations were always below 50 mgO2/L (Fig. 5), even if the influent concentration reached the maximum value of 300 mgO2/L: all the experimental units were able to remove most of the influent organic matter and comply with the Italian discharge limit. Trends of effluent concentrations in leachate irrigated reactors were comparable to the trend detected in CW, in which there was no addition of external organic matter: wastewater did not produce any additional increase in the effluent concentration. Likely, a relevant fraction of the influent COD was consumed by heterotrophic microorganism living in the substrate (e.g.: to perform the denitrification process) or adsorbed by the substrate itself. The lowest output concentrations were detected in reactors with elephant grass (P1 and P2): plants are known to favor the growth of microorganisms, which play a dominant role in the degradation of organics, by conveying the oxygen to the rhizosphere (Akinbile et al., 2012). Anyhow, it was a further confirmation that phytotreatment is a combination of different phenomena, rather than the isolated action of plants (Jones et al., 2006).

Fig. 5. COD influent and effluent concentrations in the experimental units

2.5. Nitrogen mass balance
At the end of the experimental period, fate of nitrogen in leachate irrigated columns and control column with Pennisetum Purpureum was investigated. Nitrogen mass balance for the system components (effluent, substrate soil and plants), calculated according to equation (1), is reported in Table 6.8. Inlet nitrogen was added to the system by irrigation mainly as ammonium form (Table 6). Part of the influent was found in the effluent (range: 23-46%), a fraction was adsorbed by the substrate soil (range: 12-23%), and another small fraction (range: 8-15%) was taken up by the plants. Sum of nitrogen accumulation in substrate and plants and effluent was always lower compared to the influent nitrogen, revealing the occurrence of nitrogen losses, likely due to the nitrification and denitrification processes which were already discussed. During denitrification, nitrogen was converted into gaseous form as N2 and released in the atmosphere, as already observed by Garbo et al. (2017), Lavagnolo et al. (2016), Cheng and Chu (2011).
P1 and P2 shown similar values in the final mass balance, with approximately 25% of the influent nitrogen found in the effluent. The combined role of Pennisetum Purpureum uptake and substrate accumulation accounted for another 25%; the main fraction of the influent nitrogen (approximately 50%) was removed by nitrification and denitrification. Control column PC, although subjected to the same nitrogen load in the influent, was characterized by the lowest nitrogen loss (24%) and the highest release of N in the effluent (42%): the absence of external COD addition due to leachate irrigation likely limited the development of the bacteria populations involved in the denitrification process. Nitrogen loss in CL was comparable to P1 and P2, while the fraction found in the effluent (36 %) exceeded the values found in P1 and P2 (25 and 23%, respectively): this difference could be related to the presence of the plants, which contributed in the removal of nitrogen, otherwise released with the outflow.

Table 6.  Final nitrogen mass balances in the leachate irrigated reactors with Pennisetum Purpureum

Reactor

UnitNtot in influent -
Nin


Ntot in effluent -
Nout

Plant uptake -
ΔNP
Substrate
accumulation - ΔNS
Nitrogen loss -
ΔNL
P1
mg
% (on Ntot in influent)
7950
-

-
1977
25
1162
15
938
12
3873
48
P2
mg
% (on Ntot in influent)
7950
-
1815
23
659
8
1173
14
4303
55
PC
mg
% (on Ntot in influent)
7950
-
3328
42
821
11

1876
23
1925
24
CLmg
% (on Ntot in influent)
7950
-
2881
36
-1055
13
4014
50

2.6. Heavy metals concentration in Pennisetum Purpureum biological tissues
Heavy metals concentration in Pennisetum Purpureum tissues was investigated to assess whether leachate irrigation resulted in an accumulation of such chemical elements. Essences growing in reactors P1 and P2 were analyzed and the resulted compared with those of plants grown in PC (Fig. 6). Some metals (Cd, Cr, Pb, Ni) are not visible because their value was below 1 mg/kgTS. A higher concentration of Fe and Mn was detected in tissues of plants grown in P1 and P2, but the statistical analysis did not reveal any significant increase compared to plants grown in the control PC. These results suggest that conversion of elephant grass into bio-energy is a feasible option and that risks related to the potential presence of contaminants into the biological tissues were not revealed by the experimental data.

Fig. 6. Heavy metals concentration in the biological tissues of Pennisetum Purpureum

3. CONCLUSIONS
Landfill leachate phytotreatment using elephant grass proved to be feasible under lab-scale conditions. Plants growth was not affected by leachate irrigation, which on the contrary seemed to stimulate the biomass development. Removal efficiencies of the investigated contaminants were excellent: more than 95% for TKN, ammonia and COD. Complete nitrification was observed in all the units in which nitrogen addition was applied, but also a partial denitrification in leachate irrigated reactors, which was confirmed by the final nitrogen mass balance. Final heavy metals concentration in the tissues of Pennisetum Purpureum showed an acceptable, not significant increase.

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