irrigation of landfill leachates in energy forests — a technique to recover nutrients from...

IRRIGATION OF LANDFILL LEACHATES IN ENERGY FORESTS –A TECHNIQUE TO RECOVER NUTRIENTS FROM MUNICIPAL SOLID

WASTES

LARS BRANDER, MARTIN DAHL and TORLEIF BRAMRYD∗Department of Environmental Strategy, University of Lund, Campus Helsingborg, Helsingborg,

Sweden(∗ author for correspondence, e-mail: [email protected])

(Received 2 May 2003; accepted 27 November 2003)

Abstract. From an ecological point of view it is important to close nutrient cycles by recirculatingmineral nutrients from the urban society back to agriculture and forestry, and thereby obtaining asustainable resource utilisation. A part of this cycle is illustrated by irrigation of bioreactor landfillleachates on short rotation forests. This paper presents a budget for nutrients and heavy metals,beginning with the leachates and ending with the harvested tree fraction. The hypotheses were: Theapplied minerals deliver nutrients to the trees. The nutrient content in the accumulating biomasscorresponds to the amount of mineral nutrients applied. The concentrations of heavy metals in thetrees will remain low. The uptake of elements in birch was for P 35%, Ca 1.2%, Cd 64%, Cu 10%, Mn19%, Ni 0.11%, and for Zn 26% of the supplied amounts. It was concluded that nutrients, with someexceptions, are supplied in sufficient amounts from the irrigated leachates to achieve optimal biomassgrowth, that the amounts of ions immobilised by the plants were significantly lower compared tothe applied amounts, and that the concentrations of heavy metals are not increasing in the treesafter irrigation. The overall conclusion is that a leachate irrigation system is efficient if the availablevegetated land area is large enough for effective nutrient uptake, but the nutrient ratio may need tobe balanced to meet the needs of the plants.

Keywords: Betula, birch, heavy metal, irrigation, landfill, leachate, nutrient

1. Introduction

A major problem in the urban system is that resources are transported from agri-culture and forestry to the human society without sufficient recirculation. Froman ecological viewpoint it is important to close the nutrient cycle by bringingthe mineral nutrients back to the producing ecosystems and thereby obtaining asustainable resource utilisation.

From an energy efficiency point of view the treatment of solid wastes in a land-fill bioreactor could be compared to incineration. In a bioreactor methane gas isproduced, which can be used for heat production or, for example, as car fuel. Someenergy is lost due to the undegradable material in the waste, such as plastics. Thiscan be compensated by the utilization of the biomass as an energy source, which isproduced in a forest irrigated with leachate from the bioreactor (Bramryd, 1997a).

Water, Air, and Soil Pollution 154: 213–224, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

214 L. BRANDER ET AL.

Figure 1. The cycling of nutrients between the urban society and the biomass producing ecosystems.This paper is concerned with the part within the dotted line. MSW = Municipal Solid Waste.

At the Filborna waste treatment facility outside Helsingborg, Sweden, mineralnutrients are recovered from solid wastes by anaerobic decomposition in landfillbioreactors. Under the anaerobic conditions in landfill bioreactor cells, the heavymetals will be bound as insoluble sulphide compounds and will not be prone toleak (Bramryd, 1997a). The produced leachates are collected in ponds and usedfor irrigation of a mixed stand of European birch (Betula pendula) and Coloradospruce (Picea pungens), with birch as the dominating species. An increasing shareof the leachates comes from the landfill bioreactor cells, and only minor parts fromthe old parts of the landfill. The minerals from the applied leachates provide macro-and micro-nutrients to the trees, while organic solutes in the leachates will bedecomposed by the soil organisms (McBride et al., 1989). Eventually the forestwill be harvested and possibly burned as an energy source. Another alternative isfermentation for gas production. The resulting ash or fermentation residue can bebrought back to farming or forestry since it contains only low amounts of heavymetals, depending on the low concentrations in the leachates. Thereby, the nutrientswill be brought back to the producing ecosystems with very low contamination ofheavy metals.

In this paper, a budget was constructed for nutrients and heavy metals. Thebudget concerns applied leachates and tree uptake (Figure 1). The hypotheseswere: The applied minerals deliver nutrients to the trees. The nutrient content inthe accumulating biomass corresponds to the amount of mineral nutrients applied.The concentrations of heavy metals in the trees will remain low.

IRRIGATION OF LANDFILL LEACHATES IN ENERGY FORESTS 215

2. Materials and Methods

2.1. SITE DESCRIPTION

The Filborna waste treatment facility is located outside Helsingborg in southernSweden. The plant is owned by Northwest Scania Recycling Company, NSR, whohas developed an integrated biological system for recovery of biogas and nutrientsfrom residual solid waste in bioreactor cells (Bramryd, 1997b). In these cells, amixture of domestic, commercial, and light industrial waste is treated.

Leachates collected from the bioreactor cells, and parts of the old landfill, aretransported to an aeration pond, from which they are pumped into an irrigationsystem. During the vegetation period from May to September, a mixed stand of 20–30 yr old European birch (Betula pendula) and Colorado spruce (Picea pungens),with a larger part of birch, is irrigated with leachates. The biomass productionof the spruce is low in comparison to the birch, and thus this investigation onlyconcerns the effects on birch. New self-sown birch plants emerge continuously,which affects the age structure. In the area used for the experiment the soil type isa clay soil with a pH (H2O extraction) ranging between 4.2 and 4.6.

The irrigated area is surrounded with the same type of forest, thus boundaryeffects depending on the forest edge should be prevented. A sprinkler system isused to evenly distribute the leachates over the area. The sprinklers are positioned20 cm above ground to avoid spray of leachates on the leaves (Shrive et al., 1994).An area of 1.2 ha has been irrigated over a five-year period. During dry weather,approximately 40 m3 of leachates per day are irrigated to the area. The irrigation iscancelled during days with rainfall.

2.2. FIELD SAMPLING

Each year during April, May, August, and November leachate samples were col-lected from the aeration pond. In the first year the field sampling started in Au-gust. During the final year of the study (year five) a sample was also collectedin June, after which the study ended. Thus no samples were collected in Augustand November that year. At each sampling occasion, several samples were taken atdifferent points in the pond. The samples were then pooled together before analysis.

Birch leaves were sampled in October year two, in September year four andin August year five. Stem wood-, and bark samples from birch were collected inAugust year five. A number of trees were selected at random from the area andsampled. Stem wood and bark samples were pooled together separately before ana-lysis. Within randomly selected areas in the forest stand, the breast height diameterof all birch trees were measured and the standing birch biomass was calculated forthe whole area.


2.3. CHEMICAL ANALYSES

The leachate samples were not filtrated before analysis. All vegetation sampleswere dried in a ventilated drying oven at 40 ◦C immediately after collection. Theanalyses were made immediately after pre-treatment.

The concentration of Cl was analysed by ion chromatography. The concentra-tions of P, Ca, K, Mg, Na, Cd, Cr, Ni, Pb, Cu, Fe, Mn, and Zn in the leachates wereanalysed with an ICP-AES spectrophotometer after acidifying with nitric acid. Thevegetation samples, 300 g d.w. of each plant fraction, were fully digested in 20%nitric acid (HNO3) before analysis of the same elements as in the leachates by anICP-AES instrument. The choice of elements is based on leachate contents, majorplant nutrients and major heavy metals, but is limited by incomplete analysis at thebeginning of the project. Total N was analysed according to the Kjeldahl procedure(Bremner, 1965). Total-N is used because the different forms of nitrogen in theleachates are partly transformed to organic compounds in the trees.

2.4. CALCULATIONS

The irrigated amounts of leachates are reported as annual totals, and to be able tocalculate the amount of applied ions a mean concentration was estimated for eachion and year. During the first year only the August sample represented conditionsunder irrigation. During the following years the mean concentrations were basedon the samples taken in April, May and August. Concerning year five, an annualmean value is calculated based on the samples from April, May and June. Thecalculated mean concentrations were then multiplied with the irrigated leachatequantities each year to estimate the applied amounts of ions.

The standing tree dry weight biomass of birch stem wood, stem bark, andbranches was calculated for year five (Marklund, 1988) using the diameters of allmeasured trees. As whole tree harvesting during winter is the common practice forcollection of the biomass, the total biomass of stems and branches, but not roots andleaves, for birch was calculated. Growth curves (Hägglund and Lundmark, 1981)were used to calculate the height of the trees during year two. The biomass of stemwood, stem bark, and branches in year two were then calculated by using equationsin Marklund (1988). In year two only foliage samples were taken from the trees.The nutrient concentrations in stem wood and stem bark were calculated for thisyear, assuming that the element ratio was the same for foliage as in the stem, bark,and branch fraction each year. The nutrient concentrations in the branches wereestimated by calculating a ratio between stem wood and bark for trees with a breastheight diameter <3 cm. It is assumed that such small trees have a similar stem/barkratio as branches on larger trees.

An analysis of variance (ANOVA) was performed to detect any differences inconcentrations between the years in the foliage samples. Additionally an independ-ent samples t-test was performed, where we compared the means of the first year


TABLE I

The mean concentrations (mg L−1) of ions in the irrigated leachates during theyears one to five

Year 1 Year 2 Year 3 Year 4 Year 5

(mg L−1)

Cl 400 514 540 494 383

N, total 45 47 19 6.9 4.3

P 0.065 0.42 0.33 0.31 0.26

Ca 76 46 33 13 21

K 106

Mg 19

Na 343

Cd 0.0019 0.0001 0.0004 0.0001 <0.001

Cr 0.003 0.005 0.004 0.004

Cu 0.013 0.011 0.011 0.003 0.004

Fe 0.63 1.13 0.61 1.79 0.98

Mn 0.33 0.13 0.08 1.8 1.7

Ni 0.025 0.024 0.031 0.025

Pb 0.028 0.0026 0.001 0.003

Zn 0.04 0.09 0.06 0.59 <0.001

with the means of the final year, for the ions that showed significant differences inthe ANOVA test.

3. Results

3.1. CONCENTRATIONS IN THE IRRIGATED LEACHATES

The concentrations of minerals in the irrigated leachates were calculated as meanvalues for the years one to five (Table I). Among the analysed anions Cl was themost abundant. Na was only analysed during year five, and showed higher con-centration than all other analysed cations together that year. At the same time theconcentrations of total N decreased after the second year. The concentrations ofheavy metals were generally low, and only Fe and Mn were found in considerableamounts. During year five the Cd and Zn concentrations were below detectionlimits, in this year another laboratory was used for the analysis. The leachate pHranges between 8 and 9.


TABLE II

The concentrations of elements in the foliage of birch from years two, four, and five, and in theharvest fraction (i.e. stem, bark, and branches) in year five (µg g−1 d.w.). Standard deviationsincluded for the foliage samples

Birch foliage (B. pendula) Harvest

Year 2 sd Year 4 sd Year 5 sd Year 5

(µg g−1 d.w.)

N 22572 2179

P 1639 264 2151 342 1359 118 277

Ca 6388 387 3498 1505 3713 806 921

K 9745 2137 12879 4586 11818 1048

Mg 2268 332 1964 804 1781 372

Na 86 34 81 48 89 77

Cd 0.46 0.14 0.18 0.11 0.25 0.18 0.31

Cr <0.35 0.057 0.053 0.29 0.074

Cu 4.6 0.97 3.6 0.88 3.3 1.05 2.2

Fe 68.4 22 79.8 21

Mn 1614 307 834 484 889 407 154

Ni 0.67 0.32 <0.2 0.45 0.13 0.06

Pb <3 0.1 0.27 0.85 0.28

Zn 201 61 121 66 152 58 57

3.2. CONCENTRATIONS OF ELEMENTS IN THE TREES

The concentrations of elements in the birch leaf samples are presented for eachyear (Table II). The concentrations of Cr and Pb in year two and the concentrationof Ni in year four were below detection limits. Concentrations in the harvestedfraction (i.e. stem, bark, and branches, but not foliage nor roots) are presented in thecases where complete data from both irrigation and foliage samples are available(Table II).

An ANOVA test concerning birch foliage between the years two, four and fiverevealed significant differences for the concentrations of P and Ca (p < 0.001),and of K, Mn, and Cd (p < 0.01). A t-test revealed significant decrease in theconcentrations of P, Ca, and Mn. Ni was included in this test since it was above thedetection limit during the years two and five (Table III).

During year five the total estimated standing biomass of birch, not includingfoliage nor roots, was 30 940 kg d.w., divided into 64% stem wood, 23% branches,and 13% stem bark. During year two, the total standing biomass was 8150 kg d.w.,


TABLE III

Analysis of variance (ANOVA) for the concentrations in birch foliage samples,between the years two, four, and five. An independent samples t-test compares thoseconcentrations for year two with year five, where significant differences were found.Ni was included in the t-test since it only showed concentrations above the detectionlimit during the years two and five. In the change column, + represents an increaseand – represents a decrease

ANOVA t-test

Element df F Sign. Element df t Sign. Change

P 2 19.11 ∗∗∗ P 13 2.21 ∗ –

Ca 2 15.87 ∗∗∗ Ca 13 8.85 ∗∗∗ –

K 2 6.63 ∗∗ K 13 –2.03 NS +

Mg 2 1.39 NS

Na 2 0.07 NS

Cd 2 5.73 ∗∗ Cd 13 1.87 NS –

Cu 2 1.51 NS

Mn 2 8.95 ∗∗ Mn 13 3.86 ∗∗ –

Ni 13 0.27 NS –

Zn 2 2.22 NS

df = Degrees of freedom.∗ = p < 0.05, ∗∗ = p < 0.01, ∗∗∗ = p < 0.001.NS = No significance.

divided into 65% stem wood, 23% branches, and 12% stem bark. Some of the treesincluded in year five were to small to be included in year two.

Figure 2. Nutrient budget concerning birch uptake (in stem, bark and branches) with the total appliedamounts of P, Ca, and heavy metal ions from year two to five. The birch uptake in percent of appliedamount is included in the birch uptake box. (1) = The amounts are only based on the values betweenthe years two and four. (2) = Before year four the Cr and Pb concentrations were below the detectionlimit.


3.3. THE NUTRIENT BUDGET

In the nutrient budget, the applied amounts of Cd, Cr, Ni, Pb, and Zn were totalsfrom years two to four, while for the other ions the totals are from year two to five(Figure 2).

35% of the applied amounts of P were taken up by the birch, and stored in thestem, bark, and branch fractions. The birch uptake of the applied Ca, in the samefractions, was 1.2% (Figure 2). Cr and Pb were not analysed in the birch samplescollected during year two.

4. Discussion

4.1. TYPICAL CONCENTRATIONS IN LEACHATES AND PLANTS

The differences in leachate concentrations between the investigated years were inmany cases immense (Table I). However, a comparison with literature data con-cerning landfill leachates shows that most of the concentrations were within therange of average values for municipal landfills, except for Cd, Cr, Cu, and Pb,where the concentrations in this study were low. The variations in leachate con-centrations can be large between landfills and between years. The main reasons forthese variations are landfill age and contents, site climate, season, and precipitationvariation between years (Clement et al., 1997; Fatta et al., 1998; Lema et al., 1988;Menser et al., 1983; Ragle et al., 1995; Shrive et al., 1994).

Literature data on mineral concentrations in birch foliage have been compiledfrom Alriksson and Eriksson (1998), and Hytönen et al. (1995) in ranges for eachion (Table IV) to be able to make a comparison with the concentrations in leafsamples collected five years after the start of the irrigation experiment.

4.2. THE FLOW OF ELEMENTS

The distinct decrease in nitrogen concentrations after year two in the irrigatedleachates (Table I) could be explained by nitrification, followed by denitrification.After the summer of year two, the leachates remain in the collection pond forat least three weeks before irrigation, while before year two the leachates weremore or less directly pumped from the collection pond into the irrigation system.The leachates also contain dissolved organic carbon, which can be readily usedas a carbon and energy source for denitrifying micro organisms (Wetzel, 1983).Another explanation could be denitrification during the anaerobic conditions inthe landfill bioreactors, and as an increasing part of the leachates come from thebioreactors, the concentrations of inorganic nitrogen might decrease.

Wet deposition contributes with about 20 kg ha−1 of nitrogen each year inthis area of Sweden (Naturvårdsverket, 1993). Since the area is 1.2 ha, the wetdeposition of nitrogen is approximately 25 kg yr−1. The total N concentrations in


TABLE IV

Concentrations of minerals in birch leaves collec-ted year 5. The table compares the data with con-centration ranges for B. pendula in µg g−1, com-piled from literature data (Alriksson and Eriksson,1997; Hytönen et al., 1995)

Year 5 Literature values

(µg g−1)

N 22572 25000–29700

P 1359 2900–4900

Ca 3713 8400–10500

K 11818 6900–16000

Mg 1781 2700–4800

Na 89 12

Cd 0.25

Cr 0.29

Cu 3.3 4.4–8.1

Fe 79.8

Mn 889 530–2223

Ni 0.45

Pb 0.85

Zn 152 127–347

the vegetation samples from year five were near the literature values (Table IV).This indicates that the amount of N added to the area was sufficient for the plants.

In the nutrient budget, P uptake in the plant stem, bark and branch fractionswas approximately 35% of the applied amount (Figure 2). A comparison withliterature data shows that the concentration in the birch foliage was low (Table IV).The decrease of P in the leaves (Table III) could be an indication of that P wasbound in the soil, in a form that was not available for the plants. A comparisonof the birch foliage Ca concentrations with literature values (Table IV) shows thatthe concentrations in this study were at about one third of the normal and weredecreasing (Table III), while the applied amount exceeds the uptake in birch stem,bark and branch fractions 85 times (Figure 2). This indicates that the uptake isimpaired. For instance, Ca is readily replaced by other cations at its binding sitesat the exterior surface of the plasma membrane of the root cells (Marschner, 1986).K was taken up in sufficient amounts by the birch trees (Tables II and IV) but theapplied amount was probably in excess. In the birch leaves, the Mg concentration


was low compared to literature values (Table IV). It is possible that the birch sufferfrom Mg deficiency, as uptake of Mg can be strongly depressed by other cations,such as Ca, K, and Mn. For example, Mn can block the binding sites for Mg on theroot cell membranes (Marschner, 1986).

In the applied leachates during year five, the concentration of Na was higherthan all of the other analysed cations together (Table I). It can be suspected, withreference to the high concentrations of Cl (Table I), that high amounts of Na alsowere added to the soil during the first four years of the study. Na is very mobile inthe soil, and is exchanged from the colloids when other ions are added (Brady andWeil, 1999). The Na concentrations in the birch leaves were very high (Table IV).A high concentration of Na in the soil and in the plants can prevent the uptake ofother cations, due to high osmotic pressure and unfavourable combinations of ions,such as a high Na/Ca ratio (Marschner, 1986). The impaired nutrient uptake canlead to decreased growth, which in turn leads to a situation where even smalleramounts of the applied nutrients can be taken up by the vegetation. To be ableto calculate the tree growth by using the tables in Marklund (1988), salinity wasassumed to have a negligible effect in this case.

During year five the Cd concentration was below detection limits, in this yearanother laboratory was used for the analysis. The birch uptake of Cd in the stem,bark and branch fractions was 64% of the applied amount (Figure 2), but in thefoliage samples, there was no increase in concentration (Table III). Thus, the uptakecorresponds to the increasing biomass of the trees. A comparison with literaturedata shows that the concentrations of Cu in the birch foliage were lower thannormal (Table IV). The critical deficiency level in vegetative parts is generally3–5 µg g−1 according to Marschner (1986), and the concentrations in the birchleaves were close to this level. The low concentrations of Cu, and the fact that only10% was taken up in the stem, bark and branch fractions (Figure 2), emphasisethe conclusion that Cu was bound in an unavailable form for plants. The appliedamounts of Mn were higher than the birch uptake (Figure 2), but the birch leafconcentration was decreasing (Table III). This leads to a conclusion that the uptakein the trees is inhibited. The reason could be the competition with other ions inhigh concentrations. The Mn concentrations in birch leaves were still within thenormal range after five years (Table IV), but if these processes continue, deficiencyof Mn could be a result. Only a small fraction of the applied Ni was taken up bythe birch in the stem, bark and branch fractions (Figure 2). No conclusions couldbe drawn from the results concerning Pb concentrations in the birch leaves, butthe application of Pb through the leachates was decreasing over the years. Of theapplied amount of Zn, 26% is taken up by the birch and stored in the stem, barkand branch fractions (Figure 2), while the birch leaf concentrations were near theliterature values (Table IV). This indicates that more Zn was applied, than could betaken up by the birch.

An optimal growth is necessary to maximise the nutrient uptake. After a heavyirrigation with leachates the soil becomes waterlogged. When a soil is flooded,


water occupies the soil pores, causing almost immediate deficiency of soil O2

(Kozlowski and Pallardy, 1997). Most plant species develop injury symptoms suchas wilting in a few days of waterlogging, since the root respiration is disturbed.A drop in redoxpotential as a result of waterlogging can also be responsible fortree injuries (Lambers et al., 1998; Marschner, 1986). Thus, waterlogging canlead to reduced growth (Kozlowski and Pallardy, 1997) and decreased uptake ofnutrients (Fitter and Hay, 1983), which counteracts the purpose of the vegetationfilter. During the irrigation experiment the following amounts of leachates wereapplied to the birch forest: 25,416 m3 year one, 20,545 m3 year two, 21,132 m3

year three, and 3193 m3 year four corresponding to a precipitation of 2063, 1668,1715, and 259 mm, respectively, for each of the years. A comparison with theactual precipitation of 474, 438, 455, and 977 mm in the same years shows that theirrigation was very high, especially when it is taken into account that the area isirrigated only during five months per year.

5. Conclusions

The applied leachates contain mineral nutrients essential for plant growth, even ifthe ion ratio does not correspond to the needs of the irrigated birch. Above all,the concentrations of Cl and Na were high and could thereby disturb the uptake ofother ions. As a suggestion a filter could be used to reduce the Cl and Na contentin the leachates.

The uptake of nutrients in the trees corresponds to a varying degree to theapplied amounts of mineral nutrients. The main reason is that the large amountof leachates supplied, provides the birches with larger amounts than they couldassimilate. There are two ways to handle this: either to decrease the amount of ap-plied leachates, or increase the irrigated surface. The nutrient ratio in the leachatescould be balanced by adding the nutrients that are deficient.

No increases of heavy metal concentrations could be found in the tree biomassduring the investigated five-year period.

A soil study would be desirable to obtain a comprehensive view of the system,but this paper is limited to discuss the direct relation between added elements andelements taken up by the trees. To fully understand the system a number of studieshave to be made, including effects on soil water, ground water, soil structure, ioninteractions, and soil adsorption.

References

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