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Characterization of fine fraction mined from two Finnish landfills

Tiina J. Mönkärea,*, Marja R.T. Palmrotha, Jukka A. Rintalaa

a Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101

Tampere, Finland

*corresponding author, e-mail: tiina.monkare@tut.fi, Tel: +358 50 301 4111, Fax: +358 3 364 1392,

Department of Chemistry and Bioengineering, Tampere University of Technology, P.O. Box 541, FI-33101

Tampere, Finland

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Abstract

A fine fraction (FF) was mined from two Finnish municipal solid waste (MSW) landfills in Kuopio (1- to 10-

year-old, referred as new landfill) and Lohja (24- to 40-year-old, referred as old landfill) in order to

characterize FF. In Kuopio the FF (< 20 mm) was on average 45 ± 7 w-% of the content of landfill and in

Lohja 58 ± 11 w-%. Sieving showed that 86.5 ± 5.7 w-% of the FF was smaller than 11.2 mm and the

fraction resembled soil. Total solid (TS) content was 46 – 82%, being lower in the bottom layers compared

to the middle layers. The organic matter content (measured as volatile solids, VS) and the biochemical

methane potential (BMP) of FF were lower in the old landfill (VS/TS 12.8 ± 7.1% and BMP 5.8 ± 3.4 m3

CH4/t TS) than in the new landfill (VS/TS 21.3 ± 4.3% and BMP 14.4 ± 9.9 m3 CH4/t TS), and both were

lower compared with fresh MSW. In Kuopio landfill materials were also mechanically sieved in full scale

plant in two size fraction < 30 mm (VS/TS 31.1% and 32.9 m3 CH4/t TS) and 30–70 mm (VS/TS 50.8% and

BMP 78.5 m3 CH4/t TS). The nitrogen (3.5 ± 2.0 g/kg TS), phosphorus (<1.0–1.5 g/kg TS) and soluble

chemical oxygen demand (COD) (2.77 ± 1.77 kg/t TS) contents were low in all samples. Since FF is major

fraction of the content of landfill, the characterization of FF is important to find possible methods for using

or disposing FF mined from landfills.

Keywords: Biochemical methane potential, characterization, fine fraction, landfill mining, municipal solid

waste

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1. Introduction

Landfilling has been the major method for disposing of municipal solid waste (MSW) in many regions of the

world for decades. In 2010 in the European Union (EU-27) 38% of the MSW (96 million tons) was

landfilled, while in 1995 in same countries up to 68% was landfilled (141 million tons) (Eurostat, 2011). For

example, in Sweden (population 9.6 million), there are 6 000 old landfills with an average size of 8 000 m2

(total 4 800 ha) (Hogland et al., 2010), in the Netherlands (population 16.7 million) about 3 800 abandoned

landfills with total surface of 9 000 ha (Paap et al., 2011) and Finland (population 5.4 million) has an

estimated 1 600 landfills. Usually, landfills contain MSW from households, commerce, trade and

administration (Eurostat, 2011) but also from industry (Kaartinen et al., 2013). Composition of MSW

depends besides waste management system also on region and season, mainly composing of food waste,

paper and cardboard, plastics, metal and glass. Low-income countries have higher proportion of organic

waste compared to high-income countries (IPCC, 2006).

Landfills may contain valuable materials and resources that are wasted. Landfills, especially old ones, are

sources of local water, soil and air pollution and also generate long-term methane (greenhouse gas)

emissions. In EU-28 in 2011, the waste management sector produces 3% of greenhouse gases, of which 84%

of methane emissions (4 700 Gg CH4) derives from solid waste disposal sites (EEA, 2013). It is estimated

that globally 1 500 million tons of MSW is landfilled yearly having potential to produce 50 Nm3 CH4 per ton

MSW (Themelis & Ulloa, 2007). Recently, interest in landfill mining, e.g., excavating, processing, treating

and recycling waste materials, has increased. Landfill mining offers possibility to recover landfilled

resources and also diminishes global and local pollution, especially methane emissions. Landfill mining

provides addition space for landfill or for other purposes. After processing, for example sieving, magnetic

separation and size reduction, the mined waste can be used as a raw material or energy resource or safely

disposed. Until now, landfill mining research has focused on recovering valuable metals and additional space

for landfilling or other purposes (Krook et al., 2012). When valuable materials are mined, less valuable waste

materials must be used or safely disposed. This study concentrates on the fine fraction (FF) mined from

landfills.

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The FF (particle size ranging between < 10 mm and < 25.4 mm) is 40–70 w-% of mined landfill waste

(Kaartinen et al., 2013; Quaghebeur et al., 2013; Hull et al., 2005), and is typically considered mainly soil

containing varying amounts of landfilled materials (Kaartinen et al., 2013). The FF should be characterized

so that utilization methods, such as energy and material, required processing (e.g. sieving, stabilization)

methods or final disposal methods can be identified. Although thermochemical technologies, i.e.,

incineration, gasification, pyrolysis, have recently been evaluated to process materials (also FF) recovered

from landfills (Bosmans et al., 2013), methane production potential of FF has not been studied. The methane

potential may indicate potential for methane recovery for energy utilization, or the need for stabilization of

the FF material to prevent emissions. To evaluate possibility to use FF in applications for soil type materials,

factors such as particle size and nutrients are important characteristics. Site specific contaminants such as

heavy metals are not characterized in this study, but they also effect the utilization of FF.

The aim of this study was to characterize the FF mined from two landfill sites by analyzing the content of the

FF. The characterized properties are the water content, organic content (VS) and biochemical methane

potential (BMP) of the FF. Also pH, chemical oxygen demand (COD), total nitrogen and phosphorus of the

FF were studied after leaching. The FF samples were further sieved to examine the organic matter content

and the methane potential in smaller fractions to compare these properties in various size fractions. The

results can be used in assessing the processing, use and disposal of the FF while planning landfill mining.

2. Materials and Methods

2.1 Sampling sites

The two studied landfills are located in Kuopio in central Finland and in Lohja in southern Finland. The

Kuopio landfill contains MSW landfilled between 2001 and 2011. The landfilled waste was affected by

changes in the local waste management system, as biowaste source segregation was initiated in 2004. Since

2009, MSW has been mechanically pre-treated, and only sieved underflow (< 70 mm) has been landfilled.

Regional paper, glass, hazardous waste, and metal collection systems were used during the landfill’s history.

The Kuopio landfill has a sealed bottom structure according to the EU requirements. For this study, the

landfill was sampled when the vertical gas collection system was built in July 2012. The Lohja landfill was

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landfilled between 1967 and 1989 with MSW, construction waste and soil. The area is closed with a top

cover but has no bottom structure; the gas collection system was built in 2000. The site was sampled in June

2013.

2.2 Sampling

In both sites, samples were taken from wells (0.9 m borehole) drilled with a hydraulic piling rig Casagrande

B 170. In the Kuopio landfill, the samples (six in total) were taken from three wells at two depths (referred to

as the bottom layer and the middle layer). The cutting points of the layers were chosen so that layers would

present approximately same years of landfilling. Wells are referred as KU1, KU2 and KU4, while middle

layer samples are named with number 1 (e.g. KU1.1) and bottom layer samples with 2 (e.g. KU1.2). In the

Lohja landfill, vertical samples (altogether seven samples) were taken from four wells, of which two wells

were studied as single samples, one well was divided into a three-layer samples and one well into a two-layer

sample. One sample was not studied further, because it contained only soil without any waste materials.

Depth of wells was determined so that layer between soil and waste would not be affected by sampling.

Wells were referred as LO1, LO2, LO3 and LO4. Different layers were numbered starting from the top layer

(LO1.1, LO1.2, LO1.3, LO4.1 and LO4.2). Samples, sampling depths and masses are presented in Table 1.

Due to technical failure in transporting the sample KU4.2 from auger to skip, sample mass was less than that

of other samples.

Immediately after the materials were drilled from landfill, the samples were stored for 1–2 weeks in ambient

conditions in dumpsters. During sampling and storing, water may have evaporated or poured off the samples

since the water was not collected and the dumpsters were not closed. The samples were sieved and sorted at

the site manually from approximately 600 L of sub-samples collected from the dumpsters. Samples were

manually sieved in the Kuopio landfill to separate four particle size categories (> 100 mm, 40–100 mm, 20–

40 mm and < 20 mm) and in the Lohja landfill three particle size categories the (> 100 mm, 20–100 mm and

< 20 mm). Size categories 20–40 mm and 40–100 mm were combined in the Lohja landfill because 20–40

mm was very small fraction of the Kuopio landfill (6 w-%). Samples were weighed before and after sieving.

Particles smaller than 20 mm were referred as FF. In the Kuopio landfill, the rest of the sampled material

(which was not manually sieved) was mechanically pre-treated using the same full-scale machinery that

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processed the MSW (since 2009). Three middle layer samples and three bottom layer samples were

combined before mechanical treatment (middle layer referred as KUMTP1 and bottom layer as KUMTP2).

The mechanical pre-treatment plant consisted of a shredder, a magnetic separator, a drum sieve (< 30 mm

and 30–70 mm) and a wind sieve as described in Kaartinen et al., (2013). The fractions examined were the <

30 mm and 30–70 mm drum-sieved fractions.

The sieved and sorted samples were packed in 10 L buckets and transferred to the laboratory where they

were stored at 7 °C. Maximum storage time was 6 months.

2.3 Batch assays

BMP was determined in duplicate or triplicate in 1 L glass bottles, which contained 500 mL (Kuopio

samples) or 350 mL (Lohja samples) inoculum and waste samples at a ratio of 0.5 g VS inoculum/g VSwaste. The

inoculum was digested mesophilic municipal sewage sludge from the Viinikanlahti sewage treatment plant

(Tampere, Finland). The inoculum was assayed alone, and its gas production was excluded from that of the

samples. 50 mL of 42 g/L NaHCO3 was added each bottle to adjust and buffer pH. Deionized water was

added so that the total liquid volume in all bottles was 700 mL. Bottles were flushed 2–3 min with N2 gas

before sealing. The samples were incubated at 35 °C in a water bath. The biogas produced was collected in

aluminum gas bags. The BMPs were continued until methane production became negligible (< 5 mL CH 4/d)

after 130–160 days.

2.4 Analyses

Total solid (TS) and volatile solid (VS) were analyzed according to standard methods (APHA, 1998). pH and

soluble COD were determined from the leachate obtained in a one-stage shaking leaching test, which was

performed at a liquid to solid ratio of 10 L/kg according to standard EN 12457-4 (2002), except the size

fraction was 20 mm instead of 10 mm. pH was measured with a WTW ProfiLine pH 3210 and SenTix 51

electrode. Soluble COD (GF/A filtered) was analyzed according to SFS 5504 (1988). Soluble nitrogen and

phosphorus were extracted with water at a 1:5 ratio (SFS-EN 13652, 2002). Nitrogen was analyzed with the

Kjeldahl methods (SFS-EN 13342, 2000 and SFS-EN 13654, 2002). The samples for the phosphorus

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analysis were pretreated according to standards ISO 11464 (2007) and ISO 11466 (2007), and the total and

soluble phosphorus were measured with inductively coupled plasma atomic emission spectroscopy.

Sieving in the laboratory was performed with a sieving column. The sieves were 20, 16, 11.2, 8, 5.6, 4, 2, 1

and 0.5 mm. About 1–2 kg of the samples was sieved (at the moisture content the sample had after storing)

by mechanically shaking the sieving columns. Fractions larger than 20 and 16 mm were combined before the

sieved samples were analyzed.

Methane content was measured with a Shimadzu GC-2014 gas chromatograph with a thermal conductivity

detector. The column used was Agilent Porapak N 80-100 Mesh (length 1.8 m and inner diameter 2.00 mm).

The carrier gas was helium with flow rate 25 mL/min. Temperature of injector and detector were 80°C and

oven 40°C. The biogas volume was measured using the water displacement method.

3. Results

3.1 Manually sieved fine fraction

FF (< 20 mm) constituted the major fraction of the samples being 38.0–53.9% (mean 45 ± 7%) of the mass

of the Kuopio landfill and 39.8–73.6% (58 ± 11%) of the mass of the Lohja landfill (Table 1). All 12 FF

samples from both landfills were characterized for TS and VS content as well as for pH and soluble COD in

the leaching test at a 10 L/kg liquid to solid ratio (Table 2). Soluble and total nitrogen and phosphorus were

analyzed from the mixed middle and bottom layer Kuopio samples and separately in all Lohja samples

(Table 2). Among all the samples, the TS ranged between 46% and 82% (mean 64.2%) and the VS between

4.9% and 16.8% (mean 10.3%) without clear trends according to the landfill site or height. The VS/TS ratio

was low in both landfills but it was lower in the Lohja samples (6.0–24.0%, mean 12.8%) than in the Kuopio

samples (15.5–27.3%, mean 21.3%). The Kuopio middle layer samples had higher TS and VS/TS values

(mean 61.6% and 22.4%, respectively) than the bottom layer samples (mean 51.8% and 20.3%). In the Lohja

well divided into three layers, the VS/TS ratio was higher in the bottom layer (24.0%) than in the upper layer

(8.1%), which means that organic matter content can also remain for a long time in landfills.

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From one-stage shaking leaching test, soluble COD was 2.77 ± 1.79 kg/t TS in the samples from Kuopio and

values were lower in bottom layers (0.79–1.27 kg/t TS) than in the middle layers (3.79–4.60 kg/t TS). In the

Lohja, the soluble COD was at same range as in bottom layers in Kuopio (0.53–0.96 kg/t TS). The pH of the

leachate ranged from 6.83 to 7.57 in the Kuopio samples and from 7.16 to 7.88 in the Lohja samples. The

Kuopio bottom layer sample had higher pH than the middle layer in the two sampling wells (KU1 and KU2)

while in one well (KU4) the pH was similar in both layers.

The total nitrogen was 1.4–8 g/kg TS, water soluble nitrogen < 0.012–0.425 g/kg TS, total phosphorus <

1.0–1.5 g/kg TS and water soluble phosphorus under the 10 mg/kg TS detection limit except in one sampling

point (12 mg/kg TS) in both landfills. The samples with the lower VS/TS ratio (6–8%) also had lower

nitrogen and phosphorus content, though phosphorus was under the detection limit in most of the samples.

Two samples from Kuopio (KU1.1 and KU1.2) and four samples from Lohja (LO1.2, LO1.3, LO2 and LO3),

with the high organic matter content compared with other samples in this study, were sieved in the laboratory

to study size distribution in the FF (Figure 1). Sieving showed that in the Kuopio samples 78–81 w-% of the

FF was < 11.2 mm and 51–52 w-% < 5.6 mm, and in Lohja samples, these same fractions were 88–93 w-%

and 66–74 w-% (except one sample having 40 w-% under 5.6 mm). The size distribution was similar in the

middle and bottom layer samples in the Kuopio landfill. In all sieved samples, the VS/TS ratio range (17.2 ±

6.0% in Kuopio and 14.1 ± 7.4% in Lohja) was the same in the different size categories as in the overall

samples (Figure 1).

The BMP of the 12 FF samples was assayed at 35 °C for 130–160 days with inoculum. The BMP ranged

from as low as 0.4 up to 26.6 m3 CH4/t TS (Table 2). The three middle layer Kuopio samples had three times

higher BMP (19.2–26.6 m3 CH4 / t TS, mean 22.6 m3 CH4 / t TS) than the three bottom layer Kuopio samples

(0.4–9.9 m3 CH4 / t TS, mean 6.1 m3 CH4 / t TS), and the six Lohja samples had BMP between 1.2–10.0 m3

CH4 / t TS (mean 5.8 m3 CH4 / t TS). In contrast to the Kuopio samples, in Lohja the one bottom layer

sample studied had higher BMP (LO1.3: 10.0 m3 CH4 / t TS) than the samples from upper layers (LO1.1 and

LO1.2: 1.2 and 6.7 m3 CH4 / t TS). With all FF samples, methane production was the fastest during the first

10 days (Figure 2) and up to 60–97% (mean 76%) of the methane was produced during the first 30 days of

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the 130–160 days of incubation, except for one sample from the Lohja landfill that produced 33% of the

methane during the first 30 days (sample LO1.1). Standard deviations of BMP samples were in some results

very high, or even higher than the actual results due to heterogeneous of material.

To compare the BMP of different fractions, it was also measured from four sieved Lohja samples < 11.2 mm

fraction. The BMP of the four samples was 2.7–8.7 m3 CH4 / t TS fraction, which was 63–100% of the BMP

of the < 20 mm fraction. Thus, further sieving of the FF below 11.2 mm did not increase methane

production, but decreased it compared with FF.

3.2 Mechanical plant processed material

The mined Kuopio landfill waste samples were mixed from three wells according to middle and bottom

layers to form two samples, which were then shredded and drum sieved into two size fractions (< 30 mm and

30–70 mm) in mechanical plant. These mechanically processed samples were analyzed for TS, VS and BMP.

The pH and soluble COD were also measured from size fraction < 30 mm from one-stage shaking leaching

test. All four samples contained more plastics, paper and cardboard, textiles and other materials than the FF,

because the waste was shredded before drum sieving. These samples had lower TS (mean 34.5%) than the

FF, but the VS/TS ratio was significantly higher (25.2–60.1%) than that of the FF (6–27%). The pH and

soluble COD were in middle layer < 30 mm 6.26 and 5.30 kg/t TS and in bottom layer 7.20 and 1.42 kg/t TS.

The pH of middle layer was slightly lower than in FF, and COD was in both layers higher than in FF from

same layers. The mechanically treated bottom layer samples clearly had higher methane potential than the FF

samples. The bottom layer samples produced less methane (< 30 mm 10.6 m3 CH4 / t TS and 30–70 mm 15.3

m3 CH4 / t TS) than the middle layer samples (< 30 mm 55.2 m3 CH4 / t TS and 30–70 mm 141.7 m3 CH4 / t

TS).

4. Discussion

Present and previous studies on landfill mining suggest that the FF contributes up to 40–70 w-% of

excavated landfill materials (Table 3). Contrary to the most landfills, in a landfill in Sweden the FF was only

14.8–24.7 w-%, while the largest fraction was over 50 mm (48.1–59.2 w-%) (Hogland et al., 2004) and in

Thailand in an open dumpsite 18 w-% was FF (Prechthai et al., 2008). Sieving showed that in Kuopio 29.6–

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43.7 w-% of the landfill material was smaller than 11.2 mm and in Lohja 35.0–68.4 w-%. Similarly, in the

Swedish FF (< 18 mm) more than 90% of the FF sample was smaller than 10 mm (Hogland et al., 2004), and

in New Jersey, 80% of the FF was smaller than 12.5 mm (Hull et al., 2005). The presence of large amounts

of FF could mean that the FF consists of soil used as intermediate or daily cover in landfills. An open

dumpsite might have low FF content, because intermediate covers are not used as much as in monitored

landfills. Intermediate and daily cover is a 15–30 cm layer of, for example, soil, clay or compost

(Tchobanoglous et al., 1993). According to the standard EN ISO-14688-2 (2005), the FF mined from Kuopio

and Lohja could be classified as gravel or sandy gravel, but the sieving was performed without drying; thus,

classification is approximate. The FF also increased when the age of the waste increased due to the

decomposition of the MSW, as in Lohja compared with the Kuopio landfill, or as in Shanghai, where the FF

(< 15 mm) was 10 w-% of fresh waste, 18.7 w-% of 4-year-old waste and 45.3 w-% of 10-year-old waste

(Zhao et al., 2007).

The mined FF is not only soil but also contains decomposed materials, metals and other small particles from

landfill waste. Kaartinen et al., (2013) presented the results for the concentrations of elements and total

organic carbon (TOC) in the Kuopio landfill. The organic content of the FF was 6.0–35.0% as the VS/TS

ratio and 4.7–15.1% as TOC (Table 3), thus the FF contains organic matter, but the amount is low compared

with fresh MSW. In fresh MSW, the VS/TS ratio was 76% for residual Danish household waste (Riber et

al., 2009) and about 80% for organic fraction of MSW in the United Kingdom (Zhang & Banks, 2013).

Sieving the Kuopio and Lohja samples showed that VS was evenly distributed in all 0.5–20 mm fractions.

The lower organic content suggested that organic matter had decomposed in older landfill layers compared

with new landfills like Kuopio or fresh MSW, resulting in decreased organic matter. The increasing age of

waste also decreased the TOC of the FF (Quaghebeur et al., 2013). The larger fraction in the Kuopio landfill

(mechanically treated 30–70 mm) had a higher VS/TS ratio (41.4–60.1%) compared with the FF, which

suggests that the larger fraction contained material with high organic content such as paper or cardboard,

which was also visually seen.

The total nitrogen content in both landfills (mean 3.5 ± 2.0 g/kg TS) was similar to that of two other Finnish

landfills where the total Kjeldahl nitrogen content was 4 ± 2 kg/t TS (in Kujala) and 3.9 ± 1.5 kg/t TS (in

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Ämmässuo) (Sormunen et al., 2008). Total phosphorus was below the detection limit in almost all sampling

points (< 1.0–1.5 g/kg TS) in both landfills, and similarly, in Sweden the total phosphorus was 0.8–1.6 g/kg

TS (Hogland et al., 2004). In fresh biowaste (vegetable food, animal food, yard waste) from Danish

household waste, the nitrogen content was 15–70 g/kg TS and the phosphorus content 2.0–10.0 g/kg TS

(Riber et al., 2009), which is significantly higher than that measured in landfills. When organic waste ends

up in a landfill, the nutrient content must have leached or otherwise changed form, because it is not found in

landfills. The average water soluble nitrogen (0.257 g/kg TS) and soluble COD (1.73 kg/t TS) mean that the

FF from landfill, for example, sized 10 000 t and 50 w-% of FF could leach 825 kg of nitrogen and 5.5 t of

COD. Even higher COD contents have been reported in other two Finnish landfills (Kujala and Ämmässuo,

5.6 ± 4.8 g/kg TS and 19.3 ± 11.1 g/kg TS, respectively), when studied fraction was shredded and sieved <

50 mm (Sormunen et al., 2008). These possible leachates need to be treated when mined FF is processed for

reuse.

The BMP of the FF has not been studies before unlike the calorific value of the FF, which has previously

been reported to be low, 2.2–4.8 MJ/kg TS (< 10 mm) (Quaghebeur et al., 2013) and 0.4–0.9 MJ/kg (< 18

mm) (Hogland et al., 2004), compared with fresh household waste, 15.42 MJ/kg TS (Riber et al., 2009). This

study showed that samples from old landfill (Lohja 5.8 ± 3.4 m3 CH4/t TS) produced less methane than

samples from new landfill (Kuopio 22.6 ± 3.7 m3 CH4/t TS in the middle layer and 6.1 ± 5.0 m3 CH4/t TS in

the bottom layer). Similar trend have been observed in two Finnish landfills, Kujala and Ämmässuo (Table

3) from waste samples shredded and sieved < 50 mm, where the BMP was lower in the bottom layers of

landfills (Sormunen et al., 2008). In a study conducted in Danish landfills, the fresh waste fraction with low

organic content produced similar amounts of methane as FF in this study: Shredded waste (VS 29–31%) had

a BMP of 8.6–12.7 m3 CH4/t waste and bulky mixed waste (VS 7–8%) 7.5–9.8 m3 CH4/t waste, but

combustible waste with higher organic content (VS 69–70%) produced 148.0–163.8 m3 CH4/t waste (Mou et

al., 2014). However, a much lower BMP was found in a landfill in Florida where the methane production of

waste samples aged 1 year was 0.1–0.3 m3 CH4/kg VS, but the fraction examined was much smaller (< 4.75

mm) than in this study (Kim & Townsend, 2012). This study also showed that the methane potential

increases when the particle size increases since the fraction smaller than 11.2 mm had a BMP of 2.7–8.7 m 3

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CH4/t TS, the FF < 20 mm 0.4–26.6 m3 CH4/t TS, the < 30 mm fraction 10.6–55.2 m3 CH4/t TS and the 30–

70 mm fraction 15.3–141.7 m3 CH4/t TS. Thus processing of the mined material can be used to affect the fine

fraction characteristics, especially the sieve size greatly affects the utilization and disposal methods of the

sieved materials.

5. Conclusions

This study characterized the organic content, nutrients and BMP of the fine fraction from two landfills. In

Kuopio 45 ± 7 w-% and in Lohja 58 ± 11 w-% of the mined landfill material was below 20 mm, and this

study showed that 86.5 ± 5.7 w-% of the FF was smaller than 11.2 mm and 59.7 ± 13.2 w-% of the FF was

smaller than 5.6 mm, and the material could be classified as gravel or sandy gravel. The FF cannot be

compared with soil, because the FF from landfills contains possible inorganic and organic contaminants, for

example, heavy metals, decomposed materials and other particles. The organic content and the BMP of the

FF were low in the 24- to 40-year-old landfill (VS/TS 12.8 ± 7.1% and BMP 5.8 ± 3.4 m3 CH4/t TS)

compared with the 1- to 10-year-old landfill (21.3 ± 4.3% and 14.4 ± 9.9 m3 CH4/t TS), which showed that

the organic matter content decreased when the age of the waste increased. The methane potential increased

when the particle size of the waste fraction increased as fraction < 11.2 mm had a BMP of 5.6 m3 CH4/t TS,

the FF (< 20 mm) 10.1 m3 CH4/t TS, the mechanically treated < 30 mm fraction 32.9 m3 CH4/t TS and the

30–70 mm fraction 78.5 m3 CH4/t TS. In both landfills, the nutrient content (1.4–8 g N/kg TS and < 1.0–1.5

g P/kg TS) was significantly lower than in fresh waste. The results can be utilized when evaluating the

possible methods for processing or stabilizing fine fraction before reuse as material or energy.

Acknowledgements

This study was supported by the Finnish Funding Agency for Technology and Innovation (TEKES, Grant

No. 40474/11) and the Finnish companies Ekokem Oy, BMH Technology Oy, Lassila & Tikanoja Oyj, and

Rosk’n Roll Oy Ab. PhD Kai Sormunen (Ramboll Finland Oy) and MSc Tommi Kaartinen (VTT Technical

Research Centre of Finland) are acknowledged for major role in sampling and processing the samples in

landfill.

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Table 1 Sampling points, sample depths, masses and portion of fine fraction in two studied landfills.

Sampling point Depth (m) Sample mass (t) < 20 mm (w-%)Kuopio landfillKU1.1 2–10 3.4 38.0KU1.2 10–22 7.2 49.8KU2.1 2–14 8.2 50.2KU2.2 14–26 9.9 38.0KU4.1 2–15 11.0 41.2KU4.2 15–31 3.3 53.9Lohja landfillLO1.1 2–5 8.8 39.8LO1.2 5–9 1.9 58.3LO1.3 9–13 3.5 61.6LO2 2–10 3.0 59.4LO3 2–10 4.7 56.5LO4.1 2–9.3 10.7 not studieda

LO4.2 9.3–10 1.0 73.6a = sample contained only soil and not waste materials

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Table 2 Characteristics of fine fraction and mechanically sieved samples from Kuopio and Lohja landfills with standard deviations ( ±) for TS, VS and BMP results.

TS VS VS/TS pH sCOD total N Nws total P Pws BMP 30d BMP end

% % % kg/t TS g/kg TS g/kg TS g/kg TS mg/kg TS L CH4 / kg TS L CH4 / kg TS

Kuopio landfill, fine fraction < 20 mmKU1.1 61.9 ± 1.2 16.9 ± 0.7 27.3 ± 1.5 6.89 3.85

3.5a 0.398a 1.1a < 10a 18.1± 5.2 26.6 ± 6.5KU1.2 56.6 ± 1.0 8.8 ± 0.4 15.5 ± 0.6 7.57 0.79 5.3 ± 1.7 8.1 ± 2.8KU2.1 67.3 ± 1.7 11.7 ± 0.7 17.4 ± 0.6 7.15 3.79

4a 0.345a < 1.0a < 10a 13.8 ± 0.1 19.2 ± 0.2KU2.2 46.2 ± 1.7 11.2 ± 0.6 24.3 ± 1.3 7.56 1.27 6.4 ± 0.1 9.9 ± 0.2KU4.1 55.7 ± 3.9 12.5 ± 1.8 22.7 ± 4.8 6.99 4.60

8a 0.041a 1.4a < 10a 15.3 ± 4.9 22.1 ± 8.2KU4.2 52.6 ± 2.8 11.1 ± 1.0 21.2 ± 2.0 6.83 1.17 0.4 ± 0.1 0.4 ± 0.1Kuopio landfill, mechanical treatment plant, sieved < 30 mm and 30 – 70 mmKUMTP1 <30 39.8 ± 2.2 14.7 ± 1.4 37.1 ± 4.6 6.26 5.30 n.a. n.a. n.a. n.a. 33.6 ± 4.8 55.2 ± 22.6KUMTP2 <30 47.3 ± 1.5 11.9 ± 0.6 25.2 ± 2.0 7.20 1.42 n.a. n.a. n.a. n.a. 7.7 ± 0.3 10.6 ± 3.8KUMTP1 30-70 20.8 ± 1.2 12.5 ± 1.3 60.1 ± 2.7 n.a. n.a. n.a. n.a. n.a. n.a. 99.7 ± 7.9 141.7 ± 9.7KUMTP2 30–70 29.9 ± 3.1 12.4 ± 1.0 41.4 ± 8.1 n.a. n.a. n.a. n.a. n.a. n.a. 11.1 ± 12.3 15.3 ± 19.8Lohja landfill, fine fraction < 20 mmLO1.1 76.6 ± 1.1 6.2 ± 1.0 8.1 ± 1.3 7.47 0.70 1.4 0.061 < 1.0 < 10 0.4 ± 0.8 1.2 ± 1.4LO1.2 68.8 ± 0.9 11.4 ± 0.2 16.6 ± 0.5 7.40 0.79 3.5 0.425 < 1.0 < 10 4.1 ± 1.1 6.7 ± 1.5LO1.3 59.6 ± 1.6 14.3 ± 0.8 24.0 ± 1.6 7.16 0.63 5 0.421 1.5 < 10 6.2 ± 0.5 10.0 ± 1.4LO2 63.5 ± 1.3 9.8 ± 0.4 15.4 ± 0.9 7.32 0.96 3.3 0.107 1.2 12 6.2 ± 0.9 8.9 ± 1.8LO3 81.6 ± 1.0 4.9 ± 0.4 6.0 ± 0.5 7.73 0.55 1.6 < 0.012 < 1.0 < 10 2.1 ± 1.0 2.7 ± 1.5LO4.2 80.0 ± 0.6 5.4 ± 0.1 6.7 ± 0.2 7.88 0.53 1.5 < 0.012 < 1.0 < 10 4.1 ± 0.8 5.4 ± 1.3sCOD = soluble COD, ws = water soluble, BMP 30d = biochemical methane potential at day 30, BMP end = biochemical methane potential after incubation at day 160 (Kuopio) or 130 (Lohja), n.a. = not analyzed, a = middle and bottom layer from same well were mixed for nutrient analysis

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Table 3 Amount of fine fraction (FF) and its organic content as VS/TS and TOC and biochemical methane potential (BMP) in landfills presenting different ages. In addition, results are compared with shredded landfill waste.

Landfill Age of landfill Particle size Amount of FF (w-%) VS/TS (%) TOC (%) BMP (m3 CH4/t TS) Reference

Kuopio, Finland 1–10 years < 20 mm 38.0–53.9 15.5–27.3 4.7–5.6 14.4 ± 9.9 This study; Kaartinen et al., 2013

Lohja, Finland 24–40 years < 20 mm 39.8–73.6 6.0–24.0 n.a. 5.8 ± 3.4 This study

Houthalen, Belgium 14–29 years < 10 mm 44 ± 12 n.a. 7.6–12.4 n.a. Quaghebeur et al., 2013

New Jersey, USA 1–11 years < 25.4 mm 50–52 24.4–35.0 n.a. n.a. Hull et al., 2005

Northern Italy 5–15 years < 20 mm 45–55 n.a. 10.7–15.1a n.a. Raga & Cossu, 2013

Måsalycke, Sweden 17–22 years < 18 mm 14.8–24.7 n.a. n.a. n.a. Hogland et al., 2004

Nonthaburi, Thailand 3–5 years < 25 mm 18 n.a. n.a. n.a Prechthai et al., 2008

Kujala, Finlandb 15–44 years < 20–50 mmb n.a.b 16–36 n.a. 8–22 Sormunen et al., 2008

Ämmässuo, Finlandb 1–17 years < 20–50 mmb n.a.b 55–65 n.a. 21–68 Sormunen et al., 2008

n.a = not available, a = % of dry weight, b = studied material was landfill waste which was shredded and sieved to < 20 mm for VS/TS and to < 50 mm for BMP

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Figure 1 Particle size distribution of the FF and organic matter content (as VS/TS) in different particle sizes in Kuopio landfill (A) and Lohja landfill (B).

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Figure 2 Methane production of the Kuopio FF samples (A), Lohja FF samples (B) and mechanically treated fraction from Kuopio (C).