Anaerobic treatment of sludge from a
nitrification–denitrification landfill leachate plant
E. Maran˜o´n
a,*
, L. Castrillo´n
a
, Y. Ferna´ndez
a
, E. Ferna´ndez
b
a
Chemical and Environmental Engineering Department, Higher Polytechnic School of Engineering, University of Oviedo,
Campus of Viesques, 33204 Gijo´n, Spain
b
COGERSA, 33697 Serı´n, Gijo´n, Spain
Accepted 2 August 2005
Available online 26 September 2005
Abstract
The viability of anaerobic digestion of sludge from a MSW landfill leachate treatment plant, with COD values ranging between 15,000
and 19,400 mg O
2
dm
3
, in an upflow anaerobic sludge blanket reactor was studied. The reactor employed had a useful capacity of 9 l,
operating at mesophilic temperature.
Start-up of the reactor was carried out in different steps, beginning with diluted sludge and progressively increasing the amount of
sludge fed into the reactor. The study was carried out over a period of 7 months. Different amounts of methanol were added to the feed,
ranging between 6.75 and 1 cm
3
dm
3
of feed in order to favour the growth of methanogenic flora.
The achieved biodegradation of the sludge using an upflow anaerobic sludge blanket Reactor was very high for an HRT of 9 days,
obtaining decreases in COD of 84–87% by the end of the process. Purging of the digested sludge represented
16% of the volume of the
treated sludge.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Landfilling still remains one of the main methods for
disposing of municipal and industrial solid waste. About
18 million tons of municipal solid waste (MSW) are gener-
ated annually in Spain. The degradation of the organic
fraction of the waste in the landfill in combination with
the percolation of rainwater produces a polluted liquid
called leachate. There are a number of factors that affect
the quality and the quantity of such leachates (
): seasonal weather variation, landfilling tech-
nique, compaction method, waste composition and the
age of the landfill (
Baig et al., 1999; Ehrig, 1983; Kang
The specific composition of leachates determines their
relative treatability. Various processes have been em-
ployed, such as anaerobic and aerobic biological degrada-
tion,
chemical
oxidation,
coagulation–precipitation,
activated carbon adsorption, and membrane processes
(
Haapea et al., 2002; Di Palma et al., 2002
Biological processes are quite effective in removing or-
ganic matter when applied to relatively young leachates.
The organic pollutant load of leachates generally reaches
maximum values during the first years of operation of a
landfill and then gradually decreases over succeeding years
(
Rodrı´guez et al., 2000; Warith, 2002
). The refractory or-
ganic contaminants (low ratios of BOD
5
/COD) contained
in biologically pretreated leachate and old landfill leachates
are not amenable to conventional biological processes and
must be treated by a physico-chemical process (
).
High concentrations of ammonium nitrogen are a com-
mon feature of leachates, normally around 2000 mg dm
3
.
Different techniques can be used to remove ammonium,
such as air stripping (
Berrueta and Castrillo´n, 1997
), chem-
ical precipitation (
Li et al., 1999; Li and Zhao, 2001
) or aer-
obic–anoxic biological treatment (
0956-053X/$ - see front matter
Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.wasman.2005.08.002
*
Corresponding author. Tel.: +34 985 18 20 27; fax: +34 985 18 23 37.
E-mail address:
(E. Maran˜o´n).
www.elsevier.com/locate/wasman
Waste Management 26 (2006) 869–874
). The phosphorus content is generally low, usu-
ally in the range of 16–3.5 mg dm
3
(
1997; Berrueta and Castrillo´n, 1992
). Supplementary addi-
tion of phosphorus is often necessary for the biological
treatment of landfill leachates.
The treatment processes used for landfill leachates often
involve a combination of appropriate techniques (
et al., 2003; Kargi and Pamukoglu, 2003
). However, most
of these techniques generally produce different amounts
of sludge, a residue that needs to be treated.
Different systems for the treatment of landfill leachate
can be found in the reviewed bibliography, although no
studies on the treatment of sludge from these plants. This
sludge has usually been disposed of in landfills, but this sit-
uation is being substantially modified due to the Council
Directive 1999/31/CE currently in force concerning waste
disposal. According to this Directive, the disposal of or-
ganic waste will have to progressively decrease and alterna-
tive valorisation methods, such as biological or thermal
methods, are to be applied.
Anaerobic biological methods are currently being used
for the treatment of sludge, being the oldest and most
important process for sludge stabilisation worldwide
(
Schellinkhout, 1993; Seghezzo et al., 1998; Gavala et al.,
).
Asturias is a region located in the North of Spain, with a
population of approximately 1,100,000 inhabitants. Its cli-
mate is humid, with an average annual rainfall of 900 mm.
The MSW generated in the region is managed by the Con-
sortium for the Management of Municipal Solid Waste
(COGERSA), to which all the Asturian municipalities be-
long. This waste has been disposed of at the ConsortiumÕs
Central Landfill at La Zoreda since 1986, the amount of
which has increased with time up to the present day level
of around 550,000 t/year. The volume of leachate gener-
ated has also increased, reaching levels of around
600 m
3
day
1
during periods of heavy rainfall. The COD
of the leachate has decreased with time from 80,000 mg
O
2
dm
3
at the beginning to current values of around
3000 mg O
2
dm
3
(with BOD
5
values of around 700 mg
O
2
dm
3
), indicating that leachates have a large amount
of refractory organic matter. The leachates produced are
collected and subsequently transferred to a treatment
plant. The treatment system employed consists of a pres-
surised nitrification–denitrification process, followed by
ultrafiltration to remove the nitrogen. An adsorption plant
for the removal of organic refractory matter is currently
under construction. The biogas collected from the landfill,
about 13,200 m
3
day
1
, is used as fuel, mainly for generat-
ing electricity.
The scope of the present work was to analyse the effect
of biological treatment (a nitrification–denitrification pro-
cess) on the content of ammonium nitrogen and organic
matter from the leachates generated at the landfill of Astu-
rias and to analyse the viability of anaerobic digestion of
the sludge produced in this treatment. Among high-rate
anaerobic reactors, the upflow anaerobic sludge blanket
process is widely employed, although mainly for wastewa-
ter. Hundreds of full-scale treatments plants have been in-
stalled over the past decade for the treatment of different
types of wastewater (
Zoutberg and Zerrin, 1999; Monroy
). However, the possibility of using upflow anaerobic
sludge blanket reactors to treat sludge or other types of
waste is currently under study.
used
this type of reactor for the digestion of olive oil mill efflu-
ents mixed with swine manure, obtaining COD reductions
up to 75%.
Castro-Gonza´lez et al. (2001)
employed it for
the stabilisation of excess biological sludge generated in
the treatment of wastewater from a sugarcane mill, with
COD removal efficiencies around 50%. The authors have
studied the digestion of cattle manure screened through
1 mm mesh, obtaining up to 75% COD removal (
et al., 2001; Castrillo´n et al., 2002
).
Our aim in the present study was to prove that this type
of reactor is adequate for treating the excess sludge gener-
ated in the nitrification–denitrification treatment of landfill
leachates, as an alternative to employing a CSTR unit,
widely used for this type of waste. Shorter hydraulic reten-
tion times may thus be achieved, therefore needing smaller
units. In addition, the fact that upflow anaerobic sludge
blanket reactors do not need stirring reduces operational
costs.
2. Material and methods
2.1. Landfill leachate treatment plant
The treatment system employed at the landfill under
study consists of a pressurised nitrification–denitrification
process, characterised by a high concentration of volatile
solids (14,000 mg dm
3
) and increased oxygen solubility
as a consequence of the elevated pressure (2.5–3.0 bar).
The biomass is subsequently separated by means of ultrafil-
tration. The plant treats 550 m
3
day
1
of leachate, metha-
nol being added as a source of easily biodegradable
organic matter to carry out the denitrification. The amount
of sludge generated is around 30 m
3
day
1
.
Different samples of leachates, both untreated and bio-
logically treated, as well as the sludge produced at the
leachate treatment plant, were characterised.
2.2. Equipment at laboratory scale
Studies to analyse the viability of anaerobic digestion
of the sludge produced in the biological treatment plant
were carried out at laboratory scale employing an upflow
anaerobic sludge blanket reactor made of transparent
PVC. The reactor consisted of two cylindrical sections,
the lower one jacketed and separated from the upper
one by a deflecting ring so as to facilitate phase separa-
tion. The upper part had a larger diameter and contained
the gas collector, as well as outlets for the effluent,
870
E. Maran˜o´n et al. / Waste Management 26 (2006) 869–874
recycling and other uses. Other side-outlets were ar-
ranged
along
the
lower
body
for
sample
taking
(
). The volume of the reactor up to the triphasic
separator was 9 l.
The gas collector is connected to a gasometer, which
consists of a cylindrical recipient, 24.5 cm in diameter
and 50 cm high. It is divided into two parts: the top part
is totally airtight and the bottom part has a side opening
or window that allows access to the valve controlling the
outlet of the biogas. The upper tank is also coupled to a
valve that allows the gasometer to be filled with the li-
quid above which the biogas is collected. This outlet is
also used to measure the composition of the biogas pro-
duced by means of its connection to a portable
methanometer.
2.3. Chemical analyses
The parameters analysed were chemical oxygen demand
(COD), biochemical oxygen demand (BOD
5
), ammonium
nitrogen, Kjeldahl nitrogen, phosphate
ðPO
3
4
Þ, total solids
(TS), volatile total solids (VTS), suspended solids (SS), vol-
atile acidity (VA), total alkalinity (TA), metals, gas volume
and gas composition. Standard methods were employed
(
).
Metals were determined by atomic absorption on a Per-
kin Elmer Mod. 3110 spectrophotometer.
The volumetric composition of the biogas was deter-
mined by means of a Geotechnical Instruments portable
methanometer.
2.4. Experimental work
The sludge produced in the leachate treatment plant was
supplied by the Consortium and transported to the labora-
tory in plastic containers.
The upflow anaerobic sludge blanket reactor was inocu-
lated with sludge from a similar reactor that is used to treat
cattle slurry (
). The study was carried
out over a period of 7 months. The hydraulic residence
time employed was 9 days, on the basis of results obtained
with other organic waste with a similar COD.
Start-up of the reactor was carried out with diluted
sludge (10%), the amount of sludge fed into the reactor
being progressively increased (from 10% at the beginning
to 100% at the end). Different amounts of methanol were
added to the feed, ranging between 6.75 and 1 cm
3
dm
3
of feed, in order to favour the growth of methanogenic
flora. If needed, other substances such as bicarbonate or
hydrochloric acid were added to control the pH. Phosphate
was added at the end of the experiment, when feeding undi-
luted sludge, with the aim of maintaining a C/P ratio of
around 100/0.1.
Recirculation of the effluent was carried out at the end
of the experiment (recirculation ratio: 2) to improve the
mixing in the reactor and also to decrease both the COD
and the solids content of the influent.
The aforementioned parameters were determined in the
influent and effluent in order to control the digestion process.
Once the parameters of the effluent presented stable values,
the organic loading rate (OLR) was progressively increased
by decreasing the water added to the sludge until completing
the start-up stage. The solids content inside the reactor was
likewise characterised, as well as the biogas produced.
3. Results and discussion
3.1. Characteristics of the landfill leachates and the sludge
produced in the biological treatment plant
As mentioned above, the leachates generated in the
landfill are treated by a nitrification–denitrification process
in pressurised bioreactors, followed by ultrafiltration to
separate the biomass.
shows the results obtained
at the landfill leachate plant during the period under study.
The COD of the treated leachates presented values in the
range of 1000–1500 mg O
2
dm
3
, whereas the values of
the BOD
5
were very low (60–90 mg O
2
dm
3
), indicating
that most of the organic matter is resistant to biological
degradation. The amount of NH
þ
4
-N present in the leach-
ate was considerably reduced by means of this treatment
(from values of 1350–2670 mg dm
3
to values of 34–
48 mg dm
3
). The final effluent is sent to a sewage plant
where is treated together with domestic wastewater. This
sewage plant treats around 3600 m
3
h
1
of wastewater
(259,125 equivalent inhabitants).
The production of sludge from the leachate treatment
plant is around 30 m
3
day
1
, its composition being shown
Fig. 1. Experimental equipment employed.
E. Maran˜o´n et al. / Waste Management 26 (2006) 869–874
871
in
. As can be observed, there were important vari-
ations in the solids content (18,000–32,000 g TS dm
3
).
Metals were found to be present in the sludge at generally
low concentrations.
3.2. Anaerobic digestion of the sludge in an upflow anaerobic
sludge blanket reactor
The reactor was fed with diluted sludge, increasing the
concentration once the parameters of the effluent reached
constant values. During the first 3 months, however, the
functioning of the reactor was not very stable due to the
acclimatisation period (in which the concentration of
sludge was progressively increased from 10% to 40%).
After this period, the concentration was increased to
55%, 75% and 100%. The experimental protocol employed
and the composition of the feed to the reactor in each step
are shown in
. The evolution of the COD of the
influent and effluent after the acclimatisation period is
shown in
. The COD of the influent ranged between
values of 10,600 and 20,500 mg O
2
dm
3
. From day 125
onwards, the influent COD increased from values of
around 10,600 mg O
2
dm
3
to values of 12,200 mg
O
2
dm
3
, whereas the COD values of the effluent increased
from approximately 300–2300 mg O
2
dm
3
. This increase
may be due to two different factors: the increase in OLR
and the decrease in methanol dosage (from 3 to
1.5 cm
3
dm
3
). After reaching stable COD values in the
effluent, undiluted sludge was fed to the reactor. The
COD increased to 18,200 mg O
2
dm
3
(sludge no.
2 + 1 cm
3
methanol dm
3
sludge) and subsequently to
20,570 mg O
2
dm
3
(sludge no. 3 + 1 cm
3
methanol dm
3
sludge). In this last step, the effluent was recirculated
(R = 2), with the aim of diminishing the concentration of
Table 2
Characteristics of the sludge (freshly collected samples)
Parameter
Sludge no. 1
Sludge no. 2
Sludge no. 3
pH
7.8
7.5
7.4
Total solids
17,960
19,505
32,500
Volatile solids
11,310
12,720
23,625
COD
15,000
16,120
19,420
Total alkalinity
3421
3230
4315
Volatile acidity
962
903
1610
Fe
4.3
5.4
1.3
Mn
0.5
0.4
0.9
Zn
0.6
0.7
3.1
Cu
<0.1
<0.1
0.6
Ni
0.72
0.63
1.2
Pb
0.3
0.2
1.3
Cd
<0.1
<0.1
0.2
a
All values, except pH in mg dm
3
.
Table 3
Experimental protocol for the start-up and working of the UASB reactor
Days
1–31
32–52
53–73
74–94
95–125
126–160
161–193
194–224
% Sludge
10
20
30
40
55
75
100
100
Methanol (cm
3
dm
3
)
6.7
6.7
5
4
3
1.5
1
1
COD
influent
(mg dm
3
)
7100
8100
8150
8400
10,700
12,300
18,100
20,600
NaHCO
3
(mg dm
3
)
800
800
1500
1500
1500
1000
–
–
Na
2
HPO
4
(mg dm
3
)
26
26
26
26
–
–
26
26
NH
4
Cl (mg dm
3
)
61
61
61
61
–
–
–
–
HCl (cm
3
dm
3
)
–
–
–
–
–
–
1.5
1.5
0
3000
6000
9000
12000
15000
18000
21000
100
125
150
175
200
225
days
COD,
m
g.
d
m
-3
Influent
Effluent
Fig. 2. Evolution of the COD of the reactor influent and effluent.
Table 1
Physico-chemical characteristics of the leachates before and after treatment
Parameter
Landfill leachate
Biologically treated leachate
Minimum
Maximum
Average
Minimum
Maximum
Average
PH
8.06
8.71
8.39
6.8
7.0
6.9
BOD
5
500
1600
858.3
59
89
76
COD
2440
4980
3757
1017
1510
1287
KTN
1450
5184
2442
76.6
90.2
83.4
NH
þ
4
-N
1355
2670
2132
33.8
47.8
43.2
NO
3
-N
< 0.1
356
21.9
330
600
498
a
All values, except pH in mg dm
3
.
872
E. Maran˜o´n et al. / Waste Management 26 (2006) 869–874
COD and the content in solids. Consequently, the pH of
the effluent increased to values of around 8.7. This made
it necessary to neutralise with HCl to values of 7.2, thus
avoiding a high pH inside the reactor, which would have
impeded the correct functioning of the digestion process.
The effluent COD presented values between 2500 and
3200 mg O
2
dm
3
, similar to those obtained in the previous
step. The final effluent from the digester may be recircu-
lated to the leachate treatment plant to be treated together
with the landfill leachates. The biodegradation obtained
when feeding undiluted sludge varied between 84% and
87%.
To investigate the possibility of the biodegradation tak-
ing place without the addition of methanol, this compound
was no longer added at the end of experimentation. This
gave rise to an important reduction in COD removal,
which decreased to values of 58% (results not shown here).
As expected, methanol increases the growth of methano-
genic bacteria, thus enabling the assimilation of other sub-
strates at the same time (the COD of the added methanol
represents approximately 1500 mg oxygen dm
3
).
Determination of the volatile solids evidenced the
growth of the biomass. This growth was appreciable at
start-up, but much greater at the end of experimentation.
Therefore, the reactor had to be purged to impede this
mass overflowing into the settling funnel and contaminat-
ing the effluent. Around 800 cm
3
had to be purged every
5 days during the last step of the study, representing
approximately 16% of the volume of the treated sludge.
The biomass produced per kg of COD removed was
9.2 dm
3
sludge. Considering the volume of sludge produced
in the landfill leachate treatment plant, 30 m
3
day
1
, its
anaerobic digestion will thus produce 4.8 m
3
of waste
(digestat) that can be disposed in the landfill and an effluent
(with a COD of around 3000 mg dm
3
) that can be treated
together with the landfill leachates.
The production of biogas presented values in proportion
to the removal of COD, as can be seen in
, where
COD removal and biogas production for the different or-
ganic loading rates (OLR) are represented.
shows
the daily production of biogas in m
3
biogas m
3
reac-
tor day
1
. The composition of biogas varied between val-
ues of 70–75% methane. Considering the amount of
sludge to be treated, the production of biogas will be
approximately 75 m
3
day
1
, a very small quantity com-
pared to the biogas generated and extracted at the landfill
(
132,000 m
3
day
1
).
Heavy metal ions are accumulated in anaerobic digesters
outside the bacterial cells by precipitation and adsorption
reactions and inside the cells by microbial absorption (
).
presents analytical values
of the principal metals present in the influent and the efflu-
ent of the upflow anaerobic sludge blanket reactor and in
the reactor sludge. A decrease in the concentrations of met-
als can be observed in the process due to the deposition of
metals inside the reactor.
4. Conclusions
Upflow anaerobic sludge blanket reactors may be used
for the treatment of sludge as an alternative to the low rate
reactors currently being used. With the former type of reac-
tor, very high biodegradation was obtained when digesting
sludge produced in the treatment of landfill leachates. De-
creases of up to 87% in COD were obtained for a HRT of 9
days, lower than the usual HRT employed in CSTR units.
The effluent COD presented values of 2500–3200 mg
80
82
84
86
88
90
92
1.19
1.36
2.02
2.29
OLR (kgCOD.m
-3
.day
-1
)
COD removal
0
0.05
0.1
0.15
0.2
0.25
0.3
m
3
.m
-3
.da
y
-1
COD removal
biogas production
Fig. 3. COD removal and biogas production for the different organic
loading rates (OLR) (mean values).
0.00
0.10
0.20
0.30
100
120
140
160
180
200
220
240
days
m
3
.m
-3
.day
-1
Fig. 4. Daily production of biogas in the UASB reactor.
Table 4
Metals content (mg dm
3
) in the influent, effluent and digested sludge (sludge no. 3)
Cd
Pb
Ni
Zn
Cu
Fe
Influent
0.2
1.3
1.2
3.1
0.6
1.3
Effluent
0.03
0.2
0.4
0.6
0.1
0.9
Digested sludge
0.2
2.1
1.5
4.4
2.4
1.4
E. Maran˜o´n et al. / Waste Management 26 (2006) 869–874
873
O
2
dm
3
, similar to those of the leachates. This effluent
may be recirculated to the leachate treatment plant to be
treated together with the landfill leachates.
The
production
of
biogas
was
0.29 m
3
of
bio-
gas m
3
day
1
at the end of the experiment, with a meth-
ane content of 70–75%. Purging of the digested sludge
represented approximately 16% of the volume of the trea-
ted sludge, thus reducing the waste that will have to be dis-
posed of at the landfill, in compliance with Directive 1999/
31/CE.
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Document Outline
- Anaerobic treatment of sludge from a nitrification - denitrification landfill leachate plant
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