Bioresource Technology 98 (2007) 1426 1432
BOD5 and COD removal and sludge production
in SBR working with or without anoxic phase
a,* a b
Dorota Kulikowska , Ewa Klimiuk , A. Drzewicki
a
Department of Environmental Biotechnology, University of Warmia and Mazury in Olsztyn, Sloneczna St. 45G, 10-709 Olsztyn, Poland
b
Department of Applied Ecology, University of Warmia and Mazury in Olsztyn, Oczapowskiego St. 5, 10-957 Olsztyn, Poland
Received 27 May 2004; received in revised form 2 April 2006; accepted 12 May 2006
Available online 5 July 2006
Abstract
The aim of this study was to estimate the BOD5 and COD removal efficiency and biomass yield coefficient in sequencing batch reac-
tors (SBR) treating landfill leachate.
Experiments were carried out in four SBRs at HRT of 12, 6, 3 and 2 d. Two series were performed. In series 1, the reactors were
operated in a 24 h cycle mode (anoxic 3 h, aeration 18 h, settling 2.75 h, and discharge 0.25 h). In series 2, however, the anoxic phase
was eliminated.
In both series the BOD5 removal efficiency was almost identical  over 98%. On shortening HRT from 12 to 2 d, COD removal effi-
ciency decreased from 83.1% to 76.7% (series 1). In series 2, efficiency ranged from 79.6% to 75.7%. In the reactors working with the
anoxic phase the observed biomass yield coefficient (Yobs) was nearly constant (0.55 0.6 mg VSS/mg COD). Upon elimination of the
anoxic phase, the Yobs was observed to decrease from 0.32 mg VSS/mg COD (HRT 2 d) to 0.04 mg VSS/mg COD (HRT 12 d).
Ó 2006 Elsevier Ltd. All rights reserved.
Keywords: Landfill leachate; Sequencing batch reactor (SBR); BOD5 and COD removal; Observed yield coefficient; Biomass decay rate
1. Introduction 1993) or a combination of different technologies such as
biological treatment, electron-beam radiation and chemical
A major problem usually associated with the disposal of oxidation (Bae et al., 1999; Kennedy and Lentz, 2000; Lin
waste by sanitary landfill is the pollution of groundwater and Chang, 2000).
and surface waters if leachate is discharged into these water In multi-stage systems, pollutant elimination by biolog-
bodies. Many cases of leachate impact on surface water ical oxidation is a predominant process in wastewater treat-
and groundwater quality may be linked to improper or ment technology. In this process new cells (sludge) are the
insufficient landfill technology. Careful site management one of the final products. Landfill leachate contains xeno-
can reduce the amount and strength of the leachate produc- biotic organic compounds and heavy metals (Baun et al.,
tion but cannot eliminate it. Some form of treatment is, 2004; Jensen and Christensen, 1999; Paxéus, 2000). Many
therefore, necessary if the receiving waters are to be pro- such pollutants are hydrophobic and the principal removal
tected. Currently, leachates are treated by biological and mechanism for these compounds is sorption to sludge par-
physico-chemical methods. Among the most commonly ticles and transfer to the sludge processing system. It can
used are activated sludge (Doyle et al., 2001; Hosomi negatively influence the quality of sludge composition
et al., 1989; Im et al., 2001), fluidized beds (Imai et al., and impose restrictions in relation to disposal of the excess
sludge and further waste management.
On the other hand, with rising costs of sludge disposal,
*
the minimization of sludge production has become
Corresponding author. Tel.: [phone].
E-mail address: [e-mail] (D. Kulikowska). of increasing importance. The expense of excess sludge
0960-8524/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2006.05.021
D. Kulikowska et al. / Bioresource Technology 98 (2007) 1426 1432 1427
treatment has been estimated to be 50 60% of the total cost (S.D. 38.97) and 451 mg BOD5/l (S.D. 34.04), respectively,
of municipal wastewater treatment (Egemen et al., 2001). in series 2. The BOD5/COD ratios in leachate were 0.39
Therefore, modification of the aerobic treatment process (S.D. 0.01) (series 1) and 0.38 (S.D. 0.03) (series 2). Bio-
in order to reduce biosolids production seems to be highly chemical oxygen demand rate constants (k) were deter-
interesting. mined from the first-order kinetics equation and reached
The important strategies for minimization of excess 0.44 d 1 (series 1) and 0.41 d 1 (series 2) (Fig. 1). The rate
sludge production are lysis-cryptic growth, uncoupling constant k is a parameter determining the course of BOD
metabolism and maintenance metabolism and predation and thus also the BOD5 value. In the literature, it is usually
of bacteria (Wei et al., 2003). It is commonly known that stated that the value of k depends primarily on the rate at
increase of sludge age is associated with decreased sludge which the organic substances can be oxidized biologically.
production (Loosdrecht and Henze, 1999). In addition, Thus, for instance, in the raw municipal wastewater, values
Abbassi et al. (1999) showed that reduction of excess sludge of k are much higher (0.3 0.5 d 1) than in the same waste-
production can be achieved by raising the concentration of water after biological treatment (0.2 d 1). In some indus-
dissolved oxygen in the mixed liquor. In a reactor with a trial wastewater containing primarily slowly degradable
sludge loading of 1.7 mg BOD/mg MLSS Ć d, an increase compounds, k values can be lower than 0.2 d 1 (Pitter
in the dissolved oxygen concentration from 2 to 6 mg/l and Chudoba, 1990).
caused a reduction in the amount of solids in the reactor
by about 25%. 2.2. Process configuration and system design
In activated sludge models yield and decay coefficients
must be determined empirically. Until now, investigations The investigations were carried out at bench scale in
concerned municipal wastewater. However, literature data four SBRs operated in parallel (SBR 1 SBR 4). The reac-
show a lack of investigation of the yield of activated sludge tors, with a working volume of 6 l each, were made of
in landfill leachate treatment. Toxicity of the landfill leach- plexiglass and were equipped with a stirrer at regulated
ate has been extensively studied and well documented (Ber- rotation speed (36 rpm). Dissolved oxygen was supplied
nard et al., 1996; Isidori et al., 2003; Marttinen et al., 2002; using porous diffusers, placed at the bottom of the reactors.
Silva et al., 2004). It can influence the growth and decay of The leachate was supplied to the reactors by means of a
microorganisms. peristaltic pump (ZALIMP type pp 1 05) for 4 h of the
The aim of this study was to investigate the BOD5 and cycle at 0.125 l/h (SBR 1), 0.25 l/h (SBR 2), 0.5 l/h (SBR
COD removal efficiency and biomass yield coefficient in 3) and 0.75 l/h (SBR 4). The amount of the leachate sup-
sequencing batch reactors (SBRs) treating landfill leachate.
In the experiment, anaerobic aerobic (with anoxic and aer-
800
k = 0.44 d-1
ation phases in the cycle) and aerobic systems (without
700
anoxic phase) were tested. Unlike conventional systems,
600
SBRs offer various advantages, including minimal space
500
requirements and ease of management (Irvine et al.,
400
1997). Their additional benefit is the possibility of techno-
300
logical modifications during the process, since very signifi-
200
cant changes in the chemical composition of leachate may
100
occur throughout landfill exploitation.
0
The removal efficiency for BOD5 and COD, depending
0 2 4 6 8 10 12 14 16 18 20
on reactor cycle working with or without the anoxic phase,
time [d]
was estimated at different hydraulic retention times (HRT)
(a) first-order kinetic experimental data
in the SBRs. The observed biomass yield coefficients (Yobs),
the values of biomass yield coefficients Y and biomass
800
decay rates kd were determined. k = 0.41 d-1
700
600
2. Methods
500
400
2.1. Leachate feed
300
200
Leachate used in this study was collected from a muni-
100
cipal landfill located in Wysieka near Bartoszyce in the
0
Warmia and Mazury Province, Poland. The landfill had 0 2 4 6 8 10 12 14 16 18 20
time [d]
been operated since January 1996.
first-order kinetic experimental data
Throughout the experimental period, COD and BOD5 (b)
were 1380 mg COD/l (S.D. 80.4) and 539 mg BOD5/l
Fig. 1. Experimental data, biochemical oxygen demand curves and
(S.D. 37.3), respectively, in series 1, and 1188 mg COD/l constant k determined from first-order kinetics (a. series 1, b. series 2).
BOD [mgO
2
/ l]
BOD [mgO
2
/ l]
1428 D. Kulikowska et al. / Bioresource Technology 98 (2007) 1426 1432
plied within 24 h to the reactors varied from 0.5 l (SBR 1) well-mixed sample at microscope magnifications ranging
to 3 l (SBR 4). Hydraulic retention time of the leachate ran- from ·100 to ·400, depending on organism size.
ged from 12 d (SBR 1) to 2 d (SBR 4) (Table 1). The numbers of rhizopods, ciliates and metazoans were
Two series were performed. In series 1, the reactors were estimated as arithmetic averages obtained from analysis of
operated in a 24 h cycle mode, at 3; 18; 2.75 and 0.25 h for four subsamples with volume of 0.05 ml mixed liquor. The
the anoxic, aeration, settling and discharge, respectively. In number of colonial species (Opercularia sp., Epistylis sp.,
series 2, the anoxic phase was omitted and the time of the Carchesium sp.) was calculated as the sum of all individuals
operating phases of the cycle was as follows: 21 h for aera- in a colony. Finally, total number of organisms was recal-
tion, 2.75 h for settling and 0.25 h for discharge. In the aer- culated per l (ind./l) and mg of VSS (ind./mg VSS).
ation phase, the amount of oxygen supplied to the reactor
was regulated in order to maintain the oxygen concentra- 3. Results and discussion
tion at the end of the phase at 2.5 4.0 mg O2/l. The system
was operated at room temperature (20 22 °C) for two 3.1. BOD5 and COD removal
months.
In this study, leachate originated from a municipal land-
2.3. Analytical methods fill operated for five years. It is known that the leachate
from young landfills (up to five years) contains biodegrad-
2.3.1. Chemical analysis able organic substances at very high concentration, even
Daily measurements of effluent from the SBR included: over 10,000 mg COD/l (Amokrane et al., 1997). In the
present investigation, the content of organic compounds
" chemical oxygen demand (COD) (according to Her- was up to 1400 mg COD/l, but BOD5 and COD removal
manowicz et al., 1999), efficiency was relatively high.
" biochemical oxygen demand BOD5 (according to DIN Under anaerobic aerobic conditions (series 1), the effi-
EN 1899-1/EN 1899-2 official EPA method using Oxi- ciency of BOD5 removal ranged from 99.2% (at the reten-
TopÒ made by WTW company (WTW Wissenschaft- tion time of 12 d  SBR 1) to 97.6% (at the retention time
lich Technische Werksträtten Gmbh, D-82326 of 2 d  SBR 4) (Fig. 2). The COD removal efficiency
Weilheim, Germany)), decreased from 83.1% (SBR 1) to 76.7% (SBR 4) (Fig. 3).
" volatile suspended solids (VSS) and total suspended sol- Under aerobic conditions (aeration phase only) (series 2),
ids (TSS) in the settled effluent (according to Her- the BOD5 removal efficiency was almost identical to that
manowicz et al., 1999). in series 1 (Fig. 2). However, a slight decrease in the effi-
ciency of COD removal was observed in all SBRs (Fig. 3).
The mixed reactor content was analysed for: The results of Rusten and Eliassen, 1993 on municipal
wastewater treatment in SBR indicate that an increase in
" volatile suspended solids (MLVSS) and total suspended duration of the aeration phase of the cycle from 61% to
solids (MLSS) (according to Hermanowicz et al., 1999), 67% caused a 5% increase in COD removal efficiency. In
" oxygen concentration (using an oxygen controller HI the present experiment, a higher efficiency was obtained
9142 made by Hanna Instruments (Hanna Instruments under anaerobic aerobic conditions. The highest differ-
S.p.A, 35030 Sarmeola di Rubano, Padova, Italy)). ences in the efficiency of COD removal between series 1
and 2 were obtained at HRT of 12 d (SBR 1) (Fig. 3). In
all SBRs sludge age was over 2-fold longer in series 2 than
2.3.2. Biological analysis in series 1.
The research on activated sludge biocenosis encom- In wastewater effluents, the non-biodegradable fraction
passed a microscopic analysis of quantitative composition is constituted of compounds present in the raw wastewater
of rhizopods, ciliates and metazoans. Microfauna composi- plus those non-biodegradable substances produced by the
tion was determined  in vivo . Number of organisms was microorganisms. These soluble microbial products (inter-
calculated from microscope slides, containing 0.05 ml of a mediates or final products of substrate degradation and cell
Table 1
Technological parameters in series 1 and 2
Parameters Units Series 1 Series 2
SBR 1 SBR 2 SBR 3 SBR 4 SBR 1 SBR 2 SBR 3 SBR 4
Volume of leachate influent in SBR operating cycle l 0.5 1.0 2.0 3.0 0.5 1.0 2.0 3.0
Hydraulic retention time (HRT) d 12 6 3 2 12 6 3 2
Volatile suspended solids g MLVSS/l 1.99 2.61 3.45 3.90 1.65 2.12 3.57 3.84
Solid retention time (SRT) d 33 22 15 11 80 56 38 30
D. Kulikowska et al. / Bioresource Technology 98 (2007) 1426 1432 1429
Xe effluent volatile suspended solids concentration
series 1
100
series 2
(mg VSS/l),
Veff volume of leachate effluent in SBR operating cycle
98
(l),
Vin volume of leachate influent in SBR operating cycle
96
(l), (Vin = Veff + Vw),
Cs concentration of COD in raw leachate (mg COD/
94
l),
92
Ce concentration of COD in the effluent (mg COD/l).
90
In series 1, the observed biomass yield coefficient Yobs
SBR 1 SBR 2 SBR 3 SBR 4
was almost stable (0.55 0.6 mg VSS/mg COD) (Fig. 4),
Fig. 2. BOD5 removal efficiency in series 1 and 2.
irrespective of the leachate retention time (changing from
12 d to 2 d) and the sludge age ranging from 33 d to
11 d, respectively. It means that in this series 55 60% of
90
series 1
COD was converted to biomass during the leachate treat-
series 2
85
ment process.
Under aerobic conditions (series 2), a significant
80
decrease was observed in the value of Yobs, which corre-
75 sponded with the increase in the hydraulic retention time
in the reactors. The Yobs values reached 0.32 mg VSS/
70
mg COD and 0.04 mg VSS/mg COD at the leachate
hydraulic retention times of 2 d and 12 d, respectively
65
(Fig. 4). Thus, in order to maintain the concentration of
60
volatile suspended solids in the SBR at a level comparable
SBR 1 SBR 2 SBR 3 SBR 4
with that in series 1, it was necessary to increase the sludge
Fig. 3. COD removal efficiency in series 1 and 2.
age (Table 1).
The results of a study by Lishman et al. (2000), including
both protein solution and raw sewage as substrates, indi-
decomposition) are very important owing to their influence cated that the observed yield coefficients were 35 52%
on the quality of effluent from biological treatment higher for anoxic reactors than for aerobic reactors.
processes. Anoxic conditions or anoxic zones are widely applied in
Pribyl et al. (1997) showed that the amount of soluble activated sludge systems with denitrification and phospho-
microbial products (SMP) in the effluent from a SBR rus removal.
depends on the sludge age. The minimum SMP concentra- Lee and Welander (1996) indicated the possibility of
tions could be achieved at sludge ages ranging between 5 d minimizing the sludge production in aerobic wastewater
and 15 d. At sludge ages below 5 d and above 15 d SMP treatment through manipulation of the ecosystem, so that
concentration increased. most of the bacterial biomass produced is consumed by
predating protozoa and metazoa. In this way, the biomass
3.2. Biomass yield coefficient
The observed biomass yield coefficient Yobs is the mass
0.80
of bacteria formed per mass of COD removed and it
0.70
involves the energy requirements. The value of the
0.60
observed biomass yield coefficient Yobs corresponds to net
biomass yield coefficient and can be calculated from the
0.50
following equation:
0.40
X ðV =tcÞþX ðV =tcÞ
org w e eff
Y ź ; ð1Þ 0.30
obs
ðCs CeÞ ðV =tcÞ
in
0.20
where
0.10
Yobs observed biomass yield coefficient (mg VSS/
mg COD),
0.00
Xorg volatile suspended solids in SBR (mg VSS/l), series 1 series 2
Vw volume of suspended solids disposed in SBR oper-
SBR 1 SBR 2 SBR 3 SBR 4
ating cycle (l),
tc time of SBR operating cycle (d), Fig. 4. Observed biomass yield coefficient Yobs in series 1 and 2.
5
BOD removal efficiency [%]
COD removal efficiency [%]
obs
Y
[mgVSS/mg COD]
1430 D. Kulikowska et al. / Bioresource Technology 98 (2007) 1426 1432
yield coefficients obtained were 30 50% lower than those of age, biomass yield coefficient (Y) and loss of biomass
conventional systems. expressed by means of biomass decay rate kd. The values
In the present study, elimination of the anoxic phase of both parameters (Y and kd) can be calculated from the
during the cycle period had a favourable effect on micro- following equation:
fauna (Fig. 5). In activated sludge in series 1 (SBR 1), the
1 ðCs CeÞ ðV =tcÞ
d
źY ð2Þ
kd;
highest abundance of microfauna (800 ind./mg VSS) was
H V X
org
found at 12 d HRT and at 33 d sludge age. On the other
hand, in SBR 3 and 4 at HRT of 3 and 2 d and sludge where
age of 15 and 11 d there was noticed 8-fold and 10-fold H solids retention time (SRT) (d),
smaller microfauna abundance in comparison with SBR Y biomass yield coefficient (mg VSS/mg COD),
1. However, in series 2 the abundance increased slightly Cs concentration of COD in raw leachate (mg COD/
from 878 to 986 ind./mg VSS with HRT shortened from l),
12 to 2 d. Comparing the total abundance in both series Ce concentration of COD in the effluent (mg COD/l),
it can be stated that the abundance did not decrease below Vd volume of leachate influent in SBR operating cycle
800 ind./mg VSS at sludge age not shorter than 30 d. (l),
Madoni (1991), on the basis on his own and others tc time of SBR operating cycle (d),
research, concluded that abundance of ciliates in properly V working volume of SBR (l),
functioning activated sludge plants treating municipal Xorg volatile suspended solids in SBR (mg VSS/l),
wastewater should be 106 ind./l. In the present work the kd biomass decay rate (d 1).
abundance of protozoan communities in series 1 ranged
from 3.24 · 105 to 1.59 · 106 ind./l (from 83 to 800 ind./ The results indicate that in both series the values of Y
mg VSS) and in series 2 from 1.45 · 106 to 4.77 · 106 ind/ were almost the same. The biomass yield coefficient was
l (from 878 to 986 ind./mg VSS). 0.628 mg VSS/mg COD in series 1 and 0.616 mg VSS/mg
In the present study, the highest differences in Yobs were COD in series 2 (Table 2).
seen in SBR 1 in series 1 and 2, with comparable abun- The elimination of the mixed phase (series 2) was
dance of microfauna. It means that in such conditions observed to affect the value of the biomass decay rate kd.
sludge production depends on factors other than
predation. Table 2
Values of biomass yield coefficient Y, biomass decay rate kd and specific
For a steady state system, the observed yield coefficient
maintenance coefficient m in series 1 and 2
in activated sludge is primarily determined by the sludge
Series no. Y kd (d 1) m
age. Longer sludge ages result in decrease of the sludge
(mg VSS/mg COD) (mg COD/mg VSS Ć d)
mass by endogenous metabolism. Losses of biomass by
Series 1 0.628 0.006 0.01
endogenous metabolism can be expressed by means of spe-
Series 2 0.616 0.032 0.052
cific decay rate kd. There is a correlation between sludge
1200
1000
800
600
400
200
0
1 2 3 4 1 2 3 4
SBR No.
series 2
series 1
Series 1 Series 2
Taxons Unit
SBR 1 SBR 2 SBR 3 SBR 4 SBR 1 SBR 2 SBR 3 SBR 4
ind./mg VSS 27 28 - - 32 18 23 3
Rhizopoda
ind./mg VSS 762 524 100 83 769 848 927 970
Ciliata
ind./mg VSS 11 2 - - 77 36 18 13
Metazoa
Fig. 5. Total number of microfauna in activated sludge in series 1 and 2 (The table includes composition of Protozoa and Metazoa in activated sludge.)
total numbet of microfauna [ind./mgVSS]
D. Kulikowska et al. / Bioresource Technology 98 (2007) 1426 1432 1431
In SBR working under anaerobic aerobic conditions (series the SBR working without the anoxic phase. The change
1) the value of kd reached 0.006 d 1. Under aerobic condi- of anoxic conditions for aerobic resulted in an increase
tions (aeration phase only), an over 5-fold increase in the in substrate utilization for biomass production. It
kd value (up to 0.032 d 1) was observed (Table 2). caused a significant increase in the biomass decay rate
For heterotrophic bacteria, the decay rate is in the range kd from 0.006 d 1 (series 1) to 0.032 d 1 (series 2), after
of 0.3 0.7 d 1, whereas for nitrifying bacteria and polyphos- the elimination of the anoxic phase in the work cycle of
phate accumulating organisms  0.15 0.2 d 1, according to the SBRs.
Loosdrecht and Henze (1999) (after Henze et al., 1995). 4. During municipal leachate treatment aerobic conditions
Lee and Oleszkiewicz (2003) showed experimentally that in SBRs should be considered as optimal owing to
the average autotrophic decay rates in aerobic, anoxic and clearly lower biomass production with comparable effec-
anoxic/aerobic reactors were 0.153, 0.097, and 0.058 d 1, tiveness of BOD5 and COD removal.
respectively. Siegrist et al. (1999) pointed out that at 20 °C
the decay rate of nitrification activity reduces from about
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