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© IWA Publishing 2013 Water Science & Technology
Multistage treatment wetland for treatment of reject
waters from digested sludge dewatering
M. Gajewska and H. Obarska-Pempkowiak
The paper presents the influence of sewage composition on treatment in pilot-scale facility for reject
waters (RW) from sewage sludge centrifugation. The facility consisted of mechanical (two tanks with
10 d retention each) and biological parts composed of three subsurface flow reed beds working in
batch. Two years of monitoring of the facility proved high efficiency removal of predominant
pollutants: chemical oxygen demand (COD) 75–80%, biochemical oxygen demand (BOD) 82.2–95.5%
M. Gajewska (corresponding author)
H. Obarska-Pempkowiak
Faculty of Civil and Environmental Engineering,
Gdańsk University of Technology,
11/12 Narutowicza St,
80-233 Gdańsk,
and total nitrogen 78.7–93.9% for low ratio of BOD5/COD in discharged RW. The differences in
efficiency removal were correlated with the composition of organics and nitrogen compounds rather
than with concentrations. It was assumed that high concentration of colloidal fraction of Org-N and
COD in discharged RW led to a decrease in efficiency removal.
Key words
| colloids, digested sludge, nitrogen fractions, pollutants removal, reject waters,
treatment wetlands
Return flows of reject waters (RW) from sewage sludge dewatering in conventional wastewater treatment plants (WWTPs)
usually contribute up to 20% of total nitrogen (TN) load with
relatively small hydraulic (up to 1%). Furthermore, the
remaining load of chemical oxygen demand (COD) after
anaerobic digestion is generally quite low and poorly biodegradable and another problem with RW management is
connected with its irregular generation and large fluctuation
of pollutant concentrations, even from one WWTP (Fux
et al. , ; Wett & Alex ; Gajewska & ObarskaPempkowiak , ). It was proved that ammonia nitrogen and total suspended solids (TSS) concentrations in
sludge liquors from the digested sludge dewatering in
WWTP in Luggage Point, Australia, varied from 943 to
1,710 NHþ
and from 95 to 6,132 mg TSS L1
4 -N mg L
(Fux et al. , ). In WWTP in Minworth, Great Britain,
ammonia nitrogen concentrations changed from 450 to
750 mg L1, and TSS from 220 to 2,340 mg L1 (Fux et al.
). According to Hans et al. () and Jeavons et al.
(), at two WWTPs in Switzerland the RW from the dewatering of sludge after fermentation process was similar and
contained 657 ± 56 and 619 ± 21 mg L1 (NHþ
4 -N) and
344 ± 112 and 384 ± 137 mg L1 (TSS). These properties
cause fundamental problems with RW management and
doi: 10.2166/wst.2013.306
furthermore return flows of RW could alter the activated
sludge process in conventional WWTP. As a consequence
this can cause increase in TN concentration in treated effluent
above the standards required by legislation (WFD//60/
EC). Separate treatment of high nitrogen content in this
stream can ensure stable operation of WWTPs and reduces
TN concentration considerably in final discharges to water
recipients (Laurich & Günner ; Wett & Alex ).
It was proved that treatment wetlands (TWs) are successful in both treatment and polishing of landfill leachate (which
has similar properties as RW) and has the potential to remove
not only organic carbon and nitrogen compounds, but xenobiotics and heavy metals as well (Maehlum ; Peverly et al.
; Ye et al. ; Bulc ; Wojciechowska et al. ).
Among others it was assumed that wastewaters with
very low content of biodegradable organic matter
(low biochemical oxygen demand (BOD5)/COD ratio) and
very high concentrations of ammonia nitrogen (exceeding
800 mg/L in the case of RW from sludge centrifuging) can
be treated effectively in multistage treatment wetlands
(MTWs) (Obarska-Pempkowiak et al. ; Gajewska &
Obarska-Pempkowiak ).
TW, which is inexpensive and simple in operation
technology, creates high biodiversity environment of
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
very complex transformation of discharged pollutants. A
number of processes transfer nitrogen compounds from
one point to another without results in molecular transformation, among others: (1) particulate settling and
resuspension, (2) dissolved form diffusion, (3) biomass
translocations, (4) ammonia volatilization, and (5) sorption
of dissolved nitrogen (NHþ
4 -N) on substrates (medium). In
addition to the physical transfer of nitrogen compounds
principal processes which transform nitrogen from one
form to another are as follows: (1) ammonification, (2)
transformation of nitrogen compounds vs microbiological
processes to N2 or N2O by conventional nitrification
flowed by denitrification or shorten removal pathways, (3)
assimilation, and (4) decomposition (Tanner et al. ;
Vymazal ; Sirivedhin & Gray ; Dong & Sun
; Kadlec ; Kadlec & Wallace ). Detailed comprehension of the nitrogen transfer and transformation
processes is important for understanding the removal processes in TWs.
This paper aims to present the performance of facilities
consisting of mechanical and biological parts (three-stage
TW) for RW from digested sludge dewatering treatment.
Two years of maintenance showed significant differences
in RW compositions, mainly fractions of organic nitrogen
(colloidal, dissolved and particulates) which led to
Figure 1
Table 1
significant differences in unit processes responsible for pollutants removal.
Study site
The pilot facility to treat RW from dewatering of digested
sludge in centrifuge was constructed in 2008. The investigations carried out had funding support from the EEA
Financial Mechanism (PL 0085) and the Ministry of Science
and Higher Education in Poland (E007/P01/2007/01).
It consists of two settlement tanks (each of 1 m3 volume)
in mechanical stage and three subsurface flow (SSF) reed
beds in biological stage (Figure 1).
The design details of each stage of treatment in investigated pilot plant are described in Table 1. In order to
calculate subsequent SSF beds operating in a batch it was
assumed that the beds would treat the load of wastewater
corresponding to 5 pe (person equivalent). After each
stage of treatment there was 1 m3 tank with pump for
collecting and ensuring intermittent feeding of equal,
single dose of RW for each stage (about 0.11 m3 twice
per day).
The scheme of pilot facility for reject waters treatment with sampling points. (VSSF: vertical subsurface flow; HSSF: horizontal subsurface flow.)
Design details for the pilot three-stage TW
Flow [m3/d], (pe)
Filled [mm]
Contact time [d]
0.22 (5)
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Area [m2]
Unit area [m2/pe]
Depth [m]
Hydraulic load [mm/d]
Σ 16.4
Σ 3.24
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
Water Science & Technology
Washed gravel was used as filtration materials in
all beds (Table 1). The hydraulic coefficient was
4.2 × 102 m/s1 and void rate 0.35. The beds were planted
with Phragmites australis with 5 clumps per m2, in 2008.
Sampling and analytical procedures
Significant differences in inlet concentrations of pollutants
in two sampling events were observed and caused that
studied parameters were shown for 2009 and 2010 separately. In the case of RW the predominating pollutants are
organics (COD for hardly degradable and BOD5 for easily
degradable organic matter) and nitrogen compounds
(mainly in the form of ammonium – 90% and Org-N –
10%) (Gajewska & Obarska-Pempkowiak ). Variations
of predominating pollutants in inflow (1), after mechanical
part (2) and effluent (3) are given in Figure 2, while more
detailed characteristics of RW composition are presented
in Table 2.
According to Lo () and Klimiuk et al. (),
BOD5/COD and BOD5/TN ratios bring information about
biodegradability, and they decrease in progress with the
decomposition process. The BOD5/COD ratio of RW discharged to the facility have already been very low and not
adequate for treatment according to concept of highly effective technologies, like activated sludge or attached growth
technologies. Furthermore, BOD5/COD decreased even
more after mechanical treatment, which suggests that a
small part of easily biodegradable organic matter was
trapped too in both tanks together TSS and VSS (Table 2).
The ratios obtained in this study were slightly higher than
the ones for the RW from the WWTP in Minworth, Great
Britain given by Fux et al. (), which was equal to 0.2.
This could be the result of low efficiency performance of fermentation tanks towards biogas production in the sludge
processing at WWTP (Table 2). Such low BOD5/COD is
characteristic for mature landfill leachate (with ratio of
0.5–0.3) where the easily biodegradable organics (BOD5)
have already been consumed (Bulc ; Klimiuk et al.
; Wojciechowska et al. ). Furthermore, such low
BOD5/COD ratio reflects low degradability of organic compounds which is characteristic for methanogenic sanitary
landfill, characterized by low concentration of volatile
fatty acids and relatively high concentration of humic compounds (Bulc ; Klimiuk et al. ; Wojciechowska
et al. ).
According to the literature, in TWs low carbon-to-total
Kjeldahl nitrogen ratio (BOD5/TKN below 1) is necessary
for ensuring effective nitrification process (Kadlec & Wallace ). In this study for RW the TN concentration
The composite samples of influent (1) after mechanical stage
(2) and effluent (3) were collected from April to October
2009 and from May to November 2010. Sampling was carried out according to the calculated retention time: 20 d
for mechanical stage and measured 9 d and 7 hours for
MTWs (Table 1). The construction of the outdoor pilot
plant do not favor the operations during winter due to possibility of RW freezing in distributions system.
The chemical analyses have been performed according to standards methods (Polish Environmental
Ministry Regulations of th July  with amendment
from th January ), and are comparable with
APHA () by independent laboratory (ISO certificate).
Concentrations of the following pollutants were
measured: organics (BOD5 and COD), TSS and volatile suspended solids (VSS), and nitrogen: total Kjeldahl nitrogen
(TKN), ammonia nitrogen and nitrate and nitrite.
The quantification (dissolved – DON, colloidal – CON
and particulate – PON) of organic nitrogen was based on filtration of the influent and subsequent stages effluent
through a series of filters (0.1 and 1.2 μm pore size Millipore
nitrocellulose filters) (Pagilla et al. ).
The temperature of RW and air, pH as well as dissolved
oxygen and redox potential were measured at the sampling
points onsite while collecting the samples, using the
measuring probe (WTW Multi 340i/SET).
Removal efficiency was calculated as a quotient
of pollutants load difference in influent (Linf ) and effluent
(Lout) after subsequent stages of treatment in constructed
wetland and load in influent (Linf ), η ¼ (Linf–Lout)/Linf.
The results were evaluated using the StatSoft
STATISTICA 8.0. The normality of variables was checked
using the Shapiro–Wilk test (for small amount of samples)
with p level ¼ 0.05. The data were distributed normally, for
the predominate pollutants COD and TKN, within each separate year of sampling (2009 and 2010).
Box and whiskers plots have been chosen as a graphical interpretation of the statistical analysis. Analysis of
variance tests were used to evaluate significances of
achieved results. The linear relationship was considered
significant when determination coefficient R 2 > 0.8.
Concentration of pollutants after each stage of
Figure 2
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
Water Science & Technology
Variation of organics (COD and BOD5) and nitrogen compounds in influent, after the mechanical stage and effluent in 2009 and 2010.
Table 2
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
Water Science & Technology
The average RW composition in analyzed stage of treatment in 2009 and 2010
2009 (n ¼ 10)
2010 (n ¼ 10)
596.3 ± 69.7
368.1 ± 67.4
22.3 ± 5.6
517.1 ± 83.8
449.3 ± 47.2
24.9 ± 6.7
506.7 ± 67.9
194.8 ± 42.9
16.3 ± 2.2
358.9 ± 69.6
318.3 ± 63.0
17.8 ± 7.6
1,309.3 ± 286.1
932.2 ± 113.2
79.5 ± 28.4
790.6 ± 51.1
643.9 ± 51.3
168.8 ± 21.1
0.7 ± 0.2
2.2 ± 0.2
0.1 ± 0.1
0.9 ± 0.2
2.2 ± 0.2
0.1 ± 0.1
[ C]
362.6 ± 24.1
304.1 ± 28.9
160.5 ± 23.8
362.7 ± 24.8
200.2 ± 25.1
163.1 ± 21.9
equals TKN and above requirement was achieved for both
sampling periods, as well as all stages of treatment. Moreover both BOD5/COD and BOD5/TN ratios decreases
significantly during the treatment process (mainly in MTW
processing) which indicated proportional decreased of relevant pollutants and, in consequence, confirmed efficient
removal of predominant pollutants (Table 2, Figure 2).
The examples of differences in concentrations of predominant pollutants in inflow and effluent as well as after
tanks (mechanical part) are illustrated in Figure 2. As a consequence of unstable sludge processing in conventional
WWTP (mainly fermentation) the pilot facility was discharged by pollutant concentrations variable in time.
Organic concentrations were generally higher in the
second year of operation (2010), but demonstrated lower
variability. Meanwhile ammonium nitrogen concentrations
were lower in 2010 and showed smaller variability in comparison with concentrations in 2009 (Figure 2).
Figure 3
Although the concentration of Org-N in the inflow was
considerably lower (nearly 34%) in 2010 in comparison
with 2009, the final concentration in outflow in 2010 was
significantly higher in comparison with 2009 (up to
250%) (Figure 2). Since the working conditions such as
hydraulic load and temperature of RW were similar in
both sampling period one of the reasons of lower efficiency
of Org-N and as a consequence TN removal could be the
composition of Org-N (Tables 1 and 2, Figure 2). Speciation of Org-N presented as a share of dissolved (DON),
colloidal (CON) and particulates (PON) fraction is given
in Figure 3. The variation of DON, CON, and PON fraction was substantial in both sampling events (2009 and
2010) and was mainly related with colloidal fraction of
Org-N. The inflowing RW in 2009 contained up to 36%
of CON and 41.5% of PON while in 2010 CON raised
up to 51.3% and PON was only 22.1% (Figure 3). The
final effluent in 2009 consisted mainly of DON in 82%
The share of Org-N fractions (DON – dissolved, CON – colloidal and PON – particulates) in RW treated in pilot MTW in 2009 and 2010.
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
while in 2010 the composition of effluent was more equalized but CON was still 41.5%.
Efficiency removal after each stage of treatment
Considerable reduction of pre-dominating pollutants in the
samples was general regularity confirmed by statistical
tests. Differences between influent, after mechanical stage
and effluent in 2009, 2010 and in 2009 and 2010 taken
together for organics (COD and BOD5) and nitrogen compounds were checked with the Friedman’s analysis of
variance with the significance level ¼ 0.05. In every case
the hypothesis of insignificant difference between the
three population distributions was rejected. Then insignificance of differences between every appropriate two
population distributions of inflow and after tanks or effluent of pre-dominating pollutants in 2009, 2010 and in
2009 and 2010 taken together was tested with t-test for
dependent samples or Wilcoxon matched pairs test with
regard to appropriate test assumptions, with the significance level ¼ 0.05. In every case the hypothesis of
insignificant difference between the two population
distributions was rejected.
The comparison of efficiency of pre-dominating pollutants removal in 2009 and 2010 reflected up to 25%
higher effectiveness in the first year of performance for
the total facility (Table 3). Mechanical step of the pilot
treatment also demonstrated better efficiency of pollutants
removal confirming that in 2009 the RW composition
was much easier to treat in both stages of treatment
except BOD5. It contained higher concentration of particulates (which were trapped in tanks) and dissolved the
organic matter which was more available for microbiological processes. According to the literature TWs usually
show better efficiency removal in time of operation when
Table 3
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the roots zone system is mature (Kadlec & Wallace
). In the case of this study the performance in the
second year was lower since the RW contained higher concentration of colloidal compounds which are characterized
by lower availability for transformations (Pagilla et al. ;
Kadlec & Wallace ; Gajewska & Obarska-Pempkowiak ).
The obtained results demonstrate that this particular MTW
provided an excellent pre-treatment for wastewaters with
high and variable concentrations of pollutants. The removal
of TSS and organic matter (BOD5 and COD) is both steady
and efficient, and can reach very low concentrations in the
effluents. The average concentrations of pollutants in effluent for two sampling events were as follow: 23.6 mg
TSS/L, 21.02 mgBOD5/L, 263.8 mg COD/L and 124.2 mg
TN/L and 96.1 mg NHþ
4 N/L and were much lower in case
of TSS, BOD5, COD or similar (TN) to concentrations of
adequate pollutants in raw sewage discharged to WWTP
in Gdańsk. Thus the returned flow of RW after treatment
in MTW would not alert the treatment processes in
conventional WWTP.
In both sampling periods the hydraulic working conditions were similar (Table 1). Due to the construction of
the pilot MTW above ground, the surface flow of the precipitation cannot contribute to the presented good
performance. Only direct rain falling onto the area of the
pilot plant could contribute to RW dilutions. During
sampling years average precipitations were equal to
72.8 mm/months (from minimum to maximum 24.1 to
106.7 mm/months) for 2009 and 91.7 mm/months (from
27.2 to 152.5 mm/months) for 2010. Thus, daily precipitations was on average 2.3 mm/d in 2009 and 3.0 mm/d in
Average efficiency removal of predominating pollutants in pilot plant for RW treatment, %
Figure 4
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
Water Science & Technology
The overall efficiency with regression line of predominant pollutants in first and second stage of treatment in the pilot plant in 2009 and 2010.
Table 4
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
First-order areal rates constant for TN and Org-N development with standard
deviations at MTW in 2 years
kA TN ± σ, [m/d]
kA Org-N ± σ, [m/d]
0.11 ± 0.01
0.04 ± 0.007
0.06 ± 0.01
0.03 ± 0.006
2010, which is much less than day hydraulic load of the
MTW. Taking into consideration intensive growing of vegetations and the average temperature of air in sampling
events (the average was 16.5 C in 2009 and 16.7 C in
2010) the ‘direct’ precipitation was balance by transpirations and evapotranspirations and their influence on
treatment processes could be neglected (data acquired
from the Meteorological Station of Civil and Environmental
Engineering Faculty, Gdansk University of Technology,
Substantial differences was observed in predominant
pollutants’ behavior during the treatment process within
two sampling events (Figures 2 and 3). Many unit processes
are responsible for nitrogen transformations and removal in
TWs discharged by wastewater with high concentrations of
nitrogen, mainly in the form of ammonium nitrogen, like
volatilization, sorption, nitrification and denitrification
(Vymazal ). In this investigation the volatilization of
ammonia was strongly limited in conditions of subsequently
working tanks and MTW. Covered tanks and SSF beds did
not favor air exchange and moreover pH and temperature
of RW were much lower than optimal for this process
(Table 1) (Reddy & D’Angelo ; Vymazal ; Kadlec &
Wallace ; Saeed & Sun ). In the first year of operation (2009) the reduction of both Org-N and ammonium
nitrogen was very effective in the mechanical stage
suggesting that the main unit processes responsible for
removal or transformation of nitrogen compounds could
be sedimentation in case of particulate organic nitrogen as
well as adsorption of ammonia nitrogen on suspended
In the 2010 results, both fittings and correlation were
weaker or not significant in comparison with 2009. This
could suggest that sedimentation process was of less
importance and other processes which were more complex like adsorption could be involved in pollutants
removal (Figure 4).
Similar regression models have been tested for the
second stage of treatment (conducted in MTW) for Org-N
and COD but only few of them was statistically significant,
at the same time the tests did not prove any other dependence than linear one (Figure 4). In case of organics
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represented by COD there was no significant linear relationship (R 2 < 0.2, p > 0.05) between the variation of the
concentrations in inflow to MTW (after tanks) and outflow
from the MTW.
The first-order areal rate constant kA values for TN and
Org-N during the two monitored sampling events are shown
in Table 4. Achieved results are in agreement with findings
of Kadlec & Knight () and Kadlec & Wallace ()
where kA values change in wide scope from 0.007 to
0.1 m/d. Rate constant kA for Org-N as it was expected,
was much lower than kA for TN which could suggest that
mineralization process slows down the processes of nitrogen
removal. What is more both rate constants were lower for
the second year of operations when the concentrations of
TN (both NHþ
4 -N and Org-N) were lower in comparison
to concentrations discharged in the first year but the OrgN consisted of much higher concentrations of colloidal N
fraction (Figures 2 and 3).
These achievements are consistent with the literature
data indicating very complex processes which are incorporated in TWs leading to efficient removal of pollutants. These
processes have more complex futures and are more complicated than processes in conventional highly effective
treatment plants. Moreover, concepts apply to activated
sludge or trickling filters (like oxygen demand for nitrification or easily available organic matter for denitrification)
appears to be doubtful for TWs (Kadlec & Wallace ).
The lack of simple linear regression between influent and
effluent concentrations in the case of the second stage treatment in MTW could indicate that not only concentration of
discharged pollutants but also their composition is very
important for unit processes leading to efficient pollutants
As is shown in Figure 2, in 2010 the share of colloidal
fraction in RW was much higher than in 2009. In the literature the colloids need to be transformed firstly to the
dissolved fraction and then become available for further
microbiological transformations (Ma˛ kinia et al. ;
Ma˛ kinia ). What is more, diffusion of dissolved organics and nitrogen has very important role in both physical
transfer processes as well as transformations leading to
removal of pollutants (Tanner et al. ; Kadlec & Wallace ; Ma˛ kinia et al. ). Diffusion, in biofilms
attached to the surface of medium in SSF TWs, is the
main mechanism responsible for internal supply of
oxygen, nitrate and ammonia substrates needed for nitrification and denitrification. Thus, higher concentration of
colloidal fraction slows down the diffusion and, as a consequence, is a limiting factor in removal processes.
M. Gajewska & H. Obarska-Pempkowiak
Multistage treatment wetland for treatment of reject waters
The carried-out investigation confirmed that the accepted
concepts from highly effective technologies (like activated
sludge) are not applicable to TW technology. Among
others it was confirmed that RW from digested sludge dewatering with very high concentration of ammonium nitrogen
and hardly degradable organic matter (COD) could be successfully pretreated in the facility consisting of mechanical
part and MTW. Two years of monitoring of the facility
proved high efficiency removal of predominating pollutants:
COD 75–80%, BOD 82.2–95.5% and TN 78.7–93.9%. Since
the working conditions of the pilot TW in both years were
similar and concentrations of pollutants in the second year
of operation were even lower when comparing with 2009,
the reason for TN removal decreasing in 2010 was probably
the composition of treated RW. It was indicated that the
higher concentration of colloidal fraction slowed down the
pollutant removal due to limited diffusion processes. There
is a need for further investigations in the long term,
especially during winter time, to confirm the finding of
this investigation.
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First received 14 January 2013; accepted in revised form 2 April 2013
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