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e c o l o g i c a l e n g i n e e r i n g 3 3 ( 2 0 0 8 ) 54–67
a v a i l a bl e a t w w w . s c i en c e d i r e c t . co m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e c o l e n g
Anaerobic digesters as a pretreatment
for constructed wetlands
J.A. ´ Alvarez∗, I. Ru´ ız, M. Soto
Department of Physical Chemistry and Chemical Engineering I, Campus A Zapateira, 15008,
Faculty of Science, University of A Coru ˜ na, A Coru ˜ na, Spain
a r t i c l e i n f o
Article history:
Received 11 June 2007
Received in revised form
25 January 2008
Accepted 17 February 2008
Keywords:
Anaerobic digesters
Constructed wetlands
Municipal wastewater
Clogging
a b s t r a c t
The most commonly used pretreatment technologies for constructed wetland (CW) treat-
ment of domestic sewage are septic tanks (ST) and Imhoff tanks (IT). These technologies
have frequently suffered from failures and even in normal operation they offer insufficient
removal of solids. As a result, combined ST-CW or IT-CW can experience substrate clogging,
especially when high organic loads areapplied. In thelast 7 years, theoperation of combined
systems using high-rate anaerobic digesters as a pretreatment and CW as a post-treatment
has been reported. A review of the literature indicates that CW in these combined sys-
tems operates with a similar organic loading rate (on a chemical oxygen demand basis) but
with a lower total suspended solid (TSS) loading rate. In these combined systems, the TSS
loading rate is 30–50% less than that applied in CW combined with classical pretreatment
technologies. A low TSS loading rate could prevent substrate clogging in CW.
This work presents the results of different case studies on the treatment of municipal
wastewater with high-rate anaerobic systems. Our interest is focused on the capacity of
these systems for removing suspended solids, and therefore on their potential as an appro-
priate pretreatment to avoid clogging in constructed wetlands and to improve efficiency.
Average and 95 percentile TSS concentrations of anaerobic treated wastewater were below
60 and 100 mg/l, respectively, for all configurations. Therefore, the use of high rate anaer-
obic systems as a pretreatment for constructed wetlands could delay gravel bed clogging.
Furthermore, according to the level of organic matter removal, anaerobic pretreatment pro-
vided a 30–60% reduction in the required wetland area. Both treatment alternatives can
be combined to develop low-cost, robust, and long-term systems for treating municipal
15 AT + SSF Z.b. and T.sc 1.5 – – – – 4.5 1.0 71.4 86.1 –
16 AT + SSF Z.b. and T.sc 0.75 – – – – 9.0 2.0 37.5 46.1 –
a References: (1,2,3) Sousa et al. (2001), (4,5) Sousa et al. (2003), (6,7) El-Khateeb and El-Gohary (2003), (8) Kaseva (2004), (9) Mbuligwe (2004), (10,11) Green et
et al. (2006), (14) El-Hamouri et al. (2007), (15,16) Da Motta Marques et al., 2001.b System description: UASB (Upflow Anaerobic Sludge Bed), SSF (Horizontal Subsurface flow constructed wetland), VF (Vertical flow constructed wetlan
wetland), and AT (Anaerobic treatment not specified). Referred units were connected in series, the number in parentheses indicates several units of the c Zizaniopsis bonariensis and Typha subalata.
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e c o l o g i c a l e n g i n e e r i n g 3 3 ( 2 0 0 8 ) 54–67 59
Table 3 – Comparison of the loading rate and efficiency for CW treatment of effluents from UASB and from classicalpretreatment technologies
BOD5 COD TSS TP TN NH4+-N
Worldwide experiment SSFa
Loading rate (g/m2 d) 3.9 12.0 5.4 0.39 1.76 1.06
Efficiency (%) 81 71 78 32 39 34
UASB-SSF combined systemsb
Loading rate (g/m2 d) 5.5 10.8 2.9 0.49 3.01 2.02
Efficiency (%) 78 73 63 54 53 53
a Vymazal (2005), n =66–131.b This review: mean values obtained from data in Table 2, except for experiments 10, 11, and 14 (n = 4–13).
operation period was 3 years) to conclude whether anaerobic
pretreatment can prevent gravel bed clogging. Furthermore,
information about solid accumulation or hydraulic conduc-
tivity evolution in constructed wetlands combined with UASB
is not included in referred bibliography.
In general, the performance of the systems is satisfactory
with high removal efficiencies for organic matter, suspendedsolids, nutrients and pathogens, reaching mean values (±S.D.)
TN (total nitrogen), 51 (±26)% TP (total phosphorous), and 94
(±13)% FC (data obtained from Table 2). These efficiency val-
ues are close to those found in the literature (Vymazal, 2002;
Rousseau et al., 2004a; Puigagut et al., 2007) for SSF CW treat-
ing primary settled effluents. Planted beds generally perform
better than unplanted ones (El-Khateeb and El-Gohary, 2003;
Sousa et al., 2003; Mbuligwe, 2004; Kaseva, 2004; El-Hamouri
et al., 2007). Da Motta Marques et al. (2001) f ound that plants
improve constructed wetland efficiency only under high load-
ing rates. No significant differences in efficiency between
macrophyte species were found in UASB-CW systems treatingdomestic sewage, except in some restricted cases.
The organic load rate for horizontal flow constructed wet-
lands varies from 5 to 20 (mean value of 11.4)gCOD/m2 d
and from 1.4 to 3.3 (mean value of 3.0) gTSS/m2 d, when the
study from El-Hamouri et al. (2007) is excluded. In general,
organic loading rates on a COD basis are similar to those
reported for SSF CW operating in several European countries
while loading rates of suspended solids are lower. As indi-
cated, Vymazal (2002) reported organic loading rates in the
range of 8.6–12.7gCOD/m2 d for the Czech Republic, Denmark,
Poland, and Slovenia, and TSS loading rates in the range of
3.6–5.2 gTSS/m2 d for the Czech Republic, Denmark, UK, North
America, and Poland. Vymazal (2005) reported worldwidedata, including data from Australia, Austria, Brazil, Canada,
the Czech Republic, Denmark, Germany, India, Mexico, New
Zealand, Poland, Slovenia, Sweden, the USA, and the UK.
Table 3 compares mean worldwide values reported by
Vymazal (2005) and mean values obtained from studies
included in Table 2 to UASB-SSF (or SF) combined systems.
Although the number of examples for UASB-SSF combined
systems is scarce, results suggest similar organic loading rates
and lower TSS loading rates for CW combined with UASB
pretreatment. So, UASB reactors reduce the suspended solid
loading rate from 30 to 50% compared to classical pretreat-
ment technologies. COD removal efficiency is similar while
TSS removal efficiency is lower. Nutrient loading rates (TP,
TN, and NH4+-N) are higher for CW in UASB-SSF combined
systems, which generally also have higher nutrient removal
efficiencies. This behaviour is in accordance with the fact that
UASB efficiently removes organic mater and suspended solids,
but UASB is primarily a nutrient conservative process. There-
fore, in UASB-SSF combined systems, CW will have a lower
TSS influent concentration but a higher nutrient concentra-tion. The removal of faecal coliforms has a range of 1–4 log
units and is clearly influenced by the HRT applied.
El-Hamouri et al. (2007) reported higher loading rates of
131 gCOD/m2 d and 64gTSS/m2 d for a SSF CW fed with the
effluent from a two-step UASB system. The SSF used by
El-Hamouri et al. (2007) had a high depth (0.8m) and the
system reached low nutrients removal, indicating only sec-
ondary treatment objectives. Furthermore, the reportedperiod
of operation was short (6 months) and there is no informa-
tion on the sustainability of this highly loaded SSF CW. Even
higher organic loading rate values were reached when UASB
effluents were treated in VF CW or in combined systems that
included VF CW units (Green et al., 2006). A system includ-ing a UASB followed by two VF CWs and one SSF CW reached
a high secondary treatment efficiency that had a small foot-
print, equivalent to 0.9m2 per person. An even lower footprint
of 0.13m2 per person equivalent was achieved for a scheme
that included a UASB followed by three VF CWs (Green et al.,
2006).
5. Anaerobic configurations as CWpretreatment: case studies
5.1. Anaerobic digestion processes and up flow
anaerobic digesters
The UASB reactor is the most commonly used anaerobic tech-
nology for domestic sewage treatment; and the hydrolytic
upflow sludge bed (HUSB) is an option to be considered. These
digesters have similar design features, but are primarilydiffer-
entiated by their operational conditions. Both UASB andHUSB
can be operated as a single unit or as a combined two-step or
hybrid system (see Fig. 1).
In upflow mode reactors like the HUSB and UASB, raw or
pretreated wastewater enters the bottom of the digester and
goes up until it reaches the solid–liquid–gas (S–L–G) separator,
if it exists, and finally reaches the exit level. Sedimentation,
filtration, and absorption processes enable suspended solids
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60 e c o l o g i c a l e n g i n e e r i n g 3 3 ( 2 0 0 8 ) 54–67
Fig. 1 – Schematic representation of anaerobic systems used for laboratory, pilot, and field scale applications (note that
Table 4 indicates which of these configurations are tested at lab, pilot, or full scale experiments). Abbreviations: UASB,Upflow Anaerobic Sludge Bed; HUSB, Hydrolytic Upflow Sludge Bed; CMSS, Completely Mixed Sludge Stabilization digester;
I, Influent; E, Effluent; G, Biogas; S, Sludge.
to be retained inside the digester, resulting in the sludge bed.
Because of this, suspended solids and absorbable organic mat-
tercontained in wastewater have a longer solid retentiontime
(SRT) than the liquid fraction (HRT), allowing particulate mat-
ter to be totally or partially biodegraded. In properly designed
systems, the pass of the influent through the sludge in up
flow digesters improves contact between organic substrates
and biomass, enhancing digester performance.
Depending on operational conditions, the sludge held inthe digester can reach the S–L–G separator and, eventually,
the exit level. In order to avoid the presence of great amounts
of suspended solids in the digester effluent, purges must be
periodically practiced from a point slightly below the S–L–G
separator or at an equivalent point. The frequency of this
purge is highly variable, from once a week in the case of high
load HUSB systems to a yearly purge or no purge in the case
of low load UASB methanogenic systems. In the case of HUSB
systems, additional purges maybe necessary in order to main-
tain the SRT at an appropriate value, as indicated below.
The anaerobic degradation process takes place in two
main sequential phases. Particulate organic and soluble
polymers should first be hydrolysed and subsequently acid-ified to volatile fatty acids (known as acidogenic phase,
or hydrolytic–acidogenic phase). The process can continue
through acetic acid generation from other volatile fatty acids
and through methane generation from acetic acid and hydro-
gen (known as the methanogenic phase). The overall process
for the anaerobic digestion of complex substrates may be
performed either in a single unit system (only one digester,
single-step system) or in two separated units (two digesters
connected in series, two-step system). In two-step systems,
the first step mainly deals with the substrate hydrolysis and
acidification and the second step involves the acetogenic
and methanogenic process. However, many two-step systems
respond to a partial phase separation, showing the presence
of methanogenic activity in the first step andhydrolysis in the
second step.
On the other hand, the anaerobic process may be stopped
in the first phase as a function of environmental and oper-
ational conditions. In this case, the one-step system will be
called an anaerobic hydrolytic pretreatment. The well-known
UASB system is the most commonly used design for anaero-
bic methanogenic treatment of domestic sewage. A digester
design similar to the UASB, when used under hydrolytic (non-methanogenic) conditions, is known as a HUSB reactor.
The type of substrate, influent concentration, temperature,
HRT, and SRT are the main operational parameters that define
the methanogenic or nonmethanogenic conditions. Domes-
tic sewage is a complex substrate with only a small fraction
of readily degradable matter in anaerobic conditions, making
hydrolysis the limiting step of the overall process in many
cases. Influent concentration and the applied HRT determine
the maximum achievable SRT, although the actual SRT may
be reduced through a sludge purge (Alvarez et al., 2006). Lower
influent concentration and lower HRT lead to a lower SRT.
Temperature determines the minimum required SRT for
methanogenic conditions. Methanogenic digesters operatingat 13–20 ◦C need a minimum SRT of 80 and 50 d (Henze et al.,
1995). In this way, Zeeman and Lettinga (1999) postulated that
a SRT higher than 75 d would be required for a UASB treating
municipal wastewater at 15 ◦C.
With dilute or verydilute sewage, the maximum achievable
SRT of an UASB may be equal to or below the minimum SRT
required for methanogenesis. In this case, the methanogenic
processes is partial andvolatile fatty acids (mainly aceticacid)
accumulate in the effluent of the digester. In any case, the SRT
may be reduced through a sludge purge to reduce methano-
genesis and to reach predominantly hydrolytic–acidogenic
conditions. In practice, hydrolytic conditions are established
by applying a low HRT and practicing an additional sludge
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Table 4 – Summary of the results obtained in anaerobic systems for municipal wastewater treatment
Expa Systema Volume (l) Days (samples)b T (◦C) HRT (h) SRT (d) Vup (m/h) XR (gVSS/l) Effluent pH
Single-step HUSB systems (hydrolytic pretreatment)
a For system description, see also Fig. 1. Experiments: (1) Ligero et al. (2001a); (2) Alvarez et al. (2003); (3) Alvarez (2004); (4) Ruız et al. (1998); (5, 6, 7) Alvare
Ruız et al. (1998); (10) Alvarez et al. (2004); (11) Alvarez et al. (2007); (12) Alvarez (2004); (13) Barros and Soto (2004).b Reported operation period in days, the number of samples analysed is in parentheses.c The average is followed by the minimum and maximum values in parentheses.d Removal range obtained from average removal values that corresponded to periods of different operation conditions.
e Values corresponding to the first and second step units (in two-step systems), respectively.
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62 e c o l o g i c a l e n g i n e e r i n g 3 3 ( 2 0 0 8 ) 54–67
purge if necessary. In this way, a lower HRT and a lower SRT
differentiate HUSB from UASB systems.
5.2. Description of surveyed anaerobic systems
Fig. 1 shows the different anaerobic digester configurations
analysed in this section, which include laboratory, pilot-
and field-scale applications of anaerobic digesters, singleand two-step systems, and hydrolytic and methanogenic
operation conditions. All applications were carried out in
Galiza, in northwest Spain. Attention has been paid to the
removal efficiency and effluent concentration of suspended
solids.
The main characteristics of these systems are described
below, while a detailed explanation is available in the ref-
erences indicated. All water line digesters, i.e., all digesters
fromacetic respiration in anaerobicconditions was lower than
biomass growth from complex substrates (Gujer and Zehnder,
1983). Lowgrowth will reduce solidaccumulation in CW media
and could prevent clogging phenomena. At the present time,no research has been reported on the influence of the type of
anaerobic pretreatment on the post-treatment CW operation;
and there is a need for additional studies on this subject.
7. Conclusions
One of the most significant handicaps of constructed wet-
lands for urban wastewater treatment is gravel bed clogging
after a few years of operation with poor waste pretreatment
or high organic loading rates. Another disadvantage of con-
structed wetlands is that a large superficial area is required.
Both handicaps can be minimised with an appropriate anaer-obic pretreatment.
Anaerobic plants may be operated either as hydrolytic or
methanogenic digesters. Hydrolytic digesters, at an HRT of
3–5h, remove 65–85% of TSS and 35–55% of COD, showing
a large amount of hydrolysis and acidification of influent
SS. Methanogenic digesters, operating at a HRT of 8–11h,
remove 60–90% of TSS and 40–75% of COD. A two-step system
(hydrolytic and methanogenic digesters in series) can remove
up to 80–90% of TSS and 50–65% of COD. These results corre-
spond to applications carried out in temperate climates where
wastewater temperature ranges from 13 to 20 ◦C, or in some
cases from 5 to 20 ◦C.
The average and 95th percentile TSS concentrations of anaerobically treated wastewater were below 60 and 100 mg/l,
respectively, for all configurations. Therefore, anaerobic pre-
treatment of sewage could help prevent media clogging
in constructed wetlands. Furthermore, depending on the
amount of organic matter removed, anaerobic pretreatment
can provide a reduction of 30–60% of the wetland area.
Acknowledgments
This work was supported by project CTM2005-06457-C05-
02/TECNO from the Ministery of Education and Science of
Spain.
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