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Transcript of Spain AD CW
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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
wastewater.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Sustainability of sanitation systems should be related to low
cost and low energy consumption and, in some situations,
low mechanical technology requirements. Decentralised and
low-cost processes are considered to be a better choice for
rural areas (Lens et al., 2001). Anaerobic digesters and con-
∗ Corresponding author. Tel.: +34 981 563100x16016; fax: +34 981528050.
E-mail address: [email protected] (J.A. Alvarez).
structed wetlands are treatment systems with a very small
energy input, low operational cost, and low surplus sludge
generation (Sperling, 1996; Kadlec et al., 2000; Lens et al.,
2001; Hoffmann et al., 2002). These characteristics, together
with low technological requirements, make them particu-
larly suitable for decentralised wastewater treatment in rural
areas.
0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ecoleng.2008.02.001
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The costs of construction, installation, and operation of
anaerobic digesters are lower thanthose of conventionalaero-
bic units because anaerobicdigesters do not require expensive
equipment for process maintenance andcontrol. In fact, if the
environmental conditions inside the digester are adequate,
anaerobic processes are mainly self-controlled. Additionally,
the production of excess sludge is minimal, and energy bal-
ances are quite favourable, even when heating is required, dueto the production of methane (Foresti, 2002).
The disadvantage of anaerobic digesters is that additional
treatment is necessary to polish and lower the pollution load.
Even in tropical regions (Sousa et al., 2001), and mainly in cool
to temperate climate regions (Alvarez et al., 2003), the effluent
of UASB (up flow anaerobic sludge blanket) systems requires
an effluent post-treatment to reduce organic mater, nutri-
ents, and pathogenic microorganisms. In the case of operating
temperatures below 20 ◦C, UASB systems are good at remov-
ing suspended solids; however, acetic acid accumulation in
the effluent reduces the COD (chemical oxygen demand) and
BOD(biologicaloxygen demand) removal efficiencies (Alvarez,
2004; Alvarez et al., 2006).It is of great interest to combine wetland systems with
anaerobic digesters in order to obtain sufficient treatment effi-
ciency. The most commonly used anaerobic technology for
municipal wastewater treatment is the UASB (Lettinga, 2001;
Foresti et al., 2006; Van Haandel et al., 2006). There are sev-
eral studies of systemscombining anaerobicpretreatment and
constructed wetlands, which are assessed in Section 4. UASB
reactors are the referent pretreatment anaerobic technology
used in these combined systems. However, other anaerobic
technologies may be used as sewage pretreatment for con-
structed wetlands. The hydrolytic upflow sludge bed reactor
(HUSB) is a promising alternative.
However, constructed wetlands (CW) are land-intensivetreatment systems. The use of an appropriate anaerobic pre-
treatment before constructed wetland treatment can reduce
the construction cost by about 36–40%, due to the fact that
anaerobic treatment reduces the influent organic matter and
therefore the area required for CW is decreased (Barros and
Soto, 2002). Bothanaerobic and wetlandtreatment approaches
are characterized by lowconstruction and operationcosts, low
excess sludge, and low energy demand. Therefore, both treat-
menttechnologies are complementary andhighly sustainable.
Limited organic removal efficiency in anaerobic digesters
is compensated by high efficiency in CW, while anaerobic
digesters present minimal area requirements, generally less
than 0.1 m2 /p.e. for UASB (Kivaisi, 2001).Studies have shown that one of the most important oper-
ational handicaps of constructed wetlands is gravel bed
clogging; this may occur after several years, resulting from
the treatment of raw or poorly pretreated urban wastewa-
ter. Suspended solids that are not removed in a pretreatment
system are effectively removed by filtration and settlement
within the first few metres beyond the inlet zone. Thus, a
high level of total suspended solid (TSS) removal in anaer-
obic pretreatment would contribute to avoiding or reducing
wetland clogging problems, reinforcing constructed wetland
sustainability (Vymazal, 2005; Caselles-Osorio et al., 2007).
The aim of this work is to analyse and discuss the poten-
tial of high-rate anaerobic digesters as a pretreatment for
municipal wastewater that will later be treated in constructed
wetlands. First, a brief analysis of clogging phenomena in CW
is presented, and the pretreatment technologies most often
used in combination with CW are discussed, focusing on their
potential for reducing the quantity of suspended solids intro-
duced into constructed wetlands. Next, the authors review
the literature on systems combining anaerobic digesters and
CW. Finally, detailed case studies on anaerobic pretreatmentof municipal wastewater are presented, focusing on the effi-
ciency of suspended solid removal and the potential of anaer-
obic digesters for preventing clogging and reducing CW area.
2. Substrate clogging in constructedwetlands
Substrate clogging encompasses severalprocesses that lead to
a reduction of the infiltration capacity of the gravel bed after
several years of operation. In horizontal flow (HF) wetlands,
apparent clogging and subsequent ponding near the inlet
of the treatment cells dampen the remarkable performanceof the system. This may occur after few years of operation
(Dahab and Surampalli, 2001; Caselles-Osorio et al., 2007). In
vertical flow (VF) wetlands, clogging of the substrate matrix
critically hinders the oxygen transport and therefore results
in an extremely rapid failure of the system’s ability to treat
wastewater (Langergraber et al., 2003).
The main parameters that influence the substrate clogging
process are the organic load and the suspended solid load.
Besides these main factors, the clogging risk is also controlled
by gravel size, since large gravel prevents or delays clogging
phenomena (Chazarenc and Merlin, 2005; Zhao et al., 2004).
Organic load is an indirect parameter leading to sludgepro-
duction derived from bacterial growth. Both influent sludgeand sludge generated in situ will accumulate in the gravel
bed. Literature values for the maximal acceptable organic
load fall within a wide range. For example, Winter and Goetz
(2003) indicated the area of VF constructed wetlands should
be designed for a maximum loading rate of 20 gCOD/m2 d to
avoid the clogging process. So, the clogging risk becomes a
limitation of wetland performance.
On the other hand, one of the major parameters influenc-
ing clogging is the suspended solid load (Batchelor and Loots,
1997; Dahab and Surampalli, 2001; Winter and Goetz, 2003;
Langergraber et al., 2003). Little information is available con-
cerning the maximum acceptable TSS loading rates. Values
given are only valid for one special type of substrate and can-not be used as a general guideline. For example, Dahab and
Surampalli (2001) f ound clogging in a subsurface horizontal
flow constructed wetland system after 3.5 years of treating
wastewater with a load of 1.44gTSS/m2 d. Winter and Goetz
(2003) showed that in order to avoid clogging processes in
a vertically constructed wetland, the average concentration
of TSS in the inflow should not exceed 100 mg/l, while the
suspended solid load should not exceed 5 gTSS/m2 d. These
authors thought that growth of biomass has only a minor
effect on clogging compared to the accumulation of influent
TSS.
Green et al. (2006) compared two types of pretreatments:
a UASB system and a primary decanter. They indicated that
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by using UASB effluent as feeding water for a VF CW a higher
removalratecould be achieved thanby using primary decanter
effluent, as a consequence of the relatively low TSS loading
rate resulting from the higher removal of TSS in the UASB.
These authors found that the total TSS removedin each active
cycle (until clogging occurred) was similar for the VF CW that
receivedeitherpre-settleddomestic wastewater or UASB efflu-
ent, whilethe total COD removed was aboutthreetimes higherfor the VF CW receiving UASB effluent. Therefore, it seems
that the TSS loading rate was the most influential parameter
affecting the rate of bed clogging in VF CW (Green et al., 2006).
Caselles-Osorio and Garcia (2007) compared the physico-
chemical pretreatment and primary settling for constructed
wetlands. Physico-chemical pretreatment reduced the COD to
48% and turbidity to 17% that of primary settled wastewater.
After 8 months of operation at similar hydraulic loading rates,
it was observed that the hydraulic conductivity decreased
by 20% in the subsurface flow (SSF) CW fed with settled
wastewater. The authors estimated that the physico-chemical
pretreatment extended the lifespan of the constructed wet-
land by approximately 10 years, compared to a primarydecanter pretreatment.
The effect of the influent type (dissolved glucose or par-
ticulate starch) on the efficiency of SSF CW was reported by
Caselles-Osorio and Garcıa (2006). The type of organic mat-
ter did not appear to influence the COD removal efficiency.
However, ammonia nitrogen removal was higher in the sys-
temfed with glucose than in theone fedwithstarch. Hydraulic
conductivity was lower near the inlet of the SSF CW fed with
glucose, despite the possible retention and accumulation of
starch particlesnear theinletof theotherSSF CW. The authors
hypothesized the growth and development of biofilm was
greater in the system fed with glucose than in the system fed
with starch, since glucose is a readily biodegradable carbonsource. Therefore, the biofilm growth could be an important
parameter in the evaluation of clogging phenomena, as these
authors indicated.
It is generally accepted that the application of a good
wastewater pretreatment is essential for sustainable, long-
term operation of subsurface flow constructed wetlands
(Vymazal et al., 1998; USEPA, 2000; Vymazal, 2002; Caselles-
Osorio et al., 2007). On the other hand, although VF CW
can directly treat raw domestic wastewater (Chazarenc and
Merlin, 2005), several authors also recommended wastewa-
ter pretreatment (Winter and Goetz, 2003; Langergraber et al.,
2003; Green et al., 2006).
3. Pretreatment alternatives for constructedwetlands
The main objective of pretreatment or primary treatment is
the reduction of suspended solids in wastewater, although
additional treatment effects leading to organic content reduc-
tion and, in some cases, the hydrolysis and stabilization of
the generated sludge are obtained. In this way, some pretreat-
ment technologies can reach up to 50% COD or BOD removal.
Furthermore, froma general point of view, pretreament opera-
tions are considered to be a convenient means of ensuring the
correct operation of subsequent treatment steps in both con-
ventional and natural low cost treatment approaches (Metcalf
and Eddy, 2003). However, information about the operation
and efficiency of pretreatment systems combined with CW
is scarce. Even in many scientific reports, the TSS concentra-
tion entering the CW system is not available, in contrast to
the frequent statement that the influent concentration and
loading rate of TSS are the main factors that influence clog-
ging.Classical sewage pretreatment technologies include a sep-
tic tank and Imhoff tank for small-scale installations. These
systems can achieve a TSS removal of 50–70%, generating pri-
mary effluent concentrations in the range of 50–90 mgTSS/l
when they are operated well (Metcalf and Eddy, 2003). Fur-
thermore, septic and Imhoff tanks stabilize the sludge by
anaerobic digestion, reducing the amount of sludge gener-
ated. Another classical pretreatment alternative, which is
used mainly for larger installations, is the primary decanter.
Primary decanters offer similar TSS removal of 50–70%, but
the high amount of primary sludge produced is their largest
handicap (Metcalf and Eddy, 2003). Physico-chemical treat-
ment (coagulation and flocculation followed by clarification)is an advanced pretreatment for domestic sewage, reach-
ing up to 90% TSS removal and 80% COD ( Metcalf and Eddy,
2003). However, physico-chemical pretreatment also has cer-
tain requirements that can make this process unsuitable in
the context of constructed wetlandstechnology; these include
thecost of the coagulants, energy for adding and mixing coag-
ulants, and increased sludge handling (Caselles-Osorio and
Garcia, 2007).
Until now, the most common wastewater pretreatments
for CW have been the septic tank (ST) or the Imhoff tank (IT).
When properly operated, ST and IT offer good pretreatment
levels, reaching low TSS concentrations (Neralla et al., 2000;
Vymazal, 2002). However, ST and IT frequently suffer fromfailures that decreased the treatment efficiency (Philippi et al.,
1999; Mbuligwe, 2004; Caselles-Osorio et al., 2007).
A recent survey indicates that 86% of the constructed wet-
land plants in operation in Spain use a septic tank or Imhoff
tankfor pretreatment(Puigagutet al., 2007). Thiswas observed
in spite of the fact that the majority of these CW were built
within the last 5 or 6 years. A report of recently built CW sys-
tems in Italy also indicated the use of Imhoff tanks (Masi et
al., 2006). The situation is similar in most countries where CW
systems are being used. In the case of the Czech Republic,
pretreatment for a small system usually consists of a septic or
settling tank, while pretreatment for larger systems usually
consists of an Imhoff tank (Vymazal, 2002). Settling tanks areused also in Flanders (Rousseau et al., 2004a) and Denmark
(Brix and Arias, 2005).
A summary of data on wastewater pretreatment for con-
structed wetlands is presented in Table 1. The average primary
treatment effluent concentration of SS in Czech Republic CW
systems is 65 mg/l, while the average mass-loading rate is
3.6 gTSS/m2 d (Vymazal, 2002; n = 42). Data for Denmark and
the UK (n = 77), North America (n =34), and Poland (n =6),
and the Czech Republic, indicate that the average influ-
ent concentration to CW after pretreatment ranges from 48
to 173mgTSS/l and average loading rates range from 3.6
to 5.2gTSS/m2 d (Vymazal, 2002). Vymazal (2005) reported
worldwide figures for CW, indicating an average influent TSS
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Table 1 – Effluent concentration and efficiency of TSS removal for domestic sewage pretreatment systems combined withCW
N TSS (mg/l) TSSr (%) Reference
Septic and Imhoff tanks
Primary sedimentation (to VF CW) 1 240–416 Green et al. (2006)
Septic tank (to SSF CW) 8 26–114 Neralla et al. (2000)
Septic tank (to VF CW, single-house) 3 85–124 Brix and Arias (2005)Septic tank (to SSF CW) 4 90–517 (261)a 35.2 Caselles-Osorio et al. (2007)
Settling pond (to FWS CW) 12 5–200 (25)b Rousseau et al. (2004a)
Settling tank (to VF CW) 7 13–1000 (80)b Rousseau et al. (2004a)
Settling tank (to HF CW) 2 10–400 (47)b Rousseau et al. (2004a)
Septic tank or Imhoff Tank (to SSF CW) 3 173 Puigagut et al. (2007)
Septic tank or Imhoff Tank (to SSF CW) 42 65 Vymazal (2002)
Imhoff Tank (to SSF CW) 1 146 73.0 Caselles-Osorio et al. (2007)
Imhoff Tank (to HF or VF CW) 3 26–76 Masi et al. (2006)
Range (Average) 26–1000 (123)
High-rate anaerobic digesters
UASB (to VF CW) 1 124 52 Green et al. (2006)
UASB (to SSF and FWS CW) 1 59 66.5 El-Khateeb and El-Gohary (2003)
UASB (to SSF CW) 1 189 El-Hamouri et al. (2007)
UASB (to SSF CW) 1 34–42 82–91 Barros et al. (2006)
UASB (to SSF CW) 1 38–74 (52)a 49–78 (65)a Ruız et al. (2006)Range (Average) 34–189 (92) 52–91 (68)
a Range followed by the average. N is the number of studies included.b Range followed by 50% percentile.
concentration of 107 mg/l and an average TSS loading rate of
5.4g/m2 d.
For Spanish CW-based treatment systems, TSS loading
rates range from 3 to 17gTSS/m2 d (n = 6), and the aver-
age primary treatment effluent concentration is 173 mg/l
(n = 3) (Puigagut et al., 2007). These authors highlight the
scarcity of data about TSS loading rates and influent con-
centrations, as they surveyed a total of 39 SSF systems butonly found information on TSS for a few of these systems.
Also in Spain, recent research conducted on several Catalo-
nian SSF CW systems (Caselles-Osorio et al., 2007) reports
primary effluent from septic tanks and Imhoff tanks con-
taining 90–517 mgSS-COD/l (average and standard deviation
of 238±172 mgSS-COD/l; n = 5). These SS-COD values indicate
higher TSSconcentrations. The authors indicated thatin some
cases, septic tanks used as pretreatment systems were not
working properly. Estimated TSS loading rates for Catalonian
SSF CW systems (Caselles-Osorio et al., 2007) are in the range
of 2.6–10 gTSS/m2 d, and are higher than the ranges indicated
above for other countries.
As indicated, high-rate anaerobic digesters have becomean alternative for sewage treatment in regions with a warm
climate. As a consequence, in recent years CW systems have
been applied in some occasions as a post-treatment pro-
cess for anaerobically pretreated sewage. Section 4 deals the
operation of CW treating anaerobic effluents, while Table 1
summarizes available data about TSS in UASB effluents fed
to CW systems. Data from Table 1 indicate a somewhat low
TSS concentration in UASB effluents when compared to sep-
tic and Imhoff tank effluents. However, it is not possible to
make a definitive comparison due to the scarcity of data for
UASB-CW combined systems.
A general review of high-rate anaerobic digesters treating
municipal wastewater (Alvarez et al., in preparation) indi-
cated that UASB removes about 73% of influent TSS (average
influent TSS of 241 mg/l, effluent TSS of 65 mg/l, n =127 lab
and field applications, temperature of 21.6 ◦C, HRT (hydraulic
retention time) of 8.5 h. However, mean values for perfor-
mance of field-only applications of UASB were lower (influent
TSS of 301 mg/l, effluent TSS of 102 mg/l, n = 22, temperature
of 23.8 ◦C, HRT of 6.9h). This could be due to the fact that
UASB field applications mainly correspond to tropical coun-tries where wastewater concentration is high. Furthermore,
higher temperatures in these countries lead to higher biogas
production that in turn increases sludge washout. However,
UASB offers an advanced wastewater pretreatment, which
reaches about 62% COD removal and 68% BOD removal, lev-
elsthat aremaintained in field applications. In addition, UASB
systems generate very small amounts of sludge and applied
HRTs are lower than those of some primary treatments such
as septic tank or ponds.
Different configurations of anaerobic digesters had been
studied in order to treat municipal wastewater in both cold
and warm regions. In Section 5, some of these configurations
are analysed, with special attention given to the solid removalcapability of anaerobic systems.
4. CW post-treatment of anaerobicallytreated sewage
Table 2 shows the main design and operating characteris-
tics of various constructed wetlands for UASB-CW combined
systems found in the literature. A dozen UASB-CW appli-
cations were described, although there is only information
about influent TSS for a few systems, as can be seen by com-
paring Tables 2 and 1. In addition, the operational period
reported in these studies is not long enough (the maximum
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Table 2 – Operation of constructed wetlands using anaerobic technology as a pretreatment
Ref.a Systemb Plant OLR (g/m2 d) (in CW sub-units system) CW system
TRH (d) COD TSS BOD N-NH4 TN TP COD TSS
1 UASB + SSF Juncus spp. 5 – – – – 1.32 0.16 82.9 65.0 –
2 UASB + SSF Juncus spp. 7 – – – – 1.89 0.23 81.3 56.2 –
3 UASB + SSF Juncus spp. 10 – – – – 2.64 0.32 81.6 48.0 –
4 UASB + SSF Juncus spp. 10 6.6 – – – 2.0 0.17 81.7 70.3 –
5 UASB + SSF Juncus spp. 7 9.5 – – – 2.0 0.25 76.7 66.0 –
6 UASB + SSF T. latifolia 5 5.5–13.5 1.4–3.3 1.7–4.7 0.7–1.8 1.3–3.1 0.08–0.19 78.0 79.7 7
7 UASB + SF T. latifolia 10.8 5.5–13.5 1.4–3.3 1.7–4.7 0.7–1.8 1.3–3.1 0.08–0.19 69.7 55.9 7
8 UASB + SSF Ph. Mauritianus
T. latifolia
1.9 12.3 – – 2.4 – – 56–61 – –
9 UASB + SSF T. Latifolia
Colocasia esculenta
1.2 14.5 – – 3.9 – 0.75 75–80 – –
10 UASB + VF(3x) Ph. australis 0.6 779.0 183.0 333.9 – – – 82.2 91.3 9
11 UASB + VF(×2)+SSF Ph. australis 0.4 + 1.6 73.7 17.3 31.6 – – – 82.2 91.3 9
12 UASB(×2)+SSF+SF Juncus spp. 5.0 7.7 1.7 5.2 1.2 2.3 0.2 72–83 32–52 7
13 UASB + FSF + SF Juncus spp. 2.4 16.5 5.0 10.2 – – – 70.5 77.3 7
14 UASB(×2)+SSF Ph. Australis
Arundo donax 0.54 130.1 64.1 74.6 21.4 21.4 3.7 78–82 79–80 79–82 8–9 8
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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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.)
of 74 (±12)% COD, 68 (±17)% TSS, 83 (±9)% BOD, 49 (±22)%
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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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)
1 HUSBlab 2 524 (147) 20 2.2–4.5 14–29 0.11 10–15 7.38 624
2 HUSBpilot run 1 25500 53 (23) 19–20 3–5 11.4 1.43 6.7 7.26 438
3 HUSBpilot run 2 25500 495 (250) 13–20 3–5 22.4 1.30 11.1 7.14 282
Single-step UASB systems (anaerobic treatment)
4 UASBlab 2 155 (42) 20 5–16 33–215 0.04 9.2 7.00 685
5 UASBpilot run 1 25500 69 (34) 14–15 10–11 88 0.49 11.4 7.15 282
6 UASBpilot run 2 25500 54 (28) 14 11 57 0.48 4.2 6.98 169
7 UASBpilot run 3 25500 70 (34) 20–21 4.7–5.6 38 1.02 10.7 7.15 339
8 UASB (field) 3600 325 (32) 5–18 17 46–92 0.19 3–6 7.10 1354
UASB-CMSS systems (anaerobic treatment)
9 UASB-CMSS lab 2–1.6 95 (42) 20 6–7 33–75 0.06 14.8 6.83 644
10 UASB-CMSS pilot 25500–20000 80 (41) 15–16 6–9 82.5 0.70 8.4–5.9 7.27 321
Two-step systems (anaerobic treatment)
11 HUSB + UASB pilot run 1 25500 + 2 0000 97 (42) 14–21 3–5 + 7 –14e 28–83e 1.26–0.50e 11.7–8.3e 7.14 251
12 HUSB + UASB pilot run 2 25500 + 2 0000 60 (35) 16–20 3–4 + 6 –9e 21–71e 1.27–0.52e 12.6–8.5e 7.21 367
13 UASB + UASB (field) 3600 + 3600 252 (29) 7–18 24 +24e 387 0.12–0.12e 4.4–6.5e 7.24 352
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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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
usedexceptthe Completely Mixed Sludge Stabilization (CMSS)
digester, operated in an upflow mode.
A 2l active volume digester was operated on a laboratory
scale UASB at a HRT of 5–16h (Ruız et al., 1998). In a second
study, the UASB reactor was operated in combination witha 1.6 l active volume CMSS digester (Ruız et al., 1998). The
CMSS digester was fed with sludge drawn from UASB and an
equal volume of the CMSS digester content was returned to
the bottom of the UASB. The CMSS digester was mechanically
stirred and in a thermostat-controlled bath that was 35 ◦C.
Finally, the same digester was operated as a HUSB at a HRT
of 2.2–4.5h (Ligero et al., 2001a,b). In this case, the digester
was equipped with an internal recirculation system. These
laboratory digesters were fed with raw domestic wastewater
collected from the main sewer of the city of A Coru ˜ na (Ruız et
al., 2007).
An anaerobic pilot plant was located at the municipal
wastewater treatment facility of Santiago de Compostela, andit was fed with raw domestic wastewater from this city. This
plant had a 25.5m3 active volume and it could treat municipal
wastewater from a population of about 200–300 inhabitants
when operated in methanogenic conditions or about 500–800
inhabitants when operated as a hydrolytic pretreatment. This
pilot plant was successively operated as methanogenic UASB
system (runs 1, 2, and 3) at a HRT from 5 to 11 h (Alvarez et
al., 2006), and as a hydrolytic HUSB reactor at a HRT of 3–5 h
(Alvarez et al., 2003).
In another study, this UASB was coupled with a CMSS
digesterthathad20m3 of active volume.This system is named
as the UASB-CMSS pilot plant. In this configuration, the UASB
was operated at a HRT of 6–9 h and the CMSS digester at a HRTof 16–27 d and 30–35 ◦C (Alvarez et al., 2004). The overall HRT
was in the range of 10.7–16.1 h.
A two-step pilot plant was also studied (Alvarez et al.,
2007), and consisted of a hydrolytic–acidogenic reactor (HUSB,
25.5m3) followed by a methanogenic unit (UASB, 20m3). Both
digestershad a similar design, and were differentiated by their
operating conditions. The HRT ranged from 3 to 5 h for HUSB
and from 6 to 14 h for UASB.
A field applicationof theanaerobic digester was carried out
in order to treat domesticwastewaterfrom a small community
of about 30 inhabitants (Beariz, Ourense). The operation of a
single-step UASB with 3.6 m3 of active volume was checked
(Barros and Soto, 2002). In a second study, a two-step sys-
tem (Barros and Soto, 2004) consisting of two UASB, each with
3.6m3 of active volume was used.These digestersdid nothave
a solid–liquid–gas separator.
Analytical methods were carried out according to Standard
methods (1995), as previously detailed (Ruız et al., 1998;
Alvarez et al., 2006). Sampling frequency of influent and efflu-
ent varied from once a week for field scale applications to four
or five times a week for pilot and lab scale digesters. The mon-itoring period varied from 53 to 495 days depending on the
system considered.
5.3. Operation and efficiency of anaerobic systems
Table 4 summarises the results of the different anaerobic
systems studied, and includes the main design and oper-
ation variables such as the HRT, SRT, upflow velocity, and
biomass concentration. The operation and efficiency of these
systems has been described in detail elsewhere (see refer-
ences in Table 4). Upflow velocity of the different systems
surveyed is determined by design characteristics, digestersize, and HRT applied. Design characteristics and HRT com-
bined with wastewater characteristics also determined the
SRT and the biomass concentration (XR) obtained. However,
in some operation periods of examples 3 and 12, the SRT of
the HUSB system was intentionally reduced via an additional
sludge purge. As indicated in Table 4, SRT was highly variable,
while thebiomass concentration wasgenerally between 8 and
15 gVSS/l (volatile suspended solids). Lower biomass concen-
trations were registered in somecases, either whenvery dilute
wastewater was treated (experiment 6) or in very low-load
digesters (experiments 8 and 13).
In this paper, we carried out a comparative study of
different systems focusing on the TSS removal efficiencyand effluent quality. As indicated above, this aspect is of
great importance in preventing clogging phenomena in post-
treatment wetlands. For this purpose, original data on the
influent and effluent were used.
Attention is also focused on other design and operation
variables like COD removal efficiency, effluent pH, biomass
activity, and surplus sludge generation. Other parameters, like
alkalinity, pathogens, fat, and oil were not measured in most
of the research described. Pathogen removal has scarcely been
considered in anaerobic digesters treating municipal wastew-
ater, and generally this aspect is not considered in monitoring
anaerobic digesters, although helminthic eggs were reported
to be completely eliminated in UASB (Lettinga et al., 1993). In acombined UASB-CW system treating the effluent from a small
rural community, anaerobic digesters removed less than 0.5
logunitsof faecalcoliforms, while the overall systemremoved
about 2.0 log units.
5.3.1. Single-step HUSB systems
The anaerobic hydrolysis of wastewater is a promising
pretreatment with the following advantages (Wang, 1994;
Goncalves et al., 1994; Ligero et al., 2001a,b): (a) it removes
an high percentage of SS; (b) it totally or partially stabilises
the sludge; and (c) it increases the biodegradability of the
remaining COD. The latter advantage favours the subsequent
biological elimination of nutrients (N, P).
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In laboratory-scale experiments with the HUSB, optimum
results were obtained at a HRT of 2.3h (see experiment 1
in Table 4). Over 60% of influent SS were retained in the
digester and hydrolysed. On the other hand, a pilotplant-scale
HUSB reactor treating diluted wastewater at 20 ◦C removed
more than 82% of TSS (experiment 2). Most of the solids
removed (above 81%) were eliminated by hydrolysis. In con-
trast, at lower temperatures (13–15 ◦C), the TSS retention andhydrolysis decreased (experiment 3). Furthermore, the HUSB
digester removed COD in an extension that varied from 30 to
60%.
The process is self-controlled in relation to operational
parameters like pH, biomass concentration, and activity. Since
acidification is a faster process than hydrolysis, the result of
hydrolytic pretreatment is the generation of VFA that reduces
the pH in both the sludge bed and the digester effluent. The
sludge bed pH is in the range from 5.5 to 7, which is lower
than influent and effluent pH. The HUSB effluent contained
acetic acid in a range of concentrations that varied from 60
to 110 mg/l, and the effluent pH was generally 0.2–1.0 units
lower than the influent pH. Sludge held in HUSB reactorsshowed residual methanogenic activity ranging from 0.01 to
0.02gCH4-COD/gVSSd, indicating partial separation of anaer-
obic phases. A lower SRT may be reached by an additional
purge, which in turn enhances phase separation through
a lower biomass concentration and methanogenic activity.
However, lower SRT also reduces the suspended solid hydrol-
ysis and increases the surplus sludge generation. Previous
research (Alvarez, 2004) demonstrated that influent wastewa-
ter strength strongly influences the overall efficiency (percent
TSS, COD, and BOD removal) and acidification efficiency (VFA
generation) of the HUSB reactor, while temperature only
appreciably influences the acidification efficiency. The influ-
ence of operational parameters such as HRT, SRT, and sludgeconcentration on thebehaviour of theHUSB systemis notwell
established (Alvarez, 2004). Further research on this subject is
still necessary.
5.3.2. Single-step UASB systems
The laboratory UASB system, which treats domestic waste-
water at 20 ◦C and HRT of 16 h, reached COD and TSS removal
efficiencies of 76 and 85%, respectively. An important effect of
HRT on theremovalefficiency was observed,since at thesame
temperature and 5 h of HRT, removal efficiencies decreased
to 53% of COD and 63% of TSS (experiment 4). In experi-
ments 5 and 6, a pilot plant UASB was operated at 10–11 h
HRT and 14–15 ◦C. In experiment 5, this plant achieved TSSand COD removals above 75 and 54%, respectively. In exper-
iment 6, these values decreased to 58% TSS removal and
40% COD removal. The influent concentration explained this
behaviour, since very dilute wastewater was used in exper-
iment 6. In experiment 7, the pilot UASB reached a high
level of TSS removal (81–82%) but COD removal remained low
(47–49%). In the field application (experiment 8), the full-scale
UASB reached a TSS removal of 82–96% and a COD removal of
58–93%.
Effluent VFA (mainly acetic acid) in single-step UASB sys-
temsworking at low environmenttemperatures ranged from0
to 80 mgCOD/l, while the average specific methanogenic activ-
ity of UASB sludge ranged from 0.02 to 0.05 gCH4-COD/gVSSd.
Fig. 2 – Average effluent TSS concentration (mg/l) in the
anaerobic system studied. Bars and whiskers represent
average values and standard deviations, respectively.
Number of data is shown at the bottom of the columns. To
identity system, see Table 4.
Effluent pH wasgenerally 0.1–0.3 units lower than the influent
pH. Surplus organic sludge generation ranged from 0 to 30%
of influent VSS, depending mainly on organic and hydraulic
loading andSRT. For example, no generation of surplus sludge
was found in experiments 6 and 8 (Table 4). In contrast, sur-
plus sludge reached 20% for influent VSS in experiment 5 and
up to 31% in experiment 7.
At temperatures of 20 ◦C and particularly at tempera-
tures lower than 15 ◦C, single-step methanogenic process had
some difficulties caused by low hydrolysis rates of influent
suspended solids, which accumulatedin the digestersdisplac-
ing the active methanogenic biomass (Zeeman and Lettinga,
1999). Therefore, significant amounts of volatile fatty acidsremained in the digester effluent. For example, this occurred
in experiment 7, working at a low HRT of 4–5h, when about
75 mgVFA-COD/l were registered in the treated effluent. Thus,
at a low temperature, a higher HRT must be applied in single-
step UASB systems, as experiments 5, 6, and 8 described (see
Table 4).
5.3.3. UASB-CMSS systems
The main aim of the CMSS digester, combined with the UASB,
was to enhance the biodegradation of influent solids retained
in the UASB and to increase its specific methanogenic activity.
The sludge drawn from the middle zone of the UASB entered
the upper zone of the digester and then circulated from thebottom of the CMSS digester to the bottom of the UASB (Fig. 1).
The CMSS digester temperature was set at optimum values
ranging from 30 to 35 ◦C, while the UASB operated at ambient
temperature.
The laboratory scale UASB-CMSS system (experiment 9,
Table 4) reached COD and TSS removal levels of 76% and 86%,
respectively, at a HRT of 6.2h for the UASB, improving the
results obtained in the single-step laboratory UASB (experi-
ment 4).
The UASB-CMSS pilot plant (experiment 10, Table 4) also
had increased efficiency compared to the single-step UASB,
since it slightly increased the methanogenic activity of the
sludge and reduced the excess sludge generation, which was
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Fig. 3 – Percentile distribution of influent and effluent TSS for each anaerobic configuration. Legend: (1) Single-step HUSB
system, (2) Single-step UASB system, (3) UASB-CMSS system, and (4) Two-step HUSB-UASB and UASB-UASB systems. The
numbers of data included were: (1) 282, (2) 128, (3) 41, and (4) 106.
only 7% of the influent total COD (or 11% of influent VSS).
The VFA concentration in UASB effluent was reduced to8–30 mgCOD/l. As indicated in Table 4, steady state efficiency
for TSS removal was high (63–79%). Furthermore, results sug-
gest that the relative volume of the CMSS digester could be
considerably lower than the volume of the UASB, and a plug
up flow sludge digester could be of interest (Alvarez et al.,
2004).
5.3.4. Two-step anaerobic treatment systems
At temperatures below 20 ◦C, the two-step anaerobic sys-
tem can improve the efficiency of the single digester, due to
the retention and hydrolysis of suspended organic matter in
the first step, allowing for an increase in the methanogenic
activity of the anaerobic biomass held in the second stepdigester.
The pilot-scale two-step HUSB-UASB system was operated
at a HRT varying from 5.7 to 2.8 h for the first step and from
13.9 to 6.5h forthe second step (experiment 11). For theoverall
system, TSS and COD removals ranged from 81 to 89% and 49
to 65%, respectively. Hydrolysis of influent VSS reached 59.7%,
and surplus sludge was 22% of the incoming VSS. Although
COD removal efficiency was influenced by wastewater con-
centration and temperature, the effluent TSS concentration
was mainly constant for influent COD higher than 250 mg/l.
In the second run (experiment 12, Table 4), the efficiency was
86–89% and 59–65% for TSS and COD, respectively, which was
slightly higher than in experiment 11 and was a consequenceof a higher influent concentration. Surplus sludge in this case
reached 29% of the incoming VSS. The specific methanogenic
activity was 0.01–0.02gCH4-COD/gVSSd for the sludge from
the first step and 0.05–0.06 gCH4-COD/gVSSd for the sludge
from the second step.
The field application of the two-step system also showed a
very good efficiency (experiment 13). The UASB-UASB system
operated at a HRT of 24 h for each digester and a temperature
of 7–18 ◦C. The efficiency of this low load system was 45–65%
and75–90% of CODand TSS, respectively (Table4); and surplus
sludge was not generated. Specific methanogenic activity was
0.01 and 0.02gCH4-COD/gVSSd for the sludge in the first and
second step UASB reactors, respectively.
5.4. Effluent TSS concentration of the surveyed
anaerobic systems
Fig. 2 shows the average TSS concentration in treated effluent
from each system studied. The TSS concentration of HUS-
Blab effluent was the highest (87 mg/l, see Exp. 1 in Fig. 2).
In the HUSB pilot plant, the effluent TSS concentration was
reduced to 50 and 63 mg/l depending on the operating condi-
tions (Fig. 2). These differences in TSS effluent concentration
were probably caused by the lower height of the lab scale
HUSB, which reduced the distance between the top of the
sludge bed and the effluent exit. Furthermore, the upflow
velocity (surface loading rate) is higherin the pilot-scale HUSB
reactor (1.4 m/h compared to 0.1 m/h for the lab-scale unit)
allowing better contact between the influent and the sludgebed. In practice, the lab-scale digester needs effluent recircu-
lation in order to homogenize the sludge bed and avoid bed
compaction. The pilot-scale HUSB showed a good hydraulic
flux distribution without the need for recirculation, as was
outlined by experiments on hydraulic retention time distri-
bution (Alvarez et al., 2003).
UASB systems, operating at higher HRT than HUSB sys-
tems, had average effluent TSS concentrations below 50 mg/l.
In the case of UASB-CMSS systems, values for effluent TSS
were similar to those of UASB. The lowest effluent TSS con-
centration was obtainedwith two-step systems,since the pilot
plant and field application systems had effluent TSS concen-
tration below 35 mg/l.Fig. 3 shows the percentile distribution of the influent
and effluent TSS concentration for each anaerobic configu-
ration, excluding laboratory scale experiments. Effluent TSS
concentration was below 100 mg/l for 95% of the data for all
configurations. In the case of the two-step systems, this con-
centration was 55mg/l for 95% of the data. Mean effluent TSS
concentrations ranged from 35 to 63 mg/l.
Anaerobic digesters generated pretreated effluents with a
TSS concentration that was 50% lower than that generated
by classical pretreatment technologies used in combination
with CW, as indicated above in Table 1 (mean effluent concen-
tration of 123 mgTSS/l), or as reported by Vymazal (2005) f or
worldwide experiment (107 mgTSS/l). Therefore, taking into
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account these data, all these anaerobicdigester configurations
meet the general requirements for a municipal wastewater
pretreatment capable of preventing clogging in a constructed
wetland.
The stability and reliability of these anaerobic digesters is
indicated by their behaviour when they are faced with the
wide range of influent and operational conditions that were
tested. Influent COD varied from 34 to 2700 mg/l and influentTSS from 19 to 2100 mg/l, while the operational tempera-
ture ranged from 5 to 21 ◦C. Effluent quality, however, varied
to a lesser extent, as indicated above for effluent TSS. COD
removal efficiency suffered from low influent temperatures
and organic loads, but values remainedin the ranges indicated
in Table 4. Pilot- and field-scale digesters tolerated prolonged
periods of temperaturesbelow 13 ◦C. However, prolongedperi-
ods of more than 1 month treating very dilute wastewater
(influent COD below 200 mg/l) clearly affected the efficiency
of single-step UASB, and also the stability of two-step HUSB-
UASB systems, when the biomass concentration became very
low (Alvarez, 2004).
6. Influence of anaerobic pretreatment onconstructed wetland area
Anaerobic pretreatment has two important consequences for
the quality of influent wastewater in a constructed wetland.
The first one is the high TSS removal and the maintenance of
TSS concentration in the pretreated wastewater so that it is
below 100 mg/l, as indicated above.
A second consequence is the decrease in the influent
COD concentration to the wetland by an amount that varied
from 30 to 90%, depending on the type of anaerobic digester
used, wastewater characteristics, and operational conditions(Table 4). Horizontal flow constructed wetlands can be sized
in order to meet a defined superficial COD load, for exam-
ple 12 gCOD/m2 d (Vymazal, 2005). Therefore, generally the
reduction in the wetland area required when an anaerobic
pretreatment is introduced may range from 30 to 90%.
However, a better method to measure constructed wet-
lands is one that takes into consideration the BOD5 removal
kinetic, such as the first order model (Rousseau et al., 2004b).
In this case, assuming the background concentration of BOD
is equal to zero, the constructed wetland area is calcu-
lated according to the following equation (Kadlec et al., 2000;
Rousseau et al., 2004b):
A =
F
h· kv · E
ln
BOD5i
BOD5e
(1)
where “ A” is the wetland area (m2), “F” is the volumetric flow
(m3 /d), “h” is the wetland depth (0.4–0.6m), “E” is the gravel
bed porosity (generally, 0.3), and “kv” is a first order kinetic
constant that depends on temperature (from 0.17 to 6.11 d−1,
as reported by Kadlec and Knight (1996) and by Rousseau et
al., 2004b).
In this way, the wetland area is proportional to the loga-
rithm of the quotient between the wetland’s influent BOD5
and effluent BOD5. Anaerobic pretreatment greatly modifies
this quotient but it does not influence the rest of the param-
eters present in Eq. (1). Finally, wetland effluent must meet
legal specifications, which according to the EU is a BOD5 less
than or equal to 25mg/l.
Information on BOD5 influent concentration and removal
efficiency resulting from the anaerobic treatment of munic-
ipal wastewater is scarce in the literature. A general review
of anaerobic digesters treating municipal wastewater (Alvarez
et al., in preparation) indicated that UASB removes about67% of influent BOD5. Limited data for single-step UASB and
UASB-CMSS systems treating diluted (BOD5 about 200 mg/l)
municipal wastewater at temperaturesbelow 20 ◦CshowBOD5
removals ranging from 50 to 70% (Alvarez et al., 2004, 2006). In
this case, the BOD5 entering the wetland decreases from 200
to 80 mg/l (60% reduction on average) when an UASB anaero-
bic pretreatment was applied. Therefore, the required wetland
area will be reduced by 44%, as can be calculated using Eq. (1).
More efficient anaerobic pretreatment systems could remove
about 70% of BOD5 and provide a 60% reduction in wetland
area. Even if BOD5 removal decreases to 46%, as may be the
case when HUSB reactors are used as a municipal wastewater
pretreatment, the required wetland area will be 30% less.Construction costs of CW are highly variable from place
to place but in many cases may be similar to those of some
conventional treatment technologies or may be higher when
land costs are accounted for (Rousseau et al., 2004a; Puigagut
et al., 2007). The requirement of a large amount of land is one
of the limitations to widespread adoption of CW technology
for wastewater treatment in both developed and developing
countries, and the needfor reducing investment costs through
reducing the CW area has been proposed on several occasions
(Badkoubi et al., 1998; Kivaisi, 2001; Gomez Cerezo et al., 2001;
Green et al., 2006; El-Hamouri et al., 2007). The footprint of
high-rate anaerobic digesters is very small, ranging from0.005
to0.02m2 /p.e.for the systems includedin Table 4. Thus, anaer-obic digesters may be combined with CW in order to reduce
theoverallareabelow1m2 /p.e.,as previously proposed (Barros
and Soto, 2002; Green et al., 2006). Furthermore, construction
costs of anaerobic digesters are lower than that of CW and
operation costs are very low and are comparable to that of
CW (Kivaisi, 2001; Hoffmann et al., 2002). In this way, the use
of high-rate anaerobic digesters as a first treatment step may
be a better choice than using a high-rate vertical flow CW or a
very high load horizontal flow CW that cansuffer from surface
flooding and clogging (Batchelor and Loots, 1997).
An anaerobic system is preferable as a wetland pretreat-
ment, compared to a primary decanter or a common septic
tank, as it reduces surplus sludge generation, it removes SSand BOD5 more effectively, and it offers a good way to buffer
the large fluctuations of municipal wastewater from a small
population.
The type of anaerobic process, either a hydrolytic pre-
treatment or methanogenic digestion can also influence the
performance and efficiency of CW post-treatment as the type
of substrate changes. Advanced methanogenic digestion pro-
duces an effluent that is mainly recalcitrant for anaerobic
processes in CW. Therefore, post-treatment CW could be
designedwith a lower depthin order to maximize aerobic con-
ditions; or VF CW may be of great interest. In the case of an
anaerobic hydrolytic pretreatment, as in the HUSB process,
most of the volatile suspended solids and readily biodegrad-
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66 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
able matter present in raw wastewater are converted to acetic
acid. Acetic acid may be converted in both anaerobic/anoxic
conditions or in aerobic conditions aiding in the nitrogen and
phosphorus removal process. Furthermore, biomass growth
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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