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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

mailto:[email protected]

http://dx.doi.org/10.1016/j.ecoleng.2008.02.001

http://dx.doi.org/10.1016/j.ecoleng.2008.02.001

mailto:[email protected]



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 55

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



56 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

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




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



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 –



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.




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




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



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.




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).




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




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




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-




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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