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Pilot-scale comparison of constructed wetlands operated under high hydraulic

loading rates and attached biofilm reactors for domestic wastewater treatment

M.S. Fountoulakis a,, S. Terzakis a,b, A. Chatzinotas c, H. Brix d, N. Kalogerakis b, T. Manios a,e

a School of Agricultural Technology, Technological Educational Institute of Crete, Heraklion, Greeceb Department of Environmental Engineering, Technical University of Crete, Chania, Greecec UFZ, Helmholtz Centre for Environmental Research UFZ, Department of Environmental Microbiology, Permoserstrasse 15, D-04318 Leipzig, Germanyd Department of Biological Sciences, Aarhus University, Denmarke Greek Open University, School of Science and Technology, Patras, Greece

a b s t r a c ta r t i c l e i n f o

Article history:

Received 7 July 2008

Received in revised form 23 December 2008

Accepted 5 January 2009

Available online 30 January 2009

Keywords:

Constructed wetland

Treatment performance

Nutrients

Rotating biological contactor

Packed bed filter

Four different pilot-scale treatment units were constructed to compare the feasibility of treating domestic

wastewater in theCity of Heraklio,Crete,Greece: (a) a freewatersurface(FWS)wetlandsystem, (b)a horizontal

subsurface flow (HSF) wetland system, (c) a rotating biological contactor (RBC), and (d) a packed bed filter

(PBF). All units operated in parallel at various hydraulic loading rates (HLR) ranging from 50% to 175% of

designed operating HLR. The study was conducted during an 8 month period and showed that COD removal

efficiencyof HSF was comparable (N75%) to that of RBCand PBF, whereas that of theFWS systemwasonly 57%.

Average nutrient removal efficiencies for FWS, HSF, RBC and PBF were 6%, 21%, 40% and 43%, respectively for

total nitrogen and 21%, 39%, 41% and 42%, respectively for total phosphorus. Removals of total coliforms were

lowestin FWS andPBF(1.3 logunits) andhigher inHSFand RBC (2.3to 2.6logunits).HSF showed slightlylower

but comparable effluent quality to that of RBC and PBF systems, but the construction cost and energy

requirements for this system are significantly lower. Overall the final decision for the best non-conventional

wastewater treatment system depends on the construction and operation cost, the area demand and the

required quality of effluent.

2009 Published by Elsevier B.V.

1. Introduction

There is an urgent need to implement efficient treatment of

domestic wastewater from small towns as imposed by the Water

Framework Directive 2000/60/EC (EU Parliament and Council, 2000).

However, the per capita cost for implementation of the common

activated sludge process in small communities is much higher than

that for large cities. Therefore non-conventional technologies ranging

from simple biological low rate systems such as ponds, constructed

wetlands and sand filters to complex high-rate suspended and fixed

biomass reactors have to be evaluated according to their treatment

performance, foot print, process reliability, investment and operation

costs (Boller, 1997; Colmenarejo et al., 2006; Fahd et al., 2007 ).

Constructed wetlands (CW) are used worldwide to treat municipal

wastewater (Brix, 1994a; He and Mankin, 2002; Nitisoravut and

Klomjek, 2005; Paing and Voisin, 2005; Brix and Arias, 2005; Chung

et al., 2008). CW systems are based on the functioning of natural

ecosystems and the treatment processes involve complex interactions

between soil, water, plants and microorganisms. CWs are generally

efficient in removal of organic matter (BOD) and suspended solids (SS),

but the removal of nitrogen and phosphorus is often relatively poor

(Verhoeven and Meuleman, 1999; Tanner et al., 1999; Kuschk et al.,

2003; Vymazal, 2007). Constructed wetlands can be classified into two

main categories depending on how the water passes through the

systems: subsurface-flow or surface-flow design. The most widely used

CW design in Europe is designed with horizontal subsurface flow (HSF).

However, free water surface (FWS) constructed wetlands are increas-

ingly being favoured because they are cheaper to construct and may

have higher wildlife habitat values.

There is a concern about the feasibility of wetlands to become a cost

effective method because wetlands typically require a low hydraulic

loading rate (HLR) and a long hydraulic retention time (HRT) to achieve

efficient pollutant removal. That means wetland treatment method may

need a large land area. USEPA have recommended that the organic

loading rate shouldnot exceed 6 g BODm2d1 inHSF(USEPA, 2000)and

11.2g BOD m2 d1 inFWS (USEPA,1988). These suggestions may not be

applicable when land is expensive or limited. Using high HLR to operate

the constructed wetland may potentially reduce the required area.

Recent years there is an effort to study the performance of

constructed wetlands under high HLRs. Caselles-Osorio et al. (2007a)

found that therewas no significant difference in COD removalbetween

two HSF systems operated at an HLR of 6 g COD m2 d1 and 23 g COD

m2 d1. Another study in France concluded that overloads up to ten

times the dry weather flow in vertical flow constructed wetlands are

possible while still complying with the European standards ( Molle

Science of the Total Environment 407 (2009) 29963003

Corresponding author. Tel.: +30 2810 379456; fax: +30 2810 318204.

E-mail address: [email protected] (M.S. Fountoulakis).

0048-9697/$ see front matter 2009 Published by Elsevier B.V.

doi:10.1016/j.scitotenv.2009.01.005

Contents lists available at ScienceDirect

Science of the Total Environment

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mailto:[email protected]://dx.doi.org/10.1016/j.scitotenv.2009.01.005http://www.sciencedirect.com/science/journal/00489697http://www.sciencedirect.com/science/journal/00489697http://dx.doi.org/10.1016/j.scitotenv.2009.01.005mailto:[email protected]

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et al., 2006). Furthermore, Linet al.(2005)shown that a unit consisting

of FWS and HSF wetland cells treating intensive shrimp aquaculture

wastewater effectively removed TSS, BOD and NH4N under high

hydraulic loading rates (157195 m d1).

On the other hand, there are also several compact systems using

fixed and movable bed reactors with high removal efficiencies for

BOD, SS and nitrogen that potentially can be used for small towns.These systems in general have good process stability, less footprint

requirements compared to CWs and high specific removal rates

(Helmer and Kunst, 1998; Gebara, 1999; Sirianuntapiboon, 2006).

Different types of attached biomass technologies have been widely

applied as small-scale plants, including packed bed biofilm reactors

(Mann and Stephenson, 1997; Aesoy et al., 1998; Schubert and

Wolfgang Gnthert, 2001) and rotating biological contactors (Hans-

ford et al., 1978; Ayoub and Saikaly, 2004; Tawfik et al., 2006). These

systems generally achieve high removal efficiencies when treating

wastewater from 25500 person equivalents (p.e.), however the

capital and operations costs for small towns (10005000 p.e.) may be

very high.

The aim of this work was to assess and compare the performance

of different non-conventional wastewater treatment systems incontrolled experiments under Mediterranean climatic conditions.

Four pilot-scale treatment systems were constructed: a) FWS, b) HSF,

c) RBC, and d) PBF. All units operated in parallel receiving primarily

treated municipal wastewater. In order to decrease land requirements

for constructed wetlands so that are more competitive to compact

attached biomass systems the FWS and HSF were designed to operate

at high hydraulic loading rates.

2. Materials and methods

2.1. Pilot-scale unit description

All pilot systems were constructed during summer 2006 in our

open-air laboratory with a total surface area of approximately 360 m2

.

The facility is located in Heraklion, Crete, South Greece (N 35, 19q;

E 25, 10q).

The FWS system was constructed with dimensions of 12.4 m long

and 3.4 m wide, and with three separated zones, two vegetated zones

and one deeper anoxic un-vegetated zone in the centre (Fig. 1a). A soil

layer of 40 cm depth was added in the vegetated zones, and was

planted with two species of reeds, Phragmites australis and Arundodonax. Plants were transplanted until a total cover of 40% was reached.

Then the wetland was filled with tap water to a depth of 50 cm.

Wastewater was mixed with tap water at gradually increasing

wastewater/tap water ratios until only wastewater was added after

3 weeks. The incoming wastewater entered the wetland through a

40 cm gravel layer to distribute the water across the width of the bed.

The unitwas designedto treat 6 m3 of domestic wastewater daily (HLR

c. 140 mm d1).

The constructed HSF was constructed with a length of 8.4 m and a

width of 5.4 m (Fig.1b). The average gravel porosity was equal to 0.45

and the depth of the bed was 0.45 m. The front and the effluent end of

thebed were established with 60 to 100 mm diameter gravel. TheCWs

bed consisted of 30 mm diameter gravel with a top layer of 10 mm

diameterfor supporting the vegetation. Plantspeciesin the HSFsystemwere again P. australis and A. donax. The initial wastewater addition

Fig.1. View of the pilot scale units, a) free water surface constructed wetland, b) subsurface constructed wetland, c) rotating biological contactor, d) packed bed reactor.

Table 1

Operational data on the system

HLR % of

designed

FWS HSF RBC PBF

OLR

(gCOD

m2 d1)

HRT

(d1)

OLR

(gCOD

m2 d1)

HRT

(d1)

OLR

(kgCOD

m3 d1)

HRT

(h1)

OLR

(kgCOD

m3 d1)

HRT

(h1)

50 37.8 3.84 35.1 2.88 0.53 24.0 0.37 34.3

75 56.6 2.56 52.7 1.92 0.80 16.0 0.56 22.9

100 56.3 1.92 52.3 1.44 0.79 12.0 0.55 17.1

125 46.0 1.54 42.8 1.15 0.65 9.6 0.45 13.7

150 73.1 1.28 68.0 0.96 1.03 8.0 0.72 11.4

175 142.8 1.10 132.7 0.82 2.01 6.9 1.40 9.8

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was carried out in a similar way to the FWS. The HSF was designed totreat 6 m3 of domestic wastewater daily (HLR c. 130 mm d1).

The RBC system used in this study (EKOL 4, AquaImpex) was

provided by Dialynas S.A (Fig. 1c). The plant consists of a poly-

propylene tank partitioned into a primary sedimentation section, a

storage section, a section for biological treatment with rotating

biological contactor and finally a sedimentation section. The RBC

consists of a cage filled with small plastic elements with a total surface

area of 235 m2. The volume of the sedimentation and storing sections

were 6.2 m3 and the volume of the biological contactor tank was

2.0 m3. The unit was designed to treat domestic wastewaters from

hotels, schools, and small villages with a flow of 4 m3/d.

The packed bed filter (PBF) unit used was the Advantex-AX20

(Orenco) (Fig. 1d). This system is a recirculating filter using a textile

material as the treatment media. The overall volume of the filter

media was 1.85 m3 and the volume of the recirculation tank was

2.5 m3. The specific PBF was designed to treat 3.5 m3 of wastewater

daily.

All units operated in parallel, receiving primarily (sedimentation)

treated wastewater from the sewage treatment plant of Heraklion, at

various HLRs ranging from 50% to 175% of designed HLR. The study

was carried out from February 2007to September 2007. Table 1 shows

the organic loading rates (OLRs) and hydraulic retention time (HRT)

for each system during experimental operation.

2.2. Sampling and analysis

Water samples were taken as grab samples from the influent and

from the effluent of each treatment unit two times every week

throughout a period of 8 months, totalling 51 sampling dates. Samples

were analyzed for: Chemical Oxygen Demand (COD), Total Suspended

Solids (TSS), pH, Electrical Conductivity (EC), Total Nitrogen (TN),

Nitrates (N3N), Ammonium (NH4N) and Total Phosphorus (TP),

according to APHA (1995).

Faecal bacteria were analyzed by the membrane filtration techni-

que(APHA,1995), using sterile 0.2-m pore-size Milliporefilters. Three

sample volumes of 0.01, 0.1 and 1 ml were used to increase the

probability of obtaining counts within acceptable ranges. Filtered

samples wereincubated on M-FC agar for 24 h at 44.5 C to enumerate

Faecal coliforms (FC) colonies and incubated on K-FS agar for 48 h at

35 C to assess Faecal streptococci (FS).

Statistical analyses were carried out with MicroCal Origin 7.0

(OriginLab) or Statgraphics Centurion XV (Statpoint, Inc., Virginia,

USA). Data were tested forhomogeneityof variances using Cochran's C

test. The datawas analyzedusing one and two-wayanalysis of variance

(ANOVA) to compare inlet and outlet concentrations between systems

and loading rates and the removal of COD, TSS, TN, NO3N,NH4N and

TP. For inlet and outlet water quality posthoc comparisons of means

were carried out using Scheffes test at the 5% significance level. To

detect the statistical significance of differences (Pb0.05) between

means of treatments, the Tukey test was performed.

3. Results and discussion

3.1. General

Fig. 2 presents the varying concentrations of pollutants and

suspended solids in the four units for the entire operation period. In

this study, 4851 samples have been collected for the measurements

of COD, TSS, TN, TP, NO3N and NH4N for every unit exposed to

different HLRs. The composition of the influent wastewater varied

throughout the study (Table 2). The characteristics of the influent are

typical for primarily treated domestic wastewater (Korkusuz et al.,

2007; Caselles-Osorio et al., 2007b).Fig. 2. Variation of COD, TSS, TN, TP, NNH4

+ and NNO3 in influent and effluent of

different treatment systems during operation at a loading rate of 50%, 75%, 100%, 125%,

150% and 175% of designed loading rate.

Table 2

Average composition of the wastewater loaded into the experimental treatment

systems throughout the study and results of one-way ANOVA ( P-values) testing the

mean composition between the six different loading rates (n =number of samples)

Parameter n Mean (minmax) P-value

COD (mg L1) 48 465 (1051089) 0.0022

TSS (mg L1) 50 129 (66283) 0.0019

Total-N (mg L1) 51 70 (5287) 0.0034

Total-P (mg L1) 51 15 (821) 0.0049

NH4N (mg L1) 51 11.3 (1.732.1) 0.0000

NO3N (mg L1) 50 0.85 (03.0) 0.0442

Total coliforms (CFU/100 ml)105 7 411 (174690) 0.3010

Fecal coliforms (CFU/100 ml)105 7 39 (1473) 0.1602

Fecal streptococci (CFU/100ml)105 7 46 (1085) 0.2285

: Figures in bold indicate statistical significant differences between the mean

composition at the different loading rates at the 95% confidence level.

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Effluent concentrationsfor COD, TSS, TN,TP,NO3NandNH4+N are

presented in Table 3 and the removal percentage for the four systems

and thesix loading ratesare presented in Fig. 3. Fig. 4 present charts of

pH, conductivity and air temperature during the functioning period of

the units. It can be observed that generally for all units the pH value

increased about 0.40.8 of the influent pH value. The conductivity

decreased slightly from 1.60 to 1.40 mS cm1 for two attached biofilm

units and remained almost stable for two constructed wetlands. EC

values in constructed wetlands depends on several parameters as soilor gravel composition, evaportranspiration, treatment performance

etc. Normally, during a wastewater treatment plant EC values

decreased however in constructed wetlands the evaportranspiration

and the interactions between soil and water affect on final EC values.

Air temperature for almost all cases was above 12 C. Water

temperature expecting to had a variation from the air temperature

of 03 C (Akratos and Tsihrintzis, 2007).

3.2. Organic matter removal

The outlet concentrations of COD and TSS differed significantly

between systems and depended also on loading rate, but the

dependency of loading rate differed between systems as shown by

the significant interaction term in the two-way ANOVA (Table 4).

Effluent levels of COD and TSS were significantly higher for FWS

systems compared with the other systems.

COD and TSS concentration values did not significantly differ

between theHSF, RBCand PBFunit forall testedloading rates(Table 3).

On the other hand, FWS effluent was in most cases found to have

statistical significant different concentration from the other three

systems. The mean removal efficiency of HSF for COD and TSS was

76.9% and 80.7%, 74.7% and 83.4%, 61.0% and 68.4% for low (5075% of

designed), nominal and high (125175% of designed) HLR respectively

(Fig. 3). This is in accordance with previous studies which focused on

the treatment of domestic wastewater with HSF showing removal

efficiencies about 75% for COD and 80% for TSS (Vymazal, 2005,

Babatunde et al., 2008). Furthermore, our results suggest that a

possible underestimation of designed HLRs may result in a consider-

able decrease of COD and TSS reduction. RBC and PBF removed 86.7%

90.4%, 95.1%95.4% and 74.6%72.0% of the COD for low, nominal and

high HLRs respectively. High removal efficiencies observed in nominal

HLRs for these treatment systems agree well with data from literature

(Gebara,1999; Ayoub and Saikaly, 2004). Generally, the removalof CODdecreased in allsystems forHLRs higherthan theloading thewetlands

were designed to operate. The decreasing COD removal feasibility of

treatment systems facing fluctuating HLRs is a remarkable disadvan-

tage because in high tourist regions such as Crete Island there are

significant fluctuations of influent quantity between winter (no or low

tourist period) and summer (extremely high tourist period). The unit

with the lowest removal efficiency for COD was the FWS (57.0%, 32.3%

and 20.1%). These COD removal rates were lower than mean ranges

reported in literature. For instance, a combined Typha domingensis and

P. australis wetland treating urbanwastewater in Spain removed87% of

the incoming COD (Gomez Cerezo et al., 2001). Similar results were

observed by other researchers in the USA (Verhoeven and Meuleman,

1999) and in Europe (Brix, 1994b; Tsihrintzis et al., 2007). However,

there are few other researchers who found COD removal efficiencies

relatively close to ourobservations. A wetland systemin a rural area in

Spain for sewage treatment with Typha latifolia and Salix atrocinerea

removed only 60% of the incoming COD (Ansola et al., 2003) and

Rousseau et al. (2004) reported average COD removal efficiencies of

61% for FWS treatment systems in Flanders region. The hydraulic

loading rates applied in thisstudy in orderto reduce landrequirements

were abnormally high as compared with most works.The high organic

strength of influent resulting in a high organic loading rate was the

major reason forpoor treatment feasibilityof FWS. Hiley (1995) report

that most wetlands are oxygen limited and that performance is

Table 3

Mean values of quality parameters for the effluent from each treatment system for different HLRs

System COD (mg/L) TSS (mg/L) TN (mg/L) TP (mg/L) NH4+ (mg/L) NO3 (mg/L)

Mean S.D. Mean S.D. Mean S.D. Mean S.D. Mean S.D. Mean S.D.

FWS

HLRd 50% 227.9a 97.3 58.9a 17.7 58.5a 9.2 11.3a 1.0 17.5a 4.9 0.9a 0.9

75% 228.3a 114.2 62.0a 35.6 55.8a 16.6 11.3a 1.2 10.4a 4.2 0.7a 0.5

100% 230.0a 76.5 75.9a 24.4 65.7a 8.4 11.4a 1.0 6.2a 3.0 2.1a 1.9

125% 317.9a 83.8 126.7a 72.3 87.0a 18.4 9.7a 0.9 6.9a 2.6 1.4a 1.1

150% 187.9a 101.9 88.8a 40.9 63.6a 3.4 12.4a 0.9 8.4a 2.8 1.0a 0.5

175% 294.3a 133.2 70.1a 32.5 65.8a 4.9 11.2a 1.5 15.2a 3.8 0.7a 0.4

HSF

HLRd 50% 130.5b 103.3 26.0b 6.7 68.9a 25.0 8.6b 0.7 17.4a 8.4 6.2a 7.1

75% 99.6b 49.4 20.7b 5.1 41.2a 12.3 9.2b 0.6 11.4a 1.6 3.1a 2.5

100% 85.7b 58.9 29.4b 10.2 49.4b 7.3 7.8b 0.6 7.8b 3.6 2.7a 2.2

125% 34.6b 24.8 25.7b 19.9 47.0b,c 7.8 7.9b 0.3 7.2a 4.4 1.2a 0.4

150% 126.3a 65.2 30.8b 12.6 51.2a,b 8.8 9.1b 0.8 7.8a 1.7 1.3a 0.3

175% 341.1

a

189.1 39.4

a

10.8 67.4

a

7.2 10.0

a,b

0.8 14.6

a

4.5 0.7

a

0.6

RBC

HLRd 50% 95.5b 62.8 14.6c 5.4 59.2a 9.7 8.6b,c 1.0 2.9b 1.3 25.1b 23.0

75% 43.4b 18.4 11.9b 4.5 38.4a 10.4 7.9c 0.9 1.8b 1.1 30.5b 8.0

100% 16.6b 9.7 14.9b 6.3 25.6c 5.1 8.6c 1.7 5.5b 3.1 28.1b 20.9

125% 38.4b 19.6 14.2b 7.5 61.5b 11.6 8.0b 1.3 8.6a 3.9 2.9a 2.5

150% 94.0a 63.1 26.0b 8.1 42.2b 11.4 6.8b 2.4 10.7a 1.0 1.8a 0.7

175% 178.9a 101.8 61.8a 23.1 41.4b 4.5 8.7b,c 0.8 5.2b 3.9 22.0b 8.8

PBF

HLRd 50% 51.9b 43.0 10.2c 8.6 55.0a 10.4 7.5c 0.9 1.1b 0.5 24.8b 25.8

75% 50.3b 32.2 11.1b 7.0 47.5a 13.5 8.1c 1.1 1.5b 1.1 34.6b 10.8

100% 18.3b 18.3 19.3b 8.1 28.0c 5.1 8.6c 0.7 4.4b 2.0 27.8b 13.3

125% 32.0b 16.0 19.2b 3.5 35.5c 15.3 7.8b 0.8 5.2a 2.6 3.5a 2.7

150% 111.2a 61.0 33.8b 9.8 38.0b 13.3 8.6b 1.1 7.1a 1.4 2.6a 1.6

175% 216.0a 123.1 57.5a 24.9 39.0b 17.1 8.1c 0.9 12.5a,b 1.0 5.2a 0.5

a, b, c: In each column for each loading rate, mean values followed by a different symbol are significantly different (Pb

0.05).


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enhancedif extra aerationis provided. Theoxygenrelease from rootsofmacrophytes seems to be far less than the amount needed for aerobic

degradation of the oxygen consuming substances delivered with

sewage (Brix, 1990; Brix and Schierup, 1990).

As shown in Fig. 5, there is a correlation between the loading rate

and the effluent values of the analyzed parameters. It has been

observed that there is a positive linear relationship between

increasing load and effluent COD or TSS concentration. In the case of

RBC and PBF we observed significant adverse effects to organic matter

concentration for increasing load. On the other hand the two

constructed wetlands turned out to be less sensitive to variations in

HLRs.

3.3. Nutrients removal

The effluent quality with respect to nitrogen was better for the two

attached biofilm technologies, while nitrogen values were moderate

for HSF and still high for FWS (Table 3.). Nitrate concentrations were

significantly higher for the RBC and PBF than for the two constructed

wetlands. Even though ammonium mean concentration values for

RBC and PBF were lower than for FWS and HSF, statistically there were

no significant differences. The average TN removal efficiencies of two

attached biofilm systems under nominal HLRs were 62.3% and 58.1%.

These results were in accordance with previous reported nitrogen

removal values of about 4565% (Tawfik et al., 2006; Griffin et al.,

1999). An interesting point to note was that nitrogen removal of these

systems was decreased both for low and for high HLRs operation.

According to Table 3, for low (5075%) HLRs these systems had the

lowest NH4N concentrations; the higher NO3N concentrations

suggest that the denitrification process was the rate limiting step for

nitrogen biodegradation. On the other hand during high HLRs

nitrification occurred only partially as the NO3N concentrationswere the lowest. The inhibition of the nitrification process under high

organic loading rates in attached biofilm reactors were in general

consistent to several other published reports (Gupta and Gupta, 2001;

Sirianuntapiboon, 2006).

Relatively poor nitrogen removal performance of constructed

wetlands treating domestic wastewater was often reported in

literature (Tanner et al., 1999; Rousseau et al., 2004; Brix and Arias,

2005). In this work, TN average removal of both FWS and HSF with

each examined loading rate was not above 12.7% and 27.0%,

respectively. NH4+N and NO3N measured concentrations showed

that both ammonification and nitrification processes were low. In fact,

the magnitude processes which ultimately remove the total nitrogen

Fig. 3. Removal percentage for each HLR and system.

Fig. 4. Variation of pH, electrical conductivity and air temperature during operation

period.

Table 4

Results of two-way ANOVA (F-ratios) used to test for differences in outlet water quality

between the four systems tested (main factor System) and the six loading rates (main

factor Loading) and their interaction (SystemLoading)

Parameter System Loading System Loading

COD 40.11 17.42 2.36

TSS 76.87 12.03 4.16

Total-N 37.52 12.00 5.84

Total-P 92.75 3.80 3.36

NH4N 34.76 14.65 10.89

NO3N 30.63 11.29 3.77

Total coliforms 7.20 0.67NS 1.40NS

Fecal coliforms 0.77NS 0.59NS 1.06NS

Fecal streptococci 4.55 1.07NS 0.38NS

: Pb

0.05; : Pb

0.01; : Pb

0.001; NS: Not Significant.


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from these systems is usually low and therefore removal of TN iscommonly low in single-stage constructed wetlands (Vymazal, 2007).

The concentration of TP was reduced in HSF, RBC and PBF in

relatively satisfactoryefficienciesranging from 40 to 50%. This result is

in accordance with previous works observing removal efficiencies of

about 2565% (Chen et al., 2006; Tsihrintzis et al., 2007; Vymazal,

2005; Kivaisi, 2001; Yun et al., 2004). FWS was found to have the

lower capacity to remove phosphorus. The low removal rate observed

in FWS may be due to the fact that the most important processes

involved, occur in the sediment and not in the water column

(Verhoeven and Meuleman, 1999; Kadlec, 2006). All examined

systems show a potential to decrease phosporus removal as loading

rate increase. However this potential seems to be rather independent

from the HLR indicating that the main mechanism for TP removal is

the adsorption to the porous media (Knight et al., 2000; Jing et al.,

2001, Vymazal, 2002; Akratos and Tsihrintzis, 2007).

3.4. Pathogen removal

According to Table 5, average concentrations of pathogenic bacteria

in the influent in the range of typical domestic wastewater (Karratha-

nasis et al., 2003; Decamp and Warren, 2000). Furthermore the

average concentration of fungi in the influent was 36 18 105 CFU/

100 ml. Faecal indicators were efficiently removed in all examined

systems. The averageremoval efficienciesfor TC were between 1.3 and

2.2log units.HSF showeda betterefficiency to remove pathogens than

theothersystems with average effluent concentrations of 0.3 105 and

0.4105 CFU/100 ml forFC andFS, respectively.These valuesshowthat

only HSF met the level recommended by the EPA of 1.0105 CFU/

100 ml. These efficiencies appear to be in the range described in

previous studies both for constructed wetlands (Karrathanasis et al.,

2003; Ansola et al., 2003; Keffala and Ghrabi, 2005; Ottova et al., 1997;

Sleytr et al., 2007) and attached biofilm reactors (Sagy and Kott, 1990;

Tawfik et al., 2002; Hua et al., 2003).

3.5. Criteria for optimum treatment system

A comparison of several critical parameters among the examined

non-conventionalwastewater treatmentsystems is presentedin Table 6.

Results based on per capita values suggested that the average waste-water production is 0.2 m3 per capita. Removal rates were calculated

from experimental data during the period which the systems were

operated at 100% of designed HLR. Remarked that land requirements

and construction costs evaluated based only on this specific experi-

mental process. Thevalue of construction cost for FWS and HSFwas 237

and 268 Inhab1 respectively while Rousseau et al. (2004) found

significant higher average construction cost of constructed wetlands

treating wastewater in Flanders region (392 and 1258 Inhab1 for FWS

and HSF, respectively). The overloaded designed constructed wetlands

in this work resulting lower construction cost as well as lower land

requirement. Furthermore, Rousseau et al. (2004) found a footprint for

FWS and HSF of7.0 and 4.8 m2 Inhab1 respectively whereas in this work

Table 5

Influent and effluent concentrations, and removal efficiencies for microbiological parameters for each treatment system

Parameter Influent FWS HSF RBC PBF

(CFU/10 0 ml) (CFU/10 0 ml) Removal (CFU/10 0 ml) Removal (CFU/10 0 ml) Removal (CFU/10 0 ml) Removal

x105 x105 log units x 105 log units x 105 log units x 105 log units

Total coliforms

Mean 411.5 20.4 1.3 2.6 2.2 2.9 2.2 16.8 1.4

S.D. 217.4 14.0 1.2 2.7 1.9 4.2 1.7 11.7 1.3

Faecal coliforms

Mean 56.3 5.8 1.0 0.3 2.3 4.8 1.1 5.0 1.1

S.D. 38.3 3.9 1.0 0.3 2.1 11.1 0.5 6.7 0.8

Faecal streptococci

Mean 47.9 6.0 0.9 0.4 2.1 1.5 1.5 3.4 1.1

S.D. 30.7 2.6 1.1 0.4 1.9 2.3 1.1 3.1 1.0

Table 6

Removals rate and costs of each wastewater treatment system

Treatment

system

Removal rate

(gr inhab.1 d1)

Requirements Construction

costs

( inhab.1)COD TSS N P Land

(m2 inhab.1)

Power (kWh

inhab.1 year1)

FWS 33.4 21.0 0.4 0.7 1.4 ~0 237

HSF 62.0 30.4 3.8 1.4 1.4 ~0 268

RBC 75.8 33.2 8.4 1.3 0.3 219 476

PBF 75.6 32.4 8.0 1.3 0.2 43 672

Fig. 5. Correlation charts of effluent concentration and pollutant load for COD, TSS, TN

and TP.


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the land requirement was notably lower. Although FWS found to have

the lowest construction cost and generally very low requirements, the

effluent quality characteristics are significantly poorer than from the

other three systems. The effort to operate this system in higher HLRs in

order to reduce land requirements had significant negative effect on

effluent characteristics. On the other hand RBC and PBF, which require

the smallest land area (0.20.3 m2 inhab1), showed approximately the

same effluent values both for organic matter and nutrients concentra-

tions. The quite low power requirements for PBF as compared to RBCbalance the relatively higher construction costs. The construction cost

for HSF is significant lower than thosefor thetwofixedfilm media units

and additionally found to have almost the same removal rates for COD,

TSS and TP. Consequently,the HSF wetland designed and operated under

high HLRs showed both effective wastewater treatment and low land

requirements.However nitrogen removal efficiencyis by farlower. Thus,

the choice of the optimal treatment system depends on the overall

requirements in terms of treatment efficiency.

4. Conclusions

Four non-conventional wastewater treatment systems operated

continuously with various HLRs for about 8 months. During this

period, with the exception of FWS all other units used in this studyremoved efficiently the organic matter of domestic wastewater. The

RBC and PBF performance showed lower performance efficiency

under shock loading conditions. Nitrogen reductionwas higher in RBC

and PBF than in the two constructed wetlands while phosphorous

removal was not significantly different for all examined systems.

Additionally, all studied non-conventional units showed considerable

potential for removing fecal bacteria from domestic wastewater.

Overall, treatment efficiencies of the two attached biofilm technolo-

gies are comparable while power requirements are considerable

higher for RBC. HSF wetland designed to operate under high HLRs

provide comparable effluent quality as attached biofilm technologies

whereas the construction and operation costs are lower. However

whenever the requirements of nitrogen level in the effluent are

crucial, HSF would not successfully meet these criteria. Since

overloaded designed FWS wetlands showed the lowest treatment

performance, the selection of this technology could only be justified

when land is available (lower HLRs) and the effluent quality

requirements are not very strict. Therefore, the selection of the

optimum unit depends on the demand for treatment efficiency and

the total available budget.

Acknowledgements

This research was funded by the Greek General Secretariat for

Research and Technology, and well as Prisma Domi S.A. The authors

wish to express their gratitude to Professor Vassilis Manios, Vaggelis

Theodorakopoulos and Ioannis Sabathianakis for their continued

involvement in the realisation of this project.

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