Carbon and nitrogen removal using a novel

horizontal flow biofilm system

Michael Rodgers *, Aoife Lambe, Liwen Xiao

Department of Civil Engineering, National University of Ireland, Galway, Ireland

Received 15 July 2005; received in revised form 29 April 2006; accepted 19 May 2006

Abstract

This 365-day laboratory study examined the performance of a novel horizontal flow biofilm reactor (HFBR) system in the treatment of a

synthetic domestic-strength wastewater. The HFBR system comprised two biofilm reactors positioned one above the other: Reactor 1 contained a

stack of 8 horizontal plastic sheets with 25 mm high spacing frustums and Reactor 2, under Reactor 1, contained a stack of 18 horizontal sheets with

11 mm high frustums. The sheets were arranged so that when the wastewater was pumped onto the top sheet of Reactor 1, for 10 min every hour, it

flowed along that sheet, and onto and back along the sheet underneath, and so on down through the system, resulting in biofilm development on the

sheets. The hydraulic and filtered chemical oxygen demand (CODf) loading rates were 155 l/(m2 d) and 47 g/(m2 d), respectively, based on the

system plan area. The effluent CODf concentration was 33 mg/l. Nearly full nitrification occurred with 56% total nitrogen (TN) reduction at the

loading rate of about 4.7 g TN/(m2 d). During the study period, the solids yield was only 0.1 g SS/g CODf removed. The system was easy to

construct and operate, did not clog and could be used for complete biodegradable COD removal, nitrification and partial nitrogen removal.

# 2006 Elsevier Ltd. All rights reserved.

Keywords: Biofilm; Horizontal flow reactor; Carbon and nitrogen removal; Nitrification

www.elsevier.com/locate/procbio

Process Biochemistry 41 (2006) 2270–2275

1. Introduction

There is a need for simple low-maintenance biological

technologies for treating domestic wastewater from small

isolated communities. This treatment should include the

oxidation of biodegradable carbon and nitrification, since

organic carbon and ammonia nitrogen (NH3 + NH4+) cause

oxygen depletion in receiving water bodies [1] and furthermore,

NH3 is toxic to fish and other aquatic organisms. Also, a guide

concentration for non-ionized ammonium (NH4) of 0.05 mg/l

in drinking water is recommended by the EEC [2]. In the past,

Imhoff tanks and septic tank systems were often used to treat

the wastewaters from small communities. The Imhoff tank only

provides primitive treatment and an effective septic tank system

requires a large percolation area of suitable unsaturated soil. In

temperate climates, a septic tank acts mainly as a settlement

tank with little biological treatment [3] and produces an effluent

high in organic carbon and nitrogen [4,5]. If a suitable soil

percolation area is not available, then an on-site technology

* Corresponding author. Tel.: +353 91 750462; fax: +353 91 750507.

E-mail address: michael.rodgers@nuigalway.ie (M. Rodgers).

1359-5113/$ – see front matter # 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.procbio.2006.05.019

such as a filter system, a mechanical aeration system or a

wetland [6–8] is required to treat the septic tank effluent; these

technologies could also be used to treat Imhoff tank effluent.

However, on-site filters can clog, mechanical failures can occur

in aerobic package systems [9] and large areas are required for

constructed wetlands.

Biofilm wastewater treatment reactors are now commonly

used to treat wastewaters from small communities because of

the availability of high specific surface-area plastic media and

the robustness of the biofilm process. In biofilm systems, the

microorganisms are attached in a biofilm to the surface of an

inert packing material or substratum. A biofilm develops on the

substratum as a result of: (i) transport of cells and nutrients to

the substratum by diffusion and advection; (ii) adsorption of

cells to the substratum and the consequent formation of the

biofilm; (iii) growth and other metabolic processes within the

biofilm; (iv) detachment of portions of the biofilm [10].

Biofilms can be used under aerobic, anoxic and anaerobic

conditions to biologically remove organic carbon, nitrogen and

phosphorus from wastewater. Biofilm technologies that are

normally used for nutrient removal include rotating biological

contactors, fluidised bed reactors, trickling filters, moving bed

reactors and soil percolation areas. In biofilm systems, organic

M. Rodgers et al. / Process Biochemistry 41 (2006) 2270–2275 2271

material and nutrients in the wastewater are utilised by the

biofilm as wastewater flows past and through the biofilm.

Advantages of biofilm systems over suspended growth systems

include: (i) reduced sludge production; (ii) high sludge age

benefiting slow growing bacteria, such as nitrifiers; (iii)

denitrification in anoxic zones at depth in the biofilms [11];

(iv) greater process stability [12]. The disadvantages of biofilm

systems include: (i) speed limitation in rotating biological

contactors (RBC); (ii) risk of clogging in sand and peat filters;

(iii) back washing required in biological aerated filters (BAF);

(iv) carrier separation in moving bed biofilm reactors (MBBR)

[13].

In the context of the above, the objective of this project was

to develop a biofilm wastewater treatment system – a horizontal

flow biofilm reactor (HFBR) – that could be suitable for on-site

carbonaceous oxidation and nitrification and would be simple,

robust and easily maintained. In designing the system, the

authors aimed to maximise the advantages of biofilm systems

whilst minimising their disadvantages.

2. Materials and methods

2.1. Construction of the horizontal flow biofilm reactor (HFBR)

The laboratory system consisted of a synthetic wastewater feed tank, an

electronic timer, a peristaltic feed pump and a horizontal flow biofilm reactor

system. The unit was operated in a temperature-controlled room at an average

temperature of 11 8C. The horizontal flow biofilm system consisted of two

reactors with Terram geosynthetic sheets, stacked one on top of the other with

frustums upwards (Fig. 1). The internal dimensions of each reactor tank,

constructed from plastic storage bins were 320 mm by 400 mm in plan and

240 mm deep. The top reactor, Reactor 1, comprised 8 sheets with 25 mm high

frustums, and the bottom reactor, Reactor 2, comprised 18 sheets with 11 mm

high frustums. It was estimated, on the basis of loading rates on other biofilm

systems, that COD removal with its associated thick heterotrophic growth

would occur mainly in Reactor 1, but would cause no clogging due to the 25 mm

high frustums. Also, it was considered that nitrification and its associated thin

biofilm could develop without clogging in Reactor 2 with its 11 mm high

Fig. 1. Vertical section of the horizontal flow biofilm reactor system showing

the arrangement of the sheet (310 mm � 350 mm in plan) and wastewater flow

paths. The frustum heights were 25 and 11 mm high in Reactors 1 and 2,

respectively.

frustums. The horizontal sheets, 310 mm � 350 mm in plan, were assembled

one above the other, so that wastewater flowed along one sheet, discharged to

the sheet underneath, and then flowed back along that underneath sheet, and so

on down through the two reactors. The influent end of each sheet was offset

40 mm from the discharge end of the sheet above to capture the flow. The sheets

were sealed – to prevent leakage – at the inflow end and along the two sides with

a 20 mm strip of L shaped PVC to ensure horizontal flow along the sheets. The

horizontal top surface area of each sheet was 0.1085 m2. The reactor system was

raised above the floor on a timber platform to facilitate sampling of the effluent.

Daily maintenance of the system included preparation of the synthetic

wastewater, cleaning feed tanks, feed-lines, sampling, testing and measuring

flow rates. At commissioning of the system, activated sludge from a municipal

wastewater treatment plant was mixed with synthetic feed and applied to the

system. Recycling of effluent was carried out for the first 2 days of the

experiment to generate a high biomass concentration on the sheets.

2.2. Sampling and analysis for water quality parameters

Samples were tested for water quality parameters in accordance with the

Standard Methods for the Examination of Water and Wastewater, APHA

AWWA WEF 19th Edition 1995 [14]. Total and filtered chemical oxygen

demands (CODT, CODf) were measured by the titrimetric dichromate method.

Filtered samples were obtained by filtering wastewater samples through What-

man glass fibre filters (pore size 1.2 mm). Ammonia-nitrogen was tested by

method 4500-NH3 D with a NH 500/2 WTW ion selective electrode and WTW

pH 320 m. Nitrate–nitrogen was tested by method 4500-NO3� D with a nitrate

WTW82362 ion selective electrode and WTW pH 91 m. Total nitrogen (TN)

was tested using a DR/2010 spectrophotometer (HACH company). Dissolved

oxygen (DO) was measured with an electrochemical membrane type electrode

(WTW cellOx 325) and a WTW oxi 330 m. The pH was measured using a

WTW SenTix 50 pH electrode and a WTW pH 91 digital meter. Oxidation

reduction potential (ORP) was measured by a Dolmen 23 redox combination

electrode and recorded on a WTW 330 digital meter. DO, ORP and pH

measurements were taken immediately after sample collection. All electrodes

were calibrated before and after measurement as specified by the manufac-

turers’ instructions.

Sampling of the system for water quality analysis involved collection of an

influent sample, an effluent sample from Reactor 1 and an effluent sample from

Reactor 2, on a thrice-weekly basis. Samples to monitor the performance down

through the sheets of the treatment process were collected approximately every

2 weeks from each sheet. These samples for profiles were pipetted from three

locations on each sheet and combined to form a composite sample.

2.3. Hydraulic retention time

A pulse tracer experiment using NaBr was carried out to assess the hydraulic

characteristics of a clean reactor system. The experiment was commenced by

pumping tap water onto the top sheet of Reactor 1 for 10 min every hour at the

rate of 155 l/(m2 d)—based on the top plan area of the system. For the tracer

test, a known concentration of bromide was pumped onto the top sheet for one

10-min pumping period. Tap water was applied again during subsequent 10-min

pumping periods. The pumping regime was the same as that used in the

wastewater treatment study. Samples were collected at the base of the unit

and analysed using a bromide WTW ion selective electrode and WTW pH

320 m. Sample times and volumes were recorded. The tap water was pumped

onto the unit over a period of 15 h after the bromide was applied to ensure near

full recovery of the tracer material.

Fig. 2 shows the normalised bromide concentration plot for the HFBR. The

hydraulic retention time of the clean reactor system, HRT, was calculated [15]

from the pulsed tracer experiments using the following equation and the HRT

was determined experimentally as 8.3 h:

t ¼P

itiCiViDtiPiCiViDti

where t is the mean time of passage (HRT) (h), ti the time at the ith measurement

(h), Ci the tracer concentration at the ith measurement (mg/l), Dti the time

increment about Ci and Vi is the volume at the ith measurement (l).

M. Rodgers et al. / Process Biochemistry 41 (2006) 2270–22752272

Fig. 2. Normalised plot for the bromide tracer concentration (C) exiting from a

clean horizontal flow biofilm reactor as a fraction of the inflow concentration (C0).

Table 1

Composition of synthetic wastewater used as influent to the horizontal flow

reactor system

Constituent Concentration (mg/l)

Glucose 200

Yeast 30

Dried milk 120

Urea 30

NH4Cl 60

Na2PO4�12H2O 100

KHCO3 50

NaHCO3 130

MgSO4�7H2O 50

FeSO4�7H2O 2

MnSO4�H20 2

CaCl2�6H2O 3

Fig. 3. Filtered COD concentrations in the influent, effluent from the top reactor

and from the bottom reactor against time in days.

2.4. Hydraulic and organic loading rates

The HFBR was tested over 365 days at an organic loading rate of 47 g COD/

(m2 d) and a hydraulic loading rate of 155 l/(m2 d) based on the top plan area of

the system; the hydraulic loading based on the total top surface area of the sheets

was 6 l/(m2 d). Daily, 16.8 l of synthetic wastewater was applied intermittently

to the top sheet of Reactor 1 at a rate of 70 ml/min for 10 min every hour—the

Table 2

Analytical results from the influent, and effluent from Reactor 1 (Sheets 1–8) and R

operation (Days 60–365)

CODT

(mg/l)

CODf

(mg/l)

SS

(mg/l)

TN

(m

Influent

Average 385.3 325.3 50.6 3

Std. dev. 55.6 52.1 12.8

No. samples 127 127 127 12

Effluent Reactor 1

Average 71.6 55.7 14.5 2

Std. dev. 14.3 9.1 7.2

No. samples 127 127 127 12

Effluent Reactor 2

Average 46.2 33.0 11.2 1

Std. dev. 9.7 5.8 5.6

No. samples 127 127 127 12

same as in the tracer test. The 2 areal hydraulic loading rates used in the present

system allow a comparison of its performance to be made with other systems,

e.g. 4 l/(m2 d) for a soil filter system, 40–100 l/(m2 d) for an intermittent sand

filter and 100 l/(m2 d) for a peat filter [8]. The feed wastewater was similar to

that used by Odegaard and Rusten [16] and the composition of the synthetic

wastewater is given in Table 1.

3. Results and discussion

The average influent and effluent concentrations are shown

in Table 2 for Days 60–365, when a near steady state was

reached. The strength of the feed wastewater was similar to

low-strength domestic wastewaters [17]. Overall, the HFBR

system removed 90% of the CODf and 56% of the total nitrogen

applied, and achieved almost full nitrification.

The average CODf in the influent, 325.3 mg/l, was reduced

to 55.7 mg/l in Reactor 1 and was then further reduced to

33 mg/l in Reactor 2 (Fig. 3). The average non-biodegradable

COD in the effluent was 30 mg COD/l, and this was determined

by aerating a batch sample of effluent until no change in CODf

concentration occurred [19]. Fig. 4 shows the CODf profile

down through the sheets during steady-state operation. In

Reactor 1, 61.5% of the CODf was removed in the top 5 sheets,

giving the removal rate of about 6.2 g CODf/(m2 d) based on the

eactor 2 (Sheets 9–26) in the horizontal flow biofilm system during steady-state

g/l)

NH4–N

(mg/l)

NO3–N

(mg/l)

pH DO

(mg/l)

1.6 21.6 0.5 7.8 6.5

3.7 3.4 0.2 0.2 0.6

7 127 127 127 127

2.7 21.7 0.9 8.0 5.5

2.8 2.7 0.6 0.6 0.5

7 127 127 127 127

3.9 2.2 11.8 7.7 6.8

3.2 4.7 3.6 0.5 0.6

7 127 127 127 127

M. Rodgers et al. / Process Biochemistry 41 (2006) 2270–2275 2273

Fig. 4. Profile of CODf concentrations down through the sheets of the hor-

izontal flow biofilm reactor system during steady-state operation (Days 60–

365).

Fig. 6. Profile of nitrate–nitrogen (NO3–N) concentrations down through the

sheets of the horizontal flow biofilm reactor system during steady-state opera-

tion (Days 60–365).

total surface area of the top five sheets; a further 21% was

removed in Sheets 6–8; and in Reactor 2, 6.9% was removed in

Sheets 9 and 10.

During this test series, the average organic areal removal

rates of 45.3 g CODf/(m2 d) and 1.8 g CODf/(m

2 d) were

achieved, based on the top plan area and total sheet surface area,

respectively. A removal rate of 3.1 g COD/(m2 d) was reported

for a constructed wetland system [18] for a similar wastewater.

Ammonium-nitrogen removal was also at steady state from

approximately Day 60 to 365, during which time the average

influent ammonium–nitrogen concentration was 21.6 mg/l and

the average effluent from Reactor 2, was 2.2 mg/l. There was an

increase in ammonium–nitrogen concentrations (Fig. 5) in the

top six sheets that can be attributed to ammonification of

organic nitrogen present in the influent. From Sheet 7 onwards

there was a reduction in the ammonium–nitrogen concentration

due to cell synthesis and nitrification resulting in 90% removal.

The average removal rate in Sheets 8–15 was about 0.35 g NH4-

N/(m2 d) based on the total sheet surface area, which was

comparable with the removal rates of 0.234 g NH4-N/(m2 d)

[20] and 0.756 g NH4-N/(m2 d) [21] reported in constructed

wetlands. A profile of nitrate–nitrogen concentrations is shown

in Fig. 6. The average nitrate–nitrogen concentration increased

up to 13.95 mg/l in Sheet 19, after which it reduced to 11.8 mg/l

in the final effluent.

Fig. 5. Profile of ammonium–nitrogen (NH4-N) concentrations down through

the sheets of the horizontal flow biofilm reactor system during steady-state

operation (Days 60–365).

The average TN concentration was 31.6 mg/l in the influent,

22.7 mg/l in the Reactor 1 effluent, and 13.9 mg/l in the Reactor

2 effluent, which gives a 28.2% and a 27.8% reduction in

Reactors 1 and 2, respectively. This total 56% reduction in TN

corresponded to areal removal rates of 2.7 g N/(m2 d) and about

0.1 g N/(m2 d), respectively, based on the top plan surface area

and the total sheet surface area of the system. The TN removal

rates were of a similar order to the removal rate of 0.268 g TN/

(m2 d) achieved in a constructed wetland system [18], but the

footprint area required was much smaller for the HFBR. The

removal of nitrogen in this study, which was greater than

expected, could be attributed to uptake in cell synthesis and

denitrification in anoxic zones in the biofilm.

Average dissolved oxygen concentrations increased in

Reactor 1 from 1.1 mg/l on Sheet 1 to 5.5 mg/l on Sheet 8

and in Reactor 2 from 5.8 to 6.8 mg/l. The high dissolved oxygen

concentration in the effluent indicates that there was an adequate

oxygen supply for biochemical reactions. The pH of the influent

was 7.8, suitable for carbonaceous oxidation and nitrification.

Oxidation–reduction potential (ORP) profiles (Fig. 7) show

that on Day 4, when 74% COD was removed and no nitrification

occurred, ORP values were 21.3 EH mVon Sheet 1, 192.8 EH mV

on Sheet 4 and 207.7 EH mV on Sheet 26. The increase in ORP

with sheet number is due to reducing substrate and an increase in

the dissolved oxygen concentration. This ORP range is indicative

Fig. 7. Oxidation–reduction potential (ORP) profiles down through the sheets

of the horizontal flow biofilm reactor system on Days 4, 18, 32 and Days 60–

365.

M. Rodgers et al. / Process Biochemistry 41 (2006) 2270–22752274

of biological carbonaceous oxidation, since organic carbon

compounds are easily oxidised and can be removed at low EH

values. The ORP profiles from Days 18 and 32 show increases in

EH values in Reactor 2 due to the onset of nitrification. The range

at which nitrification occurred was 200–375 EH mV, which when

combined with the pH data (range 7.5–8) corresponds with the

Pourbaix biological nitrification window [22]. ORP measure-

ments in the effluent from an activated sludge system treating

medium and low-strength wastewaters indicate that values in the

range of 270–300 EH mV are optimal for COD biodegradation,

and 350–400 EH mV are suitable for nitrification [23]. An ORP

range of 200–400 EH mV was sufficient to achieve 92% and 84%

removal of COD and ammonium-nitrogen, respectively [24].

During steady-state conditions (Days 60–365) there was little

variation in the ORP values obtained, indicating that ORP was a

very useful stable monitoring parameter for this system. The final

effluent from the reactors had ORP values of 366� 26 EH mV

indicating a relatively high oxidising status, which is suitable for

nitrification.

In the HFBR system, the solids yield (Yobs) as a fraction of

the filtered COD removed was on average 0.173 g SS/g

CODrem, with a near constant ratio of approximately 0.1 g SS/g

CODrem for Days 270–365. This value is similar to that reported

by [25] for a membrane reactor and significantly lower than that

of 0.4 g VSS/g COD reported by Droste [26] for an activated

sludge plant. The rate of increase of sludge mass in the HFBR

decreased as the study progressed. Small amounts of solids

were discharged in the effluent, 14.5 � 7.2 mg/l and

11.2 � 5.6 mg/l from Reactors 1 and 2, respectively.

The tracer test on the clean reactor system gave a HRT of

8.3 h. This result is only indicative, as the actual HRT will

change as biofilm develops in the system. However, the HRT of

8.3 h is of a similar duration to that used in conventional

activated sludge processes for nitrification.

Based on the results achieved in this laboratory study, it

would appear that an on-site HFBR system with a foot print

plan area in the range of 1.5–4.0 m2/person-equivalent and a

stack of 26–30 plastic sheets with frustums could provide

adequate biofilm for biodegradable organic carbon removal and

nitrification of domestic wastewater. The design area values

would depend on the strength of the domestic wastewater and

climate. The reactor units could be placed in a tower

arrangement with a suitable pumped distribution system to

reduce the overall foot print area, as the depths of the two stacks

of sheets used in this study were less than 400 mm in total. It

would be prudent to install a clarifier after the system. The

design values would need to be proven in field tests.

4. Conclusions

In this study, a novel horizontal flow biofilm reactor (HFBR)

system – comprising 26 sheets arranged one above the other –

was developed and tested at 11 8C for 365 days using a

domestic-strength synthetic wastewater that was applied

intermittently for 10 min every hour at hydraulic and filtered

chemical oxygen demand (CODf) loading rates of 155 l/(m2 d)

and 47 g/(m2 d), respectively, based on the system plan area.

The main conclusions are as follows:

1. T

he HFBR had an average organic areal removal rate of

45.3 g CODf/(m2 d), based on system plan area, giving a

final effluent concentration of only 33 mg CODf/l, most of

which was non-biodegradable.

2. A

lmost complete nitrification occurred.

3. T

otal nitrogen removal was 56%.

4. S

olids production during the study period was very low

towards the end of the study period at 0.1 g SS/g COD

removed and the system retained almost all of its sludge for

over 365 days.

5. N

o clogging of the sheets occurred during the study.

6. O

RP monitoring provided a useful simple indicator of steady

state development for the biofilm.

Overall, the system was simple to construct and operate with

minimal maintenance requirements, and achieved excellent

COD removal, nitrification and partial nitrogen removal with

low solids production. From this study, the system would

appear to have good potential as a suitable biological reactor for

treating domestic wastewater from small communities.

Acknowledgements

Funding from the Higher Education Authority of Ireland for

this project is gratefully acknowledged. Assistance from the

Environmental Change Institute at the National University of

Ireland is appreciated. Sincere thanks are due to R. Duffy, M.

O’Brien and G. Hynes for their help.

References

[1] Effler SW, Brooks CM, Auer MT, Doerr SM. Free ammonia and toxicity

criteria in a polluted urban lake. J Water Pollut Control Fed 1990;62:771–9.

[2] EEC. The quality of water intended for human consumption. EEC

Directive 80/778/EEC. Official Journal No. 229. Brussels, Belgium; 1980.

[3] Zaveri RM, Flora JRV. Laboratory septic tank performance response to

electrolytic stimulation. Wat Res 2002;36:4512–24.

[4] Walker WG, Bouma J, Keeney DR, Olcott PG. Nitrogen transformation

during subsurface disposal of septic tank effluent in sands. J Environ Qual

1973;2:521–5.

[5] Starr JL, Sawhney BL. Movement of nitrogen and carbon from a septic

tank system drainfield. Water Air Soil Pollut 1980;13:113–23.

[6] Gupta AB, Gupta SK. Simultaneous carbon and nitrogen removal from

high strength domestic wastewater in an aerobic RBC biofilm. Wat Res

2001;35(7):1714–22.

[7] Luederitz V, Eckert E, Martina LW, Lange A, Gersberg RM. Nutrient

removal efficiency and resource economics of vertical flow and horizontal

flow constructed wetlands. Ecol Eng 2001;18:157–71.

[8] Environmental Protection Agency (Ireland), Wastewater treatment man-

uals: treatment systems for single houses, 2000.

[9] Hanna KM, Kellam JL, Boardman GD. Onsite aerobic package treatment

systems. Wat Res 1995;29(11):2530–40.

[10] Characklis WG, Marshall KC. Biofilms New York: John Wiley and Sons

Inc.; 1990.

[11] Suwa Y, Suzuki T, Toyohara H, Yamagishi T, Urushigawa Y. Single-stage,

single-sludge nitrogen removal by an activated sludge process with cross-

flow filtration. Wat Res 1992;26(9):1149–57.

[12] Tchobanoglous G, Burton FL, Stensel HD. Wastewater engineering:

treatment and reuse, 4th ed., New York: Metcalf & Eddy Inc./

McGraw-Hill; 2003.

M. Rodgers et al. / Process Biochemistry 41 (2006) 2270–2275 2275

[13] Rodgers M. Organic carbon removal using a new biofilm reactor. Wat Res

1999;33(6):1495–9.

[14] APHA, AWWA, WPCF (American Public Health Association, American

Water Works Association and Water Environment Federation), Standard

methods for the examination of water and wastewater. 19th ed. Washing-

ton, DC, USA; 1999.

[15] Levenspiel O. Chemical reaction engineering, 3rd ed., New York: John

Wiley and Sons; 1999.

[16] Odegaard H, Rusten R. Nitrogen removal in rotating biological contactors

without the use of an external carbon source. In: Proceedings of the First

National Symposium/Workshop on Rotating Biological Contactor Tech-

nology; 1980. p. 1301–17. p. 2.

[17] Henze M, Harremoes P, la Cour Jansen J, Arvin E. Wastewater treatment:

biological and chemical processes, 3rd ed., Germany: Springer; 2002.

[18] Verhoeven JTA, Meuleman AFM. Wetland for wastewater treatment:

opportunities and limitations. Eco Eng 1999;12:5–12.

[19] Henze M, Guyer W, Van Loosdrecht M, Mino T. Activated Sludge Models

ASM1, ASM2, ASM2d and ASM3 (Scientific & Technical Reports, No.

9), IWA Publications; 2000.

[20] Cronk JK, Shirmohammadi A, Kodmur V. An evaluation of wetlands for

the treatment of dairy effluent: results from the first year of operations.

Presented at the International Winter Meeting. St. Joseph, MI, USA;

ASAE: 1994 [Pap. No. 94-2600].

[21] Skarda SM, Moore JA, Niswander SF, Gamroth MJ. Preliminary results of

wetland for treatment of dairy farm wastewater. In: DuBowy PJ, Reaves

RP, editors. Proceedings of a Workshop on Constructed Wetlands for

Animal Wastewater Management. 1994. p. 34–42.

[22] Charpentier J, Martin G, Wacheux H, Gilles P. ORP regulation and

activated sludge: 15 years of experience. Water Sci Technol 1998;

38(3):197–208.

[23] Li B, Bishop PL. Oxidation–reduction potential changes in aeration tanks

and microprofiles of activated sludge floc in medium- and low-strength

wastewaters. Water Environ Res 2004;76(5):394–403.

[24] De la Menardiere M, Charpentier J, Vachon A, Martin G. ORP as a

control parameter in a single sludge biological nitrogen and phos-

phorus removal activated sludge system. Water SA 1991;17(2):

123–32.

[25] Pollice A, Laera G, Blonda M. Biomass growth and activity in a

membrane bioreactor with complete sludge retention. Water Res 2004;

38:1799–808.

[26] Droste RL. Theory and practice of water and wastewater treatment New

York: John Wiley and Sons; 1997.