Treatment of domestic and agricultural wastewater by reed bed systems

Ecological Engineering 12 (1999) 13–25

Treatment of domestic and agricultural wastewaterby reed bed systems

Jurgen Kern *, Christine Idler

Department of Biological Engineering, Institute of Agricultural Engineering, Max-Eyth-Allee 100,14469 Potsdam, Germany

Abstract

The purification capacity and seasonal variability of two newly created reed bed systemswas monitored over 1 year. In one of the systems treating domestic wastewater, the reductionin the concentrations of chemical oxygen demand (COD), total N, total P and fecal coliformbacteria (FC) ranged from 80 to 99%. In the second system, treating a mixture of domesticand agricultural wastewater produced by a cheese dairy, treatment efficiency ranged between13 and 99%. The removal rates did not show any seasonal pattern for domestic wastewater,in contrast to the mixture of domestic and agricultural wastewater with increased removalrates in the summer. Bacteria which are involved in N metabolism was estimated by the mostprobable number-test (MPN). In both reed bed systems the MPN of proteolytic bacteria,ammonifiers, nitrifiers and denitrifiers/g DW of gravel substrate were of the orders ofmagnitude of 104–106, 105–107, 101–105 and 0–103, respectively. On the basis of themicrobial assemblage there is a need to improve the conditions for both nitrification anddenitrification to enhance the efficiency of N removal. © 1999 Elsevier Science B.V. All rightsreserved.

Keywords: Constructed wetlands; Wastewater treatment; Nitrogen dynamics; Denitrification

* Corresponding author. Tel.: +49 331 5699123; fax :+49 331 5699849;e-mail: [email protected]

0925-8574/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.

PII S0925-8574(98)00051-2

J. Kern, C. Idler / Ecological Engineering 12 (1999) 13–2514

1. Introduction

Wastewater disposal is a major problem in the new federal states of Germany,particularly in the rural areas of the state of Brandenburg. Very often thewastewater, derived from households and agricultural processes, is dischargeddirectly or indirectly into the soils due to a lack of sewerage reticulation and surfacewaters. Because the soils of Brandenburg are very sandy, the wastewater mayrapidly reach deeper layers of the soil and eventually pollute the groundwater.Groundwater concentrations of up to 50 mg l−1 NO3

−�N have already beenmeasured in various communities of Brandenburg.

In recent years there has been a growing interest in wetlands due to theirpotential in the removal of nutrients. Hanson et al. (1994) showed that up to 50%of the groundwater NO3

− may be removed by denitrification within riparianwetlands. The use of constructed wetlands opens up a vast area for the treatmentof various types of wastewater (Hammer, 1989; Moshiri, 1993). Reed bed systemshave proved to be an appropriate technology for cleaning municipal wastewater(Manfrinato et al., 1990; Brix, 1994a; Ansola et al., 1995; Cooper and Green, 1995;Li et al., 1995). However, relatively little information exists about the treatment ofagricultural wastewaters by reed beds. This may be linked with operational prob-lems that can occur as a result of high concentrations of suspended solids, N, P andfat in these wastewaters (van Oostrom and Cooper, 1990; Gray et al., 1990; Tanner,1994). To realize a high purification of agricultural wastewater, a pretreatment hasbeen considered as very important (Cronk, 1996). van Oostrom and Cooper, (1990)concluded that in the case of meat processing effluents, the use of constructedwetlands seemed to be restricted to a polishing stage after anaerobic or aerobictreatment. For dairy farm wastewater, constructed wetlands showed a considerablecleaning potential as reported by Tanner et al. (1995a,b).

With special focus on the microbial decomposition and on the fate of nitrogencompounds, this paper will discuss the performance of two reed bed systemsreceiving different types of wastewater.

2. Materials and methods

2.1. Study site and experimental design

The two reed bed systems studied each consist of two parallel reed beds, plantedwith Phragmites australis. One of these systems (two beds, each 16×4×1 m deep)treats the domestic wastewater from the School and Research Institute of Horticul-ture Großbeeren/Werder e.V. (Großbeeren) located south of Berlin (52°21%N,13°18%E). The other system (two beds, each 10×7×1.8 m deep), located at theOrganisation for Biodynamic Management Marienhohe e.V. (Marienhohe) near thevillage Bad Saarow (52°17%N, 14°01%E) receives a mixture of wastewater fromhouseholds and a cheese dairy with relative proportions of about 7:1. The waste-water flow in the Großbeeren system is horizontal, whereas at Marienhohe the

J. Kern, C. Idler / Ecological Engineering 12 (1999) 13–25 15

wastewater has a vertical flow. Fig. 1 shows the cross sections of the two reed bedsystems. In both systems the wastewater is pretreated in a settling tank. Hydraulicloading is intermittent, controlled by a siphon in Marienhohe and a float valve anda pump which are installed inside the settling tank in Großbeeren.

Both beds are rubber lined and filled to 0.6 and 1.5 m, respectively, with gravel(2–16 mm diameter) and operated as subsurface flow systems. The water table levelin the reed bed was controlled in Großbeeren 20–30 cm and in Marienhohe 100 cmbelow the gravel surface by an adjustable outflow pipe leading to a pond and intoan outlet tank, respectively. The low water table level in the Marienhohe reed bedwas a new management practice after the reconstruction of the reed bed had beenfinished in December, 1995. Formerly the reed bed that was filled with sand(0.3–2.0 mm diameter) had shown serious clogging problems.

The main data of the two reed bed systems under study are given in Table 1. Themean loading rate of wastewater treated at Großbeeren is half that treated atMarienhohe. Hydraulic residence times are 8 and 3 days in the Großbeeren andMarienhohe reed beds, respectively. After passing the reed bed and the outlet tankthe treated wastewater reaches a pond, in Großbeeren by natural flow, in Marien-hohe by pumping from the outlet tank into the pond.

Fig. 1. Cross sections of the reed bed systems at Großbeeren and Marienhohe, Germany.


Table 1Operational and design characteristics of the reed bed systems at Großbeeren and Marienhohe

Großbeeren Marienhohe

WastewaterType Domestic Domestic/agricultural

4.090.22.090.5Loading rate (m3 day−1)SettlingSettlingPretreatment

Reed bed1995, after reconstructionStart up date 1995140130Surface area (m2)

Horizontal, intermittent Vertical, intermittentFlow system0.0290.015Mean hydraulic load (m3 m−2 day−1)Gravel (2–5 mm diameter)Gravel (2–3 mm diameter)Substrate (upper layer)

Gravel (8–16 mm diameter) Gravel (8–16 mm diameter)Substrate (lower layer)Gravel depth (m) 0.6 1.5

2.2. Sampling

Wastewater was grab sampled from the settling tank and from the outlet tank ofeach reed bed system at 3-month intervals over a 1-year period. The samples weretransported in polyethylene bottles to the ATB laboratory where the analyses werestarted within 24 h of collection.

Gravel substrate, 0–20 cm in Großbeeren and 0–20 cm and 20–40 cm inMarienhohe, was taken by a groove corer from six different places in the reed bedsto form a composite sample with equal proportions from the various depths. Gravel

Table 2Selected physical and chemical properties regarding the gaseous nitrogen release during incubations ofthe gravel substrate from the reed beds (0–20 cm)

MarienhoheGroßbeeren

9/9612/95 6/96 4/96 7/96

20.0 8.80Soil temperature (°C at 10 cm depth) 15.2 10.30.9 1.10.9ndLoss on ignition (% per dry weight) 0.7

nd8.4 7.8 nd 8.2pHnd 145 80B1N2O evolution −1

(18) (16)(B1)nmol N2O kg DW−1 ·h−1 (1)18371173096964Denitrification rate 9339

(150)(60) (1054)(8) (608)nmol N2O kg DW−1 ·h−1

1319 99Denitrification potential 17688 25349 22851(1364)(29)(790) (1246)nmol N2O kg DW−1 ·h−1 (2409)

Data for N2O evolution, denitrification rate and denitrification potential are average rates (n=3) with1 S.D. in parentheses.nd, Not determined.


samples did not contain plant roots as reflected by low values of loss on ignition(Table 2).

The substrate was handled with sterile instruments to prevent bacterial contami-nation of the samples, which were destined for microbiological investigations.

2.3. Chemical and microbiological methods

Water samples were analyzed using standard methods. Chemical oxygen demand(COD) was measured after acid digestion, total Kjeldahl nitrogen (TKN) bydigestion using selenium catalyst. Colorimetric techniques were used to measure,NH4

+�N (indophenol blue method), NO3−�N (sulfanilamide method) and total P

(molybdenum blue method). Total N was calculated as the sum of TKN andNO3

−�N. Fecal coliforms (FC) were counted according to WHO (1975) afterincubation of 48 h at 44°C on a lactose/bromothymol blue agar. Gravel substratesamples were studied for groups of N-metabolizing bacteria. The number ofculturable bacteria was estimated in the case of proteolytic bacteria after incubationof 48 h by counting colony forming units (CFU), grown on a casein/milk powdermedium. The number of ammonifiers, nitrifiers and denitrifiers, were calculated bythe most probable number method (MPN) after a 4-week incubation according toHarrigan and McCance (1976). The following media were prepared for incubation.Ammonifiers: 10.0 g peptone, 5.0 g NaCl and filled up to 1000 ml with distilledwater. Nitrifiers: 1.0 g K2HPO4, 0.5 g MgSO4, 1.0 g NaCl, 5.0 g CaCO3, and filledup to 1000 ml with distilled water. Before the start of incubation 50 ml of a solutionof (NH4)2SO4 (2%) was autoclaved separately, and added to the sterile solution ofthe remaining ingredients. Denitrifiers: 1.0 g glucose, 1.0 g yeast extract, 0.2 gMgSO4, 0.1 g NaCl, 0.2 g KNO3, 10 ml of a 0.1% FeSO4/EDTA solution, 10 ml ofa 0.5 M KH2PO4 buffer and filled up to 1000 ml with distilled water.

Gaseous N release from the gravel substrate was measured after an anaerobicincubation in gastight 125 ml glass vials for 24 h. Three assays, each with threereplicates, were applied to obtain the natural N2O evolution, the denitrification rateand the denitrification potential (+1 mmol NO3

−). Denitrification rate and deni-trification potential were measured using the acetylene-blockage technique in con-trast to the N2O evolution measured without acetylene. The difference between thedenitrification rate and the denitrification potential has been checked to be statisti-cally significant by a Mann–Whitney Rank Sum Test. For further details see Kernet al. (1996). Gas samples of 1 ml were injected into a gas chromatograph (FisonsInstruments, GC 8340) equipped with a 3 m long packed column (Haye Sepe D100/120 mesh). Oven temperature was programmed between 70 and 120°C. N2Owas detected by a 63Ni electron capture detector (ECD) operating at 320°C. Thecarrier gas was Helium with a flow rate of 15 ml min−1. Working standardsconsisted of N2O diluted in He (Scotty Speciality Gases).

3. Results and discussion

The purification capacity of the reed bed systems at Großbeeren and Marien-


Fig. 2. Influent and effluent concentrations in the reed bed systems at Großbeeren and Marienhohe.Percentage reductions are shown above effluent concentrations and the nitrate component of effluenttotal N identified.

hohe, expressed in terms of influent and effluent concentrations, was monitoredover four seasons during 1995/1996 (Fig. 2). Generally the wastewater ofGroßbeeren was less polluted than that of Marienhohe, except the influent total N


concentration was always higher in Großbeeren due to the dominance of toilet water.Considering the difference between influent and effluent, the removal rates in the reedbed of Großbeeren generally exceeded those of Marienhohe during the whole studyperiod. The reduction in the concentrations of COD, total N, total P and FC rangedfrom 80 to 99% and from 13 to 99% in Großbeeren and Marienhohe, respectively.

Due to the fatty constituents of the cheese dairy wastewater in the influent atMarienhohe, it showed high COD concentrations of between 500–800 mg l−1. CODremoval was highest in summer probably reflecting the impact of temperature on themicrobial processes. This seasonal pattern also occurred for total N, total P and FCremoval. The removal of total N at Marienhohe ranging between 13 and 63%, wasrelatively low (Fig. 2). A significant amount of the nitrogen in the effluent was inthe form of NH4

+ (24.8 mg N/l) and NO3− (31.8 mg N/l), indicating that nitrification

and denitrification were incomplete (Table 3).High numbers of proteolytic (10−4–10−6) and ammonifying bacteria (10−5–

10−7) reflect an intensive mineralization of organic N, although there seems to beno correlation between the total N of the influent (Fig. 2) and the bacterial numbersin the gravel substrate (Fig. 3). Much lower however, were the numbers of nitrifiersand denitrifiers. Compared with Großbeeren, the reed bed in Marienhohe seemed tohave a relatively high number of nitrifiers with a large range (10−1–10−5) reflectinga high nitrifying potential. This may be linked generally with high NO3

−-Nconcentrations in the effluent. However, considering the mean NH4-N concentrationsin the effluent, the removal rate in Großbeeren (95.8%) is obviously higher than thatin Marienhohe (66.6%). Consequently, the estimated number of nitrifiers does notappear to reflect the real activity of ammonium oxidizing bacteria. On the otherhand, the reduction of NO3

− to N2 and N2O at Marienhohe might have been retardedby low numbers of denitrifying bacteria, and an environment that was not suitablefor denitrification. Denitrification at Marienhohe was probably limited by thepresence of atmospheric oxygen, which could penetrate the reed bed easily, due tothe coarse gravel substrate, the low water table level and the intermittent verticalwater flow. To overcome these problems and to enhance the N removal bydenitrification, further efforts have to be made to improve the design criteria atMarienhohe. A combination of two stages with low and high water table levels mightbe helpful. Recently, different strategies for advanced nitrogen removal fromdomestic wastewater have been tested by Laber et al. (1997). By recirculation ofnitrified effluent from the soil filter back to the settling tank at the influent, theremoval rate of total N could be enhanced from 34 to 72%. Another approach addingmethanol as a carbon source for denitrifying bacteria let increase the removal rateof total N during intermittent loading from 28 to 78%.

In spite of a low MPN of denitrifiers and low total N removal under laboratoryconditions, there was high denitrifying activity at Marienhohe, which was muchhigher than at Großbeeren (Table 2).

This may reflect the more favorable conditions for denitrification achieved by theanaerobic atmosphere within the incubation vessels. It is not clear, however, whythe MPN of denitrifiers, which was obtained under anaerobic conditions as well,was so low. Principally, the MPN test for denitrifiers may be criticized because it


Tab

le3

Ave

rage

influ

ent

and

efflu

ent

conc

entr

atio

nsan

dco

rres

pond

ing

rem

oval

rate

sof

chem

ical

oxyg

ende

man

d,to

tal

N,

NH

4+-N

,N

O3−

-N,

tota

lP

and

feca

lco

lifor

ms

inth

ere

edbe

dsof

this

and

othe

rst

udie

s

Tot

alN

NH

4�N

NO

3�N

Tot

alP

Fec

alco

lifor

ms

Ref

eren

ces

Typ

eof

was

tew

ater

Hyd

raul

iclo

adC

OD

(m3

d−1

per

m−

2)

0.01

5D

omes

tic

9.9

2900

Thi

sst

udy,

Gro

ßbee

ren,

0.7

Influ

ent

(mg

l−1)

183.

420

9.9

317.

66.

81.

226

Ger

man

y25

.618

.87.

8E

fflue

nt(m

gl−

1)

Rem

oval

(%)

87.9

99.1

91.9

91.0

95.8

0

0.02

9C

hees

eda

iry/

dom

esti

c10

.484

000

0T

his

stud

y,M

arie

nhoh

e,In

fluen

t(m

gl−

1)

628.

297

.074

.10.

64.

733

000

Ger

man

y31

.824

.8E

fflue

nt(m

gl−

1)

63.1

88.8

66.6

054

.896

.1R

emov

al(%

)85

.935

.0

0.04

7M

unic

ipal

4.8

2.1

0.9

8100

Li

etal

.(1

995)

,C

hina

Influ

ent

(mg

l−1)

547.

014

.40.

48

0.6

Effl

uent

(mg

l−1)

103.

02.

05.

173

.255

.399

.981

.264

.659

.4R

emov

al(%

)

van

Oos

tron

and

Coo

per

(199

0),

Mea

tpr

oces

sing

0.04

615

.0N

ewZ

eala

ndIn

fluen

t(m

gl−

1)

94.0

125.

015

31.0

11.0

330.

099

.094

.0E

fflue

nt(m

gl−

1)

27.0

78.0

21.0

0.0

Rem

oval

(%)

Che

ese

dair

y0.

022

0.4

10.4

3410

Zus

tan

dSc

honb

eron

(199

4),

537.

726

.0In

fluen

t(m

gl−

1)

14.9

1.3

5.0

37Sw

itze

rlan

d95

.314

.210

.4E

fflue

nt(m

gl−

1)

51.9

98.9

0.0

Rem

oval

(%)

30.4

45.3

82.2


Tab

le3

(Con

tinu

ed)

Tot

alN

NH

4�N

NO

3�N

Tot

alP

Fec

alco

lifor

ms

Ref

eren

ces

Typ

eof

was

tew

ater

CO

DH

ydra

ulic

load

(m3

d−1

per

m−

2)

Dai

ryfa

rm0.

2111

.214

35T

anne

ret

al.

(199

5a),

Influ

ent

(mg

l−1)

21.6

38.2

—88

Tan

ner

etal

.(1

995b

),—

—E

fflue

nt(m

gl−

1)

*71.

074

.093

.9N

ewZ

eala

ndR

emov

al(%

)*7

5.0

Dai

ryfa

rm0.

073

11.2

1890

Tan

ner

etal

.(1

995a

),38

.2In

fluen

t(m

gl−

1)

21.6

Effl

uent

(mg

l−1)

—45

0T

anne

ret

al.

(199

5b),

——

*37.

076

.2N

ewZ

eala

nd*4

8.0

Rem

oval

(%)

*34.

0

*%

rem

oval

isba

sed

onm

ass

flow

s.


relies on the development of a gas bubble without specific analysis as done by theacetylene-blockage technique. A low MPN can also be explained by a mediumwhich was not appropriate for denitrifiers. The intensity of denitrification seems tobe highly dependent on the supply of a suitable carbon source (Isaacs and Henze,1995; Laber et al., 1997). That means, the high denitrifying activity at Marienhohe,

Fig. 3. Mean number of bacteria in 0–20 and 20–40 cm depth samples of gravel substrate from the reedbeds at Großbeeren and Marienhohe. nd, Not detected.


measured as accumulation of N2O, may be due to the high influent COD at thissite.

The natural N2O evolution rate was less than 1% of the denitrification rate whenthe acetylene block technique was used (Table 2). An important factor controllingthe N2/N2O ratio is the pH value (Gale et al., 1993). Because the pH of the gravelin both the Großbeeren and Marienhohe reed beds, was high (7.8–8.4), denitrifica-tion could run mainly to N2 (Table 2). The denitrification potential was significantlyhigher (P=0.05) than the denitrification rate except at Großbeeren on June 1996,indicating that denitrifying enzymes were not saturated with NO3

− during thedenitrification test.

The removal of N in constructed wetlands is very often lower than for COD(Table 3). The operational design has to guarantee both oxic conditions fornitrification and anoxic conditions for denitrification. In contrast to the reed bedsat Marienhohe, in which primarily denitrification was limited, other studies haveshown that N removal in wetlands can also be limited above all by insufficientnitrification (van Oostrom and Cooper, 1990; Zust and Schonborn, 1994). Thewastewater studied by Zust and Schonborn (1994), which was derived from a cheesedairy and therefore had characteristics similar to those of Marienhohe, showednearly the same concentration of COD and total P. High numbers of FC in theinfluent and effluent at Marienhohe may be explained by the lack of disinfectants,which are never applied during the production of milk and cheese. Nevertheless, thepercentage efficiency for the reduction of FC showed similar results to those inother reed beds (Table 3).

Another factor which affects the purification process is the hydraulic loadingrate, which typically ranges between 0.025–0.050 m3 m2−1 day in reed beds (Brix1994b). An increase of the hydraulic loading rate within this range correspondswith a decrease in the removal rate as shown in Table 3 and reported by Phipps andCrumpton (1994) and Tanner et al. (1995a,b). Despite some uncertainties in dailyhydraulic loadings we can assume low hydraulic loading rates in the reed bedsunder study. In contrast to Marienhohe, the Großbeeren system had a greatervariability due to the varying school attendance of the Research Institute. Thismight have affected the removal rates, as possibly found in September 1996 wherethere was a slight decrease in the removal rates of COD, total N and total P.

Although the data set is limited, the results give some information about the earlyperfomance of two newly created reed bed systems. Rootzone depth has not beenmeasured. However, due to the short period of operation of both systems, plantroots should not have reached the maximum depth and thus the purificationcapacity is likely to become considerably higher than it was during the study period.

Acknowledgements

This study is part of a cooperation between the Institute of AgriculturalEngineering Bornim e.V., the School and Research Institute of HorticultureGroßbeeren/Werder e.V. and the Organisation for Biodynamic Management


Marienhohe e.V. The authors acknowledge the technical assistance of GundulaCarlow, Ulrike Knuth and Harald Fuchs.

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Treatment of domestic and agricultural wastewater by reed bed systems