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Volume 11 No.1 2002REPRINT pp. 27 - 29

ROLE OF DUCKWEED (Lemna minorL.)HARVESTING IN BIOLOGICAL PHOSPHATE REMOVA LFROM SECONDARY TREATMENT EFFLUENTS

E. bek H. H asar

An gerstr. 12853 54 Freising - GermanyPho ne: ++49 (0) 8161 -484 20Fax: ++49 - (0) 8161-4842 48Email: p arlar@psp-parlar.dewww.psp-parlar.de

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ROLE OF DUCKWEED (Lemna minorL.)

HARVESTING IN BIOLOGICAL PHOSPHATE REMOVAL

FROM SECONDARY TREATMENT EFFLUENTS

Erdal bek and Halil Hasar

Firat University, Faculty of Engineering, Department of Environmental Engineering, 23119-Elazig, TURKEY

ABSTRACT

The conventional wastewater treatment plants canpartly remove phosphorus. When the effluents of secon-

dary treatment are discharged to particularly inland waterbodies, they cause eutrophication in the receiving envi-ronment. Phosphate is a limiting factor and must beremoved from the effluent before being discharged. Thecapacity of duckweed (Lemna minor L.) in phosphateremoval from secondary effluents was studied in labora-tory to understand the mechanism of biological phosphateuptake. Orthophosphate can be efficiently removed ifduckweed is frequently harvested. The initial phosphateconcentration decreased from 15 mg L-1 to 0.5 mg L-1 atthe end of an 8 days period.

KEYWORDS:Biological phosphate uptake, duckweed, harvesting.

INTRODUCTION

The discharge of nitrogen and phosphate to rivers,lakes and dams causes massive but undesired growthof algae and plants called eutrophication [1]. This situa-tion disturbs the natural balance in water body. Phosphateis the most important limiting factor. Thus, this problemhas to be solved by reducing its concentration beforedischarge into receiving environments such as inlandrivers, lakes and dams. Further treatment of secondary

effluent is required to meet the discharge limits for phos-phates (0.5-1.0 mg L-1).

Advanced treatment processes are generally applied,particularly chemical precipitation. A number of secon-

dary/ biological phosphorus removal processes, such asA/O process, PhoStrip or SBR, have been developed,additionally to chemical treatment. These processes are

effective but often unsuitable for mass removal of pollut-ants. Alternative reliable technologies which are easy tooperate and have low capital and operating costs include

aquatic plants for treatment of small community waste-water, which cannot be sewered to a centralised treat-

ment system.

In recent years, a number of aquatic plants and wet-land systems, such as Salvina natas [2], Lamna gibba,Eichhornia crassipes [3], Typha glauca [4], Phragmitesaustralis [5], Melaleuca quinquenervia [6], and Lemnaminor[2, 7, 8] have been reported to remove nutrientsfrom aquatic environment.

In this study harvesting has been done after 4 and2 days periods to increase the efficiency of phosphate

removal using duckweed (Lemna minor) and the roleof frequent harvesting on treatment capacity has beenestablished.

MATERIALS AND METHODS

Duckweed

Duckweed belongs to the family of floating mono-cotyledons and consists of 4 genera (Lemna, Spirodela,Wolffia, Wolffiella) and 28 species [9]. They are green,small-sized (1-3 mm) and have short but dense roots(1-3 cm).

Duckweed grows even at water temperatures of5-7 C and atmospheric temperatures of 1-3 C [7, 10] butalso at temperatures up to 30 C (optimal 20-30 C).

Therefore, it is among the most vigorously growing plantson earth [11]. Duckweed reached a doubling of frondnumber in 4 days under laboratory conditions (24 C;12 h dark and light photo period [7]). Typical pH rangefor growth is 4.5-7.5 and growth is completely inhibited

only at pH values higher than 10 [12]. Water contentof duckweed is 94-95 % [7], protein content is high (240 -410 g kg-1, and fiber content low (60-90 g kg -1). Harvey

and Fox [13] determined the nitrogen (40-60 g kg

-1

) andphosphorus content (3-29 g kg-1) in duckweed harvestedfrom wastewater ponds.

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Duckweed samples used in this study were collectedfrom the secondary settling tank in Elazig City domestic

wastewater treatment facility.

Treatment

Duckweed was washed with an excess of pure water.The surface of three laboratory reactors was covered by

duckweed and the height of water in reactors was kept tobe 4 cm. The plexiglass reactors (15 x 45 x 15 cm) weretreated batchwise under following conditions (simulatedsummer temperature of 27 4 C; 12 h dark and lightphoto periods; total treatment time 8 days). The reactorwater samples collected were analysed every day.

Detention time of duckweed was 8 days in the firstreactor, 4 days and 2 days in the second and third one.After harvesting, new and prewashed duckweed wasinserted. Water volume reduction by volatilisation was

compensated by addition of pure water. The initial pH ofeffluents in the experimental studies was about 6.5

Orthophosphate analysis

Reactor water samples were filtered (0.22 mm Milli-pore filters) prior to measurement according to the ascor-bic acid Standard Method [14].

RESULTS AND DISCUSSION

Phosphate removal by duckweed to prevent eutrophi-

cation in receiving environment was studied becausethe effluents of treatment plants normally contain highphosphate concentration. Frequent harvesting affects Premoval positively. The P uptake capacity realised bydifferent periods of harvesting is summarised in Table 1.

TABLE 1 - Variation of orthophosphate concentrations with time.

Orthophosphate concentrations (mg L-1)

Time (days) Without harvesting Harvesting period of 4 days Harvesting period of 2 days0 12.0 15.0 15.0

1 9.3 12.2 11.5

2 8.0 9.1 8.8

3 7.2 7.8 5.9

4 6.5 6.9 4.25 6.0 4.7 2.5

6 6.5 3.5 1.5

7 7.1 2.8 0.8

8 8.2 2.2 0.5

FIGURE 1 - Orthophosphate uptake versus time according to frequent harvesting.

0

20

40

60

80

100

0 1 2 3 4 5 6 7 8Time, days

O-PhosphateUptake,

%

not harvested

harvested 1 per 4 days

harvested 1 per 2 days

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In duckweed experiments without harvesting, after5 days a P uptake of 50 % was observed. Then P was

released by duckweed into the reactor water again (Fig. 1;P increased from 6.0-8.2 mg L-1 (Table 1)). The absorp-

tion capacity of duckweed collected from domesticwastewater treatment facility and used in this study isrestricted due to higher phosphorus content in wastewaterthan in natural water. Therefore, the growth of duckweedwas kept down by frequent harvesting.

Duckweed has been harvested in 4 days periods in the

second reactor and diurnal P analyses on samples haveshown that P removal was 54 % before harvesting. Aftercovering with new duckweed harvested on the 8 th day,overall removal efficiency was 85.3 % (P concentrationdecreasing from 15 mg L-1 to 2.2 mg L-1).

The removal in the third reactor (2 days harvestingperiods) increased quite strikingly (Fig. 1). From P analy-ses removal efficiency was calculated reaching 41.3 %,72 %, 90 % and 96.7 % after 2, 4, 6, and 8 days (initial Pconcentration decreased from 15 mg L-1 to 0.5 mg L-1).

In laboratory experiments phosphate can be success-fully removed when duckweed is frequently harvested.

REFERENCES

[1] Sidat, M., Kasan, H.C. and Bux, F., 1999, Laboratory-ScaleInvestigation of Biological Phosphate Removal from Munici-pal Wastwater; Water SA 29, 459-462.

[2] Bayhan, H., Aka, L., Altay, A., and Sakar, S., 1996, 'YzenSu Bitkileri ile Atiksulardan Ntrient Giderimi', Tarim evreliski leri Symposium. Mersin, Turkey, pp. 589-598.

[3] Mandi, L., 1994, Marakesh Wastewater Purification Experi-ment Using Vascular Aquatic Plants Eichhornia crassipes andLemna gibba, Wat. Sci. Tech. 29, 283-287.

[4] Davis, C.B. and Van Der Valk, A.G., 1983, 'Uptake and

Release of Nutrients by Liming and Decomposing Typhaglauca Godr.', Tissues at Eagle LaIce, Iowa. Aquat. Bot. 16,75-89.

[5] Drizo, A., Frost, C.A., Grace, J., 1997, 'Phosphate andAmmoniumby Constructed Wetlands with Horizontal Subsur-face Flow Using Shale as a Substrate', Wat. Sci. Tech. 35,95-102.

[6] Bolton, K.G.E. and Greenway, M., 1997, 'A Feasibility StudyofMelaleuca Trees for Use in Constructed Wetlands in Sub-tropieal Australia', Wat. Sci. Tech. 35, 247-254.

[7] Bonomo, L., Pastorelli, G. and Zambon, N., 1997, 'Advan-tages and Limitations of Duckweed-Based WastewaterTreatment Syystems', Wat. Sci. Tech.35, 239-"46.

[8] Hasar, H., Ipek, U., Obek, E. and Saatci, Y., 2000, Useof Duckweed (Lemna minorL.) in Advanced Treatment, SuKirliligi Kontrol Dergisi 10, 9-13.

[9] Sculthorpe, C.D., 1967, The Biology of Aquatic VascularPlants, Edward Arnold, London UK.

[10] Brix, H., 1993, 'Wastewater Treatment in ConstructedWetlands: System Design, Removal, Processes and TreatmentPerformance', Mashiri, G.A. (eds.), Constructed Wetlands forWater Ouality Improvement, Lewis Publishers, Boca Raton,Florida, USA, pp. 9-22 .

[11] Boniardi, N., Vatta, G., Rota, R., Nano, G. and Carra, S.,

1994, 'Removal of Water Pollutants by Lemna gibba',The Chem.Eng. 54, 41-48.

[12] Zirschy, J. and Reed, S.C., 1988, 'The use of Duckweed forWastewater Treatment',J. WPCF60, 1253-1258.

[13] Harvey, R.M. and Fox, J.L., 1973, 'Nutrient removal usingLemna minor', J. WPCF45, 1928-1938.

[14] APHA, 1992, Standard Methods for the Examination ofWater and Wastewater, 18th edn. Am. Publ. Health Assoc.,Washington D.C., USA.

Received for publication: October 23, 2001Accepted for publication: December 27, 2001

CORRESPONDING AUTHOR

Halil HasarFirat UniversityFaculty of EngineeringDepartment of Environmental Engineering23119-Elazig, TURKEY

e-mail: hhasar@firat.edu.tr

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