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PERFORMANCE OF DUCKWEED-COVERED SEWAGE

LAGOONSI. OXYGEN BALANCE AND COD REMOVAL

F. AL-NOZAILY1, 2*, G. ALAERTS1M and S. VEENSTRA1

1International Institute for Infrastructural, Hydraulic and Environmental Engineering, POB 3015,2601 DA, Delft, The Netherlands and 2Sana'a University, Engineering Faculty, Sana'a, Yemen

(First received 1 March 1999; accepted in revised form 1 January 2000)

AbstractLaboratory scale experiments were performed in a non-continuous batch reactor system with0.841.2 l domestic sewage exposed to constant light intensity, temperature and humidity. Thetreatment performance of duckweed (L. gibba )-covered sewage lagoons (DSL) was studied within aCODtotal range of 200500 mg/l (113294 mg CODlt/l), in 10, 30, 70 and 95 cm deep reactors, andliquid mixing intensity (power dissipation) of 0, 0.3, 1.0, 2.3 and 34.1 W/m3. The duration of eachexperiment was 20 days with biomass harvesting every 5 days. COD removal at extreme depths andextreme mixing intensities was compared with that in covered control reactors without duckweed.Removal of CODlt did not dier in duckweed-covered and control reactors. The role of duckweedcover was marginal in changing the redox potential or the DO. COD removal l

r

(kg CODlt

/ha 20days) correlated strongly with initial surface load ls (kg CODlt/ha). Concentration removal (as mgCODlt/l) was also proportional to initial CODlt concentration. For a given CODlt mass input,increasing depth up to 1 m aected DSL performance only by increasing surface load, and not byhampering oxygen transfer. Mixing (up to 2.3 W/m3) raised CODlt removal. Therefore, at depthsbeyond 70 cm, moderate mixing is recommended. The rst-order kinetic removal rate coecient forCODlt was 0.040.06 d

1. 7 2000 Elsevier Science Ltd. All rights reserved

Key wordsduckweed, Lemna, mixing, depth, kinetics, sewage lagoons, COD

INTRODUCTION

Aquatic plant-based wastewater treatment lagoons

are engineered systems in which aquatic plants in

association with bacteria can purify wastewater.

The oating aquatic plants with great potential

include duckweeds. Tackholm (1974) reported the

presence of Lemna sp., Spirodela sp. and Wola sp.

in the eastern Mediterranean and Middle East

region. L. minor (Landolt and Kandeler, 1987) and

L. gibba (own observation) are the only species

encountered in Yemen (the area of our interest) so

far.

Duckweed-covered sewage lagoons (DSL) have

been studied under laboratory conditions (Oron et

al., 1984; Reed et al., 1987; Zirschky and Reed,

1988; Vroon and Weller, 1995). Their long-termperformance under full-scale eld conditions has

been documented only for Mirzapur, Bangladesh

(PRISM, 1992; Alaerts et al ., 1996). The DSL

removes organic matter primarily through aerobic

heterotrophic oxidation. For this it needs the active

diusion or transportation of oxygen into the liquid

phase. Rao (1986) suggested that aquatic weeds act

as a ``biolter'' by providing attachment opportu-

nities for aerobic heterotrophic bacteria.

Dissolved oxygen (DO) transfer is inuenced byreactor depth, time of the day, and the degree of

wind-induced turbulence of the water surface (Mor-

ris and Barker, 1977). In duckweed lagoons (0.5

1.5 m) re-aeration through the surface might be

obstructed by the duckweed mat (O'Brien, 1981;

Zirschky and Reed, 1988). The full scale DSL was

reported to have a fairly constant high DO of 24

mgO2/l along the whole length of the pond

suggesting adequate re-aeration, which in this case

might have been caused by the low BOD concen-

tration of approximately 100 mg/l at the inlet

(Alaerts et al., 1996).

The optimal depth of a DSL has to be related tothe ratio of the oxygen consuming wastewater

volume to the duckweed-covered surface area. The

latter determines the O2 ux into the wastewater,

and may thus enhance COD removal in the water

column. The vertical transport uxes of oxygen and

nutrients in the water column and the volume-to-

surface area ratio determine the maximal depth that

can be applied. In general, despite available chemi-

cal and microbiological information, knowledge is

still missing on the design and optimal operational

conditions. Oron et al. (1988) suggested that COD

Wat. Res. Vol. 34, No. 10, pp. 27272733, 20007 2000 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0043-1354/00/$ - see front matter

2727

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PII: S0043-1354(00)00003-8

*Author to whom all correspondence should be addressed.Fax: +31-15-2122921; e-mail: [email protected]

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reduction increases with decreasing depth from 30

20 cm. Vroon and Weller (1995) reported no eect

of depth on COD removal at all in the range of 15

60 cm.

The aim of this study is to assess the feasibility of

DSL for sustainable sanitation in developing

countries, in particular the Republic of Yemen. The

eect of depth and mixing intensity on the COD

and nutrient removal of the DSL for dierent sew-

age concentrations is investigated in order to pro-

vide more clarity on design and operational

parameters of DSL. This paper discusses (1) the

eect of depth (up to 1 m), mixing and sewage con-

centration on COD removal, and (2) the levels of

DO and pH in the DSL. Nutrient removal is dis-

cussed in Part II.

MATERIALS AND METHODS

Experiments were performed in a non-continuous batchreactor system with settled domestic wastewater in vesselsof 0.841.2 l with dierent depth, mixing intensity and

wastewater strength. Two groups of experiments were con-ducted (Table 1). Group 1 experiments allowed rationaliz-ing the number and design of the Group 2 experiments, inview of the long duration of each experiment. Group 1experiments used medium-strength settled sewage withCODtotalI 250 mg/l and studied system performance fortwo extreme depths (10 and 95 cm) and mixing intensity(G = 0 and 58 s1 [34.1 W/m3]). Results suggested thatdepth does not aect COD removal; it only alters the in-itial CODtotal surface load ls,t (approx. 250 [at 10 cm] and2400 kg/ha [at 95 cm]) (with variable CODtotal massinput). In Group 2 experiments, depths of 30, 70 and 95were chosen to dierentiate between the inuence of initialCODtotal concentrations (200500 mg/l) and initial ls,t(approx. 14004400 kg CODtotal/ha). In experiments 2/1and 2/2 comparable surface loadings were applied but at a

dierent COD concentration. The range of power dissipa-tion between 0.75 and 3.0 W/m3 for aerated pond design,enabling solids re-suspension (Arceivala, 1998) was trans-lated into a velocity gradient G (s1) and consequentlyinto a degree of turbulence (Appendix A) (Camp andStein, 1943). CODlt was typically used as a parameter toquantify organic matter, as it is less dependent on variableand less relevant amounts of particulate matter. The

CODlt/BOD5,total ratio of settled sewage was 1.3.CODtotal /BOD5,total was typically 1.6 for settled inuentand 2 for euent.

To study in Group 1 the role of the duckweed in O2supply, triplicate reactors with and without duckweed(control) were used in parallel. Control reactors were cov-ered with non-transparent plastic sheets to simulate duck-weed mat. pH and DO were measured regularly and algalgrowth was monitored visually. In Group 2, no control

reactors were used. Duplicates were found to be statisti-cally suciently precise (se averaging 03% both in thecase of duplicates and triplicates).

Municipal raw wastewater (collected from Berkel-Rodenrijs or TNO Delft treatment plants, both in TheNetherlands) was settled for 24 h to remove settleablematter. L. gibba was chosen because it is available in theMiddle East and Yemen, and is most resilient to waste-water conditions (Al-Nozaily and Alaerts, in prep.).Clones were collected from Delft city canals and placed inwastewater for two weeks prior to the experiments toallow them to acclimatize. Healthy clones were selectedand used to create full cover in the duckweed reactors.

Depending on the situation, mixing occurred by mag-netic stirring or rotating paddle. Energy input was calcu-lated according to equation (A2) (Appendix A). In theGroup 2 experiments, a rotating paddle was used based

on work by Coulson and Richardson (1987). Disturbancesof the duckweed cover were prevented by surrounding therod on the surface with an immobile PVC ring. Verticalbaes attached to the reactor wall halted the rotation ofthe duckweed mat.

To suppress algal growth, duckweed was rinsedthoroughly after each harvest before reintroduction intothe reactor. In addition, 2 mgCuSO4/l was added as rec-ommended by Edwards et al. (1992). This Cu concen-tration can be tolerated by heterotrophs (Bolton andKlein, 1961), while the relative growth rate (RGR) of L.gibba was found to be not noticeably aected. Light wasmaintained at 1302 10 mE/m2 s using HPIT 400W Hglamps under a time regime of 18/6 h on/o. Ambient andwater temperatures were 212 48C and 19248C, respect-ively, while humidity was 40210%.

Liquid samples were collected at 5, 30 and 65 cm depthevery 5 days. Loading rates and ``inuent'' concentrationspertain to the initial reactor content. ``Euent'' valuespertain to the liquid in the reactor at the end of the exper-iment. Duckweed was harvested every 5 days by nettingthe biomass, rinsing it with tap water and drying the bio-mass by paper tissue (wet weight). Thereafter the initialstock density (5002 100 g wet wt/m2) was restored in

Table 1. Overview of experimental conditions in Groups 1 and 2a

Exp. no. Depth(cm)

Volume(l)

Surface area(m

2)

E(W/m

3)

G(s

1)

CODtotal(mg/l)

ls,t(kg CODtotal/ha)

CODlt(mg/l)

Mass input(gCODlt)

ls(kg CODlt/ha)

1/1 10 0.8 0.008 0 0 I 250 I 250 13826 0.11 138261/2 95 41.2 0.043 0 0 I 250 I 2400 167215 6.88 15872143

1/3 34.1 58 15723 6.47 1492229

2/1/1 30 13.0 0.043 0 0 I 500 I 1400 29428 3.82 8822242/1/2 0.3 52/1/3 1.0 102/1/4 2.3 15

2/2/1 70 30.0 0.043 0 0 I 200 I 1400 11328 3.39 7912562/2/2 0.3 52/2/3 1.0 102/2/4 2.3 15

2/3/1 95 41.2 0.043 0 0 I 500 I 4400 23028 9.48 21852762/3/2 1.0 10

aWastewater characteristics and COD surface loading ls and ls,t at the beginning of the experiments. Average2se.

F. Al-Nozaily et al.2728

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every reactor. The increment from each replicate wasmixed, and 510 g wet wt was taken for further analysis.Evapotranspiration was measured and compensated byadding tap water daily. Temperature, redox potential, DOand pH were measured every 23 days, and light intensitywas checked every 5 days.

BOD5 was analyzed using the Winkler method. TheCOD analysis was based on the closed reux technique(acid destruction at 1508C for 2 h) and colorimetry at

600 nm (Perkin Elmer 550 S, US) (APHA, 1992). Filteredsamples were prepared by ltering over GF/C (1.2 mmpores) glass ber lter paper. The DO was measured byWTW OX 196 (Germany). Redox potential was measuredby platinum electrode AG 9100 from Metrohm Herisau(Switzerland). The light intensity was measured at the sur-face level of the experimental reactors by using a LI-CORradiation sensor, type SB (UWQ 4681, Campbell ScienticLtd, UK). Air humidity was measured daily with a hy-grometer.

SPSS software was used for statistical analysis. Com-parisons among mean values were made by analysis ofvariance (one-way ANOVA), signicant ANOVAs werefollowed by mean comparisons using Tuky's honestly sig-nicant dierence test. Statistical analyses are reported assignicant when P 0.05.

RESULTS

The CODlt concentration varied between 113

and 294 mg/l with a loading rate of ls 1382185 kg

CODlt/ha (Table 1). No stratication occurred in

the non-mixed reactors. The duckweed cover mar-

ginally inuenced redox potential and DO at an in-

itial ls ofr1492 kg CODlt/ha. At 95 cm with

a high ls of 1492 (experiment 1/3) and 1587 kg

CODlt/ha (experiment 1/2), the redox potential

after 20 days was 24827 mV and 30922 mV

in duckweed-covered reactors, and 2932 16 mV

and 3062 2 mV in control reactors. Statistical

analysis did not reject the hypothesis that the redoxpotential in the deeper duckweed and control reac-

tors was equal. However, in shallow reactors of

10 cm depth with low ls of 138 kg CODlt/ha (ex-

periment 1/1), DO after 20 days was 3.9 2 0.1

mgO2/l in duckweed-covered reactors and 0.720.1

mgO2/l in control reactors, a statistically signicant

dierence.

The initial ls and the degree of mixing, aected

the amount of oxygen that diused into the reactor

(Group 2 experiments, Table 2). A higher nal DO

correlated with lower ls and intensied mixing.

The pH in the water column of duckweed and

control reactors ranged continuously between 7 and

8. There was no day/night rhythm, nor any statisti-

cally signicant dierence. Due to the control of

algal growth, only marginal amounts of Cyanobac-

terium Chroococcus (NERC, 1978) were found

attached to the duckweed.

The CODlt removal rate lr over 20 days in 10 cm

deep duckweed-covered and control reactors was 86

22 and 9123kg/ha 20 days, respectively (Table 3).

In 95 cm deep reactors, removal in duckweed and

control reactors lr was 8932 76 and 8172 57 kg

CODlt/ha 20 days, respectively. These results are

statistically speaking equal, which suggests that the

presence of a duckweed mat did not aect CODltremoval, in spite of the presence of DO in duck-

weed reactors compared to control.

The removal loading lr (as kg CODlt/ha 20

days) is linearly proportional to ls, while the per-

centual removal ZCOD varied between 51 and 82%

irrespective of the initial ls (Fig. 1). At 30 and

70 cm depth, E= 0, and similar ls of 882 and 791

kg CODlt/ha (experiments 2/1/1 and 2/2/1,Table 3), lr was 633 and 511 kg CODlt/ha 20 days,

a statistically not signicant dierence. On the

other hand, for initial ls of 2185 kg/ha, at depth of

95 cm and E = 0 (experiments 2/1/1 and 2/3/1), lrwas 1235 kg/ha 20 days, which is a statistically sig-

nicant dierence.

Concentration removal (as mg CODlt/l 20 days)

as a function of dierent reactor depth showed no

statistically signicant dierence (experiments 1/1

and 1/2, Table 3). This conrmed that reactor

depth, as separate variable did not aect the

CODlt concentration removal rate in duckweed-

covered reactors within the range of 10100 cm.Concentration removal increased linearly with

increasing inuent concentration (Fig. 2). Three dis-

tinct sets of CODlt concentration removal values

were statistically signicantly dierent. At initial

CODlt of 113167, 230, and 294 mg/l, the CODltremoval ranged 8124, 14224 and 22825 mg/l 20

days, respectively. The CODlt removal followed

rst-order kinetics with respect to time. The rate

coecient (k ) increased signicantly with increasing

CODlt concentrations from 0.04 to 0.06 d1 (Fig

3). This suggests that concentration, and thus load-

ing determined the rate of COD removal.

Table 2. DO of Group 2 experiments, after 20 daysa

Exp. no. ls (kg CODlt/ha) DO (mg/l)

E=0 E=0.3 E=1.0 E=2.3

2/2 791 1.120.2 a 3.320.9 m 6.220.7 o 5.520.2 o2/1 882 0.220.1 b 2.320.2 n 3.320.8 n 4.220.7 p2/3 2185 0.020.0 b 0.020.0 n

aAverage2se. Dierent letters indicate signicant dierence of the value at P 0.05. Signicance was tested among depths at E=0 (a,

b), and among the combinations of depth and mixing for E > 0 (m, n, o, p).

Performance of duckweed-covered sewage lagoonsI 2729

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CODlt removal increased with mixing intensity

within the range of 02.3 W/m3 by approximately

10% (Table 3). Intensive mixing (34.1 W/m3) did

not signicantly improve the CODlt removal rate

at a reactor depth of 95 cm (Table 3).

DISCUSSION

Depth variation in the range of 10100 cm did

not aect COD removal in DSL. Similar con-

clusions were reported by others albeit within much

smaller depth ranges (Oron, 1988; Vroon and

Table 3. CODlt removal rate in duckweed-covered and control reactors at dierent depth and with dierent mixing intensity in Group 1and 2 experiments

a

Exp. no. E (W/m3) Depth (cm) ls (kg CODlt/ha) CODlt removal

(mg/l 20 days) lr (kg/ha 20 days) ZCOD (%)

1/1D 0 10 13826 8622 a 8622 a 62C 9123 a 9123 a 66

1/2D 0 95 15872143 9428 a 893276 b 56C 8626 a 817257 b 51

1/3D 34.1 95 1492229 8324 a 789238 b 53C 83211 a 7892104 b 53

2/1/1 0 30 882224 211221 c,d 633224 d 722/1/2 0.3 23429 d 702224 e,f 802/1/3 1.0 234224 d 684224 e 782/1/4 2.3 24024 d 720224 f 82

2/2/1 0 70 791256 7323 a 511256 g 652/2/2 0.3 8626 b 602256 h 762/2/3 1.0 8022 b 560256 i 712/2/4 2.3 8228 b 574256 i 73

2/3/1 0 95 2185276 13024 a,b 1235276 j 572/3/2 1.0 147214 b,c 1397276 k 64

aAverages 2se. Dierent letters indicate signicant dierence at P 0.05. CODlt loading ls at the beginning of the experiments.

D=duckweed reactor, C=control reactors.

Fig. 1. CODlt removal lr (as kg/ha 20 days) and ZCOD (as %) as a function of CODlt initial loadinglS in duckweed reactors, and corresponding reactor depth (cm). Dotted lines indicate the range of %

removal.


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Weller, 1995). The higher DO in duckweed-covered

than in control reactors has been attributed by

others to translocation of oxygen in the plants

through their roots (Vroon and Weller, 1995; Ko r-

ner et al., 1998). However, as duckweed lowers

NH+4 N levels below 2 mg N/l so that nitrication

exerts a lower oxygen demand, and as substantial

nitrication occurred only in the control reactor

(see Part II), it is more likely that the lower demand

in the duckweed reactor caused the higher DO.

Also, given the high volume-to-surface/mat ratio

applied here (at >10 cm depth), it is unlikely that

oxygen translocation plays an important role. Else-

where, DO has been reported to range between 2

and 4 mgO2/l in a low loaded full-scale (80100 kg

COD/ha days after settling) DSL (Alaerts et al.,

1996) with a depth of 0.40.9 m, and with continu-

ing re-aeration by wind and natural mixing. Vroon

Fig. 2. CODlt concentration removal (as mg/l 20 days) as a function of CODlt initial concentration induckweed reactors, and corresponding reactor depth (cm). Average of all experiments2se.

Fig. 3. CODlt removal rate as a function of experiment duration time (d) in duckweed and controlreactors at the three distinct sets of 294, 230 and 113167 mg CODlt/l (average of all experiments

2se).


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and Weller (1995) reported DO of 0 mg/l. In reactors with ls < 800 kg CODlt/

ha, oxygen consuming substances are most likely to

have been fully oxidized by the end of the exper-

imental period.

The pH in all reactors ranged between 7 and 8.

This fact and the equally stable DO conrmed the

absence of pronounced algal photosynthetic ac-

tivity. The observed small quantities of algae did

not signicantly interfere. Vroon and Weller (1995),

Alaerts et al. (1996) and Ko rner et al. (1998) also

reported similar pH ranges, as well as the absence

of a diurnal prole or stratication.

lr was not statistically signicantly dierentbetween duckweed-covered and control reactors.

This suggests that the duckweed cover did not play

a role in the CODlt removal. Attachment of het-

erotrophic biomass to the plant roots was less sig-

nicant at high depths given the high volume-to-

surface ratio. The ux of oxygen into the liquid is

the deciding and rate limiting factor for COD

removal processes.

CODlt removal rates were inuenced primarily

by initial CODlt concentration, as well as by ls.

Higher CODlt concentration probably stimulates

heterotrophic bacterial metabolization according to

rst-order kinetics. The ls, which depends on con-

centration and depth, determines the overall oxygen

demand exerted. At ls > 800 kg CODlt/ha oxygen

tends to become rate limiting, though this did not

cause detectable deviations in the linear correlation

between the removal rate and the initial BODltconcentration.

The CODlt removal eciency ZCOD (after 20

days) was comparatively low at 5070% when com-

pared with conventional algae-based sewage

lagoons and the full-scale DSL (Alaerts et al .,

1996). This can be attributed partly due to an in-

adequate supply of oxygen, and to the fact that

removal performance in this study was calculated

against CODlt in the inuent. If removal resultswere adjusted to CODtotal, overall removal rates

would increase to 8090% and above 90% when

calculated on BOD5,total.

Oron et al. (1988) studied COD removal in DSL

systems of 20 and 30 cm using a comparable settled

wastewater with initial CODtotal of 318 mg/l, which

corresponded to a ls,t of 636 and 954 kg/ha, re-

spectively. Removal rate on CODtotal lr,t was 328

and 402 kg/ha 10 days. Vroon and Weller (1995)

conducted outdoor batch experiments at depths of

15, 30, 45 and 60 cm with settled wastewater at

CODtotal of 252, 241, 236 and 235 mg/l, which cor-

responded to ls,t of 378, 723, 1062 and 1410 kg/ha.

They achieved a lr,t of 280, 432, 450 and 460 kg/ha

11 days at a water temperature of 16208C, which

is signicantly lower than in our experiments.

Mandi (1994) in continuous outdoor experiments at

14 cm depth in summer in Marrakesh, Morocco,

using raw wastewater with ls,t of 427 and 622 kg

CODtotal/ha achieved lr,t of 270 and 324 kg/ha 7

days (water temperature was not measured during

the experiments).

Other researchers reported higher results. This

could be attributed to higher water temperatures or

longer retention times. A pilot plant (Italy) with ls,tof 7268 kg CODtotal/ha (316 mg/l) of settled waste-

water and a depth of 2.3 m yielded a lr,t of 5796

kg/ha 16 days in summer at water temperature of

158C, but of only 3634 kg/ha 15 days in winter at a

water temperature of 5108C (Bonomo et al., 1997).

Alaerts et al. (1996) in a full-scale DSL of 0.51 m

depth reported a lr,t of 1399 kg/ha 20 days at an in-

itial ls,t of 1788 kg CODtotal/ha. All previous stu-dies used settled municipal sewage to exclude

comparatively large variable amounts of particu-

lates. Ko rner et al. (1998), using non-settled munici-

pal wastewater at 208C, reported much higher lr,tof 38, 81 and 202 kg/ha 3 days at ls,t of 49, 104

and 251 kg CODtotal/ha, respectively. These are less

representative of eld conditions because of the pre-

sence of settleable matter and the very shallow

water depth of 3.3 cm.

The reaction constant for CODtotal removal in

1 m deep facultative ponds with an inuent of

504 mg CODtotal/l non-settled sewage at HRT of

16.8 days in Portugal is 0.35 d1 (Gomes de Sousa,

1987), this is much higher than the k of 0.04

0.06 d1 found here. This dierence can be attribu-

ted partly to the fact that our results are based on

CODlt. In addition, the kinetics appear fairly inde-

pendent of the dierences in oxygen transfer (aera-

tion) associated with shallow to deep reactors (10

100 cm), in DSL as well as in algae-based systems.

This suggests that under our experimental con-

ditions oxygen transfer could be a rate limiting fac-

tor.

CONCLUSIONS

A mediumdeep to deep DSL system (50150 cm)essentially functions as a facultative lagoon with

respect to COD removal. The role of duckweed is

marginal in removing COD from wastewater.

DSL depth in the range of 10100 cm had no

other eect than that it increased the surface load-

ing ls (as kg CODlt /ha) (by increasing the CODltmass input, in the experimental set-up). Surface-re-

lated processes, notably oxygen transfer, were not

strongly aected.

A linear relationship was established between

applied and removal CODlt surface loading. The


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best tting linear equation is lr=0.53 ls+66 with

r 2=0.98. Another linear relationship was also

established between applied and removal CODltconcentration. The best tting linear equation is:

CODlt removal=0.82 (CODlt initial)25, with

r 2=0.93.

The concentration removal rates depended on in-

itial surface loading ls, which in turn depends on

initial CODlt concentration. This conrms that

CODlt removal is largely determined by volume-re-

lated microbial processes and not by surface-related

duckweed uptake or oxygen uxes.

ls determined the overall oxygen demand; at in-

itial ls > 800 kg CODlt/ha the oxygen supply

might become rate limiting. The CODlt removal

rate followed rst-order kinetics, with a reaction

constant k of 0.040.06 d1.CODlt surface loading

removal lr was in the range of 86 to 1397 kg/ha 20

days at the ls of 138 and 2185 kg/ha, respectively.

Euent quality ranged between 27 and 100 mg

CODlt/l, equivalent to 1763 mg BOD5,total/l.

A depth of up to 11.5 m is not likely to limitadequate DSL performance with respect to COD

degradation. Thus, depth of DSL should be chosen

as a function of inuent water quality and desired

euent quality. Mixing (up to 2.3 W/m3) h a s a

moderately benecial eect by raising DO, favoring

COD removal up to 10%.

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APPENDIX A. QUANTIFICATION OF DISSIPATEDPOWER

Mechanical mixing can be quantied by determining thevelocity gradient G (Camp and Stein, 1943):

G

E

Vm

sA1

where: G is the velocity gradient (s1); E is the dissipatedpower (W/m3); m is the kinematic water viscosity (kg s/m2); V is the liquid volume (m3).

If a rotating object is used for mixing, G can be relatedto the rotor's rotational velocity:

G

1a2CdrA1X5prn

3

Vm

sA2

where: Cd is the drag coecient (I1.4 under turbulentcondition); r is the density of water (1000 kg/m3); A is theprojected area of the object (m2); r is the tip radius of theobject (m); n is the mixing speed of the object (rps).


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