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Performance of duckweed-covered sewage lagoons—I. Oxygen balance and COD removal
PII: S0043-1354(00)00003-8
Wat. Res. Vol. 34, No. 10, pp. 2727±2733, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
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PERFORMANCE OF DUCKWEED-COVERED SEWAGE LAGOONSÐI. OXYGEN BALANCE AND COD REMOVAL F. AL-NOZAILY1, 2*, G. ALAERTS1M and S. VEENSTRA1 1
International 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)
AbstractÐLaboratory scale experiments were performed in a non-continuous batch reactor system with 0.8±41.2 l domestic sewage exposed to constant light intensity, temperature and humidity. The treatment performance of duckweed (L. gibba )-covered sewage lagoons (DSL) was studied within a CODtotal range of 200±500 mg/l (113±294 mg COD®lt/l), in 10, 30, 70 and 95 cm deep reactors, and liquid mixing intensity (power dissipation) of 0, 0.3, 1.0, 2.3 and 34.1 W/m3. The duration of each experiment was 20 days with biomass harvesting every 5 days. COD removal at extreme depths and extreme mixing intensities was compared with that in covered control reactors without duckweed. Removal of COD®lt did not dier in duckweed-covered and control reactors. The role of duckweed cover was marginal in changing the redox potential or the DO. COD removal lr (kg COD®lt/ha 20 days) correlated strongly with initial surface load ls (kg COD®lt/ha). Concentration removal (as mg COD®lt/l) was also proportional to initial COD®lt concentration. For a given COD®lt mass input, increasing depth up to 1 m aected DSL performance only by increasing surface load, and not by hampering oxygen transfer. Mixing (up to 2.3 W/m3) raised COD®lt removal. Therefore, at depths beyond 70 cm, moderate mixing is recommended. The ®rst-order kinetic removal rate coecient for COD®lt was 0.04±0.06 dÿ1. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐduckweed, 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-term performance 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 *Author to whom all correspondence should be addressed. Fax: +31-15-2122921; e-mail: [email protected]
as a ``bio®lter'' by providing attachment opportunities for aerobic heterotrophic bacteria. Dissolved oxygen (DO) transfer is in¯uenced by reactor depth, time of the day, and the degree of wind-induced turbulence of the water surface (Morris 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 2±4 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 concentration of approximately 100 mg/l at the inlet (Alaerts et al., 1996). The optimal depth of a DSL has to be related to the 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-tosurface area ratio determine the maximal depth that can be applied. In general, despite available chemical and microbiological information, knowledge is still missing on the design and optimal operational conditions. Oron et al. (1988) suggested that COD
2727
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F. Al-Nozaily et al.
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 sewage concentrations is investigated in order to provide more clarity on design and operational parameters of DSL. This paper discusses (1) the eect of depth (up to 1 m), mixing and sewage concentration on COD removal, and (2) the levels of DO and pH in the DSL. Nutrient removal is discussed in Part II. MATERIALS AND METHODS
Experiments were performed in a non-continuous batch reactor system with settled domestic wastewater in vessels of 0.8±41.2 l with dierent depth, mixing intensity and wastewater strength. Two groups of experiments were conducted (Table 1). Group 1 experiments allowed rationalizing the number and design of the Group 2 experiments, in view of the long duration of each experiment. Group 1 experiments used medium-strength settled sewage with CODtotal 1 250 mg/l and studied system performance for two extreme depths (10 and 95 cm) and mixing intensity (G = 0 and 58 sÿ1 [34.1 W/m3]). Results suggested that depth does not aect COD removal; it only alters the initial CODtotal surface load ls,t (approx. 250 [at 10 cm] and 2400 kg/ha [at 95 cm]) (with variable CODtotal mass input). In Group 2 experiments, depths of 30, 70 and 95 were chosen to dierentiate between the in¯uence of initial CODtotal concentrations (200±500 mg/l) and initial ls,t (approx. 1400±4400 kg CODtotal/ha). In experiments 2/1 and 2/2 comparable surface loadings were applied but at a dierent COD concentration. The range of power dissipation between 0.75 and 3.0 W/m3 for aerated pond design, enabling solids re-suspension (Arceivala, 1998) was translated into a velocity gradient G (sÿ1) and consequently into a degree of turbulence (Appendix A) (Camp and Stein, 1943). COD®lt was typically used as a parameter to quantify organic matter, as it is less dependent on variable and less relevant amounts of particulate matter. The
COD®lt/BOD5,total ratio of settled sewage was 1.3. CODtotal /BOD5,total was typically 1.6 for settled in¯uent and 2 for euent. To study in Group 1 the role of the duckweed in O2 supply, triplicate reactors with and without duckweed (control) were used in parallel. Control reactors were covered with non-transparent plastic sheets to simulate duckweed mat. pH and DO were measured regularly and algal growth was monitored visually. In Group 2, no control reactors were used. Duplicates were found to be statistically suciently precise (se averaging 0±3% both in the case of duplicates and triplicates). Municipal raw wastewater (collected from BerkelRodenrijs or TNO Delft treatment plants, both in The Netherlands) was settled for 24 h to remove settleable matter. L. gibba was chosen because it is available in the Middle East and Yemen, and is most resilient to wastewater conditions (Al-Nozaily and Alaerts, in prep.). Clones were collected from Delft city canals and placed in wastewater for two weeks prior to the experiments to allow them to acclimatize. Healthy clones were selected and used to create full cover in the duckweed reactors. Depending on the situation, mixing occurred by magnetic stirring or rotating paddle. Energy input was calculated according to equation (A2) (Appendix A). In the Group 2 experiments, a rotating paddle was used based on work by Coulson and Richardson (1987). Disturbances of the duckweed cover were prevented by surrounding the rod on the surface with an immobile PVC ring. Vertical baes attached to the reactor wall halted the rotation of the duckweed mat. To suppress algal growth, duckweed was rinsed thoroughly after each harvest before reintroduction into the reactor. In addition, 2 mgCuSO4/l was added as recommended by Edwards et al. (1992). This Cu concentration can be tolerated by heterotrophs (Bolton and Klein, 1961), while the relative growth rate (RGR) of L. gibba was found to be not noticeably aected. Light was maintained at 130 2 10 mE/m2 s using HPIT 400W Hg lamps under a time regime of 18/6 h on/o. Ambient and water temperatures were 21 2 48C and 19±248C, respectively, while humidity was 40210%. Liquid samples were collected at 5, 30 and 65 cm depth every 5 days. Loading rates and ``in¯uent'' concentrations pertain to the initial reactor content. ``Euent'' values pertain to the liquid in the reactor at the end of the experiment. Duckweed was harvested every 5 days by netting the biomass, rinsing it with tap water and drying the biomass by paper tissue (wet weight). Thereafter the initial stock density (500 2 100 g wet wt/m2) was restored in
Table 1. Overview of experimental conditions in Groups 1 and 2a Depth (cm)
Volume (l)
Surface area (m2)
E (W/m3)
G (sÿ1)
CODtotal (mg/l)
ls,t (kg CODtotal/ha)
COD®lt (mg/l)
Mass input (gCOD®lt)
1/1 1/2 1/3
10 95
0.8 41.2
0.008 0.043
0 0 34.1
0 0 58
1 250 1 250
1 250 1 2400
13826 167215 15723
0.11 6.88 6.47
13826 15872143 1492229
2/1/1 2/1/2 2/1/3 2/1/4
30
13.0
0.043
0 0.3 1.0 2.3
0 5 10 15
1 500
1 1400
29428
3.82
882224
2/2/1 2/2/2 2/2/3 2/2/4
70
30.0
0.043
0 0.3 1.0 2.3
0 5 10 15
1 200
1 1400
11328
3.39
791256
2/3/1 2/3/2
95
41.2
0.043
0 1.0
0 10
1 500
1 4400
23028
9.48
2185276
Exp. no.
a
Wastewater characteristics and COD surface loading ls and ls,t at the beginning of the experiments. Average 2se.
ls (kg COD®lt/ha)
Performance of duckweed-covered sewage lagoonsÐI every reactor. The increment from each replicate was mixed, and 5±10 g wet wt was taken for further analysis. Evapotranspiration was measured and compensated by adding tap water daily. Temperature, redox potential, DO and pH were measured every 2±3 days, and light intensity was checked every 5 days. BOD5 was analyzed using the Winkler method. The COD analysis was based on the closed re¯ux technique (acid destruction at 1508C for 2 h) and colorimetry at 600 nm (Perkin Elmer 550 S, US) (APHA, 1992). Filtered samples were prepared by ®ltering over GF/C (1.2 mm pores) glass ®ber ®lter paper. The DO was measured by WTW OX 196 (Germany). Redox potential was measured by platinum electrode AG 9100 from Metrohm Herisau (Switzerland). The light intensity was measured at the surface level of the experimental reactors by using a LI-COR radiation sensor, type SB (UWQ 4681, Campbell Scienti®c Ltd, UK). Air humidity was measured daily with a hygrometer. SPSS software was used for statistical analysis. Comparisons among mean values were made by analysis of variance (one-way ANOVA), signi®cant ANOVAs were followed by mean comparisons using Tuky's honestly signi®cant dierence test. Statistical analyses are reported as signi®cant when P R 0.05. RESULTS
The COD®lt concentration varied between 113 and 294 mg/l with a loading rate of ls 138±2185 kg COD®lt/ha (Table 1). No strati®cation occurred in the non-mixed reactors. The duckweed cover marginally in¯uenced redox potential and DO at an initial ls of r1492 kg COD®lt/ha. At 95 cm with a high ls of 1492 (experiment 1/3) and 1587 kg COD®lt/ha (experiment 1/2), the redox potential after 20 days was ÿ248 2 7 mV and ÿ309 2 2 mV in duckweed-covered reactors, and ÿ293 2 16 mV and ÿ306 2 2 mV in control reactors. Statistical analysis did not reject the hypothesis that the redox potential in the deeper duckweed and control reactors was equal. However, in shallow reactors of 10 cm depth with low ls of 138 kg COD®lt/ha (experiment 1/1), DO after 20 days was 3.9 2 0.1 mgO2/l in duckweed-covered reactors and 0.7 2 0.1 mgO2/l in control reactors, a statistically signi®cant 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 intensi®ed mixing. The pH in the water column of duckweed and
2729
control reactors ranged continuously between 7 and 8. There was no day/night rhythm, nor any statistically signi®cant dierence. Due to the control of algal growth, only marginal amounts of Cyanobacterium Chroococcus (NERC, 1978) were found attached to the duckweed. The COD®lt 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 893 2 76 and 817 2 57 kg COD®lt/ha 20 days, respectively. These results are statistically speaking equal, which suggests that the presence of a duckweed mat did not aect COD®lt removal, in spite of the presence of DO in duckweed reactors compared to control. The removal loading lr (as kg COD®lt/ha 20 days) is linearly proportional to ls, while the percentual 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 COD®lt/ha (experiments 2/1/1 and 2/2/1, Table 3), lr was 633 and 511 kg COD®lt/ha 20 days, a statistically not signi®cant 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), lr was 1235 kg/ha 20 days, which is a statistically signi®cant dierence. Concentration removal (as mg COD®lt/l 20 days) as a function of dierent reactor depth showed no statistically signi®cant dierence (experiments 1/1 and 1/2, Table 3). This con®rmed that reactor depth, as separate variable did not aect the COD®lt concentration removal rate in duckweedcovered reactors within the range of 10±100 cm. Concentration removal increased linearly with increasing in¯uent concentration (Fig. 2). Three distinct sets of COD®lt concentration removal values were statistically signi®cantly dierent. At initial COD®lt of 113±167, 230, and 294 mg/l, the COD®lt removal ranged 8124, 14224 and 22825 mg/l 20 days, respectively. The COD®lt removal followed ®rst-order kinetics with respect to time. The rate coecient (k ) increased signi®cantly with increasing COD®lt concentrations from 0.04 to 0.06 dÿ1 (Fig 3). This suggests that concentration, and thus loading determined the rate of COD removal.
Table 2. DO of Group 2 experiments, after 20 daysa Exp. no.
2/2 2/1 2/3 a
ls (kg COD®lt/ha)
791 882 2185
DO (mg/l) E=0
E=0.3
E=1.0
E=2.3
1.120.2 a 0.220.1 b 0.020.0 b
3.320.9 m 2.320.2 n ±
6.220.7 o 3.320.8 n 0.020.0 n
5.520.2 o 4.220.7 p ±
Average 2se. Dierent letters indicate signi®cant dierence of the value at P R 0.05. Signi®cance was tested among depths at E=0 (a, b), and among the combinations of depth and mixing for E > 0 (m, n, o, p).
2730
F. Al-Nozaily et al.
Table 3. COD®lt removal rate in duckweed-covered and control reactors at dierent depth and with dierent mixing intensity in Group 1 and 2 experimentsa Exp. no.
E (W/m3)
Depth (cm)
ls (kg COD®lt/ha)
COD®lt removal (mg/l 20 days)
lr (kg/ha 20 days)
ZCOD (%)
1/1 D C
0
10
13826
8622 a 9123 a
8622 a 9123 a
62 66
1/2 D C
0
95
15872143
9428 a 8626 a
893276 b 817257 b
56 51
1/3 D C
34.1
95
1492229
8324 a 83211 a
789238 b 7892104 b
53 53
2/1/1 2/1/2 2/1/3 2/1/4
0 0.3 1.0 2.3
30
882224
211221 c,d 23429 d 234224 d 24024 d
633224 702224 684224 720224
d e,f e f
72 80 78 82
2/2/1 2/2/2 2/2/3 2/2/4
0 0.3 1.0 2.3
70
791256
511256 602256 560256 574256
g h i i
65 76 71 73
2/3/1 2/3/2
0 1.0
95
2185276
1235276 j 1397276 k
57 64
7323 8626 8022 8228
a b b b
13024 a,b 147214 b,c
a
Averages 2se. Dierent letters indicate signi®cant dierence at P R 0.05. COD®lt loading ls at the beginning of the experiments. D=duckweed reactor, C=control reactors.
COD®lt removal increased with mixing intensity within the range of 0±2.3 W/m3 by approximately 10% (Table 3). Intensive mixing (34.1 W/m3) did not signi®cantly improve the COD®lt removal rate at a reactor depth of 95 cm (Table 3).
DISCUSSION
Depth variation in the range of 10±100 cm did not aect COD removal in DSL. Similar conclusions were reported by others albeit within much smaller depth ranges (Oron, 1988; Vroon and
Fig. 1. COD®lt removal lr (as kg/ha 20 days) and ZCOD (as %) as a function of COD®lt initial loading lS in duckweed reactors, and corresponding reactor depth (cm). Dotted lines indicate the range of % removal.
Performance of duckweed-covered sewage lagoonsÐI
2731
Fig. 2. COD®lt concentration removal (as mg/l 20 days) as a function of COD®lt initial concentration in duckweed reactors, and corresponding reactor depth (cm). Average of all experiments 2se.
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; KoÈrner et al., 1998). However, as duckweed lowers NH+ 4 ±N levels below 2 mg N/l so that nitri®cation exerts a lower oxygen demand, and as substantial nitri®cation 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. Elsewhere, DO has been reported to range between 2 and 4 mgO2/l in a low loaded full-scale (80±100 kg COD/ha days after settling) DSL (Alaerts et al., 1996) with a depth of 0.4±0.9 m, and with continuing re-aeration by wind and natural mixing. Vroon
Fig. 3. COD®lt removal rate as a function of experiment duration time (d ) in duckweed and control reactors at the three distinct sets of 294, 230 and 113±167 mg COD®lt/l (average of all experiments 2se).
2732
F. Al-Nozaily et al.
and Weller (1995) reported DO of <0.1 mgO2/l in higher loaded (378±1410 kg COD/ha with settling) laboratory-scale experiments of 0.15±0.6 m depth without mixing, which is comparable to our results. Euent DO was in¯uenced by initial ls and by mixing. Initial ls of below 800±900 kg/ha (at zero mixing) was found to exert a suciently low oxygen demand to cause some oxygen to be in excess beyond the heterotrophic demand, and ®nal DO to be >0 mg/l. In reactors with ls < 800 kg COD®lt/ ha, oxygen consuming substances are most likely to have been fully oxidized by the end of the experimental period. The pH in all reactors ranged between 7 and 8. This fact and the equally stable DO con®rmed the absence of pronounced algal photosynthetic activity. The observed small quantities of algae did not signi®cantly interfere. Vroon and Weller (1995), Alaerts et al. (1996) and KoÈrner et al. (1998) also reported similar pH ranges, as well as the absence of a diurnal pro®le or strati®cation. lr was not statistically signi®cantly dierent between duckweed-covered and control reactors. This suggests that the duckweed cover did not play a role in the COD®lt removal. Attachment of heterotrophic biomass to the plant roots was less signi®cant at high depths given the high volume-tosurface ratio. The ¯ux of oxygen into the liquid is the deciding and rate limiting factor for COD removal processes. COD®lt removal rates were in¯uenced primarily by initial COD®lt concentration, as well as by ls. Higher COD®lt concentration probably stimulates heterotrophic bacterial metabolization according to ®rst-order kinetics. The ls, which depends on concentration and depth, determines the overall oxygen demand exerted. At ls > 800 kg COD®lt/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 BOD®lt concentration. The COD®lt removal eciency ZCOD (after 20 days) was comparatively low at 50±70% when compared with conventional algae-based sewage lagoons and the full-scale DSL (Alaerts et al., 1996). This can be attributed partly due to an inadequate supply of oxygen, and to the fact that removal performance in this study was calculated against COD®lt in the in¯uent. If removal results were adjusted to CODtotal, overall removal rates would increase to 80±90% 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, respectively. 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 corresponded 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 16±208C, which is signi®cantly 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,t of 7268 kg CODtotal/ha (316 mg/l) of settled wastewater 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 5±108C (Bonomo et al., 1997). Alaerts et al. (1996) in a full-scale DSL of 0.5±1 m depth reported a lr,t of 1399 kg/ha 20 days at an initial ls,t of 1788 kg CODtotal/ha. All previous studies used settled municipal sewage to exclude comparatively large variable amounts of particulates. KoÈrner et al. (1998), using non-settled municipal wastewater at 208C, reported much higher lr,t of 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 presence 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 in¯uent of 504 mg CODtotal/l non-settled sewage at HRT of 16.8 days in Portugal is 0.35 dÿ1 (Gomes de Sousa, 1987), this is much higher than the k of 0.04± 0.06 dÿ1 found here. This dierence can be attributed partly to the fact that our results are based on COD®lt. In addition, the kinetics appear fairly independent of the dierences in oxygen transfer (aeration) associated with shallow to deep reactors (10± 100 cm), in DSL as well as in algae-based systems. This suggests that under our experimental conditions oxygen transfer could be a rate limiting factor. CONCLUSIONS
A medium±deep to deep DSL system (50±150 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 10±100 cm had no other eect than that it increased the surface loading ls (as kg COD®lt /ha) (by increasing the COD®lt mass input, in the experimental set-up). Surface-related processes, notably oxygen transfer, were not strongly aected. A linear relationship was established between applied and removal COD®lt surface loading. The
Performance of duckweed-covered sewage lagoonsÐI
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 COD®lt concentration. The best ®tting linear equation is: COD®lt removal=0.82 (COD®lt initial)ÿ25, with r 2=0.93. The concentration removal rates depended on initial surface loading ls, which in turn depends on initial COD®lt concentration. This con®rms that COD®lt removal is largely determined by volume-related microbial processes and not by surface-related duckweed uptake or oxygen ¯uxes. ls determined the overall oxygen demand; at initial ls > 800 kg COD®lt/ha the oxygen supply might become rate limiting. The COD®lt removal rate followed ®rst-order kinetics, with a reaction constant k of 0.04±0.06 dÿ1.COD®lt 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 COD®lt/l, equivalent to 17±63 mg BOD5,total/l. A depth of up to 1±1.5 m is not likely to limit adequate DSL performance with respect to COD degradation. Thus, depth of DSL should be chosen as a function of in¯uent water quality and desired euent quality. Mixing (up to 2.3 W/m3) has a moderately bene®cial eect by raising DO, favoring COD removal up to 10%. REFERENCES
Alaerts G. J., Mahbubar Rahman M. and Kelderman P. (1996) Performance analysis of a full-scale duckweedcovered sewage lagoon. Wat. Res. 30(4), 843±852. Al-Nozaily F. and Alaerts G. J. Duckweed-covered Sewage Lagoon performance on domestic wastewater in Sana'a using Lemna gibba. (In prep.). APHA (1992) Standard Methods for the Examination of Water and Wastewater, 18th ed. American Public Health Assoc, New York. Arceivala S. J. (1998) Wastewater Treatment for Pollution Control, 2nd ed. Tata McGraw Hill, NewDelhi. Bolton R. L. and Klein L. (1961) Sewage Treatment, Basic Principles and Trends. Butterworths, London. Bonomo L., Pastorelli G. and Zambon N. (1997) Advantages and limitations of duckweed-based wastewater treatment systems. Wat. Sci. Tech. 35(5), 239±246. Camp T. R. and Stein P. C. (1943) Velocity gradients and internal work in ¯uid motion. J. Bost. Soc. Civil Eng. XXX(4), 219±237. Coulson J. M. and Richardson J. F. (1987) Liquid±liquid system. In Chemical Engineering, Vol. 2, p. 814. Pergamon Press. Edwards P., Hassan M. S., Chao C. H. and Pacharaprakiti C. (1992) Cultivation of duckweed in septage-loaded earthen ponds. J. Biores. Tech. 40, 109±117. Gomes de Sousa J. M. (1987) Wastewater stabilization lagoon design criteria for Portugal. Wat Sci. Tech. 19(12), 7±16. KoÈrner S., Lyatuu G. B. and Vermaat J. E. (1998) The in¯uence of Lemna gibba L. on the degradation of organic material in duckweed covered domestic wastewater. Wat. Res. 32(10), 3092±3098.
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Landolt E. and Kandeler R. (1987) The Family of Lemnaceae, a Monographic Study. Vol. 2, Veroeentlichungen des geobotanisches Institutes der ETH Zurich. Stiftung Rubel 95, 638. Mandi L. (1994) Marrakesh wastewater puri®cation experiment using vascular aquatic plants Eichhornia crassipes and Lemna gibba. Wat. Sci. Tech. 29(4), 283±287. Morris P. F. and Barker W. G. (1977) Oxygen transport rates through mats of Lemna minor and Wola sp. and oxygen tension within and below the mat. Can. J. Bot. 55, 1926±1932. NERC (1978) A Beginners Guide to Freshwater Algae, 3rd ed. Natural Environmental Research Council, Cambridge, pp. 36±37. O'Brien J. W. (1981) Use of aquatic macrophytes for wastewater treatment. J. Env. Eng. Div., ASCE 107(EE4), 681±698. Oron G., Wildschut L. R. and Porath D. (1984) Wastewater recycling by duckweed for protein production and euent renovation. Wat. Sci. Tech. 17(4±5), 803±817. Oron G., De Vegt A. and Porath D. (1988) Nitrogen removal and conversion by duckweed grown on wastewater. Wat. Res. 22(2), 179±184. PRISM (1992) The Shobuj Shona Village Enterprise Project, Progress report, The PRISM Group, Dhaka. Rao S. V. R. (1986) A review of the technological feasibility of aquacultures for municipal wastewater treatment. Intern. J. Env. Stud. 27, 219±223. Reed S. C., Middlebrooks E. J. and Crites R. W. (1987) Natural Systems for Waste Management and Treatment. McGraw-Hill, New York. Tackholm V. (1974) Students Flora of Egypt, 2nd ed. Cairo University, Printed by Cooperative Printing Co., Beirut. Vroon R. and Weller B. (1995) Treatment of domestic wastewater in a combined UASB-reactor duckweed pond system., Doktoraal verslagen, series Nr. 95±07, Dept. Env. Tech., Agric. University Wageningen, The Netherlands. Zirschky J. and Reed S. C. (1988) The use of duckweed for wastewater treatment. J. WPCF 60(7), 1253±1258.
APPENDIX A. QUANTIFICATION OF DISSIPATED POWER Mechanical mixing can be quanti®ed by determining the velocity gradient G (Camp and Stein, 1943): s E G Vm
A1
where: G is the velocity gradient (sÿ1); E is the dissipated power (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 related to the rotor's rotational velocity: s 1=2Cd rA 1:5prn3 G Vm
A2
where: Cd is the drag coecient (11.4 under turbulent condition); r is the density of water (1000 kg/m3); A is the projected area of the object (m2); r is the tip radius of the object (m); n is the mixing speed of the object (rps).
Wat. Res. Vol. 34, No. 10, pp. 2727±2733, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
www.elsevier.com/locate/watres
PERFORMANCE OF DUCKWEED-COVERED SEWAGE LAGOONSÐI. OXYGEN BALANCE AND COD REMOVAL F. AL-NOZAILY1, 2*, G. ALAERTS1M and S. VEENSTRA1 1
International 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)
AbstractÐLaboratory scale experiments were performed in a non-continuous batch reactor system with 0.8±41.2 l domestic sewage exposed to constant light intensity, temperature and humidity. The treatment performance of duckweed (L. gibba )-covered sewage lagoons (DSL) was studied within a CODtotal range of 200±500 mg/l (113±294 mg COD®lt/l), in 10, 30, 70 and 95 cm deep reactors, and liquid mixing intensity (power dissipation) of 0, 0.3, 1.0, 2.3 and 34.1 W/m3. The duration of each experiment was 20 days with biomass harvesting every 5 days. COD removal at extreme depths and extreme mixing intensities was compared with that in covered control reactors without duckweed. Removal of COD®lt did not dier in duckweed-covered and control reactors. The role of duckweed cover was marginal in changing the redox potential or the DO. COD removal lr (kg COD®lt/ha 20 days) correlated strongly with initial surface load ls (kg COD®lt/ha). Concentration removal (as mg COD®lt/l) was also proportional to initial COD®lt concentration. For a given COD®lt mass input, increasing depth up to 1 m aected DSL performance only by increasing surface load, and not by hampering oxygen transfer. Mixing (up to 2.3 W/m3) raised COD®lt removal. Therefore, at depths beyond 70 cm, moderate mixing is recommended. The ®rst-order kinetic removal rate coecient for COD®lt was 0.04±0.06 dÿ1. 7 2000 Elsevier Science Ltd. All rights reserved Key wordsÐduckweed, 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-term performance 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 *Author to whom all correspondence should be addressed. Fax: +31-15-2122921; e-mail: [email protected]
as a ``bio®lter'' by providing attachment opportunities for aerobic heterotrophic bacteria. Dissolved oxygen (DO) transfer is in¯uenced by reactor depth, time of the day, and the degree of wind-induced turbulence of the water surface (Morris 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 2±4 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 concentration of approximately 100 mg/l at the inlet (Alaerts et al., 1996). The optimal depth of a DSL has to be related to the 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-tosurface area ratio determine the maximal depth that can be applied. In general, despite available chemical and microbiological information, knowledge is still missing on the design and optimal operational conditions. Oron et al. (1988) suggested that COD
2727
2728
F. Al-Nozaily et al.
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 sewage concentrations is investigated in order to provide more clarity on design and operational parameters of DSL. This paper discusses (1) the eect of depth (up to 1 m), mixing and sewage concentration on COD removal, and (2) the levels of DO and pH in the DSL. Nutrient removal is discussed in Part II. MATERIALS AND METHODS
Experiments were performed in a non-continuous batch reactor system with settled domestic wastewater in vessels of 0.8±41.2 l with dierent depth, mixing intensity and wastewater strength. Two groups of experiments were conducted (Table 1). Group 1 experiments allowed rationalizing the number and design of the Group 2 experiments, in view of the long duration of each experiment. Group 1 experiments used medium-strength settled sewage with CODtotal 1 250 mg/l and studied system performance for two extreme depths (10 and 95 cm) and mixing intensity (G = 0 and 58 sÿ1 [34.1 W/m3]). Results suggested that depth does not aect COD removal; it only alters the initial CODtotal surface load ls,t (approx. 250 [at 10 cm] and 2400 kg/ha [at 95 cm]) (with variable CODtotal mass input). In Group 2 experiments, depths of 30, 70 and 95 were chosen to dierentiate between the in¯uence of initial CODtotal concentrations (200±500 mg/l) and initial ls,t (approx. 1400±4400 kg CODtotal/ha). In experiments 2/1 and 2/2 comparable surface loadings were applied but at a dierent COD concentration. The range of power dissipation between 0.75 and 3.0 W/m3 for aerated pond design, enabling solids re-suspension (Arceivala, 1998) was translated into a velocity gradient G (sÿ1) and consequently into a degree of turbulence (Appendix A) (Camp and Stein, 1943). COD®lt was typically used as a parameter to quantify organic matter, as it is less dependent on variable and less relevant amounts of particulate matter. The
COD®lt/BOD5,total ratio of settled sewage was 1.3. CODtotal /BOD5,total was typically 1.6 for settled in¯uent and 2 for euent. To study in Group 1 the role of the duckweed in O2 supply, triplicate reactors with and without duckweed (control) were used in parallel. Control reactors were covered with non-transparent plastic sheets to simulate duckweed mat. pH and DO were measured regularly and algal growth was monitored visually. In Group 2, no control reactors were used. Duplicates were found to be statistically suciently precise (se averaging 0±3% both in the case of duplicates and triplicates). Municipal raw wastewater (collected from BerkelRodenrijs or TNO Delft treatment plants, both in The Netherlands) was settled for 24 h to remove settleable matter. L. gibba was chosen because it is available in the Middle East and Yemen, and is most resilient to wastewater conditions (Al-Nozaily and Alaerts, in prep.). Clones were collected from Delft city canals and placed in wastewater for two weeks prior to the experiments to allow them to acclimatize. Healthy clones were selected and used to create full cover in the duckweed reactors. Depending on the situation, mixing occurred by magnetic stirring or rotating paddle. Energy input was calculated according to equation (A2) (Appendix A). In the Group 2 experiments, a rotating paddle was used based on work by Coulson and Richardson (1987). Disturbances of the duckweed cover were prevented by surrounding the rod on the surface with an immobile PVC ring. Vertical baes attached to the reactor wall halted the rotation of the duckweed mat. To suppress algal growth, duckweed was rinsed thoroughly after each harvest before reintroduction into the reactor. In addition, 2 mgCuSO4/l was added as recommended by Edwards et al. (1992). This Cu concentration can be tolerated by heterotrophs (Bolton and Klein, 1961), while the relative growth rate (RGR) of L. gibba was found to be not noticeably aected. Light was maintained at 130 2 10 mE/m2 s using HPIT 400W Hg lamps under a time regime of 18/6 h on/o. Ambient and water temperatures were 21 2 48C and 19±248C, respectively, while humidity was 40210%. Liquid samples were collected at 5, 30 and 65 cm depth every 5 days. Loading rates and ``in¯uent'' concentrations pertain to the initial reactor content. ``Euent'' values pertain to the liquid in the reactor at the end of the experiment. Duckweed was harvested every 5 days by netting the biomass, rinsing it with tap water and drying the biomass by paper tissue (wet weight). Thereafter the initial stock density (500 2 100 g wet wt/m2) was restored in
Table 1. Overview of experimental conditions in Groups 1 and 2a Depth (cm)
Volume (l)
Surface area (m2)
E (W/m3)
G (sÿ1)
CODtotal (mg/l)
ls,t (kg CODtotal/ha)
COD®lt (mg/l)
Mass input (gCOD®lt)
1/1 1/2 1/3
10 95
0.8 41.2
0.008 0.043
0 0 34.1
0 0 58
1 250 1 250
1 250 1 2400
13826 167215 15723
0.11 6.88 6.47
13826 15872143 1492229
2/1/1 2/1/2 2/1/3 2/1/4
30
13.0
0.043
0 0.3 1.0 2.3
0 5 10 15
1 500
1 1400
29428
3.82
882224
2/2/1 2/2/2 2/2/3 2/2/4
70
30.0
0.043
0 0.3 1.0 2.3
0 5 10 15
1 200
1 1400
11328
3.39
791256
2/3/1 2/3/2
95
41.2
0.043
0 1.0
0 10
1 500
1 4400
23028
9.48
2185276
Exp. no.
a
Wastewater characteristics and COD surface loading ls and ls,t at the beginning of the experiments. Average 2se.
ls (kg COD®lt/ha)
Performance of duckweed-covered sewage lagoonsÐI every reactor. The increment from each replicate was mixed, and 5±10 g wet wt was taken for further analysis. Evapotranspiration was measured and compensated by adding tap water daily. Temperature, redox potential, DO and pH were measured every 2±3 days, and light intensity was checked every 5 days. BOD5 was analyzed using the Winkler method. The COD analysis was based on the closed re¯ux technique (acid destruction at 1508C for 2 h) and colorimetry at 600 nm (Perkin Elmer 550 S, US) (APHA, 1992). Filtered samples were prepared by ®ltering over GF/C (1.2 mm pores) glass ®ber ®lter paper. The DO was measured by WTW OX 196 (Germany). Redox potential was measured by platinum electrode AG 9100 from Metrohm Herisau (Switzerland). The light intensity was measured at the surface level of the experimental reactors by using a LI-COR radiation sensor, type SB (UWQ 4681, Campbell Scienti®c Ltd, UK). Air humidity was measured daily with a hygrometer. SPSS software was used for statistical analysis. Comparisons among mean values were made by analysis of variance (one-way ANOVA), signi®cant ANOVAs were followed by mean comparisons using Tuky's honestly signi®cant dierence test. Statistical analyses are reported as signi®cant when P R 0.05. RESULTS
The COD®lt concentration varied between 113 and 294 mg/l with a loading rate of ls 138±2185 kg COD®lt/ha (Table 1). No strati®cation occurred in the non-mixed reactors. The duckweed cover marginally in¯uenced redox potential and DO at an initial ls of r1492 kg COD®lt/ha. At 95 cm with a high ls of 1492 (experiment 1/3) and 1587 kg COD®lt/ha (experiment 1/2), the redox potential after 20 days was ÿ248 2 7 mV and ÿ309 2 2 mV in duckweed-covered reactors, and ÿ293 2 16 mV and ÿ306 2 2 mV in control reactors. Statistical analysis did not reject the hypothesis that the redox potential in the deeper duckweed and control reactors was equal. However, in shallow reactors of 10 cm depth with low ls of 138 kg COD®lt/ha (experiment 1/1), DO after 20 days was 3.9 2 0.1 mgO2/l in duckweed-covered reactors and 0.7 2 0.1 mgO2/l in control reactors, a statistically signi®cant 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 intensi®ed mixing. The pH in the water column of duckweed and
2729
control reactors ranged continuously between 7 and 8. There was no day/night rhythm, nor any statistically signi®cant dierence. Due to the control of algal growth, only marginal amounts of Cyanobacterium Chroococcus (NERC, 1978) were found attached to the duckweed. The COD®lt 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 893 2 76 and 817 2 57 kg COD®lt/ha 20 days, respectively. These results are statistically speaking equal, which suggests that the presence of a duckweed mat did not aect COD®lt removal, in spite of the presence of DO in duckweed reactors compared to control. The removal loading lr (as kg COD®lt/ha 20 days) is linearly proportional to ls, while the percentual 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 COD®lt/ha (experiments 2/1/1 and 2/2/1, Table 3), lr was 633 and 511 kg COD®lt/ha 20 days, a statistically not signi®cant 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), lr was 1235 kg/ha 20 days, which is a statistically signi®cant dierence. Concentration removal (as mg COD®lt/l 20 days) as a function of dierent reactor depth showed no statistically signi®cant dierence (experiments 1/1 and 1/2, Table 3). This con®rmed that reactor depth, as separate variable did not aect the COD®lt concentration removal rate in duckweedcovered reactors within the range of 10±100 cm. Concentration removal increased linearly with increasing in¯uent concentration (Fig. 2). Three distinct sets of COD®lt concentration removal values were statistically signi®cantly dierent. At initial COD®lt of 113±167, 230, and 294 mg/l, the COD®lt removal ranged 8124, 14224 and 22825 mg/l 20 days, respectively. The COD®lt removal followed ®rst-order kinetics with respect to time. The rate coecient (k ) increased signi®cantly with increasing COD®lt concentrations from 0.04 to 0.06 dÿ1 (Fig 3). This suggests that concentration, and thus loading determined the rate of COD removal.
Table 2. DO of Group 2 experiments, after 20 daysa Exp. no.
2/2 2/1 2/3 a
ls (kg COD®lt/ha)
791 882 2185
DO (mg/l) E=0
E=0.3
E=1.0
E=2.3
1.120.2 a 0.220.1 b 0.020.0 b
3.320.9 m 2.320.2 n ±
6.220.7 o 3.320.8 n 0.020.0 n
5.520.2 o 4.220.7 p ±
Average 2se. Dierent letters indicate signi®cant dierence of the value at P R 0.05. Signi®cance was tested among depths at E=0 (a, b), and among the combinations of depth and mixing for E > 0 (m, n, o, p).
2730
F. Al-Nozaily et al.
Table 3. COD®lt removal rate in duckweed-covered and control reactors at dierent depth and with dierent mixing intensity in Group 1 and 2 experimentsa Exp. no.
E (W/m3)
Depth (cm)
ls (kg COD®lt/ha)
COD®lt removal (mg/l 20 days)
lr (kg/ha 20 days)
ZCOD (%)
1/1 D C
0
10
13826
8622 a 9123 a
8622 a 9123 a
62 66
1/2 D C
0
95
15872143
9428 a 8626 a
893276 b 817257 b
56 51
1/3 D C
34.1
95
1492229
8324 a 83211 a
789238 b 7892104 b
53 53
2/1/1 2/1/2 2/1/3 2/1/4
0 0.3 1.0 2.3
30
882224
211221 c,d 23429 d 234224 d 24024 d
633224 702224 684224 720224
d e,f e f
72 80 78 82
2/2/1 2/2/2 2/2/3 2/2/4
0 0.3 1.0 2.3
70
791256
511256 602256 560256 574256
g h i i
65 76 71 73
2/3/1 2/3/2
0 1.0
95
2185276
1235276 j 1397276 k
57 64
7323 8626 8022 8228
a b b b
13024 a,b 147214 b,c
a
Averages 2se. Dierent letters indicate signi®cant dierence at P R 0.05. COD®lt loading ls at the beginning of the experiments. D=duckweed reactor, C=control reactors.
COD®lt removal increased with mixing intensity within the range of 0±2.3 W/m3 by approximately 10% (Table 3). Intensive mixing (34.1 W/m3) did not signi®cantly improve the COD®lt removal rate at a reactor depth of 95 cm (Table 3).
DISCUSSION
Depth variation in the range of 10±100 cm did not aect COD removal in DSL. Similar conclusions were reported by others albeit within much smaller depth ranges (Oron, 1988; Vroon and
Fig. 1. COD®lt removal lr (as kg/ha 20 days) and ZCOD (as %) as a function of COD®lt initial loading lS in duckweed reactors, and corresponding reactor depth (cm). Dotted lines indicate the range of % removal.
Performance of duckweed-covered sewage lagoonsÐI
2731
Fig. 2. COD®lt concentration removal (as mg/l 20 days) as a function of COD®lt initial concentration in duckweed reactors, and corresponding reactor depth (cm). Average of all experiments 2se.
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; KoÈrner et al., 1998). However, as duckweed lowers NH+ 4 ±N levels below 2 mg N/l so that nitri®cation exerts a lower oxygen demand, and as substantial nitri®cation 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. Elsewhere, DO has been reported to range between 2 and 4 mgO2/l in a low loaded full-scale (80±100 kg COD/ha days after settling) DSL (Alaerts et al., 1996) with a depth of 0.4±0.9 m, and with continuing re-aeration by wind and natural mixing. Vroon
Fig. 3. COD®lt removal rate as a function of experiment duration time (d ) in duckweed and control reactors at the three distinct sets of 294, 230 and 113±167 mg COD®lt/l (average of all experiments 2se).
2732
F. Al-Nozaily et al.
and Weller (1995) reported DO of <0.1 mgO2/l in higher loaded (378±1410 kg COD/ha with settling) laboratory-scale experiments of 0.15±0.6 m depth without mixing, which is comparable to our results. Euent DO was in¯uenced by initial ls and by mixing. Initial ls of below 800±900 kg/ha (at zero mixing) was found to exert a suciently low oxygen demand to cause some oxygen to be in excess beyond the heterotrophic demand, and ®nal DO to be >0 mg/l. In reactors with ls < 800 kg COD®lt/ ha, oxygen consuming substances are most likely to have been fully oxidized by the end of the experimental period. The pH in all reactors ranged between 7 and 8. This fact and the equally stable DO con®rmed the absence of pronounced algal photosynthetic activity. The observed small quantities of algae did not signi®cantly interfere. Vroon and Weller (1995), Alaerts et al. (1996) and KoÈrner et al. (1998) also reported similar pH ranges, as well as the absence of a diurnal pro®le or strati®cation. lr was not statistically signi®cantly dierent between duckweed-covered and control reactors. This suggests that the duckweed cover did not play a role in the COD®lt removal. Attachment of heterotrophic biomass to the plant roots was less signi®cant at high depths given the high volume-tosurface ratio. The ¯ux of oxygen into the liquid is the deciding and rate limiting factor for COD removal processes. COD®lt removal rates were in¯uenced primarily by initial COD®lt concentration, as well as by ls. Higher COD®lt concentration probably stimulates heterotrophic bacterial metabolization according to ®rst-order kinetics. The ls, which depends on concentration and depth, determines the overall oxygen demand exerted. At ls > 800 kg COD®lt/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 BOD®lt concentration. The COD®lt removal eciency ZCOD (after 20 days) was comparatively low at 50±70% when compared with conventional algae-based sewage lagoons and the full-scale DSL (Alaerts et al., 1996). This can be attributed partly due to an inadequate supply of oxygen, and to the fact that removal performance in this study was calculated against COD®lt in the in¯uent. If removal results were adjusted to CODtotal, overall removal rates would increase to 80±90% 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, respectively. 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 corresponded 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 16±208C, which is signi®cantly 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,t of 7268 kg CODtotal/ha (316 mg/l) of settled wastewater 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 5±108C (Bonomo et al., 1997). Alaerts et al. (1996) in a full-scale DSL of 0.5±1 m depth reported a lr,t of 1399 kg/ha 20 days at an initial ls,t of 1788 kg CODtotal/ha. All previous studies used settled municipal sewage to exclude comparatively large variable amounts of particulates. KoÈrner et al. (1998), using non-settled municipal wastewater at 208C, reported much higher lr,t of 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 presence 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 in¯uent of 504 mg CODtotal/l non-settled sewage at HRT of 16.8 days in Portugal is 0.35 dÿ1 (Gomes de Sousa, 1987), this is much higher than the k of 0.04± 0.06 dÿ1 found here. This dierence can be attributed partly to the fact that our results are based on COD®lt. In addition, the kinetics appear fairly independent of the dierences in oxygen transfer (aeration) associated with shallow to deep reactors (10± 100 cm), in DSL as well as in algae-based systems. This suggests that under our experimental conditions oxygen transfer could be a rate limiting factor. CONCLUSIONS
A medium±deep to deep DSL system (50±150 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 10±100 cm had no other eect than that it increased the surface loading ls (as kg COD®lt /ha) (by increasing the COD®lt mass input, in the experimental set-up). Surface-related processes, notably oxygen transfer, were not strongly aected. A linear relationship was established between applied and removal COD®lt surface loading. The
Performance of duckweed-covered sewage lagoonsÐI
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 COD®lt concentration. The best ®tting linear equation is: COD®lt removal=0.82 (COD®lt initial)ÿ25, with r 2=0.93. The concentration removal rates depended on initial surface loading ls, which in turn depends on initial COD®lt concentration. This con®rms that COD®lt removal is largely determined by volume-related microbial processes and not by surface-related duckweed uptake or oxygen ¯uxes. ls determined the overall oxygen demand; at initial ls > 800 kg COD®lt/ha the oxygen supply might become rate limiting. The COD®lt removal rate followed ®rst-order kinetics, with a reaction constant k of 0.04±0.06 dÿ1.COD®lt 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 COD®lt/l, equivalent to 17±63 mg BOD5,total/l. A depth of up to 1±1.5 m is not likely to limit adequate DSL performance with respect to COD degradation. Thus, depth of DSL should be chosen as a function of in¯uent water quality and desired euent quality. Mixing (up to 2.3 W/m3) has a moderately bene®cial eect by raising DO, favoring COD removal up to 10%. REFERENCES
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APPENDIX A. QUANTIFICATION OF DISSIPATED POWER Mechanical mixing can be quanti®ed by determining the velocity gradient G (Camp and Stein, 1943): s E G Vm
A1
where: G is the velocity gradient (sÿ1); E is the dissipated power (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 related to the rotor's rotational velocity: s 1=2Cd rA 1:5prn3 G Vm
A2
where: Cd is the drag coecient (11.4 under turbulent condition); r is the density of water (1000 kg/m3); A is the projected area of the object (m2); r is the tip radius of the object (m); n is the mixing speed of the object (rps).