Published in J. Environ. Qual. 32:1583-1590 (2003).
© 2003 ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA
REVIEWS AND ANALYSES
The Capacity of Duckweed to Treat Wastewater
Ecological Considerations for a Sound Design
Sabine Körner*,a,
Jan E. Vermaatb and
Siemen Veenstrac
a Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 301, 12587 Berlin, Germany
b Institute for Environmental Studies, Vrije Universiteit, De Boelelaan 1087, 1081 HV Amsterdam, the Netherlands
c Water Company Overijssel WMO, Oude Veerweg 1, 8000 GW Zwolle, the Netherlands
* Corresponding author (hilt{at}igb-berlin.de).
Received for publication July 18, 2002.
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ABSTRACT
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Duckweed species are promising macrophytes for use in sustainable wastewater treatment due to their rapid growth, ease of harvest, and feed potential as a protein source. This paper reviews growth rates of different duckweed species on wastewater and ammonia toxicity to duckweed and summarizes insights into the mechanism of organic matter and nutrient removal. Results were gained from laboratory experiments in small, shallow, duckweed-covered semicontinuous batch systems. Growth rates on different types of wastewater vary considerably among different species. Ammonia is toxic for duckweed in both the ionized and un-ionized forms. Duckweed, however, can be used to treat wastewater containing very high total ammonia concentrations as long as certain pH levels are not exceeded. The degradation of organic material is enhanced by duckweed through both additional oxygen supply and additional surface for bacterial growth. The duckweed mat with attached bacteria and algae is, independent of the loading rates, responsible for three-quarters of the total nitrogen (N) and phosphorus (P) loss in very shallow systems. Based on our results we suggest that full-scale pilot plants with duckweed should be shallower than the range encountered in the literature. A harvesting schedule that allows doubling times of 2 to 3.5 d, maintenance of a full coverage, and plug flow conditions are recommended.
Abbreviations: BOD, biological oxygen demand • COD, chemical oxygen demand
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INTRODUCTION
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MACROPHYTE-BASED wastewater treatment systems have several potential advantages compared with conventional treatment systems (Brix and Schierup, 1989). The use of duckweed species has been advocated because they have rapid growth rates (Hillman, 1961; Landolt, 1986) and achieve high levels of nutrient removal (Sutton and Ornes, 1975; Reddy and De Busk, 1985; Alaerts et al., 1996). In addition, low fiber and high protein contents (Landolt, 1986; Oron et al., 1986; Hammouda et al., 1995) of duckweed make it a valuable fodder (Culley and Epps, 1973; Skillicorn et al., 1993; Haustein et al., 1994). Furthermore, duckweed can easily be harvested, is comparatively cold tolerant (Landolt, 1986), and appears to suppress algal growth (Hammouda et al., 1995). Duckweed wastewater treatment systems have been studied for dairy waste lagoons (Culley et al., 1981; Whitehead et al., 1987), raw and diluted domestic sewage (Skillicorn et al., 1993; Mandi, 1994; Hammouda et al., 1995; Alaerts et al., 1996), secondary effluent (Harvey and Fox, 1973; Sutton and Ornes, 1975), sewage-loaded ponds (Edwards et al., 1992), and fish culture systems (Porath and Pollock, 1982). Several full-scale systems are in operation in Taiwan, China, Bangladesh, Belgium, and the USA (Edwards, 1980; Zirschky and Reed, 1988; Gijzen and Veenstra, 2000). Economic use of harvested duckweed has been tested with variable results (e.g., Edwards et al., 1992; Skillicorn et al., 1993). To our knowledge, larger-scale profitable exploitation has not been reported so far.
Nevertheless, a number of problems are still unresolved and prevent an efficient use of duckweed for wastewater treatment. First, a systematic screening of the performance of different duckweed species has not been performed yet. Second, the sensitivity to potentially toxic high concentrations of NH+4 and NH3 in wastewater has not been examined with sufficient experimental rigor. Third, the contribution of duckweed-related mechanisms to nutrient removal and organic matter degradation has generally been addressed as a "black box," without due attention to the different processes involved.
This paper reviews the results from a number of laboratory experiments performed between 1996 and 1998 using small-scale duckweed-covered batch systems (Vermaat and Hanif, 1998; Körner and Vermaat, 1998; Körner et al., 1998, 2001). We intend to provide an update that we feel to be appropriate more than a decade after the review of Brix and Schierup (1989). Experiments concentrated on domestic wastewater, both because of its worldwide prevalence and its low phytotoxicity compared with various types of industrial sewage. To arrive at a high ratio of duckweed biomass (surface) to volume of wastewater, water depth in our systems was kept low (3.3 cm), whereas duckweed-covered treatment systems in practice have depths between 30 and 150 cm (Koles et al., 1987; Zirschky and Reed, 1988; Alaerts et al., 1996).
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PERFORMANCE OF COMMON DUCKWEED SPECIES ON DIFFERENT TYPES OF WASTEWATER
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Duckweeds belong to the fastest-growing angiosperm plants on earth (Hillman and Culley, 1978). Maximum growth rates of Lemnaceae are species- and clone-specific. Maximum relative growth rates (RGR) of 0.73 to 0.79 d-1 were measured in lesser duckweed (Lemna aequinoctialis Welw.) and Indian duckweed [Wolffia microscopica (Griffith) Kurz], which correspond to doubling times between 20 and 24 h. Lowest maximal growth rates are observed in submerged species (Landolt and Kandeler, 1987). For comparison, RGR values of angiosperm herbaceous plants range between 0.031 and 0.365 d-1 (Lambers and Poorter, 1992), whereas algae grow at rates between 0.26 and 2.84 d-1 (Nielsen and Sand-Jensen, 1990).
The growth of common duckweed species, including fat duckweed (Lemna gibba L.), common duckweed (Lemna minor L.), star duckweed (Lemna trisulca L.), great duckweed [Spirodela polyrhiza (L.) Schleiden], and spotless watermeal [Wolffia arrhiza (L.) Horkel ex C.F.H. Wimmer], on different types of wastewater (300–442 mg L-1 chemical oxygen demand [COD], 14–52 mg Kjeldahl nitrogen L-1, and 7–9 mg total P L-1) compared with a standard mineral growth medium (1/10 Huttner) was considerably different. All species yielded less on the two artificial wastewaters (sucrose, propionic acid, acetate, and milk powder) than on the mineral medium, whereas the submerged species (star duckweed) performed poorly on all media (Table 1). The smallest species (spotless watermeal) performed highly variably (low r2). Only great duckweed and fat duckweed performed equally well on domestic sewage compared with the mineral medium (Vermaat and Hanif, 1998). The latter species has been selected frequently for trials on wastewater, but no reference gives explicit reasons for this selection (Sutton and Ornes, 1975; Oron et al., 1987; Boniardi et al., 1994).
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TOXICITY OF TOTAL AMMONIA TO DUCKWEED
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Ammonia is one of the major constituents of domestic wastewater and concentrations commonly range from 10 to 50 mg total ammonia N L-1, but might be as high as 200 mg total ammonia N L-1 in domestic wastewater in arid and semiarid countries or industrial wastewater (Konig et al., 1987; Veenstra et al., 1995). Despite being the preferred N source for duckweed plants (Porath and Pollock, 1982), it may become one of the parameters inhibiting the growth of duckweed in wastewater (Bitcover and Sieling, 1951; Lüönd, 1983).
Total ammonia in aqueous solution consists of two principal forms, the ammonium ion (NH+4) and un-ionized ammonia (NH3), with relative concentrations being pH- and temperature-dependent. The knowledge of the relationship between total ammonia, pH, and relative growth rate of duckweed is therefore needed to assess its possible use for the treatment of wastewater with high total N concentrations. Most of the studies made on the toxicity of ammonia for duckweed (Wang, 1991; Monselise and Kost, 1993; Clement and Merlin, 1995; Caicedo et al., 2000) suffer from a lack of pH control, which precludes any distinction between the effects of NH3 and NH+4.
The toxicity of total ammonia on fat duckweed can be attributed to the effect of the un-ionized NH3 alone at NH3–N concentrations greater than 1 mg L-1 (Körner et al., 2001). In this range the toxic effect of NH+4–N can be disregarded. The maximum tolerance level for un-ionized ammonia was detected around 8 mg NH3–N L-1. At NH3–N concentrations below 1 mg L-1, the ionized form (NH+4–N) contributed to the toxicity as well. A maximum tolerance level for NH+4 could not be detected, because this would require very low pH levels to keep NH3 concentrations low (Körner et al., 2001). A predicted contour plot (Fig. 1) suggests that fat duckweed can be used to treat wastewater containing very high total ammonia concentrations as long as certain pH levels are not exceeded: up to pH levels of 7.8, a substantial production of 55 kg dry wt. ha-1 d-1 can be achieved, which is comparable with values found in full-scale ponds (Alaerts et al., 1996). Wastewater treatment using fat duckweed becomes impossible at pH levels above approximately 9.8. As a consequence, treatment efficiencies in duckweed-covered wastewater can be increased by lowering the pH values, for example, by anaerobic pretreatment by means of (anaerobic) ponds or sophisticated (anaerobic) reactors (Veenstra et al., 1995). Algal growth, which increases the pH, should thus be prevented by maintaining a closed duckweed cover. This may also be an efficient solution to the generation of new biological oxygen demand (BOD) in algae-dominated stabilization ponds (Arceivala, 1998).
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Fig. 1. Isopleth contour map of relative growth rates of fat duckweed (in kg kg-1 dry wt. d-1) as a function of pH and total ammonia concentrations in domestic wastewater at 23°C (y axis starting at 10 mg L-1 total ammonia). Data from Körner et al. (2001).
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DEGRADATION OF ORGANIC MATERIAL
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Duckweed-covered systems can attain BOD and COD removal rates between 50 and 95% (Oron et al., 1987; Zirschky and Reed, 1988; Mandi, 1994; Boniardi et al., 1994; Alaerts et al., 1996; Körner et al., 1998). The role of duckweed in the removal of organic material has been a subject of controversy. Indirect effects like provision of surface and substrate for bacterial growth and change of the physicochemical environment in the water (Godfrey et al., 1985) as well as the possibility of direct removal of small organic compounds by heterotrophic growth were supposed (Hillman, 1961; Culley and Epps, 1973; Frick, 1994; Boniardi et al., 1994; Hammouda et al., 1995; but see below).
Brix and Schierup (1989) assumed that duckweed, compared with, for example, water hyacinth [Eichhornia crassipes (Mart.) Solms-Laub.], would play a less important role in the treatment process as the plants lack extensive root systems and therefore provide a smaller substrate surface for attached microbial growth. Fat duckweed, however, was found to significantly enhance COD removal in shallow batch systems (Körner et al., 1998). Reconsideration of the dimensions of a treatment system therefore appears necessary when duckweed will be applied in ponds.
Bacteria present in the duckweed system must play a role because significant differences between duckweed-covered systems and controls disappeared after killing (at least part of) the bacteria with antibiotics (ampicillin) or sterilization. The degradation coefficient of 0.65 d-1 in these shallow systems probably represents the maximum value possible. Lower values are reported for duckweed systems in practice (0.11 d-1) at a temperature of 24°C and a COD between 140 and 320 mg L-1, though these were still higher than those using a combination of bacteria and algae without duckweed (0.04 d-1; Xu et al., 1991).
It was not possible to artificially simulate the influence of duckweed by applying either additional surface for bacterial growth, additional oxygen supply, or both (Fig. 2)
. Apparently, duckweed cannot be replaced easily by artificial surfaces for bacterial growth and/or artificial oxygen supply by pumps. Possible explanations include (i) a difference in the attached bacterial community, where bacteria on duckweed would apparently be more effective in degradation of organic matter; or (ii) a difference in the way oxygen is diffused into the reactor, where the duckweed roots and lower frond surfaces provide oxygen at a "microsite level" within the biofilm.
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Fig. 2. Exponential chemical oxygen demand (COD) removal curves in differently treated wastewater. Multiple comparisons of fitted curves were made across all possible combinations. The COD removal was significantly faster in treatments containing duckweed compared with the control, whereas treatments with artificial duckweed, air bubbling, or both were not significantly different from either the control or the duckweed treatment. For curve fit parameters see Körner et al. (1998). Data points represent means of four replicates; for clarity, standard errors were omitted. Data from Körner et al. (1998).
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Körner et al. (1998) found that dissolved organic carbon (DOC) levels were similar with and without axenic and nonaxenic duckweed. Since rapidly assimilated organic compounds are generally in the dissolved form (i.e., DOC), this suggests that heterotrophic uptake of simple organic compounds by fat duckweed does not play an important role in the removal process of dissolved organic compounds in wastewater (Körner et al., 1998), contrary to the postulations of Culley and Epps (1973) and Hammouda et al. (1995).
A dense cover of duckweed on the water surface was suggested to inhibit both oxygen entering the water by diffusion from the air and photosynthetic production of oxygen by phytoplankton because of the poor light penetration (Culley and Epps, 1973; Morris and Barker, 1976; Brix and Schierup, 1989). Zirschky and Reed (1988) stated that BOD removal could decrease in ponds covered with duckweed because of the limited oxygen transfer into the water. Alaerts et al. (1996), however, found that the water column in a duckweed-covered sewage lagoon system with a low BOD load always remained aerobic. Shallow duckweed-covered batch systems containing 100% wastewater even contained significantly more oxygen than the uncovered controls (Körner et al., 1998). This probably occurred because of oxygen leaking from the roots in these shallow containers that maximized the ratio of duckweed surface to wastewater volume.
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NITROGEN AND PHOSPHORUS REMOVAL
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Different components of duckweed-covered systems contribute to the removal of the major nutrients N and P. Nitrogen and P losses can be attributed to uptake by duckweed, its attached biofilm (algae and bacteria), the biofilm attached to the walls of the systems, and sedimentation of particulate N and P. Volatilization of NH3 could be important in systems with high pH and high ammonia concentrations, but a closed duckweed mat is assumed to prevent this process. Nitrogen inputs by N fixation were calculated at only 1 to 2 mg N m-2 d-1 for naturally occurring duckweed–cyanobacterial associations (Duong and Tiedje, 1985) and can therefore be excluded as an important component.
Considerable effort has been invested in assessing treatment efficiencies of duckweed systems and the necessary mass balances were made up with variable success (e.g., Oron et al., 1986, 1987; Reddy and Smith, 1987; Zirschky and Reed, 1988; Boniardi et al., 1994; Vatta et al., 1995; Alaerts et al., 1996). In general, effluents of treatment systems with duckweed meet the desired standards. Removal efficiencies of 34 to 99% for N and 14 to 99% for P were reported for systems using fat duckweed (Sutton and Ornes, 1975; Oron et al., 1986; Hammouda et al., 1995; Vatta et al., 1995; Alaerts et al., 1996; Körner and Vermaat, 1998; Vermaat and Hanif, 1998).
Nutrient removal coefficients show a linear correlation to the initial nutrient concentrations until a certain concentration, at which point the nutrient probably became limiting and removal was much faster (Fig. 3)
. This concentration was reached for total P (<2.2 mg L-1), whereas for N no limitation occurred within the studied range of concentrations (
11.6 mg N L-1). Duckweed can be directly responsible for 16 to 47% of the total N removal (Reddy and De Busk, 1985; Alaerts et al., 1996; Körner and Vermaat, 1998; Vermaat and Hanif, 1998) depending on the initial ammonium concentration and provided that ammonia concentrations are not toxic (Fig. 3). Nitrogen uptake rates of fat duckweed vary between 45 and 1670 mg N m-2 d-1 (Culley et al., 1981; Zirschky and Reed, 1988). The direct contribution of duckweed to P removal can vary between 9 and 61% (Reddy and Smith, 1987; Alaerts et al., 1996; Körner and Vermaat, 1998; Vermaat and Hanif, 1998) (Fig. 4)
. Phosphorus uptake rates of fat duckweed vary between 8 and 220 mg P m-2 d-1 (Culley et al., 1981; Reddy and Smith, 1987; Zirschky and Reed, 1988; Körner and Vermaat, 1998; Vermaat and Hanif, 1998). Nitrogen and P removal by duckweed uptake are mainly realized by newly grown tissue, not by increasing the tissue N or P content (Körner and Vermaat, 1998). This points to the importance of harvesting schemes (Rejmankova et al., 1990) to maintain vigorous growth. Removal rates therefore depend on growth rates, which depend (in part) on initial N and P concentrations (Fig. 5)
. Typical nutrient contents of duckweed in sewage are on the order of 22 to 63 g N kg-1 dry wt. and 3 to 14 g P kg-1 dry wt. (Körner and Vermaat, 1998; Vermaat and Hanif, 1998).
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Fig. 3. Nitrogen and phosphorus removal coefficients of duckweed-covered wastewater in dependence of the initial nutrient concentrations (85, 72, 46, 25, and 12 mg Kjeldahl N L-1 and 14, 10, 4, 2, and 1 mg total P L-1). One data point was excluded for the curve fit of Kjeldahl N. Data from Körner and Vermaat (1998).
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Fig. 4. Contribution of the different components and processes of a duckweed-covered system to the total N and P loss in shallow duckweed-covered batch systems. The terms Ndw and Pdw are uptake by duckweed; Nad and Pad are uptake by biofilm attached to duckweed; Naw and Paw are uptake by biofilm attached to the walls and/or sediment; and Nnd and Nnw are N loss due to nitrification–denitrification by biofilm attached to duckweed (nd) and walls (nw). Data from Körner and Vermaat (1998).
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Fig. 5. Nitrogen and phosphorus removal rates of fat duckweed, biofilm (uptake by attached algae and bacteria), and coupled nitrification–denitrification (n/d) of attached bacteria in duckweed-covered domestic wastewater dependent on the initial nutrient concentrations. Removal rates of duckweed refer to the surface of the system whereas those of the biofilm and nitrification–denitrification refer to the growth surface available on duckweed and walls of the system. For the curve fit of the N removal by duckweed, one data point (Experiment 1 without duckweed growth) was excluded. Data from Körner and Vermaat (1998).
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The indirect contribution of duckweed to the nutrient removal by attached algae and bacteria in the biofilm and the loss by nitrification–denitrification account for 35 to 46% and 31 to 71% of the total N and P loss, respectively (Fig. 4). Nitrification–denitrification rates in biofilms of duckweed-covered systems vary between 10 and 50 mg N m-2 d-1 depending on the initial N concentration in the sewage (Fig. 5). They appear to be low compared with nitrification–denitrification activities in other compartments like river sediments or biofilms on submerged macrophytes (30–4000 mg N m-2 d-1; Schreiner, 1984; Perchtold et al., 1988; Christensen et al., 1990; Eriksson and Weisner, 1996; Körner, 1997). The influence of the coupled nitrification–denitrification on the total N loss of the duckweed-covered system was comparable with that of the duckweed itself in small and shallow batch systems (Körner and Vermaat, 1998). Ammonium removal in deeper duckweed-covered systems was mainly attributed to nitrification (Reddy and De Busk, 1985).
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DESIGN CONSIDERATIONS
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Within the context of wastewater treatment, duckweed ponds can be qualified as secondary or tertiary treatment, meaning that pretreatment by sedimentation is always recommended to remove suspended solids (up to 65–70%) and a large part of the organic matter (30–45% of BOD). In case pretreatment is practiced in high-rate anaerobic reactors, removal rates can rise up to 75% and 80% for suspended solids and BOD respectively, while it also may ensure effective precipitation (and thus removal) of heavy metals (Arceivala, 1998; Gijzen and Veenstra, 2000).
In practice, pond depth, the organic surface loading rate, sewage temperature, and the hydraulic retention time are crucial design parameters of duckweed ponds. Based on our laboratory experiments we suggest that duckweed systems should be shallower than the range encountered for full-scale systems in the literature (Table 2) to maximize duckweed–wastewater exchange. We realize that shallower systems cause increased area demands and thus suggest that local evapotranspiration estimates be incorporated in design considerations. The implications of shallow-depth pond configurations on costs are substantial; local prices for land, excavation, and labor largely determine its economic feasibility. Basically, shallow-depth duckweed ponds tend to be favorable only in decentralized sanitation schemes with low land prices. Retention times applied in full-scale pilot plants with duckweed vary considerably (Table 2). We argue here that retention times could be kept at the lower end of this range in shallower systems, based on our own laboratory findings. Domestic sewage is known to vary substantially along with climate and public wealth, but also over days and weeks in the same collection system (Vermaat and Hanif, 1998; Körner et al., 1998). Organic loading rates and effluent characteristics of duckweed-covered systems therefore also vary considerably (Table 2). Retention times to be applied should guarantee to keep EU standards for discharge in surface water (Council of the European Communities, 1991) (Table 2). Plug flow conditions enhance nutrient removal rates (Reed et al., 1988), but neither the applied horizontal flow rate nor the ammonia concentration should be prohibitive for duckweed growth. Recirculation of effluent might create operational flexibility to lower toxicity effects and provide a buffer to pH fluctuations (Shelef and Kanarek, 1995).
Pathogen removal is of utmost importance in case of effluent reuse as well as for duckweed use as a fodder crop. In algal-based ponds pathogen removal is induced by the penetration of UV radiation and indirectly by the daily pH and dissolved oxygen fluctuations due to the photosynthetic activity. Duckweed ponds perform less well due to the intense duckweed cover preventing sunlight penetration in the water column (Gijzen and Veenstra, 2000). The inclusion of tertiary maturation ponds is recommended in case pathogen reduction has not yet reached World Health Organization guidelines. Culley and Epps (1973) suggested that dissolved oxygen also plays a crucial role in disinfection through its release of free oxygen radicals. An additional benefit of duckweed lies in the low concentration of suspended solids in the effluent, which makes it a valuable source of water for use in (drip) irrigation systems (Taylor et al., 1995).
A harvesting schedule is needed to maintain vigorous growth and nutrient removal. It should be tuned to the relevant relative growth rate of the duckweed applied, leading to doubling times ranging between 2 and 3.5 d. Removal of half the biomass or cover every third day is a practical option. A full cover of the pond will generally be ensured, since the plants will spread out. Floating duckweed may be heaped up to one side in larger ponds by wind action. In such cases floating barriers (e.g., those manufactured by Lemna International, Minneapolis, MN) should be used to maintain a fully covered pond. Potential economic outlets for the harvested material are use as feed for fish, chicken, and ducks (Landolt and Kandeler, 1987) or organic manure (Culley et al., 1981). Smith and Moelyowati (2001) anticipate a great future for duckweed ponds in tropical countries due to the high ambient temperatures, the desire to reuse treated effluent in agriculture, the relatively low per capita total cost, and the potential to stimulate local employment and income generation.
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CONCLUSIONS
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Duckweed species perform differently on different types of wastewater, with fat duckweed and great duckweed showing practical potential on domestic wastewater. The toxicity of total ammonia, a major constituent of domestic wastewater, to duckweed is a result of both ionized and un-ionized forms at low NH3 concentrations. Relative growth rates of fat duckweed decrease linearly with increasing NH3 concentrations up to a maximum level (8 mg N L-1), above which duckweed was found to die. Fat duckweed can be used to treat wastewater containing high total ammonia concentrations as long as pH levels do not exceed a level of about 8. The degradation of organic material in terms of COD (and therefore also of BOD) is faster in shallow duckweed-covered batch wastewater treatment systems compared with uncovered systems. Heterotrophic uptake of small organic compounds by duckweed itself can be excluded, whereas the provision of both additional oxygen supply and additional surface for bacterial growth explain this higher degradation. Removal of N in duckweed-covered systems can mainly be attributed to (i) N uptake by duckweed, (ii) N uptake by the attached biofilm on duckweed and walls of the system, and (iii) coupled nitrification–denitrification by these biofilms. Each of these processes accounts for approximately one-third of the total loss, and these relative contributions were independent of the initial N concentrations. Several potentially important processes were excluded in our experiments. In particular, nitrification–denitrification by bacteria on suspended flocculates could become important with increasing depth, as could NH3 volatilization, with increasing retention time and at high pH. The relative contributions to P removal (P uptake by duckweed and P uptake + adsorption by the attached biofilm) depended more strongly on initial P concentrations with a low direct contribution of duckweed at both high and low initial P concentrations. For N as well as for P, on average only one-quarter of the total losses were not directly or indirectly connected to duckweed.
Based on our results we suggest that full-scale pilot plants with duckweed should be shallower than the range encountered in literature (<50 cm). We recommend a harvesting schedule that allows doubling times of 2 to 3.5 d, maintenance of a full coverage, and plug flow conditions.
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ACKNOWLEDGMENTS
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Patrick Denny, Huub Gijzen, Fred Kruis, Don Lindenburg, Peter van der Steen, and Frank Wiegman, our former colleagues at IHE, are kindly acknowledged for their reflections and support, as are our MSc students who have participated in the research: Jose Casal, Sanjev Kumar Das, Khalid Hanif, Gwendolyn Kyoburungi, Getrud Lyatuu, Anna Mdamo, Julia Rosa Caicedo, and Sonia Silva. Thanks to Ellen Roberts and three anonymous reviewers for valuable comments on the manuscript.
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