Wal. Sci. T«h. Vol. 40. No.3. pp.341-347. 1999 C 1999 Publishedby Elsevier Science Ud on behalf of the IAWQ Printed in Great Britain. All rights reserved 0273-1223/99 $10.00 + 0.00
NUTRIENT SINKS IN A CONSTRUCTED MELALEUCA WETLAND RECEIVING SECONDARY TREATED EFFLUENT Keith G. E. Bolton and Margaret Greenway School ofEnvironmental Engineering. Griffith University. Nathan. 4 JJJ. Australia
This study examined N. P and K partitioning in the sinks of a 130 m constructed Melaleuca wetland after receiving secondary treated sewage effiuent for 21 months. The sinks examined were: 1) biomass. which was further partitioned into the harvestable above ground portion and the roots; 2) sediment; 3) gravel and; 4) the clay base. Gravel was the major nutrient storage sink, however this was a function of the high gravel particle mass (525 kg m·2) rather than high nutrient concentrations. M. altemifolia trees had the highest biomass due to high growth rates. higher planting densities, and low Iitterfall. M. quinquenervia trees were severely attacked by a sap sucking Hetcropteron (Eucerocoris. suspectus} which stunted growth, but resulted in a cumulative litterfall mass three times that of the M. alternifolia trees. The sediment sink was strongly influenced by litterfall, with the sediment sink in the M. quinquenervia terraces storing more than twice the nutrients in the sediment sink of the M. a/temi/olia terraces. Because of their higher growth rates and above ground biomass fraction, and their potential to produce tea tree oil, M. altemifolia is most suitable for constructed wetlands incorporating a harvesting regime. Because of their high rate of transfer from the biomass to sediment sink via litterfall, M. quinquenervia is more suited to non-harvested constructed wetlands. iC 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved
KEYWORDS Constructed wetland; Melaleuca; nutrient sink; secondary treated effiuent; tea tree oil. INTRODUCTION Nutrients are major pollutants in many wastewaters. There are numerous potential fates for nutrients entering a constructed wetland via the wastewater. These fates determine the removal capability of these substances from the wastewater (Kadlec and Knight, 1996). The major nutrient removal pathways in constructed wetlands are 1) loss to the atmosphere, 2) drainage from the wetland, and 3) retention in the wetland sinks . Nutrient accumulation in a wetland sink is a function of both the mass of the sink, and the concentrations of nutrients in the sink. Whilst a number of studies have investigated nutrient accumulation by wetland macrophytes (e.g. Duarte, 1992; Behrends et al., 1994; Greenway, 1997), little work has investigated nutrient partitioning and translocation in other wetland sinks. This paper quantifies nutrient and metal accumulation in the sinks of a surface flow constructed Melaleuca wetland in Loganholme, south east Queensland, Australia, which had received secondary treated effiuent for twenty one months. For the purpose of this study, the constructed Melaleuca wetland system has been divided into four wetland sinks: (I) the tree biomass sink; (2) the sediment sink; (3) the gravel sink, and (4) the clay base sink. The tree biomass sink is further partitioned into the readily harvestable above-ground 341
K. G. E. BOLTON and M. GREENWAY
biomass, and below-ground biomass. The gravel sink is further partitioned into intra-gravel sediment, and gravel particles (adsorbed portion). Me/a/euca trees readily coppice when cut which makes them suitable for repeated harvesting to remove nutrients from the wetland. Furthermore, M. a/ternifo/ia produces tea tree oil, a commodity in demand by the health-care industry (Southwell, 1988; Murtagh, 1990), and this adds financial incentive to harvest the biomass. The sediment sink is comprised of detrital accumulation above the gravel surface, and consists primarily of decomposing organic material. Transfer of nutrients and metals from the biomass sink to the sediment sink occurs via litterfall (Figure I).
biomass si nk to se diment sink tran sfer
above ground biomass
biomass sink - '"'
.~.- sed i m e n t sink
·ci 6 ' 6 : 6 ff"'tl ·-,: -
. ~... .
'~~~~~~~ .0000".: 0 l) •
, . 6 •
. 6 .. 0
0 , _
o .. ..
' . ~ ~
-gravelI sediment d . sink . -grave a sorption sink
~- - - - - - ---- - -- - - - ---- --- - - ----- -~
IIIIIIIIIIIIIIIIIIII lf- ciay sink Figure I. A schematic model of the nutrient sinks in the Loganholme constructed Melaleuca wetland.
MATERIALS AND METHODS Constructed Me/a/euca wetland The Mela/euca wetland was constructed during May 1994 at Loganholme Water Pollution Control Centre. It is situated on a clay drainage site with a slope of 5.6°, and consists of six terraces in series. Terraces are 4 m wide, and the total wetland area is 130 m2. The walls and base of the constructed wetland are made from compacted clay, and the terraces are filled to 40 em depth with mixed gravel. Since planting, the trees were continuously waterlogged with secondary treated effluent which flowed through the wetland at a rate of approximately 300 L h'l. Typically, effluent contained 5-9 mg N L'I, 4-6 mg P L'I, and 10-20 mg K L'I. Perspex V-notch weirs were inserted at the outlet of each terrace, and the water level was maintained at 10-20 em above the gravel. The top three terraces are planted with M. quinquenervia in an alternate pattern at a density of 2 trees m'2 which simulates dense natural stands. The next three terraces are planted with M. a/ternifo/ia (tea tree) in an alternate pattern at a density of 3.5 trees m'2 simulating high tea tree plantation densities. The tree biomass sink Tree growth was monitored every three months. Growth parameters measured were height, girth diameter at 15 em height and branch number. Two litter traps of 0.25 m2 area were placed at random in each terrace, excluding the edge rows. Litter was collected and weighed at three month intervals, then stored for later nutrient and metal analysis. On February 15 1996, after twenty one months of growth, two trees were randomly selected from each terrace, excluding the edge row (a total of 6 trees per species). The selected trees were gently pushed over, then pulled out of the gravel. Broken primary roots were retrieved, however fine roots were inevitably lost. Roots were washed, and harvested trees were partitioned into roots, stems,
Nutrient sinks in a constructed Melaleuca wetland
branches, leaves and bark . These port ions were bagged, then dried for 48 hours in forced air ovens at 80° C. The three monthly litter samples, and the plant components were completely ground using a hammer mill. 0.4g subsamples were digested and analysed for TKN, total P and total K. The sediment sink The flow of water through the constructed wetland was temporarily stopped to facilitate collection of sediment, gravel and clay samples. Two sampling sites per wetland terrace were selected randomly avoiding edge rows, and a steel quadrat of area 20 cm by 20 ern was placed over the sampling site. Plant litter within the quadrat was removed by hand, and the remaining sludge-like sediment was scooped up with a spoon taking care to discard roots and gravel. Sediment was dried at 80°C for 48 hours, weighed, then ground using a hammer mill . 0.4 g subsamples from each sampling site were digested and analysed for TKN, total P and total K. Further I g subsamples were placed in a muffle furnace at 550°C for thirty minutes then reweighed to calculate total volatile solids. The gravel sink The particle size distribution of the gravel is displayed in Table 1. Prior to construction of the wetland, 10 samples of gravel (initial) were collected and stored in a cupboard for two years in a sealed container. During the sampling period (21 months after construction), a 10 cm diameter PVC tube with a sharpened edge was twisted into the gravel matrix at the sampling sites in order to extract a core sample (final). Live roots were removed from the gravel core . All gravel samples were rinsed with deionised water, and the rinse water was sieved then filtered (1 film filter paper) to collect the intra-gravel sediment. The sediment was dried, weighed, then ground in a ring grinder. From each gravel sample, 0.4 g subsamples of intra-gravel sediment, and 5 g sub samples of washed gravel were digested and analysed for TKN, total P and total K. Initial nutrient concentrations were subtracted from final concentrations to calculate accumulated nutrient concentrations. Nutrients extracted from the washed gravel were considered to be adsorbed nutrients. Further 1 g subsamples of intra-gravel sediment and 50 g subsamples of washed gravel were placed in a muffle furnace at 550°C for thirty minutes and one hour, respectively, then re-weighed to calculate total volatile solids. Table 1. The composition of the gravel used in the constructed Melaleuca wetland at Loganholme Water Pollution Control Centre
Gravel diameter >4.75 rom - <12 rom >2 rom - <4.75 mm >1 mm-<2mm >0.5 mm - <1 mm
% of gravel composition 24% 43% 13% 10% 10%
The clay sink During construction of the wetland, ten samples of clay were taken from the clay base to a depth of 5 em (initial). They were dried, ring-ground, then stored at -10°C for two years. During the sampling period a core of clay from the wetland base was sampled to a depth of 5 em (final) . Clay samples were dried for 24 hours at 105°C then ring-ground. 0.4 g subsamples of clay were digested and analysed for TKN, Total P and total K. Initial nutrient concentrations were subtracted from final concentrations to calculate accumulated nutrient concentrations. A further 10 g subsample from each sampling site was placed in a muffle furnace at 550°C for thirty minutes then re-weighed to calculate total volatile solids. The N, P and K capacity (g m·2) of each sink was calculated by multiplying the sink mass (kg m,2) by its N, P and K concentration (g kg").
K. G. E. BOLTON and M. GREENWAY
RESULTS Tree growth Although they were more densely planted, M alternifolia trees were taller, had thicker stems, and had more branches than M quinquenervia trees after 21 months exposure to secondary treated effiuent (Table 2). Growth of both species was retarded during winter. Table 2. Growth parameters of Melaleuca trees receiving secondary treated effiuent in the Loganholme constructed wetland for 21 months Species
M. alternifolia M. quinquenervia
Height mean ± standard deviation 2890 ± 305 1605 ± 275
Girth diameter Mean ± standard deviation
52 ± 10 48 ± 14
Branch number mean ± standard deviation 485 ± 140 101 ± 50
M quinquenervia trees were severely damaged by the sap-sucking Heteropteron Eucerocoris suspectus, resulting in high leaf loss. In contrast, the M. alternifolia trees remained relatively free from insect attack. Consequently, after 21 months growth, the M. quinquenervia trees had shed three times more litter (0.48 ± 0.07 kg m·2) than the M alternifolia trees (0.16 ± 0.06 kg m"), This represented a transfer from the biomass to sediment sink of 12.2 g N m", 2.8 g P m-2 and 5.0 g K m'2 in the M. quinquenervia terraces, and 8.5 g N m·2, 1.0 g P m· 2 and 1.9 g K m· 2 in the M. alternifolia terraces. Mass and nutrient concentrations of wetland sinks Gravel had the highest total mass per square metre followed by clay. Due to higher tree growth and higher planting densities of the M. alternifolia trees, plant biomass per square metre in the M alternifolia terraces was more than three times greater than plant biomass in the M. quinquenervia terraces. Furthermore, the M alternifolia trees partitioned a higher proportion of their biomass above-ground than the M quinquenervia trees (with above ground/below ground biomass ratios of 2.2 and 1.6 respectively). Because of their higher litterfall rates, the sediment mass in the M quinquenervia terraces was 1.6 times greater than sediment mass in the M. alternifolia terraces. Gravel particles had the lowest concentrations of accumulated nutrients, however nutrient concentrations in the intra-gravel sediment was two orders of magnitude greater than the gravel particles (Table 3). Table 3. Mass and nutrient concentrations of the nutrient sinks in the Loganholme constructed Melaleuca wetland receiving secondary treated effiuent for 21 months Nutrient sink and species Biomass M. altemifolia (AG) M. alternifolia (BG) M. quinquenervia (AG) M quinquenervia (BG) Sediment M. alternifolia
N concentration Mgkg-·
P concentration mgkg· 1
K concentration mgkg· 1
3.31 ± 0.65 1.51 ± 0.35 0.93 ± 0.22 0.59 ± 0.17
7.7 ± 1.2 9.1± 1.8 8.0 ± 1.7 9.2± 3.1
1.1 ± 0.2 2.0±0.3 1.5 ± 0.2 2.6±0.6
4.0 ± 1.0 4.0±0.8 4.0± 1.0 3.9±0.8
1.59 ± 0.43 2.49 ± 0.78
6.4 ± 1.8 9.4 ± 0.7
1.9 ± 0.3 2.3 ± 0.3
2.3 ± 0.5 2.2 ±0.4
Gravel M. alternifolia (Ads) 525± 19 0.01 ± 0.00 0.02±0.01 M alternifolia (IGS) 3.66 ± 1.1 6.2 ± 1.6 1.4 ± 0.5 M. qutnquenervia (Ads) 525 ± 19 0.06± 0.03 0.03 ± 0.01 M. qutnquenervia (IGS) 4.33 ± 1.1 6.0 ± 1.0 1.3 ± 0.6 Clay M. IIlternifolill 72± I 0.4 ± 0.2 0.11 ± 0.04 M. qutnquenervie 72± 1 0.3±0.1 0.09 ± 0.03 AG - above ground; BG - below ground; Ads = adsorbed portion; IGS = intra-gravel sediment
0.01 ± 0.00 4.3 ± 1.0 0.04±0.01 3.9 ± 0.9 0.11 ± 0.03 0.10 ± 0.03
Nutrient sinks in a constructed Melaleu ca wetland
Sediment contained 82% ± 9% volatile solids, and the intra-grav el sed iment contain ed 73% ± 12% volat ile solids. In contrast, the washed gra vel contained < I% and the cla y contained 3% vo latile sol ids . Nutrient part itioning in the constructed wetland sinks.
In the M. alternifolia terraces, tree biomass sink capacity exceeded the sed iment sink capacity for all nutrients. In contrast, in the M . quinquenervia terraces, the sedime nt sink capacity exceeded the biomass sink capacity for both N and P. For both speci es , more than half of the nutrients were stored in the aboveground component. Gravel and clay were both significant nutrient sinks, and combined, these two sinks stored more than half of the total accumulated nutrients. In the gravel sink , the bulk of the N and K was stored in the intra-gravel sediment, however approximately two thi rds of the P was adsorbed to the gravel surface (Figure 2).
M . altem lfo lla
M . qulnquenervla Tota l N stora g e
b io mass
M. quinq uenervia terra ces t 06 l: N rn'
M. a/tertii/alia terraces 91l: Nm" 28%
g rav el
-- nO ",.wl
M . al temlfolla
M . qulnquenervla
bio m as s
Total P stora ge M. quinqu enervia terraces 34 g Pm"
M. alternifolia terraces 3 1 g Pm"
M. al temlfolla
M. qu/nq uen erv la Total K s to ra ge M. quinquenervia terraces S2 g K m"
M. alternifo lia terraces 4\ g Km'
Figure 2. Nutrient part ition ing in the sinks of the constru cted Melaleuca wetl and.
DISCUSSION Nutrient accumulation in the wetland sinks pro vided an important nutri ent removal mechanism in the co nstructed Melaleuca wetland. With mean wastewater N, P and K con centrations o f 7, 5 and 15 mg L' \ resp ectively, and a me an flow rate of 300 L h' l (0.5 ML ha' i d' l), nutricnt sto rage in the wet land sinks
K. G. E. BOLTON and M. GREENWAY
accounted for 46% of the N, 21% of the P and 11% of the K which flowed through the wetland via secondary treated effiuent over the 21 month growth period. Constructed Me/a/eu ca wetlands may therefore be suitable for effiuent polishing. The tree biomass sink stored a considerable portion of the accumulated nutrients, with the M. alternifolia trees accumulating approximately one third of the total NPK mass within their terraces. Furthermore, continued growth of both species beyond the 21 month sampling period indicates that, if left unharvested, the biomass sink would increase its capacity. Because ofthe lower growth of both species during winter, it is likely that nutrient accumulation by the biomass would be reduced, and this should be taken into account when designing stringent nutrient budgets. Since one half of the nutrients were partitioned into the above ground harvestable portion, tree harvesting would therefore provide a significant means of nutrient removal from the wetland system, particularly from the M. altemifolia terraces. Since M. alternifolia can also produce tea tree oil, it is an excellent choice the use in a constructed wetland which incorporates a harvesting programme. In the constructed wetland environment, the two Me/a/euca species displayed different biomass accumulation strategies. With its high growth rates, and lower litterfall, M. alternifolia was a nutrient accumulator. Because of the high defoliation and resultant lower growth caused by E. suspectus damage, M. quinquenervia was a nutrient transferor. Furthermore, M. a/ternifo/ia partitioned a higher proportion of bioaccumulated nutrients into the aboveground harvestable biomass portion. These different bioaccumulation strategies can be utilised when designing a constructed wetland. If a harvesting regime is to be used in the management of a constructed Me/a/euca wetland, then M. alternifolia would be the best choice of the two species examined. If harvesting is not incorporated, M. quinquenervia trees may be more suitable because they more rapidly transfer nutrients from the tree biomass sink to the long-term sediment sink. A caution to those considering incorporating a nutrient and/or tea tree oil harvesting regime into a constructed Me/aleuca wetland: it is important to discontinue waterlogging a week prior to harvesting, but to keep the gravel substrate well watered until coppice buds are 5 em long. If the Me/a/euca trees remain waterlogged after harvesting, there will be high tree mortality (Bolton, submitted).
The gravel was the major sink.for all three nutrients examined, however this was largely due to its high mass and volume rather than overall nutrient concentration. Within the gravel matrix, the nutrients were strongly concentrated in the intra-gravel sediment with nutrient concentrations approximating those of the sediment sink. above the gravel surface. The high proportion of volatile solids (73%) in the intra-gravel sediment infers that the accumulated nutrients are in a predominantly organic form. Intra-gravel sediment is likely to result from sloughed biofilm, sloughed roots, and leaching from the above-gravel sediment. The low concentrations of adsorbed nutrients is typical of gravel, as its relatively low surface area contains few adsorption sites compared with soil and clay (Kadlec and Knight, 1996). Similar to the intra-gravel sediment, the above-gravel sediment contained a high proportion of organic material, indicating that it was composed primarily of litter and microbial fauna and flora rather than inorganic substances. The sediment sink was strongly influenced by litterfall, with the sediment sink of the insect-struck M. quinquenervia terraces accumulating more than twice the mass and nutrients than the sediment sink of the M. a/ternifo/ia terraces. The high severity of E. suspectus infestations on M. quinquenervla therefore provided a significant pathway of transfer from the biomass sink to the sediment sink. Bolton and Greenway (l997b) hypothesised that the high rate of infestation of E. suspectus in constructed wetland grown M. quinquenervia is the result of high nutrient concentrations in their leaves leading to a higher desirability to this sap sucking insect. The higher sediment load in the M. quinquenervia terraces may also be partly due to the fact that they were situated upstream from the M. a/ternifolia terracesa number of studies have noted that sediment accumulates in the upper reaches of constructed wetlands (e.g. Kadlec, 1994; Bolton and Greenway I997a). Since the sediment sink. provides long-term storage for nutrients (Kadlec, 1994), the high litterfall of M. quinquenervia can be put to use in a constructed wetland provided that it is situated in the range that E. suspectus occupies. Since the clay sink. was comprised primarily of inorganic substances, this indicates that the stored nutrients were predominantly adsorbed to the clay particles. This is to be expected, since clay has a much greater
Nutnent sinks in a constructed Melaleuca wetland
surface area than gravel and therefore has a higher density of binding sites for nutrients (Brady and Weil, 1996). The capacity of the clay sink would therefore be defined by the adsorption capacity of the clay minerals. ACKNOWLEDGEMENTS Funding and facilities for this project were jointly provided by Griffith University, Logan City Council and The Department of Employment Education and Training. Many thanks are due to the staff at Loganholme Water Pollution Control Centre, and to Scott Byrnes of Griffith University for their excellent technical assistance and advice. REFERENCES Behrends , L. L, Bailey, M. J., Bulls, M. 1., Coonrod, H. S. and Sikora, F. J. (1994) . Seasonal trends in growth and biomass accumulation of selected nutrients and metals in six species of emergent aquatic macrophytes. In: Preprint of the 4th International Conference on Wetland Systems for Water Pollution Control, Guangzhou, PRC, 6-10 November, pp , 274-289 . Bolton, K. G. E. (submitted). From Wastes to Resources. Constructed Melaleuca wetlands for sewage treatment works. PhD thesis, School of Environmental Engineenng, Griffith University. Bolton, K. G. E. and Greenway, M. (l997a). Constructed Melaleuca wetlands: an integral component for sewage treatment works . In Proceedings ofBNR3, Brisbane Convention Centre, Brisbane. 30 November - 4 December 1997. Bolton, K. G. E. and Greenway, M. (1997b). A feasibility study of Melaleuca trees for use in constructed wetlands in subtropical Australia . Wat. Sci. Tech., 35(5), 247-254 . Brady, N. C. and Weil, R. R. (1996). The Nature and Properties ofSoils. Eleventh Edit ion. Prentice-Hall International, Inc. New Jersey, USA, pp. 740. Duarte , C. M. (1992). Nutrient concentration ofaquatic plants : Patterns across species. Limnol, Oceanogr. , 37(4) ,882-889. Greenway, M. (1997) . Nutrient bioaccumulation in wetland plants receiving municipal effiuent in constructed wetlands in tropical Austral ia. Wat. Sci. Tech., 35(5), 135-142. Kadlec:, R. H. (1995) . Overview: Surface flow constructed wetlands. Wat. Sci. Tech., 32(3) , 1-12. Kadlec, R. H. and Knight, R. L. (1996) . Treatment Wetlands. CRC Press, Inc. Florida , USA. Murtagh, G. J. (1990) . Tea-tree oil- plantation production. Agfact first edition 1990. NSW Agriculture and Fisheries. Southwell, I. A. (1988). Australian tea tree: oil of Melaleuca terpenen-4-o1 type . Chern. in Aust. November 1988,400-402.