Phosphorus removal rates in bucket size planted wetlands with a vertical hydraulic flow

PII: S004-3135(49)70021-1X

Wat. Res. Vol. 32, No. 4, pp. 1280±1286, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00

PHOSPHORUS REMOVAL RATES IN BUCKET SIZE PLANTED WETLANDS WITH A VERTICAL HYDRAULIC FLOW I. R. LANTZKE*, A. D. HERITAGE, G. PISTILLO and D. S. MITCHELL{ CSIRO Division of Water Resources, Grith, NSW 2680, Australia (First received February 1996; accepted in revised form June 1997) AbstractÐRate studies of orthophosphate removal from wastewater in Schoenoplectus validus Vahl. (A Love and D Love) planted bucket-sized vertical up¯ow wetlands occurs simultaneously through three removal processes. In order of decreasing rate, these are reversible sorption to the gravel substratum, reversible conversion to complex phosphorus compounds, and irreversible uptake by plant roots. Mixing the rhizosphere liquid, using forced convection, accelerated orthophosphate removal; concentrations falling from 110 mg litreÿ1 to <0.2 mg litreÿ1 in <48 h. The rate determining step in systems without rhizosphere liquid mixing appears to be di€usive transport of phosphate. Over two years, initial removal rates, and minimum reactive phosphorus concentrations changed little, but aged or high BOD systems sometimes released reactive phosphorus or tannins holding large amounts of phosphorus. The relevance of these mechanisms to total wetland assimilative capacity, and e‚uent quality, are evaluated. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐarti®cial wetland, emergent aquatic macrophyte, wastewater treatment, removal rate, phosphorus, Schoenoplectus validus

INTRODUCTION

The mechanisms of phosphorus (P) removal in planted constructed wetlands are incompletely understood (Juwarkar, et al., 1992; Knight et al., 1992), although mass-balance studies have quanti®ed signi®cant phosphorus accumulation in the plants and substratum (Breen, 1990; Rogers et al., 1990; Sharma, 1992). Figure 1 shows a conceptual model of the reaction paths considered available for phosphorus removal by subsurface ¯ow, constructed wetlands. However, uncertainty exists over the magnitudes of these reactions and of microbial conservation of phosphate to soluble polyphosphates or organic phosphorus compounds (nrP), changing reactive phosphorus (rP):nrP ratios (Moriarty and Boon, 1990). Determining removal rates of rP, and ®nal reactant and product P concentrations, could identify the rate determining removal processes, primary uptake sites, and minimum retention times, as experimental rates of orthophosphate (PO4) removal can be used to evaluate possible mechanisms and operating conditions. *Author to whom all correspondence should be addressed. Permanent address: 4 Ailsa St, Wembley Downs, WA 6019, Australia. {Murray Darling Freshwater Research Centre, Albury, NSW 2640, Australia.

Root uptake is an active process, principally occurring at root hairs (Sorrell and Orr, 1993), PO4-concentration-independent above a very low threshold (probably R0.05 ppm) (B.K. Sorrell pers. comm.). Unless di€usion controlled, the rate of PO4 removal by roots will be concentration independent in the range considered here. Bio®lm assimilation of PO4 at molar C:P ratios below about 80 is likely to be carbon-limited, so PO4 removal would be faster in high BOD wastewaters. The rate should be PO4-concentration-independent at low BOD but proportional once carbon availability ceased to be limiting. If PO4 sorption onto fresh gravel is rate determining, the kinetic form should be ®rst order (ln(co/ c) = k1t; co and c reactant concentrations at times 0 and t respectively, k1 a constant and t time), for approach to equilibrium, since the opposing desorption increases with increasing sorbed PO4. In soils the initial fast step is limited by availability of sorption sites, with the theoretical and observed rates described by a linear log[PO4] vs log t relationship, (where [x] = concentration of species x) and sorption follows the Freundlich adsorption equation (Barrow and Shaw, 1975). System ageing may indicate the signi®cance of sorption and precipitation. Sorption should result in slower uptake and higher equilibrium PO4 concentrations as sites become progressively saturated, although, if variable charge uptake sites are

1280

Phosphorous removal in constructed wetlands

1281

This paper reports the mechanisms operating in these systems; the relative PO4 removal rates of the major processes; and the in¯uence of wetland age, BOD, and temperature upon them.

METHODS AND MATERIALS

Fig. 1. Possible reaction paths for removal of orthophosphate in planted constructed wetlands (rP, reactive phosphorus; nrP, nonreactive phosphorus).

involved, loss of sorptive sites will also be timedependent (Barrow, 1974; Barrow, 1983a). Insoluble organic residues would progressively ®ll pore spaces, as the proposed chemical precipitates will not redissolve or transfer to plants in the timescale considered here. Small upwards vertical ¯ow systems (VFWs), with strong plant growth, have given large reductions in total phosphorus (TP) over less than 12 months (Breen, 1990; Rogers et al., 1990; Sharma, 1992). Over 2 months Breen (1990), using 10-litre buckets, with daily internal mixing (pers comm.), retained >95% of in¯uent TP load, with 67% of in¯uent P in young, vigorously growing, Typha orientalis Presl. P precipitates were insigni®cant, 98% of the removed P being found in plants, bio®lms, or sorbed to the gravel. With two shoot harvests, over 10 months of sustained Schoenoplectus validus Vahl. (A Love and D Love) growth, Sharma (1992) obtained above-ground plant P of 95% of in¯uent sewage TP using 25-litre buckets, loaded at a mean 90 mg TPmÿ2 dÿ1. To reconcile the diverse results on P removal by constructed plant wetlands, and to establish a quantitative model for the processes, a series of detailed investigations has been undertaken, using VFWs.

Rates of PO4 removal from wastewater in identically loaded unplanted (control) and planted (experimental) buckets were determined over 2 years. Variables were bucket and plant age; nutrient mixing; BOD, varied by addition of glucose; and temperature. Rates were calculated from changes in rhizophere concentrations of rP, nrP, NH+ 4 , O2, and glucose (in high BOD experiments), using batch loading; di€erent stages of plant growth; and two BODs. In¯uent variability was eliminated by using a synthetic sewage based on Hoagland's solution. Sampling times and operating conditions were those expected to provide maximum information in the available time. Four series of kinetic runs were conducted using VFWs (Breen 1990) planted with Schoenoplectus validus in washed river gravel (3±7 mm diam.) as substratum; a central ®lling tube to the bottom; over¯ow spiggots 30 mm below the gravel surface; and interstitial sampling ports. Insulated buckets and a thermostatted growth cabinet were used, (except Series 2 which operated in a glasshouse), with lighting set for 12 h per day. Light intensity (PAR) ranged between 550 mMmÿ2 sÿ1 at mid-leaf height to 650 mMmÿ2 sÿ1 at leaf tip height. In runs 1 and 3±10 the S. validus were actively increasing in biomass, but in run 2 they had completed seed setting. Experimental systems See Table 1 for details. System hydrology Trials showed forced cycling of rhizosphere liquid eliminated depth e€ects on sample composition. Mixing was achieved by pumping liquid from the out¯ow spiggots back into the central in¯uent column. Dye studies con®rmed complete mixing in all systems after cycling twobucket volumes. A ¯ow rate of 3.6 litre hÿ1 was used in all mixing experiments (Series 2±4). Trials of the e€ect of pumping rate on PO4 uptake showed that low ¯ows caused most change, but short spells of zero, or high speed, pumping during a run appeared to be smoothed out in the ®nal data. Speed of pumping was not investigated in detail and further investigation of pumping e€ects is warranted.

Table 1. Details of buckets and treatments. C = unplanted control; E = experimental; B1 and B2 = buckets 1 and 2 Experimental treatments Run no. 1 2 3 4 5 6 7 8 8 9 10

C E C B1 B2 C E C&E C&E C&E C&E C E C&E C&E

Bucket age

Gravel mass. (kg)

Drainage vol. (SD) (litres)

No. of buckets

Temp. (8C)

8 wks 8 wks 6 mths 6 mths 6 mths 12 mths 12 mths 12 mths 12 mths 12 mths 12 mths 23 mths 23 mths 23 mths 23 mths

10.6 10.6 36 36 36 36 36 36 36 36 36 36 36 36 36

2.052 0.03 1.912 0.05 6 2 0.5 6 2 0.5 6 2 0.5 6.4 2 0.3 5.4 2 0.2 5.4 2 0.2 5.4 2 0.2 5.4 2 0.2 5.4 2 0.2 6.6 2 0.5 4.3 2 0.1 4.3 2 0.1 4.3 2 0.1

2 3 1 1 1 2 3 3 3 3 3 3 3 3 3

30 20.1 30 20.1 15-30 15±30 15±30 20 20.1 20 20.1 20 20.1 20 20.1 20 20.1 20 20.1 20 20.1 20 20.1 20 20.1 30 20.1

1282

I. R. Lantzke et al. Table 2. Analytic methods

Method Ammonia Reactive phosphorus Total phosphorus Oxygen Glucose

Analysis

Additional information

FIA; di€usion method Un®ltered samples/molybdenum blue method (ascorbic acid reductant) Un®ltered samples/persulphate digestion Dissolved oxygen meter Spectrophotometric

(Tecator application note 50/84) Technicon Industrial Method No. 329-74W/B APHA (1989) Orion Model 820/Radiometer with E50o46 electrode Phenol-sulphuric acid (Dubois et al., 1956)

RESULTS

System establishment characteristics and experimental operation Experimental buckets were planted with 19 seedlings of S. validus, and between experiments, control and experimental buckets were watered daily, using an automated trickle system. Twenty ®ve litre buckets (Series 2±4) were fertilized weekly with 1±1.5 litre Hortico ``Aquasol'', (P 4%, K 18%, N 23% and trace elements) at 12 mg-P litreÿ1 concentration. Series 1 buckets were loaded with 25 mg-P (``Aquasol'') twice weekly. For kinetic measurements, thermastatted buckets were drained of all freely ¯owing liquid, then ®lled with thermally equilibrated solution, after which forced convection commenced (except Series 1). Completion of ®lling was taken as the starting time of reactions. Initial concentrations were calculated from the volume and concentration of nutrient added, and the total drainage volume of each bucket. Water levels were topped up with deionized water twice per day. Results were not corrected for increases in nutrient concentration due to evapotranspiration (110% volume loss per day in planted systems), as the increases (5%) were within the general accuracy of the study. In Series 4, interference from tannin accumulation was minimized by ¯ushing buckets between runs with seven volume changes of tap water. Hoagland's nutrient solution, bu€ered with 0.01 M sodium citrate, was used as arti®cial sewage. It supplied all nitrogen as NH+ 4 (in an 8:1 N:P weight ratio) and gave control bucket pHs of 6.1±6.9. In planted systems the pH initially fell rapidly to 5.3±6.4, then stabilized. Increased BOD was achieved by adding glucose. Substratum characterization The mineral composition of the gravel substratum was determined as >95% quartz, minor amounts (<5%) of albite type feldspar, traces (21/2%) mica, and traces of an undetermined clay mineral. Shaken ¯ask phosphate sorption determinations (20 g in 50 ml of 6 mg-P litreÿ1 PO4 solution, made 31.4 mM in KCl to maintain the ionic strength similar to that of the nutrient; pH 7.8±7.9), extrapolated to an equilibrium time of 120 h. Applying the Langmuir equation gave a maximum adsorption of 20 mgPgÿ1. Chemical analyses See Table 2 for analytical details. Several interferences occurred with analyses for phosphorus from buckets older than one season or with coloured out¯ows, as samples collected after 4±5 half-lives sometimes darkened slowly precipitating rP containing brown gum. Making fresh samples about 1 M in H+ retained rP available for analysis, without releasing other P. Standard persulphate digestion (APHA, 1989) of low BOD out¯ows gave good recoveries of TP and identical results to digestions using selenium catalysed sulphuric acid. Coloured and high BOD samples inconsistently gave low TP values, but adding 1 ml of 20 g litreÿ1 ascorbic acid per 50 ml of warm digest gave good TP values.

Phosphorus dynamics Rates of rP removal did not ®t log[PO4] vs t, or log t, or 1/[PO4] vs t equations, but with rhizosphere mixing the second-order kinetic equation gave the best description, being linear for 3±5 halflives. Orthophosphate removal in isothermal VFWs Series 1 Two-month-old systems. Dye studies showed very limited, irregular liquid movement; changes in rP concentration, were variable. Removal was faster in the upper portions of all treatments, and faster in planted than control buckets. Subsequent examination showed plant roots con®ned to the upper portions of experimental buckets. After 48 h, concentrations of rP were less in planted root zones, (2 mg litreÿ1), than at the bottom or at equivalent depths in unplanted buckets (4 mg litreÿ1) and greatest (15 mg litreÿ1) at the unplanted bottom (SD 0.7±0.9). At all depths, 20± 30% of TP was nrP in planted buckets and in the upper portion of unplanted buckets, and 40% at the bottom of the latter. Uneven P removal in unplanted buckets is tentatively attributed to redox e€ects, where after only 5 h oxygen concentrations (0.4 mg litreÿ1), were less than half those at the bottom of planted buckets. The di€erence in oxygen concentrations may have resulted from the larger volumes of (oxygenated) evapotranspirational make-up water added to planted buckets. Estimates of the initial rates of rP removal, using ®rst half-life (t1/2) values are compared in Table 2 with t1/2 values with nutrient cycling (Run 10, below) (Table 3). This shows cycling increased the rP removal rate from 10- to 50-fold.

Table 3. First half-life (h) for removal of rP in 9-litre unmixed and 25-litre mixed buckets at 308C Sampling location Treatment

Top

Middle

Unmixed, unplanted Unmixed, planted Mixed, unplanted Mixed, planted

13 5

24 6 0.8 10.1

Bottom 37 16

Phosphorous removal in constructed wetlands

1283

Table 4. Parameters for reactive phosphorus removal from the bucket wetlands at 6, 12 and 23 months (C, unplanted; B1, B2, buckets 1 and 2; Exp, planted) Initial concentration

12 months 3 4 5 6 7 23 months 8 9 10

First t1/2 (h)

rP (mg litreÿ1) (at h)

nrP (mg litreÿ1) (at h)

Treatment

PO4-P (mg litreÿ1)

C B.1 B.2

low BOD

11.9 12.0 13.2

6.8 13.5 17.5

1.4 (72) 1.7 (72) 3.0 (72)

Ð Ð Ð

C Exp C Exp C Exp C Exp C Exp

low BOD

7.9 9.3 7.9 9.3 8.0 9.3 9.9 11.6 9.9 11.6

0.8 0.4 0.4 0.2 1.0 0.3 2.6 0.5 0.6 0.2

0.3 (94) 0.1 (94) 0.4 (45) 0.12 (45) 0.9 (52) 0.1 (52) Ð Ð Ð Ð

0.15 (94) 0.05 (94) 0.17 (45) 0.05 (45) 0.1 (46) 0.3 (46) Ð Ð Ð Ð

Run no. 6 months 2

Minimum concentration

Glucose (mg litreÿ1)

C Exp C Exp C Exp

low BOD high BOD high BOD low BOD low BOD high BOD low BOD

7.6 7.2 7.6 7.0 6.6 6.0

176 176 176 176

120 120

Orthophosphate removal in VFWs with forced convection

1.1 0.2 1.1 0.2 0.8 10.1

<1.5 (>25) 0.6 (6) <0.8 (>26) 0.5 (3.2) <0.9 (>23) 0.5 (1.1)

0.2 0.2 0.4 0.2 0.3 0.3

(23) (23) (23) (23) (23) (23)

Series 2 Six-month-old systems. The rP concentration in all buckets fell rapidly (as observed for all systems with forced convection; Table 4). Removal in bucket 2 (slower cycling) was slower than in the control and bucket 1, which had similar, faster, cycling rates. Removal rates in these latter, were little di€erent, suggesting PO4 uptake by plants is slight after seed setting. In all buckets rP decreased towards small (1±2 mg litreÿ1), ®nite concentrations. Series 3 Twelve-month-old systems. The rate of rP removal in ®ve sequential kinetic runs (3±7; Table 4; Fig. 2) using low and high BOD was similar to, but faster than, that at 6 months. Within the accuracy of the data (210%) there was no di€erence in PO4 removal rate directly attributable to high BOD.

Series 4 Twenty-three-month-old systems. The initial rates of rP removal from low and high BOD nutrient, at 208C, were similar to those at 1 year (Table 4). At 308C initial removal doubled in planted systems, and increased 30% in unplanted systems. At later times (Fig. 3), rP concentrations increased in planted buckets. High BOD increased the rate of this release; after 20 h rP concentrations were approximately double those in low BOD. The higher temperature had a similar e€ect. This release prevented estimation of the free rP concentration of the sorption equilibrium in older (>1 y) planted rhizospheres and is under further investigation. At 208C nrP formation commenced in the ®rst 45 min in low and high BOD, and in 5±10 h reached similar equilibrium values (low, 0.2 mg litreÿ1; high, 0.3 mg litreÿ1). These were higher than at 12 months (planted, 0.05 mg litreÿ1). At 308C the equilibrium nrP concentrations (ca 0.25

Fig. 2. Change in rP concentrations in the rhizosphere of 12-month-old planted and unplanted buckets, during spring, using forced convection (SE, standard error).

Fig. 3. Change in rp concentrations in the rhizophere of planted and unplanted, 23-month-old buckets.

1284

I. R. Lantzke et al. Table 5. Initial rates, (in mg hÿ1 bucketÿ1) of rP removal, (210%), and nrP formation, (220%), at 23 months Run No. BOD Temp.

8 Low 208C rP

Control Exp.

24 30

9 High 208C nrP 0.3 0.2

rP 24 26

mg litreÿ1) did not di€er greatly in planted or control buckets, or from the 208C values. Initial rates of rP removal and nrP formation (Table 5), calculated from concentrations-vs-time curves, indicate little e€ect of BOD, or temperature increase, on the initial rP removal rate in all systems, and show nrP formation more than an order of magnitude slower than rP removal during this period of maximum removal. The similarity of equilibrium nrP concentrations at both temperatures shows rates of micro¯ora processing of rP into nrP are balanced by the rates of nrP hydrolysis. Also the e€ective rate of nrP accumulation is independent of bulk rP concentration: nrP concentrations remained steady from 5 to 24 h, while rP concentrations rose one- to three-fold in planted buckets and fell one-fold in unplanted buckets. DISCUSSION

Mechanism of phosphate removal Despite the lack of adequate ®t of the expected kinetic equations to the results, orthophosphate removal can be described fully by three parallel removal paths: gravel sorption; micro¯ora processing; and macrophyte uptake. The major site for initial PO4 uptake must be gravel, as sorption is the only process with the necessary speed; complex kinetic order; and capacity. Microbial PO4 removal is more than one order of magnitude slower, shown by the formation of soluble nrP, and absence of a direct BOD e€ect. Plant uptake is still slower (see below). Precipitation of insoluble nrP would not show the observed rapid initial removal rate, decreasing with time, and would involve much greater volume loss, which between 12 and 23 months was nil in control and 20 26% in experimental buckets. Over the time taken to establish sorption equilibrium (48±150 h) the major quantity of PO4 removal will be to gravel, with sorption determining rP concentrations over short time intervals, although the rate of PO4 removal is the sum of all three processes. Large di€erences between planted and unplanted rP removal rates result from gravel rejuvenation between intermittent loads, due to steady plant PO4 uptake not to rapid plant uptake. Removal rates in frequently loaded buckets were suciently similar

10 Low 308C nrP 0.5 0.2

rP 22 24

nrP 1.8 0.3

for di€erences due to plant uptake to appear only after 24 h. Taking the di€erence between planted and control TP concentrations after 46 h in Run 4 (Fig. 2) as due to macrophyte uptake, gives plant P uptake as only 0.6 mg Phÿ1g (above ground dry weight)ÿ1. The large increase in PO4 removal during forced convection to give a rate following second-order kinetics (i.e. proportional to [PO4]2), implies a rate determining step of two PO4 ions in the activated complex. This is hard to visualize; it seems more probable the rate is di€usion limited, i.e. PO4 assimilation is much faster than its transport rate. Forced convection will decrease di€usion times since the di€usion rate is dependent on both concentration and di€usion distance. With constant liquid mixing, PO4 concentrations adjacent to removal sites are maximized, and increased ¯ow reduces the thickness of the hydrodynamic boundary layer surrounding gravel surfaces. However, plots of log (CtÿC1) vs log t, (Ct and C1=[PO4-P] at t = t and t = 1), expected to be linear for di€usion dependent removal of PO4 from a well-mixed solution of limited volume (Crank, 1964), were not observed; perhaps faster mixing will reveal some other rate determining PO4 removal process. The temperature coecient of 3% per degree suggests it is still di€usion limited, as di€usion rates increase about 2% per degree, whereas chemical reactions increase by 5±15% per degree. The extent of sorption to this essentially quartz gravel is surprising, as silica soils usually provide limited PO4 sorptive capacity (Gerritse and Scho®eld, 1989). However, the wetland PO4 removal mechanism di€ered from that in soils. Barrow's (1983b) soil sorption equation requires HPO2ÿ to be the sorbed species, but plotting 4 log[HPO2ÿ 4 ] vs log t gave lines equally as curved as the corresponding log[PO4-P] vs log t plots (linear <3 half-lives).

Other variables Bio®lm Although net nrP formation is independent of rP concentration (at least down to about 0.5 mg-PO4-P litreÿ1), good bio®lm±liquid contact gave lower equilibrium nrP concentrations (0.05± 0.4 mg litreÿ1 depending on age and treatment) than no mixing (0.6±2.0 mg litreÿ1). It also seems likely that adequate bio®lm±liquid contact will enable

Phosphorous removal in constructed wetlands

hydrolysis of many of the complex phosphates in wastewater. Temperature Between 20 and 308C the overall removal rate showed a temperature coecient of 3% per degree, with little change in equilibrium nrP concentration. If these e€ects apply down to around 108C, winter retention times need only increase 40% to achieve similar phosphorus removal, with similar nrP concentrations. BOD Although the direct e€ect of high BOD on the rate of PO4 removal and equilibrium nrP concentrations was small, high BOD in planted buckets, older than 12 months, had signi®cant indirect e€ects, causing increases in the free rP concentration. The e€ect is similar to, and probably correlated with, wetland ageing. Age The e€ects of ageing are complex and still under investigation. Firstly, the long-term rhizosphere capacity was larger than the shaken-¯ask predictions of 20± 25 mg-P gÿ1, an e€ect apparently related to the gravel. Series 4 buckets had been loaded with 55 mg gÿ1 yÿ1, and similar gravels, regularly loaded with sewage, carried 85 mg-Pgÿ1 (Sharma, 1992). In soils, repeated loading with high PO4 concentrations is still followed by repeated sorption (Barrow, 1983a), explained as resulting from slow solid-state di€usion, of HPO2ÿ 4 , into the adsorbing particles. Secondly, the initial rates of PO4 removal increased over the 2 years (Table 4), and minimum rP concentrations remained low, even in controls, which is consistent with increased capacity. Finally, aged planted buckets released rP; iron; coloured minerals; and insoluble material, sometimes containing strongly held P. Old roots were important, not just high BOD, since no unplanted control showed these e€ects. With sucient rhizosphere ¯ushing, insoluble material was not visible; slower ¯ushing produced a colloid or ®ne suspension. The e€ect was also observed in bucket (Sharma, 1992) and larger tank VFWs (Heritage, 1992), loaded with sewage, and could arise from higher micro¯ora levels with old roots, or local Fe(III) reduction. Similar periods of net phosphorus export from aged, constructed wetlands have been observed with both planted and unplanted gravels (Mann, 1991; Tanner et al., 1995). E‚uent oxygen concentrations in Series 4 did not speci®cally implicate Fe(III), being >0.1 mg litreÿ1 until well after both Fe and rP release were established, although lower values occurred at later times. These side and subsequent reactions were not investigated further, but appear similar to iron catalysed oxidation of phenols and ``tannic acid'' recorded by Stumm and Morgan (1981), and the formation of stable ``dissolved humic material-Fe

1285

(III)-phosphate'' complexes which release PO4 if the iron is reduced (Cotner and Heath, 1990). The signi®cance of these release reactions for sustained constructed wetland operation may lie in the need for aeration of the upper layers of gravel, as obtained by batch loading and allowing draw-down by evapotranspiration. Applications While experimental data here does not apply to larger systems, knowledge of the mechanisms operating, their relative rates, and the variables in¯uencing e‚uent TP concentrations can be applied to their performance. Minimum phosphorus in out¯ows E‚uent TP consists of nrP and rP, with minimum nrP determined by bio®lm reactions, while the minimum rP concentration attainable is that of the sorption equilibrium. Once minimum rP is reached its concentration will only decrease at a rate determined by plant uptake, less desorbed PO4. Retention times Only general estimations are possible because of the complex relationship between system hydrology, gravel characteristics and phosphorus concentrations. Potential retention time reductions with rhizosphere mixing (PO4 110 mg litreÿ1) can be gauged from the half-lives of unmixed, (5±16 h) and mixed systems (0.2±0.5 h). Annual assimilative capacity and macrophyte uptake To maintain removal, sorption must be maintained by removing PO4. Since shoot harvesting provides the only irreversible PO4 removal path, plant standing crop probably limits the system's annual capacity. Plant removal is limited by a relatively small capacity and seasonality of uptake (run 2) (Loneragan et al., 1976; Gopal and Sharma, 1988) Sharma (1992) harvesting small VFWs removed an annual standing TP crop of S. validus containing 15±22 g mÿ2 (1200 kg haÿ1). However, macrophyte uptake could constitute the minimum capacity, if the increased substratum sorption observed with ageing is sustainable and not o€set by re-release mechanisms. AcknowledgementsÐThe authors wish to thank the Land and Water Resources Research and Development Corporation for ®nancial support, Mr D. Erskine and Drs D. Baldwin, K. Bowmer, G. Davies, R. Gerritse and an unnamed referee for helpful advice; Mr M. Raven, CSIRO Division of Soils, Adelaide, for mineralogical analyses and Mr A. J. Chick and Ms V. Patten and R. Smith for chemical analyses. One of the authors (IRL) thanks Edith Cowan University for secondment to the MDFRC.

REFERENCES

APHA (1989) Standard Methods for the Examination of Water and Wastewater. 17th edn. American Public Health Association, Washington, DC. Barrow N. J. (1974) E€ect of previous additions of phosphate on phosphate adsorption by soils. Soil Sci. 118, 82±89.

1286

I. R. Lantzke et al.

Barrow N. J. (1983a) On the reversibility of phosphate sorption by soils. Soil Sci. 34, 751±758. Barrow N. J. (1983b) A mechanistic model for describing the sorption and desorption of phosphate by soils. Soil Sci. 34, 733±750. Barrow N. J. and Shaw T. C. (1975) The slow reactions between soil and anions. 2. E€ect of time and temperature on the decrease in phosphate concentration in the soil solution. Soil Sci. 119, 167±177. Breen P. F. (1990) A mass balance method for assessing the potential of arti®cial wetlands for wastewater treatment. Wat. Res. 24, 689±697. Cotner J. B. and Heath R. T. (1990) Iron redox e€ects on photosensitive phosphorus release from dissolved humic materials. Limnol. Oceanogr. 35, 1175±1181. Crank J. (1964) The Mathematics of Di€usion, p. 52. Oxford University Press, London. Dubois M., Gilles K. A., Hamilton J. K., Rebers P. A. Smith F. (1956) Colorimetric method for determination of sugars and related substances. Analyt. Chem. 28, 350±356. Gerritse R. G. and Scho®eld N. J. (1989) A simple leaching model for phosphate in small catchments of the Bassendean sands, and its potential use for management and planning. In Proceedings of the Swan Coastal Plain Groundwater Management Conference (Edited by Lowe G.), pp. 191±200. Western Australian Water Resources Council, Perth. Gopal B. and Sharma K. P. (1988) Seasonal changes in concentration of major nutrients elements in the rhizomes and leaves of Typha elephantina Roxb. Aquat. Bot. 20, 65±73. Heriage A. D. (1992) Treatment of sewage by constructed vertical-¯ow wetlands (VFWs). Report on pilot scale experiments at Grith, to Cassiro, May 1992. CSIRO, Grith, Australia. Juwarkar A. S., Mamta V., Meshram J., Bal A. S. and Juwarkar A. (1992). Wastewater treatment in constructed wetlands. In Wetland Systems in Water Pollution Control (Proceedings), Organising Committee, Sydney. pp. 35.1±35.4.

Knight R. L., Kadlec R. H. and Reed S. (1992) Wetland for wastewater treatment data base. In Wetland Systems in Water Pollution Control (Proceedings), pp. 15.1± 15.20. Organising Committee, Sydney. Loneragan J. F., Snowball K. and Robson A. D. (1976) Remobilization of nutrients and its signi®cance in plant nutrition. In Transport and Transfer Processes in Plants (Edited by Wardlaw I. F. and Passioura J. B.), pp. 463± 469. Academic Press, New York. Mann R. A. (1991) Phosphorus removal by constructed wetlands: substratum adsorption. In Constructed Wetlands and Water Pollution Control (Edited by Cooper P. F. and Findlater B. C.), pp. 97±105. Pergamon Press, Oxford. Moriarty D. J. W. and Boon P. I. (1990) Interactions of seagrasses with sediment and water. In Biology of seagrasses (Edited by Larkum A. W. D., McComb A. J. and Shepherd S. A.), pp. 500±535. Elsevier, Amsterdam. Rogers K. H., Breen P. F. and Chick A. J. (1990) Hydraulics, root distribution and phosphorus removal in experimental wetland systems. In Constructed Wetlands and Water Pollution Control (Edited by Cooper P. F. and Findlater B. C.), pp. 587±590. Pergamon Press, Oxford. Sharma K. P. (1992) Harvesting Schoenoplectus validus Vahl. (A Love and D Love) shoots for nutrients removal from constructed wetlands. Final Report to DITAC. CSIRO, Grith, Australia. Sorrell B. K. and Orr P. T. (1993) Proton exchange and nutrient uptake by roots of the emergent macrophytes Cyperus involucratus Rottb., Eleocharis sphacelata RBr. and Juncus ingens N.A. Wakef. New Phytol. 125, 85±92. Stumm W. and Morgan J. J. (1981) Aquatic Chemistry: an introduction emphasizing chemical equilibria in natural waters. 2nd edn. Wiley-Interscience, New York. Tanner C. C., Clayton J. S. and Upsdell M. P. (1995) E€ect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlandsÐII. Removal of nitrogen and phosphorus. Wat. Res. 29, 27±34.