This study aimed to assess the quantity and quality of water in a surface flow constructed wetland in Australia's far north Queensland. Owing to tropical climate in the region, the wetland provided dual functions: retention of a treated wastewater for zero discharge during the dry season and tertiary treatment prior to discharge during the wet season. Rainfall data, permeability of wetland soil, evaporation, inflow and outflow were analysed in a water balance analysis; the results showed that based on a 72-year-average rainfall pattern, daily wastewater inflow of 85 m3/d is the maximum this wetland can cope with without breaching its discharge certificate. In water quality analysis, the K-C* model was used to predict changes of biochemical oxygen demand (BOD, suspended solids (SS), total nitrogen (TN), total phosphorus (TP) and faecal coliforms (FC) in the wetland. Model predictions were compared with field sampling results. It was found that the wetland was effective in removing FC (>99.9%), TN (70.7%) and TP (68.2%), for which the predictions by the K-C* model were consistent with field testing results. However, significant disparities between the predictions and testing results were found for BOD and SS. A revised K-C* equation was proposed to account for the internal generation of organics in constructed wetlands with a long retention time.

INTRODUCTION

Constructed wetlands are engineered systems to mimic the conditions in natural wetlands. When they are used in wastewater treatment, constructed wetlands utilise common processes taking place in the natural wetlands, such as uptake by macrophytes, sedimentation, chemical precipitation, sorption into soils and other wetland media, and microbial activities, to remove pollutants from the wastewater (Kadlec & Wallace 2009; Li et al. 2012).

Since the mid-1980s, constructed wetlands and ponds have been increasingly used in urban areas for the storage and treatment of stormwater (Lee & Scholz 2007) and a variety of wastewaters (Sun & Cooper 2008; Babatunde et al. 2010). When constructed wetlands are used for such purposes, they have a significant advantage over conventional energy-intensive techniques, due to lower energy consumption, lower operation and maintenance costs and aesthetic appearance (Brix 1999). Constructed wetlands complement other techniques used in integrated urban water management, making them a promising technique for water-sensitive urban design (Somes et al. 2000).

According to the latest development in taxonomy, constructed wetlands for treatment purpose are categorised into two groups: surface flow and subsurface flow systems (Fonder & Headley 2013). Subsurface flow wetlands, effectively functioning as bioreactors or biofilters, are more commonly used in European countries. Having a water depth typically lower than 0.5 m (Vymazal 2007), surface flow wetlands are similar to shallow aerobic ponds, and they are more widely used in countries where land surfaces are relatively abundant, such as the USA and Australia. Relative to a natural wetland of equivalent surface area, either type of constructed wetland should provide greater pollutant removal efficiency in the treatment of stormwater or wastewater (Reed et al. 1988; Cooper et al. 1996). However, such efficiency can only be achieved by appropriate design. The most critical tasks of constructed wetland design are to enable desired flow pattern of water and incorporate suitable pollutant removal dynamics to predict system performance, although other tasks (such as the selection of wetland media and plants) can also affect its functioning (Sun & Saeed 2009).

Although a large number of constructed wetlands have been built and operated, their design (especially regarding pollutant removal efficiencies) is still largely based on experience or simplified correlations of pollutant concentrations in their inflows and outflows. The flow of wastewater in the wetland is often assumed to fit ideal plug-flow or continuously stirred tank reactor pattern, with the quantity of inflow matching outflow, and the change of water quantity (due to rainfall, evapotranspiration, etc.) neglected. Development towards model-based constructed wetland design has been made (Rousseau et al. 2004; Langergraber 2011), but such an approach has not been widely adopted by practitioners, primarily due to the lack of key parameter values. With increasing applications of constructed wetlands in urban water management, it is critical that a thorough examination is carried out for the validity and reliability of current design equations. This study aims to: (1) assess the seasonal change of water quantity in a surface flow wetland in a tropical region; and (2) examine the validity of a common design equation.

METHODS

System description

The constructed wetland is located close to the township of Toomulla in far north Queensland, Australia. Combined with a compact activated sludge unit, the wetland forms the final part of the treatment facility for Toomulla's domestic wastewater. Treated wastewater from the activated sludge unit overflows by gravity, via a chamber and a pipe buried underneath the wall, to enter into the bottom of the wetland on the east side. A weir, located on the west side, is used to control the water level in the wetland. Only in periods of intense rainfall is water in the wetland allowed to discharge from the weir into a nearby creek.

Contained by a wall of compact clay, the wetland was initially constructed to be an open water system for evaporation purpose, with a design area of 13,046 m2 and water storage capacity of 6,693 m3. However, at the time it was assessed in July 2012 and July 2013, the wetland was partly vegetated (covering approximately 50% surface area) by emergent plants (predominately Typha genus) along its perimeter, whereas only the central area maintained an open water surface, making it function essentially as a type 1 surface flow wetland (Fonder & Headley 2013).

Because the facility is close to the Great Barrier Reef, a world heritage site, it is essential that a high treatment standard is maintained. The activated sludge plant treats the wastewater to secondary standard before it enters into the wetland. The wetland is then used for storage, evaporation and final polishing of the wastewater, prior to discharge into the creek. According to the certificate issued to this treatment facility, zero discharge from the wetland must be maintained throughout the dry season (June–October). The discharge of treated wastewater from the wetland is only allowed during the wet season (November–May), provided that strict water quality criteria are met.

After over 10 years of operation, seasonal discharge pattern and water quality in the wetland became a cause of concern. As a result, this project was launched to study: (1) the water balance in the wetland, so that the timing and quantity of wastewater discharge can be more accurately predicted; (2) the change of water quality in the wetland, so that assessment can be made on whether, and how, the wetland is providing tertiary treatment function; and (3) the development of water quantity and quality models useful for upgrading the wetland.

Water balance modelling and soil permeability testing

The water balance modelling, which was performed using Microsoft Excel, had the following tasks completed: (1) collection and analysis of precipitation and evaporation data in the region; (2) in situ testing of wetland soil permeability to determine water infiltration into the groundwater; and (3) estimation of water quantity changes in the wetland with various inflow values. The following equation was used for the water balance analysis: 
formula
1
In Equation (1), A = wetland surface area (m2); E = evapotranspiration rate (m/d); P = precipitation rate (m/d); Qb = bank loss rate (m3/d); Qc = catchment runoff rate (m3/d); Qgw = infiltration to groundwater (m3/d); Qi = wastewater inflow rate (m3/d); Qo = wastewater outflow rate (m3/d); Qsm = snowmelt rate (m3/d); V = water storage in wetland (m3); and t = time (d).
Equation (1) illustrates that the water storage of a surface flow wetland is a function of the inflow, outflow and characteristics of the wetland basin and retaining wall. It is rare that all of the parameters in Equation (1) contribute to the change of water storage on this particular site, and some parameters can be neglected as they are deemed insignificant. The Toomulla wetland is constructed with a wall of impervious clay, forming a ridge to prevent the loss of stored wastewater and flow of rainwater into the wetland. Therefore, Qb and Qc can be neglected. Also, Qsm is negligible in the tropical region, giving a simplified water balance Equation (2) 
formula
2

Two in situ soil permeability tests were carried out in July and August 2012, to measure if there was any leaching of water through the wall and bottom of the wetland. In the tests, saturated permeability coefficient, Ksat (cm/min), was measured with a constant-head method (Talsma–Hallam method), as recommended by Australian/New Zealand Standard (2012) AS/NZS 1547:2012. The rate of water infiltration from the wetland into the groundwater (or surface water, if applicable) was considered a constant that can be calculated by combining Ksat value, wetland surface area and hydraulic gradient.

Water quality modelling and sample collections

For water quality modelling, two steps were taken: (1) six sets of influent and effluent water samples were collected during the dry season in 2012 and 2013; and (2) the analytical data of these samples were used to check the validity of the K-C* equation (3) that correlates the outflow and inflow concentrations of specific pollutants (such as 5-day biochemical oxygen demand (BOD5)) in the wetland. The sampling was carried out in July 2012 and July 2013 when the mean maximum and minimum ambient temperature was 25.1 and 13.7 °C, respectively. The influent samples were collected from the overflow chamber between the activated sludge unit and the wetland, whereas the effluent samples were collected from the water body, close to the weir. When the samples were collected, no discharge of wastewater from the weir into the creek was taking place. Shortly after collection, the samples were analysed for pH, BOD, suspended solids (SS), total nitrogen (TN), total phosphorus (TP) and thermo-tolerant faecal coliforms (FC) (org/100 mL) 
formula
3
In Equation (3), Ah = surface area of the wetland (ha); Q = daily wastewater flow rate (m3/d); Ci = inflow concentration of any specific pollutant (mg/L); Co = outflow concentration of the pollutant (mg/L); C* = background concentration of the pollutant (mg/L); and K = reaction constant of the pollutant (m/y). Assuming steady wastewater flow (i.e. neglecting the effect of evapotranspiration and precipitation on Q), the K-C* equation allows the prediction of pollutant concentration changes, provided that its inflow concentration, surface area of the wetland, flow rate and the two coefficients (K and C*) are known. Values of K and C* of some common pollutants are presented in Table 1. The values of background concentration C* in Table 1 were obtained from the US EPA (2000) manual for constructed wetland design; they are the typical values recommended for surface flow wetlands. The K values in Table 1 were recommended by Kadlec & Wallace (2009). The influent concentrations (Ci) from six sets of water samples were used in Equation (2) to compute the theoretical Co values, which were then compared with the analysed values of wastewater samples collected from the outlet of the wetland, to check the validity of the K-C* equation for predicting BOD, SS, TN, TP and FC changes.
Table 1

Values of background concentrations and reaction constants used in the K-C* equation

  BOD5 (mg/L) SS (mg/L) TN (mg/L) TP (mg/L) FC (cfu/100 mL) 
C* (mg/L) 0.3 200 
K (m/y) 34 1,000 22 12 77 
  BOD5 (mg/L) SS (mg/L) TN (mg/L) TP (mg/L) FC (cfu/100 mL) 
C* (mg/L) 0.3 200 
K (m/y) 34 1,000 22 12 77 

RESULTS AND DISCUSSION

Water balance in the wetland

To estimate water storage change in the wetland, the 72-year-averaged monthly rainfall data (mm/month) and 2009 daily rainfall data (mm/d) were obtained from the Bureau of Meteorology for the Townsville region (latitude 19.2° S, longitude 146.77° E). These rates were then converted to an equivalent volume of water spread over the entire surface area of the wetland. Evapotranspiration rate was assumed to vary insignificantly from the evaporation rate; thus, evaporation rate was used in Equation (2) instead of evapotranspiration. The evaporation data for Townsville region, based on 72-year-average temperature in each month, were also obtained from the Bureau of Meteorology.

On-site hydraulic conductivity tests found that the retaining clay wall of the wetland was impermeable, but measurement of the hydraulic conductivity of bottom soil gave a Ksat value of 5.445 × 10−4cm/min and hydraulic gradient of 0.2846 m/m. These values, combined with wetland surface area, indicate that water leaches from the wetland into groundwater at a rate of 29.1 m3/d (Qgw in Equation (1)).

To estimate the amount of water discharge from the wetland in a random year in the future, the 72-year-average monthly rainfall pattern was used. At the beginning of the year, it was assumed that the wetland was at full water storage capacity (i.e. in Equation (2) V0 = 6,693 m3). When Qi is at the design level (20 m3/d, based on 50 households having an average annual water consumption of 600 kL per household, with 25% wastewater discharge to sewer), the water balance analysis found that zero discharge can be maintained in March–December, as long as the rainfall pattern in the region fits the 72-year-average pattern. As the inflow of wastewater increases, discharge from the wetland into the creek will steadily intensify, as shown in Table 2. When the inflow into the wetland reaches 85 m3/d, the objective of maintaining zero discharge in the dry season (June–October) cannot be reached. If wastewater inflow increases further to over 132 m3/d, discharge from the wetland will be continuous throughout the year. It should be noted that the water balance analysis results are based on transpiration rate via plants being negligible (i.e. 0 m3/d) and the rate of infiltration into groundwater not exceeding 29.1 m3/d; any increase in these two values can give the wetland a greater capacity for maintaining zero discharge.

Table 2

Predicted monthly water discharge rate Qo at different inflow Qi rate, based on 72-year-average rainfall and evaporation pattern

Qi (m3/d) Qo (m3/month) 
20 40 60 80 84.9 85.0 100 120 140 
Jan 129 749 1,369 1,989 2,140 2,144 2,609 3,229 3,849 
Feb 1,121 1,681 2,241 2,801 2,938 2,941 3,361 3,921 4,481 
Mar 44 664 1,284 1,436 1,439 1,904 2,524 3,144 
Apr 29 32 482 1,082 1,682 
May 443 1,063 1,683 
Jun 478 1,078 1,678 
Jul 272 892 1,512 
Aug 561 1,181 
Sept 532 
Oct 243 
Nov 582 
Dec 523 1,606 
Annual total 1,250 2,474 4,274 6,074 6,544 6,561 9,548 14,872 22,172 
Qi (m3/d) Qo (m3/month) 
20 40 60 80 84.9 85.0 100 120 140 
Jan 129 749 1,369 1,989 2,140 2,144 2,609 3,229 3,849 
Feb 1,121 1,681 2,241 2,801 2,938 2,941 3,361 3,921 4,481 
Mar 44 664 1,284 1,436 1,439 1,904 2,524 3,144 
Apr 29 32 482 1,082 1,682 
May 443 1,063 1,683 
Jun 478 1,078 1,678 
Jul 272 892 1,512 
Aug 561 1,181 
Sept 532 
Oct 243 
Nov 582 
Dec 523 1,606 
Annual total 1,250 2,474 4,274 6,074 6,544 6,561 9,548 14,872 22,172 

On record, 2009 was one of the wettest years (annual rainfall 72.8% higher than long-term average) in the Townsville region. However, rainfall in 2009 mostly occurred in 3 months (Jan, Feb and Dec; totalling 1,884 mm), with only 106 mm distributed in the rest of the year (Mar–Nov). If future rainfall follows the 2009 pattern, discharge of wastewater from the wetland will only occur in the Jan–Feb period, provided that wastewater inflow does not exceed 97 m3/d, as illustrated in Figure 1. If the inflow exceeds 97 m3/d, discharge will occur in July, which would breach the discharge certificate.

Figure 1

Predicted monthly discharge from the wetland (left) and change of water storage (right) in the wetland, based on 2009 rainfall pattern and wastewater inflow being constant at 97 m3/d.

Figure 1

Predicted monthly discharge from the wetland (left) and change of water storage (right) in the wetland, based on 2009 rainfall pattern and wastewater inflow being constant at 97 m3/d.

Overall, the water balance analysis demonstrated that: (1) the volume of continued inflow of the municipal wastewater has a significant impact on the amount and timing of water discharge from the wetland; and (2) the amount of annual rainfall is not directly linked to the timing of wastewater discharge; instead, the distribution of rainfall has a more significant effect on the timing and quantity of the discharge.

Assessment of water quality in the wetland

In field testing, the average pH value of the wastewater was found to reduce slightly in the wetland, from 6.85 ± 0.08 in the inflow to 6.69 ± 0.14 at the outlet point. Other water quality parameters (BOD5, SS, TN, TP and FC), however, changed significantly according to the analytical results of six water samples collected from the site. Figure 2 illustrates the percentage reduction, or increase, of BOD5, SS, TN and TP levels in the wetland.

Figure 2

Percentage removal (mean ± SD; negative value indicating increase in concentration) of BOD5, SS, TN and TP in the wastewater from the inlet to the outlet.

Figure 2

Percentage removal (mean ± SD; negative value indicating increase in concentration) of BOD5, SS, TN and TP in the wastewater from the inlet to the outlet.

As shown in Figure 2, BOD and SS values increased in the wetland during the sampling period. The increases were possibly caused by the growth of algae in the wetland. In contrast, TN and TP were significantly reduced, demonstrating the function of the wetland as a ‘sink’ of inorganic nutrients.

The reduction of coliforms was nearly 100% (from 170,000 to less than 10 cfu/100 mL), demonstrating that the wetland was very effective in removing microorganisms. From a public health point of view, the removal of pathogenic microbes (especially protozoan pathogens and helminth worms) is of particular importance in a tropical region. Surface flow constructed wetlands are known to offer an effective combination of physical, chemical and biological processes for the removal of pathogenic organisms. As wastewater passes through a wetland, pathogens are retained in soil and biomass. Once the microorganisms are entrapped, their numbers drop rapidly by natural die-off, predation and ultraviolet radiation. In addition, root metabolites from macrophytes have been mentioned to have an antibiotic effect on pathogens (Cooper et al. 1996).

Similar to many municipal wastewater treatment plants, the Toomulla treatment plant receives influent flow rates that vary according to time and seasonal factor. The inflow of treated wastewater into the wetland (Qi value in Equation (2)) was ungauged and could not be directly measured in this study. To compare the testing results with predictions by the K-C* equation, 20 m3/d (design flow rate) and 85 m3/d (the maximum flow rate to safeguard compliance with discharge certificate) were applied in the equation to calculate pollutant concentrations at the outlet of the wetland. Table 3 compares the predictions by the K-C* equation with the actual sample testing results.

Table 3

Comparison of pollutant concentrations at wetland outlet obtained from sampling (mean ± SD) with the values predicted by the K-C* model

  BOD (mg/L) SS (mg/L) TN (mg/L) TP (mg/L) FC (cfu/100 mL) 
Measured inflow concentration 3.5 ± 1.4 5.2 ± 2.9 4.1 ± 0.9 4.4 ± 0.4 1,70,000 ± 20,000 
Measured outflow concentration 6.8 ± 1.2 6.7 ± 4.3 1.2 ± 0.3 1.4 ± 0.4 <10 
Predicted outflow concentration when Qi = 20 m3/d 3.7 5.9 2.0 0.30 200 
Predicted outflow concentration when Qi = 85 m3/d 3.7 5.9 2.0 0.33 200 
  BOD (mg/L) SS (mg/L) TN (mg/L) TP (mg/L) FC (cfu/100 mL) 
Measured inflow concentration 3.5 ± 1.4 5.2 ± 2.9 4.1 ± 0.9 4.4 ± 0.4 1,70,000 ± 20,000 
Measured outflow concentration 6.8 ± 1.2 6.7 ± 4.3 1.2 ± 0.3 1.4 ± 0.4 <10 
Predicted outflow concentration when Qi = 20 m3/d 3.7 5.9 2.0 0.30 200 
Predicted outflow concentration when Qi = 85 m3/d 3.7 5.9 2.0 0.33 200 

Table 3 shows that a 3.25-fold increase in daily wastewater inflow has a negligible impact on the quality of the effluent from the Toomulla wetland; this is due to two factors: (1) influent pollutant concentrations Ci being close to background pollutant concentrations C* and (2) the long water retention time in the wetland. The accuracies of K-C* model predictions for TN, TP and FC are relatively satisfactory, compared to its predictions for the changes of BOD and SS.

In tropical climates, due to high water evaporation rate, a surface flow constructed wetland is likely to have a long water retention time which, combined with sufficient sunlight, can cause an increase in organics concentration. It is inappropriate to use the K-C* equation to predict the changes of BOD and SS in such a system. When the K-C* equation is used for BOD calculations, possible improvement to the equation is by introducing a regeneration element to account for the input of organics from algal activities (Equation (4)); this is similar to the element of ammonia regeneration from aggregate nitrogen in wetland soil, as proposed by Mayo & Bigambo (2005). 
formula
4

In Equation (4), CTN and CTP are the average concentrations (mg/L) of TN and TP in the wetland; Rreg (L/mg) represents a BOD regeneration coefficient that needs to be derived from field data. The rationale of Equation (4) is that combining the concentrations of TN, TP and the regeneration coefficient allows a crude quantification of internal generation of organics. A preliminary calibration of Rreg from data in Table 3 yields an Rreg value of 0.405 L/mg. It must be noted that the internal generation of organics in a treatment wetland is extremely difficult to predict. Equation (4) only makes a tentative effort to include this factor. Further development and calibration of Rreg value is needed to validate or refute this equation.

CONCLUSIONS

Water balance calculations were carried out for a surface flow constructed wetland in a tropical climate, taking account of precipitation, infiltration into groundwater, evaporation, and wastewater inflow and outflow. Assuming that a 72-year-average rainfall pattern fits a random year in the future, a daily inflow of 85 m3/d is the maximum that the wetland can cope with, if zero discharge is to be maintained between June and October in that year, as required by a discharge certificate. Monthly distributions of annual rainfall have a significant effect on the timing and amount of wastewater discharge. Water quality testing showed that the wetland was effective for removing coliforms, TN and TP from the wastewater, but not for BOD5 and SS. It is inappropriate to use the K-C* model to predict the changes of BOD5 and SS in a surface flow wetland that has a long water retention time; input of organics and solids into the water in the wetland must be considered.

ACKNOWLEDGEMENTS

This research was supported by CAS/SAFEA International Partnership Programme for Creative Research Teams. The authors thank staff in Townsville Water (in particular, Cameron Tully, Anna Whelan and Mark Vis) for assisting with site access and water sample analysis. Amanda Maddocks and Alison Ambrey are thanked for assisting with rainfall data collection, soil permeability testing and proofreading the manuscript.

REFERENCES

REFERENCES
Australian/New Zealand Standard
2012
Onsite Domestic Wastewater Management
.
AS/NZS 1547:2012, Standards New Zealand, Wellington, New Zealand
.
Cooper
P. F.
Job
G. D.
Green
M. B.
Shutes
R. B. E.
1996
Reed Beds and Constructed Wetlands for Wastewater Treatment
.
WRc Publications
,
Swindon, UK
.
Kadlec
R. H.
Wallace
S.
2009
Treatment Wetlands
.
CRC Press, Boca Raton, FL
,
USA
.
Langergraber
G.
2011
Numerical modelling: a tool for better constructed wetland design?
Water Science and Technology
64
(
1
),
14
21
.
Reed
S. C.
Middlebrook
J. E.
Crites
R. W.
1988
Natural Systems for Waste Management and Treatment
.
McGraw-Hill
,
New York, USA
.
Somes
N. L. G.
Fabian
J.
Wong
T. H. F.
2000
Tracking pollutant detention in constructed stormwater wetlands
.
Urban Water
2
(
1
),
29
37
.
US EPA
2000
Constructed Wetlands Treatment of Municipal Wastewaters
.
EPA/625/R-99/010
,
US Environmental Protection Agency, Cincinnati, OH, USA
.
Vymazal
J.
2007
Removal of nutrients in various types of constructed wetlands
.
Science of the Total Environment
380
,
48
65
.