Wal.Sci. Tech. Vol. 32, No.3, pp. 87-93. 1995.
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AN EVALUATION OF POLLUTANT
REMOVAL FROM SECONDARY TREATED SEWAGE EFFLUENT USING A CONSTRUCTED WETLAND SYSTEM P. R. Thomas*, P. Glover** and T. Kalaroopan** * Department of Environmental Management and Ecology, La Trobe University, Wodonga, Victoria 3690, Australia ** Kiewa Murray Region Water Authority, Wodonga, Victoria 3690, Australia
ABSTRACT Pilot scale investigations were carried out to examine the pollutant removal efficiency of a constructed wetland receiving secondary treated sewage effluent. Four constructed wetland cells were established. three of them planted with either Schoenoplectus validus, Juncus ingens or both species of macrophytes, and the fourth serving as an unvegetated control cell. Although there was a significant improvement in the emuent quality during the initial ten month period of monitoring, results to date have not indicated any overall trend for pollutant removal by a particular plant species. Biocbemical oxygen demand and chemical oxygen demand removals averaged between 71-75% while suspended solids removals were around 85% in the macrophyte cells. Ammonia reductions were in the range 17-24% but better nitrate reductions between 6580% were obtained. Phospborus removal bas been low (13%) in all four of the wetland cells and bore hole samples bave sbown DO groundwater conlamination with nitrogen or phosphorus from the wetland system to date.
KEYWORDS Constructed wetlands; Juncus ingens; nutrient removal; pollutant removal; Schoenoplectus validus; sewage treatment. INTRODUCTION The increasing concern about the adverse effects of pollution on many waterways together with the emphasis on wastewater recovery and reuse has resulted in all wastewater treatment plants coming under heavy scrutiny in order to satisfy stringent effluent standards. After secondary treatment in a conventional sewage treatment plant, normally some 25-75% of phosphorus, 90% of nitrogen and about 10% of both organic matter and suspended solids remain in the effluent making it one of the main potential sources of nutrients to rivers. To remove nutrients and other pollutants effectively from wastewaters some form of tertiary treatment unit which is economical and easy to operate is necessary in the treatment stream, and constructed wetlands have the potential to provide such a method of wastewater treatment (Finlayson, 1983; Scholes et al., 1986; Brix, 1987; Davies, 1988).
P. R. THOMAS et al.
SEWAGE TREATMENT USING CONSTRUCTED WETLANDS A constructed wetland system has the positive characteristics of a natural wetland and it duplicates the physical, chemical and biological processes in the natural system. This system shows promise for application in small rural communities as a tool for waste management because it has the potential to provide tertiary treatment to reduce biochemical oxygen demand, suspended solids, nutrients and pathogens. In the operation of the system pretreated wastewater is allowed to come in contact with an engineered bed in which various aquatic macrophytes native to the local area are grown. The purpose of the macrophytes is to help maintain the hydraulic conductivity of the supporting substrate and to assist with the transfer of atmospheric oxygen to the root zone creating aerobic microzones around the roots and anoxic and anaerobic zones away from them. This results in both aerobic and anaerobic processes which facilitate the breakdown of the organic matter, and removal of nitrogen through nitrification and denitrification. Phosphorus removal in many constructed wetlands is not effective because the gravel media offer limited contact opportunities between the wastewater and the soil, and due to the short hydraulic retention times. Aquatic plant species for use in constructed wetlands should be selected based on the following criteria (Mitchell, 1978): rapid and relatively constant growth rate; ease of propagation; capacity for absorption of pollutants; tolerance of hyper-eutrophic conditions; and ease of harvesting and potential usefulness of harvested material. Examples of aquatic macrophytes that have been used in the constructed wetland system are: Floating plants: - Eichhornia crassipes (Water Hyacinth) - Spirodela (Duckweed) - Salvinia molesta (Salvinia) - Hydrocotyle umbellata (Pennywort) Emergent plants: - Schoenoplectus validus (Great Bulrush) - Juncus ingens (Giant Rush) - Phragmites (Common Reed) - Typha spp. (Cumbungi or Cattail) Although Eichhornia crassipes, because of its significant ability to absorb nutrients from wastewater, is used in aquatic macrophyte-based wastewater treatment systems in the United States of America (De Buskand Reddy, 1987) and in the Caribbean (Thomas and Phelps, 1987) it is declared as a noxious weed in Australia. As a result, in recent years, research into wetland systems has concentrated on surface flow and subsurface flow constructed systems with emergent plants (Breen, 1989; Mitchell et al., 1990). DESIGN AND OPERATION Although there has been no consensus regarding the design criteria for subsurface flow wetlands the principal design approach is based on BOD5 removal (first-order plug flow kinetics) and the appropriate land area required may be calculated from the mathematical model (USEPA. 1993)
Pollutant removal using a constructed wetland system
where As =surface area (m2 ) Q = daily average flow (m 3/d) C, = influent BODS (mg/l) C, =effluent BODS (mg/l) n =effective porosity of the media (% as a decimal) d = average depth of flow in bed (m) k T = rate constant at temperature TOC (per day) Constructed wetlands are about 0.6 m deep and the bottom should be lined with an impervious layer to prevent seepage if there is no clay layer. The inlet zone should have a buried perforated pipe and the treatment zone should contain clean gravel. The longitudinal section of a single cell subsurface flow wetland is shown in Fig. 1.
PERFORATID LATERAL PIPE
Figure 1. Longitudinal section of a subsurface flow wetland.
PILOT STUDIES - WODONGA SEWAGE TREATMENT FACILITY The sewage treatment facility in Wodonga, Australia has been under increasing pressure to reduce the quantity of nutrients and other pollutants in its treated sewage effluent before being discharged to the river. Consequently it was decided to conduct some pilot studies in the use of constructed wetlands as an economical alternative to other forms of tertiary treatment. The pilot subsurface flow constructed wetland system at the Wodonga treatment facility consists of four cells in parallel each 27.0 m long x 3.6 m wide x 0.6 m deep containing emergent vegetation growing in 0.5 m deep gravel media with a bed slope of 0.7%. This configuration was chosen to accommodate the constructed wetland system within the available land adjacent to the treatment facility resulting in an aspect ratio (length:width) of 7.5: l. Each of the three cells were planted with either Schoenoplectus validus, Juncus ingens or both species of plants in August 1993, the fourth serving as an unvegetated control cell. The cells with Schoenoplectus validus and Juncus ingens contain 10 mm and 14 mm size gravel respectively as bed media while 20 mm size gravel is used for the other two cells. The effective hydraulic conductivity varied from about 1500 m 3/m2Jd for the 10 mm gravel to about 2500 m3/m 2/d for the 20mm gravel. The inlet to each cell consists of a subsurface perforated teeinlet perpendicular to the direction of flow and the outlet structure is a V-notch weir for each cell. It was not necessary to line the bottom of the cells with an impervious layer because of the presence of heavy clay in the area of construction. The macrophytes were obtained from the Murray-Darling Freshwater Research Centre and were planted by hand with an initial spacing of 0.5 m and in rows of I m apart in August 1993. Part of the secondary treated sewage effluent from the treatment facility is used as the inflow to each of the four cells and monitoring of the parameters biochemical oxygen demand (BOD ), suspended solids (5S), ammonia (NH ), nitrate (NO ),
P. R. TIl0MAS et al.
total phosphorus (TP) and chemical oxygen demand (COD) from the inflow and outflow of the wetland system started in December 1993 and the preliminary results obtained to October 1994 are presented in this paper. In order to ascertain for any groundwater contamination from the operation, two bore holes each 5.5 m deep were erected, one upgradient and the other downgradient of the wetland cells. RESULTS AND DISCUSSION Since the monitoring began, flow to the wetlands has been varied to give different hydraulic retention times with one day being the lowest value. The secondary treated wastewater from the sewage treatment plant showed poor quality from time to time because of the acceptance of effluent from a meat works to the treatment plant during this monitoring period. This affected the performance of the wetland system which was less than twelve months old and in the developing stage. The mean concentrations of the various parameters in the inflow and outflow of the constructed wetlands are presented in Table 1. The water balance was made up mainly of inflow, outflow and evaporative losses, because of the very low permeability of bottom soil and lack of rainfall during this initial monitoring period. Table I. Mean concentrations in the inflow and outflow of the constructed wetlands at Wodonga Sampling period December 1993 to October 1994
N0 3 mgIL
Influent Effluent from: Schoenoplectus validus (10 mm gravel)
Juncus ingens (14 mm gravel)
Mixed Species (20 mm gravel)
Unvegetated Control (20 mm gravel)
Note: Values in brackets indicate range In the vegetated cells BODs removal efficiencies averaged between 71-75% whereas in the unvegetated control cell BODs removal averaged 83%. Figure 2 illustrates the relationship between BODS loading and BODs removal rates for all four cells. In general the cell with both plant species, Schoenoplectus validus and Juncus ingens, appears to offer a better BODS removal than the cell with either Schoenoplectus validus or Juncus ingens. Analysis of COD removal showed similar trends. Since BODs removal is enhanced under aerobic conditions, it is reasonable to assume that the superior efficiency obtained in the control cell was due to the presence of oxygen in the voids component of the 20 mm gravel media. The cell (20 mm gravel media) with the mixture of two plant species had slightly elevated BODs levels contributed by the decay of some of the vegetation. Suspended solids removal averaged 87% with Schoenoplectus validus and 85% with the combination of the two species whereas with both Juncus ingens and the unvegetated control cells a reduction of 92% was obtained. Recent results show suspended solids removal rates of 95-99% from the cells indicating better performance with full grown macrophytes.
Pollutant removal using a constructed wetland system
in.on. _poole. oon.trol
100 ElOD 5
Figure 2. BOD 5 loading and BODs removal rates for the different cells.
Reduction in ammonia was poor, 17-24% for vegetated cells and 34% for the control cell, however, nitrate removal was better with Juncus ingens offering the highest reduction of 83%. Although significant ammonia removal is expected through nitrification-denitrification mechanisms from the constructed wetland, it is believed that the oxygen in the gravel media was insufficient to convert the ammonia to nitrate. Generally all the cells have been performing satisfactorily in regard to denitrification, with effluent nitrate levels dropping to as low as 1.1 mg/I in the cell with Schoenaplectus validus (Fig. 3). 20
"'a C> ~
oS -S 10 .". C>
;;; ~ ~
10 . 0 ~ltr_'tG
Figure 3. Nitrate input and output levels for the four cells.
Phosphorus removal in constructed wetlands is by adsorption onto soil, incorporation into microbial mass, and some plant uptake. It is normally low in many systems unless their design is based on very large areas
P. R. mOMAS et at.
with soil media for that purpose. The results obtained support this, showing a mean removal efficiency of 13 % in all of the cells and the relationship between phosphorus input and phosphorus output for the wetland system is shown in Fig. 4. The 5.5 m deep bore hole upgradient of the wetland system did not have any water and the samples collected from the downgradient bore hole indicated a mean total phosphorus level of 0.5 mg/l and nitrate levels < 1.0 mg/1. This did not give any indication of the possibility of groundwater contamination from the constructed wetland system. During this initial period of monitoring surface flows were observed at the inlet end of the wetland cells from time to time. This may be either due to excessive organic loading or due to the aspect ratio of 7.5:1 for each of the cells, resulting in inadequate hydraulic gradient and recently a value of 3:1 or less has been recommended to provide sufficient hydraulic gradient (Reed and Brown, 1992). 11.0 '-&n'Vaaet_tod
.....Ii::e..... c:> c:>
B.O 6.0 4.0 3.0
Figure 4. Phosphorus input and output levels for the four cells.
CONCLUSIONS Constructed wetlands have the potential to provide an economically feasible and simple method of polishing secondary treated wastewater where a consistent high quality effluent is not always required. In the subsurface flow wetland systems the liquid level is maintained below the media surface with fewer problems from odours, insect vectors and public exposure. They can be integrated into wastewater treatment systems for small communities in relatively close proximity to the public, particularly in rural areas. Results to date from the pilot studies have not indicated any overall pattern for pollutant removal either by a particular plant species or by a gravel size. However, phosphorus removal has been low in all four of the wetland cells. ACKNOWLEDGEMENT The Authors wish to thank La Trobe University, Albury-Wodonga campus, for the financial assistance and the Rural City of Wodonga for the facilities to carry out this project. REFERENCES Breen, P. F. (1989). Hydrology, structure and function of natural wetlands. Proceedings of the Applied Ecology and Conservation Seminar Series. La Trobe University, October-December, 31-38.
Pollutant removal using a constructed wetland system
Brix, H. (1987). Treatment of wastewater in the rhizosphere of wetland plants - the rootzone method. Wat. Sci. Tech., 19(1), 107118. Davies. T. H. (1988). Reed bed treatment of wastewaters: a European perspective. Water, 15(1), 32-33, 39. De Busk, T. A. and Reddy, K. R. (1987). BODs removal in floating aquatic macrophyte-based wastewater treatment systems. Wat. Sci. Tech., 19(12), 273-279. Finlayson, C. M. (1983). Use of aquatic plants to treat wastewater in irrigation areas of Australia. Proceedings of AWWA 10th Federal Convention, Sydney. Australian Water and Wastewater Association, 25.1-25.9. Mitchell, D. S. (1978). The potential for wastewater treatment by aquatic plants in Australia Water, 5(3). 15-17. Mitchell, D. S., Breen, P. F. and Chick, AJ. (1990). Artificial wetlands for treating wastewaters from single households and small communities. Adv. Wat. Pollut. Control, no. II, P. F. Cooper and B. C. Findlater (ed.), pp. 383-389. Pergamon, Oxford. Reed. S. C. and Brown, D. S. (1992). Constructed wetland design - the first generation. Water Environment Research, 64(6), 776781. Scholes, J. D., Kerr, R. J. and Nuttall. P. M. (1986). Treatment of wastewater by aquaculture systems. Australian Water Resource Council Project No 30113, Final Report. Department of Resources and Energy, Canberra. Thomas, P. R. and Phelps, H. O. (1987). Aeration and water hyacinths in waste stabilization ponds. Wat. Sci. Tech.• 19(12),265271. United States Environment Protection Agency (1993). Subsurface flow constructed wetlands for wastewater treatment - a technology assessment EPA832-R-93-008.