Nutrient removal mechanisms in constructed wetlands and sustainable water management

Nutrient removal mechanisms in constructed wetlands and sustainable water management

~ Pergamon War Sci. Tech Vol. 40 , No.2, pp . 121-128, 1999 10 1999 Published by Elsevier Science Ltd on behalfof the IAWQ Pnnted in Great Bntam, Al...

576KB Sizes 0 Downloads 40 Views

~

Pergamon

War Sci. Tech Vol. 40 , No.2, pp . 121-128, 1999 10 1999 Published by Elsevier Science Ltd on behalfof the IAWQ Pnnted in Great Bntam, All rights reserved 0273-1223/99 $20.00 + 0.00

PII: S0273-1223(99)00478-3

NUTRIENT REMOVAL MECHANISMS IN CONSTRUCTED WETLANDS AND SUSTAINABLE WATER MANAGEMENT K. Sakadevan and H. J. Bavor Water Research Laboratory-Centre for Water and Environmental Technology. University of Western Sydney Hawkesbury, Richmond. NSW-1753. Australia

ABSTRACT Constructed wetland systems are used to treat domestic and industrial wastewater and agricultural runoff. In this field study the influence of hydraulic loading, retention time, water column depth and phosphorus (P) concentrations of influent wastewater on P and nitrogen (N) removal was examined in experimental constructed wetland systems. Five constructed wetlands with a surface area of 150 mI , were developed in the field. Results showed that P concentrations of water from outlets of all five systems decreased (from as low as 11% decrease for the system which received wastewater with average P concentration of 8.2 mg PL" to as high as 48.9% decrease for the system which received wastewater with average P concentration of 3.58 mg P LO') compared to the influent water. The N concentrations in the outlet water were also decreased in all five systems (from as low as 26.3% decrease to as high as 77.5% decrease) compared to the influent water. The total P and N in the wetland sediment increased in the first year in all five systems but were unchanged at the end of the second year. The study showed that low hydrau lic loading and greater reten tion times positively enhanced removal of P and N from wastewater in constructed wetland systems . 0 1999 Published by Elsev ier Science Ltd on behalf of the IAWQ. All rights reserved

INTRODUCTION Wastewater recycling serves an important function in water resources management by providing quality water for irrigation. industrial and urban water requirements. Reliable low cost and low technology methods are necessary for successful implementation of wastewater recycling. Constructed wetland systems. a low cost and low technology option, have been used successfully for recycling and managing domestic and industrial wastewater (Kadlec, 1985; Brix, 1987; Cooper and Findlater, 1990; Fisher, 1990; Cooke, 1992), urban stormwater runoff (Livingston, 1989; Cutbill, 1994) and agricultural runoff (Higgins et al, 1993; De Laney. 1995). In Australia and East Asia interest in the use of wetlands for water pollution control is growing rapidly (Bavor and Mitchell, 1994; Roser and Bavor, 1994; Sakadevan and Baver, 1998). Most studies on constructed wetlands have focused mainly on the net removal of pollutants such as bacteria, suspended solids, biological oxygen demand and nutrients such as phosphorus (P) and nitrogen (N) from wastewater during its passage through a wetland (Wetzel, 1993). Although such studies have generally confirmed the effectiveness of constructed wetlands for pollutant removal and have identified a range of treatment efficiencies of wetlands, they provide little information on the processes involved in pollutant removal (Hosomi et al. 1994). An understanding of processes involved in pollutant removal is necessary for both improving the design of wastewater wetlands and predicting their long term sustainability for pollutant removal/reduction under a range of environmental conditions, soil/substrate types and hydraulic and pollutant loading. This is particularly important for the removal of nutrients such as P and N, because P and N are key nutrients limiting blue-green algal blooms and surface water quality. As the nutrient 121

K. SAKADEVAN and H. J. BAVOR

122

transformation and transport processes, and factors controlling these processes, are better understood, the design and operational parameters of constructed wetland systems may be modified to enhance the removal/reduction processes in the system. The objective of the study was to examine, quantitatively, the transformation of P and N in soil-plant-water of experimental constructed wetland systems receiving secondary treated sewage effluent, in order to establish the removal/reduction performance of constructed wetland systems. MATERIALS AND METHODS Constructed wetland systems The study was carried out at the constructed wetland research facility at the University of Western SydneyHawkesbury, Richmond, New South Wales, Australia, 60 km north-west of Sydney. Five free water surface flow systems (FWS) of identical design were constructed (approximately 30 m x 5 m) with a 400 mm subsoil (clay) layer and a topsoil layer of 300 mm. (Fig.l, UI, U2, U3, U4 and U5). The systems were divided into seven segments and planted with three different macrophyte species, Phragmites australis (P), Schoenoplectus validus (S) and Triglochin procera (T) (Fig.I). The macrophyte species were planted in a similar layout in all five systems (Fig.I). At the beginning ofthe experiment no macrophytes were planted in the open water at the outlet end of all five systems (Fig.I ), but with time macrophytes were established within the open water areas.

Figure 1. Experimental study site at Richmond showing the five constructed wetland systems.

Five different operating conditions (treatments) were established (Table I) to test the ability of constructed wetlands to polish secondary treated sewage effluent from the Richmond Sewage Treatment Plant. The configurations were designed to allow comparisons of the effect of effluent hydraulic loading rate (high vs low), phosphorus concentration (high vs low), retention time (long vs short) and water column depth (high vs low), on P and N transformations within soil-plant-water systems. The low P concentration in the wastewater applied to Systems 3 and 4 was achieved by alum dosing (treating secondary effluent with aluminium sulphate and removing P as aluminium phosphate). Systems I, 2 and 5 were used to compare high vs low effluent hydraulic loading, Systems I and 2 for comparing high vs low water column depths,

Nutrient removal mechanisms in constructed wetlands

123

Systems 2, 3, 4 and 5 were used to compare high vs low P concentrations and Systems 1, 2, 4 and 5 were used to compare long vs short retention times. Table 1. The operational parameters for constructed wetland systems at Richmond System

1 2 3 4 5

Hydraulic Loadin (Lday" R ) 15,000 6,000 6,000 2,000 2,000

Depth (ern) 50 20 20 20 20

Retention Time (days) 5 5 5

P Concentration low (l mg L') high (8 mg L') High High Low Low High

IS 15

Soils Soils (both topsoil and subsoil-clay) used in the study were on-site native soils. Some chemical characteristics of soils, which are important for use in constructed wetland systems for nutrient removal, are given in Table 2. Table 2. Chemical characteristics of soils used in constructed wetland systems Property pH Total Organic Carbon (mg C kg'! soil) Total N (mg N kg'! soil) Total P (mg P kg'! soil) Inorganic P (mg P kg'! soil) Maximum P adsorption (my P kg" soil) Exchangeable Ca (cM+ kg' soil) Exchangeable Mg (cM+ k t soil) Exchangeable K (cM+ kg' soil) Oxalate extractable AI (cM+ kg" soil) Oxalate extractable Fe (cM+ kg'! soil)



Topsoil 6.2 8700 731 380 1153 4.01 0.66

Subsoil-clay 7.2 2867 521 180 34 1723 3.66 1.55

0.77 3.14

1.2 1.14

3))

o

o

Soil and water measurements Six sub- and topsoil samples were collected from each system at the beginning of the experiment (October 1993, before wastewater was applied to the system) and were combined to give one composite soil sample each for topsoil and subsoil for each system. Triplicate soil cores were collected from all seven segments in each wetland system (21 cores for each system) using a 10 cm (internal diameter) soil sampler at the end of the first year (September 1994) and at the completion of the study (September 1995). Soil cores were divided into top 0-30 cm and >30 ern below the top soil. All soils were air dried and total P and total Kjeldahl N were determined. The difference between initial and final soil P and N was used to calculate P and N accumulation and/or loss in the soil. The inflow and outflow of water in all five systems was quantitatively measured at the inlet with a tipping bucket system and at the outlet using a gauged V-notch weir. The measurements of water flow were recorded daily for both inflow and outflow for all five systems. The rainfall and pan evaporation data were obtained from the University of Western Sydney-Hawkesbury's meteorological station. Water samples were collected every two weeks from both inlets and outlets of all five wetland systems and total P and total N were measured.

K. SAKADEVAN and H. J. BAVOR

124

Analytical measurement The total P and total Kjeldahl N for soil samples was measured by first digesting soil samples with a mixture of sulphuric acid, K2S04 and selenium (Kjeldahl digest) followed by assay of the digest using a Flow Solution III flow injection analyser (Method Numbers 4500 and 4500-P, American Public Health Association, 1992). The total P and N for water samples were determined as described above. Total carbon for soils was measured using a CNS analyser (Model NA 1500 NCS, Fisons Instrument). The total Ca, Mg, K, Fe and Al in soils was measured using atomic adsorption spectrophotometry, after digesting with concentrated nitric acid. The exchangeable Ca, Mg and K were measured by extracting soil samples with neutral ammonium acetate, pH=7.0 (1:10 ratio of soil and extract). Oxalate extractable Fe and AI were measured as above by extracting soils with 0.3 M ammonium oxalate, pH=3.25 (Saunders, 1965). All extracts for metal analysis were acidified with 50% concentrated hydrochloric acid before analysis. RESULTS AND DISCUSSION Water balance Water balance calculations for the study period (from October 1993 to September 1995) showed marked differences between inputs and outputs (Table 3) particularly in Systems 2, 3, 4 and 5, which showed up to 50% loss of water. Evapotranspiration measurements conducted in summer and winter on systems during zero flow periods, however, indicated that very high losses were occurring through evapotranspiration. Different effiuent hydraulic loading and depth for each system would give different evapotranspiration for all five systems. Some error would have been introduced through extrapolation to annual losses of the two seasonal evapotranspiration measurements. Table 3. Water balance for the study period System

Water In

1 2 3 4 5

8434 4548 4682 2022 1674

Rainfall (kilolitres) 108 108 108 108 108

Water Out 8028 2604 3186 1282 1126

Phosphorus concentrations in influent and effiuent waters The total P concentrations of the influent water for Systems 1,2 and 5, which received wastewater with high P concentrations, ranged from 6.6 mg P L'\ to 13.9 mg P L'I during the experimental period with a median P concentration of 8.2 mg P L·1 (Figs 2a, b and e). The average P concentration of influent water during the experimental period was 8.2±0.8 mg P L'\ (Table 4). The influent to Systems 3 and 4, which received alumdosed wastewater with low P concentrations, had total P concentrations ranging from 0.17 mg P L'\ to 5.26 mg P L·t (Table 4). The average influent P concentrations applied to Systems 3 and 4 was 3.5±2.7 mg P L·t (Figs 2c and d). The high standard deviation for the average influent P concentrations for Systems 3 and 4 was mainly due to the inclusion of some high P concentration values obtained for influent water samples before alum dosing was carried out for these two systems, and also due to the fact that the alum dosing system was malfunctional on some occasions. There was little variation in the concentration of P in the inflow water within a period of one day (8.0 to 8.2 mg P L·t over 16 hours). This suggested that the P concentration of influent water did not fluctuate significantly over time scales of <1 day.

Nutrient removal mechanisms in constructed wetlands

125

CalSystem 1

so

so

40

~

30 Z

g

.'"

20 ~

o

l:

10

200

400 Cumulative days

Z

600

Cb)System 2

so

• •••

... ..• ..• 9:0.•• •



": o·

J.

o

c:l 0

9:P 0

000 0

o

40

0

••

0

o

0

o

0

0

M~~o,~ c:Qo~

8J"O'

30

~

! 20

l.

10

Z

c::P

c:l

.... ~

-.po

o ••

0

Il;I 0

so



••

0 0

200

400 Cumulative days

600

Ce) System 3

so

o

••

•• ••

•··0" "0·.

••o

.• _.



o •



-

•• rlJ o

Dr

.

50 40

g

o l:P•

I:b

20 10

200

400 Cumulative days

:;

30 Z

e

E

Z

600

Cd) System 4

so

.

:; 40 110 !30

e ,g !

..

••

':..• • ••

••





•• rlJ

• • •••••

••

20

110 10

••

.

• •

o I:b

50 40

~

30 20

00

..'S

10

0

200

400 Cumulative days

600

.... Z

..

..a:,

!

g Z

K. SAKADEVAN and H. J. BAVOR

126

(ej System 5 50 .;- 40

..

.....l

,! 30

E

,.g

...:s 20

• .. • ... .. .. ... .. - . ••



~





c

••

50

-



40

••

30

c

20

... ... 10

10

400

200

~

~

;Z;

!

~

~

;Z;

600

Cumulative day.

Figure 2. Concentration ofP in the influent (.) and effluent (0) waters and N in the influent (.) and effluent (0) waters for Wetland Systems 1,2,3,4 and 5.

Table 4. Mean phosphorus and nitrogen concentrations of influent and effluent water for all five experimental systems

System

Influent

Phosphorus Effluent

1 2 3 4 5

8.18±0.6 8.21±0.6 3.58±2.8 3.45±2.6 8.26±1.1

7.3±1.8 7.1±1.7 1.8±1.4 1.8±1.4 6.4±1.8

%P Removal mgL,1 11.0 13.4 48.9 47.0 22.7

Influent

Nitrogen Effluent

±S.D 35.4±4.9 35.7±4.9 33.3±5.6 33.3±5.6 35.2±5.1

26.1±9.8 21.2±8.1 19.4±9.8 7.5±6.4 8.1±5.7

%N Removal 26.3 37.8 41.7 77.0 77.0

During the experimental period, the average P concentrations of water from the outlets of all five systems were lower than the P concentrations of inlet waters (Table 4), even though P concentrations increased in the outlet waters on some individual occasions (Figs 2a, b and e). In systems receiving wastewater with high P concentrations, the largest decrease in P concentration occurred in System 5 (22.7%) and the lowest decrease in P concentration was found for System 1 (11%) with System 2 more than System 1 (13.4%). The apparent reason for less than 25% P removal in Systems 1, 2 and 5 is that the high P concentrations in the influent water over-loaded the system uptake adsorptive capacity. Previous studies have shown that P removal in constructed wetlands is strongly influenced by the P concentration of the influent water (Hosomi et al., 1994). In addition to the influent P concentration, the water column depth (high depth in System 1), hydraulic loading (high for System 1) and retention time (low for Systems I and 2) reduced P removal in these three systems. The water column depth (50 ern) and hydraulic loading (15,000 L day") of System 1 were greater than that of System 2 (20 ern and 6000 L day", respectively) and 5 (20 ern and 2000 L day", respectively) (Table I). Even though the retention time for Systems I and 2 were similar, the combined effect of water column depth. hydraulic loading and retention time in System 2 influenced greater P removal in System 2 than in System 1. The P removal efficiency of System 5 was greater (22.7%) than Systems 1 and 2 because it has lower water column depth than System 1, greater retention time than Systems I and 2 and lower hydraulic loading than Systems 1 and 2, favorable for greater P removal. Nitrogen concentrations in influent and effluent waters The total N concentrations of influent water for Systems 1. 2 and 5 (high P input systems) varied from 24.8 mg N L'I to 49 mg N L't with a median of 33.0 mg N L,t (Figs 2a. b and c). The average influent N concentration during the study period was 35.4±4.9 mg N L'! (Table 4). Similarly in Systems 3 and 4, which received alum-dosed wastewater. the average N concentrations decreased only by 2 mg N L'! to 33.3±5.6 mg

Nutrientremovalmechanisms in constructedwetlands

127

t

N L- • As with P, the total N concentrations of influent water varied little over a period of one day (35.3 to 35.6 mg N L-t over 16 hours). On average, the total N concentrations of water from the outlets in Systems 1,2 and 5 were lower than the respective inflows during the study period (Table 4). Unlike P, the total N concentrations of the outlet water decreased on all individual occasions throughout the study period (Figs 2a, b, c, d and e). The largest N removals were observed for Systems 4 and 5 (77% for both Systems 4 and 5) and the lowest N removal was observed for System 1 (26.3%) (Table 4). Since all systems received similar N loading, the N removal efficiency was more related to hydraulic loading, water column depth and retention time. The results showed a clear dependence of removal efficiency on hydraulic loading and this is being used to derive N removal algorithms for these kinds of systems. The results also showed the combined effect of hydraulic loading, retention time and water column depth on N removal in constructed wetland systems. Soil phosphorus The total P in soil increased in all five systems in the first year irrespective of P loading (Table 5). All systems accumulated similar amounts ofP in the soil. The amount ofP in the soil was unchanged during the second year (Table 5). The reason for no change in P level in the soil was unclear even though P concentration in water decreased. Phosphorus accumulation in the soil was not influenced by the P loading, retention time or water column depth. It appeared that P accumulation in the soil mainly occurred through adsorption ofphosphate as shown by the presence ofO.5M H2S04 extractable phosphate in the soil (data not given). In all five systems more than 50% of soil P was present as 0.5M H2S04 extractable phosphate (data not shown). Table 5. Initial and final phosphorus contents in soil for all five systems System

Initial soil P

1 2 3 4 5

156±30 156±30 156±30 156±30 156±30

End of first year soil total P

End of second year soil total P

216±29 223±63 203±33 224±69 223±53

230±6 225±11 206±11 231±23 228±6

Soil nitrogen The total N in the soil increased in all five systems in the first year (Table 6). The largest increase was observed in System 5, which was associated with greater removal of N from wastewater (Tables 4 and 6). The standard deviation associated with total soil N was also high. An influence of hydraulic loading, retention time and water column depth on N accumulation in the soil was not observed in the study. The soil N was not increased in the second year (Table 6). One possible explanation is that N removed from water might have been lost through volatilisation and also through nitrification/denitrification. There are micro aerobic and anaerobic sites which exist under flooded conditions in constructed wetland systems and these micro sites may contribute to the loss ofN through nitrification/denitrification from the soil. Table 6. Initial and final nitrogen contents in soils for all five systems System

Initial SoilN

End of First year Soil total N

1 2 3 4 5

384±34 38.4±34 384±34 384±34 384±34

572±104 474±76 471±95 468±78 549±38

End of second year Soil total N

gNm-i±S.D.

562±111 492±143 523±116 459±171 577±173

K. SAKADEYAN and H. J. BAYOR

128

CONCLUSIONS In the past, factors controlling P and N removal from wastewater using constructed wetland systems have not been well established even though there is considerable interest in developing constructed wetlands for pollution control. The results from this study confirm that constructed wetlands remove P and N from wastewater and that the efficiency of removal may be influenced by the design of the wetland. Low hydraulic loading and greater retention times positively enhance the removal of P and N from wastewater in constructed wetland systems. Even though wetland design may positively influence P and N removal from wastewater, nutrient accumulation in the sediments of constructed wetland systems was not demonstrated to have been closely linked with the design (hydraulic loading, retention time and water column depth) of the constructed wetland system. REFERENCES Bavor, H. J. and Mitchell, D. S. (1994). Wetland systems in water pollution control. Wat. Sci. Tech., 29(4). Brix, H. (1987). Treatment of wastewater in the rhizosphere of wetland plants- the root zone method. Wat. Sci. Tech., 19(1/2), 107-118. Brix, H. (1994). Use of constructed wetlands in water pollution control: Historical development, present status and future perspectives. Wat. Sci. Tech., 30(8), 209-223. Cooke, J. G. (1992). Phosphorus removal processes in a wetland after a decade of receiving a sewage effluent. J. Environ. Qual. 21, 733-739. Cooper, P. F. and Findlater, B. C. (1990). Constructed Wetlands in Water Pollution Control. Pergamon Press, New York, NY. Cutbill, L. B. (1994). The potential for urban stormwater runoff treatment by constructed wetlands. In: 4th International Conference on Wetland Systems for Water Pollution Control. Guangzhou, China. IAWQ, 677-686. De Laney, T. (1995). Benefits to downstream flood attenuation and water quality as a result of constructed wetlands in agricultural landscapes. J. Soil Wat. Conser., 50, 620-626. Faulkner, S. P. and Richardson, C. J. (1989). Physical and chemical characteristics of fresh water wetlands. In: Constructed Wetlands for Wastewater Treatment: Municipat, Industrial and Agricultural, D. A. Hammer (Ed.). Lewis Publishers, Chelsea, MI. pp. 41-72. Fisher, P. J. (1990). Hydraulic characteristics of constructed wetlands at Richmond, NSW, Australia. In: Constructed Wetlands in Water Pollution Control, P. F. Cooper (Ed.), Pergamon Press, Oxford, pp. 21-31. Gersberg, R. M., Elkins, B. Y., Lyon, S. R. and Goldman, C. R. (1986). Role of aquatic plants in wastewater treatment by artificial wetlands. Wat. Res., 20(3), 363-368. Higgins, M. J., Rock, C. A., Bouchard, R. and Wengrezynek, B. (1993). Controlling agricultural runoff by use of constructed wetlands. In: Constructed Wetlands for Water Quality Improvement, G. A. Moshiri (Ed.), Lewis Publishers, 359-367. Hosomi, M., Murakami, A. and Sudo, R. (1994). A four-year mass balance for a natural wetland system receiving domestic wastewater. Wat. Sci. Tech., 30(8), 235-244. Kadlec, R. H. (1985). Aging phenomena in wastewater wetlands. In: Constructed Wetlandsfor Wastewater Treatment: Municipal, Industrial and Agricultural, D. A. Hammer (Ed.), Lewis Publishers, Chelsea, MI, 239-247. Livingston, E. H. (1989). Use of wetlands for urban stormwater management. In: Constructed Wetlands for Wastewater Treatment- Municipal, Industrial and Agricultural, D. A. Hammer (Ed.), Lewis Publishers, Chelsea, MI, 253-264. Reddy, K. R. and Patrick, Jr. W. H. (1984). Nitrogen transformation and loss in flooded soils and sediments. CRC Critic. Rev. Environ. Con., 13,273-309. Richardson, C. J. and Marshall, P. E. (1986). Processes controlling movement, storage, and export of phosphorus in a fen peat land. Ecolog. Monograph., 56, 279. Roser, D. J. and Bavor, H. J. (1994). SWAMp™. A computensed decision support system for employing wetlands in the biological removal of nutrients and other water pollutants. In: Biological Nutrient Removal 2. Albury, Australia, pp. 227-232. Sakadevan, K. and Bavor, H. J. (1998). Phosphate adsorption characteristics of soils, slags and zeolite to be used as substrates in constructed wetland systems. Wat. Res., 32, 393-399. Saunders, W. M. H. (1965). Phosphate retention by New Zealand soils and its relationship to free sesquioxides, organic matter and other soil properties. NZ. J. Agric.Res., 8, 30-57. Standard Methods for the Examination of Water and Wastewater (1992). 18th edn, APHAlAWWAlWPCF, Washington DC, USA. Wetzel, R. G. (1993). Constructed wetlands: Scientific foundations are critical. In: Constructed Wetlands for Water Quality Improvement, G. A. Moshtri (Ed.). Lewis Publishers, pp. 3-7.