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Rootzone dynamics in constructed wetlands receiving wastewater: a comparison of vertical and horizontal flow systems
~
Pergamon
WO!. Sci. Tech. Vol. 32, No.3. pp. 281-290.1995.
0273-1223(95)00629-X
Copyright e 1995 IAWQ Printed in Great Britain. All rights reserved. 0273-1223195 59'50 + 0'00
ROOTZONEDYNAMUCSIN CONSTRUCTED WETLANDS RECEIVING WASTEWATER: A COMPARISON OF VERTICAL AND HORIZONTAL FLOW SYSTEMS Peter F. Breen and Alan J. Chick Cooperative Research Centre for Freshwater Ecology, 68 Ricketts Road, Mt Waverley, Victoria 3 / 49, Australia
ABSTRACT Proftle conditions were exami ned in both small experimental and pilot scale sub-surface flow wetlands. The study systems differed in their hydraulic design. The experimental systems had a vertical up-flow design whereas the pilot system was a horizontal flow trench design. Both systems were found 10 have significant physical, chemical and biological gradients within the sub-surface profile. System age and plant root density appear to he important factors in determining profile differentiation within the experimental systems. Root densities were found to be partitioned between the upper and lower layers on a 70%/30% split. respectively. However, in the experimental systems as the systems aged and root densities increased beyond 112- 25l g.m-2 chemical water quality differentiauon in the profile disappeared . Pilot scale systems were found 10 have physical gradients wilbin t.be proftle as evidenced by hydraulic shan -circuiting. Venical root density distribution is proposed as a major cause of this condition in horizontal flow systems.
KEYWORDS Wetlands (artificial or constructed ); wastewater treatment; nutrient removal; aquatic macrophyte : rootzone: wetland hydraulics; Schoenople ctus validus; Eleocharis sphacelata , INTRODUCTION The performance of artificial wetlands treating wastewaters, although generally successful, has been variable (Nichols, 1983; Conley et al., 1991). It has been argued that this has occ urred because systems have been designed and constructed in the absence of sufficient treatment process understanding (Heliotis & De Witt, 1983: Middlebrooks, 1987). Breen (1990) has addressed some of these inadequacies in understanding by determining the relative importance of chem ical and biological processes in the treatment performance of experimental artificial wetlands. Breen & Chick ( 1989) have examined some physical aspects by evalua ting the influence of system hydraulics on performance . The present study investigates aspects of the profile variability of vertical and horizontal flow artificia l wetlands. 28 1
282
P. F. BREEN and A. J. CHICK
MATERIALS AND METHODS The experimental wetlands consisted of 20 I plastic buckets filled with washed river gravel (3-7 mm diameter) and planted with Schoenoplectus validus (Vahl) A. & D. Love. The systems had a vertical upflow hydraulic format. Influent was introduced to the bottom of the bucket via a central tube. Effluent was collected at the top of the bucket via three equidistant peripheral drainage tubes located 3 ern below the gravel surface. Breen (1990) gives a more detailed description of the experimental systems and their hydraulic behaviour. A pool of 36 planted experimental systems were set up on 19 December 1985 and were established for approximately 6 months prior to receiving their normal operational loading of 1 I.d-1 of primary settled domestic sewage . This loading rate resulted in a theoretical retention time of six days. A group of 12 control systems (i.e. no plants) were set up at the same time and operated in an identical way to the planted system. The systems were batch loaded daily (0800-0900 h) and operated continuously from 1 July 1986. The systems exhibited plug flow hydraulics during batch loading and mixed fully between daily batch loading events (Breen, 1990). All experimental systems were housed in a glasshouse to protect them from extraneous inputs such as rain and excessive dust. Position in the glasshouse was randomly allocated prior to each test period. Because some of the tests took place in summer in inland temperate Australia, it was necessary to control the daytime temperature of the glasshouse by whitewashing and evaporative cooling. No winter or night heating was employed and the glasshouse was always fully ventilated. Air temperature in the glasshouse was monitored with a Wilko Lambrecht thermograph in a Stevenson screen. The experimental systems were sampled at depths of 0.10 m and 0.25 m within the 0.35 m profile. Samples were extracted using a 100 ml syringe prior to the morning batch loading period. The experimental systems to be sampled in this manner were randomly selected from the pool of available systems. The systems were assessed by analysing the top and bottom samples for: filterable reactive phosphorus (FRP); total phosphorus (TP); filterable total phosphorus (FTP); total Kjeldahl nitrogen (TKN); filterable total Kjeldahl nitrogen (FfKN); ammoniacal nitrogen (AN); total oxidised nitrogen (TON); sulphate (SO), winter only; dissolved oxygen (DO). Filterable non-reactive phosphorus (FNRP) :: FTP - FRP; filterable organic nitrogen (FON) :: FfKN - AN; total nitrogen (TN) :: TKN + TON. This rootzone profile study was co-incident with a mass balance study in which systems that had similar histories were randomly selected from the pool, harvested and partitioned. The harvest involved hand sorting the entire bucket into aboveground parts, rhizomes and roots. The root tissues were further partitioned into upper and lower halves by cutting the systems into horizontal halves, each of which was sorted separately for root biomass. The partitioned root biomass is reported in this study. The root tissues were dried at 75°C and weighed for dry weight biomass. The trench system was a pilot scale unit located at Wodonga, Victoria. The system was 50 m long, 2 m wide and 0.5 m deep, plastic lined, filled with crushed quartz gravel (5-10 mm diameter) and planted with Eleocharis sphacelata R.Br. The system was planted in September 1984 and was approximately 22 and 24 months old at the respective sampling dates of 22-28 June 1986 and 19-24 August 1986. The system was loaded with a continuous flow of secondary treated domestic sewage from a wastewater treatment pond at 4.3 kl.d- 1 which resulted in a theoretical retention time of seven days. The system was continuously operated from September 1984. The trench was sampled at the inflow, outflow and internally at three positions along the trench located at 12.5 m, 25.0 m and 37.5 m from the head. At each position along the trench a six point sampling grid was established. At each position, sampling points were located in a grid pattern within the profile at depths of 0.1 m and 0.4 m, and at 0.5 rn, 1.0 m and 1.5 m widths across the trench. Interstitial water samples were extracted at intervals ranging from 0.5 to 6.0 h using a 100 ml syringe and analysed for AN, TON and FRP. Influent and effluent was sampled at 0600, 1200, 1800 and 2400 h for three consecutive days and analysed for TN, AN, TON, FON, TP, FRP and FNRP.
Rootzone dynamics in constructed wetlands
283
The hydrological characteristics of the system were examined using eriochrome acid red as a tracer. A 20 I slug of 0.1 g.l-I dye was introduced to the trench over I hr. The slug was continuously pumped in a mixed stream with the influent. Samples were taken at all points within the trench at intervals varying from 0.5 to 6.0 h. All samples were analysed using APHA (1985) Standard Methods. RESULTS AND DISCUSSION Results from both the experimental and pilot scale trench systems indicate that for particular variables significant differences can occur between different vertical positions in the profile. However very few differences are evident within the profile of the experimental control systems which suggests that the presence of plants is a major factor determining profile variability. Winter results from the planted experimental systems (221 days old) show that significant differences (at the 5% level) occur between the top and bottom zones of the profile for FRP, TP, AN, TN, DO, root biomass and root density (Table 1). The summer results from similar systems (412 days old) show that with the exception of root biomass and root density these differences are no longer evident (Table 2). With the exception of TON there were no differences between the top and bottom zones of the profile in the control systems for any variable. In winter top and bottom TON differences in the control systems were significantly different, but the large standard deviation in the summer results prevented a significant difference being statistically established. Although the absolute levels were low, the relatively high TON concentrations in the upper zones of the control systems probably reflects surface aeration acting as an oxygen source for nitrification. Results from the trench systems (Table 3) for both mid and late winter show significant differences occur between the concentrations of AN, TON and FRP from the upper and lower zones of the profile. The very large build-up of TON in the upper zone during the midwinter trial, accompanied by a decrease in AN, indicates that substantial nitrification had been occurring. The accumulation of TON also indicates that conditions were apparently not suitable for denitrification. Under most natural conditions denitrification is usually capable of utilising any nitrate at a rate greater than its production and consequently high concentrations of TON are unusual. This later situation was apparently occurring during the late winter trial where AN concentrations in the upper zone of the final half (25-37.5 m section) of the trench were significantly less than those in the lower zone, but TON concentrations in the upper zone, while increased, were still low in comparison with the midwinter trial. This pattern suggests that any nitrification that may have been causing a reduction in AN concentrations was accompanied by denitrification which prevented any accumulation of TON. FRP results also show a significant elevation in concentration in the upper zone during the late winter trial. The explanation for this pattern is not clear if active denitrification processes were occurring; anaerobic heterotrophs may have been mineralising organic matter accumulated in the rootzone and generating inorganic fractions like FRP. Anaerobic conditions could also have released orthophosphate from the ion exchange system of the gravel matrix by reducing the ferric oxide films which typically form most of the adsorption sites. The overall level of performance from the planted experimental systems for the winter and summer trials, as measured by percentage concentration reduction, was very similar (Table 4). This suggests that temperature, the most prominent seasonal variable, was not the major factor responsible for differences in the concentration of variables within the profile. The control systems however did show some apparent seasonal variation for TN. Water temperatures for both the experimental and trench systems are shown in Table 5. Temperatures are clearly very different for the experimental systems in winter and summer. These results indicate that under certain conditions wetland wastewater treatment is feasible in the Australian temperate winter. Differences in the profile conditions between winter and summer may reflect differences in other important features such as system age or maturity. This possibility is exemplified by the root biomass data. There was a major increase in root biomass over the winter to summer period. Interestingly however the relative vertical distribution of root tissues for the winter and summer period remained very similar with approximately 70% of the biomass occurring in the upper zone and 30% in the lower zone.
P. F. BREEN and A. J. CHICK
284
Table I. Winter experimental system mean water quality and rootzone biomass FNRP
TP
AN
TON
FON
TN
SO
DO
RB
RD
0.20 0.10 48.8
0.41 0.15
0.41 0.29 23.4
0.03 0.02 1.7
0.54 0.33 30.9
I. 75 0.75
16.9 6.3
1.2 1.0
251 59
1.7 0.5
0.40 0.17 46.0
0.08 0.08 9.2
0.87 0.27
2.42 1.60 54.0
0.01 0.01 0.2
0.75 0.46 16.7
4.48 1.68
17.0 6.7
bdl
0.37
0.13
1.2
12.3
0.12
0.20
13.5
0.8
bdl
0.19 30.8
0.04 10.8
0.3
2.2 91.3
0.05 0.9
0.17 L5
2.1
0.7
0.30 0.14 27.3
0.12 0.04 10.9
I.l
Standard Deviation Proportion Of Total (%)
0.16
II. 7 1.6 90.8
0.01 0.004 0.08
0.29 0.25 2.3
12.8 1.6
0.5 0.5
INFU1ENT
4.4
0.19
5.7
21.7
0.01
1.7
27.5
27.8
0.1 77.2
0.13 3.4
0.6
OA 78.8
0.01 0.04
0.2 6.1
0.5
4.4
FRP INTERSTlTIAL WATER PLANTED Top (X) 0.04 Standard Deviation 0.02 Proportion Of Total (%) 9.8
Bottom (X
)
Standard Deviation Proportion OfTotal (%)
CONTROL Top (X) Standard Deviation Proportion OfTotal (%)
Bottom (x )
69.2 0.8 0.02
112 35 30.8
bdl
NA bdl
(X)
Standard Deviation Proportion Of Total (%)
Units mg.1-l except RB detection limit
~m·2
& RD mg.cm-J ;Inflllfnt n=5, EOlllfnt n=20 (planted)/n=lO (control); bdl below
Table 2. Summer experimental system mean water quality and rootzone biomass FRP INTERSTITIALWATER PLANTED top (X) <0.03 Standard Deviation Proportion Of Total (%) 9.1
FNRP
TP
AN
TON
FON
TN
DO
RB
RD
0.16 0.04 48.5
0.33 0.13
0.02 0.01 1.4
0.01 0.01 0.7
0.89 0.38 61.8
IA 0.39
0.4 OA
1172 17 68.3
7.8 0.1
0.55 0.29
0.03 0.01 2.1
0.01 0.01 0.7
1.0 0.38 71.8
1.4 0.33
bdl
545 150 31.7
4.0 1.1
bdl
Bottom (X) Standard Deviation Proportion OfTotal (%)
<0.03 5.5
0.12 0.07 21.8
CONTROL
Top
2.63
0.21
3.3
7.3
0.11
1.0
8.8
Standard Deviation Proportion OfTotal (%)
0.10 79.2
0.11 6.3
0.15
1.5 82.2
0.08 1.3
OAI 11.5
1.5
Bottom (X) Standard Deviation Proportion Of Total (%)
2.5
0.17 0.08 5.7
3.0 0.20
6.8 1.69 80.0
0.03 0.03 OA
0.95 0.37 11.2
8.5 2.0
bdl
O.19 84.1
INFIAJENT
3.2
0,32
4.7
15.5
0.02
1.1
23.6
bdl
0.21 67.5
0.15 6.8
0.39
1.3 65.6
001 0.01
0.58 4.6
2.1
(X)
(X) Standard Deviation Proportion Of Total (%)
Units mg.l-I except RB ~m-2 & RD mg.cm-J ;Inflllfnt n=5, EOlllfnt n=20 (planted)/n= I a (control); bdl below detection limit
The major changes in the chemical conditions in the profile of the experimental planted systems between the winter and summer periods were a marked decrease in bottom FRP and TN, and profile AN. All these changes probably reflect increased plant uptake due to increased plant biomass and improved wastewaterrootzone contact. The changes in the control systems included a marked increase in profile FRP and TP from winter to summer. This reflects the progressive saturation of orthophosphate adsorption sites on the gravel ion exchange system (Breen, 1990). Other winter to summer changes in the control systems included a decrease in profile AN and TN, and an increase in FaN.
285
Rootzone dynamics in constructed wetlands
Table 3. Trench system mean water quality Water Quality
Interstitial Water
Input 12.5m
Output 31.5m top bottom
25m
top
bottom
top
bottom
53.6 2.5 0.95 6.9 0.31
54.9 2.4 1.0 0.92 1.1 0.22
39.2 4.3 21.1 8.4 6.1 0.24
49.6 3.8 1.4 1.0 1.2 0.49
41.6 6.1 12.1 2.1 6.4 0.61
54.6 2.1 0.61 0.31 1.0 0.33
50.1 0.68 6.3 0.41 6.1 0.31
61.2 0.69 0.12 0.03 1.9 0.10
64.2 1.9 0.10 0.04 8.0 0.16
41.4
58.6
1.3
1.3
0.89 0.06 12.5 0.61
0.01 0.01 8.4 0.24
38.6 2.1 1.2 0.08 11.1 0.41
51.6 1.9 0.11 O.oJ 9.0 0.18
43.1 0.53 0.91 0.03 8.0 0.14
21 June 1986
AN
(X)
Standard Deviation (X) TON Standard Deviation
(x)
FRP
Standard Deviation 18 August 1986
eX)
AN
Standard Deviation eX) TON Standard Deviation FRP eX) Standard Deviation
51.3 2.1 0.411 0.11 6.8 0.65
1.5
60.4 3.9 0,01
0.01 1.0 0.21
Units mg.l-I; n = 20
Table 4. System performance: mean percentage reduction in concentration SYSTEM EXPERIMENTAL Planted top*
TN
top* bottom
93.6 83.1 51.0 53.3
Effluent
8.1
bottom
Control
TP
TN
Winter 92.1 84.6 18.8 80.5
93.9 94.0 62.6 64.2
-1.2
30.6
93.0 88.3 29.1 35.1 Late Winter
MidWinter
TRENCH
TP Summer
-14.8
• equivalent to system eftIuent
Table 5. Summary of water temperatures (oq over the study period SYSTEMS Experimental" Winter Sununer Trench U MidWinter Late Winter
Mean
Standard deviation
Median
Range
8.3 22.4
1.1
8.1
3.8
22.8
6.6 - 9.6 18.0- 21.0
9.1 11.4
1.7 1.9
9.5
7.8 - 13.0
11.1
8.0- 14.2
• Interstitial water temperature recorded over the study period ··Influent v.ater from treatment plant operation records 1980-1986. mid winter
n=1O. late winter n=l1
These changes in the nitrogen fractions reflect the increased biological diversity and nutrient transformation capacity of the control systems with increased temperature and system age, and highlight the differences between Nand P removal mechanisms in the absence of plants. The dye tracer data (Fig. l) from the trench systems indicate that there were major hydraulic differences between the upper and lower zones. The upper and lower dye traces at 12.5 m were more or less coincident. although the majority of the flow occurred in the upper zone. However at the 25 m and 37.5 m points along
286
P. F. BREEN and A. J. CHICK
the trench the majority of the flow occurred in the lower layers of the trench. The upper and lower dye peaks were also non-synchronous indicating differential flow paths which lead to dead zones and shortcircuiting. The peak transit times for the greatest peaks at the 12.5 rn, 25 m and 37.5 m positions along the trench were approximately I. 2 and 3 days early. respectively. further confirming short-circuiting. For a 7 day retention time plug flow theoretical peak transit time would be approximately 1.75. 3.5 and 5.25 days respectively. If the 70%/30% respective split between root biomass in the upper and lower zone of the experimental systems is applicable to root density distributions in the trench systems it would clearly explain the channelling of flow to the lower layers. For a horizontal flow system if the upper zone has 40% more root biomass in the pore spaces of the gravel, flow will naturally be directed to the lower zone which would be the path of least resistance. Figure 2 illustrates the impact of root density distribution on hydraulic resistance and flow in trench systems. The chemical stratification evident in midwinter, particularly the TON data. illustrate. the extent to which vertical flow partitioning can isolate the upper and lower zones of a horizontal flow system.
: :1 .
o" w a:
AUGUST lUll
JUNE HIIIII
o
l2.$m
TOP
C BOTTOM
200
0
F~
g
0
ii:
w
37.11111
I
o
I
I
I
I
2
:I
4
II
II
I
I
I
I
I
I
0
2
3
4
II
II
TIME(d)
Figure I. Longitudinal series of dye tracer concentration-time curves for the top (0) and bottom (0) sampling positions of the central plane of the trench.
The comparison of influent and effluent can be a useful way of assessing the processes occurring in a treatment system. Similarly the comparison of interstitial water composition can indicate the importance of particular treatment processes at particular points within a system. In winter the composition of interstitial water in the top and bottom layers of the experimental systems were very different (Table I). In the top layer FNRP represented 48.8% of the TP, whereas in the bottom layer FRP was the major P fraction at 46.0% of the TP. Similarly for nitrogen FON (30.9%) was the largest fraction in the top layer of the experimental systems whereas AN (54.0%) was the dominant fraction in the lower layers. The influent was dominated by FRP (77.2%) and AN (78.8%). For the experimental systems with a vertical up-flow format this pattern suggests that during the winter trial influent was not significantly processed until it moved into the upper zone of the system. In summer with the exception of FNRP. the composition of interstitial water in the upper and lower zones of the experimental systems was very similar (Table 2). This suggests that in winter different processes were occurring within the system rootzone profile whereas in summer the whole profile was operating in a similar way. As previously discussed the major difference between the vertical condition of the profiles between winter and summer was root biomass. This observation also suggests that a critical root biomass may exist. The
287
Rootzone dynamics in constructed wetlands
results from the winter trial suggest that the critical root biomass for substantial plant mediated treatment appears to be between 112-251 g.m·2. This was the root biomass range for the lower and upper zones, respectively, of the experimental systems in the winter trial (Table 1). The relative nutrient composition of influent and effluent of the trench systems suggests little plant processing of the influent was occurring (Table 6). The nutrient reduction data support this observation (Table 4). With the exception of midwinter TON concentrations the percentage composition of effluent from trenches in both winter trials was similar to the experimental control systems, particularly in the summer trial. This suggests that plant processes were having little influence on effluent quality from the trench. The hydrological data (Fig. 1) which shows that most of the flow occurs in the lower zone of the trench. where there were least roots, would also support this suggestion. In addition the temperatures in the winter trench trials and the winter experimental trial were similar (Table 5) and suggest that temperature per se was not necessarily the main determining factor in the comparative performance of the trench and experimental systems. Table 6. Trench system mean influent/effluent quality FRP
FNRP
TP
AN
TON
FON
TN
21 June 1986 (X) INFLUENT Standard Deviation Proportion of Total (%)
6.8 0.7 79.1
1.0 0.3 11.6
8.6 0.3
57.3 2.1 89.3
0.5 0.1 0.8
6.7 1.1 10.4
64.2 3.0
(X) EFFLUENT Standard Deviation Proportion of Total (%)
6.7 0.3 77.0
0.9 0.2 10.3
8.7 0.2
50.1 0.7 85.5
6.3 0.5 10.8
3.0 1.2 5.1
58.6 1.5
18 August 1986 (X) INFLUENT Standard Deviation Proportion of Total (%)
7.0 0.2 79.6
0.9 0.2 10.2
8.8 0.2
60.5 3.9 89.1
0.1 0.01 0.1
7.1
67.9 4.0
10.5
(X) EFFLUENT Standard Deviation Proportion of Total (%)
8.0 0.2 79.2
0.9 0.1 8.9
10.1 0.2
43.7 0.5 92.8
0.9 0.03 1.9
2.8 1.1 6.0
1.1
47.1 L7
Units mg l-I; n = 20
The results from trials on both the experimental systems and the pilot scale trench system indicate that significant chemical gradients can occur within the belowground profile of these systems. This confirms previous results (Fetter et al., 1976; Bowmer, 1987; Rogers et al., 1991) although the present study suggests the differences may be more significant than those found by these other authors, and that these profile differences can vary with time. In the present study the vertical differences in particular chemical variables, can be attributed to a combination of the distribution and density of roots within the profile of systems and the system hydraulics. Fetter et at. (1976) while not addressing the particular role or function of plants in their systems, suggested from a comparison of planted and unplanted system performance, that plants only increase treatment performance by several percentage points. Bowmer (1987) suggested that plants may influence system performance by channelling flow around the root mass and thus causing short-circuiting. From the present study it would appear that this observation is a general feature of horizontal flow systems. However the reports of Breen (1990), Rogers et at. (1990 and 1991) indicate that under certain circumstances plant uptake can be the major nutrient removal pathway and consequently plants can be an important and highly beneficial part of such systems. The results from the reported trials exemplify both the positive and negative influence that plants can have on different system designs and their performance.
P. F. BREEN and A. J. CHICK
288
The existence of a critical root biomass for effective plant mediated nutrient removal has a significant impact on system establishment time and the effective operational depth of wetland systems. To have the entire rootzone effective in nutrient removal, systems have to be established long enough to allow between 112 and 251 g.m- 2 of root biomass to accumulate in the designed rootzone depth. For the experimental systems with a 0.35 m profile depth between 221-412 days were required. Plant root densities have an inverse depth relationship. Consequently system depths greater than those at which root biomass in the 112-251 g.m- 2 range can develop, may be superfluous to plant mediated nutrient removal processes. The idea of a critical root biomass and its associated active plant depth concept suggests that for the purposes of plant mediated nutrient performance, system volume and retention time should be calculated on active rootzone depth and not on true system depth. System profiles deeper than the active rootzone may however be designed for other purposes such as an anaerobic filter for BOD removal or as a flow distribution zone. Rogers et al. (1990) has shown in experimental systems similar to those used in the present study that root density distribution is dependent on system hydraulic format. In systems with a vertical downflow format it was found that more than 50% of the root biomass occurred in the top 0.05 m of the profile. Systems with a vertical upflow format had a more even root density, although still clearly depth dependent. As a result the determination of effective system depth is also dependent on the hydraulic design objectives and how these may influence plant growth and behaviour. The flow stratification and short-circuiting in trench systems results in both reduced retention times and less wastewater-rootzone contact. It has already been mentioned that in the experimental systems plant uptake is a major nutrient removal mechanism (Breen, 1990; Rogers et al., 1990 and 1991). Consequently for plant mediated processes to be involved in determining treatment performance, wastewater-rootzone contact must be optimised. This is the case in the experimental systems. The ideal hydraulic condition for a batch loaded design like the experimental systems is either plug flow or fully mixed. The vertical upflow experimental systems have been assessed as mixing fully between batch loading events (Breen & Chick, 1989; Breen, 1990; Rogers et al., 1990). The upflow hydraulic design of the experimental systems also maximises wastewater-rootzone contact by directing outflow through the root dense upper layers. Similar experimental systems with a downflow format have been assessed as approximating plug flow (Rogers et al., 1990). With a vertical flow format these systems also maximise wastewater-rootzone contact. In the trench systems however the horizontal flow hydraulic design sets up the conditions for flow partitioning and short-circuiting and minimises spatial and temporal wastewater-rootzone contact. low
shallow
high
PROFILE DEPTH
ROOT DENSllY & HYDRAULIC RESISTANCE
flow distribution
curve FLOW
high
deep
WETLAND ROOTZONE low
Figure 2. Subsurface horizontal flow wetland model showing root density and hyraulic resistance gradients and their effect on plug flow.
The high concentration of nutrients in the profile of the trench system can be used to illustrate several features of trench function that are significant in system design and operation. An important feature to recognise is that system hydraulics is the unifying process that connects the various treatment and nutrient removal mechanisms as water is obviously the carrier for all wastewater constituents. Flow partitioning in the trench system resulted in the spatial separation of the upper and lower layers of the system. This appears to have had several results. Firstly the accumulation of TON in the upper layers of the system, particularly in midwinter, suggests that the microbial process of nitrification and denitrification were poorly connected.
Rootzone dynamics in constructed wetlands
289
Secondly at least in late winter FRP also accumulated in the upper layers of the system. The exact reason for this is not clear, although it is thought that increased organic decomposition during this initial period of increasing temperature may be involved. CONCLUSIONS It is apparent that major chemical and biological variations occur within the belowground profile of artificial wetland systems. The spatial variations measured within the test systems also underwent temporal change. The temporal changes appear to be both cyclical and related to seasonal factors, and unidirectional and associated with system maturation and age.
Variations within the profile are largely the result of plant activity and the interaction of plants on system hydraulics. In the experimental systems chemical differences within the profile seemed to be associated with absolute root densities not necessarily the relative root density distribution. Consequently there appears to be a critical root density for effective plant mediated nutrient removal. Data from the experimental systems suggest the critical root density is between 112 and 251 g.m- 2 which in the experimental systems took between 221 and 412 days to accumulate. These findings could influence system design and operation by determining effective rootzone depths and establishment times. In the trench systems chemical variations with the profile are at least partly determined by the normal vertical distribution of root density causing an associated vertical variation in hydraulic resistance. This vertical variation in hydraulic resistance, due to roots filling up interstitial space within the substratum, results in flow partitioning and progressive diversion of flow away from the rootzone, that is to areas of least hydraulic resistance (Fig. 2). Consequently it is concluded that horizontal flow designs are theoretically and practically incompatible with vertically uniform plug flow and hence with plant mediated nutrient removal. The accompanying conclusion is that to optimise plant mediated processes and system performance wastewater-rootzone contact must be maximised. The vertical flow format of the experimental systems is an option for achieving this design objective. ACKNOWLEDGMENTS The project was funded by Sydney Water Board. The work was undertaken at the Griffith Laboratory of CSIRO, Division of Water Resources. Particular thanks are extended to Dr D.S. Mitchell, Dr K.H. Bowmer, Ms S. Naumovski, and Ms RH. Shields. REFERENCES APHA (1985) Standard Methods for the Examination of Water and Wastewater, 16th edn, American Public Health Association. Washington, D.C. Bowmer, K. H. (1987) Nutrient removal from effluents by an artificial wetland: influence of rhisosphere aeration and preferential flow studied using bromide and dye tracers. Water Research, 21, 591~599. Breen, P. F. (990) A mass balance approach for assessing the potential of artificial wetlands for wastewater treatment. Water Research, 24, 689---{)97. Breen, P. F. & Chick, A. J. (989) Wastewater treatment using artificial wetlands: the hydrology and treatment perfonrumce of horizontal and vertical flow systems. Proceedings Australian Water and Wastewater Association 13th Federal Convention. The Institute of Engineers, Australia, pp 167-171. Conley, L. M., Dick, R. I. and Lion, L. W. (1991) An assessment of the rootzone method of wastewater treatment. Research Journal WPCF, 63,239-247. Fetter, C. W., Sloey, W. E. and Spangler, F. L. (976) Potential replacement of septic tank drain field by artificial marsh wastewater treatment system. Groundwater, 14, 396-402. Heliotrs, F. D. and De Witt, C. B. (1983) A conceptual model of nutrient cycling in wetlands used for wastewater treatment: a literature analysis. Wetlands, 3, 134-152. Middlebrooks, J. (1987) Research and development needs for utilisation of aquatic plants for water treatment and resource recovery: engineering status. In: Aquatic Plants for Wastewater Treatment and Resource Recovery, Reddy, K. R. and Smith, W. H. (eds), pp ICXI9-10 I 1. Magnolia Publishing Inc., Florida. Nichols. D. S. (1983) Capacity of natural wetlands to remove nutrients from wastewaters. Journal WPCF, 55(5), 495-505. 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. Cooper, P. F. and Findlater, B. C. (eds.), pp. 587-590. Pergamon Press. Oxford.
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Rogers, K. H., Breen, P. F. and Chick, A. J. (1991) Nitrogen removal in experimental wetland treatment systems: evidence for the role of aquatic plants. Research Journal WPCF, 63(7), 934--941.
Pergamon
WO!. Sci. Tech. Vol. 32, No.3. pp. 281-290.1995.
0273-1223(95)00629-X
Copyright e 1995 IAWQ Printed in Great Britain. All rights reserved. 0273-1223195 59'50 + 0'00
ROOTZONEDYNAMUCSIN CONSTRUCTED WETLANDS RECEIVING WASTEWATER: A COMPARISON OF VERTICAL AND HORIZONTAL FLOW SYSTEMS Peter F. Breen and Alan J. Chick Cooperative Research Centre for Freshwater Ecology, 68 Ricketts Road, Mt Waverley, Victoria 3 / 49, Australia
ABSTRACT Proftle conditions were exami ned in both small experimental and pilot scale sub-surface flow wetlands. The study systems differed in their hydraulic design. The experimental systems had a vertical up-flow design whereas the pilot system was a horizontal flow trench design. Both systems were found 10 have significant physical, chemical and biological gradients within the sub-surface profile. System age and plant root density appear to he important factors in determining profile differentiation within the experimental systems. Root densities were found to be partitioned between the upper and lower layers on a 70%/30% split. respectively. However, in the experimental systems as the systems aged and root densities increased beyond 112- 25l g.m-2 chemical water quality differentiauon in the profile disappeared . Pilot scale systems were found 10 have physical gradients wilbin t.be proftle as evidenced by hydraulic shan -circuiting. Venical root density distribution is proposed as a major cause of this condition in horizontal flow systems.
KEYWORDS Wetlands (artificial or constructed ); wastewater treatment; nutrient removal; aquatic macrophyte : rootzone: wetland hydraulics; Schoenople ctus validus; Eleocharis sphacelata , INTRODUCTION The performance of artificial wetlands treating wastewaters, although generally successful, has been variable (Nichols, 1983; Conley et al., 1991). It has been argued that this has occ urred because systems have been designed and constructed in the absence of sufficient treatment process understanding (Heliotis & De Witt, 1983: Middlebrooks, 1987). Breen (1990) has addressed some of these inadequacies in understanding by determining the relative importance of chem ical and biological processes in the treatment performance of experimental artificial wetlands. Breen & Chick ( 1989) have examined some physical aspects by evalua ting the influence of system hydraulics on performance . The present study investigates aspects of the profile variability of vertical and horizontal flow artificia l wetlands. 28 1
282
P. F. BREEN and A. J. CHICK
MATERIALS AND METHODS The experimental wetlands consisted of 20 I plastic buckets filled with washed river gravel (3-7 mm diameter) and planted with Schoenoplectus validus (Vahl) A. & D. Love. The systems had a vertical upflow hydraulic format. Influent was introduced to the bottom of the bucket via a central tube. Effluent was collected at the top of the bucket via three equidistant peripheral drainage tubes located 3 ern below the gravel surface. Breen (1990) gives a more detailed description of the experimental systems and their hydraulic behaviour. A pool of 36 planted experimental systems were set up on 19 December 1985 and were established for approximately 6 months prior to receiving their normal operational loading of 1 I.d-1 of primary settled domestic sewage . This loading rate resulted in a theoretical retention time of six days. A group of 12 control systems (i.e. no plants) were set up at the same time and operated in an identical way to the planted system. The systems were batch loaded daily (0800-0900 h) and operated continuously from 1 July 1986. The systems exhibited plug flow hydraulics during batch loading and mixed fully between daily batch loading events (Breen, 1990). All experimental systems were housed in a glasshouse to protect them from extraneous inputs such as rain and excessive dust. Position in the glasshouse was randomly allocated prior to each test period. Because some of the tests took place in summer in inland temperate Australia, it was necessary to control the daytime temperature of the glasshouse by whitewashing and evaporative cooling. No winter or night heating was employed and the glasshouse was always fully ventilated. Air temperature in the glasshouse was monitored with a Wilko Lambrecht thermograph in a Stevenson screen. The experimental systems were sampled at depths of 0.10 m and 0.25 m within the 0.35 m profile. Samples were extracted using a 100 ml syringe prior to the morning batch loading period. The experimental systems to be sampled in this manner were randomly selected from the pool of available systems. The systems were assessed by analysing the top and bottom samples for: filterable reactive phosphorus (FRP); total phosphorus (TP); filterable total phosphorus (FTP); total Kjeldahl nitrogen (TKN); filterable total Kjeldahl nitrogen (FfKN); ammoniacal nitrogen (AN); total oxidised nitrogen (TON); sulphate (SO), winter only; dissolved oxygen (DO). Filterable non-reactive phosphorus (FNRP) :: FTP - FRP; filterable organic nitrogen (FON) :: FfKN - AN; total nitrogen (TN) :: TKN + TON. This rootzone profile study was co-incident with a mass balance study in which systems that had similar histories were randomly selected from the pool, harvested and partitioned. The harvest involved hand sorting the entire bucket into aboveground parts, rhizomes and roots. The root tissues were further partitioned into upper and lower halves by cutting the systems into horizontal halves, each of which was sorted separately for root biomass. The partitioned root biomass is reported in this study. The root tissues were dried at 75°C and weighed for dry weight biomass. The trench system was a pilot scale unit located at Wodonga, Victoria. The system was 50 m long, 2 m wide and 0.5 m deep, plastic lined, filled with crushed quartz gravel (5-10 mm diameter) and planted with Eleocharis sphacelata R.Br. The system was planted in September 1984 and was approximately 22 and 24 months old at the respective sampling dates of 22-28 June 1986 and 19-24 August 1986. The system was loaded with a continuous flow of secondary treated domestic sewage from a wastewater treatment pond at 4.3 kl.d- 1 which resulted in a theoretical retention time of seven days. The system was continuously operated from September 1984. The trench was sampled at the inflow, outflow and internally at three positions along the trench located at 12.5 m, 25.0 m and 37.5 m from the head. At each position along the trench a six point sampling grid was established. At each position, sampling points were located in a grid pattern within the profile at depths of 0.1 m and 0.4 m, and at 0.5 rn, 1.0 m and 1.5 m widths across the trench. Interstitial water samples were extracted at intervals ranging from 0.5 to 6.0 h using a 100 ml syringe and analysed for AN, TON and FRP. Influent and effluent was sampled at 0600, 1200, 1800 and 2400 h for three consecutive days and analysed for TN, AN, TON, FON, TP, FRP and FNRP.
Rootzone dynamics in constructed wetlands
283
The hydrological characteristics of the system were examined using eriochrome acid red as a tracer. A 20 I slug of 0.1 g.l-I dye was introduced to the trench over I hr. The slug was continuously pumped in a mixed stream with the influent. Samples were taken at all points within the trench at intervals varying from 0.5 to 6.0 h. All samples were analysed using APHA (1985) Standard Methods. RESULTS AND DISCUSSION Results from both the experimental and pilot scale trench systems indicate that for particular variables significant differences can occur between different vertical positions in the profile. However very few differences are evident within the profile of the experimental control systems which suggests that the presence of plants is a major factor determining profile variability. Winter results from the planted experimental systems (221 days old) show that significant differences (at the 5% level) occur between the top and bottom zones of the profile for FRP, TP, AN, TN, DO, root biomass and root density (Table 1). The summer results from similar systems (412 days old) show that with the exception of root biomass and root density these differences are no longer evident (Table 2). With the exception of TON there were no differences between the top and bottom zones of the profile in the control systems for any variable. In winter top and bottom TON differences in the control systems were significantly different, but the large standard deviation in the summer results prevented a significant difference being statistically established. Although the absolute levels were low, the relatively high TON concentrations in the upper zones of the control systems probably reflects surface aeration acting as an oxygen source for nitrification. Results from the trench systems (Table 3) for both mid and late winter show significant differences occur between the concentrations of AN, TON and FRP from the upper and lower zones of the profile. The very large build-up of TON in the upper zone during the midwinter trial, accompanied by a decrease in AN, indicates that substantial nitrification had been occurring. The accumulation of TON also indicates that conditions were apparently not suitable for denitrification. Under most natural conditions denitrification is usually capable of utilising any nitrate at a rate greater than its production and consequently high concentrations of TON are unusual. This later situation was apparently occurring during the late winter trial where AN concentrations in the upper zone of the final half (25-37.5 m section) of the trench were significantly less than those in the lower zone, but TON concentrations in the upper zone, while increased, were still low in comparison with the midwinter trial. This pattern suggests that any nitrification that may have been causing a reduction in AN concentrations was accompanied by denitrification which prevented any accumulation of TON. FRP results also show a significant elevation in concentration in the upper zone during the late winter trial. The explanation for this pattern is not clear if active denitrification processes were occurring; anaerobic heterotrophs may have been mineralising organic matter accumulated in the rootzone and generating inorganic fractions like FRP. Anaerobic conditions could also have released orthophosphate from the ion exchange system of the gravel matrix by reducing the ferric oxide films which typically form most of the adsorption sites. The overall level of performance from the planted experimental systems for the winter and summer trials, as measured by percentage concentration reduction, was very similar (Table 4). This suggests that temperature, the most prominent seasonal variable, was not the major factor responsible for differences in the concentration of variables within the profile. The control systems however did show some apparent seasonal variation for TN. Water temperatures for both the experimental and trench systems are shown in Table 5. Temperatures are clearly very different for the experimental systems in winter and summer. These results indicate that under certain conditions wetland wastewater treatment is feasible in the Australian temperate winter. Differences in the profile conditions between winter and summer may reflect differences in other important features such as system age or maturity. This possibility is exemplified by the root biomass data. There was a major increase in root biomass over the winter to summer period. Interestingly however the relative vertical distribution of root tissues for the winter and summer period remained very similar with approximately 70% of the biomass occurring in the upper zone and 30% in the lower zone.
P. F. BREEN and A. J. CHICK
284
Table I. Winter experimental system mean water quality and rootzone biomass FNRP
TP
AN
TON
FON
TN
SO
DO
RB
RD
0.20 0.10 48.8
0.41 0.15
0.41 0.29 23.4
0.03 0.02 1.7
0.54 0.33 30.9
I. 75 0.75
16.9 6.3
1.2 1.0
251 59
1.7 0.5
0.40 0.17 46.0
0.08 0.08 9.2
0.87 0.27
2.42 1.60 54.0
0.01 0.01 0.2
0.75 0.46 16.7
4.48 1.68
17.0 6.7
bdl
0.37
0.13
1.2
12.3
0.12
0.20
13.5
0.8
bdl
0.19 30.8
0.04 10.8
0.3
2.2 91.3
0.05 0.9
0.17 L5
2.1
0.7
0.30 0.14 27.3
0.12 0.04 10.9
I.l
Standard Deviation Proportion Of Total (%)
0.16
II. 7 1.6 90.8
0.01 0.004 0.08
0.29 0.25 2.3
12.8 1.6
0.5 0.5
INFU1ENT
4.4
0.19
5.7
21.7
0.01
1.7
27.5
27.8
0.1 77.2
0.13 3.4
0.6
OA 78.8
0.01 0.04
0.2 6.1
0.5
4.4
FRP INTERSTlTIAL WATER PLANTED Top (X) 0.04 Standard Deviation 0.02 Proportion Of Total (%) 9.8
Bottom (X
)
Standard Deviation Proportion OfTotal (%)
CONTROL Top (X) Standard Deviation Proportion OfTotal (%)
Bottom (x )
69.2 0.8 0.02
112 35 30.8
bdl
NA bdl
(X)
Standard Deviation Proportion Of Total (%)
Units mg.1-l except RB detection limit
~m·2
& RD mg.cm-J ;Inflllfnt n=5, EOlllfnt n=20 (planted)/n=lO (control); bdl below
Table 2. Summer experimental system mean water quality and rootzone biomass FRP INTERSTITIALWATER PLANTED top (X) <0.03 Standard Deviation Proportion Of Total (%) 9.1
FNRP
TP
AN
TON
FON
TN
DO
RB
RD
0.16 0.04 48.5
0.33 0.13
0.02 0.01 1.4
0.01 0.01 0.7
0.89 0.38 61.8
IA 0.39
0.4 OA
1172 17 68.3
7.8 0.1
0.55 0.29
0.03 0.01 2.1
0.01 0.01 0.7
1.0 0.38 71.8
1.4 0.33
bdl
545 150 31.7
4.0 1.1
bdl
Bottom (X) Standard Deviation Proportion OfTotal (%)
<0.03 5.5
0.12 0.07 21.8
CONTROL
Top
2.63
0.21
3.3
7.3
0.11
1.0
8.8
Standard Deviation Proportion OfTotal (%)
0.10 79.2
0.11 6.3
0.15
1.5 82.2
0.08 1.3
OAI 11.5
1.5
Bottom (X) Standard Deviation Proportion Of Total (%)
2.5
0.17 0.08 5.7
3.0 0.20
6.8 1.69 80.0
0.03 0.03 OA
0.95 0.37 11.2
8.5 2.0
bdl
O.19 84.1
INFIAJENT
3.2
0,32
4.7
15.5
0.02
1.1
23.6
bdl
0.21 67.5
0.15 6.8
0.39
1.3 65.6
001 0.01
0.58 4.6
2.1
(X)
(X) Standard Deviation Proportion Of Total (%)
Units mg.l-I except RB ~m-2 & RD mg.cm-J ;Inflllfnt n=5, EOlllfnt n=20 (planted)/n= I a (control); bdl below detection limit
The major changes in the chemical conditions in the profile of the experimental planted systems between the winter and summer periods were a marked decrease in bottom FRP and TN, and profile AN. All these changes probably reflect increased plant uptake due to increased plant biomass and improved wastewaterrootzone contact. The changes in the control systems included a marked increase in profile FRP and TP from winter to summer. This reflects the progressive saturation of orthophosphate adsorption sites on the gravel ion exchange system (Breen, 1990). Other winter to summer changes in the control systems included a decrease in profile AN and TN, and an increase in FaN.
285
Rootzone dynamics in constructed wetlands
Table 3. Trench system mean water quality Water Quality
Interstitial Water
Input 12.5m
Output 31.5m top bottom
25m
top
bottom
top
bottom
53.6 2.5 0.95 6.9 0.31
54.9 2.4 1.0 0.92 1.1 0.22
39.2 4.3 21.1 8.4 6.1 0.24
49.6 3.8 1.4 1.0 1.2 0.49
41.6 6.1 12.1 2.1 6.4 0.61
54.6 2.1 0.61 0.31 1.0 0.33
50.1 0.68 6.3 0.41 6.1 0.31
61.2 0.69 0.12 0.03 1.9 0.10
64.2 1.9 0.10 0.04 8.0 0.16
41.4
58.6
1.3
1.3
0.89 0.06 12.5 0.61
0.01 0.01 8.4 0.24
38.6 2.1 1.2 0.08 11.1 0.41
51.6 1.9 0.11 O.oJ 9.0 0.18
43.1 0.53 0.91 0.03 8.0 0.14
21 June 1986
AN
(X)
Standard Deviation (X) TON Standard Deviation
(x)
FRP
Standard Deviation 18 August 1986
eX)
AN
Standard Deviation eX) TON Standard Deviation FRP eX) Standard Deviation
51.3 2.1 0.411 0.11 6.8 0.65
1.5
60.4 3.9 0,01
0.01 1.0 0.21
Units mg.l-I; n = 20
Table 4. System performance: mean percentage reduction in concentration SYSTEM EXPERIMENTAL Planted top*
TN
top* bottom
93.6 83.1 51.0 53.3
Effluent
8.1
bottom
Control
TP
TN
Winter 92.1 84.6 18.8 80.5
93.9 94.0 62.6 64.2
-1.2
30.6
93.0 88.3 29.1 35.1 Late Winter
MidWinter
TRENCH
TP Summer
-14.8
• equivalent to system eftIuent
Table 5. Summary of water temperatures (oq over the study period SYSTEMS Experimental" Winter Sununer Trench U MidWinter Late Winter
Mean
Standard deviation
Median
Range
8.3 22.4
1.1
8.1
3.8
22.8
6.6 - 9.6 18.0- 21.0
9.1 11.4
1.7 1.9
9.5
7.8 - 13.0
11.1
8.0- 14.2
• Interstitial water temperature recorded over the study period ··Influent v.ater from treatment plant operation records 1980-1986. mid winter
n=1O. late winter n=l1
These changes in the nitrogen fractions reflect the increased biological diversity and nutrient transformation capacity of the control systems with increased temperature and system age, and highlight the differences between Nand P removal mechanisms in the absence of plants. The dye tracer data (Fig. l) from the trench systems indicate that there were major hydraulic differences between the upper and lower zones. The upper and lower dye traces at 12.5 m were more or less coincident. although the majority of the flow occurred in the upper zone. However at the 25 m and 37.5 m points along
286
P. F. BREEN and A. J. CHICK
the trench the majority of the flow occurred in the lower layers of the trench. The upper and lower dye peaks were also non-synchronous indicating differential flow paths which lead to dead zones and shortcircuiting. The peak transit times for the greatest peaks at the 12.5 rn, 25 m and 37.5 m positions along the trench were approximately I. 2 and 3 days early. respectively. further confirming short-circuiting. For a 7 day retention time plug flow theoretical peak transit time would be approximately 1.75. 3.5 and 5.25 days respectively. If the 70%/30% respective split between root biomass in the upper and lower zone of the experimental systems is applicable to root density distributions in the trench systems it would clearly explain the channelling of flow to the lower layers. For a horizontal flow system if the upper zone has 40% more root biomass in the pore spaces of the gravel, flow will naturally be directed to the lower zone which would be the path of least resistance. Figure 2 illustrates the impact of root density distribution on hydraulic resistance and flow in trench systems. The chemical stratification evident in midwinter, particularly the TON data. illustrate. the extent to which vertical flow partitioning can isolate the upper and lower zones of a horizontal flow system.
: :1 .
o" w a:
AUGUST lUll
JUNE HIIIII
o
l2.$m
TOP
C BOTTOM
200
0
F~
g
0
ii:
w
37.11111
I
o
I
I
I
I
2
:I
4
II
II
I
I
I
I
I
I
0
2
3
4
II
II
TIME(d)
Figure I. Longitudinal series of dye tracer concentration-time curves for the top (0) and bottom (0) sampling positions of the central plane of the trench.
The comparison of influent and effluent can be a useful way of assessing the processes occurring in a treatment system. Similarly the comparison of interstitial water composition can indicate the importance of particular treatment processes at particular points within a system. In winter the composition of interstitial water in the top and bottom layers of the experimental systems were very different (Table I). In the top layer FNRP represented 48.8% of the TP, whereas in the bottom layer FRP was the major P fraction at 46.0% of the TP. Similarly for nitrogen FON (30.9%) was the largest fraction in the top layer of the experimental systems whereas AN (54.0%) was the dominant fraction in the lower layers. The influent was dominated by FRP (77.2%) and AN (78.8%). For the experimental systems with a vertical up-flow format this pattern suggests that during the winter trial influent was not significantly processed until it moved into the upper zone of the system. In summer with the exception of FNRP. the composition of interstitial water in the upper and lower zones of the experimental systems was very similar (Table 2). This suggests that in winter different processes were occurring within the system rootzone profile whereas in summer the whole profile was operating in a similar way. As previously discussed the major difference between the vertical condition of the profiles between winter and summer was root biomass. This observation also suggests that a critical root biomass may exist. The
287
Rootzone dynamics in constructed wetlands
results from the winter trial suggest that the critical root biomass for substantial plant mediated treatment appears to be between 112-251 g.m·2. This was the root biomass range for the lower and upper zones, respectively, of the experimental systems in the winter trial (Table 1). The relative nutrient composition of influent and effluent of the trench systems suggests little plant processing of the influent was occurring (Table 6). The nutrient reduction data support this observation (Table 4). With the exception of midwinter TON concentrations the percentage composition of effluent from trenches in both winter trials was similar to the experimental control systems, particularly in the summer trial. This suggests that plant processes were having little influence on effluent quality from the trench. The hydrological data (Fig. 1) which shows that most of the flow occurs in the lower zone of the trench. where there were least roots, would also support this suggestion. In addition the temperatures in the winter trench trials and the winter experimental trial were similar (Table 5) and suggest that temperature per se was not necessarily the main determining factor in the comparative performance of the trench and experimental systems. Table 6. Trench system mean influent/effluent quality FRP
FNRP
TP
AN
TON
FON
TN
21 June 1986 (X) INFLUENT Standard Deviation Proportion of Total (%)
6.8 0.7 79.1
1.0 0.3 11.6
8.6 0.3
57.3 2.1 89.3
0.5 0.1 0.8
6.7 1.1 10.4
64.2 3.0
(X) EFFLUENT Standard Deviation Proportion of Total (%)
6.7 0.3 77.0
0.9 0.2 10.3
8.7 0.2
50.1 0.7 85.5
6.3 0.5 10.8
3.0 1.2 5.1
58.6 1.5
18 August 1986 (X) INFLUENT Standard Deviation Proportion of Total (%)
7.0 0.2 79.6
0.9 0.2 10.2
8.8 0.2
60.5 3.9 89.1
0.1 0.01 0.1
7.1
67.9 4.0
10.5
(X) EFFLUENT Standard Deviation Proportion of Total (%)
8.0 0.2 79.2
0.9 0.1 8.9
10.1 0.2
43.7 0.5 92.8
0.9 0.03 1.9
2.8 1.1 6.0
1.1
47.1 L7
Units mg l-I; n = 20
The results from trials on both the experimental systems and the pilot scale trench system indicate that significant chemical gradients can occur within the belowground profile of these systems. This confirms previous results (Fetter et al., 1976; Bowmer, 1987; Rogers et al., 1991) although the present study suggests the differences may be more significant than those found by these other authors, and that these profile differences can vary with time. In the present study the vertical differences in particular chemical variables, can be attributed to a combination of the distribution and density of roots within the profile of systems and the system hydraulics. Fetter et at. (1976) while not addressing the particular role or function of plants in their systems, suggested from a comparison of planted and unplanted system performance, that plants only increase treatment performance by several percentage points. Bowmer (1987) suggested that plants may influence system performance by channelling flow around the root mass and thus causing short-circuiting. From the present study it would appear that this observation is a general feature of horizontal flow systems. However the reports of Breen (1990), Rogers et at. (1990 and 1991) indicate that under certain circumstances plant uptake can be the major nutrient removal pathway and consequently plants can be an important and highly beneficial part of such systems. The results from the reported trials exemplify both the positive and negative influence that plants can have on different system designs and their performance.
P. F. BREEN and A. J. CHICK
288
The existence of a critical root biomass for effective plant mediated nutrient removal has a significant impact on system establishment time and the effective operational depth of wetland systems. To have the entire rootzone effective in nutrient removal, systems have to be established long enough to allow between 112 and 251 g.m- 2 of root biomass to accumulate in the designed rootzone depth. For the experimental systems with a 0.35 m profile depth between 221-412 days were required. Plant root densities have an inverse depth relationship. Consequently system depths greater than those at which root biomass in the 112-251 g.m- 2 range can develop, may be superfluous to plant mediated nutrient removal processes. The idea of a critical root biomass and its associated active plant depth concept suggests that for the purposes of plant mediated nutrient performance, system volume and retention time should be calculated on active rootzone depth and not on true system depth. System profiles deeper than the active rootzone may however be designed for other purposes such as an anaerobic filter for BOD removal or as a flow distribution zone. Rogers et al. (1990) has shown in experimental systems similar to those used in the present study that root density distribution is dependent on system hydraulic format. In systems with a vertical downflow format it was found that more than 50% of the root biomass occurred in the top 0.05 m of the profile. Systems with a vertical upflow format had a more even root density, although still clearly depth dependent. As a result the determination of effective system depth is also dependent on the hydraulic design objectives and how these may influence plant growth and behaviour. The flow stratification and short-circuiting in trench systems results in both reduced retention times and less wastewater-rootzone contact. It has already been mentioned that in the experimental systems plant uptake is a major nutrient removal mechanism (Breen, 1990; Rogers et al., 1990 and 1991). Consequently for plant mediated processes to be involved in determining treatment performance, wastewater-rootzone contact must be optimised. This is the case in the experimental systems. The ideal hydraulic condition for a batch loaded design like the experimental systems is either plug flow or fully mixed. The vertical upflow experimental systems have been assessed as mixing fully between batch loading events (Breen & Chick, 1989; Breen, 1990; Rogers et al., 1990). The upflow hydraulic design of the experimental systems also maximises wastewater-rootzone contact by directing outflow through the root dense upper layers. Similar experimental systems with a downflow format have been assessed as approximating plug flow (Rogers et al., 1990). With a vertical flow format these systems also maximise wastewater-rootzone contact. In the trench systems however the horizontal flow hydraulic design sets up the conditions for flow partitioning and short-circuiting and minimises spatial and temporal wastewater-rootzone contact. low
shallow
high
PROFILE DEPTH
ROOT DENSllY & HYDRAULIC RESISTANCE
flow distribution
curve FLOW
high
deep
WETLAND ROOTZONE low
Figure 2. Subsurface horizontal flow wetland model showing root density and hyraulic resistance gradients and their effect on plug flow.
The high concentration of nutrients in the profile of the trench system can be used to illustrate several features of trench function that are significant in system design and operation. An important feature to recognise is that system hydraulics is the unifying process that connects the various treatment and nutrient removal mechanisms as water is obviously the carrier for all wastewater constituents. Flow partitioning in the trench system resulted in the spatial separation of the upper and lower layers of the system. This appears to have had several results. Firstly the accumulation of TON in the upper layers of the system, particularly in midwinter, suggests that the microbial process of nitrification and denitrification were poorly connected.
Rootzone dynamics in constructed wetlands
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Secondly at least in late winter FRP also accumulated in the upper layers of the system. The exact reason for this is not clear, although it is thought that increased organic decomposition during this initial period of increasing temperature may be involved. CONCLUSIONS It is apparent that major chemical and biological variations occur within the belowground profile of artificial wetland systems. The spatial variations measured within the test systems also underwent temporal change. The temporal changes appear to be both cyclical and related to seasonal factors, and unidirectional and associated with system maturation and age.
Variations within the profile are largely the result of plant activity and the interaction of plants on system hydraulics. In the experimental systems chemical differences within the profile seemed to be associated with absolute root densities not necessarily the relative root density distribution. Consequently there appears to be a critical root density for effective plant mediated nutrient removal. Data from the experimental systems suggest the critical root density is between 112 and 251 g.m- 2 which in the experimental systems took between 221 and 412 days to accumulate. These findings could influence system design and operation by determining effective rootzone depths and establishment times. In the trench systems chemical variations with the profile are at least partly determined by the normal vertical distribution of root density causing an associated vertical variation in hydraulic resistance. This vertical variation in hydraulic resistance, due to roots filling up interstitial space within the substratum, results in flow partitioning and progressive diversion of flow away from the rootzone, that is to areas of least hydraulic resistance (Fig. 2). Consequently it is concluded that horizontal flow designs are theoretically and practically incompatible with vertically uniform plug flow and hence with plant mediated nutrient removal. The accompanying conclusion is that to optimise plant mediated processes and system performance wastewater-rootzone contact must be maximised. The vertical flow format of the experimental systems is an option for achieving this design objective. ACKNOWLEDGMENTS The project was funded by Sydney Water Board. The work was undertaken at the Griffith Laboratory of CSIRO, Division of Water Resources. Particular thanks are extended to Dr D.S. Mitchell, Dr K.H. Bowmer, Ms S. Naumovski, and Ms RH. Shields. REFERENCES APHA (1985) Standard Methods for the Examination of Water and Wastewater, 16th edn, American Public Health Association. Washington, D.C. Bowmer, K. H. (1987) Nutrient removal from effluents by an artificial wetland: influence of rhisosphere aeration and preferential flow studied using bromide and dye tracers. Water Research, 21, 591~599. Breen, P. F. (990) A mass balance approach for assessing the potential of artificial wetlands for wastewater treatment. Water Research, 24, 689---{)97. Breen, P. F. & Chick, A. J. (989) Wastewater treatment using artificial wetlands: the hydrology and treatment perfonrumce of horizontal and vertical flow systems. Proceedings Australian Water and Wastewater Association 13th Federal Convention. The Institute of Engineers, Australia, pp 167-171. Conley, L. M., Dick, R. I. and Lion, L. W. (1991) An assessment of the rootzone method of wastewater treatment. Research Journal WPCF, 63,239-247. Fetter, C. W., Sloey, W. E. and Spangler, F. L. (976) Potential replacement of septic tank drain field by artificial marsh wastewater treatment system. Groundwater, 14, 396-402. Heliotrs, F. D. and De Witt, C. B. (1983) A conceptual model of nutrient cycling in wetlands used for wastewater treatment: a literature analysis. Wetlands, 3, 134-152. Middlebrooks, J. (1987) Research and development needs for utilisation of aquatic plants for water treatment and resource recovery: engineering status. In: Aquatic Plants for Wastewater Treatment and Resource Recovery, Reddy, K. R. and Smith, W. H. (eds), pp ICXI9-10 I 1. Magnolia Publishing Inc., Florida. Nichols. D. S. (1983) Capacity of natural wetlands to remove nutrients from wastewaters. Journal WPCF, 55(5), 495-505. 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. Cooper, P. F. and Findlater, B. C. (eds.), pp. 587-590. Pergamon Press. Oxford.
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Rogers, K. H., Breen, P. F. and Chick, A. J. (1991) Nitrogen removal in experimental wetland treatment systems: evidence for the role of aquatic plants. Research Journal WPCF, 63(7), 934--941.