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Author(s)
First Name
Niall
Middle Name
David
Surname
O’Luanaigh
Role
PhD Research Student
Affiliation
Organization
Department of Civil, Structural & Environmental
Engineering, Trinity College Dublin, Dublin 2, Ireland
URL
www.tcd.ie
Email
noluanai@tcd.ie
Author(s)
First Name
Laurence
Middle Name
William
Surname
Gill
Role
Senior Lecturer
Affiliation
Organization
Department of Civil, Structural & Environmental
Engineering, Trinity College Dublin, Dublin 2, Ireland
URL
www.tcd.ie
Email
gilll@tcd.ie
Author(s)
First Name
Titiksh
Middle Name
Surname
Patel
Role
PhD Research Student
Affiliation
Organization
Department of Civil, Structural & Environmental
Engineering, Trinity College Dublin, Dublin 2, Ireland
URL
www.tcd.ie
Email
patelt@tcd.ie
Author(s)
First Name
Paul
Middle Name
Surname
Johnston
Role
Senior Lecturer
Affiliation
Organization
Department of Civil, Structural & Environmental
Engineering, Trinity College Dublin, Dublin 2, Ireland
URL
www.tcd.ie
Email
pjohnston@tcd.ie
Author(s)
First Name
Bruce
Middle Name
Surname
Misstear
Role
Senior Lecturer
Affiliation
Organization
Department of Civil, Structural & Environmental
Engineering, Trinity College Dublin, Dublin 2, Ireland
URL
www.tcd.ie
Email
bmisster@tcd.ie
Publication Information
Pub ID
ASABE will
complete
Pub Name
Eleventh National Symposium on Individual and Small
Community Sewage Systems
Pub Date
ASABE will
complete
A comparison of conventional septic tank systems and
alternative horizontal subsurface-flow reed bed systems in
the treatment of domestic wastewater
Niall O’Luanaigh, Laurence Gill, Titiksh Patel, Paul Johnston, Bruce Misstear
Department of Civil, Structural & Environmental Engineering, University of Dublin, Trinity College
Abstract. In Ireland, the most prevalent domestic wastewater treatment application in unsewered areas
is the conventional septic tank system with percolation area. However, concern has been expressed over onsite effluent discharging into highly permeable soils which, although permitted in current Irish EPA
guidelines, has generated debate as to whether an insufficient level of treatment in the subsoil’s unsaturated
zone is being achieved. In situations where a septic tank installation is considered unsuitable according to a
rigorous site assessment, some form of secondary treatment system can be installed to improve the effluent
quality before discharge to the subsoil. Horizontal subsurface-flow reed bed systems are one such
technology receiving significant attention recently, being deemed an effective and low-cost alternative for
secondary treatment applications. On-site research was thus carried out to assess and compare the
treatment capabilities of freely-draining sandy subsoils receiving both septic tank and secondary effluents in
tandem with a treatment assessment of a horizontal subsurface-flow reed bed receiving heavily loaded
septic tank effluent. Results over a 12-month period have shown the reed bed to remove only 47% of the
organic load but achieve 2-3 log removal in total coliforms and E.coli. The majority of nitrification is seen
to occur in the first 0.3m of subsoil for both the trenches receiving septic tank effluent and secondary
effluent respectively. The research also shows that in general the septic tank effluent has received a
comparable quality to the secondary effluent in terms of E.coli by the time the point of discharge to
groundwater is reached in the subsoil.
Keywords. On-site wastewater treatment, STE, septic tank effluent; SE, secondary effluent; percolation
area, freely draining, horizontal subsurface flow reed bed, evapotranspiration
Introduction
Domestic wastewater from over one third of the population in Ireland, or 400,000 dwellings, is treated
by on-site systems (Department of the Environment et al., 2004). Given that over 25% of all water supplies
is currently provided by groundwater (EPA, 2005), there is a great need to protect this resource from the
risks posed by contamination arising from both diffuse (generally agricultural) and point (manure and silage
storage, on-site wastewater treatment systems) sources. Traditionally the dominant management approach for
on-site systems has been to settle wastewater in a septic tank prior to subsurface distribution via a percolation
area (soil treatment system). A septic tank provides limited primary treatment to wastewater via anaerobic
breakdown of organic material with its effluent typically containing high levels of nutrients, biochemical
oxygen demand (BOD) as well as some carry over of suspended solids (SS). However, it is the remaining
biological constituents of the wastewater, namely pathogenic bacteria, infectious viruses and protozoa that
are, as Daly (2003) states, the single greatest threat to groundwater in Ireland and which in turn pose huge
health risks to the general public in the form of waterborne diseases. As such, it is role of the percolation
area and its inherent subsoil characteristics to provide a sufficient contaminant attenuation zone and ensure
long-term protection from groundwater pollution.
Recommendations published by Ireland’s Environmental Protection Agency (EPA, 2000) are aimed at
defining subsoil conditions that will provide an acceptable level of treatment for on-site domestic
wastewater effluent in order to protect groundwater resources from contamination. This risk assessment
approach is composed of a stringent site assessment which includes a desk study, an on-site trial hole
inspection and a percolation test. The latter is required to ascertain the assimilation capacity of the subsoil
and is represented in terms of a ‘T-value’, obtained from the standard falling head percolation test
(Mulqueen and Rodgers, 2001). A low T-value (1 to 5) denotes a coarse-grained soil that is highly
permeable in nature – typically sand or gravel, while a high T-value (45 to 50) is indicative of a less
permeable clay-based soil. Currently, one of the growing concerns amongst water authorities, practitioners
and other bodies surrounds the discharge of on-site effluent into the more freely draining soils (with Tvalues of between 1 and 5), where although permitted under the current guidelines, there exists an intuitive
feeling that the effluent is percolating at an excessive rate. This ultimately creates the risk of insufficient
nutrient and pathogenic removal and an increased potential for groundwater contamination. Many studies
have highlighted the importance in the formation and development of an organic biomat layer at the soilgravel interface along the base and wetted sides of the percolation trenches and deemed it to be a crucial
factor in the retention of the effluents’ constituents (Bouma et al., 1972; Ziebell et al., 1974; Bitton and
Gerba, 1984, Gill et al., 2005). The level of its development is thought to be dependent on a combination of
both the subsoil’s grain size and the strength of the applied effluent to be treated.
In cases where septic tank effluent (STE) is deemed unsuitable for immediate discharge to a percolation
area (e.g. high water table, shallow bedrock), secondary effluent (SE) can be achieved by an alternative
system prior to further treatment in the subsoil. One such system which is receiving much research attention
over the past two decades in terms of efficiency and design improvements is the horizontal subsurface-flow
(HF) reed bed. Despite this, there is a reluctance in Ireland amongst a number of water authorities to permit
the use of wetlands as stand-alone secondary treatment systems given the scarcity of reliable long-term
performance data relevant to Ireland and its climate and their apparent poor winter treatment performance
(Healy and Cawley, 2002). HF reed beds comprise a shallow basin lined with an impervious material which
is filled with a substrate, usually soil or gravel, and planted with vegetation tolerant of saturated conditions.
The wastewater to be treated is introduced at one end and flows horizontally through the substrate below its
surface before being discharged via an outlet structure. Treatment occurs as a result of a number of physical,
chemical and biological processes during the water’s passage through the bed (Davison et al., 2005). HF
reed beds, with gravel as its chosen media, are regarded as particularly good in the removal of BOD5 and
SS. However, the majority of research to date has been carried out using domestic wastewaters with low (50
-150 mg L-1) BOD5 inflow concentrations (Vyzamal, 2001), probably due to the use of combined (surface,
rain and wastewater) sewer systems (Griggs and Grant, 2001). Undiluted septic tank effluent, in
comparison, has a BOD5 of between 150 and 400 mg L-1. Little nitrification is also achieved across the bed,
due to its poor oxygenation capacity, which can have a detrimental impact on groundwater quality if the
receiving subsoil is highly permeable and freely draining.
As such a three-year project funded by the EPA was undertaken to examine and compare the
performance of a freely-draining subsoil, with a T-value of between 1 and 5, in treating both septic tank
effluent (STE) and secondary effluent (SE). A gravel based horizontal subsurface-flow reed bed was
constructed to provide secondary treatment with its treatment efficiency as a stand-alone system in an Irish
climate assessed in parallel. This paper details the findings from the initial 12-month period of sampling
(April 2006 – April 2007).
Materials and Methods
Site Selection, Design and Construction
In selecting a study site in the field to carry out the research, compliance with the EPA recommendations
for site acceptability was a prerequisite. Site location and a trial hole inspection had to satisfy Groundwater
Protection Schemes (Department of the Environment et al., 2004) and EPA guidelines (EPA, 2000). In
addition, it was essential that the subsoil had a T-value of between 1 and 5 which would indicate a highly
permeable freely-draining medium. A dwelling with at least four people was also desirable to ensure a
generous hydraulic loading over the whole system was maintained throughout the sampling period. The
study site (Figure 1) finally chosen fulfilled all the aforementioned criteria, with the subsoil having a Tvalue of 4 and a total of 6 people occupying the house.
U/S Groundwater BH
Lysimeter
16m
Distribution
Box 3
Tensiometer
HF Reed Bed
Polishing Filter Percolation Trenches
Septic Tank
Distribution
Box 1
Rain Gauge
Weather Station
Distribution
Box 2
Secondary Treatment Percolation Trenches
20m
D/S Groundwater BH
Figure 1. Schematic diagram of study site and instrumentation layout
In the construction phase, a two-chambered septic tank was installed with half the septic tank effluent
(STE) fed to secondary percolation trenches and the other half of the flow directed to the constructed reed
bed via a distribution device. The secondary effluent from the reed bed was thereafter discharged to a set of
polishing filter percolation trenches. In essence, the site could thus be regarded and analysed as two separate
treatment systems :
1)
2)
a conventional septic tank system where the percolation area receives septic tank effluent (half
the hydraulic load)
a septic tank and reed bed system where the subsequent percolation area receives secondary
effluent (half the hydraulic load)
The percolation systems were built according to EPA specifications (EPA, 2000) and consisted of 110
mm diameter perforated PVC pipework bedded in 500 mm of gravel, 250 mm of which was below the pipe,
in a 450 mm wide trench at a slope of 1:200. The base of the percolation trench was set at 0.8m below
ground level. The achievement of an equal loading rate on each trench of course depended on an even
distribution of the effluent within the distribution device. This proved challenging given the fact that the
efficiency of a number of distribution devices (e.g V-notch concrete distribution box, flow splitter) were
being analysed on-site simultaneously as part of a separate research study.
The basis of the design for the constructed reed bed (Figure 2) was centred on WRc guidelines (Cooper
et al., 1996), a document which outlines the design features, methodology, removal mechanisms and
operational requirements of a number of different constructed wetland configurations. Combining the theory
of a plug-flow BOD5 removal model (first-order kinetics) and Darcy’s law yields an areal bed design figure
of 5 m2 per person for secondary treatment. In treating a 3PE hydraulic load, the dimensions for the reed bed
were set at 6 x 2.5 x 0.6 m (l x b x h) accordingly. The bed was sealed with an impervious butyl rubber liner
and filled with 5 – 15 mm washed pea gravel. The inlet structure had dual free discharge outfalls to promote
an even spread of the influent across the inlet end of the bed and thus prove critical in maximizing hydraulic
efficiency. A constant water level 50 mm below the surface was maintained throughout by the standpipe in
the outlet structure. An increased gravel size (15 – 30 mm) was also distributed locally around the inlet and
oulet pipes to reduce the potential for clogging and blockage. The bed was then planted with Phragmites
Australis (common reed) in blocks of 4 per m2.
Phragmites
Level Control
Inlet
W.L
Outlet Structure
0.6m
Outlet
Impervious Liner
Slope 1%
Figure 2. Elevation of subsurface horizontal-flow reed bed treatment system on study site
Instrument Installation and Sampling Methodology
Automatic portable samplers (Isco 6712 model and Bühler Montec xian 1000 model) were programmed
to take a sample of the STE and SE from the horizontal-flow reed bed every 3 hours over a 24-hour period.
The resulting eight samples were then mixed to provide a representative diurnal composite sample.
Sampling at the inlet of the septic tank was not carried out, with experience showing that the results
achieved are practically meaningless in terms of quantity and quality given the sporadic flows received from
the household. The positioning of the samplers upstream and downstream of the reed bed enabled the
changes in effluent quality across the wetland to be assessed. In order to monitor the hydraulic efficiency of
the distribution box, tipping bucket instrumentation with dataloggers were installed underneath each of the
four outfalls of the box. This enabled a very accurate determination (to a sensitivity of one tip every 150 ml)
to be made of the effluent distribution to both the reed bed and percolation area. Sixty suction lysimeters
(Soilmoisture Equipment Corporation) were installed in the percolation area along the trenches at nominal
depths of 0.3, 0.6 and 1.0 m below the invert of the percolation trenches respectively. In two of the three
trenches receiving STE, lysimeters were positioned at intervals of 0, 5, 10, 15 and 20 m, while in two of the
three polishing filter trenches, lysimeters were placed at 0, 2.5, 5, 10 and 15 m. Given such concentrated
positioning, a detailed picture of the movement, distribution and fate of the effluent in the unsaturated zone
could be established from both sets of trenches. Careful installation of the lysimeters was vital, however, to
avoid creating artificial preferential flowpaths (macropores) which would otherwise compromise the
treatment of the percolating effluent. Groundwater monitoring holes upstream (U/S) and downstream (D/S)
of the entire percolation area were also bored to assess the pollutant effect of the system on the underlying
aquifer. To obtain a profile of soil moisture tension, eighteen tensiometers (Soil Measurement Systems)
were installed to the three same depths as the lysimeters in the positions shown in Figure 1. In order to
assess the meteorological effects (evapotranspiration and precipitation levels) on both the reed bed and
percolation area, a weather station (Campbell Scientific) and rain gauge (Casella) were mounted on site.
Parameters recorded were rainfall, temperature, relative humidity, wind speed, wind direction, solar
radiation and net radiation.
On the morning preceding all sampling events, the automatic samplers were programmed to run while
the lysimeters were put under a suction of 50 cbar (50.7 kPa) using a vacuum-pressure hand pump. The
specific suction pressure was adopted from advice given by Soilmoisture Equipment Corporation to Gill et
al. (2005) that the practical limit for water flow in soils is approximately 65 cbar. Soil moisture sampling
was carried out 24 hours later using the vacuum-pressure hand pump with a 1000 ml conical flask and
rubber bung with an extraction tube attached. The total volume recovered from each lysimeter was recorded
and samples collected in 300 ml sterilised plastic sample bottles. At successive lysimeters it was ensured
that the extraction equipment was cleaned with sterilised water. The diurnal STE and SE composite samples
were also collected while tensiometer and meteorological readings were recorded. All samples were then
brought immediately to the laboratory for chemical and microbiological analysis.
Analysis and Results
Laboratory Analysis
The STE, SE and soil moisture samples were analysed for chemical oxygen demand (COD), ammonium
(NH4-N), nitrite (NO2-N), nitrate (NO3-N), orthophosphate (PO4-P) and chloride (Cl) while the STE was
also tested for total kjeldahl nitrogen (TKN) to ascertain the fraction of organic and inorganic nitrogen
present. A Merck Spectroquant Nova 60® spectrophotometer and associated reagent kits (USEPA approved)
were used for the chemical analysis. If parameter concentrations were outside the detectable range for a
specific reagent the samples were diluted with a known volume of distilled water. Bacteriological analysis
was also carried out on all samples in the laboratory for total coliforms and E. coli using the Idexx Colilert®18 procedure. E. coli was chosen as an indicator bacterium for faecal contamination whereby its presence
and attenuation kinetics should be suggestive of the behaviour of human pathogens.
Flow Analysis
Wastewater generation per capita is dependent on a number of factors including climate, time of day,
year and country. In Ireland, the EPA guidelines specify a value of 180 L d -1 as the typical daily hydraulic
load per person to an on-site wastewater treatment system for single houses. Research has shown this figure
to vary substantially, with Gray (1995) quoting water consumption values in Ireland of between 65 and 180
L d-1. The average domestic wastewater generation measured on the study site was 118.7 L d-1 which is in
line with similar investigations recently carried out by Gill et al. (2005) on other test sites in unsewered
areas throughout Ireland. They yielded equally low values ranging from 60.2 L d -1 to 123.0 L d-1. As such its
thought the EPA’s higher design loading rate is deemed to be deliberately conservative in light of the illegal
but commonly occurring practice of builders connecting the household’s roof water into the wastewater
treatment system.
Reed Bed Analysis
Loading rates at the inlet and outlet of the reed bed were calculated on a daily basis according to a mass
balance study incorporating the effects of precipitation and evapotranspiration across the reed bed. This analysis
was carried out based on Equation (1) and Figure 3 (IWA, 2000).
Qo  Qi  ( P  ET ) A
ET = Evapotranspiration (mm d-1)
Qi = Inflow(L d-1)
(1)
P = Precipitation (mm d-1)
Qo = Outflow(L d-1)
A = Area (m2)
Figure 3. Water balance approach to determine evapotranspiration measurements across the reed bed
Reference crop evapotranspiration (ET0) was approximated from weather data, collected from the onsite weather station, using the Penman-Monteith equation (FAO, 1998). Bavor et al. (1988) reported a linear
relationship between evaporation from subsurface-flow gravel reed beds and Pan Evaporation but the
correlation coefficient was very low. Additional work is required to clarify a potential crop factor between
the vegetated bed’s evapotranspiration and ET0. Consequently a factor of 0.8 in summer and 1.0 in winter
have been applied, as is commonly used for surface flow wetlands, to estimate evapotranspiration from the
reed bed for the purposes of this paper.
Method of Soil Moisture Sample Analysis
As chloride does not play a significant role in any geochemical reactions (Marshall et al., 1999), its
indicative use in the research was as a tracer to ascertain the extent of the wastewater effluent spread across
the percolation area. The results of the laboratory analysis for Cl at the various sample positions along both
sets of trenches (secondary and polishing filter) were averaged at the same depth plane at which they were
recorded. A clear indication could thus be found of any potential differences in loading rates between
sampling distances on the same depth plane, thus highlighting any variances within the plane. From this,
one of the two following methods, both of which assume isotropic and homogeneous soil properties, was
chosen as the most representative conceptual model for the analysis of the attenuation of the percolating
effluent.
(i)
(ii)
Planar average: this method involved the averaging, across each set of trenches, of the concentrations
of each parameter across all sampling positions at the 0.3, 0.6 or 1.0 m depth planes. The difference
between the average loading rates was then compared for the different depth planes.
Depth average: the average loading rate, across each set of trenches, of each parameter within each
plane was calculated at the different sample distances thought to be receiving effluent along the
trenches and the corresponding differences in loading rates between the planes compared.
ET0 levels calculated from weather station data were combined with the recorded rainfall figures and the
soil moisture deficit (SMD) to determine actual evapotranspiration (AE) measurements. Where the SMD was
greater than 40 mm the AE was considered to occur at a slower rate than ET0 and was calculated using the
Aslyng scale (Keane, 2001). Daily effective rainfall, or recharge, was then calculated by subtracting the daily
AE and accumulated SMD figures from the daily rainfall measurement.
Analysis of the Cl data across the secondary treatment percolation trenches showed consistent loading at
the 0, 5 and 10 m sample positions along all three planes but a decrease in levels at the 15 and 20 m points
(Figure 4). This is highlighted by Figure 5 which compares the average Cl loading rates across each depth
plane at the 0, 5 and 10 m sample positions. This suggests that the planar average method across the first half
of the percolation trenches (0 – 10 m) was the more representative way of assessing the effluent attenuation.
200
180
160
0m (0.3m)
0m (0.6m)
0m (1.0m)
5m (0.3m)
5m (0.6m)
5m (1.0m)
10m (0.3m)
10m (0.6m)
10m (1.0m)
15m (0.3m)
15m (0.6m)
15m (1.0m)
20m (0.3m)
20m (0.6m)
20m (1.0m)
-1
Cl Loading (g d )
140
120
100
80
60
40
20
09/03/2007
07/02/2007
08/01/2007
09/12/2006
09/11/2006
10/10/2006
10/09/2006
11/08/2006
12/07/2006
12/06/2006
13/05/2006
13/04/2006
14/03/2006
0
Figure 4. Average Cl loading rates at all depth planes across all sample positions on secondary treatment
percolation trenches
Further scrutiny of the soil moisture tension values from the tensiometers installed on the secondary
percolation trenches at the front (2.5 m), middle (7.5 m) and end (17.5 m) showed that the effect of dilution
by recharge events on contaminant concentrations on the first half of the percolation area (0 – 10 m) was
minimal (Figure 6). As such, physical, chemical and biological processes were the predominant mechanisms
in the attenuation of effluent at the 0, 5 and 10 m sample positions. This was in contrast to the 15 and 20 m
sample positions where it was suspected little effluent had been recorded. At these positions in the
percolation area, the tensiometers reacted to the variations in effective rainfall over the sampling period.
160
Cl Loading (mg L -1)
140
120
100
0.3m
80
0.6m
60
1.0m
40
20
09/03/2007
18/01/2007
29/11/2006
10/10/2006
21/08/2006
02/07/2006
13/05/2006
24/03/2006
0
0.00
-70
8.00
0.3m
0.6m
1.0m
09/03/2007
07/02/2007
08/01/2007
09/12/2006
40.00
09/11/2006
-350
10/10/2006
32.00
10/09/2006
-280
11/08/2006
24.00
12/07/2006
-210
12/06/2006
16.00
13/05/2006
-140
Effective Rainfall (mm)
0
13/04/2006
Soil Moisture Tension (mBar)
Figure 5. Average Cl loading rates measured on the three depth planes (0.3, 0.6 and 1.0 m) at the
0, 5 and 10 m sample positions
eff. rain
Figure 6. Soil moisture tension plotted against effective rainfall for the 2.5m tensiometer position
Analysis of Cl concentrations along the polishing filter percolation trenches showed that the majority of
the effluent has been recorded at the 0 m sample position. As such, it was decided the optimum approach for
parameter analysis in this case was to use the depth average method. This is confirmed by Figure 7 which
clearly shows the heavy concentration of effluent at the front sample position on the 0.3 m depth plane.
Similar plots are experienced at the 0.6 and 1.0 m depth planes. This, one suspects, is due to the reduced
organic loading on the polishing filter trenches as a result of the additional secondary treatment provided by
the reed bed. This has inhibited the formation of a biomat preventing distribution of the effluent along the
base of the trenches. This poor spread could also have been aided by the hydraulic loading rate although
mass balance measurements across the reed bed show the average discharge to the polishing filter trenches
to be only marginally lower than the flow received by the secondary percolation trenches (ratio 1 : 1.04).
300.00
Cl Concentration (mg/l)
250.00
0m
200.00
2.5m
150.00
5m
100.00
10m
15m
50.00
09/03/2007
18/01/2007
29/11/2006
10/10/2006
21/08/2006
02/07/2006
13/05/2006
24/03/2006
0.00
Figure 7. Average Cl concentrations at the 0.3 m depth plane at the five sample positions along the
polishing filter percolation trenches
0.3m
0.6m
1.0m
Effective Rainfall (mm)
40.00
09/03/2007
-350
07/02/2007
32.00
08/01/2007
-280
09/12/2006
24.00
09/11/2006
-210
10/10/2006
16.00
10/09/2006
-140
11/08/2006
8.00
12/07/2006
-70
12/06/2006
0.00
13/05/2006
0
13/04/2006
Soil Moisture Tension (mBar)
Tensiometer readings on the polishing filter trenches conclusively show the prominent effect of recharge
on those areas not thought to be receiving much effluent (Figure 8).
eff. rain
Figure 8. Soil moisture tension plotted against effective rainfall for the 12.5 m tensiometer position
Chemical Analysis
Tables 1 and 2 list the average loading rates of each chemical parameter across both the reed bed and the
designated depth planes through both percolation areas. The concentrations for the secondary treatment
percolation trenches were averaged over the 0, 5 and 10 m sample positions while the 0 m sample position
concentrations alone were averaged for the polishing filter percolation trenches. The discrepancy between
loadings to the reed bed and those to the secondary treatment trenches is due to the unpredictable
distribution efficiency of the STE in the tested distribution devices on site despite monitoring on a regular
basis.
Table 1. Average loading rates of chemical parameters across the secondary treatment percolation
trenches
Parameter
Loading Rate (g d-1)
STE
0.3m plane
0.6m plane
1.0m plane
214
45.5
37.6
36.8
COD
NH4-N
5.3
2.5
0.7
30.4
NO2-N
1.3
0.4
0.0
0.1
NO3-N
11.7
11.3
16.8
0.3
5.1
0.3
0.1
0.1
Ortho-P
Table 2. Average loading rates of chemical parameters across the reed bed and polishing filter
percolation trenches
Parameter
Loading Rate (g d-1)
Reed Bed in Reed Bed out (SE)
0.3m plane
0.6m plane
1.0m plane
158
84
20.8
19.3
15.5
COD
NH4-N
24.9
22.9
0.5
1.3
0.3
NO2-N
0.1
0.0
0.1
0.2
0.0
NO3-N
0.2
0.2
21.9
22.7
13.0
4.0
2.5
0.2
0.2
0.1
Ortho-P
The installation of the constructed reed bed as a secondary treatment system reduced the organic
loading on the subsequent percolation area by 47%. When compared with comparable stand alone
secondary treatment systems on the Irish market (e.g. peat filter) (Gill et al. 2005), however, the reed bed
appears to under-perform considerably with regard to organic removal. An average COD effluent
concentration of 230 mg L-1 suggests it to be on the high side of a secondary treated effluent. Moreover, on
the percolation area receiving STE, over twice the reed bed’s organic load reduction (79%) was seen to have
occurred in the first 0.3 m of the subsoil presumably due to the attenuation and filtering capacity of the
formed biomat. It is suggested that the partial removal in organics achieved by the reed bed is responsible
for the poor formation and development of a biomat layer along the base and side-walls of the polishing
filter trenches. Nevertheless, 75% of organics were removed between the SE and 0.3 m depth plane as a
result of assumed aerobic heterotrophic biodegradation and physicochemical processes.
TKN measurements at the septic tank outlet clearly showed that the majority of the nitrogen (80%) in
the raw effluent underwent ammonification in converting to its inorganic NH4 form. Clear signs of
nitrification coupled with the greatest reduction in total inorganic nitrogen load were noted between the STE
and 0.3 m depth plane in the secondary percolation trenches. A load reduction of 40% was achieved, with
only an additional 3% of inorganic nitrogen removed by the 1.0 m plane. In fact, average total-N loads at the
point of discharge to groundwater were similar for both levels of treatment with 17.5 g d -1 passing the 1.0 m
depth beneath the trenches receiving STE, compared to 13.3 g d-1 for the trenches receiving SE. The primary
reason for this similarity is due to the inability of the horizontal flow reed bed to produce a nitrified effluent,
or more significantly, provide any appreciable reduction in nitrogen load.
The attenuation of ortho-P within the subsoil treatment system is a function of the soil’s texture and
mineralogy while its removal from the percolating effluent is controlled by soil adsorption and mineral
precipitation (Robertson, 2003). The ability of a soil to fix phosphorous is highly dependent on its clay
content. It was rather surprising, therefore, to find low concentrations of ortho-P in the top layer of the
subsoil and throughout given its freely draining nature. A reduction in load of 94% was measured between
the STE and the 0.3 m plane and 92% between the SE and 0.3 m plane. Arias et al. (2001) has suggested
that the P-removal efficiency with respect to sands is often high initially but then decreases after some time
after the P-sorption capacity of the sand is used up. The more likely reason in the P-reduction, however, can
be found in the on-site trial hole where, on closer inspection, it was noted that an overlying layer of
SILT/CLAY had slightly protruded to below the invert of the percolation trenches and as such the clay
content of the subsoil initially receiving effluent was increased locally. A combination of sorption onto the
media and biomass growth is said to enhance P-removal in the early stages of a wetland which may be
suggestive of the 38% decrease in load across the reed bed to date. The nutrient uptake, mobilisation and
storage by the macrophyte roots and rhizomes is thought to be negligible given their limited growth and
expansion in the bed substrate.
Bacteriological Analysis
Tables 3 and 4 present the Total Coliforms (TC) and E. coli concentrations measured at the septic tank
and reed bed outlets in tandem with measurements across both sets of percolation trenches (secondary and
polishing filter). The average concentration of total coliforms in the STE was 1.15 x 108 with mean E. coli
concentrations of 8.27 x 106, both values of which are in line with findings from other studies (Canter &
Knox, 1985; Ziebell et al., 1974). The installation of the horizontal-flow reed bed has greatly reduced the
bacterial load on the polishing filter trenches with an average 2-3 log removal (99.5%) in total coliforms and
2 log removal (99%) in E. coli reported across the wetland. Despite this impressive performance, relatively
high concentrations of pathogens still remain in the SE which would deem it unsuitable for discharge to
groundwater prior to further (tertiary) treatment in the subsoil. No seasonal variation in bacterial removal
across the reed bed appears to have taken place in its first 12 months of operation although it’s known that
wetland systems take a number of years to fully mature and develop (IWA, 2000). It’s thought that a greater
knowledge and insight into the removal mechanisms and meteorological effects on the reed bed will be
gained over the next 12 - 24 months.
The bacteriological results from the percolation trenches underline the ability of the highly permeable
subsoil and its associated biomat to remove enteric bacteria from the percolating effluent. Although the
subsoil receiving secondary effluent from the reed bed is treating a lower bacterial load, the total coliform
levels at two of the three depth planes (0.3 and 1.0 m) are somewhat higher than their corresponding planes
on the percolation area receiving septic tank effluent. This highlights the extent of the role of the biomat
formation given that it’s the first line of defence against the migration of pathogenic organisms in the
subsoil towards groundwater. One of the lysimeters on the 0.6 m depth plane of the percolation area
receiving septic tank effluent experienced a high E. coli count (387.3 MPN 100 mL-1) on an early sampling
date. However, a reduction in enteric bacteria concentration with time was evident at this position
suggesting the initial occurrence may have been due to the presence of a preferential flowpath which
initiated the free movement of the bacteria to the lysimeter’s porous cup but which became blocked by
biofilm over time thereafter. In excluding this value, complete removal of E. coli was achieved across the
three depth planes. Negligible E. coli readings were also recorded along the polishing filter trenches with
depth apart from isolated incidences of bacteriological contamination on the 1.0 m depth plane. BS5930
(British Standards Institute, 1981) analysis of soil samples at this depth showed the lysimeter cup to be on
the boundary of a sandy SILT / sandy GRAVEL medium. With the subsoil having a high sand content, it’s
possible that with an associated large percentage of pore spaces, bacteria migration through the local subsoil
may have occurred. Coupled with this is the likelihood that the assumed reduced biomat formation under the
polishing filter trenches had the effect of increasing the hydraulic conductivity and load per unit area. The
occurrence of such an incident had a resulting negative effect on the groundwater quality on that date with a
high of 100.5 MPN 100mL-1 recorded at the downstream groundwater monitoring point. Despite this, of the
four samples taken downstream of the percolation area, the remaining three tested negligible for E. coli (<10
MPN 100mL-1), i.e. less than one single bacterium was picked up on each of the three sampling days.
Table 3. Results of TC analysis across both the secondary (0, 5 and 10 m sample positions) and
polishing filter (0 m sample position) percolation trenches
Total Coliforms
Average
Range
No. of
Samples
(MPN 100mL-1)
(MPN 100mL-1)
9
STE
1.15 x 108
106 - 108
Secondary Percolation Trenches
2215
40.4 - 24196
23
Red Depth Plane (0.3m)
2319
13.5 - 24196
21
Blue Depth Plane (0.6m)
892
20.3 - 2419.6
21
Black Depth Plane (1.0m)
SE (Reed Bed)
Polishing Filter Trenches
Red Depth Plane (0.3m)
Blue Depth Plane (0.6m)
Black Depth Plane (1.0m)
9
104 - 106
5.85 x 105
12
20
16
410.6 - 24196
36.4 - 2419.6
307.6 - 24196
6127
2096
1895
Table 4. Concentrations of E. coli measured on sample planes and at groundwater points
No. of Samples with E.coli concentration (MPN 100mL -1 )
No. of
Samples
<10
10-100
100-1000
>1000
Percolation Area
Red Depth Plane (0.3m)
Blue Depth Plane (0.6m)
Black Depth Plane (1.0m)
23
21
21
21
18
21
2
0
0
0
3
0
0
0
0
Polishing Filter
Red Depth Plane (0.3m)
Blue Depth Plane (0.6m)
Black Depth Plane (1.0m)
12
20
16
12
20
13
0
0
2
0
0
1
0
0
0
U/S Groundwater Point
D/S Groundwater Point
3
4
1
3
2
0
0
1
0
0
Conclusions
Planar and average depth analysis has shown the biomat development under the percolation trenches of
an on-site wastewater treatment system to differ with applied effluent strength. SE, with its reduced organic
content, reached only the front (0m sample position) of the polishing filter trenches in contrast to the
percolation trenches receiving highly concentrated STE where the flow had spread to a distance of 10m.
Close examination of the reduction and removal of both chemical constituents and microbial indicator
organisms from separately applied STE and SE at various depths across the freely-draining percolation area
show there to be similar performance despite contrasting organic and bacterial influent loadings. The
evidence from this research indicates that the majority of organic, ortho-P and inorganic nitrogen load
removal occurs in the distribution gravel around the percolation pipe and across the first 300mm in the
subsoil of both the secondary and polishing filter percolation trenches. Complete nitrification appears to
occur within this distance (0.3m) in the trenches receiving SE, however, no appreciable reduction in total
inorganic-N takes place until the discharge point to groundwater is reached. Total-N loadings at this 1.0m
position vary little for both strength effluents with an average of 17.5 g d-1 recorded for the trenches
receiving STE and 13.3 g d-1 for the trenches receiving SE. Isolated incidences of E.coli contamination at
the 1.0m depth plane were found in the freely draining subsoil treating SE, indicating a potential risk to
groundwater pollution in those highly loaded areas where biomat development is negligible.
The performance of the horizontal subsurface-flow reed bed as a stand-alone secondary treatment
system has been shown to be very limiting in an Irish context with regard to organic removal when the STE
is highly concentrated (average COD concentration of 521 mg l-1). Given the associated lack of nitrification
and nominal total-N load removal across the bed, the research to date indicates that the only clear benefit in
the installation of a reed bed prior to a receiving polishing filter is in the removal of the STE’s indicator
bacteria, with 2-3 log coliforms recorded for both total coliforms and E.coli at the bed’s outlet.
The monitored wastewater flow per capita (118.7 L d-1) was found to be considerably less than the EPA
design daily hydraulic load of 180 L d-1, supporting similar investigations carried out in Ireland recently.
Acknowledgements. The authors wish to acknowledge the Irish Environmental Protection Agency for
funding this research study under the Environmental Research Technological Development and Innovation
(ERTDI) Programme as part of the National Development Plan 2000-2006.
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