Retrofitting a Saturated Zone (RSAZ) in stormwater biofilters: impact on
pollutants removal
Y. Zinger1*, G. T. Blecken2 , T. D. Fletcher1, A. Deletic1, M. Viklander2
1
Centre for Water Sensitive Cities, Department of Civil Engineering, Monash University,
Victoria, Australia, 3800
2
Urban Water, Department of Civil, Environmental and Natural Resources Engineering,
Luleå University of Technology, 971 87 Luleå, Sweden
* Corresponding author, e-mail yaron.zinger@monash.edu
ABSTRACT
Elevated nitrogen levels are one of the main triggers for eutrophication of receiving water
bodies. While efficient nitrification occurs in freely drained stormwater biofilters,
denitrification is often lacking, which has been identified as the main reason for insufficient
N removal or leaching. Furthermore, it has been previously shown that the choice of plant
species may influence N removal, with some species potentially resulting in a net leaching,
particularly when the filter media itself has relatively high levels of nitrogen. Thus,
retrofitting of a submerged zone (SAZ) into poorly performing N removal biofilter might
enhance anaerobic conditions for sufficient denitrification. The principal aim of this study
was to evaluate the influence of a retrofitted SAZ (RSAZ) on nitrogen removal by stormwater
biofilters, whilst verifying that the SAZ did not diminish the removal of other pollutants.
Fifteen mature biofilter columns planted with Dianella revoluta and Microleana stipoides
were chosen since they showed the poorest nitrogen removal of a range of plant species (TN
leaching of up 200% and up to 500% leaching of NOX for both plants), compared to the
control group of Carex appressa (TN removal > 67% and NOx removal >94) biofilter
columns. The biofilter columns were sampled over a year before and three months after
retrofitting the SAZ. Samples were analysed for TSS, TN and the nitrogen species NH4 and
NOx (nitrite and nitrate), TP and filterable reactive phosphorus (FRP), and Cu, Pb and Zn.
The RSAZ columns showed a mean 370% increase in NOx removal capacity for Dianella
(+180% for Microlaena), while the control Carex columns enhanced their NOx removal by
35%. Unsurprisingly, TP removal (FRP driven) was reduced in all treatments due to presence
of organic matter in filter media within the RSAZ. Overall, retrofitting of SAZ seems to
enhance N removal such that plant selection becomes less critical, allowing a range of plants
to be used, considering other factors such as biodiversity.
KEYWORDS
Biofilter; stormwater; saturated zone (SAZ); nitrogen, nitrate, phosphorus, heavy metals
INTRODUCTION
Stormwater quality treatment in stormwater biofilters has generally been found to be effective
and reliable. For example, TSS, heavy metal and (commonly) phosphorus removal is very
efficient and exceeds often 80 or 90% (Davis, Shokouhian et al. 2001; Muthanna, Viklander
et al. 2007; Hatt, Fletcher et al. 2009). However, nitrogen removal has commonly been less
efficient (Henderson, Greenway et al. 2007; Bratieres, Fletcher et al. 2008), and net leaching
has been observed (Hunt, Jarrett et al. 2006; Blecken, Zinger et al. 2010). This obviously has
major impacts for receiving waters since N discharges may increase eutrophication if primary
production is N controlled. A lack of anaerobic denitrification has been identified as the main
reason for insufficient N removal (Zinger, Deletic et al. 2007; Zinger, Fletcher et al. 2007;
Bratieres 2008). Thus, to enhance anaerobic denitrification in the filter media, the
introduction of a saturated zone (SAZ) in the filter, combined with a carbon source (acting as
an electron donor to facilitate denitrification), has been suggested and successfully tested
(Kim, Seagren et al. 2003; Dietz and Clausen 2006; Zinger, Fletcher et al. 2007).
However, since the SAZ has been developed recently, its implementation, design and
function is not yet included in most design guidelines (e.g. Prince George's County 1993;
U.S. EPA 2004; Melbourne Water 2005; Prince George's County 2007). Hence, the vast
majority of existing stormwater biofilters lack a SAZ and may have poor nitrogen removal
capability (particularly if the media has a high nitrogen content and the choice of plants has
not focussed on those capable of nitrogen retention; Bratieres et al., 2008; Zinger, 2007). To
address this, a saturated zone could be retrofitted to existing biofilter systems, simply by
elevating their outlet. However, in contrast to biofilters initially designed with a SAZ which
usually include a carbon source to facilitate denitrification (Kim, Seagren et al. 2003; Zinger,
Fletcher et al. 2007; Yang, McCoy et al. 2010), adding such a carbon source is technically
not possible when retrofitting a SAZ, which might damage the filter media structure.
Thus, the principal aim of this study was to evaluate the influence of a retrofitted SAZ
(RSAZ) without an additional carbon source, on nitrogen removal by stormwater biofilters.
Furthermore, the potential impacts on TP removal are of particular concern, as P could
potentially be released under sub-oxic conditions in the SAZ (Correll 1999). Metal and TSS
removal is unlikely to be impacted by the SAZ (Blecken, Zinger et al. 2009; Blecken, Zinger
et al. 2009). However, to evaluate the impact of a retrofitted submerged zone
comprehensively, metal, phosphorus and TSS removal were also included in this study.
METHODS
Fifteen biofilter columns (inner diameter of 375 mm, and a total height of 1300 mm), made
up of 900 mm PVC stormwater pipe and 400 mm transparent Plexiglas were constructed. The
~700 mm deep filter layer consisted of sandy loam (d50 of approximately 0.25 mm). Below it,
140 mm of two transition layers consisted two 70mm layers of double-washed medium and
course sand. The bottom drainage layer (7mm gravel) with an embedded drain pipe (ø 50mm)
discharged to the outflow/sampling port.
The columns were placed in a greenhouse (located in Melbourne) with open mesh on the
sides maintaining exposure of the columns to the local climatic conditions, but with a
transparent roof ensuring that the filters did not receive inflow from precipitation. Seminatural stormwater was applied to the filters twice-weekly to ensure typical stormwater
concentrations for the target pollutants (Duncan 2003; Taylor, Fletcher et al. 2005). It was
prepared by adding sediments from natural stormwater pond (<300 µm) and chemicals to
dechlorinated tap water as described by (Zinger, Fletcher et al. 2007). After approximately
one year of operation as standard freely draining biofilters configuration, all 15 biofilter
columns were retrofitted with a saturated zone (RSAZ) level of 450 mm at the bottom of the
biofilter, in which was optimised by Zinger, Fletcher et al. (2007). The RSAZ was created
simply by elevating the outflow port, which created a submerged zone in the filter material up
to the outflow height (Figure 1).
Clear Perspex top
400
Sandy loam
900
.
Transition layers:
RSAZ
70
70
70
450
1. Medium sand
2. .Course sand
Drainage- layer
.
-1:
Figure 1: left, Constructed biofilter columns in FAWB
greenhouse
FAWB
greenhouse.
before retrofitting the SAZ. Right, Biofilter columns design with outflow
from standard set-up (bottom ouflow pipe) and outflow after retrofitting
(RSAZ=450mm). All measurements in mm.
The 15 biofilter columns were divided into three groups of five replicates, based on their
vegetation type (Dianella revolute, Blueberry lily; Microlaena stipoides , Weeping Grass;
Carex appressa), herein labelled as D, M and C according to the genus name. The columns
planted with Carex appressa and Microlaena stipoides were dosed with 50 L and those
planted with Dianella revoluta with 25 L stormwater at each application, based on the
observation in their previous testing using ‘standard configuration’ (without SAZ), that
Dianella is generally adapted to drier conditions. Biofilters planted with Dianella revoluta
and Microlaena stipoides were chosen since they showed the poorest nitrogen removal of a
range of plant species in the standard biofilter configuration within the previous study
conducted (negative removal up to -250 %; see Bratieres et al, 2008). At the other end of the
scale, filters planted with Carex appressa had shown highly efficient nitrogen removal (>
70%) (Zinger, Fletcher et al. 2007; Bratieres, Fletcher et al. 2008), making them effectively a
‘control’ in this study (with and without SAZ).
Before retrofitting the saturated zone, six effluent samples were taken after a normal dosing
event. Two months after introducing the RSAZ, three of the regular twice-weekly dosing
events were sampled and analysed monthly, over a period of three months. The samples were
analysed for TSS, total nitrogen and the nitrogen species NH4 and NOx (nitrite and nitrate),
total phosphorus (TP) and filterable reactive phosphorus (FRP), and Cu, Pb and Zn. Both
effluent concentrations and removal percentages were analysed. To detect the influence of the
retrofitted SAZ on the different biofilter configurations, a univariate analysis of variance was
performed. The factors vegetation type (D, M, C), submerged zone (no-SAZ, SAZ), their
interaction (group x SAZ), inflow concentrations and run-time were analysed.
RESULTS AND DISCUSSION
All filter configurations remained very effective in removal of heavy metals and TSS after
retrofitting, with removals typically exceeding 95% (Figure 3). Whilst the RSAZ was
generally deleterious for removal of phosphorus, it had a strong positive impact on nitrogen
removal (ANOVA; p<0.05). Removal of copper was also high, and it’s removal increased by
2-9% while RSAZ was introduced. Importantly, NOx removal was substantially improved
(p<0.001) due to favourable anaerobic conditions that favoured facultative denitrifiers. The
RSAZ columns showed an increase of 370% in mean NOx removal for the previously poorly
performing Dianella columns and +180% for Microlaena), while the Carex columns
enhanced their NOx removal by 35% (given their already high removal levels). This allowed
even the previously poorly performing Dianella columns to demonstrate effective nitrogen
removal, but not enough for Microlaena columns to show net removal of TN (leaching of
~10%). On the other end, an NH4 removal of around 12% (representing an 88% reduction)
was observed, along with a 50% reduction for columns planted with Dianella and Microlaena
respectively, while Carex removal dropped by only 2%. This behaviour was attributed to the
nature of organic nitrogen removal (mineralization) and NH3 (nitrification), both of which are
activated by aerobic bacteria and inhibited by the anaerobic conditions imposed by the
RSAZ. However, most importantly, the TN effluent concentrations decreased significantly
over the standard columns (Figure 2), in which improved TN mean removal by 100% and
40% for Dianella and Microlaena (respectively).
No additional carbon source to facilitate denitrification was added within the retrofitted SAZ.
This omission did not diminish denitrification and subsequent N removal. This is contrary to
the findings of Kim et al. (2003) who showed efficient nitrate and nitrite removal in biofilters
depended on the addition of organic carbon. It is assumed that the organic matter-rich filter
material used in this study (sandy loam; ~4% organic carbon), combined with accumulated
organic carbon from the stormwater influent and root and microbial turnover have provided a
sufficient level of carbon to maintain the denitrification process. Custom-designed biofilter
columns (planted with Carex) with a SAZ were tested in a previous study by Zinger et al.
(2007). In that study the columns with an embedded carbon source (as electron donor) in the
SAZ performed slightly better than the Carex columns in this study, after retrofitting the
SAZ. Regardless, the effective N removal in the RSAZ columns without an embedded carbon
source is an important finding since the carbon added to a biofilter will eventually deplete.
The results of this study show that biofilters may become self-supplying after some operation
time due to organic carbon inputs and turnover of plant and microbial material. These results
suggest that long-term effectiveness for nitrogen removal may be maintained.
10
6
4
2
C
D M
Non
Zn (µg/L)
Pb (µg/L)
8
0
Group
SZ
12
6
Cu (µg/L)
C D M
Retrofitted
5
5
10
4
8
3
6
2
4
1
2
0
Group
SZ
C
D M
Non
C D M
Retrofitted
4
3
3
2
2
1
1
D M
Non
C D M
Retrofitted
NH4 (mg/L)
NOx (mg/L)
4
C
0,4
5
TN (mg/L)
0
Group
SZ
0,3
0,2
0
Group
SZ
C
D M
Non
C D M
Retrofitted
0
Group
SZ
0,1
C
D M
Non
C D M
Retrofitted
FRP (mg/L)
TP (mg/L)
0,3
0,3
0,2
0,2
C
D M
Non
C D M
Retrofitted
5
0,4
0,4
0,0
Group
SZ
TSS (mg/L)
4
3
2
0,1
0,1
0,0
Group
SZ
C
D M
Non
C D M
Retrofitted
0,0
Group
SZ
1
C
D M
Non
C D M
Retrofitted
0
Group
SZ
C
D M
Non
C D M
Retrofitted
Figure 2: Interval plot of the pollutant effluent concentration of the three groups before and after
retrofitting of the SAZ (Non: standard columns, sampling runs 1 to 6; Retrofitted: columns after
retrofitting, sampling runs 7 to 9), inflow concentrations are given by the dashed lines (as long as it
within the plot range). C= Carex, D = Dianella, M = Microlaena.
While the RSAZ had a positive effect on the removal of TN and NOx, TP removal was
diminished by it (Figure 3). All columns showed a mean reduction of 25-45% removal and no
net leaching behaviour was noted. Decreased FRP removal contributes to the poor TP
removal; Microlaena columns displayed the highest reduction in FRP removal (-102%),
compared to Dianella (-45%). Carex columns showed a somewhat constant FRP removal of
85%. Carex, apparently, was able to diminish the effect of RSAZ on FRP leaching, probably
due to effective uptake of phosphorus by its dense and deep roots, compared to the shallow
root network of Microlaena and Dianella. However, Carex columns showed a reduction of
TP removal, combined, with less efficient TSS removal, which implies a reduction in
particulate P removal. This can be explained by the deeper penetration of the Carex roots,
creating preferential flow due to roots turnover in which could suspend and flush fine
sediment to the outflow. In practical terms, the removal of TSS remains very high (exceeding
97% for all groups), so only the change in TP removal is of practical importance.
Figure 3: Interval plot of the pollutant removal of the three groups before and after
retrofitting of the SAZ (Non: standard columns, sampling runs 1 to 6; Retrofitted: columns
after retrofitting, sampling runs 7 to 9). C= Carex, D = Dianella, M = Microlaena.
It is clear that elevated anaerobic conditions imposed by the RSAZ induced leaching of
dissolved phosphorus (FRP) and subsequently of TP. This behaviour may be attributed to
iron-reducing bacteria activity (Baldwin and Mitchell 2000). Despite the large reduction in
net uptake of phosphorus, positive phosphorus removal was still achieved, with
approximately mean 25% and 30% of influent phosphorus being removed by Dianella and
Microlaena columns respectively, whereas Carex demonstrated 60% TP removal.
Of the metals tested, Cu outflow concentrations (and Zn in a lesser degree) were decreased
significantly by the introduction of the RSAZ. All groups showed enhanced Cu removal by
RSAZ; the Carex columns performed especially well, showing a higher improvement of 11%
which might mainly have been caused by the high affinity of Cu to organic matter (Cuorganic matter complexation). In fact, Carex seems to enhance metal removal more than the
other plants tested, which could be attributed to fine roots and high biomass traits of Carex,
which perform as a source of organic matter allowing organic complexation with metals
(Alloway 1995) and therefore enhanced removal. In addition, these traits also allow a higher
surface area for direct and indirect biological uptake of metals. The principal conclusion is
that all metals are removed efficiently by all groups throughout the experiment, meaning that
the influences of factors such as vegetation type and the retrofitted SAZ have low practical
importance.
CONCLUSIONS
Retrofitting a SAZ into standard biofilters enhances total N removal significantly due to
creation of partly anaerobic conditions within the RSAZ, facilitating improved denitrification
and subsequent nitrate removal. Furthermore, a significant difference of N removal between
the biofilters planted with different species has been shown. However, retrofitting of SAZ
enhance N removal of bad performing plants, such that plant selection becomes less critical,
allowing a range of plants to be used, considering other factors such as biodiversity.
Saturated zone can be retrofitted without supplementary carbon source (to support efficient
denitrification) as long as the filter media contain sufficient organic content (~4%). This way,
the biofilter become carbon sustainable after some time of operation also due to organic
carbon inputs, turnover of plant and microbial material. These results suggest that long-term
effectiveness for nitrogen removal may be maintained.
Total P removal was diminished by the RSAZ, but the removal of TSS and metals was in
general not affected in practical terms. While, the results suggest that retrofitting SAZ in
existing standard biofilters is recommended if the existing filter has inadequate N removal
and if N discharges pose a potential threat to the receiving environment. Yet, if P
concentrations are of principal concern, retrofitting a RSAZ might be counterproductive. If
no special circumstances are present, retrofitting is not required to maintain acceptable TSS
and metal removal. These findings also indirectly strengthen the need for a low organic
content transition layer above the SAZ design water table within future custom made
biofilters in order to prevent reduction and subsequent leaching of P.
In addition to safer water quality discharge by biofilters which incorporating RSAZ, it will
also provide other known benefits of having saturated zone, such as buffering of antecedent
dry weather periods between rainfall events, resulting in better plant growth and potentially
less variability in pollutant removal performance (avoiding drying out of the media).
REFERENCES
Alloway, B. J. (1995). "Heavy metals in soils." Blackie Academic and Professional, London, UK.
Baldwin, D. S. and A. M. Mitchell (2000). "The effects of drying and re-flooding on the sediment and
soil nutrient dynamics of lowland river-floodplain systems: a synthesis." Regulated Rivers:
Research & Management 16(5): 457-467.
Blecken, G. T., Y. Zinger, et al. (2010). "Laboratory study on stormwater biofiltration: nutrient and
sediment removal in cold temperatures." J. Hydrol. 394(3-4): 507-514.
Blecken, G. T., Y. Zinger, et al. (2009). "Impact of a submerged zone and a carbon source on heavy
metal removal in stormwater biofilters." Ecol. Eng. 35(5): 769-778.
Blecken, G. T., Y. Zinger, et al. (2009). "Influence of intermittent wetting and drying conditions on
heavy metal removal by stormwater biofilters." Water Res. 43(18): 4590-4598.
Bratieres, K., T. D. Fletcher, et al. (2008). "Nutrient and sediment removal by stormwater biofilters; a
large-scale design optimisation study." Water Res. 42(14): 3930-3940.
Bratieres, K., Fletcher, T. D., Deletic, A., & Zinger, Y. (2008). "Nutrient and sediment removal by
stormwater biofilters; a large-scale design optimisation study." Water Research 42(14): 39303940.
Correll, D. L. (1999). "Phosphorus: A rate limiting nutrient in surface waters." Poultry Sci. 78(5):
674-682.
Davis, A. P., M. Shokouhian, et al. (2001). "Laboratory Study of Biological Retention for Urban
Stormwater Management." Water Environ. Res. 73(1): 5-14.
Dietz, M. E. and J. C. Clausen (2006). "Saturation to improve pollutant retention in a rain garden."
Environ. Sci. Technol. 40: 1335-1340.
Duncan, H. (2003). Chapter 3 - Urban Stormwater Pollutant Concentrations and loads Australian
Runoff Quality Symposium. Lake Hume Resort Riverina Hwy, Albury.
Hatt, B. E., T. D. Fletcher, et al. (2009). "Hydrologic and Pollutant Removal Performance of
Biofiltration Systems at Field Scale." J. Hydrol. 365(3-4): 310-321.
Henderson, C., M. Greenway, et al. (2007). "Removal of Dissolved Nitrogen, Phosphorus and Carbon
from Stormwater by Biofiltration Mesocosms." Water Sci. Technol. 55(4): 183-191.
Hunt, W. F., A. R. Jarrett, et al. (2006). "Evaluating bioretention hydrology and nutrient removal at
three field sites in North Carolina." J. Irrig. Drain. Eng. 132(6): 600-608.
Kim, H., E. A. Seagren, et al. (2003). "Engineered Bioretention for Removal of Nitrate from
Stormwater Runoff." Water Environ. Res. 75(4): 355-367.
Melbourne Water (2005). WSUD Engineering Procedures: Stormwater, CSIRO Publishing.
Muthanna, T. M., M. Viklander, et al. (2007). "Heavy metal removal in cold climate bioretention."
Water Air Soil Pollut. 183: 391-402.
Prince George's County (1993). "Design manual for the use of bioretention in stormwater
management." The Prince George's County, Maryland, USA.
Prince George's County (2007). "Bioretention Manual." Environmental Services Division,
Department of Environmental Resources, The Prince George's County, Maryland, USA.
Taylor, G. D., T. D. Fletcher, et al. (2005). "Nitrogen composition in urban runoff--implications for
stormwater management." Water Research 39(10): 1982.
U.S. EPA (2004). "Stormwater Best Management Practice Design Guide. Volume 2. Vegetative
Biofilters. Cincinatti, OH, USA." EPA/600/R-04/121A. Office of Research and Development.
United States Environmental Protection Agency, Cincinatti, OH, USA.
Yang, H., E. L. McCoy, et al. (2010). "Dissolved nutrients and atrazine removal by column-scale
monophasic and biphasic rain garden model systems." Chemosphere 80: 929-934.
Zinger, Y., A. Deletic, et al. (2007). The effect of various intermittent dry-wet cycles nitrogen
removal capacity in biofilters systems rainwater & urban design 2007. Sydney.
Zinger, Y., T. Fletcher, et al. (2007). Optimization of the nitrogen retention capacity of stormwater
biofiltration systems. Novatech. Lyon, French.