Treatment efficiency of biofilters; results of a large-scale column study

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1 Treatment efficiency of biofilters; results of a large-scale column study T. Fletcher 1 Y. Zinger 1, A. Deletic 1, Katia Bratières 2 1 Facility for Advancing Water Biofiltration (FAWB), Institute for Sustainable Water Resources, Bldg 60, Dept. of Civil Engineering, Monash University, VIC 3800, Australia 2 Laboratoire de Génie Civil et d Ingénierie Environnementale, INSA Lyon, France ( tim.fletcher@eng.monash.edu.au, yaron.zinger@eng.monash.edu.au; ana.deletic@eng.monash.edu.au;) Abstract In order to evaluate the optimal design of biofilters for treatment of sediment, nitrogen and phosphorus, 140 biofilter columns were constructed, using different plant species, different depths and types of filter media, along with different storm volumes and input concentrations. All biofilters tested were found to be highly effective for removal of TSS, reducing inflow concentrations by an average of 98%. Total phosphorus was reduced by an average of 80%, whilst nitrogen removal was much more variable, including some configurations which yielded a net increase in nitrogen concentration. However, careful selection of plants and media type was able to achieve a simultaneous reduction of 50-70% of nitrogen and 90% of phosphorus. Carex appressa and to a lesser extent Melaleuca ericifolia performed very well in nutrient removal, whilst Dianella revoluta, Leucophyta brownii and Microlaena stipoides did not, within the nine months of testing, effective in facilitating nitrogen removal. Appropriate sizing of biofilters relative to their catchment area, as well as careful selection of plants for climate condition may be critical for biofilter performance. Further research will be undertaken to determine whether relatively poorly performing biofilter designs can be improved by retrofitting a saturated anaerobic zone, to promote denitrification and enhance drought tolerance. 1.0 Introduction Urban stormwater is recognised as the primary source of degradation for receiving waters creeks, rivers, lakes, estuaries and bays in populated areas of Australia (Commonwealth of Australia, 2002). In particular, eutrophication due to excess nutrients is a threat to major bays such as Moreton Bay, Port Phillip Bay and Cheseapeake Bay (Taylor et al., 2005). Of the range of water sensitive urban design technologies for stormwater, biofiltration is becoming one of the most widely applied, due to its flexibility in terms of size and configuration. Despite this, limited data are available to predict the performance of stormwater biofilters in nutrient removal. More importantly, there has been very little research aimed at optimising the long-term nutrient uptake of stormwater biofilters. When Lloyd et al. (2001) dosed a gravel-based biofilter with artificial stormwater, they achieved a 47% reduction in total phosphorus, but little total nitrogen removal, due to leaching of organic nitrogen. Nitrate reduction was only 29%, suggesting that conditions conducive for denitrification (the conversion of nitrate into N 2 gas) were not present. Henderson et al. (2005) compared nutrient removal by three biofilter media types (gravel, sand and sandy loam), with and without vegetation. In the vegetated columns, they showed nitrogen removal ranging from 63-77%, whilst phosphorus removal averaged 85-94%. However, they observed that unvegetated mesocosms leached nitrogen. Regardless of the presence of vegetation, gravel media were less effective for nutrient removal than sand or sandy loam. Denman et al. (2006) studied biofilter media with and without trees, and also showed that the presence of vegetation was critical to determining nutrient removal behaviour.

2 Davis et al. (2001) showed that biofilters could reduce phosphorus concentrations by around 80%, and nitrogen by 63%. By adding an anaerobic zone at the base of the biofilter column (a permanently saturated zone), Hunho et al. (2003) were able to boost nitrogen removal to around 80%, as a result of promoting denitrification under low-oxygen conditions. Sharkey et al. (2005) also trialled an anaerobic zone, but did not show any substantial improvement in nitrogen reduction (µ=40% removal). In summary, whilst a number of studies have been undertaken on biofiltration performance and behaviour, they have all been relatively small scale and limited in scope. In response to the need for better guidance on the design of stormwater biofiltration systems, the Facility for Advancing Water Biofiltration (FAWB) has been conducting a fully replicated study which tests the influence of: 1. The presence and type of vegetation, 2. The depth and type of filter media, and 3. The magnitude of storms and inflow concentrations; on the removal of TSS, TP and TN within stormwater biofilters. The study also looked at the removal of heavy metals (not reported here), the influence of an anaerobic zone on nitrogen removal (Zinger et al., this volume; Zinger et al., 2007), and the evolution of hydraulic conductivity over time (Le Coustumer et al., 2007). The aim is to provide clear guidance on the optimal design of stormwater biofilters. 2.0 Methods 2.1 Experimental set-up Biofilter columns were constructed from 375mm diameter PVC pipe, with a Perspex top section to allow light for plant growth (Figure 1). Each column had a gravel drainage layer at the base, along with a transition layer (of sand), and a soil filter media layer. In total, 140 columns were constructed, in order to test the following factors (with five replicates of every configuration): 1. Effect of plant species (5 species: Carex appressa, Dianella revoluta, Microleana stipoides, Leucophyta brownii and Melaleuca ericifolia); 5 replicates of each species, and 5 replicates with no vegetation (as a control). 2. Effect of filter media (i) normal sandy-loam with target K s of 150 mm/hr, (ii) sandy loam with 10% mulch and 10% compost, (iii) sandy loam with 10% mulch and 10% compost and a lower ph and (iv) sandy loam with 10% vermiculite and 10% perlite; exploded mineral materials which enhance adsorption capacity); 5 replicates of each media x 3 species (Carex, Melaleuca and Microleana). 3. Effect of filter depth (3 depths of sandy loam are being tested: 300, 500 and 700mm, all with the 200mm sand/gravel drainage layer underneath); 5 replicates of each depth x 3 species (Carex, Melaleuca & Microleana). 4. Inflow concentration: normal and high (see Table 3) on 3 species (Carex, Melaleuca and Microleana) and unvegetated media. 5 replicates of each 5. Effect of storm volume (low, normal and high, sized to equate to a biofilter of 4%, 2% and 1% of its catchment area, respectively) x 3 species x 5 replicates (storm volumes were selected assuming a biofilter:catchment area ratio of 2%). Columns were planted in January 2006 (6 plants per column), and watered as required for 6 months to allow establishment.

3 Perspex (400mm) Soil filter media (700, 500 or 300mm) Medium sand (70mm) Coarse sand (70mm) Gravel (70mm) Figure 1. Details of biofilter column construction. Soil filter type and depth varied according to the configuration being tested. 2.2 Experimental procedure Twice-weekly dosing with stormwater commenced in July 2006 for 6 weeks before the sampling campaign began. Five sampling runs (at approximately 8 week intervals) have been undertaken to date. Semi-synthetic stormwater is applied twice weekly, to minimize variations in inflow concentration, whilst maintaining realistic composition. Sediment from a stormwater wetland inlet pond is collected and mixed with dechlorinated tap water. Concentrations are then topped up to achieve typical urban stormwater quality concentrations (Duncan, 2006; Taylor et al., 2005); see Table 1. Table 1. Standard pollutant concentrations; these concentrations were doubled for testing the effect of high concentration. Sediment & nutrients Concentration Concentration Heavy metals (mg/l) (mg/l) TSS 150 Cd TP 0.35 Cr TN 2.1 Cu 0.05 TDN 1.6 Fe 3 NH Mn 0.25 NOx 0.75 Ni 0.03 PON 0.5 Pb 0.14 DON 0.59 Zn 0.25 Org-N 1.09 To ensure consistent concentrations are applied to each column, a mixing tank is used to convey the water to distribution points, where a given volume (25 or 50L for normal and high ) is applied in 5 x 5L passes to each column. Every column receives the same proportion of water from the start and end of the dosing run. Composite samples are taken from each column according to a protocol established using a tracer pilot study. The composite is made up of five 200 ml sub-samples taken during the outflow; one after 1L and another four sub-samples after every five litres. Samples were analysed for TSS, TN and all nitrogen species, TP and FRP, and heavy metals (Pb, Zn, Cu, Fe), in a NATA accredited laboratory.

4 2.3 Data analysis Data from five sampling runs were available for analysis. Before the influence of design configuration could be tested, it was necessary to determine if there was any change in behaviour over time, as the systems matured. Repeated Measures Analysis of Variance (RM- ANOVA) showed that concentration reductions changed significantly with time. As a consequence, analysis of design configuration was separately undertaken for both the overall dataset and the most recent sampling run (No. 5). For phosphorus and nitrogen, the analysis is undertaken for both the totals (TP and TN) and individual species, in order to understand the (a) transformation processes occurring within the filter media and (b) bioavailability of nutrients discharged from the biofilter into its receiving waters. Analysis is undertaken on removal data (rather than raw concentrations). Often, removal is a distorted measure of performance because it is mathematically dependent on the input concentration (Taylor et al., 2006). However, for this experiment, input concentrations were consistent between columns, thus removing this effect. The effect of design configuration was assessed using ANOVA, with individual contrasts assessed using either Tukey s or Tamhane s T2 post-hoc tests (dependent on whether the ANOVA satisfied (or not, respectively) the assumption of homogeneity of variance (Zar, 1999). Significance is considered where p< Results 3.1 Effect of design on performance Total suspended solids are very effectively removed by all biofilter configurations tested with an average removal (μ) of 96% across all treatments and sampling runs, and a coefficient of variation (CV) of only 2.2%. Whilst there were significant differences between species (e.g. removal was lessfrom columns vegetated with Melaleuca), the practical implications are negligible, since all vegetated columns exceeded 96% removal in all runs. This is not surprising and reflects similar findings by Hatt et al. (2007; 2006). It explains in part also why soil-based biofilters are so effective in removing heavy metals, since a high proportion of the metals are attached to sediment, which are effectively filtered out (ibid). Phosphorus concentrations are also effectively reduced, which is not surprising, given the high level of TSS removal; there is an overall average (for all 5 sampling runs) removal of - 80% of Total Phosphorus (TP) and 68% of PO 4 (FRP; Filtrated Reactive Phosphorus). Notwithstanding this, several design parameters have a significant influence on performance. For example, columns planted with Carex appressa show greater removal of both TP and PO 4 than any other species or than the unvegetated control columns (Figure 2a). Differences between other species are, however, relatively minor, and not statistically significant. Removal of TP and PO 4 decrease with increasing storm volume (ANOVA p<0.01 in each case), which has implications for the specification of biofilter:catchment area ratio. Filter depth had no significant effect on removal of phosphorus, suggesting that most of the phosphorus removal occurs in the upper portion (Hatt et al., 2006). However, the type of filter media was significant, with the best removal coming from the Sandy Loam and the Sandy Loam + Vermiculite/Perlite. The addition of compost material resulted (Figure 2b) in a net production of phosphorus, due to leaching.

5 TP PO4 Reduction (a) Figure 2. (b) SL SLMC SLMCpH Media Type Mean and 95% confidence interval of reductions during Run 5 in TP and PO 4 relative to (a) vegetation type and (b) media type (SL-Sandy loam, SLMC - Sandy Loam + Mulch/Compost, SLVP - Sandy Loam + Vermiculite/Perlite). As would be expected (simply as a mathematical artifact), the percentage reduction in phosphorus was higher when the input concentration was highest. However, the output concentrations are very similar (0.06mg/L for normal input concentration of 0.35mg/L and mg/l when the high input concentration of 0.70mg/L was used). Again, this shows that the biofilters are capable of removing phosphorus down to a consistent background concentration. The background concentration is, as shown in Figure 2b, dependent on properties of the soil media. Nitrogen removal is a much more complex process, because of the many species of nitrogen, and the ability for them to readily change from one form to another. Of the five species tested, Carex provided the best removal (Figure 3), significantly better than the unvegetated control (Tukey s HSD p<0.01) and the other species (Tukey s p<0.03). Melaleuca also performed relatively well, whilst Leucophyta seems, at this stage, to offer little advantage over unvegetated soil media. Ammonia is effectively removed by all configurations (even the unvegetated controls), as is particulate organic nitrogen (PON), which is not surprising given the high level of TSS removal. However, there is for many of the columns (those not vegetated with Carex or Melaleuca, a net production of oxidized nitrogen. The input concentration of nitrogen has no significant influence on removal performance. However, the storm volume was important, with better removals of TN, NH3, NOx (all p<0.01) and PON (p=0.05) for low volumes than during either standard or high volumes. Interestingly, whilst filter depth has no influence on removal of ammonia or organic nitrogen, increasing filter depth may result in a net production of nitrate, which significantly influences the overall nitrogen reduction. In part, this observation is explained by the time taken for plants to reach maturity in terms of occupying the soil media; see section 3.2. The filter media type also has a significant influence on nitrogen removal (p<0.01), with the standard Sandy Loam filter media performing better than those with added compost, due to the effect of leaching of organic nitrogen from the organic matter. SLVP

6 100% 50% Mean Removal (%) 0% -50% -100% Sampling 1 Sampling 2 Sampling 3 Sampling 4 Sampling 5 Figure 3. Carex -150% Dianella Microleana Leucophyta / Non-Vegetated -200% Melaleuca Non-Vegetated -250% Influence of vegetation type on reductions in TN over time. 3.2 Evolution of performance over time A key question is how long a biofilter will take to mature. For TSS and TP, there is no change over time. However, as seen in Figure 3, there are significant interactions between establishment time and vegetation. Whilst Carex produced very good nitrogen removal from the first sampling run, it took until Run 5 (approximately 14 months after planting) for Melaleuca to begin to demonstrate effective N removal (Figure 3). The difference shows the difference rates of root development between the two species. Carex rapidly establishes a dense root network, whilst the Melaleuca root network is relatively sparse and takes longer to significantly occupy the soil volume. For the unvegetated columns (and those with other species), nitrogen leaching worsens over time, as a result of organic matter leaching from the soil. 4.0 Discussion 4.1 Optimising removal of target pollutants The results show that biofiltration systems can be very effective for removing TSS, TP and TN from stormwater. Any soil-based filter will be effective in reducing TSS concentrations by more than 90%. Where TSS is the only target pollutant, the key determinant of overall performance will be the percentage of mean annual flow treated. In this case, selecting a filter media with a relatively high infiltration rate will help to maximise performance, provided that the material is capable of remaining stable, and not leaching fine material. Similarly, metals removal in any soil-based filters have been shown to exceed 90% (Hatt et al., 2006 and observed (although not reported here) in this study also), and so soil-based biofilters are appropriate for catchments with land-uses likely to generate high levels of metals. If TP is the target pollutant, then most configurations are capable of reducing inflow concentrations by around 85%. Whilst choice of vegetation can be used to improve this removal to around 90% (if Carex appressa is used), the most important factor is to ensure that there is not excessive organic matter, from which leaching of phosphorus is likely to occur. Given this, it is suggested that additional compost or mulch material not be added directly into sandy loam filter media. Given that careful selection of vegetation can compensate for any media influence on nitrogen removal, it is recommended that based on these results, a sandy loam media, planted with

7 Carex or Melaleuca will optimise removal of both N and P. For example Carex was regularly able to remove over 50% of TN, exceeding 70% after 8 months in operation; Melaleuca, whilst slow to establish, was able to reduce TN concentrations by 50%. Other species tested were not able to prevent a net production of nitrogen, possibly due to their root architecture, or because they are not as well suited to the current watering regime. In part, Carex s high nitrogen removal capacity may be due to its extremely high root density. For Melaleuca, the good performance may be due to the presence of arbuscular mycorrhizal (AM) fungi, which increase the absorptive surface of the plant root system thereby providing access to soil derived nutrients from sources not otherwise accessible to roots (Smith & Read, 1997). Filter depth seems not to be a limiting factor for pollutant removal. This has important implications, given the practical constraints on fitting biofiltration systems into the existing drainage network (where the pipe invert may be relatively shallow). An important finding is that performance diminishes with larger storm volumes. The implication of this observation is that under-sizing a biofilter will result in lower performance. Whilst no absolute recommendation can be made, it is clear that a biofilter with an area of 4% of its catchment area will perform better than one of 2%. Final specification will be a compromise between performance and available space. One modification which has been demonstrated (Zinger et al., 2007) to be very effective for treatment of both TN and TP is to add a sand-based anaerobic layer in the bottom of the biofilter (of between 300 and 600mm, depending on available depth), with a small amount of organic carbon (the quantity being calculated according to required C:N ratio to achieve optimal denitrification) 4.2 Future research The results of this study have given some very useful insights into optimal biofilter designs for the removal of sediment and nutrients. However, a few important questions remain. For example, the results to date represent biofilters which are approximately 14 months old (6 months of establishment and 8 months of sampling). The columns will be tested for another six months, to determine if the differences between configurations remain constant. Whilst we have demonstrated the potential for biofilters to be effective in nutrient removal, we believe that further optimization of nitrogen and phosphorus treatment is possible. In particular, we plan to test whether retrofitting a permanently saturated anaerobic zone to the base of the biofilter can achieve the same improvements in nitrogen removal (without diminishing phosphorus treatment) shown by Zinger et al. (2007) in custom-made systems. We will also test whether plants which performed poorly during this watering regime, perform better once less water is applied. We also plan to investigate the specific role of the plant roots in determining the nutrient removal performance, and to quantify the evapotranspiration loss in biofilters, in order to determine their potential for addressing not only the water quality impacts of urban stormwater, but also the hydrologic impacts. 5.0 Conclusion Vegetated soil-based biofilters are shown to be effective for treatment of stormwater, reducing inflow TSS and TP concentrations by approximately 98 and 80% respectively. Removal of nitrogen of at least 50% is also achievable, but depends on vegetation selection. For both nitrogen and phosphorus, Carex is shown to be a very effective choice, whilst Melaleuca shows signs, after taking some time to mature, of also facilitating high levels of N and P removal. Selecting filter media which do not have excessive levels of organic matter will help

8 to prevent leaching of phosphorus. Importantly also, biofilter performance will be reduced if the biofilter is small relative to its catchment. Whilst no definitive guidance is yet available, a sizing of around 2% of the catchment area appears to give satisfactory performance for all of TSS, TP and TN. Further research is being undertaken to increase nutrient removal, particularly through the retrofitting of an anaerobic zone to poorly-performing biofilters. 6.0 References Commonwealth of Australia. (2002). The value of water: Inquiry into Australia's urban water management. Report of the Senate Environment, Communications, Information Technology and the Arts Reference Committee. Canberra, Australia: The Parliament of the Commonwealth of Australia. Davis, A. P., Shokouhian, M., Sharma, H., & Minami, C. (2001). Laboratory study of biological retention for urban stormwater management. Water Environment Research, 73(5), Denman, L., Breen, P. F., & May, P. B. (2006, April 3-7th, 2006). WSUD in local government - implementation guidelines, insitutional change and creating an enabling environment for WSUD adoption. Paper presented at the 7th Urban Drainage Modelling and 4th Water Sensitive Urban Design Conference, Melbourne, Australia. Vol. 2, pp Duncan, H. P. (2006). Urban stormwater quality. In T. H. F. Wong (Ed.), Australian Runoff Quality. Sydney, Australia: Institution of Engineers, Australia (available from Hatt, B. E., Deletic, A., & Fletcher, T. D. (2007). Stormwater reuse: designing biofiltration systems for reliable treatment. Water Science and Technology, 55(4), Hatt, B. E., Siriwardene, N., Deletic, A., & Fletcher, T. D. (2006). Filter media for stormwater treatment and recycling: the influence of hydraulic properties of flow on pollutant removal Water Science and Technology, 54(6-7), Hunho, K., Seagren, E. A., & Davis, A. P. (2003). Engineered bioretention for removal of nitrate from stormwater. Water Environment Research, 75(4), Le Coustumer, S., Fletcher, T. D., Deletic, A., & Barraud, S. (2007). Hydraulic performance of biofilters: first lessons from both laboratory and field studies. Paper presented at the Novatech, Lyon, France, June 24-28, Lloyd, S. D., Fletcher, T. D., Wong, T. H. F., & Wootton, R. (2001, June 2001). Assessment of pollutant removal in a newly constructed bio-retention system. Paper presented at the Second South Pacific Stormwater Conference, Auckland. Vol. 1, pp Sharkey, L. J., & Hunt, W. F. (2005). Hydrologic and water quality performance of four bioretention cells in central north Carolina. Paper presented at the Managing Watersheds for Human and Natural Impacts; Engineering, Ecological and Economic Challenges, Williamsburg, Virginia, USA. pp Smith, S. E., & Read, D. J. (1997). Mycorrhizal symbiosis (2nd ed.). Cambridge, UK: Academic Press. Taylor, G. D., Fletcher, T. D., Wong, T. H. F., & Breen, P. F. (2005). Nitrogen composition in urban runoff - implications for stormwater management. Water Research, 39(10), Taylor, G. D., Fletcher, T. D., Wong, T. H. F., & Duncan, H. P. (2006). Baseflow water qualty behaviour: implications for wetland performance monitoring. Australian Journal of Water Resources, 10(3), Zar, J. H. (1999). Biostatistical analysis (4th ed.). New Jersey, USA: Prentice Hall. Zinger, Y., Fletcher, T. D., & Deletic, A. (this volume). The recovery of nitrogen removal after different drying periods in biofilter systems. Paper presented at the Conference on Rainwater and Urban Design, Sydney, NSW, August, Zinger, Y., Fletcher, T. D., Deletic, A., Blecken, G. T., & Viklander, M. (2007). Optimization of the nitrogen retention capacity of stormwater biofiltration systems. Paper presented at the Novatech, Lyon, France, June 24-28, 2007.