Why stormwater biofilters?
Treament of stormwater pollution and retention of volumens and peak flows becomes an increasingly important issue in urban drainage. One sustainable solution to face this challenge is the implementation of stormwater biofilters - rain gardens.
A typical stormwater biofilter consists of a vegetated swale or basin, underlain by a filter medium, often sandy loam. Stormwater infiltrates, is filtered by the vegetation and filter media. The treated water is either infiltrated into the surrounding soil or collected in a drainage pipe at the bottom of the filter and then discharged to a recipient or the existing sewer system. The increased implementation of rain gardens would contribute to the development of sustainable cities.
The performance of a biofilter is dependent on the capacity of the system to remove pollutants (quality aspect) and the hydraulic conductivity (quantity aspect). These attributes are influenced by the physical, chemical and biological processes in the biofilter. Furthermore, biofilters should be architectonically integrated into the streetscape.
It was shown that rain gardens are able to remove significant pollutant loads from both stormwater and snowmelt water even under cold weather conditions.
The development of reliable stormwater treatment technologies for cold weather regions is particularly important due to high contamination levels in winter runoff. Thus, our results are very important for an increased implementation of biofilters in cold and temperate climates with winter conditions since our study is the first one worldwide about cold weather performance of rain gardens.
We tested the rain garden treatment performance in cold temperatures using column tests in thermostat-controlled climate rooms at Luleå University of Technology. Furthermore, at NTNU experiments have been conducted at the pilot rain garden facility.
Our results show that sediment, phosphorus and total Cd, Cu, Pb and Zn are removed excellently by biofilters even at cold temperatures (removal often in excess of 95%).
Most often even dissolved metals are removed well. However, in some cases low removal and seldom even leaching of dissolved metals was detected. Anyway, rain gardens are clearly more effective in treating dissolved metals than many comparable technologies as e.g. stormwater ponds.
Only nitrogen was not treated well; neither in cold nor in warmer temperatures. However, introducing a submerged zone might solve this problem; though the effect of colder temperatures on nitrogen treatment in the submerged zone has not been investigated yet.
Fate of metals in the filter
Our results confirmed the finding that sediment, metals and particle bound stormwater pollutants do not ingress very far into the filter media but are trapped close to the top of the filter.
This is very important for
• Design recommendations: consequently, a lower filter depth than the commonly recommended 80 - 90 cm is sufficient which minimises e.g. construction costs.
• Filter maintenance: By scraping and replacing ca. the upper 10 cm of the filter material a high fraction of the already retained metals can be removed and thus replacing the whole filter material can be delayed.
Introducing a submerged zone at the bottom of the filter involves several benefits for biofilter treatment.
Our results have shown that the submerged zone combined with a carbon source
• enables effective nitrogen treatment by promoting combined nitrification and denitrification due anoxic conditions in the submerged zone. Oxic conditions in standard biofilters do not support denitrification and thus nitrogen treatment is rather bad.
• even improves metal treatment. Especially the relatively poor Cu treatment was elevated since Cu absorption was enhanced due to increased formation of solid Cu-organic matter complexes
• minimizes the negative effects of extended dry periods on the treatment
A submerged zone can even be retrofitted into existing biofilters. Those can thus be upgraded if they are exposed to dry conditions or if nitrogen treatment is insufficient.
Intermitted drying / wetting
Extended drying reduces metal treatment. However, this effect can be minimised by introducing a submerged zone.
Our research showed that extended drying of more than about 3 to 4 weeks reduces the metal treatment performance of rain garden systems. This is especially important for rain garden implementation in dry countries as e.g. Australia where effective treatment is of special concern due to lack of water.
However, we could show that introducing a submerged zone can considerably reduce (for Cu and Zn treatment) or even eliminate (for Pb treatment) the negative effects of extended drying.
The research was conducted in close collaboration with the Norwegian University of Science and Technology (Tone Muthanna), Trondheim, Norway and Monash University, Civil Engineering, Facility for advancing water biofiltration (Yaron Zinger, Ana Deletic and Tim Fletcher), Melbourne, Australia.
The Rain Garden project was supported by:
Stiftelsen J.Gust.Richerts minne
Staff at Luleå University of Technology
Papers in international journals
Blecken, G.-T., Zinger, Y., Deletic, A., Fletcher, T. D., Viklander, M., (2010): Laboratory study on stormwater biofiltration: Nutrient and sediment removal in cold temperatures. Journal of Hydrology 394 (3-4), 507-514. DOI: 10.1016/j.jhydrol.2010.10.010
Blecken, G.-T., Marsalek, J., Viklander, M. (submitted): Laboratory study of stormwater biofiltration in low temperatures: total and dissolved metal removals and fates. Submitted to Water, Air and Soil Pollution. Accepted for publication after minor revision.
Blecken, G.-T., Zinger, Y., Deletic, A., Fletcher, T. D., Viklander, M. (2009). Impact of an anoxic zone and a carbon source on heavy metal removal in stormwater biofilters. Ecological Engineering 35 (5), 769-778. DOI: 10.1016/j.ecoleng.2008.12.009
Blecken, G.-T., Zinger, Y., Deletic, A., Fletcher, T. D., Viklander, M. (2009). Influence of intermittent drying and wetting conditions on heavy metal removal by stormwater biofilters. Water Research 43, 4590-4598. DOI: 10.1016/j.watres.2009.07.008
Muthanna, T.M., Viklander, M., Thorolfsson, S.T. (2008). Seasonal climatic effects on the hydrology of a cold climate rain garden. Journal of Hydrological Processes 22 (11), 1640-1649. DOI: 10.1002/hyp.6732
Blecken, G.-T., Viklander, M., Muthanna, T. M., Zinger, Y., Deletic, A., Fletcher, T. D. (2007). The influence of temperature on nutrient treatment efficiency in stormwater biofilter systems. Water Science and Technology 56 (10), 83-91. DOI: 10.2166/wst.2007.749
Muthanna, T.M., Viklander, M., Gjesdahl, N., Thorolfsson, S.T. (2007). Heavy metal removal in cold climate bioretention. Water, Air, and Soil Pollution 183, 391-402. DOI: 10.1007/s11270-007-9387-z
Muthanna, T.M., Viklander, M., Thorolfsson, S.T. (2007). An evaluation of applying existing bioretention sizing methods to cold climates with snow storage conditions. Water Science and Technology 56 (10), 73-81. DOI: 10.2166/wst.2007.745
Muthanna, T.M., Viklander, M., Blecken, G-T., Thorolfsson, S.T. (2007). Snowmelt pollutant retention in bioretention areas. Water Research 41 (18), 4061-4072. DOI: 10.1016/j.watres.2007.05.040
Blecken, G.-T., Zinger, Y., Deletic, A., Fletcher, T. D., Viklander, M. (2010). Laboratory studies on metal treatment efficiency of stormwater biofilters. International Short Course: Advances in Knowledge of Urban Drainage from the Catchment to the Receiving Water - Technical Solutions in Stormwater Management. June 2010. University of Calabria, Rende, Italy.
Blecken, G.-T., Zinger, Y., Deletic, A., Fletcher, T. D., Viklander, M. (2010). Effect of retrofitting a saturated zone on the performance of biofiltration for heavy metal removal - preliminary results of a laboratory study. NOVATECH 2010, Lyon, France
Blecken, G.-T., Zinger, Y., Deletic, A., & Fletcher, T. D., Viklander, M. (2008). Heavy metal removal by stormwater biofilters: can it withstand alternative drying and wetting conditions? 11th ICUD conference, Edinburgh, Scotland, UK
Blecken, G.-T., Viklander, M., Muthanna, T. M., Zinger, Y., Deletic, A., Fletcher, T. D. (2007). Biofilter treatment of stormwater: temperature influence on the removal of nutrients. NOVATECH 2007, Lyon, France.
Zinger, Y., Blecken, G.-T., Fletcher, T. D., Deletic, A., Viklander, M. (2007). Optimisation of the nitrogen retention capacity of stormwater biofiltration systems. NOVATECH 2007, Lyon, France.
Blecken, G.-T. (2010). Biofiltration technologies for stormwater quality treatment. Doctoral thesis. LTU.