More effective solution to remove stormwater pollutants

Biofilters with indigenous South African plants such as palmiet, mat sedge, soft rush, cobra lily and vleibiesie are more efficient than unvegetated soils in removing pollutants from urban stormwater which can have a detrimental impact on aquatic systems, posing a major human and environmental health risk.

This is one of main findings of a recent study at Stellenbosch University (SU) conducted by Dylan Jacklin, Isobel Brink, both from the Division of Water and Environmental Engineering in the Department of Civil Engineering) and Shayne Jacobs from the Department of Conservation Ecology and Entomology.

The study is the first to investigate nutrient and metal removal efficiency of indigenous South African plant species. The findings were published recently in the journal AQUA.

“We found that biofilters with certain indigenous plants consistently removed greater pollutant loads than unvegetated soil for the duration of our experiment. These biofilters were significantly more efficient than unvegetated soil in the removal of both nutrients and metals.

“In addition, the efficiency of unvegetated soil to remove metals from the water declined over time, whilst this was not the case with the biofilters,” says lead researcher Mr Jacklin.

He and his co-authors investigated the individual efficiencies of nine indigenous plant species and unvegetated soil exposed to varying concentrations (loads) of nutrients and metals based on published figures of stormwater runoff pollution concentrations and loads.

The researchers say biofiltration is a form of ecological engineering that utilises plants to detoxify, degrade and/or remove pollutants from the environment.

“The use of plant biofiltration has become increasingly popular within green infrastructure (GI) due to its ability to slow stormwater runoff rates, reduce runoff volumes, decrease sediment transport (caused by, among others, erosion from failing conventional infrastructure systems) and retain pollutants prior to discharge into watercourses.

“In contrast to conventional infrastructure approaches that typically consider stormwater as a substance to dispose of rather than a resource to protect, improved urban stormwater management seeks to treat nonpoint source pollution, reduce hydrologic disturbance and utilise stormwater as a supplementary resource.”

As part of their study, the researchers constructed 90 biofilter columns from PVC piping based on typical recommended urban design guidelines. Within the columns, a drainage outlet was covered by a 70 mm drainage zone below a 100 mm layer of coarse sand to prevent the clogging of sediment in the drainage outlet.

This was then topped with a 500 mm mixture of loamy sand, perlite and compost in which the plants could grow. Within this layer, with time, a biofilm forms which helps to improve the ability of the plant biofilters to remove pollutants.

The inner wall of each column was abraded to “force” the water to flow/infiltrate through the mixture of loamy sand, perlite and compost, thus near the plant roots. Each column was also sealed at the base, allowing effluent extraction for analyses.

The columns were flushed with five litres of tap water per day to stimulate the discharge of excess nutrients and metals which may have lingered within the columns. Plants that could mature rapidly during the experimental period and could tolerate inundation and periods of drought were used in the study.

Regarding their results, the researchers say unvegetated soil biofilters consistently removed the lowest amounts of nutrient and dissolved metal loads, with an average pollutant removal percentage of 45,7%.

“All the vegetated biofilters outperformed unvegetated soil for mean removals, with the worst performer being the cobra lily, which had a mean removal of 56,3%, only 10,6% greater than unvegetated soil.

“The most efficient biofilters (based on average percentage load removals) were palmiet, soft rush and mat sedge, removing on average 89,8, 85,3 and 78,6% of urban stormwater pollutant loads, respectively.

“For the removal of ammonia-nitrogen and orthophosphate (the simplest and most common form of phosphorus in water), all vegetated biofilters outperformed unvegetated soil.”

They add that more dissolved metals were removed than nutrients.

The researchers say that given their general removal capacity, indigenous South African plants should be included in sustainable GI initiatives to reduce nutrient and metal loads in urban stormwater runoff.

“The country’s extensive natural biodiversity offers untapped potential of indigenous species’ use in plant biofilters.”

According to the researchers, small-scale biofilters can be integrated in local GI systems throughout a spatially limited urban area because shallow biofiltration systems are more easily constructed and potentially homogenous with existing drainage infrastructure.

This adaptation in urban planning and design is effective, yet sensitive to issues of water protection and environmental conservation, they add.