Using biochar to purify runoff in road verges of urbanised watersheds: A large-scale field lysimeter study

Urban runoff from traffic areas is a major source of pollution that degrades the quality of adjacent surface waters. Green infrastructure provided by the substantial amount of roadside land at urban fringe areas can be used to better manage and infiltrate this urban runoff. When establishing urban green areas, recycled materials should be preferred in order to achieve economically feasible and environmentally responsible solutions. Wastewater treatment plants within e.g. the EU yield substantial amounts of solids containing nutrients and metals that need to be utilised in a sustainable way. However, soil composted from such sewage sludge is being used widely in constructing green infrastructure, such as parks and road verges, which may jeopardise their use for stormwater management even though the effects of sewage sludge at road verges on the quality of runoff waters have not been subjected to scientific examination. Biochar has been suggested to retain pollutants and may also meet the criteria of being recycled material. We established artificial biofilter structures, mimicking road verges, in large-scale field lysimeters under cold climatic conditions in southern Finland to study the ability of biochar to retain pollutants leaching from composted sewage sludge and from infiltrating artificial stormwater. The topmost 15 cm consisted of an organic layer of either natural peat (Peat) or soil composted from sewage sludge either mixed with birch-derived biochar (3% by volume) or without this biochar (Comp+bc and Comp, respectively). At the end of the 1st growing period grasses growing in the lysimeters had taken up to 32% of phosphorus in the top soil containing compost. Leaching of phosphorus did not differ between the treatments, while nitrogen (N) leaching was ten time larger from Comp than Peat. Leaching of heavy metals, such as nickel and copper, was also significantly higher from compost soils than peat, but biochar significantly reduced metal leaching from compost (up to 50%). Two years after establishment, lysimeters were irrigated with artificial stormwater mimicking runoff from roads with heavy traffic. Comp leached more N compared to Peat. However, biochar significantly reduced N load infiltrated through the compost by 44%. Mixing sewage sludge-originated compost with biochar, and adding a 5 cm thick layer of biochar underneath the organic soil layer can substantially reduce leaching of N and heavy metals. However, given the substantial amount of roadsides in urban fringe areas, the extensive use of sewage sludge and other N-rich materials in such areas should be considered carefully.