pH Buffering in Stormwater Infiltration Systems-Sustainable Contaminant Removal with Waste Mussel Shells

Storm runoff is a major vector for transporting urban contaminants, especially metals, and continues to be a leading cause of urban waterways degradation. Stormwater treatment systems in New Zealand and Australia are primarily designed to remove total suspended solids and heavy metals to low levels, principally through bioinfiltration. In Christchurch, the second largest city in New Zealand, more than two thirds of the water, including stormwater, infrastructure is currently being rebuilt following the devastating 2010-2011 earthquakes. Despite increased use of bioinfiltration systems for this purpose, there is a dearth of knowledge about their treatment performance or water quality dynamics. This paper reports enhanced treatment efficacy in bioinfiltration stormwater systems by including an alkaline waste product, mussel shells, in the substrates. Experimental systems with mussel shells significantly increased the metal removal efficacy, hardness, and pH, which also have implications for reducing the potential ecotoxicological effects of stormwater. Mussel shell systems resulted in lower dissolved metal fractions in the treated effluent because metals shifted to the particulate states facilitated by hardness buffering. This resulted in greater metal removal afforded by increased filtration. Using locally available waste products can reduce the amount and transport impacts of waste going to landfills and offset costs associated with the construction of stormwater treatment systems, while concurrently improving stormwater treatment. The long-term capacity of such systems to enhance metal removal using waste mussel shells should be examined by monitoring larger pilot-scale systems in situ under different seasonal events.