The hydraulic characteristics of a subsurface flow stone biofilter for treating polluted runoff from an informal settlement

Polluted runoff from informal settlements in developing countries poses a growing challenge due to the elevated and variable nature of contaminants, particularly nutrients and pathogens, introduced to the environment. Cost-effective and scalable treatment systems with the ability to reduce nutrient and other pollutant concentrations in contaminated runoff are desirable. Biofilters are passive water treatment systems that have the potential to achieve this. The Franschhoek Water Hub, a research site for nature-based solutions, features six large biofiltration cells designed to remediate runoff from an informal settlement. Due to their large size, understanding hydraulic behaviour and validating the design proves challenging. To address this, a scaled-down version of the Water Hub's biofilters was constructed to inform design criteria for purpose-built filters. The pilot-scale subsurface flow biofilter, filled with 8–11 mm aggregate stone, had an available volume of 225 L. Pulse tracer studies conducted at various flow rates demonstrated that the system approximated plug flow behaviour. Lower flow rates resulted in deeper tracer infiltration, which is crucial for maximising the distribution of nutrients within the filter bed. This research contributes to the effective design and operation of biofiltration systems, which hold promise for addressing surface water contamination issues in resource-constrained regions. HIGHLIGHTS Pulse tracer studies at various flow rates on a pilot-scale subsurface flow biofilter, mimicking the design of a field-scale system, investigated fluid flow behaviour in a field-scale system.; The extent of plug flow behaviour and degree of mixing in the radial direction increased with decreasing flow rate.; Low operational flow rates ensure greater utilisation of the active filter bed.;