An innovative method for estimating settling velocity of particles in stormwater using absorbance measurements and modelling

The purpose of the study was to develop a simple and rapid method for measuring the average settling velocity of particles in stormwater ponds. Water samples with suspended particles were collected from the bottom of a stormwater pond in Lund, Sweden. The absorbance in various samples was measured over 24 h (at 600 nm wavelength) and then translated into total solids concentrations. A one-dimensional model based on the Mason-Weaver equation was coded in Python and solved. The absorbance measurements were used to calibrate the model, thereby quantifying settling velocities and dispersion coefficients for samples from different zones of the stormwater pond. The quantified settling velocities ranged from 40 to 200 mm/h for fast-settling particles and from 2 to 8 mm/h for slow-settling particles. The developed methodology provided a mass estimate of the two modelled particle groups (i.e., fast- and slow-settling particles). Based on the model estimates, fast-settling particles dominated all samples, constituting 70-90% of the total solid mass. Due to its simplicity and inexpensiveness, this methodology is a potential alternative to more demanding and complicated methodologies used for measuring particle velocities in sedimentation systems.