Detection of Residual Flocculant Concentrations in Construction Stormwater Runoff

Abstract

Flocculants provide an innovative solution for managing construction stormwater runoff with their efficiency in reducing turbidity in the effluent. With proper application and dosage, these chemicals improve the performance of sediment control practices in capturing fine-sized soil particles. Flocculants function by providing electromagnetic charges to attract soil particles into large molecular bridges of flocs. Following this process, suspended particle removal can be achieved through gravitational forces. Despite the benefits of using flocculants, their implementation may create a potential risk of polluting downstream waterbodies and harming aquatic life in case of a high residual concentration in discharge. Determining residual concentrations is challenging for practitioners because of a lack of field applicable testing methods that work across a variety of flocculant types. This study developed a field applicable methodology for detecting residual flocculant concentrations by using settling velocity as an identifier of flocculant concentration. In total, 14 products comprised of polyacrylamide, sodium montmorillonite, chitosan, agricultural gypsum, and alum-based products were evaluated. Known flocculant concentrations ranging from 0% to 30% of the manufacturer’s recommended dosage, were mixed with a fine soil passing through No. 200 sieve and allowed to settle in a graduated cylinder to record the settling velocity. Regression analysis was conducted on the experimental data to develop concentration versus settling velocity relationships which indicated that more than 90% of known concentration values are dependent on the settling velocity. The study provides a framework for practitioners to identify residual flocculant concentrations in field conditions by relating settling velocity with chemical concentrations.