Abstract
Recent studies have shown that turbidity in construction site runoff can be greatly reduced by chemical turbidity control. This study evaluated the performance of chitosan-based biopolymer (dual polymer system, DPS) vs. anionic polyacrylamide (PAM) for turbidity reduction and characteristics of flocculated sediments using two soils from North Carolina, USA. The soils were Coastal Plain sand (CPS) and Piedmont Sandy loam (PSL), representing smectitic and kaolinitic mineralogy, respectively. A series of jar tests for DPS (charging agent + chitosan) and two commercial PAM products were conducted to find optimal concentration for turbidity reduction in the respective soil suspension (20 g L−1 in soil loading). After determining the optimal flocculant concentrations, the soil suspensions treated with DPS, PAM, and no flocculant (control) were investigated for turbidity change over settling time, floc stability, floc growth, settleable solids through Imhoff cone test, and particle size distribution by a laser diffraction method. Both flocculants were effective in reducing turbidity (> 90%) in PSL suspensions while PAM outperformed DPS in CPS suspension. Settleable solids volumes increased with flocculant treatments by 23–41% relative to untreated soil suspensions (13 mL for CPS and 20 mL for PSL), indicating efficacy of flocculant-assisted particle settlings. PAM-treated particle size was greater (115 µm, median diameter) than DPS (84 µm) in PSL suspension, both being 3–5 times greater than untreated suspension (24 µm). Repeated stirring resulted in floc growth with PAM but not with DPS. Our results suggested that practitioners using flocculants to treat turbid water in construction site need to perform tests with different flocculants to determine the optimal treatment on their project. While PAM has advantage over DPS as a single flocculant treatment, DPS could be an alternative due to its eco-friendly features.
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