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Pumping sediment-laden water from excavations is often necessary on construction sites. This water is often treated by pumping it through geotextile dewatering bags. The bags are not designed to filter the fine sediments that create high turbidity, but dosing with a flocculant prior to the bag could result in greater turbidity control. This study compared two systems for introducing flocculant: passive dosing of commercial solid biopolymer (chitosan) and injection of dissolved polyacrylamide (PAM) in a length of corrugated pipe connected to the bag. The biopolymer system consisted of sequential porous socks containing a “charging agent” followed by chitosan in the corrugated pipe with two levels of dosing. The dissolved PAM was injected into turbid water at a flow-weighted concentration at 1 mg L −1 . For each treatment, sediment-laden turbid water in the range of 2000 to 3500 nephelometric turbidity units (NTU) was pumped into the upstream of corrugated pipe and samples were taken from pipe entrance, pipe exit, and dewatering bag exit. Without flocculant treatment, the dewatering bag reduced turbidity by 70% but the addition of flocculant increased the turbidity reduction up to 97% relative to influent. At the pipe exit, the low-dose biopolymer was less effective in reducing turbidity (37%) but it was equally effective as the high-dose biopolymer or PAM injection after the bag. Our results suggest that a relatively simple treatment with flocculants, either passively or actively, can be very effective in reducing turbidity for pumped water on construction sites. [ABSTRACT FROM AUTHOR]

Due to stringent water quality regulations on stormwater discharges, there is increasing interest in chemically-assisted settling of suspended sediments at construction sites. This study investigated settling characteristics of flocculated sediment by polyacrylamide (PAM) in a top-loading settling tube. Studied sediment materials were obtained from construction sites in North Carolina, USA: Coastal Plain loamy sand (CPLS), Piedmont sandy clay loam (PSCL), Piedmont silt loam (PSL), and Mountain clay loam (MCL). The four different sediment suspensions mixed with and without dissolved PAM were introduced to the top of the column individually. During a 1-h settling period, samples were taken at 1-m depth from surface at various times and analyzed for total suspended solids (TSS). Flocculated sediment by PAM greatly increased its settled TSS fraction up to 95–97% only in 1-min settling period compared to those of unflocculated sediment (16–72%). The settling improvement by PAM was profound in the finer-textured soils (PSL and MCL) by increasing their median particle settling velocity (>2 cm s −1 ) compared to unflocculated counterparts (<1.1 cm s −1 ). Estimated surface area requirement of sediment basin suggested that the basins receiving flocculated sediment could be reduced in size (surface area) by 2- to 4-times compared to those receiving unflocculated sediment. Our results suggests that current sediment basin design could be modified when chemically-assisted settling is implemented, taking up less space and cost in construction sites. [ABSTRACT FROM AUTHOR]

Abstract: High levels of turbidity and fine suspended sediments are often found in stormwater discharges from construction sites even when best management practices (BMPs) for sediment control are in place. This study evaluated turbidity reduction by three check dam types: 1) rock check dam representing a standard BMP, 2) excelsior wattle representing a fiber check dam (FCD), and 3) rock check dam wrapped with excelsior erosion control blanket (rock + excelsior ECB) representing an alternative FCD. Three check dams (all same type) were installed in a lined, 24-m ditch on a 5–7% slope and three consecutive simulated stormwater flows were run in the ditch. Additional tests were performed by adding granular polyacrylamide (PAM) on the check dams in the same manner using two sediment sources differing in clay content. Without PAM treatment, significantly higher effluent turbidity (>900 nephelometric turbidity units (NTU)) exited the ditch with rock check dams than with excelsior wattles or rock + excelsior ECBs (<440 NTU). The extent of sediment deposition between the check dam types was in the order of excelsior wattle > rock + excelsior ECB > rock check dam, indicating better water pooling behind the wattle. The PAM treatment reduced turbidity substantially (>75% relative to no PAM treatment) for all check dam types and it was very effective in excelsior wattles (<57 NTU) and rock + excelsior ECBs (<90 NTU) even during the third storm event. This study demonstrates that the passive treatment of runoff with PAM on FCDs (or rock + excelsior ECB) in construction site ditches can be very effective for sediment retention and turbidity reduction. [Copyright &y& Elsevier]