Design Guidance and Development of Construction Stormwater Technologies

Abstract

As one of the largest sources of nonpoint source pollution, soil erosion has become an increasingly important topic in the U.S. Construction sites. Erosion and sediment control (ESC) practices are commonly used on construction sites to reduce soil erosion and sediment discharge. To avoid eroded sediment discharging into receiving surface water bodies, the environmental protection regulations and technologies are developed by federal, state, and local agencies to capture the eroded sediments from the construction stormwater. The regulations contain the design, installation, inspection, and maintenance of ESC practices and the technologies include their innovation and application in the industry. This dissertation explores the improvements of design guidance of ESC and construction stormwater technologies through the development of SILTspread: a silt fence design tool, bench scale tests of different lamella settlers and electrocoagulation technology, the innovation of ESC technology corresponding to the application of flocculants in the lamella settlers. Erosion and sediment controls, typically passive “best management practices” (BMPs), are the most popular methods used to eliminate soil erosion and reduce the environmental impacts. As a type of BMPs, silt fence retains soil particles from disturbed areas by forming the impoundments. A tool was developed to assist silt fence design. The tool simplifies the application of the silt fence design approach by incorporating the automation of hydrologic design calculations, silt fence segment volumetric sizing, and estimation of maintenance needs. This developed tool is applicable for various types of projects and locations, due to the ability to manually enter the basic site-specific data under a 2-yr, 24-hr rainfall event. In addition to improved design guidance, traditional erosion and sediment control practices can benefit from advanced treatment mechanisms used in other water treatment operations. For example, lamella settler and electrocoagulation technologies. Lamella settlers (LSs) are a type of water treatment system that consist of a set of inclined plates installed for wastewater treatments. This research sought to bridge the gap between theoretical knowledge and practical application of lamella settlers by conducting experiments to determine the settling performance across several design factors. A Full-Factorial Method (FFM) statistical analysis was conducted to estimate influence of different design factors (e.g., sediment concentration, particle settling distance, and residence time) on efficiency of evaluated treatments. The design factors were independent variables used for analysis, and the calculated turbidity reduction rates between influent and effluent water samples were dependent variables. The results of this research indicated that the optimal turbidity removal rates for all five soil samples was achieved through the use 1.5 hr residence time in RC (with 0.18 cm settling distance provided by a 1.27 cm of plate spacing). The calculated turbidity removal rates for different types of soils were correlated corresponding to the calculated settling velocities and measured particle sizes Dx. The higher turbidity removal rates were found when the settling velocity and Dx reduction were higher in the soil sample. In statistical approach, a model was developed based on turbidity reductions to assist designers by changing the values of design parameters (inflow concentration, settling distance, and residence time) to meet the desired turbidity of outflow at the design stage. Ultimately, results obtained from this research effort provide design guidance for developing field-scale lamella systems to treat polluted stormwater runoff. Electrocoagulation is a water-treatment technology that uses an electrochemical anode corrosion process to destabilize and remove contaminants. Electrocoagulation has been shown to have higher contaminant removal efficiency than conventional coagulation and has widespread applications for treatment of a variety of wastes. This research determines and evaluates the optimal design parameters of EC through the observations of turbidity measurements. According to the results of turbidity reduction rates, the optimal condition is using aluminum electrodes for 0.75 min. at 2 cm (0.8 in.) cell spacing and 39 A/m2 (3.6 A/ft2) current density with 90 min. residence time after 10 sec of rapid mixing. Besides the ability of turbidity removal, another factor considered was energy requirements for different metal cells. In this study, the total cost considers material, cell degradation and electric power for different metals. According to the results of turbidity removal, aluminum was the optimal cell material. Further research is necessary to investigate how to improve the sediment removal performance of lamella settlers and EC. Combining these two treatment mechanisms has the potential to decrease the required size of the lamella settler while simultaneously maximizing the sedimentation process. In this research, bench-scale experiments were conducted using electrocoagulation as pretreatment for a lamella settler reactor. Synthetic silica filler at concentrations of 500 mg/L, 1,000 mg/L, and 5,000 mg/L were used to evaluate treatment efficiency at 0.5-h, 1.0-h, and 1.5-h residence times. The collected data, including turbidity, total suspended solids, and particle size distribution were used to statistically characterize the system’s sediment removal performance. It was found that an optimized electrocoagulation lamella settler reactor with 1.27-cm (0.5 in) plate spacing and 1.5-h residence time reduced turbidity by up to 98% and total suspended solids by 99% in the effluent when compared to the base condition (using a LS with 31.8 cm [12.5 in.] plate spacing for 0.5-hr residence time without the pretreatment of the electrocoagulation). In addition, particle size distribution analyses indicated a decrease in the D90 value by 84%, indicating that the optimized reactor was effective in capturing larger-diameter soil particles. To validate laboratory results with synthetic sediment-laden influent, stormwater samples were collected from a construction site and treated in the developed EC+LS system. Turbidity and TSS reduction rates of the field-collected stormwater runoff were 50% and 66%, respectively. In addition, the field collected samples had a Na reduction of 51%, indicating a potential application for treating urban stormwater with high levels of deicing materials. Compared to chemical-based flocculation, the EC+LS system demonstrated similar capability in removing sediment. Through the outcomes of these four research studies, the designs of ESC were analyzed and improved by investigate new tool and technology. These improvements are sought to bridge the gap between theoretical knowledge and industrial application of ESC practices. This dissertation provides several recommendations for the design of ESC practices and introduces new technology to assist industry professionals for the field implementation.