Improvements in Construction Stormwater Treatment using Flocculants

Abstract

Construction stormwater runoff constitutes an increased risk for downstream water bodies if unmanaged sediment-laden discharge exits a construction site. Federal and state regulations emphasize the significance of erosion and sediment controls on job sites and require the implementation of effective stormwater pollution prevention plans. Regulations aim to prevent impairment of receiving waterbodies by requiring the management of construction stormwater with proper design, implementation, and maintenance of erosion and sediment control practices.

Temporary sediment control practices are designed to capture sediment particles and reduce the turbidity of discharge; however, commonly used sediment control practices have limited performance in capturing fine-sized sediment particles. Flocculants are chemicals that can be introduced to construction stormwater runoff to enhance the performance of the sediment control practices by improving the capture of suspended sediment. These chemicals create a bridging mechanism between particles to form larger flakes and facilitate settlement. Although flocculants can be highly effective in reducing turbidity, improper dosing may risk polluting downstream water bodies and may create risks for aquatic life. The effectiveness of flocculants for stormwater management has been investigated; however, a large knowledge gap exists on guidance for application rates and dosage for construction site applications.

This dissertation explores practical methods to enhance guidance for proper selection, use, and application of flocculants in construction stormwater management by developing design guidance on dosage rates and application techniques. This research evaluates construction stormwater treatment with flocculants through (1) state of the practice survey, (2) soil assessments, (3) bench-scale experiments, (4) flume experiments, and (5) large-scale evaluations.

The use of flocculants has been adapted by several State Departments of Transportation within the past decade on active construction sites to capture fine-sized sediment particles and minimize construction stormwater-related pollution in downstream waterbodies. However, the perception of agencies on flocculants varies due to the existing knowledge gap on flocculant usage and potential environmental consequences of overdoses. A state-of-the-practice survey was conducted to understand the current perspective of state agencies on flocculant usage and identify specific concerns and guidance needs. Survey results indicated that only 39% of state departments of transportation allow flocculant usage on construction sites. The majority of these agencies (55%) follow manufacturer guidance on dosage and the most common concern for flocculant dosage is the potential risk of polluting downstream waterbodies and damaging aquatic life. The dissertation details the methodology for identifying the performance of different flocculant types across various soil samples collected from named map units through bench-scale experiments for providing guidance on dosage and product selection. In total, 14 different products were used for bench-scale experiments, which included polyacrylamide, bentonite, sodium montmorillonite, alum, agricultural gypsum, and chitosan-based flocculants. Best performing products for 15 unique soils were identified with a match test study, which ranked products based on their performance. Following match test experiments, dosage experiments were conducted by ranging manufacturer recommended concentration values from 0% to 200% for observing the behavior of flocculants in underdose and overdose conditions. Results indicated that polyacrylamide and chitosan-based products work most effectively across the 15 tested soil samples compared to other tested products. Testing results also showed the potential of flocculants to perform well in underdose conditions and increase turbidity in overdose conditions.

Monitoring flocculant concentrations in discharge provide a supportive control mechanism to prevent possible overdoses and maintain proper dosage throughout flocculant applications on sites. However, only 23% of state agencies surveyed require monitoring residual flocculant in downstream water bodies. A field applicable residual concentration detection method was developed by using a turbid water sample with a specific testing soil. Settling velocities of each product were correlated with known concentration injections ranging from 0% to 30% of manufacturer dosage recommendation and standardized residual concentration plots were formed. Optimum dosage delivery mechanisms were evaluated through flume experiments by using block, sock, granular, and stock solution flocculant forms. A 40 ft (12.2. m) long flume was designed and constructed at the Auburn University Stormwater Research Facility (AU-SRF). Agitation and mixing requirements were identified with clear water and sediment introduction tests on 5% and 1% slopes by using 0.1 ft3/s (0.003 m3/s) controlled flow rate throughout the flume testing. Mimicked rock check dams were used within the flume for determining proper agitation. Testing results indicated that the use of ditch checks for flocculant applications in channelized flow significantly improves the agitation and mixing by providing up to 96% turbidity reduction.

Large-scale evaluations were accomplished with a collaborative effort of Auburn University Stormwater Research Facility (AU-SRF) researchers on in-channel sediment basin application. Flocculant application on a sediment basin testing apparatus was evaluated by using semi hydrated polyacrylamide-based flocculants in block form. Three flocculant blocks were installed within the forebay of the inflow channel upstream of the basin to maintain contact with the introduced flow. The performance of flocculants within the sediment basin application was evaluated through turbidity reduction and residual concentration measurements. Testing results indicated that flocculant usage provided a 90% turbidity reduction, which was 8% more than the MFE-I treatment, and showed low residual concentration values from 5 to 8 mg/L exist in the discharge point.

Effective implementation of flocculants on construction sites is possible through proper dosage, dosage delivery mechanisms, and application. This research provides a framework for practitioners to establish effective flocculant implementation that would successfully treat construction stormwater. Findings of this study allow improvements on flocculant usage in construction stormwater treatment through new and improved guidelines as well as increasing the knowledge on the use of flocculant in the erosion and sediment control industry.