N.C. STATE UNIVERSITY

College of Agriculture and Life Sciences
N.C. Agricultural Research Service
N.C. Cooperative Extension Service

Feasibility and Cost Effectiveness of Using Gypsum as a Chemical Flocculant to Maximize Sediment Removal at Urban Sites

Prepared by: D. Hesterberg, J.W. Gilliam, J.E. Parsons, and D.K. Cassel

Long-term objective

1. To determine the most effective watershed and sedimentation-basin design criteria for reducing sediment discharge from urban construction sites into streams.

Short-term Objective.

1. To develop a cost-effective method for using chemical flocculants to more rapidly settle suspended particles in sedimentation basins.

2. To determine whether gypsum-induced flocculation of dispersible particles from watershed soil samples can be used to accurately design sedimentation basin treatments.

3. To develop a prototype test kit that can be used by engineers and field technicians to evaluate the amount of gypsum needed in a sedimentation basin to maximize the benefit-to-cost ratio of this treatment.

Accomplishments

1. A preliminary field evaluation showed that surface applying a moderate amount of gypsum to turbid water in a sedimentation basin reduced the fine-sediment load by 250-fold in 3-1/2 hours.

Significance

This project is being conducted with funding from the North Carolina Department of Environment, Health and Natural Resources (DEHNR) - Land quality Section in cooperation with Mr. Warren Faircloth of the Orange County Planning Department - Erosion Control Division. Research evaluating the combination of gypsum treatment and a new sedimentation-basin discharge system developed by Mr. Faircloth is being done at construction sites in orange County. The preliminary results on one basin indicate that gypsum has potential to be an inexpensive and effective chemical agent for removing fine suspended particulates from urban runoff water. Because bare soils exposed at construction sites are more acceptable to erosion and transport into freshwater systems, more effective removal of sediments in on-site basins will substantially improve the quality of water discharged from these sites.

Future Plans

Research under this project will systematically evaluate the effectiveness of gypsum treatments for reducing sediments from different soils, and develop onsite methods for determining treatment rates. If the gypsum treatments are successful, future efforts will be directed toward developing automated technologies for applying gypsum to basins.