Saturday, 15 July 2006

Chemical Treatment to Reduce Turbidity in Borrow Pit Discharges.

Joshua W. Vetter and Richard A. McLaughlin. North Carolina State Univ, PO Box 7619, Raleigh, NC 27695-7619

Sediment is one of the leading pollutants of surface water in North Carolina. A significant source of this pollution can be attributed to construction sites. On many construction sites it is common to encounter turbid water in borrow pits and other excavations that must be removed for construction to proceed as scheduled. The current treatments for excavation discharge water can be effective in removing much of the large sediments; however these processes are ineffective in reducing turbidity, which consists of very fine soil particles that remain suspended in the discharge water. In the Coastal Plain of North Carolina nearly all excavations will require the removal and treatment of water due to the presence of a high water table throughout much of the region. The objective of our research is to develop a more effective system for decreasing the turbidity of excavation discharge water in an effort to minimize the negative impact that construction currently has on nearby receiving waters. In conducting this research we are evaluating various physical and chemical methods for reducing turbidity, including alternative stilling basin designs and the use of polyacrylamide (PAM). Two sediment sources, varying in chemical properties, from active construction sites located in the Coastal Plain region of North Carolina are being used to evaluate the effectiveness of the various treatments. A screening of a number of PAM products, varying in both charge densities and molecular weight, demonstrated that a cationic, high molecular weight PAM (NALCO 9909) was the most effective in reducing the turbidity generated from both sediment sources followed by a neutral, high molecular weight PAM (SF N300). Stilling basin design testing was conducted at the Sediment and Erosion Control Research and Education Facility (SECREF) at North Carolina State University. Tests involved a simulated borrow pit operation in which turbid water (500 - 700 nephelometric turbidity units (NTU)) was mixed in a large basin using one of the sediment sources and pumped into a stilling basin at rates of 100 gpm and 50 gpm. Preliminary test results have indicated that a concentration (5 ppm) of cationic PAM (NALCO 9909) combined with jute/coir baffles in the stilling basin can produce turbidity reductions of 96%. Further lab work will be conducted to relate the effectiveness of the PAMs to the chemical and physical properties of the sediment sources. The greater effectiveness of the cationic PAM compared to anionic PAM can be attributed to the mineralogical composition and surface chemistry of the Coastal Plain sediment.

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