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Chemical treatment systems (CTS) are implemented in areas where traditional, physical erosion and sediment control practices will not meet water quality goals for construction site runoff . Th ey are not intended to replace traditional erosion and sediment control (ESC) practices, which are the most important and cost-effective approaches to reducing sediment loads in stormwater discharge.
The purpose of CTS is to reduce the amount of suspended sediment which would be released using conventional erosion control systems. Th is sediment consists of clays and fi ne silts which are very slow to settle even under ideal conditions in settling basins. Th e primary mechanism is the introduction of chemical flocculants into runoff , resulting in a binding of the suspended clays and silts together into larger particles which settle more quickly or can be filtered from the stormwater.
Flow control through CTS is of vital importance for the proper dosing of stormwater runoff . All normal hydrologic analyses must be done to insure that reasonable peak flows are accounted for along with typical flows from designed storm events. Once analysis is adequately addressed, then the project can deal with the complexities of design and dose rate requirements.
Proper dosing, mixing, and settling time are needed for CTS to be effective. Also, matching the right flocculant to a specific sediment and water chemistry is important. In addition, disposal and final stabilization of the flocculated materials must be planned for in advance and monitored during system operation.
These best management practices (BMPs) focus on the design and use of active CTS, and do not directly address the use of passive systems. Active systems involve treating pumped stormwater using chemicals that are metered into the flow at a known dose rate. Passive systems use the flow of stormwater to dissolve the flocculant from a solid form (blocks, granules, socks, etc.) prior to a mixing and settling system. While the use of passive systems is not generally discouraged, this document is focused on active systems only.
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