Erosion and Sediment Control
Using Anionic Polymer Gels to Achieve Maximum Turbidity Reduction from Construction Projects
Federal and state regulations have required construction projects to mitigate turbidity in discharge water for several decades. The negative impacts from offsite discharge of sediment include light extinction, oxygen reduction, habitat smothering, heavy metals, bacteria, pesticides, eutrophication, and fish kills.
Anionic polyacrylamide (PAM) technologies have evolved to meet this challenge. These PAMs are environmentally benign and are very adaptable for development project applications. The most recent delivery systems are capable of routinely reducing turbidity 80-90% or more at a cost-effective rate. This presentation will compare various PAM mixing technologies that use polymer gel blocks as the flocculant. Some of these include: ditch mixing systems, baffle grid systems, swirl tank systems, baffle tank treatments, and pipe mixing. Comparisons will be made regarding turbidity removal %, pre and post treatment turbidity, and volume of water treated.
Polymer treatment systems consist of three parts:
1) Correct PAM blend matched to soil/water
2) Properly designed mixing device for maximum polymer agitation
3) Particle capture system (dispersion field, particle capture system, jute baffles)
Mixing systems range from a simple ditch to engineered in-pipe mixing devices with air-injection. Almost all of these PAM mixing systems have demonstrated the ability to remove 80-90% of project turbidity. Some are capable of consistently removing 95% or more of sediment particles in water. All of them require rapid agitation to increase polymer interaction with target soil particulate. Improved technology reduces the footprint of the treatment system and in some instances allows for more treatment volume than previously achieved.
The simplest system is a ditch mixing system. A trench is excavated or materials are assembled to make a trench (hay bales, concrete blocks). Polymer blocks are placed in the ditch and a jute particle capture system or dispersion field is added at the end of the ditch to capture soil agglomerations and PAM flocculant. This system can be used in either passive (gravity/runoff) treatment or active (pumped) treatment.
A baffle grid system is next in development stages for treatment. These can be used in passive or active treatment systems as well. A large ditch is excavated to direct runoff to the baffle grid system. The ditch widens to a rectangular area where wooden baffles are constructed. Immediately downstream from the baffle system will be a series of rectangular frames covered with jute, burlap, coconut or other natural fiber material. This is where the particulate will attach to the material and clean water allowed to discharge.
Tank mixing and swirl mixing technologies use containers to agitate the polymer blocks. Tanks can be circular or similar to frac-tanks. Pumped water is passed over polymer blocks and discharged through a series of particle capture curtains. Flow of water can be horizontal through weir tanks or vertical in the case of a upwell swirl tank. Clarified water is discharged offsite through a particle capture system or to a holding pond.
In-pipe mixing systems produce the maximum polymer mixing in the smallest technology footprint. Polymer blocks are reconfigured in a series of crimped links contained in a netting material. These links are placed inside of the dewatering pipes for polymer mixing. Nearly instantaneous agitation can be achieved within these systems. This is then discharged through a particle curtain or tank containing capture materials.
Gel-block PAM treatment systems produce consistent turbidity reduction in an economical, environmentally safe manner. They are a very flexible and adaptable technology for project management.