How To Turn Stormwater Runoff from Gravy to Chardonnay

摘要

Sediment basins are the single best form of sediment control, but the outlet structure discharge rate is key to optimizing performance. Several options have been utilized to control the discharge rate and dwell time within the basin, but the standard method of discharge has traditionally been a perforated vertical standpipe. Like a bucket with holes in it, a perforated standpipe discharges at the greatest rate from the bottom holes where the pressure is highest. Since gravity pulls sediment to the bottom of the basin the traditional standpipe will discharge the "dirtiest" water first. Also, typical perforated standpipes will discharge a full sediment basin in one to three days. Although the traditional design can remove up to 80% of the sediment entering the basin, the sediment removal efficiency can be significantly improved. First, since clay size particles take around 24 hours to overcome the drag force of water and fall one foot in a water column, increasing dwell time in the basin is important. Dwell times should be at least 7 days to allow the force of gravity to overcome the drag force and settle fine sediment. Secondly, the discharge point for storm water should be as high in the water column as possible to release the "cleanest" water first. By discharging only the water on the surface of the basin with a skimmer, gravity will have an opportunity to pull the sediment to the bottom of the basin. Also, a skimmer can be easily sized to release storm water very slowly over 7 days at a constant rate less than 0.6 cfs. Skimmers have been tested and shown to remove up to 95% of the sediment entering a basin. Lastly, the typically cloudy discharge water from skimmers, containing only the finest clay particles, can be made clear with the use of anionic polyacrylamide polymer (PAM). A skimmer is the perfect metering device for properly dosing wildly varying storm water flows. The electric charge or force between clay particles allows them to overcome the force of gravity and stay in suspension. PAM polymer will attract the clay particles like nails to a magnet to create a floe which will become big enough to overcome the drag force and drop the floe to the bottom of a second, basin, sediment fence ring, sediment trap or wood chip dike ring. With the use of a properly sized sediment basin, skimmer, PAM and a second stilling basin, sediment laden construction storm water runoff gravy can be turned into "chardonnay".