The US Environmental Protection Agency has indicated that it will be making new requirements for reducing turbidity in construction site runoff in the near future. While it is not clear how this will be achieved, recent studies has shown that significant reductions in turbidity can be achieved through a variety of passive treatment approaches using polyacrylamide (PAM). Placing PAM powder or blocks in water conveyances such as ditches, diversions, storm drains, and slope drains provides opportunities for treating turbid runoff prior to settling in a sediment basin. Flocculated sediment has very different characteristics than untreated sediment, with most of the suspended material being collected into flocs. Previous studies have shown that flocculated sediment can be settled in a stilling basin with a detention time of 1.5 hours, much smaller than a typical design of 24 hours. The flocs are the size of sand particles but much less dense, so it is important that the flow be conducive to settling through the use of baffles. However, little is known about the different characteristics of flocs formed from different materials and under different conditions. This project is designed to determine the range of flocculated sediment characteristics important to settling and the impact they can have on basin design. There will be five aspects of the project: Determination of particle characteristics (size, density, settling rate) of flocculated soils with different particle size distribution and mineralogy. Test relationships between flow, flocculant delivery method, and basin trapping efficiency. Determine the influence of basin geometry and baffles on floc settling. Characterization of particle sizes in untreated and treated (PAM) runoff on construction sites. Validate the performance of the flocculation system (treatment with PAM + sediment basin) under field conditions. This will provide the information needed to adjust the basin design to account for changes in sediment characteristics when it is flocculated. It is likely the basin will be smaller and wider than current designs.
There is increasing interest in controlling turbidity in construction site runoff using various chemical flocculation treatments. Flocculated sediment is likely to behave much differently that untreated sediment, however, which could suggest changes to sediment basin design. This project was initiated to characterize the effects of chemical flocculation on sediment properties and to evaluate how it might affect basin design. The project was conducted in three phases: laboratory studies, field-scale studies under controlled conditions, and monitoring on active construction sites. In a laboratory column study, flocculated sediment was found to largely settle 1 m (3.2’) within several minutes, with similar or better sediment removal rates for unflocculated sediment settling after an hour. The addition of polyacrylamide (PAM) shifted the particle size distribution to larger sizes for all but one soil, which was mostly sand with little clay. The uniformity coefficient was greatly reduced, primarily due to a shift from smaller to larger particle sizes. In field tests, the best flocculation system was a combination of fiber check dams and a jute liner, with PAM applied to the liner material. It is important to note that the ditch was lined with a geotextile, so the only source of sediment was that which was added to the water and the loading was much lower than typical on a construction site. Basin configurations (2:1, 1:2, and 2:1 with rising floor) did not change water quality at the outlet for flocculated sediment but the 1:2 configuration reduced sediment at the outlet compared to the 2:1 configurations for unflocculated sediment. There was evidence that passive treatment using PAM reduced sediment delivery to basins at three construction sites, but generally the sediment loads in the ditches were too high for effective treatment. Using data from the laboratory tests, the calculated surface area requirement to attain similar sediment retention for current designs could be reduced considerably if the sediment is completely flocculated. However, in order to attain this condition, construction sites would have to be managed closely in order to ensure the sediment was flocculated prior to entering the basin.
- Flocculating sediment was shown to significantly shift the particle size distribution to larger sized, greatly increasing settling rates for treated sediment.
- Using current sizing formulas, sediment basins will capture much more sediment if it is flocculated. Conversely, attaining the current retention rate based on particle size could be achieved in basins with less than half the current surface area. This presumes the sediment is all flocculated prior to discharge into the basin. Monitoring on active construction sites demonstrated the difficulty in achieving thorough flocculation as sites are currently managed.
- The design of the basin does not affect effluent water quality when the sediment is flocculated, but the 1:2 configuration did increase sediment capture for unflocculated sediment compared to the standard 2:1 geometry. This assumes functioning, porous baffles per current design specifications. The ""sideways"" configuration may be more easily fit onto sites with limited area.
- Attempts to characterize flocculated sediment on active sites were usually thwarted by construction site activities that reduced or eliminated the flocculation potential of systems installed. Construction sites will have to be managed differently if the flocculation systems are to be successful.
