Construction of Underground Stormwater Conduit Overcomes Challenges in Florida Wetlands

Aug. 20, 2014

Haleakala Construction was charged with installing a large cement pipe section to serve as an underground conduit carrying stormwater away from a stretch of I-75. This project is located in Tampa, FL, on a 5-mile stretch of I-75 surrounded by 50 acres of sensitive wetlands. The Florida Department of Transportation (FDOT) has strict guidelines in place regarding the nephelometric turbidity unit (NTU) levels of waters discharged in these sensitive areas. The first challenge came when the existing stormwater basin exceeded capacity and threatened to overflow into the conduit area. The existing stormwater basin contained water with NTU levels to high to discharge into the adjacent wetlands, so a treatment system had to be installed to increase stormwater capacity.

Haleakala had worked out a detailed plan to deal with the challenge. Pure Polymer worked with the contractor on a limited footprint to install a passive system that was implemented at first; however, after heavy rains the system needed to move greater amounts of water, so a semi-passive system was introduced, capable of handling 600 to 1,000 gallons per minute (gpm). At the end of the construction conduit, the turbid water discharges into a 4-acre stormwater detention pond that is 12 feet deep. During the excavation process, water is continually pumped away from the well points; pumping water from the ground allows a dry excavation and access to equipment and materials.

First Challenge
Initially, the company Professional Shoring recommended and provided the contractor with a 3,000-gallon weir tank to reduce sediment loads of pumped stormwater before it discharged into the stormwater treatment lake during Haleakala’s dewatering operations. Samples taken at various collection points showed an average level of 192 NTUs. The discharge threshold was 29 above background. Background samples collected showed a turbidity level of approximately 7 NTUs, requiring the contractor to discharge water with no more than 36 NTUs during the dewatering operations. It was obvious that the use of the weir tank would reduce the NTU levels sufficiently.

Another company, Florida Dewater, was then brought in by the contractor to advise on setting up a system to meet FDOT requirements. The suggested solution was to employ a passive treatment system using HaloKlear natural biopolymer products that are approved for use by FDOT. HaloKlear was contacted by the contractor after being recommended by Professional Shoring, which works frequently with the company Florida Dewatering and had seen the products work in similar situations. Flow rates were too high for polyacrylamides (PAMs) to work effectively, as the turbidity levels and type are constantly changing with such high flow rates. However, the HaloKlear products can work in a variety of soils as well as in changing conditions common on excavation projects. PAMs work by ionic bridging, which relies on existing cations in the water. HaloKlear DPS eliminates the need to rely on cations and anions by adding both chemistries so the performance is more consistent. Additionally, once both chemistries are added, the charges of both these chemistries are neutralized. To satisfy regulation on this project a HaloKlear field residual test kit was used several times, showing that no cationic polymers where leaving the effluent. Haleakala agreed to use a passive treatment channel to treat the turbid water on the north side of the job. This area was for sediment capture and final polishing of the influent.

Passive Treatment System
The channel area is approximately 15 feet wide and 120 feet long. To set up the treatment channel area, a 6-mil vinyl liner was placed on the surface of the channel. An excelsior blanket with wattles was placed directly on top of the vinyl liner. A fabricated 6-inch-wide PVC pipe was placed across the channel at the top end. The pipe had 2-inch holes cut approximately every 4 inches. These holes were to aid in the dissipation of effluent velocity and help create even sheet flow within the passive channel.

The turbid water flows through the holes into the initial dissipation area, where no treatment takes place, then enters the DPS treatment area in the manifold where the DPS socks are located. First the DBP-2101 anionic polymer sets the charge by distributing an even amount of anionic material in the supernate; next, the water travels to the GelFloc sock where an even amount of cationic charge is distributed throughout the supernate. These ions in the water attract one another rapidly, capturing suspended solids in the process. Once the two interact, the charge is neutralized, as shown in testing done by the University of Central Florida and funded by FDOT and the Florida Department of Environmental Protection to analyze BMPs, chemicals, and other approved products for the Florida designers’ and reviewers’ manual.

The system proved effective beginning on day one. The initial influent readings showed 100 to 120 NTUs, and the water was running at approximately 300 gpm. Multiple turbidity readings taken throughout the day at the end of the passive treatment channel showed 5 to 12 NTUs after treatment, well below the 28 NTUs mandated by FDOT. The system was so effective that the contractor chose to increase the water flow to 800 to 1,000 gpm that evening. With this increased flow rate, there was at least 3 inches of water flowing above the excelsior blanket, rending the media ineffective. To fix the situation, an additional blanket was added the next day to help accommodate the increased flow. After an additional three layers of excelsior blanket were added on the second day, the effluent level was 8.78 NTUs at the FL DOT inspection, well below required levels for discharge into the adjacent wetland.

To meet construction completion dates, and because of increased water from daily rain events, which threatened operations because the sediment basin was at or near capacity, the contractor needed not only to increase the flow of water for treatment but also to run the system 24 hours a day to stay on schedule. The contractor elected to use a manifold with additional sock kits with a tank/sediment bag for capturing the fines; this was done to double and triple the flow rate. On the third day of operations, Florida Dewatering and HaloKlear replaced the blanket in the passive channel and built the manifold. The goal of the manifold system is to treat the turbid water before it enters the weir tank and to use the passive treatment channel, with blanket, wattles, and rock check dams, as a backup settling area to ensure low NTU effluent.

Second Challenge
The system worked effectively until day 10, when construction operation hit a very fine layer of clay. After discussions with the FDOT and the contractor, it was decided that an additional sediment bag would be implemented to deal with the added turbidity. The sediment bag, 10 feet wide by 15 feet long, was placed within the passive treatment channel. The bag was put into action with the pump pushing 800 gpm of water through it. The holes in the bag are generally about 30 to 60 microns, and the clay particles were 1 to 4 microns; most sediment was captured in the bag but the contractor wanted a secondary system to capture any fines escaping the sediment bag. The discharge from the bag went directly into the passive channel where the water was treated with HaloKlear GelFloc. The initial influent (before bag and after bag) had a level of 1,241 NTUs because of the fine clay, but after flowing through the passive treatment channel, readings were at 1.82 NTUs, well below the FDOT limit.

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