Erosion and Sedimentation Meet Chemical Intervention

Chocolate milk: two words a construction contractor never wants to hear on the job, unless he’s looking in his refrigerator. A project manager in Pennsylvania was plagued with chocolate milk, and it wasn’t the kind your children ask for at dinner. He had sedimentation basins full of the rich, brown-colored water–runoff water containing large amounts of ultra-fine clay soil particles from a 100-ac.-plus construction site. Under scrutiny of both the county conservation district and state regulatory agency, it seemed that nothing the company tried caused the fine particles to settle from the water before it discharged from the sedimentation basins. The company had repeat violations for failing “to prevent sediment or other pollutant discharge into waters of the state.”

Engineers from Sear-Brown’s Water & Environment Division in State College, PA, were contracted to help the construction company find some solutions–quickly. An initial review of the soils information available for the project area by Sear-Brown soil scientist Steve Levitsky revealed no obvious characteristic leading to the problems with solids settling. A review of the E&S plan by civil designer Otto Herr indicated that all erosion control structures were sized properly and located in accordance with existing standards. A site visit confirmed that the structures were constructed per their design.

Chemical Intervention
The county’s resource conservationist, who learned of the use of polyacrylamides at a conference he recently attended, suggested the use of chemical treatment to augment settling in the basins. Solid forms of polyacrylamides can be placed in drainage ditches to treat water as it flows into sediment traps and basins to improve solids settling. The water picks up some amount of the chemical as it flows over the solid form of polyacrylamide, and by the time it reaches the basin, the chemical causes soil particles to begin to flocculate, increasing settling in the basins. The resource conservationist recommended this technology to the construction company, which investigated the application. Based on the estimated flows in the ditches, however, an enormous quantity of solid polyacrylamides would be required. The company learned that other chemical treatment methods existed that might prove to be more applicable to the challenging site conditions. A chemical vendor was contacted, who recommended a chemical in liquid form. The chemical was to be either applied using a drip from a 5-gal. bucket or dispensed automatically using a hydraulically activated feed system.

Enter Sear-Brown’s Water & Environment Engineering Manager Jim Dewolfe and Project Technician Jodie Womer. We collected water samples from the sedimentation basins and conducted extensive jar testing with a suite of candidate flocculants. Jar testing clearly proved that some chemicals were much more effective at dropping the solids out of the water than others. Many different types of chemicals were evaluated, not only for their treatment effectiveness but also for other factors.

First, the environmental effects were evaluated based on environmental studies available and on information provided in the chemicals’ material safety data sheets. They also were evaluated on ease of use, including parameters such as freezing point, effective dose range, necessary mixing, and effects of resuspension on the effectiveness of the chemical. Each of these parameters translated into a specific consideration for effectiveness on-site. For example, superb treatment might be achieved with flocculant A, but if there is an effective dose range of only a few parts per million, how likely is it that the site and weather conditions will allow that narrow dose to actually be achieved? Reaction rate was important as well, as the point of chemical addition and distance to the sedimentation basin were considered. The reaction rate needed to be just right to prevent solids from settling immediately and dropping out in the channel, or taking so long that settling wouldn’t occur until after water was discharged from the basins. A lower freezing point is also desirable for cold weather conditions. The more forgiving the chemical is, the better the results that can be achieved. Often (almost without fail, as it seemed to site personnel) it would begin raining in the middle of the night, and chemical treatment would need to be started by one or two staff members who had been called out of bed to apply it–in the rain–with only the lights of their pickup truck to help them. The more foolproof the method, the better.

A decision matrix was developed to rank each chemical for each parameter to be evaluated. Different categories could be weighted to show relative importance. For example, environmental effects or optimum dose range could be given a higher priority than settled solids density. The chemical with the highest overall ranking would be best suited to the application. Finally, a polyaluminum hydroxychloride blend, provided by Westwood Chemical of Middletown, NY, was chosen. It had the best combination of chemical and physical characteristics, environmental safety, treatment effectiveness, and operational flexibility.

“The Wominator”
After the chemical was selected, finding a practical method of application was necessary. As mentioned, a hydraulically activated feed system initially was used to dispense the chemical into the ditch leading to the largest basin. The system used the water flowing in the ditch to power a pump that dispensed the chemical. The manufacturer supplies both variable-dose and fixed-dose systems. However, the cost of installing enough systems to treat the large construction site, coupled with the operational issues such as freezing over the winter, revealed the need to evaluate other chemical feed systems.

Eager to invent, the Sear-Brown team developed a chemical feed system that affectionately was dubbed “The Wominator” by site personnel. The system was officially named the SB-RABO (Sear-Brown’s Remote Automated Battery Operated) chemical feed system. The main components of this system included a 12-V deep-cycle marine battery to supply power, a 5-gal. bucket of the selected treatment chemical, a 12-V peristaltic pump to pump the chemical into the stream of water, and a float switch mounted in the channel to activate the pump. The design was simple but effective. Most of the system was mounted onto a handcart fitted with shelving brackets, so the system was portable with the exception of the float-switch assembly. Because the pump was single speed, the chemical dose was adjustable using a system of tubing that included a recycle line back into the chemical supply bucket. The flows to the channel and to the recycle were adjustable using valves. Increasing the recycle flow decreased the chemical dose, and vice versa. The system was equipped with a three-position on/off/auto switch, allowing automatic or manual control of power. To keep the pump out of the weather, a 55-gal. plastic garbage can was simply placed upside down over the top of the cart. A small, rechargeable flashlight was included for assistance when the system needed attention at night. An LED signaled when the pump was on so that staff could look across the site at night and tell if the pump was running.

The float switch was mounted at the top of a slope pipe, leading into a sedimentation basin; however, depending on the application, it could be adaptable to other areas as well, such as within an interceptor channel. The float switch was housed in a plastic floor drain for support and protection and to create a still area for water to collect and activate the switch. An area was dug out to hold the switch at a level where water flowing into the slope pipe would cause the float switch to come on and actuate the pump. The drain was held in place by driving two pieces of rebar into the ground and wiring them to the drain.

Site personnel who would be maintaining the systems (three systems on this particular site) were involved in the placement and setup of each system. They learned how each component worked so that if any adjustments needed to be made, such as adjusting the activating height of the float switches or adjusting the chemical doses with the feed and recycle lines, they understood how to make them. After working with the systems, they got a good feel for which chemical dose (slow drip, fast drip) worked in each basin. They also were shown what “good floc formation” looked like and what to look for as clues of underdosing or overdosing the chemical’s optimal dose range. An operations and maintenance manual was prepared and provided with the systems, including a checklist for operating the systems, photos and descriptions of each system component, and product and warranty information from individual components such as the pump and float switch.

Parts for the systems were obtained from such suppliers as Cole Parmer, McMaster-Carr, and a local home improvement store. The final price tag for one complete system was much more attractive to the contractor than was the cost of the other hydraulically activated system. The new system also was adjusted easily for optimal treatment, less susceptible to freezing conditions, flexible in placement, easy to install, and portable.

In-Situ Soil Stabilization
To augment chemical addition in the sedimentation basins, Sear-Brown evaluated a type of in-situ soil stabilization. A very diluted solution of a flocculant was sprayed onto areas of disturbed soil to help hold it in place. This was tested on a small scale by diluting some of the chemical and spraying it onto a small (about 2-yd.²) square plot. A second plot was outlined and not treated with the chemical. The next day, after the plot had dried somewhat, both areas were “rained on” using a garden hose, and the water washing off of each area was observed. The water washing off of the treated area was noticeably clearer than the untreated area, having picked up fewer solids from the bare soil.

A different type of chemical (“chemical A”) was used for this application than that applied to the water flowing into the sedimentation basins (“chemical B”). Chemical A, a cationic polyelectrolyte in aqueous solution, was used on the actual construction site initially and had been proven to be effective. It had an appropriate rate of reaction from the time it was diluted to the appropriate dose until the time it reached the soil. This chemical doesn’t react until it comes into contact with the soil particles (rather than as soon as it is diluted). Chemical B, used in the basins, begins to hydrolyze as soon as it is diluted with water to the appropriate dose, at which point it essentially begins to lose its effectiveness. This causes an issue with application and determining the effective dose, as the dose basically is changing continually. For this reason, chemical A was preferred. Using one chemical on the land and another in the basins would cause the chemicals to come together in the basin. Interactions between the two have been investigated through jar testing them in combination. At concentrations of each chemical near and above the concentrations being applied to the land and to the basin, no negative interactions were observed. It was concluded that using one chemical on the land and another in the basins would not have a negative influence on the effectiveness of either chemical. Each one was slightly better suited for its own application, so the two different chemicals were used.

A calculated dilution was prepared by mixing some of the chemical with water in the company’s water truck. This mixture then was sprayed onto the disturbed areas at a prescribed rate. The application was effective in helping to hold the fine soil particles in place on the site, reducing the total amount of solids entering sedimentation basins. Reducing this solids loading enabled the rest of the E&S controls to not have to work so hard to control runoff from these problem areas of fine soil particles.

The aforementioned treatment can be especially effective on areas that continue to be disturbed, where vegetation cannot yet be established. Seeding and mulching areas still in disturbance are not practical; however, the application of water plus flocculant to these areas does not hinder any construction activities. The dilution also can be modified by increasing the concentration so that less water is used when applying to disturbed areas already nearly saturated with water.

Bottom Line: Results

With nature being unpredictable and site conditions frequently changing because of both the weather and ongoing construction activities, chemical addition to aid solids settling is not a cure-all for every E&S nightmare–a point that cannot be overemphasized. In this case, chemical addition was initiated only after careful evaluation of a selection of candidate flocculants. A chemical finally was selected, based not only on performance but also on potential environmental effects and other critical parameters. Everyone on-site who would be responsible for maintaining the chemical dispensing systems was thoroughly trained in the setup, operation, and safety of the systems.

The bottom line of this project is that it worked. There was a drastic difference in the water downstream after chemical addition was employed. The water leaving the sedimentation basins was considerably cleaner than in previous months. Just downstream, there was little evidence of the huge earth-disturbing activities nearby.

Success was due to a number of factors. First, the chemical and dose were selected very carefully after extensive jar testing, using water from the sedimentation basins and evaluation of site conditions. Having the “any flocculant will do–let’s add a chemical” attitude would likely land a contractor in a world of trouble. Second, a chemical addition system was selected carefully based on the conditions under which it would be used. Again, a one-size-fits-all approach would not have worked here. Finally, the site personnel were committed to making sure the systems worked. They paid very close attention to the performance of each system and carefully monitored site conditions, coordinating treatment with the construction activities that were occurring. They realized that chemical treatment was not foolproof, and they made a commitment to using the technology responsibly and maximizing its effectiveness.