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Marshalltown: Alliant Energy

Large Energy Companies - Settling Ponds, Dredging, & Water Work

Introduction

Hard Hat Services (HHS) has been providing water and environmental solutions to a large energy company and its subsidiaries since 2003. HHS has been involved in a variety of projects at 9 separate locations with this client and its subsidiaries.

Our recent experience includes: A geotechnical stability investigation, slurry wall installation, and environmental permitting for the bag house were performed at the Burlington, Iowa location. HHS was involved from start to finish on the design and build of fuel oil piping after detecting a leak at the Ottumwa, Iowa facility. Hard Hat remedied an algae problem in the settling basin at the Marshalltown, Iowa station, and designed and implemented pH adjustment systems for the Clinton and Dubuque, Iowa facilities.

Below are two case studies that capture HHS’s ability to innovate and design sustainable solutions for energy companies.

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Ottumwa Settiling Pond and Water Treatment

Ottumwa

Background

One facility, located in Ottumwa, Iowa, has a generating capacity of 675 megawatts. The generating station uses steam from coal fired boilers for power production, and pulls in water from the adjacent Des Moines River. Water that quenches bottom ash from the boilers flows to the on-site settling pond, along with cooling water tower blow down, the OGS floor drains, chemical cleaning of boilers, the boiler water pretreatment system, discharge from the sanitary wastewater treatment plant, and stormwater runoff. As process water enters the settling ponds, solids drop out of suspension and the sediments build up over time.

Hard Hat’s Contribution

Hard Hat Services (HHS) was retained by the client to conduct settling pond investigative work and prepare a Pond Maintenance Plan.

HHS completed a Pond Maintenance Plan in November 2006. The investigation included a bathymetry survey, studying the various flows that were emptying into the settling pond, and sampling the pond sediment to determine how and where the solids were settling out. Hard Hat was able to predict a lifespan for the existing pond based on the solids input.

HHS designed a solution to optimize the use of their settling pond to ensure present and future compliance with the NPDES permit.

The settling pond was redesigned to include three separate basins. There is a primary pond, where approximately 95% of the solids are removed. A secondary s-shaped settling basin was designed. By using an s-shape design in the space available, the water is flowing through a much longer path, allowing additional fine solids to drop out of suspension. The s-shape of the basin also allows for easy maintenance, making all the areas accessible for routine dredging. In addition to the primary and secondary basins, a polishing pond was added at the end of the flow path. This accompanying feature gives the client the option, if needed, of dosing the water with a polymer to provide a final stage of suspended solids removal before discharge.

HHS also developed an innovative concrete ash collection basin to improve bottom ash handling, in order to reduce the amount of bottom ash filling the secondary basin. Water that flushes out ash from the boiler passes through the collection basin, where the ash drops out and can be easily removed before the water enters the settling basin.

Future Modifications

The generating station in Ottumwa, Iowa, uses a clarifier to treat incoming river water that will be used as process water, which is another source of water discharged into the basins. HHS is in the process of designing a plate and frame filter press to remove the sludge generated from the treatment of the river water. Removing the sludge via the filter press prevents this material from being discharged into the settling basins, which in turn will reduce the frequency of maintenance dredging. As a sustainable environmental bonus, the sludge will be high in lime and organics, and can be re-used as an excellent fertilizer alternative that will be available to local agriculture when the system is installed.

As these changes come online over the next few years, HHS and our clients are working together to be proactive about maximizing solids removal and minimizing solids discharge into the Des Moines River.

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Dredging in Cassville

Cassville

Background

A generating station near Cassville, Wisconsin receives shipments of coal via Mississippi River hopper barges. During the movement from the barge to the hopper incidental amounts of coal may miss landing back in the hopper barge or in the conveyor hopper, falling in the water. Over many years, spilled coal has accumulated under the dock area such that full barges are hindered from being staged against the dock mooring cell at the unloader.

The Solution

Hard Hat Services (HHS) proposed dredging the area immediately under the clamshell to an elevation similar to the surrounding riverbed. The dredge area was about 150ft long along the bank by 100ft wide (from the loader to the channel).

The project area was located near a known historical mussel bed (Family Unionidae) that supports a diverse community of nearly 30 species, including the federally endangered Higgin’s Eye mussel (Lampsilis higginsii) and numerous species listed as rare in Wisconsin. Although the dredging area had historically been affected by tow/barge traffic and coal spill, it was possible that mussels still inhabited the area in and adjacent to the dredge site. HHS communicated with the U.S. Fish and Wildlife Service and the Wisconsin Department of Natural Resources (WDNR) to determine that a mussel survey of the site should be performed to determine potential impacts to the freshwater mussel resources.

SurveyorsHHS was contracted to perform the mussel survey activities, which included species characterization, abundance and distribution, and habitat conditions. Both HHS and a representative from WDNR were onsite supervising the study. It was discovered that the site provided only marginal habitat conditions, with a substrate of primarily sand. Mussels were found mostly downstream of the existing coal loader. One federally endangered Higgin’s Eye was found at the deepest part of the downstream edge of the study area. All 194 mussels collected during the study were relocated to a site upstream under the direction of the WDNR.

After the mussel study was completed and it was decided that dredging activities would not be detrimental to the mussel population, HHS coordinated the fieldwork activities in order to minimize the delays to the client’s coal unloading process.

A barge was spudded adjacent to the area to facilitate dredging. A silt curtain was draped around the project area to keep suspended sediment from affecting habitat conditions downstream. The area was dredged down to pre-existing elevations, and the material was placed into a barge and then transferred to the coal pile.

Preventative Measures

HHS also engineered a solution to control incidental coal fallback into the Mississippi River during future unloading of coal barges, a flexible chute that deflects the fallback into the hopper barge. The design uses a six-foot wide section of conveyor belt suspended by structural steel and two suspension cables to create a chute below the area between the lip of the receiving hopper and the river barge. The chute is twelve feett wide to capture the incidental fallback that occurs at the closure of the grab bucket. This added design component provides a sustainable solution to the coal fallback problem, allowing WP&L’s operations to continue running smoothly, while protecting the river environment at the same time.

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