"The most cost effective piece of equipment ever brought onto this property." - Mark Shea, Assistant Supervisor of Waste Treatment
In an unusual feat for a hydraulic dredge, a Series 370 Ellicott® "DRAGON®" cutterhead dredge excavated four different materials from two power plant settling ponds: fly ash, bottom ash, lime, and lime mixed with fly ash. The dredge moved over 100,000 cubic yards in just three months.
The City of Springfield, Illinois owns and operates the City Water Light & Power (CWLP) Dahlmann Generating Station. Burning coal generates three waste products: fly ash, bottom ash, and lime from the water purification plant. In the past, these waste materials were stored permanently in ponds in a dry or wet state. Recently, beneficial uses have been found for these
products. Bottom ash is used to manufacture blasting media and roof shingles; fly ash is used as a fill material for highway construction; lime is used on Illinois farm fields to neutralize acid soils and improve crop yields.
There is one great problem however: all of these products are flushed to the settling ponds with water, and hence they tend to settle out over large areas in the pond, making them very hard to recover. Previously the City excavated these materials using back-hoes located along the edges of the ponds, but this equipment has only limited reach so the bulk of the material - up to
90% - remained in the ponds under water.
The CWLP, after investigation into numerous excavation technologies, opted to mine the material using an Ellicott® Series 370 "DRAGON®" cutterhead dredge. They decided to dredge first their largest settling pond (approximately 3000 feet long x 1000 feet wide) which contained both bottom ash and fly ash. The City leased the dredge from Ellicott®, and operated it with its
own personnel who had been trained "on-site" by Ellicott® field service engineers at no additional cost to the City.
With the dredge the CWLP could mine material under water up to 2000 feet away from the staging area where the material was removed from the pond for stacking and drying. Because the 12-inch pipeline was discharging up to 50% solids in the slurry to this area at up to 4000 gpm, and the City did not know if the material would settle out in the staging area or flow back to the main pond, a decision to start there first was difficult. However, after two weeks of dredging, the City determined that the bottom ash and fly ash were settling out very well in the staging area and could be easily excavated by the back-hoes, which could now work in just one area. This was a major breakthrough in the recycling program since 100% of the material in the ponds could be excavated with the dredge and dried for beneficial use.
Dredge Efficiency Saves Over $200,000
Because a portion of the fly ash had migrated to the large pond spillway structure, the dredge was also used to clean out this area to permit better settling of solids in the pond and hence a cleaner effluent. Mr. Mark Shea, project manager of the dredging operation for the City, had nothing but praise for the Ellicott® Series 370 dredge. "Not only are we dredging material for recycling, but we are getting a cleaner effluent stream at costs far below that of conventional excavation using back-hoes and dozers. During this short dredging period, the City has saved over $285,000 vs. conventional techniques, a 50% savings."
The City spent just $1.80 per cubic yard of material removed compared to the lowest outside contractor quote of $3.85 per yard.
The second and smaller pond, which receives effluent water from the larger fly ash/bottom ash pond, also contains lime slurried-in from their SO2 water purification process. The City did not know how well the Series 370 dredge would perform in the very viscous, thick lime consolidated in the pond. There was also 2000 feet of pipeline involved, so the city installed a flow meter (to measure slurry gpm) and a nuclear density gauge to measure the percentage of solids in the slurry. At the discharge point 2000 feet away, the flow was 4100 gpm, and the lime slurry consistency in the pipeline was 50% solids by weight. Even though the 370 can dredge to a 20-foot depth, dredging depths were held to 15 feet. The City also found that the lime was
mixed with fly ash, and the Series 370 pumped this material at 50% solids by weight.
Because of the ability of the dredge to move the lime at high solids content, it may be possible to transport the lime in liquid form and place it directly onto surrounding farm fields. This is now common practice in the mid-west, and the process is growing.
Mark Shea commented, "We have proven that the Ellicott® Series 370 dredge can move three of our products very cost-effectively, and we have a market for these products. There was virtually no dredge downtime even though we were working in the colder winter months. It was probably the most cost-effective piece of equipment ever brought onto this property."
Dredging fly ash, bottom ash, and lime, once almost impossible to recover, and doing it all with just one piece of machinery was a major breakthrough for the City of Springfield, Illinois. Not only was this done with 50% cost savings compared to conventional techniques, but also in many situations where conventional techniques will not work at all.