A Better Way

Sanford South Water Resource Center installs a gasifier that provides environmentally friendly biosolids handling and substantial energy savings

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A new gasification system at the Sanford South Water Resource Center (WRC) in Sanford, Fla. is far ahead of the curve in bio-solids management. It uses a new gasification technology that is expected to save up to $13 million on natural gas in the next 20 years.

It’s all part of the city’s forward-thinking philosophy. “Biosolids have always been a concern,” says utility director Paul Moore who oversees the 2.0-mgd advanced secondary treatment facility. “We’ve asked ourselves, ‘Is there another way we can address it and be environmental stewards?’ So far, we think the path we’ve chosen is a pretty good one.”

Going to gasification

The startup of the gasification system in May follows earlier efforts to stay on top of environmentally friendly biosolids management in central Florida. In 2008, the city installed an indirect, batch biosolids dryer at the Sanford South WRC to meet Class AA standards established by the Florida Department of Environmental Protection (FDEP).

The biosolids dryer greatly reduces the amount of material to be managed. It converts about 32 tons of wet biosolids per day into a dried, reusable product at about 93 percent dry solids. The amount processed includes biosolids transported in from the city’s other treatment plant (7.3-mgd capacity).

Yet despite the reduction in volume, city officials felt a need to do more, since all signs point toward stricter biosolids regulations. The city also wanted to address the rising cost of natural gas, the main fuel for the indirect batch drying system. Gas to process the biosolids at Sanford South cost about $280,000 in 2008.

The city considered a variety of options before turning to MaxWest Environmental Systems Inc., which designs, builds, owns, and operates waste-to-energy gasification facilities throughout North America. MaxWest installed a gasification system that transforms the dry materialized biosolids to volatile syngases and a mineralized ash suitable for use as a soil amendment. The ash can also be used for commercial applications or landfilled.

Cost avoidance

The gasification system has been used in a number of energy-intensive industries but is new to municipal wastewater treatment. MaxWest supplied the capital to build the system and took responsibility for operation and maintenance. Under a 20-year contract, the city pays MaxWest about $280,000 per year, with an annual three-percent increase.

During the next 20 years, the city expects to save $8 to $13 million by avoiding escalating natural gas prices. It plans to save more by avoiding the expense of hauling biosolids to the land application site.

“We looked at projections for natural gas costs and we saw that it was going to have a significant impact on the utility budget, and the city had no control over it,” says Benjamin Fries, vice president of CPH Engineers Inc., which designed and built the Sanford South WRC and was the lead engineer for the gasification system design.

The system is a win-win because it eliminates greenhouse gases like carbon dioxide and nitrous oxide that are emitted when the material decays. The gasification process also reduces the city’s carbon footprint, and addresses potential groundwater and air pollution associated with the land application. Additionally, it eliminates air emissions from the transportation of biosolids.

Capturing energy

In the gasification process, dry and wet biosolids are first routed to a wet/dry storage hopper, where they are mixed and blended in the proper proportions to generate the appropriate feedstock stream.

The feedstock then goes into a ceramic-lined primary gasifier chamber. In the oxygen-starved, temperature-controlled environment of the gasifier, the feedstock is converted into a uniform mixture of syngases (carbon monoxide, hydrogen, and methane). The ash settles to the bottom of the chamber, where it is skimmed into a discharge chute and conveyed to a dump container.

The syngases are routed to a thermal oxidizer, where they are converted into a low-nitrous-oxide gas stream. In the thermal oxidizer, the syngases are reacted in four stages and converted to thermal energy in the form of hot air. This air then enters a diversion stack at 1,600 degrees F and is further routed to a dropout box/heat exchanger.

The heat exchanger captures the thermal energy in a thermal fluid, which is returned to the dryer at 450 degrees F to dry the wet biosolids. The cooled thermal fluid leaving the dryer is recirculated back to the heat exchanger. Excess thermal energy is routed to a cooling tower that brings the temperature down to below 100 degrees F. The cooled air is then reintroduced to the diversion stack, where it is vented to the atmosphere with no visible steam plume.

Each day, the system produces about 200 pounds of ash, which is landfilled. The city is working with the University of Central Florida to test its suitability as a soil amendment. In addition, the city hopes to create a new revenue stream by processing wet biosolids from other municipalities. The drying and gasification system is rated to process up to 55 wet tons of material per day.

The right path

So far, the gasification system has met the city’s expectations. Moore says the decision to install the gasifier was relatively easy given the savings and an arrangement that puts the responsibility for performance on MaxWest. “The gasifier costs us no more than it did to purchase natural gas each year, so it’s a no-brainer,” he says. “MaxWest knows how the gasifier works. I know it’s supposed to work and save us money.”

Although it has only been seven months since startup, Moore says the decision to install a gasification system appears to be the right one. “We’ll know for sure in about another year,” he says. “I think we’re on the right path.”



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