The regenerative emissions treatment system at the Buckman Regional Residuals Biosolids Treatment Facility suffered from frequent clogging of the ceramic block media.

The media at the facility, owned by JEA (formerly Jacksonville Electric Authority), had to be cleaned frequently and changed out roughly every 18 months, adding to operating costs and causing shutdowns lasting up to two weeks.

Today, the three 17-year-old regenerative thermal oxidizer (or RTO) cells use ceramic blocks with a pattern of channels much less susceptible to clogging. The new blocks have been in place for three years with only periodic cleaning, significantly increasing uptime and reducing maintenance costs.

Centralized processing

JEA, a publicly owned water, sewer, and electric utility in Jacksonville, Florida, serves some 240,000 water customers and 186,000 sewer customers. Its Buckman Regional Residuals Biosolids Treatment Facility stands at the 30 mgd (design) Buckman Water Reclamation Facility, which treats an average of about 22 mgd in an activated sludge process.

The biosolids facility processes solids from the Buckman Water Reclamation Facility and four other JEA clean-water plants. The material is delivered by truck or pipeline and first passes through three Alfa Laval gravity belt thickeners on its way to three anaerobic digesters.

The digested biosolids are dewatered on three centrifuges (Andritz Separation) that produce cake at 20 percent solids for feeding to an Andritz Separation drum drying system 70 pelletizer. Finished pellets are stored in silos; a contractor purchases the inventory and trucks it out. Production averages about 70 tons per day.

Treating emissions

The waste gas from the pelletizer is treated in the RTO cells to burn off air pollutants and odors. “It’s basically a giant catalytic converter that cleans the waste gas before it exits the plant,” says Dale Wiles, maintenance coordinator at the biosolids facility.

RTOs burn natural gas but employ ceramic block media that serves as a heat sink to augment combustion, reducing gas usage. The waste gases pass through channels in the ceramic blocks; the natural gas burner at Buckman heats the blocks to 1,600 degrees F. However, efficiency is lost when the channels become clogged with silica powder, a byproduct of siloxanes (components of personal care products) found in wastewater.

At startup in 2000, the Buckman RTO cells used conventional monolithic ceramic blocks with straight-through, quarter-inch square channels. “We would have to change those blocks out about every year and a half because they would clog up with silica,” Wiles says. Looking for an alternative, the JEA team in 2005 turned to Koch Knight, a company specializing in corrosion-proof materials and environmental heat transfer equipment.

Improving airflow

After consulting with Koch Knight RTO experts, JEA operators installed Type 28 FLEXERAMIC ceramic structured media, composed of vertically aligned corrugated ceramic sheets, in one of the three cells. The structured packing reroutes the hot gases to help negate imbalances from gases moving at different velocities and limit buildup of particulate matter.

After six months of running the unit with both types of blocks, the operators saw that the cell with structured media resisted clogging, improved airflow and enabled effective cleaning in less time than the previous material.

“Several variables affect airflow in RTOs,” says Charles Crosby, manager of byproduct services at JEA. “But we were convinced that the FLEXERAMIC media blocks were superior to the old style. We went from cleaning the RTO cells about every three weeks to about every four months.” Although the structured media had less surface area than the previous material, the greater airflow, improved distribution, and reduced clogging led to more effective use of the surface area and a lower pressure drop.

JEA operators looked to further optimize efficiency by turning alternating layers of blocks 90 degrees, forcing the gases to change direction and spread more evenly. The media then acts as a large static mixer, spreading the gases across the bed. The JEA team used 1-inch-wide FLEXISADDLE packing to fill the voids between the cubic media blocks and the round walls of the RTO cells.

Optimizing efficiency

After several years of improved uptime and efficiency, the JEA team learned about Type 88 FLEXERAMIC ceramic structured media, offering still larger corrugated passages. In 2015, they outfitted all three RTO cells with a total of 2,400 cubic feet of that media and used both 1-inch and 3-inch FLEXISADDLE ceramic random media. This time, they stacked the rows of blocks without alternating the layers’ orientation.

“We’ve had the Type 88 FLEXERAMIC structured media in there for the last three years without any changeout,” Wiles says. “About every six months, we do a shutdown, water-wash with fire hoses back through the blocks, and just start from fresh. We have another set of blocks on hand because we know the silica will eventually overtake it, but so far, it has been a great change.”

Natural gas consumption in the RTOs has not increased, and JEA officials estimate savings averaging $100,000 per year from lower maintenance and less frequent media replacements.  

Doug Popek, Koch Knight sales manager, observes, “Between our media design and the way the operators have installed the product, JEA has figured out how to get the maximum efficiency out of the RTO.”