February 2012, Vol. 24, No.2

Problem Solvers

Southerly sets the standard with solids disposal efforts

Problem: Increased solids volume after construction project increased overall capacity.
Solution: Pumps and sliding frames enable cake disposal.

 

The Southerly Wastewater Treatment Plant, which serves most of the greater Columbus, Ohio, area, recently completed a 5-year, $350 million expansion. The expansion nearly tripled its peak capacity from 431,500 to 1.25 million m3/d (114 to 330 mgd). The plant has earned numerous awards for plant and employee performance, but its solids disposal program truly makes Southerly a standout.

Using a quartet of heavy-duty pumps and a number of sliding-frame components from Schwing Bioset Inc. (Somerset, Wis.), cake can either be routed directly to incineration or sent to a pair of storage silos. Once in the silos, the material is available for truck loading and transport, either to an existing composting operation or directly to the landfill.

 

Change they can use 

Centrifuges installed several years before the expansion successfully handle the increased flows to the solids handling area, according to Jeff Hall, assistant plant manager.

“That upgrade was implemented both to replace aging equipment, as in the case of the centrifuges, and to add functionality to other areas, like the transportation of solids,” Hall said. “In the past, primary solids were gravity-thickened while older centrifuges thickened the waste activated sludge [WAS]. The new units now thicken both the primary solids and the WAS. This new approach boosts the solids content of the resulting dewatered cake to about 20% to 25%, a nice improvement over the 17% to 21% solids content with the older system.”

Additional changes included installation of new cake pumps, a pair of storage silos, and sliding frames at two points in the solids handling process.

 

The routes to disposal 

Southerly’s pumps and silos assist in transporting cake to several disposal options. As material exits the centrifuges, it is routed to any of four KSP 45V(HD)L-SFMS pumps, which route it either directly to the incinerators or to storage silos.

“Even though incineration is the most efficient method of disposal, we still try to keep the compost operation fed with as much as it needs, since that is the better use of the product,” said Carmon Allen, solids supervisor 2 at Southerly. The solids pumps at Southerly are designed to generate a force sufficient to move cake the long distances needed for either incineration or storage. It is approximately 91 m (300 ft) to the multihearth incinerators, which have operating temperatures of 760°C (1400°F), and about 122 m (400 ft) to the storage silos, Allen said. Equipped with a solids-flow measuring system, the pumps are able to measure to within 5% the amount of solids that are pumped to the incinerator. This simplifies the plant’s U.S. Environmental Protection Agency reporting requirements for the incinerator.

“Material headed to the silos, however, has an additional challenge to overcome,” Allen said. Once it reaches the base of the silo, “the cake has to go straight up another 100 ft [30 m] to enter the top of the structures, so the force needed to do that is really pretty impressive. I don’t think any regular equipment would be up to a task like that; these are definitely the right pumps for the job.”

 

Giving it the slip 

Despite maximum operating pressures of 7585 kPa (1100 lb/in.2) for each pump, the extended distances at Southerly prompted Schwing Bioset to make accommodations to help move the solids along. The company added a pipeline lubrication system with a 360-degree annular groove that evenly injects a thin film of water around the entire annulus of the pipe. The water separates viscous and sticky materials from the inner wall of the pipeline. This reduces friction loss in the pipeline and lowers pipeline operating pressures by as much as 50%. This lubrication also means less energy use to move the solids and less wear on parts.

Parts such as the pumping rams, poppet valve discs, and seats are lasting for 6 months, said Tom Thomas, maintenance supervisor 2 at Southerly. “That’s about 4000 hours of wear-part life, which is outstanding, given what they’re being asked to pump.”

 

Silo efficiency 

Prior to the installation of its two silos, Southerly relied upon a smaller holding vessel, a belt-fed, hopper-equipped component that used a series of screws to load trucks sitting under the discharge chute. City officials say the new silos are larger — providing about 75% more capacity — as well as far more efficient.

The new silos can load a truck in 5 minutes instead of the 45 minutes that was required before. Because the city pays a contractor to haul biosolids, reducing loading times lowers overall hauling costs. Trucks now spend more time hauling and less time waiting to be loaded. This means more trucks are loaded per day at a lower cost.

Inside the sliding-frame silo, hydraulic cylinders move an elliptical frame across the silo floor. The frame’s action not only breaks any bridging that can occur above the extraction screw, but it also pushes and pulls material toward the silo center for discharge. The cake then is fed into a twin screw feeder for discharge into trucks.

“Each silo holds better than 1500 tons [1360 Mg] of cake, so even if one of the incinerators went down and there was an interruption in the trucking operation, we’d still have a nice short-term storage option while things get back up again,” Allen said. “It’s really all about flexibility, and these silos afford us that.”

Due to the sheer size of the silos, each is equipped with three extraction screw conveyors at the bottom, enabling the trucks to be evenly loaded without having to be moved back and forth.

The silos also include an odor- and splash-control shroud that minimizes the need for odor control in the truck-loading building, reduces the chance of material splatter during loading, and confines any splatter to the area immediately adjacent to the trucks.

After loading, the solids are hauled either to a landfill or the composting operation. “Today, we are reusing about one-third of the solids we handle through the composting operation,” Hall said. “We are generating revenue from a product that was once simply discarded. However, it is also a plus from an environmental perspective. Any time you can reuse something rather than just burying it or burning it, you are making a positive impact.”

 

©2012 Water Environment Federation. All rights reserved.