Big batch

Sulphur plant's second phase to be finished by end of year

By Karla Wall

McInnis Bros. of Minden will complete phase two of a $28.5 million expansion project at the Sulphur Wastewater Treatment Plant by the end of this year, highlighted by the construction of one of the largest concrete sequential batch reactors in the country.

Project manager Sam Bethea said work began on the second phase in May 2005.

The first phase of the project, completed about three years ago, required the installation of an influent transfer pump, tertiary filters, ultraviolet disinfecting unit and a sludge building, said Wayne Harris, project engineer with Meyer and Associates of Sulphur. McInnis Brothers also handled the first phase of the expansion, estimated at $6.9 million.

The current phase will convert the plant from a two-stage anaerobic treatment process to a post-equalization process using sequential batch reactors, Harris said. The change will boost the facility's capacity by 50 percent, from 6 million gal. per day (mgd) to 9 mgd, Harris.

The additions to the plant includes a four-cell SBR basin, a 72-ft. by 34-ft. blower room, a new headworks building and a 33-ft.-deep lift station.

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McInnis Bros. project superintendent Richard Rachal said the SBR basin is 400 ft. long and 200 ft. wide. The SBR tanks are 30 ft. high and have a storage capacity of 23 ft. The structure is built on 594 16-in.-diameter auger cast pilings sunk to 30 ft.

About 13,000 cu. yds. of 4,000-psi concrete were used for the massive SBR unit.

The mix is ACI-determined 350-2R5 standard for high acidic mixes, said Warner Hanks, quality control manager for concrete supplier Port Aggregates. Steel gang forms were used for the basin walls to create a smoother finish.

The differing angles of the walls within the SBR unit created some problems, Bethea said. The interior walls are 12 in. thick at the bottom and 3 in. thick at the top, tapered to withstand the force of water from either side. The exterior walls are tapered to the outside to withstand pressure from the inside.

Bethea said the challenge was the intersection of interior and exterior walls. Because they're angled differently, joining them was a bit of a problem.

"(The contractors) drew a template on the floor of the basin," Bethea said. "They measured and re-measured. They ended up using plywood to reinforce the intersections. The problem was that the walls created a large span for the plywood to go across. It made it tricky to keep the forms from bowing."

Concrete pours have typically taken place from 10 p.m. until 2 a.m. to take advantage of cooler temperatures, said Ed Theriot of Port Aggregates. Roughly 20 pours had taken place as of early August.

The blower room is constructed of 12-in. heavy duty CMU. It will house five blowers, one for each of the SBR basins as well as a backup unit. The blowers will inject air into the process and break down sewer content. Each unit, 76 in. in length and 34 in. wide, is powered by 150-hp motors.

Bethea said the blowers are mounted on 4,000-psi concrete pads inside the headworks building.

The headworks building, constructed of 4,000-psi concrete, will separate solids from the sewer using two stainless steel bar screen filters situated on top of the headworks structure.

Also located on top of the structure will be a de-grit chamber that will remove sand granules from the wastewater, Bethea said.

"Sand is the second most damaging thing to plant equipment, right behind solids," he added.

Four pumps, two 250 hp and two 125 hp, will be located 30 to 40 ft. from the headworks structure. The vertical, variable speed pumps will be located in a wet well built around the lift station, Harris said.

The trench is 35 ft. deep and lined with 4,000 concrete. The bottom of the trench will be 3.5 in. thick and walls will be 3 in. thick. The trench was excavated using a cofferdam with sheet piles driven to 6 ft. deep.

The variable speed motors will increase pumping speed as the water rate increases, Harris said. The pumps will be situated below the lift station and the motors and discharge heads will be located on top of the lift station. The pumps will be suspended by columns.

Roughly half of the existing equipment in the facility, much of which is at least 20 years old, will be demolished to convert the facility, Bethea said.

"We're building a new facility next to the existing one," Harris said. "As soon as the new facility is on line, we'll demolish a lot of the old one and convert it to a post-equalization and sludge treatment facility."

The existing lift station, with its three 60-in.-diameter screw pumps, will be demolished after the new equipment is on line. Also scheduled for demolition are the primary and final clarifiers, as well as the existing biological oxidation unit tower, the dissolved air flotation system and the existing headworks.

"The existing clarifier will be converted to a post-equalization basin, and the old anaerobic digester will be taken offline," Harris said. "We'll take out the boiler and the floating top, and install aerators and sludge pipe and decanter systems."

"We're going to remove all the equipment in the aeration basin and we're going to remove all the dividing walls in the basin," Bethea said.

Ductile iron pipe ranging from 30-in. to 48-in. in diameter will be used to complete the facility. About 400 ft. of 48-in. interceptor pipe will be used; 1,200-1,500 ft. of 42-in. process pipe will be used to take water from the SBR to the post-equalization basin; and about 800 ft. of 30-in. process pipe will be needed from the pump station to the headworks.

Bethea said the project has experienced delays due to Hurricane Rita, a subsequent worker shortage and a rainy July.