PC Construction and CDM Construct World's Largest Thermal Hydrolysis Facility at DC Water
DC Water's Blue Plains Advanced Wastewater Treatment Plant in Washington, D.C., recently became the first facility in North America and the largest in the world to incorporate a unique thermal hydrolysis process system that turns wastewater into power. The new system saves millions of dollars while helping the environment in multiple ways.
A joint venture of PC Construction, based in Garner, North Carolina, and CDM Constructors, based in Boston, Massachusetts, delivered design and construction of main process train improvements, which included working with Norway-based Cambi to install 32 thermal hydrolysis reactors; construct four enormous concrete digester tanks that hold 3.8 million gallons each and stand 80 feet tall; and navigate a congested site without interrupting the facility's around-the-clock operations.
Wastewater to Clean Power
According to DC Water CEO and General Manager George S. Hawkins, "This project embodies a shift from treating used water as waste to leveraging it as a resource."
The $470 million project included three parts: the main process train, a combined heat and power (CHP) facility to transform biogas into power, and a dewatering facility to turn the biosolids into a usable end-product.
The main process train starts with facilities to blend, screen, and dewater raw sludge (the solids left over at the end of the wastewater treatment process) before it passes into Cambi's Thermal Hydrolysis Process (THP).
The THP tanks heat the sludge to more than 320 degrees under as much as 138 pounds of pressure for 22 minutes. When the materials move to a flash tank, the temperature and pressure drop dramatically, weakening the cell walls and the structure between cells to make the energy easily accessible to microbes in the digester tanks, the last step in the main process train.
The sludge digests over a period of two weeks. Methane produced by the process is vented out to the CHP facility, where three large turbines produce electricity, as well as steam that goes back to power the thermal hydrolysis process. The final facility dewaters the digested sludge into a pathogen-free, marketable Class A biosolid.
Compared to Blue Plains' previous lime stabilization treatment system, the Cambi system cuts the amount of waste biosolids in half. That means a significant reduction in trucking costs to farms in Virginia where the biosolids are reused for fertilizer. DC Water eventually plans local uses for the cleaner, Class A product.
In addition, with more than 10 megawatts of clean, renewable energy generated in the process, "We have the ability to make about one-third of our electricity requirements to run this plant," said Brent Christ, DC Water's Supervisor, Construction.
With those cost reductions, as well as chemical savings from eliminating the old lime stabilization process, DC Water expects to save $22 million per year. The new system also shrinks the plant's carbon footprint by one-third.
Design-Build for Faster Returns
Although DC Water typically utilized design-bid-build delivery, with the economic benefits of the thermal hydrolysis system they wanted to get online as soon as possible. The $215.1 million main process train became one of the first projects the facility procured as design-build.
"We wanted to go with that delivery option because it offers time savings on the front end," Christ said. In this case, the design-build team started construction of foundations while they finalized design for the buildings and equipment.
DC Water received six responses to their Request for Qualifications and narrowed the field to three joint ventures. Before PC Construction/CDM submitted their proposal, they sent a group of designers, estimators, and constructors to tour Cambi's research and development plant and three operating facilities in England and Ireland. "We got an up-close and personal understanding of what would be required," said Mike Angeli, Construction Executive for PC Construction.
DC Water awarded the main process train contract in June 2011 and crews broke ground in January 2012. Although DC Water pre-selected and pre-negotiated the thermal hydrolysis system, Cambi served as a subcontractor to the PC/CDM joint venture. "Cambi fabricated, installed, started, and ran the thermal hydrolysis system," Angeli said. "We connected on one side and picked it up on the other side."
115-Foot Shafts and 80-Foot Walls
The four anaerobic digesters (the tanks where the biosolids go after they exit the Cambi system) presented some of the biggest design and construction challenges. To begin, "The first 30 feet of soil in that area was discounted by the geotech engineers," Angeli said.
In order to support each of the 80-foot high tanks in clay silt, PC Construction installed 44 drilled shafts, each four feet in diameter and 115 feet deep. "All of the buildings on the main process train project are on piers; most of them sit on auger cast piers but the digester tanks with that heavy load are on drilled shafts," Angeli explained. "The auger cast piles range from 100- to 200-ton capacity. The drilled shafts were 400-ton capacity piles."
In addition to foundations, the team faced dilemmas with the tank structure. "If we poured the walls cast-in-place, the wall thickness would have been 3 feet at the bottom, getting progressively smaller on the way up, with 18-inch thickness at the top," Angeli said. "That's a lot of concrete placements and a lot of construction joints. You run a high risk of leakage with all the head pressure against a standard concrete construction joint."
Instead, the team subcontracted with VSL, a Denver, Colorado-based company specializing in post-tensioned tanks, to design the tanks and perform the post-tensioning. "The wall thickness went down to 15 inches and was uniform all the way up," Angeli said. "We poured the walls in four 20-foot lifts, in full circumference, and were able to eliminate all vertical construction joints."
Inside the concrete are plastic ducts, like flexible PVC pipe, Angeli said. "They go around the tank, with increasing spacing between the ducts as you go up the tank where liquid pressure decreases. The first tendons they pulled were the verticals, drawing the four pours together. Once they stressed all the vertical tendons, there was a pattern for stressing the circumferential ones. This process guaranteed a bottle-tight connection in the tank. The walls didn't tie into the slab, however, because when you tension those tendons, it compresses the tank and the walls move on the base slab."
With the thin, post-tensioned cable walls filled with reinforcing steel and duct, self-consolidating concrete became the best option to ensure good consolidation. DC Water had never used self-consolidating concrete before. "We think it's a good product going forward," Christ said. "Because it's not as porous as other concrete, it's good for holding in liquids."
On top of each digester, a steel dome spans the 100-foot diameter. "We couldn't build them on the ground and lift them into place because there was no room and they're way too heavy to lift," Angeli said. "If we built them in place, we'd have to erect, pressure-test, sand-blast, and paint them 80 feet in the air."
Instead, crews built the domes at the bottom of the tanks. Once they finished, "We put water in the bottom of the tank and pressure-tested them with air," Angeli said. "After pressure-testing and completion of the coating, we closed all the vents and got the dome airtight, then hooked a shop vac to one of the vents and blew air into the headspace. It only took 2 inches of pressure and the dome floatedbut it wasn't necessarily balanced because the nozzles on the top are not perfectly symmetrical."
To achieve balance, workers painted lines on the concrete tank walls to indicate the level point. "When the dome floated, we could see which spots were higher and we would send a workman or two to stand at the high spots," Angeli explained. "We shifted people around until the dome was balanced. Everyone wrote down their weight where they stood, then we weighted those spots with sandbags."
Next, crews filled the tank with water to lift the dome to the top, then welded on the outriggers to fix the cover in its final position.
Congestion and 24-Hour Operations
Throughout the work site, space was severely limited. "We had tankage on two sides, a building on the third side, and another contractor building the combined heat and power plant on the fourth side," Angeli said.
In addition, all the work occurred with the plant running 24 hours a day. Receiving and storage took place at a warehouse 30 minutes away in Maryland. Workers parked a mile away and rode construction buses to the worksite.
PC Construction relied heavily on a Peiner SK405 Tower Crane with a 220-foot jib. "It had a smaller base than the conventional track crane we usually use," Angeli said. "It took up very little space on the congested site and gave the operator improved visibility."
Up and Running
When the digesters were completed in September 2014, DC Water began phasing in the new thermal hydrolysis system. Crews trucked 2 million gallons of Class A biosolids from another treatment plant a few miles away.
"It was lucky for us that we had a local source," Christ said. "We needed Class A biosolids because they have a unique kind of microorganism and we didn't want to start off with contamination from unwanted bacteria. We slowly ramped up, adding in biosolids from our process. By February, we had 100 percent of the biosolids flow going through the main process train and turned off our old lime stabilization system."
Commissioning and training for the main process train, CHP building, and dewatering facility will continue through early 2016. "This project had 130 service manuals associated with the various equipment," Christ said. "We held 70 unique vendor training sessions for the various pieces of equipment and subsystemsand that's just the maintenance training. There was additional operational training. We installed more than 15,000 new pieces of equipment for the whole project."
Angeli added, "This is the most complex project I've ever done, and I've been with PC Construction for 40 years."