Precast concrete proves itself at a new sewer district in Indiana.
By Bridget McCrea
Building an entirely new wastewater treatment facility from scratch is a monumental task that involves a combination of coordination, patience and persistence to achieve. The city of Rockfield, Ind., recently built a 30,000-gallon-per-day plant to handle the growing municipality’s wastewater treatment needs. The municipality decided to make the switch from individual septic systems to a citywide wastewater treatment system, supported by the new facility.
Precast concrete was the material of choice for the treatment tanks – a decision that not only eased the burden of installation and backfilling, but saved the municipality time and money. Designed by GRW Engineers of Indianapolis, the wastewater facility includes five 18,000-gallon precast septic and dosing tanks that can handle 30,000 gallons of daily capacity.
Jim Shields, PE, project construction engineer at GRW, said the facility was built around a recirculating media filter used to “dose” or treat the wastewater four times before discharging it. The precast dosing tanks collect the clean, filtered water in a four-to-one ratio: three parts clean water, and one part “gray” water from the septic tank.
“From there, you’re basically just recirculating that water over the top of the media filter,” said Shields. “The dosing tank basically serves as a volumetric storage basin.”
Shields said his firm was selected by the Rockfield Regional Sewer District to assess the municipality’s needs and utilize a preliminary report prepared by another engineer. “We came in and evaluated that engineer’s report and their options,” Shields said. “Then we came up with the collection and treatment system that was subsequently built.”
Before choosing precast concrete, Shields said the engineers considered fiberglass tanks, but ruled out the material based on cost. “The whole idea was to have the system built off site and installed in components,” he said. “We were trying to avoid a lot of specialty contractors and/or construction employees on site. Instead, we wanted it built off site, brought to the job and installed.”
Mark Wieser, vice president of Wieser Concrete Products Inc., Maiden Rock, Wis., said that early in the bidding phase, the municipality didn’t realize that precast concrete tanks large enough for the project were even available. “We showed them what their options were in terms of the standard tanks,” Wieser said. “We showed them how much more cost effective it would be to use precast.”
Taking the time to educate the potential customer about the merits and capabilities of precast concrete paid off for Wieser, who ultimately won the bid to produce the tanks. Shields said precast won out on cost and was used to manufacture the five 18,000-gallon tanks. And while the material cost for fiberglass versus precast concrete was comparable, he said installing fiberglass tanks was cost prohibitive. That’s because fiberglass tanks would require granular bedding backfill around the tanks, which would necessitate the hauling of such materials to the job site.
“With precast, we were able to use the native soil that was excavated and backfilled around the tanks,” said Shields, “making it a less expensive option.” Comparing fiberglass to precast concrete, he estimates that installation and construction cost about 90 cents per gallon, compared to $1.30 per gallon for fiberglass. The choice of precast concrete also helped move the project along faster – complete installation took just two days, allowing them to progress to the next phase of the project.
Wieser noted that all tanks were vacuum tested and required post-installation testing according to ASTM C1227 “Standard Specification for Precast Concrete Septic Tanks.” There were few other design challenges and no special load considerations for the precast concrete side of the project, which was completed in June 2004.
Terry Piatt, project manager of RE Crosby Inc., Fort Wayne, Ind., helped with the installation. He said the firm has worked on similar projects in the past, many of which utilized cast-in-place concrete. He said it didn’t take long to see the benefits of using precast concrete on this particular project.
“The advantage for us is that weather doesn’t play as much of a factor as it does with poured-in-place,” said Piatt. “You can basically get your hole open, bring the material in and have it all set in a day. Then we started the backfill right away, rather than keeping a hole open for a month or more while pouring the concrete.”
Piatt said the time saved translated into labor saved that wouldn’t have otherwise been achieved. Installation went smoothly and quickly, with the contractor having only to open the holes in the ground, put in the precast structures and start hooking up pipes and installing filters.
“Overall, precast made our job a lot easier,” Piatt said, adding that the project owners are anxiously waiting waiting for another contractor to complete a collection system so they can begin using the new plant. “When we had our walk-through recently,” he said, “they appeared to be very pleased.”
Shields agreed and said the treatment plant is nearly finished. Homes in the community are currently being hooked into the collection system, which was bid as a separate contract from the treatment plant itself. “Everything is going very well with the precast portion of the project playing a role in that success.”
Project Name: Septic and dosing tanks for a 30,000-gallon per day wastewater treatment plant
Owner: Rockfield Regional Sewer District, Rockfield, Ind.
Engineer: GRW Engineers, Indianapolis, Ind.
Contractor: RE Crosby Inc., Fort Wayne, Ind.
Precast Manufacturer: Wieser Concrete Products Inc., Maiden Rock, Wis.*
* Wieser Concrete Products is a certified plant under NPCA’s Quality Assurance/Plant Certification program.