OPTION #2: Value engineering on repairs
OWNER: Walcott Public Works Department
Why does a rural community of 1,500 residents need 1.3-mgd wastewater treatment capacity?
Because it's also home to the world's largest truck stop. Every day, more than 5,000 people including Walcott residents use the 220-acre “Iowa 80” megaplex of heavy-rig museum and store, fast-food restaurants, hotels, business center, movie theater, barber shop, and dentist and chiropractor offices.
And because the tiny hamlet is located in an area with heavy soils and high ground-water. The city battles chronic sewer system inflow and infiltration from pipes and manholes, broken laterals, and residential foundation drains and sump pumps.
In 2005, faced with compliance issues primarily related to ammonia, Walcott's father-and-son public services team of John and Scott Brockmann contracted with Stanley Consultants Inc. to replace six aerated lagoons treating 0.34 to 2.4 mgd with an energy-efficient, packaged Aero-Mod Inc. activated-sludge treatment system that removes biological nutrients by converting ammonia to nitrate and nitrate to nitrogen gas.
Once the $4.86 million improvements were up and running, the managers decided that additional valving would allow better control of solids withdrawal from the system's two aerobic digesters. The treatment train sends sludge from each digester to a belt filter press for dewatering via a common pipe. Each week, the press processes about 1,000 gallons of nominally 2% digested solids to 18% cake that the city gives to anyone who needs fertilizer.
A valve on the withdrawal pipe from each digester prior to the pipes joining and going to the press would allow operators to draw from only one digester or balance flows between the two. But how to install a valve without taking both digesters offline and pumping them out? That would disrupt operations for at least a week.
The first potential workaround was an insertion valve, a common fix in the drinking water industry that doesn't require isolating a portion of the system. But finding a contractor willing to use potable water equipment on a wastewater application was difficult, and getting them to come to the relatively remote location was costly.
Stanley Consultant Principal Environmental Engineer Jay Brady, PE, and the Brockmanns came up with another option: plug the digester sludge withdrawal pipes and install an underground valve with valve box on each pipe to allow manual valve operation from above.
The Brockmanns' public services team would excavate the pipe. Then Stanley Consultants would send a diver into each digester to install and confirm the integrity of a temporary inflatable plug with an attached rope at the discharge outlets. Public services would then install the plug valves. Finally, Stanley Consultants would deflate the plugs and pull them out of the digesters.
Both valves were installed in a day for less than $4,000 including labor and materials.
Structural Engineer-Commercial Diver Ryan Bell, PE, had never done such a procedure. He routinely studies the design plans of client projects, blindfolding himself to practice a particular operation step by step so his mind and body automatically replicate the movements at the jobsite. But he didn't consider how little traction the digesters' slick floor would provide.
“It's like being inside a Jell-O pudding cup,” he says. “I'd never been in that consistency of fluid before. Imagine trying walk in that — there's nothing to push off of.” He wore a vulcanized rubber suit with surface-supplied air diving equipment; and was hosed down alternately with a high-pressure washer, soap, and chlorine for 45 minutes afterward.