When it comes to wastewater treatment plants, grit tanks are important components that rid the influent wastewater of solids, such as road sand and other debris. “At the Manchester, New Hampshire Wastewater Treatment Plant, wastewater is dropped into large tanks with air diffusers that work to decrease the density of the influent. The solids fall to the bottom of the tank and then a conveyor system moves all solids out from the influent before it enters the facility,” explained Robert “Junior” Belisle of John W. Egan Company.
As part of a larger project at the facility, Belisle and his crew were hired by the general contractor to prepare and coat the plant’s four grit tanks, including the interior walls and floors of the tanks, all appurtenant channels, boxes, and other structures. Each tank is 70-feet-long by 10-feet-wide by 10-feet-deep (21.3 m x 3.1 m x 3.1 m) and has a tapered bottom and roof.
“The grit tanks are what I like to call an awkward diamond shape,” observed Belisle, who added that the tapered design would actually pose a bit of a logistical challenge as the project began. “We had to strategically place the scaffolding in the very center of each tank where there is a flat channel that is about 2-feet [0.6 m] wide. Our crew set up pivoted staging jacks and a constructed system scaffolding with safety rails by Safway,” said Belisle.
Arriving on the scene to begin the first phase, the John W. Egan crew found that the tanks had already been drained and washed down by the general contractor. The concrete tanks had existing coal tar epoxy coating that was worn down. “The condition of the concrete underneath the coating varied depending on the location in the tank. The head space where there was the most exposure to hydrogen sulfide was in pretty bad shape. The concrete in the middle of the tank was in decent shape, but the bottom where the grit collects definitely had abrasion damage,” stated Belisle.
Limited Downtime, Limited Space
Another challenge that the crew had to contend with was the tight timeframe. “The wastewater facility was in use the entire time, so only two tanks could be down at one time. The job was divided into two phases over the course of eight months. Each phase was approximately four weeks in length. We needed to get the tanks back in service as soon as we could,” said Belisle. He and his crew of three to four men worked diligently to meet milestones and keep the job on schedule.
In addition to racing the clock, space was at a premium for this job. The laydown area was very limited, and according to Belisle, all equipment had to be stacked in a single file line, making logistics a bit challenging at times.
One piece of good news for the crew, though, was that containment was relatively easy. “The very nature of the tank made it almost entirely self-contained. There is only a 4-foot [1.2 m] gap in the ceiling of the tank. We used tarps for the containment and hooked up a 40,000 cfm [1,133.0 m³/min.] Entech dust collector when we abrasive blasted the concrete,” explained Belisle.
Clean and Prep
The first step in preparing the concrete tanks to receive the new Sherwin-Williams coating system was for the team to power wash the entire area. Using a 5,000 psi (34.5 MPa) Mi-T-M power washer and an alkaline-based detergent manufactured by Great Lakes Laboratories and distributed by Sherwin-Williams, the crew removed all loose coating. It was then time for the team to abrasive blast the substrate to remove all remaining coating material and loose concrete.
“We used an Axxiom Schmidt trailer-mounted blast pot with Black Beauty abrasives to achieve a NACE 6/SSPC-SP-13/ICRI CSP 3–6 surface profile,” said Belisle. That’s Society for Protective Coatings (SSPC) Surface Preparation (SP) and International Concrete Repair Institute (ICRI) Concrete Surface Profile (CSP).
The crew was outfitted with Bullard blast helmets and Tyvek protective suits during the blasting process. Once the blasting was complete, the crew cleaned up all waste blast media using a 2,300 cfm (65.1 m³/min.) industrial vacuum manufactured by Vector Vacuums.
With the concrete prepped and ready to go, the crew repaired all areas of deterioration or where bugholes existed in the concrete. Sherwin-Williams’ Patching Compound for Concrete and Steel Seam FT 910 were trowel applied where necessary.
Short Window
According to Belisle, one of the most important components of the Sherwin-Williams coating system is the Corobond 100 primer. This 100 percent solids epoxy primer helps prevent off-gassing. It was applied using a Titan 30:1 airless, gas-powered sprayer at a dry film thickness of 4–6 mils (101.6–152.4 microns). “It wasn’t as simple as spray applying the coating, however. This primer has a very short recoat window, so to counteract that, we blew clean silica sand over the uncured coating to re-establish the surface profile. Doing this provided an unlimited recoat window,” explained Belisle. The silica sand was sprayed on with a pneumatic broadcast gun by Kraft Tool.
With the recoat window issue addressed, the John W. Egan team applied Sherwin-Williams Cor Cote SC Mortar epoxy coat at an average dry film thickness of 125 mils (3,175.0 microns). This epoxy mortar coat was spray applied on both vertical and horizontal surfaces using a WIWA 12:1 airless pump and then troweled.
Some areas of the grit tanks received an additional coating system layer. In the headspace, where hydrogen sulfide exposure is greatest, and on the walls and floors of the sump pits where more abrasion resistance is needed, another coat of Sherwin-Williams Cor Cote SC Resin was applied at an average dry film thickness of 20 mils (508.0 microns).
Belisle pointed out that safety protocols were followed at all times on the job. “Gloves, safety glasses, and boots were worn at all times during the application process. Tyvek suits were also worn, especially when applying the patching material and mortar coat. When we spray applied any of the coatings, the guys wore 3M half-face respirators,” said Belisle.
Time for Some Warm Air
Since the work was done in a tank environment, the New Hampshire weather patterns were less of an issue than they would have been in a more open area. However, with the last two tanks, colder temperatures did prove to be a bit of a challenge. “The temperatures were dropping — although there was no snow quite yet — and applying the coating could have become very difficult. Luckily, the air diffusers for the grit tanks were already installed. Because of this, we were able to blow warm air from the facility into the tanks. This kept the ambient temperature in the tanks where it needed to be for the coating system application,” stated Belisle.
After the work on each tank was complete, but before it was put back in service, Belisle and his superintendent used a DE Stearns high-voltage holiday detector to ensure that there were no bugholes or holidays in the Sherwin-Williams system. Any areas that required repair were addressed immediately, and the John W. Egan team then turned the tank over to the Manchester Wastewater Treatment Plant. “Although there were definitely some challenges on this job, all in all, everything went really well. The right coating system was chosen, and our crew was able to meet all deadlines and turn the tanks over on schedule,” said Belisle.
