Caring for Concrete

Deterioration in large wastewater treatment structures is a costly problem. An expert offers some practical advice on fixing, repurposing and reconfiguring.

The most expensive part of a wastewater treatment plant is not the mechanical equipment — it’s the massive concrete tanks and other structures in which the treatment processes function. So says Kent Nichols, wastewater treatment practice manager with the Weston & Sampson environmental and infrastructure consulting firm based in Peabody, Mass.

Nichols observes that concrete structures in the nations’ treatment plants are aging — and meanwhile new and tighter permit requirements are forcing plants to revisit their treatment processes. For plant operations teams, that brings a variety of challenges, not the least of which is deciding whether existing buildings and tankage can accommodate the processes needed to meet current and future nutrient limits.

Nichols talked about the preservation, repair and repurposing of concrete structures in an interview with Treatment Plant Operator.

TPO: How would you describe the basic challenge of dealing with concrete structures in treatment plants?

Nichols: Most plants get pretty good funding to do repairs and upgrades of their mechanical equipment, but many plants lack the budget, time and sometimes the capacity to do maintenance on the physical structures. So what happens is that they live with the tankage and buildings while they go through three or four cycles of changing out the pumps, motors and other devices. All the while the structures are aging.

TPO: What happens to concrete structures as they age?

Nichols: We’re talking about aggressive environments. Conditions vary some depending on the region, but a constant in the business is degradation of organic material, causing formation of hydrogen sulfide that causes surface corrosion. You also get some pH conditions that can be challenging.

And everybody is exposed to weather conditions. Where I am in the northeast, the freeze-thaw cycle is a big thing. Concrete structures need to be designed for proper expansion. These are large structures — big tanks with thick walls and deep foundations. It can be a challenge to create expansion joints to control cracking.

One common thing we see in tankage is cracks on the surface of the concrete. In some cases, those cracks are just a result of natural aging, but often they result from the fact that the original expansion joints weren’t constructed properly. It’s a real challenge, and it tends to be greater in regions with a lot of freezing and thawing cycles.

TPO: In general, how would you rate the condition of concrete structures in treatment plants today?

Nichols: It’s highly variable. Many treatment plants have structures that go back to the 1960s when the original rules were put in place, and there were a lot of primary treatment systems that wouldn’t meet the standards set by the Clean Water Act of 1972.

From the 1970s to the 1980s, we built a lot of new plants and new structures to address the need for secondary treatment and introduce biological processes. So a lot of plants that existed in the 1960s were amended in the 1970s and 1980s, and now they have a mix of concrete that is 50 years old, and other concrete that is 25 to 30 years old.

Curiously enough, in plants we’ve worked in, some of the concrete that is 50 years old is in just as good or better condition than concrete that’s 25 to 30 years old. It tends to be mainly about how the concrete was prepared and set, and the type of environment it has been subjected to.

In the later upgrades, we find the designers were good at putting coatings on the concrete to prevent those aggressive conditions from attacking the surface. We find that a lot of original concrete, if it was constructed well, if it had good oversight, and if it was a good mix design, is still pretty sound.

TPO: What are the consequences of concrete deterioration?

Nichols: The big thing is it starts to affect structural stability. The places where you see it the most are in closed tanks, where we see a loss of mass on the surface. When that happens, the surface starts to erode and you’ll actually get damage to the shallow rebar. When you lose enough mass to affect the rebar, the rebar starts to fall apart. Once you’ve exposed one face of the rebar, your stability is really in question.

TPO: How do you help treatment plants deal with aging concrete structures?

Nichols: It’s a challenge for a variety of reasons. Probably the biggest one is that most plants were built or upgraded in the 1970s and 1980s and, mainly for financial reasons, a lot of communities didn’t get around to rehabbing or repairing those plants every 20 years, which was the original intended life.

Now a lot of plants are in desperate need of repair or modernization, and at the same time there are very significant changes in the discharge permits the plants are facing, so the treatment plant has to do a different job than was originally envisioned. That means you have to go back and re-evaluate. What structures do you have? What do you need for structures? What types of processes will allow you to stay within the footprint and tank sizes you have?

You have aging infrastructure and constantly tightening discharge permits, aggravated by the fact we’re in some difficult financial times. So since structures are the most expensive things to build in a plant, you’re left with a need to try to adapt them to the new permit conditions, but not spend extra money on them.

TPO: Why are these structures so costly?

Nichols: First, large structures that contain a lot of concrete are inherently expensive to build. Second, most such structures are built in excavations, and that creates all sorts of additional challenges. Wastewater treatment plants frequently are not on pristine sites, so you can run into poor soil conditions or things previously buried on the site. Old sludge burial areas are pretty common on treatment plant sites. So are landfill areas. Poor-quality urban fill is also a common problem.

TPO: How should a clean-water agency proceed when dealing with aging concrete structures and new permit requirements?

Nichols: The first step is good planning. When you’re extremely limited in the money you can spend, the last thing you want to do is change things without a lot of forethought. The new treatment processes may be basically similar, but when you add the need to deal with nitrogen and phosphorus, those processes are very different. They’re more space intensive, they’re more biologically intensive, they require more chemical addition, and they require a lot of systems you didn’t envision originally. In selecting process changes, you need to look not just at what your permit is now, but what your discharge limits are likely to be in the next couple of permit cycles.

TPO: What is the role of operators in this kind of planning process? Nichols: You need to rely on the operators. We find a lot of communities miss the opportunity to involve operators in the design. Operators know which systems they can live with and which ones they can’t. They have a good feeling for what the life of the structures and systems are like — they look at the structures from day to day. They can be very helpful in evaluating not just where you are today, but where you think you’ll end up in the future, based on permit changes.

TPO: What’s involved in assessing the condition of the structures themselves?

Nichols: The first step is surface testing. Frequently, with modern systems, we can do some non-invasive testing where we take scans of the walls and check the density of the concrete. There are technologies that provide new ways of seeing the strength of concrete and the rebar.

We generally find there’s no real substitute for doing some core samples, where you actually take a section out and look at the structural stability. There are a variety of tests we can run on core samples. We can look at the condition of the concrete at various depths. Often we can grab pieces of the rebar and check out its condition. We can get a good sense of how the original concrete structure was built. That helps us understand where the structure is in its life and how sound the structure was to begin with.

TPO: What can be done to remediate a structure that has undergone some damage?

Nichols: If you find that the structure’s physical configuration will work with what you need for new permit conditions and any process changes you’re going to make, then you have a few options. If the structure is in generally good shape, you can just do surface protection. There have been some great advances in surface coatings.

You do a pretty intensive surface cleaning to take off any loose material. You identify the cracks and use a specialized process to inject the thicker cracks, and maybe finger-seal the smaller cracks. Then you go back and do a protective corrosion inhibition coating on the concrete. In very intense situations, you can apply a built-up coating.

There are a lot of specialized liners available. They include epoxies, bitumen-based materials, and flexible lining systems that will bridge gaps if you’ve got movement in a tank. We find that on a solid tank surface, something with a hard finish performs well. The epoxies in particular are pretty tenacious. They also allow you, once you get a coating on a tank, to do a better job of cleaning the surface later.

TPO: What about tanks that have more severe surface damage?

Nichols: One option if you have a loss of surface mass is to use a spray-applied amendment, like shotcrete. Then you still finish that off with a surface coating to protect it. Tankage has to be treated carefully. You don’t want to put on any type of mass if it won’t hold, and you don’t want to put it in a place where over time the environment is just going to destroy it again.

TPO: What if the structural damage is severe enough so that these coating actions aren’t workable?

Nichols: If in situ repair techniques won’t work, then you need to do more significant reconstruction, in a lot of cases demolishing the tanks. If you’re smart about it, you can actually use the better pieces of the structures and change the configuration to be suitable for more modern treatment process technologies.

TPO: What about repurposing of concrete tanks?

Nichols: We see a lot of repurposing. A lot of old anaerobic digesters years ago were taken out of service and changed over to different modes of operation. Many were changed to aerobic digesters or gravity thickeners. I’ve seen places where they changed a circular digester tank out to be a room for pumping equipment. In one case they built a chemical feed room inside an old digester.

The curious thing is that this is starting to come full circle, because now more than anything, energy use drives a lot of the business of upgrading plants. The need to pay attention to energy efficiency and use opportunities for renewable energy drives a lot of communities back to anaerobic digestion with the ability to capture methane gas as an energy source for combined heat and power systems.

TPO: How would you summarize your advice for treatment plants in dealing with structures?

Nichols: The biggest challenge going forward is finding a way to fund projects. When you’re at the end of the life expectancy of a wastewater treatment facility, that results in some kind of large capital project, and the challenge with large capital projects is mostly funding-related. The good news is that most wastewater systems have enterprise funding, where the budget is independent of the local tax rate. But even then the need to be very fiscally conservative is pressing.

The big thing we push is to really understand the inventory of what you have for physical structures and process equipment and to follow a sound planning process. Really understand what spaces you have and will need, what the condition of your structures is, what your permit needs will be in the future, and what capacity you will need. Wrap that together and come up with an effective plan that tells you what to do.



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