Water storage tanks 101: Getting the most out of your tank

My job is to provide utilities—from managers to water operators—basic advice to make sure those tanks are functioning properly. As a certified water operator with a current T2/D2 license in New Hampshire, I have the technical perspective of both an engineer and an operator when it comes to tackling the challenges of working with water storage tanks. I make sure to keep my certifications up, and I take the same classes as the operators that I help. I understand a lot of the paperwork that many fellow engineers never see.

Storage tanks are not passive elements in a utility’s distribution system. They are dynamic and need respect. Tanks provide buffering storage during high-use times and supply for fire flows. They hold a supply to help communities through their max day demands or busier timeframes. Tanks provide storage because pumps and treatment plants can’t always supply the demand that communities need at those high-use times. Properly sizing, maintaining, and operating storage tanks goes a long way to providing optimum water quality to customers.

In this blog, let’s address common misconceptions about how a tank functions. What’s really happening inside the storage tank? Why is it vital to understand the flow and mixing characteristics? Tanks are a balancing game—is yours sized right? How was it sized? Let’s also discuss the importance of using proper terminology and the concerns of many operators retiring. That “soup can” needs some love!

What’s happening inside your water storage tank?

Water storage tanks are a critical part of every water distribution system, providing the necessary storage to meet the daily and emergency needs of a community. Most people look at these facilities as strictly storage, yet the truth is much more. Distribution storage tanks provide a key function in water quality. It’s critical to understand all the impacts a water storage tank has on your water quality. Let’s explore some key details about the inside of tanks here.

1. Stratification and tank color

Is your water tank homogenous, aka well-mixed? That determines water quality. Just like lakes, ponds, and reservoirs, storage tanks stratify. Unlike lakes and ponds, tanks don’t stratify in one level. Instead of looking like stacked layers of a cake—horizontal bands—they look more like a plaid fabric. They stratify in vertical bands and horizontal bands, with the warmer waters closest to the shell and water line, and the coldest waters in the bottom center of the tank.

Stratification should be a concern, as it can lead to water quality issues and potentially unsafe drinking water. This is because the water is harder to treat when it isn’t homogenous.

Did you know that the color of your tank affects water temperature? This is the reason behind stratification. Dark colors like navy, forest green, tank blue, and stainless steel collect sunlight and heat up fast—think of cars from the old days that were mostly metal and burned you if you leaned against them in the summer. Tanks work just like that. The shell of a steel tank heats faster than the shell of a concrete tank. And color matters. But not to worry. A light-colored tank can help to minimize thermal gain.

2. Sediments, biofilms, and stagnation

Stratification isn’t the only hurdle to overcome. There are also sediments, biofilms, and stagnation (or dead spots). Sediments happen. They need to be removed, and they will reduce your disinfectant residuals over time. This means that your disinfectant doesn’t last as long, which could affect your water quality and lead to unsafe water. Most utilities try to have their tanks inspected on a three-to-five-year basis because regulations call for it. This is a good basis for how frequently to get a detailed inspection report. This report will look at everything from coating thickness and adhesion to structural integrity, as well as compliance with federal, state, and local regulations. But are you getting your tank cleaned out at that same time? If your sediments are more than two inches in depth, you may need to clean more frequently. 

Biofilms are one of the hardest problems to deal with in tanks. They grow on the walls, floors, and columns. Once you have them, they’re hard to remove. We can drain the tank, scrub it down, disinfect it, and put it back in service, but if you don’t remove your biofilms from the source of origination, they come back. Biofilms, just like sediment, reduce disinfectant residuals.

Lastly, let’s discuss pockets of stagnation. These are weird dead spots that occur, normally due to flow patterns within the tank. These dead spots create water quality nightmares. If the water isn’t moving through those areas, the disinfectant residuals will disappear over time. Fixing these via minimization requires mixing.

We help clients with these problems frequently. I often hear something like this when they call: “We have to dump our tank every week to get our disinfection residuals up.” When we look at their tank, how it functions, what they’re doing, and where they’re sampling, 90 percent of the time their problems stem from the above tank characteristics. Once those problems are addressed, the lost water issue goes away, disinfection residuals hold, and, in some cases, chemical uses go down.

In the end, there are many ways to make any tank function better. Most of the time, proper mixing will solve the problem.

Understanding the flow and mixing characteristics of your water storage tank

Water storage tanks are very complex for the work they perform. You need proper mixing to get homogeneous water. Federal and state regulations can include vague terms that mean water storage tank mixers are required in tanks. For example, I’ve seen:

• Active mixers are required
• Mixing is required
• Well-mixed
• The water must be homogenous

Tank mixing has come a long way in the last 20 years. Water storage tank mixers can be categorized into two groups: active mixers and passive mixers.

Passive mixers rely solely on the water flowing into your tank to mix the tank. If your turnover time isn’t high—like multiple times per day—these won’t mix your tank. Active mixers use power to continuously mix the water. They should be used whenever you have low turnover times.

But mixer design isn’t just dependent on turnover time. It’s also dependent on the shape of the tank, size of the tank, obstructions within the tank, depth of water, and maintenance. Most government agencies now require that mixers be designed by an engineer. This is where having a “tank bestie” (aka an engineer who primarily works on tanks) can help you out. They’ll correct the misconception that baffles and recirculation systems are mixers (they’re not). Your tank bestie will steer you away from mixers that require food-grade oil to lubricate them or that are only intended for shallow water depths. Plus, your tank bestie will fill out the permit paperwork for you. Who doesn’t like less paperwork?

How does your tank bestie figure out what mixer type you need? They’ll probably recommend computational fluid dynamic (CFD) modeling of your exact tank. You provide drawings, flow rates, and other data. Using our mixer know-how, we build a model of your tank. We let the model run for about four days to produce two hours of real-time simulation. Then we shift mixers around until we get the right number and location. Our modeling produces a report that shows the type of mixer you need. We’ll use that report as the basis of design for drawings and specifications for procurement.

Water storage tanks are a balancing game—is yours sized right?

It’s best to move toward homogeneous water within the tank to provide ideal water quality to the customer. Routine cleaning, inspections, and data collection are critical to understand the degree of mixing and to providing exceptional water quality from the tank.

Enter: Goldilocks. What does a classic fairytale about getting things “just right” have to do with that soup can in the pantry? Too large of a water storage tank and there are potential issues with water quality. Too small and there are problems with available supply. So, size is important.

If you’re looking at a new tank, hydraulic analysis—the old-school math method combined with some system modeling—is a great way to determine the best size. Once the size is determined, the next thing your tank bestie will determine is the best shape of the tank. Your tank bestie will review the amount of dead water your new tank will have—10 percent or less is the goal—and the overflow elevation. Then, they’ll provide you with options based on the best fit.

The importance of using the right terms when discussing your water storage tank

Take time to understand the terminology around your water storage tank. Learn about the shape and materials of your tank. There are seven types: underground, reservoir, standpipe, pedestal spheroid, ellipsoidal, composite, hydropillar, and custom. Each type of tank has impacts on water quality. Learning the basic terms will help owners and operators have meaningful conversations with technical experts.

If you can’t identify your tank’s shape or materials, it can be challenging for consultants, manufacturers, or other industry experts to understand what you’re trying to convey and what you need. I’ve run into a few situations where an operator has incorrectly referred to the drain on the bowl of the tank as “my belly drain,” which sounds more like an issue with the operator’s body (perhaps they should see a doctor?).

Correct terminology makes those important conversations much easier.

Water industry changes and the effect on water storage tanks

In the water industry, we’ve seen many retirements during the past two decades. Unfortunately, we’re not seeing enough water operators coming in to replace the ones who are leaving.

According to research by Joseph W. Kane and Adie Tomer, water occupations are often older than the national median in the US. The national median sits at 42.2 years old, with water occupations slightly higher at 42.8 years old. Water treatment operators, however, are significantly older on average, with a median age of 46.4 years.

So, this means that utilities may be understaffed. Also, as operators retire and younger ones take on new responsibilities, the less-experienced operators may not be as familiar with their water storage tanks.

Because of that lack of experience, it seems like utilities aren’t maintaining their tanks properly. New superintendents of water systems may not know water storage tank maintenance and basic operational practices, like how often they should power wash their tank (it should be once a year). They may also not understand the regulatory paperwork that they need to file. Unfortunately, water storage tanks don’t come with a manual.

Thankfully, my team and I are here to help. I’m happy to answer questions about water storage tank maintenance, CFD modeling, or other topics I’ve touched on here. Reach out to us anytime.

A vital part of the water distribution system

I hope you’ve learned a bit more about water storage tanks, those vital—but possibly ignored—parts of your water-distribution systems. Let’s make sure your water storage tank is functioning properly, so your customers are receiving the best possible water quality.

Tanks, like forgotten soup cans, deserve our care and attention. I wish you good luck with yours.

A version of this blog was published in Water Online.