Tank Mixing Systems and Water Age
By Marisol Vega, Municipal water operations manager. Reviewed by Amanda McCallister, editor.
Commercial Diving Contractors Alabama | Commercial Diving Contractors Alaska Last Tuesday I stood on the roof of Tank 4, a 1.5-million-gallon welded steel reservoir on the west side of our district. The surface temp read 72°F; the sample from the bottom drain came up at 58°F. A fourteen-degree split inside one tank isn't a curiosity. It's a failure mode. Tank 4 isn't our worst tank. It just happens to be the one I was inspecting. I manage 40 finished-water storage tanks for a municipal utility, and every one of them is a small reactor. Water age is the time between a drop entering the tank and leaving it for someone's tap. We aim for less than five days, but in the dead corners of some tanks we've measured water that's 20 to 30 days old. That's where chlorine residual dies, temperature stratifies, and disinfection byproducts climb. Before you spend money on a water storage tank mixing system, understand what it does and what it doesn't do. It doesn't replace cleaning. It doesn't fix a leaking roof vent. What it does is keep the whole volume turning over so the water that entered today isn't still sitting here in June. That's the job.
What Water Age Does to a Tank
When water sits, chlorine reacts. In a stratified tank the bottom layer stays cold and dense while the top layer warms, and warm water wants to stay on top. That bottom water becomes a holding cell for nutrients. We see total coliform start to show up in those bottom samples before anything ever leaves the tank, and in summer the THM level can double between the inlet and the outlet. My state's MCL for total trihalomethanes is 80 parts per billion; HAA5 is 60. I've seen a 1-MG tank with a 35-day dead zone push HAA5 from 42 to 59 parts per billion over a single month. Call it a near miss, because that's exactly what it was. Stratification isn't new. I remember a tank we drained years ago that had 3 feet of loose sediment on the bottom, and the drain line was plugged with a gray clay that smelled like a riverbank. That tank had a recirculation pump pulling from the side and shoving it back in, which only short-circuited from inlet to outlet. You can put a 5-hp pump in a tank and still not mix the middle if the nozzles are aimed wrong.
How a Real Mixing System Works
Good tank mixing is bulk movement, not jetting. The standard design is low speed and high volume: an axial-flow pump that draws water from one level and discharges it across the surface or down the structure. A 1/2-hp solar-powered unit can turn over a 1-MG tank in roughly 30 to 40 hours, depending on tank geometry and the normal flow from demand. That sounds slow, but it's the right time scale. You don't want to stir sediment up off the floor; you want to blend the whole water column. We've installed SolarBee units in three tanks and PAX Water Blenders in two others. Both manufacturers make dependable equipment, but the right choice depends on the tank. Here's the digression every operator will understand: inlet and outlet placement matters as much as the mixer. If the inlet is at the top and the outlet is at the bottom, the newest water will find the shortest path no matter how many impellers you bolt to the floor. We repositioned two inlet risers to discharge near the bottom and outfitted the outlets with baffled risers that draw from mid-depth. The mixer can't overcome a bad pipe layout.
Choosing a System: What to Look For
Start with the tank's actual drawdown pattern. Pull your SCADA data for a month and look at how often the water level moves. A tank that cycles every two days needs a different mixer than one that sits for a week. Then calculate turnover time the way we do: volume divided by the mixer's pumping rate. For a 1-MG tank and a 500-gpm mixer, that's 2,000 minutes, or 33 hours. Add in the normal in-and-out flow from demand, and you're probably in good shape. But don't trust one calculation. Watch the temperature profile after installation. Other practical factors are power source, access, and maintenance. Solar-powered units avoid trenching but need unobstructed sun, and the battery will fail eventually. Electric units are easier to control but add a confined-space entry point and require conduit to the tank. Whatever you buy, choose a unit with a lifting eye and a hatch-mounted base because you will pull it out for cleaning eventually. Ask the manufacturer for case studies from utilities similar to yours. Not similar in population. Similar in tank size, pipe age, and source water. A mixing system that works on a deep clearwell in Oregon may not work on a shallow standpipe in Texas. The good vendors will tell you that.
What We Measure After Installation
Don't just install it and forget it. We run a two-month verification protocol. On a Saturday when demand is low, I shut the tank's inlet and let the mixer run for 24 hours. Then we sample at four depths: one foot below the surface, mid-depth, two feet off the floor, and at the drain itself. We check free chlorine, temperature, pH, and, if the lab can handle the turnaround, total THM and HAA5. My target is less than a two-degree difference from top to bottom and a chlorine residual spread of no more than 0.2 mg/L across those points. If you're still seeing a six-degree split, the mixer is too small, too slow, or placed wrong. After verification, I set up monthly sampling and keep a running chart of the outlet residual. The biggest change is not the average—it's the variance. Before the mixer, our summer residual at Tank 4 would swing from 0.1 to 0.9 mg/L. Afterward it sits in a tight band around 0.7. That consistency is what keeps me from getting a phone call about taste-and-odor complaints on a hot afternoon.
Don't Skip the Low-Tech Work
Mixing systems are tools, not magic. If a tank hasn't been drained and cleaned in years, a mixer will just stir up sediment and make your first flush look terrible. We put Tank 7 on a five-year cleaning cycle, but we check the floor at every annual inspection. When sediment depth is over six inches, we schedule a washout early. Also, mixers need to run continuously. We tried cycling them on a timer to save power, and the tanks stratified again within a day. So now we let them hum. Keep a spare power cord on hand, keep the float sensors clean, and inspect the roof vents and hatch seals every quarter. A tight lid, a screened overflow, and a working mixing system are the cheapest insurance a utility can buy for its oldest tank.
Frequently Asked Questions
How do I know if my tank needs a water storage tank mixing system? Pull a summer temperature profile from top to bottom. More than a 5-degree difference and a 0.3 mg/L residual drop from inlet to outlet means you have stratification and water age. Check the drawdown cycle and inlet/outlet placement first. What's the difference between a recirculation pump and a tank mixing system? A recirculation pump pushes water through a pipe and back into the same tank, which usually creates a short circuit. A mixing system moves the whole water column in a slow, high-volume pattern. One stirs a path; the other turns over the volume. Can I install a mixer without draining the tank? Usually yes. Most units fit through a standard 24-inch hatch or mount through a side wall, though confined-space entry is still often required. Follow OSHA 1910.146 and your utility's procedures. Ask about side-entry mixers if you want to avoid interior work. How long before I see results? You'll see temperature and residual changes in the first week, but the full effect takes about a month because you need to see different demand cycles. Our Tank 4 stabilized after six weeks.