Medora Corporation

Water Storage Tank Mixing- Active or Passive?


Courtesy of Medora Corporation

First off, providing a complete mix for your potable water storage tanks should be a 100% best practice for every tank all the time. Because mixing ensures that no water get “old” and that chlorine from new water entering the tank is spread throughout the whole tank. So mixing not only solves water quality and treatment problems, it helps prevent problems. Since proactive mixing for your tanks can alleviate a lot of future headaches, this question begs to be asked,

“Which mixing strategy is better, Active or Passive?”

Let’s start with some basic mixing definitions and how they relate to each other.

Active Tank Mixing
Active mixing can be defined as fluid disturbance that can be applied at any time and for any length of time via an external energy source mechanism (i.e. GridBee® SolarBee® potable water mixers). To further simplify in terms of water storage, it means you have the ability to mix when you want for as long as you want. This is incredibly important for consistent and reliable around the clock water treatment.

Passive Tank Mixing
Passive mixing is like active mixing's kid brother… They’re related but just nowhere near the same. Passive mixing is also a fluid disturbance via an external energy source mechanism; however, it is dependent on other external conditions and cannot be applied at any time or for any length of time (i.e. check valve or other nozzle type systems that work only by re-directing inflow or outflow water). Mixing all but disappears in a passive system except for fill & drain cycles. Water quality issues solved by mixing occur most often between fill & drain cycles making passive mixing strategies nearly worthless during this most important in-between time frame. Passive systems just don’t offer the mixing consistency needed for effective treatment.

Let’s take a look at a few active versus passive mixing head to head comparisons.

Quality Of Mix
Advantage- Active.
Significant Advantage- GridBee® SolarBee®

As stated above, passive systems don’t mix between fill & drain cycles (that alone is a huge problem); whereas, active systems can operate any time and for any length of time. The operational flexibility of active systems win out in every case but there is more to the story to consider. Just because an active mixing strategy is being used doesn't mean it is providing the best mix for your storage tank. Special attention should be paid to understand just how an active system mixes (i.e. complete floor to surface sheet and near-laminar flow systems) when choosing the appropriate equipment.

Equipment Installation & Deployment
Advantage- Active.
Significant Advantage- GridBee® SolarBee®

Passive systems can only be installed during tank construction or with the existing tank drained and out of service. This can be an enormous undertaking and a general pain especially for water systems without sufficient extra supply and/or fire protection capacity to accommodate an out-of-service tank.

Active mixing equipment can also be integrated during any phase of the tank life (construction or post-construction). The big question is whether the tank needs to be taken out of service and this is where you will see some differentiation. While some equipment claims to be deployable with water and in service, the truth can be a lot more gray (use of divers, say 'with water” or “in service' but still need to drain the tank anyway to anchor equipment to the floor). Medora Corporation understood the deployment problems for in-service tanks from the get go and made GridBee® SolarBee® equipment the only potable tank mixers on the market that can truly be installed and deployed with water… and in service… no shades of gray.

Energy & Maintenance Costs
Advantage- Active.
Significant Advantage- GridBee® SolarBee®

Most passive systems (especially check valves) make claims of “no outside energy source needed” or “no maintenance needed”. This falls back right into “really?” territory and claims like these should be taken with a grain of salt.

1) Passive “No Outside Energy Source Needed” Claim: This claim is misleading. It sounds like no additional energy is needed but in fact they are relying on your existing pumps to do all the work. The pumps are already there so it must save operational costs right? In reality, not so much. There is most definitely an additional energy cost and a potentially hefty one at that. The nature of a check valve system increases head pressure. This lowers pump efficiency so it increases the horsepower needed to move each gallon of water, plus it lowers the gallons per minute of the fill rate which in turn increases pump run time to fill the tank.

2) Passive “No Maintenance” Claim: Again, this is partially true but very misleading. 'No maintenance' on a bad and ineffective mixing strategy doesn't really save anything. And this claim doesn't take into account any added maintenance costs by increasing the workload and run time of the fill pump.

How is any of that better?
Active mixing systems do need an outside energy source and will require maintenance (some equipment more than others) but comparing a check valve system total cost to that of a GridBee® SolarBee® active mixing solution (low equipment cost, low installation & deployment cost, low energy cost, and low maintenance cost) GridBee® SolarBee® equipment comes out the best and most cost effective mixing option…hands down.

Bonus Category- Disinfectant Boosting Capability
Extreme Advantage- GridBee® SolarBee®
I almost didn’t include disinfectant boosting capability in this blog entry. Passive mixing systems I know of don’t even offer this capability but then again, neither do most active mixing systems as well. GridBee® SolarBee® mixers were designed to provide great mixing AND for use as a great in-tank water quality management tool. If a tank needs additional disinfectant, it’s better to apply during active mixing and it’s the best to apply directly through the mixer itself. No dispensing at the hatch. No disinfectant hot spots. No disinfectant dead zones. No driving around in the tank with a row boat pulling a gunny sack full of HTH tablets. GridBee® SolarBee® makes it easy and effective to boost residual at any time.

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