Saturday, September 7, 2024

By Larry L. Bell, CEO–ICS Bell Industries President, Bell Mixing Systems

After a 50 + year career as an operator, engineer and now the owner of a manufacturing company, I know that when a liquid process treatment tank is drained the best way to start evaluation of the tank is to look to see if residual materials lay on the bottom of the tank. This simple first task tells a lot about the incoming liquid matrix, the mixing / treatment equipment and the operational strategies being conducted.

If a process tank bottom is covered by piles or layers of residuals, then I know that the mixing system is not performing optimally. In the wastewater industry, excess residuals mean that not all of the tank volume is available for the intended treatment process. This results in lower carbon, nitrogen and phosphate removal, higher energy cost, higher maintenance cost, difficult solids settleability / handling and more issues too numerous to mention here. On my company’s website, I have posted a paper written nine years ago by Mr. Wyatt Troxel that discusses the process benefits of maintaining good mixing in wastewater plants. The bottom line of this paper is that complete tank mixing improves treatment in each and every step of the total treatment process. Bad mixing in one tank step creates a “domino effect” of treatment issues downstream.

Engineering professors that I have been privileged to learn from and work with the past few decades usually mention that mixing is largely overlooked in the grand scheme of wastewater treatment. They usually refer to most treatment processes as being “mixing limited.” That is a polite / politically correct way of saying that mixing is inadequate and that compensating strategies need to be considered during a project’s design. However, in private conversation, words such as “abysmal” are used to describe mixing by many commonly used mixing products. To save time, I will dispense with the niceties and get right to the blunt facts.

FACT # 1: Fine bubble aeration systems aerate but can not mix effectively. In order to expect bubbles to lift solids upward to create a mixing action, bubble diameters must be many times larger than the 1-2 millimeter diameter bubbles that fine bubble aeration create. The buoyancy (or lifting force) of fine aeration bubbles is only a few ounces. It is physically impossible for aeration bubbles to lift solids such as floc upward to create a homogenous mixture in the entire volume of an aeration tank. That is why diffusers are placed one or more feet off of the floor. If placed on the floor, the diffusers will get covered by solids. Drain any fine bubble aeration tank and you will see piles of sludge below and between rows of diffusers. These solids add endogenous oxygen demand that the oxidation system must overcome. To do that, additional diffusers and larger blowers are added into the tank design. This, of course, adds cost to the project but does nothing to improve mixing. Ten times the number of diffusers can be added to the basin and nothing will change (other than the power bill) because the size of the bubble is what lifts solids. Aeration engineers stubbornly refuse to recognize that aeration engineering principles are completely different from mixing engineering principles. It seems to me that profit is the main incentive.

FACT # 2: Any mixing device that pushes water horizontally does not mix efficiently to maintain a homogeneous distribution of solids. About 30 years ago, I saw an effort by pump companies to turn into mixing companies. They used pump related components to build mixers for use in aeration basins and process selector cells. The result was the water moved horizontally in the tank, solids would settle on the floor or mound in the tank middle and, then, turn septic releasing toxics (i.e. hydrogen sulfide, etc.) into the water distressing the bio-organisms that are supposed to treat the waste. Eventually, black solids would rise to the tank surface and create a scum layer. I interpreted this effort by project engineers as an attempt to solve the mixing problem and I was pleased to see the effort. However, it was obvious that it would take many more mixers to generate enough energy to mix. It was a losing strategy then and is still a loser now. It is not smart to try to beat Mother Nature. But, yet, these devices are still used.

One problem with this approach is that maintenance for these mechanical devices is really expensive. A Canadian customer referred to their now abandoned mechanical mixers as their “series 5000 mixers.” They said it cost them $5,000 per mixer every other year for each mixer to be repaired. Another problem is operator safety. These systems require high voltage motors to be submerged in tanks and it often is a dangerous task just to bring the mixing assemblies up to a level where they can be worked on. Operators have complained for decades about such dangerous working conditions and have also complained about the expense of equipment that obviously does not work effectively. Influential manufacturers stonewall attempts to solve problems when their profit is threatened.

FACT # 3: Large diameter gas bubbles have more buoyancy (lifting force) than do small diameter bubbles. Large gas bubbles will travel up through liquid depths farther than do smaller bubbles. For the purposes of mixing liquids ladened with solids in wastewater applications, bubble diameters need to be at least 24 inches in diameter or greater to be able to travel from a tank floor to the surface of the liquid which is necessary to be of use when mixing. A 24-inch diameter, round air bubble mass has a lifting force of 268 pounds in water. A 36-inch air bubble has a lifting force of 905 pounds in water. A 48-inch bubble’s force is 2,145 pounds. These natural, vertical forces make bubbles useful to lift solids, sludges and dense liquids to disperse them throughout the vessel with the aid of gravity. The lifting force of large bubbles can break-up grease blankets in lift station wet wells, disperse scum blankets in aeration tanks and mix thick sludges (up to 10%) in sludge holding tanks. Large bubble mixing can save Billions of energy and maintenance dollars!

Large bubbles do not aerate liquids which makes them useful to mix anoxic and anaerobic zones in BNR applications. In potable water tanks, large bubbles DO NOT air strip chlorine disinfectants from stored water basins. The cost of producing the large air bubble is a fraction of the cost of producing fine bubble aeration bubbles or buying mechanical mixers. Imbedding a large bubble mixing system within an aeration system has resulted in plant electrical costs being reduced by over one-half and the capital cost of the aeration equipment reduced by similar significant levels. Large bubble mixing is non-shearing making it viable for use to mix neat polymer storage and chemical tanks.

A recent installation in an 18 MGD plant design, enabled the utility to stop using alum altogether to meet their phosphate permit limits and lowered their monthly electric bill from a $90,000-$100,000 range to a $40,000-$50,000 range. That was the result when only one-third of the aeration basin was converted.

Large bubble vertical mixing uses the natural forces of buoyancy and gravity to mix liquid process tanks. The technology is the answer in the short term to make inefficient current technology more cost effective. The technology is the link for existing plants to survive until new technology is developed in the future. No matter what that technology turns out to be, thorough mixing of wastewater process tanks will always be necessary.

When I look back over the 5 decades that I have been in the water industry and consider the plant designs that have existed during that time, I am appalled at what has been allowed to become the current “Best Available Technology” equipment. Billions of municipal tax dollars have been wasted needlessly and engineering standards have fallen to the same corrupt political influences that have decimated many regulatory agencies and programs. There are many very large corporations that have been allowed to garner massive influence. Many of these companies have successfully impeded innovation and squashed attempts to develop technology that works and is cost effective.