Wastewater treatment tanks come in all shapes, sizes and construction. They also employ many different strategies physically, chemically and biologically to achieve the desired end result. However, all of the strategies have ONE COMMON NEED and that is the wastewater must be uniformly exposed to the treatment entity that does the actual physical, chemical or biological function. If only part of the volume of wastewater is exposed, then the treatment process tank is not achieving its potential treatment capabilities and the energy used is being wasted.
This is where mixing comes into the picture and, unfortunately, mixing has been the most overlooked aspect of wastewater treatment for decades. Until about 15 years ago, no mixing system had really worked well to mix wastewater energy efficiently. In order to achieve even a minimum amount of acceptability, huge amounts of energy were necessary. The cost to adequately mix using mechanical motor driven systems was so high that municipal and industrial users quickly turned away from such methods. Other extremely energy inefficient manufacturers quickly stepped into fill the void based upon their reputation in producing other types of equipment (but not mixing equipment). Lots of claims were made and lots of money was spent to “sell” equipment that just never worked well.
ENTER NOW LARGE BUBBLE MIXING, A NATURAL PROCESS GIVEN TO US BY OUR MOTHER EARTH! This type of mixing takes advantage of the natural forces of gravity and buoyancy. My companies have spent the past 15 years educating and promoting this very simple answer that works in every type of tank and treatment at very low energy levels. What should have been a “no brainer” type of answer was met with resistance by those who stood to lose very lucrative incomes from “selling” what can only be described as “JUNK!” The worst thing about this situation is that municipalities and their citizens have been duped and consequently, FLEECED.
Here is the solution to all of this nonsense……
In order to mix anything placed into water, the solids in the water must be lifted to the surface of the water and continuously moved around until a homogeneous concentration of solids is suspended in every part of the vessel. Large diameter bubbles (>24”diameter) can do this. Smaller bubbles can not do this in large treatment tanks that have a liquid depth more than 10 feet.
Large diameter bubbles have more lifting force than do small diameter bubbles. Thus, it has been proven through field research that….”larger bubbles mix and small bubbles do not mix energy efficiently.” On the other hand, field research has also proved that large bubbles do not disperse oxygen or other gases into water as the large bubble passes through it. So, it can be said with certainty that…..”small bubbles (< 1 inch diameter) aerate more efficiently than larger diameter bubbles. “
Those of us who research where to use various technologies in the most efficient ways now know that we can use a combination of systems to both aerate AND mix water. Large bubbles will keep solids in suspension and aeration technologies can be used to disperse oxygen into the water at the same time. If there are “selector” cells where low or no oxygen concentrations in water are desired, then all that is needed is Large Bubble Mixing. Also, Large Bubble Mixing enhances aeration systems by keeping the small bubbles in the tank so their Oxygen Transfer Efficiency will double or triple.
To convert existing tanks into a more efficient process, all one needs to do is identify where solids are accumulating and create a map where the problem locations are. Then. mixing equipment can be added. This is true whether the process is, for instance, an oxidation ditch, a conventional activated sludge tank or a sludge digestion tank. Most often, conversions using Large Bubble Mixing result in significant electrical energy reduction. The conversion pays for itself in just a few months.
From the standpoint of biology, complete mixing of tanks most often results in a better growth environment for the “bugs” who consume the carbon, nitrogen and PO4 wastes in water. If the micro-organisms are less stressed, then they will grow more quickly, consume more of the contaminants, grow fatter and settle out of the flow of water more quickly. All of these seemingly very simple things result in better water going to the receiving water courses, less solids handling problems and less energy used.
By completely mixing tanks, the solids (sludge) that accumulate on the bottoms are swept away and moved downstream in the treatment plant process. Historically, the amount of sludge that accumulates on tank bottoms reduces the tank volume available for the real treatment workers (bugs) to use. When the sludge is eliminated in tanks, the % of BOD, ammonia nitrogen and PO4 removed improves by about the same percentage as the tank volume recovered by mixing the tank completely.
Whatever future waste treatment technologies turn out to be, there will always be a need for complete tank mixing. Large Bubble Mixing will be the answer with those technologies. Large Bubble Mixing will be “IN THE MIX” of future treatment plants.