Flash or Rapid Mixing:

FLASH MIXING DESIGN RETENTION TIMES (MAXIMUM FLOW RATE):  

The key or prime factor of the design is defining the maximum and typical flow rate.  Almost all treatment facilities have some sort of flash mixing design, whether its a static mixer, a top entering mixer, or some other type of mixing device.  The prime objective for flash mixing is to introduce the coagulant, to quickly disperse into a pin-floc and to quickly discharge it to the floc basins.  

Some coagulants, such as the most common, acidic filter alum or aluminum sulfate {Al₂(SO₄)₃.14H₂O}, also requires the addition of lime into the flash mixing chamber to compensate for the source waters hardness factor due to alums chemical reaction with the hardness in the water.  Aluminum Sulfate functions best in the narrow pH rante between 5.8 and 7.  Other common coagulants include Soda Alum or Sodium Aluminate {Na₂Al₂O₃}, Copperas or Ferrous Sulfate {FeSO₄.7H₂O}, Ferri-Chlor or Ferric Chloride {FeCl₃} and Ferrifloc or Ferric Sulfate {Fe₂(SO₄)₃ .9H₂O}.  Ammonia Alum {Al₂(SO₄)₃.(NH₄)₂ SO₄.24H₂O} and potash alum {Al₂(SO₄)₃.K₂SO₄.24H₂O} are also widely used dependent upon cost and availability but these salts have some limitations compared to the iron coagulatants.  There are also numerous more expensive poly-type coagulants that are available that have numerous performance enhancements and capabilities.   

Flash mixing process related problems can be numerous but the most common problems are related to improper or uncontrolable retention times in the flash mixing chamber.  As an example, although in theory a certain G-factor can be determined for a static mixer design at peak flow, it would take a very enlightened argument to explain exactly what portion of the energy is attributable to frictional losses in an empty pipe, what is attributable to non-useful drag on the internal mixing elements, and what is attributable to actual mixing.  Again, this is assuming peak flow conditions.  The reality is that most treatment plants don't operate at peak flow, where flow turndown, dependent on the season, may be anywhere from 2:1, and most often well beyond 2:1.  Now it becomes necessary to have multiple static mixers to subdivide the total flow dependent to achieve and maintain optimal performance.  The reality is the loss of process control, which can be compensated by elevating your chemical usage, or by changing to a potenially more expensive chemical source, or living with the result.

So what is the majic surrounding designing a top entering flash mixing chamber at 40 seconds at peak flow.  Obviously, economics are involved as there is a direct relationship between energy per unit volume and retention time.  In other words, the smaller your retention time, the higher your energy per unit volume requirements will be.  In regard to process control and optimal chemical usage due to fluctuations in flow rates (retention times), a top entering mixer can be equipped with a variable speed device to lower the energy per unit volume to accomodate a specific flow rate.  Flow turndown of 2:1 and beyond can now be readily handled with little effect to the pin floc. 

Can a retention times be too long?  

Technology 1960's vs present technology?

Can older technology be better? Using PBT's.

Problems related to the G-Factor

09.21.23