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Flash or Rapid Mixer Engineering Design Concepts:

FLASH MIXING DESIGN RETENTION TIMES ( at MAXIMUM FLOW RATE):  

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Flash/Rapid Mixer Tank: 40 Seconds

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Design Basis: Maximum Flow 2.6 MGD (million gallons per 24 hours day)

The key or prime factor of the designing a flash mixer is to defining the maximum and typical flow rate.  The above WTP diagram was designed to handle a maximum flow rate of 2.6 million gallons per day (24 hours basis) using Continuous Stir Flow Reactors (CSFR) or Mixers for both the Flash or Rapid Mixer and the two (2) downstream Flocculator Basins.  The upstream flocculator being denoted as the Primary Flocculator, and the downstream being denoted as the Secondary Flocculator.        

Almost all treatment facilities have some sort of flash mixing design.  The prime objective of  flash mixing is to introduce the coagulant, to quickly disperse into an efficient pin-floc and to quickly discharge it to the floc basin for further down stream processing (see flocculation).

There are many types of mixing devices in use to perform flash or rapid mixing.  We present the CSFR design as the most effective design available.  Static Mixers have numerous marketable advantages, but static mixers are limited by the laws of both chemistry and physics, and as such, cannot be applied, or claimed to be, as equivalent to CSFR, due primarily to residence time distribution   It is for this reason that downstream process problems occur with the use of static mixers as the primary flash mixer.  Once installed, processing plants have no choice but to live with their water quality as the mixing capabilities are dependent upon flow rate alone.  In an effort to counter these effects, processing plants are required to change to more expensive designer coagulants.  During upset conditions (higher turbidity levels of surface water, for example) and during flow turn-down periods, the only opiton is to elevate dosing as there is no way to adjust the mixing intensity, or a way to enhance the residence time distribution available within the static mixer.  Elevated dosing is not only a more expensive continual cost, it mostly exacerbates the problem.  Static mixers must be designed to handle maximum flow conditions where almost all are continuously operated a flow rates well below maximum condition (see static mixer).  Since you cannot change the laws of physics, at lower flow rates, the result is less mixing.    

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 range 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.   

Now lets explore why the use of a top entering mixers design, as part of the CSFR system, with residence times of 40 seconds (at peak flow) or more, is the preferred method of Flash or Rapid mixing.  It is agreed that filter alum immediately reacts with the hardness (lime) within the process water where the pH immediately drops like a stone as there is never enough hardness (lime) within almost all water sources to complete the reaction.  Now lets further explore the limitations of this reaction.  Keep in mind that all water sources are not the same as some sources are considered soft water and some are considered hard water sources.  At 0o centigrade, hydrated lime Ca(OH)2 , the most common additive used to counteract the filter alum reaction has 0% solubility in water.  Said another way, lime does not dissolve readily in water.  So the reaction is actually controlled by the process of dissolving lime within water so that it can then react with the filter alum and that takes time.  In simple terms, water treatment process related problems can be numerous but the most common problems are related to improper or uncontrollable retention times with flash or rapid mixing chamber.  

So what is the magic 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 required and the retention time to complete the reaction at peak efficiency.  Generally, 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 accommodate a specific flow rate.  Flow turndown of 2:1 and beyond can now be readily handled with little effect of developing an efficient pin floc to minimize overall chemical usage.  Just another factor to explain why the performance of a static mixer cannot be compared to a top entering CSFR.    

OTHER FLASH MIXING TOPICS:

RESIDENCE TIME DISTRIBUTION - Can a retention times be too long in a flash mixing chamber?  

Technology 1960's vs. present technology?

Can older technology be better? Using PBT's (Pitch Blade Turbines) and when to apply them?

Problems related to the flawed G-Factor concept! 

04.12.11 

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