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Fine Bubble Diffusers

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FINE BUBBLE DIFFUSER FUNDAMENTALS

Diffused aeration systems are frequently classified into two major categories according to the diffuser's pore/bubble size, i.e.fine-pore diffusers and medium/coarse diffusers.  

     Medium/coarse diffused aeration systems are used in foul-prone applications.

     Most types and brands are suitable for activated sludge applications, but each has its own best applications.  The key is to size the unit properly for each application. Once OTR characteristics are established, the sizing is fairly straightforward. Other factors include alpha factor, impact of floc size and settleability, impact on effluent TSS, etc. The key phrase is "if properly sized/selected."

FINE BUBBLE DIFFUSER LOADING RATES

     For conventional activated sludge of average rate, i.e. medium rate, it is generally recommended approximately 50 lbBOD/1,000 cu.ft. as maximum.  For process stability and better assurances of performance, [fine pore/fine bubble] diffused aeration systems favor the use of low f/m and that is generally restricted to about 10-15 lbBOD/1,000 cu.ft.  This significantly lower rate sizing tip takes into account process recommendations (extended aeration) as well as diffuser technology old hands recommendations.

DIFFUSED AERATION BASIN DESIGN AND TANK DEPTH

     Design of the aeration tanks is also important for optimum efficiency.  Fine bubble diffusers can work at 2.5 water depth but deeper basins will give greater efficiency and superior results on capital costs.  Diffusers are directly dependent on liquid depth for their aggregate efficiency.  As a result, if the basin depth is doubled, it will approximayely use the same horsepower but it will take only roughly one-half the number of required diffusers, i.e. capital cost of the diffusers is about 50%.  Using deeper basins, e.g. 5m, and larger volumes offer much better assurances of performance.  It must be said that one of the authors once witnessed a probably still exisiting wastewater treatment plant at an edible oil plant having a detention time of ... [only] ten (10) minutes.

 

MIXING AND DIFFUSED AERATION SYSTEMS

     A common rule of thumb indicates the air volume required to mix a tank is 0.12 cfm of air per sq.ft. of flat floor area in the tank  This is a way of showing or representing the actual energy requirements per square foot of tank floor.  The reason the 0.12 cfm per square foot of tank is used is to eliminate the variable of depth from the calculation.  Regardless of the depth of the tank one ends up with a proportional energy per unit volume.  Using a value of 20-30 cfm of air per 1,000 cu.ft. as some engineers do, may be a disproportionate amount of energy requirement for deep tanks which may not be realistic.  One must understand the 0.12 value as pertaining to biological solids only.  If it were a tannery wastewater with a lot of heavy material or difficult solids, one could suggest the energy level for mixing be increased to 0.15 or possibly even approach 0.2 cfm per floor square foot.  This is a determination based on the characteristics of the solids to be handled.  For instance, for aerobic digesters one could/would specify0.2 to 0.3 cfm per square foot because of the high concentration of solids and more difficult mixing conditions.

     Most types and brands are suitable for activated sludge applications, but each has its own best applications. The key is to size the unit properly for each application.  Once OTR characteristics are established, the sizing is fairly straightforward. Other factors include alpha factor, impact of floc size and settleability, impact on effluent TSS, etc. The key phrase is "if properly sized/selected."

Need Some Help? answers@engineeringfundamentals.com - James C. Young Environmental - Balestie, Irwin & Balestie