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Membrane Diffuser Solutions for Wastewater Treatment Systems

In the aeration basin of a typical wastewaterthe course of 2005 in two dairies, one paper
treatment plant there are both organic andmill, one post aeration basin, a brewery, a
inorganic matter that can impair the functionlandfill leachate treatment plant, and a
of fine bubble diffusers. Eventually thisnumber of municipal sewage treatment plants.
requires either additional energy to overcomeIn most of the cases, PTFE layered EPDM was
high membrane headloss, or reducing theselected due to the failure of previous
oxygen  mass  transfer  to  the  process.technologies to avoid fouling to a sufficient
degree that the plant could operate
The rate and type of fouling depends onefficiently.
whether the plant is treating industrial or
municipal wastewater, as well as on theRosso and Stenstrom (in their paper Economics
process. Typically diffuser types foul moreof Fine Pore Diffuser Aging) have empirically
rapidly in low MCRT plants such as nonstudied the extent of fouling and cleaning
nitrifying conventional processes than inintervals of various diffuser media in a wide
high MCRT plants such as in nutrient removalarray of municipal sewage treatment plants
processes  like oxidation ditch, BNR and SBR.and have found that aF rates between
cleanings of membranes even in municipal
Diffuser media which have been readilyplants are much greater than common
available in the market include porous typesperception, dropping from an average alpha in
such as aluminum oxide, porcelain, ABS anda low MCRT plant of 0.50 to less than 0.40
Polyethylene, and non-porous types EPDM,after up to 2 years and stabilizing to less
Silicone  and  Polyurethane.than 0.35 thereafter. At this time
specifically in low MCRT plants they have
Most diffuser manufacturers have taken afound that the difference in aF between
targeted rather than blanket approach toporous and non-porous fine bubble media do
diffuser fouling problems. For example, in anot  vary  significantly.
dairy WWTP, it is expected that there will be
significant calcium fouling, therefore it isStamford Scientific has recorded case studies
common to use a flexible membrane diffuserwhere diffusers did not require cleaning,
rather than a hard porous type which mayhowever, it was the operator's curiosity to
prove  more  difficult  to  keep  clean.look at the diffusers that drove them to
drain the tanks and inspect them. In both
In some cases manufacturers have recommendedcases, the surface bubble pattern was
lower roughness coefficient materials such asconsistent with new and clean diffusers, the
PU rather than EPDM in such applications todissolved oxygen concentrations had not
reduce surface adhesion of calcium, gypsum,changed from new, and the diffuser headloss
and silicas to the membrane. However, thereappeared  not  to have changed significantly.
have always been trade-offs in the selection
of a diffuser media other than porous typesThere are tests and evidence that shows
or EPDM. For example PU and Siliconelittle to no change in a between a new PTFE
formulations that have been used often have aand an aged PTFE membrane in a typical
relatively high headloss and lower SOTE thanmunicipal plant setting. This also includes
EPDM. Silicone is also prone to tearevidence that there is little to no change in
propagation, add most PU is resistant to onlyDP, with the help of independent research and
40 C. Only EPDM provides desirable physicalthe addition of further empirical examples.
properties and bubble sizes consistent withIf this can be proven, wastewater plants of
high SOTE. It should be noted that any resultall sorts that install PTFE layered membranes
above 7% SOTE/m is considered high, and thesewill be looking at energy savings over the
tests were conducted at a diffuseroperating life of the plant of 30 to 40% with
submergence  of  4.7m.the added benefit of reduced maintenance and
perhaps less frequent replacement
PTFE layered EPDM membranes were introducedrequirements.
in late 2004 and were installed throughout



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