All Fouled Up - Investigating PTFE Layered EPDM Membranes |
April 16, 2007 |
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Posted by Barry Nagassar at April 16, 2007 11:51 AM |
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In the aeration basin of a typical wastewater treatment plant there are both organic and inorganic matter which can impair, over time, the function of fine bubble diffusers, thus requiring either additional energy to overcome high membrane headloss, or reducing the oxygen mass transfer to the process.
The rate and type of fouling depend on whether the plant is treating industrial or municipal wastewater, as well as on the process. Typically diffusers types foul more rapidly in low MCRT plants such as non nitrifying conventional processes than in high MCRT plants such as in nutrient removal processes like oxidation ditch, BNR and SBR.
Diffuser media which have been readily available in the market include porous types such as aluminum oxide, porcelain, ABS and Polyethylene, and non-porous types EPDM, Silicone and Polyurethane.
Most diffuser manufacturers have taken a targeted rather than blanket approach to diffuser fouling problems. For example, in a dairy WWTP, it is expected that there will be significant calcium fouling, therefore it is common to use a flexible membrane diffuser rather than a hard porous type which may prove more difficult to keep clean.
In some cases manufacturers have recommended lower roughness coefficient materials such as PU rather than EPDM in such applications to reduce surface adhesion of calcium, gypsum, and silicas to the membrane. However there have always been trade-offs in the selection of a diffuser media other than porous types or EPDM.
For example PU and Silicone formulations that have been used often have a relatively high headloss and lower SOTE than EPDM, and Silicone is prone to tear propagation, while most PU is resistant to only 40 C. Only EPDM provides desirable physical properties and bubble sizes consistent with high SOTE.
In Figure 1, see the proximity of SOTE of EPDM and PTFE layered EPDM in an independent test conducted by ATC, SA of Spain on SSI disc diffusers1. It should be noted that any result above 7% SOTE/m is considered high, and these tests were conducted at a diffuser submergence of 4.7m.
PTFE layered EPDM membranes were introduced in late 2004 and were installed throughout the course of 2005 in two dairies, one paper mill, one post aeration basin, a brewery, a landfill leachate treatment plant, and a number of municipal sewage treatment plants. In most of the cases, PTFE layered EPDM was selected due to the failure of previous technologies to avoid fouling to a sufficient degree that the plant could operate efficiently.
Rosso and Stenstrom have empirically studied the extent of fouling and cleaning intervals of various diffuser media in a wide array of municipal sewage treatment plants and have found that F rates between cleanings of membranes even in municipal plants are much greater than common perception, dropping from an average alpha in a low MCRT plant of 0.50 to less than 0.40 after up to 2 years and stabilizing to less than 0.35 thereafter.2 At this time specifically in low MCRT plants they have found that the difference in a F between porous and non-porous fine bubble media do not vary significantly.
This is the first of a two part series on wastewater management solutions. Part two will look at more empirical evidence in support of PTFE layered EPDM membranes.
About the Author
Mr. Tom Frankle
Stamford Scientific International Inc.
4 Tucker Drive
Poughkeepsie, NY 12603
Telephone: 845.454.8171
Fax: 845.454.8094
Email: info@stamfordscientific.com
Web site: http://www.stamfordscientific.com/
Mr. Frankle is part of Stamford Scientific International Inc. which maintains production and support facilities for various products including: fine bubble diffusers, disc diffusers, and tube diffusers.
Notes:
1. “Clean Water Oxygen Transfer Tests, Scientific International”, October 2005, by Ian Trillo of Asesoria Tecnica y Control, S.A. Clean water tests were commissioned by SSI.
2. “Economics of Fine Pore Diffuser Aging”, by Diego Rosso and Michael Stenstrom, Accepted for Publication Water Environment Research.
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Comments
Hello Mr Frankle
I am with SPAN, the water and wastewater regulator in Malaysia. I was looking up on some information on silicone membrane. Most of the information that had surface shows that silicone membrane easily tears in sewage use.
I am looking into options of deregistering membrane silicone products that had used for aeration in the wastewater treatment plants. The operators had raised their concern on the frequent failure (tear) of this product. However before proceeding with the deregistration we are giving the benefit of doubt to the supplier on the positve information available on the silicone. Unfortunately none so far.
What is your suggestion on this matter?
Thank you,
Punita Nook Naidu
Posted by: Punita Nook Naidu at March 26, 2008 10:54 PM
