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