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