Membrane Diffuser Solutions for Wastewater Treatment Systems

In the aeration basin of a typical wastewater treatment plant there are both organic and inorganic matter that can impair the function of fine bubble diffusers. Eventually this requires either additional energy to overcome high membrane headloss, or reducing the oxygen mass transfer to the process.

The rate and type of fouling depends on whether the plant is treating industrial or municipal wastewater, as well as on the process. Typically diffuser 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. Silicone is also prone to tear propagation, add most PU is resistant to only 40 C. Only EPDM provides desirable physical properties and bubble sizes consistent with high SOTE. 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 (in their paper Economics of Fine Pore Diffuser Aging) 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 aF 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. At this time specifically in low MCRT plants they have found that the difference in aF between porous and non-porous fine bubble media do not vary significantly.

Stamford Scientific has recorded case studies where diffusers did not require cleaning, however, it was the operator's curiosity to look at the diffusers that drove them to drain the tanks and inspect them. In both cases, the surface bubble pattern was consistent with new and clean diffusers, the dissolved oxygen concentrations had not changed from new, and the diffuser headloss appeared not to have changed significantly.

There are tests and evidence that shows little to no change in a between a new PTFE and an aged PTFE membrane in a typical municipal plant setting. This also includes evidence that there is little to no change in DP, with the help of independent research and the addition of further empirical examples. If this can be proven, wastewater plants of all sorts that install PTFE layered membranes will be looking at energy savings over the operating life of the plant of 30 to 40% with the added benefit of reduced maintenance and perhaps less frequent replacement requirements.