Blog-Technical
Disc Diffuser Basics
Early disc diffusers in Australia were either of true porous ceramic material or of porous sintered plastic. Membrane disc diffusers did not appear here until about 1987.
A large number 300mm dia porous carborundum discs were installed at Melbourne Water’s Eastern Treatment Plant in about 1974, then at the Lower Molonglo plant in Canberra in 1976. The ETP ceramics were replaced many years ago because of underdrain plenum leakage problems, but the latter are still in service.
210mm dia sintered expanded plastic porous diffusers were introduced by Nokia in about 1980, are still operating at several plants in Australia, but are no longer made.
Sanitaire manufacture 230mm dia porous ceramic diffusers that fit in the same bodies as their membrane diffusers, but these are relatively rarely used now. Membrane discs offer a far bigger turn-down capability than porous discs, and have the great advantage of being able to tolerate repeated interruptions to air supply without clogging.
A wide choice of membrane disc diffusers is now available in Australia, in diameters of 170-300mm, though larger diameter discs are occasionally used overseas. In all cases the membrane disc overlies a backing plate on which it seals when air pressure ceases. A wide variety of diffuser bodies are available, with various means of fixing these to the supporting lateral pipework.
Disc diffusers must be closely spaced over the aeration tank floor area to prevent cells of swirling hydraulic currents from carrying the bubbles too quickly to the tank surface and thus reducing effective contact time. Disc diffuser systems are therefore usually associated with large numbers of pipe ‘laterals’ over the whole tank floor area.
EDI has developed a system of ‘4-P’ to ‘8-P’ disc diffuser mountings through which many discs can be fitted more economically to stub cross-pipes on a much lesser number of laterals. This system has already been deployed to good effect by Aer-Force in many installations in Australia.
Disc membranes are usually held in place by threaded locking rings which screw down on the diffuser bodies. This allows the membranes to be simply replaced at the end of their typical 5-10-year service life. Aer-Force can supply replacement discs to fit nearly all diffuser installations in Australia.
EPDM is the most commonly used membrane material in use for disc diffusers, but some manufacturers offer silicone as an alternative. There is little evidence that these offer longer service life, and their oxygen transfer efficiencies are often lower. EDI can offer Teflon-coated or Teflon-impregnated EPDM membranes at additional cost for problem wastewaters.
Aer-Force can offer test rigs to measure the relative long-term performance of different membrane materials or coatings.
Tom Lawson (December 2020)