Blog-Technical
Tube Diffuser Basics
The earliest tubular diffusers used in Australia were imported from Germany in about 1983 and were of the porous type with external diameters of 70mm and individual lengths of 750mm. They were clamped in pairs either side of stainless-steel square tubing, and held in place by long tie-rods extending the whole pair length.
Tubular membrane diffusers became available in Australia from late 1985, had about the same overall dimensions, and were also clamped in pairs either side of square tubing. In this case, however, the membranes were mounted on PVC pipe bodies and held in place by stainless steel bands at each end. The pipe bodies were typically bonded to threaded end pieces and were open at either end to eliminate unwanted buoyancy forces. Their tie rods are short.
Porous tube diffusers have not been imported into Australia for new plants since about 1990 largely because of the relative advantages of membrane diffusers in terms of turn-down, weight and air supply interruptability.
Early membrane diffusers did not perform well under the relatively warm temperatures in Australia compared with in cooler climates in Europe because of EPDM material shrinkage. So new compounds had to be developed over the years to better retain their mineral oil softeners and hence suffer less shrinkage.
In recent years larger tubular membrane diffusers have become available from EDI in USA in diameters of up to 125mm and individual lengths of up to 1500mm. These have better membrane compounds and very large air handling capacities and so are still in widespread use. Depending on the duties to be served (continuous air flow or intermittent) these can be supplied in advanced EPDM compounds, or in PU (polyurethane).
EPDM compound membranes are favoured for continuously aerated applications, and can be supplied with Teflon or other coatings for special needs. PU membranes are better suited for intermittent aeration duties, but we recommend pilot trials with test rigs that we can supply for applications with problem wastewaters.
Although other manufacturers have used silicone membranes for tube and other diffusers, long term tests in Australia have not shown these to have better long-term performance either in terms of oxygen transfer efficiencies or in backpressure increases.
The largest tubular diffusers supplied by EDI can handle air flows of up to 50Nm3/h per pair, or about the same as twelve 230mm dia disc diffusers for similar oxygen transfer efficiencies. This greatly reduces the need for extensive submerged grid pipework, and can deliver significantly reduced installed costs.
Tom Lawson (September 2020)