How the micro-pore membrane works, body and membrane construction, EPDM and silicone membrane selection, confirmed disc diameters, performance notes, basin layout, operation and maintenance.
← Back to Disc AeratorA disc aerator — also called a disc diffuser, a membrane disc diffuser or a micro-pore disc aerator — is a fine-bubble aeration device that supplies oxygen to aerobic biological treatment. It is one of the three standard body shapes in a diffuser grid, alongside the tube diffuser and the plate diffuser, and it is the shape used where bubble release has to be distributed evenly over the floor of a basin in individual, well-defined points.
The unit is a flat circular body carrying an elastic micro-pore membrane on its upper face. Air is fed to the underside of the body, lifts the membrane away from its seat, and escapes through the pores in the membrane as a ring of fine bubbles. When the blower stops the membrane relaxes back onto its seat and closes the pores, so the diffuser seals itself against the tank contents.
Our range covers disc diameters of 215 mm, 260 mm and 300 mm, with EPDM or silicone membranes, and the recommended combinations are 215 mm EPDM, 260 mm EPDM and 300 mm silicone. Disc aerators are used in aerobic tanks, aerated biofilters, activated sludge systems and aquaculture aeration. Other diameters, membrane materials and connection arrangements are supplied to order — state the basin, the airflow and the medium on the enquiry and we will confirm what is available before quoting.
Air from the blower passes along the distribution main to the branch pipe and enters each diffuser through its air connection. Inside the body the air fills the space beneath the membrane and presses the membrane upwards off its seat. The elastic membrane stretches, the micro-pores or slits punched through it open, and the air is released into the water as a ring of fine bubbles around the diffuser.
Because the opening is a perforation in an elastic membrane rather than a fixed hole in a rigid plate, the pores are held closed by the elasticity of the material whenever the air pressure is below the opening pressure. When the blower stops, the membrane relaxes back onto its seat and seals the pores against the water. This has two practical consequences: wastewater does not drain back through the diffuser into the air main, and the pores are closed to the mixed liquor while the air is off, so they stay cleaner and clog less than fixed orifices. It is also the reason the diffuser can be used in basins that are emptied and refilled, and in intermittent aeration processes.
Oxygen transfer takes place at the surface of the bubbles as they rise through the mixed liquor, so the amount of oxygen dissolved into the water depends on how much bubble surface is in contact with the water and how long the bubbles stay in contact with it. A fine bubble has a much larger surface area for the same volume of air than a coarse bubble, which is why a micro-pore diffuser transfers oxygen at a higher efficiency per unit of air than a coarse-bubble device. The rising bubble plume also draws water upwards with it and sets up a circulation pattern in the basin, so the diffuser both oxygenates and mixes the tank contents: the number of units, their spacing and the water depth are what set the circulation achieved.
The bubble diameter released by a micro-pore membrane diffuser of this class is commonly in the region of 1–3 mm, and the release is even around the full circumference of the disc, so the bubble pattern on the water surface is a set of rings rather than a few large plumes. See section 6 for what this means in terms of transfer efficiency and pressure loss, and why those figures are confirmed per project.
A disc aerator of this type is assembled from the following parts:
Diffusers, branch piping, gaskets and supports are supplied to suit the air main and the basin layout, so the number of units, the spacing and the connection arrangement are set by the grid you need rather than by a fixed catalogue assembly.
The membrane is the working part of the diffuser and the part that wears, so the material has to match the service conditions as well as the process duty. Both materials in our range are elastic elastomers perforated with micro-pores, and both seal themselves when the air stops.
| Membrane | General character | Typical selection |
|---|---|---|
| EPDM | Ethylene propylene diene monomer rubber. Elastic, resistant to heat, ozone and to acids and alkalis, and the standard membrane material for municipal and industrial wastewater aeration. It is the usual first choice for general service, and it is the material of the recommended 215 mm and 260 mm units in our range. | Municipal sewage, general industrial wastewater, aerobic tanks, aerated biofilters and aquaculture aeration at ordinary wastewater temperatures. |
| Silicone | Silicone rubber. Retains its elasticity over a wider temperature band and is more tolerant of high temperature than EPDM, with good resistance to ageing and to oxidation. It is the material chosen where the wastewater is warmer than EPDM comfortably handles or where the service conditions are demanding, and it is the material of the recommended 300 mm unit in our range. | Higher-temperature wastewater, industrial effluent where the basin runs warm, and duties where a longer in-service life is wanted. |
The membrane grade, thickness, hardness and pore design for the unit you order are confirmed on request, and the service temperature range of the membrane is stated with the quotation, because the acceptable temperature depends on the compound and on the medium, not on the material family alone.
| Parameter | Value |
|---|---|
| Diffuser type | Micro-pore disc aerator (membrane disc diffuser), fine bubble |
| Disc diameter | 215 / 260 / 300 mm |
| Membrane material | EPDM or silicone |
| Recommended combinations | 215 mm EPDM, 260 mm EPDM, 300 mm silicone |
| Other diameters | On request |
| Body material | On request — moulded thermoplastic of this diffuser class |
| Membrane thickness and hardness | On request |
| Membrane retention | Retaining ring over the membrane edge — arrangement confirmed per order |
| Air connection type, size and thread | On request — confirmed with the air main design |
| Branch pipe / saddle arrangement | On request |
| Airflow per unit (design range) | On request |
| Service area per unit | On request |
| Bubble diameter | On request — fine, of the order of 1–3 mm for this diffuser class in general |
| Oxygen utilisation / transfer efficiency | On request |
| Head loss (dynamic wet pressure) | On request |
| Recommended water depth | On request |
| Service temperature range | On request |
| Mounting | On the air branch pipe on the tank floor, in rows or banks; also for mounting through a filter plate where required |
| Application | Aerobic tanks, aerated biofilters, activated sludge, aquaculture aeration |
| Unit of supply | Per set, with branch piping and accessories quoted separately |
| MOQ and price | On request |
Disc aerators are built to suit the basin, so every dimensional and performance figure is confirmed in writing against the ordered configuration and your tank design. Where a figure is not confirmed for your duty it is marked On request rather than estimated.
The oxygen actually dissolved in the basin depends on the water depth above the diffuser, the airflow per unit, the number and spacing of diffusers, the mixed-liquor characteristics and the water temperature. Deeper water gives the bubbles a longer contact time and therefore more transfer per unit of air, but it also raises the blower discharge pressure the compressors have to overcome. Diffuser density and layout decide whether the air is spread over the basin floor or concentrated in a few plumes. For these reasons oxygen utilisation and standard oxygen transfer rate are performance figures for a specific tank layout, and they are confirmed per project rather than published as a single number.
Head loss (dynamic wet pressure) is the pressure needed to open the membrane and pass the air through the pores. It rises as the airflow per diffuser rises, so the diffuser head loss, the water depth and the losses in the air main have to be added together when the blower duty point is calculated. A diffuser operated far beyond its design airflow does not simply produce more bubbles: the pressure requirement climbs, transfer efficiency per unit of air falls and the membrane is worked harder. The design airflow range for the ordered unit is therefore confirmed on request and should be respected when the grid is set out.
Aeration is normally the largest single electrical load in a biological treatment plant, and fine-bubble diffusion is selected because a finer bubble gives more oxygen transfer for the same volume of air, which reduces the air — and therefore the power — required for a given oxygen demand. The saving is realised only if the diffusers are correctly sized, evenly laid out, kept clean and operated at the design airflow, which is what sections 7 and 8 describe.
The membrane is a wearing part. Its working life depends on the quality of the air supply, the number of start-stop cycles, the temperature and the chemical character of the wastewater, and on whether the diffusers are run within their design airflow. Service life is therefore not published as a single figure; it is established by the operating history of the plant, and the practical measure is to keep the air supply clean and to hold spares so that an individual membrane can be renewed when its bubble pattern deteriorates.
These are general good-practice steps for a floor-mounted fine-bubble diffuser grid, not a substitute for the project installation procedure. Confirm disc diameter, connection arrangement, spacing and design airflow against the ordered configuration before detailed engineering.
To quote accurately and to confirm that the diffuser suits the duty, we need the following. The more of it you can send, the faster the quotation and the smaller the risk of a mismatch on site.
Price is on request, so that the quotation reflects the diameter, membrane, connection arrangement and quantity actually ordered rather than a catalogue price for a unit that may not match your basin.
This reference is compiled from the standard construction and operating description of the micro-pore membrane disc diffuser type, from general published practice for fine-bubble aeration in water and wastewater treatment, and from the confirmed data for our own range. Material-family statements and behaviour described as general practice are given for guidance only. Figures that apply to the unit we supply — diameter, membrane material and thickness, connection, design airflow, service area, oxygen utilisation, head loss, water depth and temperature range — are issued in writing with the quotation and confirmed against the ordered configuration and your basin design. Where a figure is not confirmed for your duty it is marked On request rather than estimated.