FINE BUBBLE DIFFUSERS

Description

Fine bubble diffusion is an energy efficient method of introducing air into a liquid effluent. In this way, aerobic bacteria are supplied with oxygen so that soluble organic pollutants can be consumed and BOD values reduced.

The air diffusers are made of porous ceramic or plastic materials or incorporate perforated synthetic rubber membranes. In operation, the diffusers are laid at intervals on the floor of a tank in which effluent is to be aerated. Air is then fed from a low-pressure air blower through interconnecting pipes to emerge from the diffusers as very small bubbles.

The total surface area of the bubbles produced in fine bubble aeration is very great and this enables a large amount of oxygen to pass into solution in the effluent. The rising bubbles also mix the effluent. The diffusers may need occasional cleaning with formic acid to remove lime and iron and condensate may need to be flushed routinely from the pipework.

Application

Fine bubble diffusers are used in activated sludge processes for the treatment of settled sewage liquor and wastewaters from manufacturing industries such as the food, brewing and petroleum industries.

Porous ceramic and rigid plastic diffusers are more suitable for continuous operation. Some perforated membrane diffusers are suited to intermittent operation. As the airflow drops the perforations close, preventing the penetration of water into the pipework. Diffusers with slightly coarser perforations are available for sludges. Such diffusers may be used in solids contact tanks to encourage flocculation of solids from trickling filters and in balancing tanks to avoid odour formation and to assist mixing.

Mixing may not be as vigorous as with some other aerators, but different circulation and mixing patterns can be established in the tank by arranging the diffuser panels or discs in particular patterns on the floor of the tank, or by adding horizontal mixing devices.

Sizing

The oxygenation efficiency of fine bubble diffusers is about 2-3 kg O2/kWh. A typical installation to treat a daily effluent flow containing 2500 kg of BOD might require about 400 aerators sited about 3 m below the surface in a 3000 m3 tank. It would deliver about 3500 m3/hour of air from compressors rated at about 90 kW. When oxygen demand is low the air rate can be turned down to save energy.