Diffuser basics

Why maintain diffusers?

Aerobic wastewater treatment – otherwise known as ‘activated sludge’ – relies on the efficient delivery of air to provide sufficient oxygen to the biomass to carry out treatment ‘work’. The majority of modern treatment plants use fine bubble diffusers which deliver many fine bubbles, ideal for oxygen transfer.

However, the delivery of air can make up over 50 % (and as high as 70 %) of the total power use at a typical activated sludge plant.

In a study undertaken by Aqua Audit for a large Water Authority in NSW the savings from monitoring and maintaining diffusers can be significant. The calculated savings were four times greater than the cost of undertaking testing and maintenance activities.

Source: Energy Efficiency Opportunities in Wastewater Treatment Facilities, NSW Office of Environment and Heritage, 2019.

But what happens to them after a few years’ service?

A clean, new diffuser will deliver an even flow of fine bubbles from its surface. Each bubble has a large surface area to volume ratio – perfect for oxygen delivery.

As membranes age, the diffuser material deteriorates, and slits or pores become clogged

Diffusers slowly accumulate biological growths and inorganic fouling while their membranes may shrink and harden. As a result, oxygen transfer efficiency slowly deteriorates and increases blower power consumption rises.

This has a several impacts:

1. Increased Pressure
2. Larger bubbles
3. Uneven aeration

… all factors contribute to dramatically
reduced oxygen transfer efficiency.

Increased blower pressure significantly increases the workload throughout the system, resulting in:

• Higher power consumption.
• Greater risk of diffuser ‘blow outs’ or tears in the membrane, jeopardising aeration efficiency and capacity.
• Additional blower wear and tear and costly unplanned maintenance.
• Less ‘buffer’ to maximum pressure limits, thus increasing blower shutdowns.

Larger Bubbles

What is happening at the micro level?

As an individual diffuser fouls bubbles must pass through the biofilm resulting in an increased bubble size and worse efficiency.

Source: Garrido-Baserba et al (2015). Linking biofilm growth to fouling and aeration performance of fine-pore diffuser in activated sludge. https://doi.org/10.1016/j.watres.2015.12.011

Uneven Aeration

Fouling, and therefore aeration intensity, can become uneven across the tank with boils or ‘dead spots’. If uneven aeration is very bad, it can generate strong floor-to-surface currents or spiral roll that ‘push’ the bubbles to the surface faster, reducing bubble contact time.

This results in:

• Energy required to blow air through the system can increase every year (on average by 8 to 10% p.a.) a significant, and cumulative, increase.
• Potential failure to meet DO setpoints resulting in licence breaches.