Key points
Wastewater treatment accounts for about 2-3% of the electricity consumption in developed nations, equating to approximately 60 terawatt-hours (tWh) annually. In the municipal wastewater treatment sector, the most significant portion of energy usage occurs in the biological treatment phase, typically ranging from 50 to 60% of a plant's total consumption.
To put that into context, around 30 to 40% of energy usage in an average-sized city is allocated to wastewater treatment operations, with aeration – the introduction of air to foster microbial growth – accounting for up to 25% of this.
Aeration certainly plays a crucial role in the secondary treatment process for both municipal and industrial wastewater treatment. Indeed, if you don’t aerate a plant’s incoming organic matter then the treatment process will fail. But it places a heavy burden on the plant’s electricity supply by taking up more than half of all the electricity it uses.
The statistics are all the more alarming in the knowledge that wastewater treatment facilities rarely operate at their full design capacity. In fact, they run on average at only one-third of their potential. There’s clearly room for improvement.
But the good news is that aerators can be made much more efficient with today’s technology. It all starts with pairing the motors used to run pumps, compressors and blowers with variable speed drives (VSDs), also known as variable frequency drives (VFDs) or, simply, drives.
In pursuit of Wastewater Aeration precision
But before we discuss the vital role of drives, let’s explore the challenges of aeration in further detail.
As we know, aeration in wastewater treatment involves introducing air into the wastewater to stimulate the growth of aerobic microorganisms, aiding in the breakdown of organic matter and improving the water's overall quality. This process enhances oxygen levels, facilitates biological degradation, and is a crucial step in achieving effective wastewater purification.
To perform aeration efficiently, you need to achieve the optimum range of dissolved oxygen in the wastewater. Too much dissolved oxygen is a waste of energy and won’t make the aerobic digestion process any more effective.
In fact, when plants over-aerate their basins, the microbes are getting too much oxygen and getting bigger and bigger, requiring more sewage to feed on. It’s just not efficient. Neither is too little oxygen; the bacteria just aren’t able to biodegrade the incoming organic matter within a reasonable time.
The goal, therefore, is to reach and maintain the optimum dissolved oxygen range, which is particularly difficult when there are so many factors at play.
Compound problems with Wastewater Aeration
Many things can disturb a plant’s aeration processes. These include the continuous and uncontrolled variation in volume of incoming loads; diurnal variation due to population activity; seasonal changes, like heavy rains; and industrial slug loads. Such fluctuations make it extremely difficult to optimise equipment and process control and can all lead to significant problems for plants, spanning from hours to even months.
When faced with these continuous and uncontrolled disturbances, operators can often fall back on equipment oversizing. But the larger the machinery, the more expensive it is to buy and run. It’s also noisy and – since its larger size is only useful sometimes – highly inefficient. A smarter solution would be to optimise appropriately sized machinery to cope with the fluctuating demands.
The solution starts with a drive
There are various different designs of wastewater treatment aerators, which perform the common function of creating a greater amount of contact between air and water. All contain electric drives and motors to power the pumps, mechanical aerator and air blowers used in the aeration process.
Equipment without a drive generally runs pumps, compressors and blowers at full speed, even when demand is lower. A bit like pressing the accelerator when your foot is on the brake, it’s a wasteful use of energy and it puts unnecessary strain on equipment. This is where variable speed drives can be a game-changer.
A drive is a device that controls the speed and torque of an electric motor by varying the frequency and voltage of the power supplied to it. It achieves this by converting fixed frequency AC power to variable frequency AC power using power electronics. This allows for efficient and precise motor control, enabling the motor to operate at different speeds to match the specific needs of the treatment process.
The ability to adjust a motor’s speed according to real-time demand is particularly important for wastewater aeration, which, as we have seen, requires operators to reach and maintain the optimum dissolved oxygen levels, despite a multitude of disturbances.
Faced with the same challenge, drives ensure that air pumps and compressors run at the most efficient rate to match the required flow rate and, alongside dissolved oxygen probes, they maintain precise oxygen levels in aeration basins, regardless of external factors.
Case study: the pitfalls of a traditional on/off approach
Here’s a real-world example. The aeration control strategy for a surface aerator at one particular wastewater treatment plant was typically traditional. That is, a dissolved oxygen probe and transmitter combination that measured the amount of oxygen in the basin and fed this data back to the controller. The controller would then compare this with the optimal dissolved oxygen range and turn the direct online motor on or off depending on the levels.
It didn’t work. Data revealed that this on/off control approach was largely incapable of maintaining the desired dissolved oxygen level and, most of the time, there were high levels of inrush current.
The plant chose to install ABB ACQ580 water drive for the motors, to give greater flexibility of oxygen transfer, together with a new control system to improve the system’s response to the oxygen demand. This more than solved the problem.
Remarkable results
The flexibility of a drive compared to a fixed speed drives results in optimised energy consumption, enhanced efficiency and precise control over aeration levels, ultimately improving the overall performance of the aeration treatment process.
The energy savings are significant. For example, we have seen that the process of aeration typically requires over half of the electricity consumed in a typical wastewater treatment facility. Energy savings achieved by using drives can be as high as 50% of the electricity cost.
The introduction of drives into aeration control also lowers parts and maintenance costs by eliminating the need for control valves, which are high-maintenance components. Operating blowers at lower pressures also reduces stress and vibrations on mechanical system components.
Lower blower speeds results in lower environmental noise pollution, which is particularly crucial for applications running around the clock in densely populated areas.
An important step towards a sustainable industry
With results like these, it’s no wonder that the water industry now widely employs VSDs to manage aeration and many other essential water treatment and distribution processes.
Ultimately, reaching net zero is a key goal for the water and wastewater industry and applying variable speed drives to aeration is one powerful way of helping the sector towards a more sustainable, energy-efficient future.
Discover more about variable drives for the water industry, here
