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Digging Deeper: Rethinking Air Power

With energy efficient aeration a priority, is it time for a rethink on the way the industry sizes its blowers?

Submersible mixing means that many aerators may be larger than neededSubmersible mixing means that many aerators may be larger than needed

by Hugh Vaughan, UK & Ireland Director, Landia

With aeration accounting for a staggering 50-70 per cent of a treatment plant’s power consumption, energy-efficient aeration should be a priority for everyone in the industry.

Historically, and still to this day, the well-worn ‘handbook’ for determining the SOTR (standard oxygen transfer rate) for the use of Fine Bubble Diffused Air Systems in an Activated Sludge Process runs something along the lines of:

1) Calculate the air flow required for the oxygen demand; 2) Then, calculate the air flow required for mixing; 3) Use the greater of those two numbers to size your blower - and that’s it!

But is it time to tear up that handbook? With more and more treatment plants now on a path towards becoming energy neutral, or at least becoming much more efficient to bring them into the 21st century, I believe so.

Traditionally, plug-flow aeration basins for the nitrification of activated sludge were not built with submersible mixers in mind. Continuing to follow ‘the old handbook’ can all too often result in also using equipment such as a blower that is way over-sized for mixing, which wastes large amounts of energy, especially during the night when loads are so much lower.

If that first number in the calculation - which asks for the amount of air required for nitrification - is greater than the second number for how much air is required for mixing, then fine. But if that second number is greater than the first, then there is an opportunity for power savings. Air flow through diffusers works great for oxygen transfer, but it’s terribly inefficient for mixing. Instead it’s time to bring in a submersible mechanical mixer, which can keep solids in suspension with just a fraction of the horse power that the blower would consume. The use of large-bladed submersible flow makers in a plug-flow aeration basin will further enable the plant to reduce aeration during low-load periods (summer/winter or night time), without losing mixing in the tank.

For many, optimizing the all-important process will naturally outrank energy efficiency concerns; even though up to a staggering 25 per cent of a whole city’s total energy use can be consumed by aeration at its treatment plants.

Savings in oxidation ditches

One company that also uses submersible mixers to actively help its customers reduce their aeration-led energy bills without compromising on their process is Lakeside Equipment Corporation (established 1928), who back in 1964, first introduced the ‘oxidation ditch process’ to the United States.

Lakeside’s Vice President, Warren Kersten says: “To optimize the process for oxidation ditches and save energy, we use rotor aerators, supplemented by Landia’s submersible mixers. During the aerobic phase we have the mixers switched off, but then later when we switch them on, we turn the rotors off so as not to add oxygen”.

In more recent aeration handbooks, you’ll find all sorts of advice on energy efficiency, largely about controls and speeds, including the use of high-speed turbo blower systems and air-bearing technology to efficiently produce air flow, but the introduction of a mixer – designed for the purpose - can play a vital processing and economic role.

“The mixers are put into operation purely for mixing, which unlike other equipment is truly optimized to keep solids in suspension and prevent the tank from going septic,” continues Kersten. “For blowers, variable speed drives may help reduce the amount of power being used, but at the end of the day, a blower is designed to add oxygen, not for mixing. They’re not dual-purpose. We’ve created a cycle so that when the DO (Dissolved Oxygen) drops below 0.5ppm, the bacteria switch from using the free O2 provided by the rotors to attacking the nitrate molecule to obtain their oxygen. We then switch the rotors back on. It enhances the process, and has the added bonus of reducing energy costs”.

Energy savings achieved with this cyclic aerobic/anoxic operation using Lakeside Magna Rotors during the aerobic phase and Landia mixers during the anoxic phase will, according to Kersten, reduce oxygen requirements by around 27 percent. This saves considerable power costs over the typical 20-year life of the project. Based on a 1 MGD plant, this can reduce energy costs by approximately £25,000 per year.
For both oxidation ditches and plug-flow aeration basins, most treatment plants prefer to work with DO levels of activated sludge from 0.5ppm, even up to 2ppm. Anything above that - be it 3, 4 or 5ppm - is simply more aeration than required, and a waste of energy.

In addition to wasting energy, too much dissolved oxygen can cause an upset in anaerobic and/or anoxic selectors by having a high DO in the return activated sludge or mixed liquor recycle. It can also allow certain filamentous organisms to flourish that reduce the effectiveness of the final clarifiers due to a higher settled sludge volume and sludge volume index.

The DO level acts as a buffer, giving the bacteria just enough to work with, but there is really no logic in anything over 2ppm. Too much oxygen may inhibit the bacteria where nitrates are used for the O2 source, so you won’t achieve denitrification. This could mean having to add another carbon source, such as methanol, which further adds to costs and labour.

Substantial off-peak savings

On capital costs, adding one or two mixers should reduce the number of operating blowers. At appropriate times, especially at night, switching off two blowers for two mixers creates savings that easily runs into tens or hundreds of thousands of pounds – and much better mixing is achieved. To bring about the best possible conditions for the process, you must have enough liquid velocity to adequately mix the reactor biomass. Initially, mixers mean more capital costs, but energy savings make the return on investment (ROI) very short indeed.

If you’ve decided to address those perhaps unnecessarily high energy costs and enhance your plant’s activated sludge process with a mixer, then my advice is to look for a low speed mixer with a stainless steel propeller.

With low speed, a mixer will gently mix the sludge without causing floc shear, whereas the shearing by a high speed mixer will disrupt and damage the process. Also, the initial capital cost of a mixer with a stainless steel propeller shouldn’t be a deterrent because it has such a long lifetime. Recently, we’ve been replacing motors that have come to the end of their natural life, but can still use the propellers after 20 or so years in an oxidation ditch because there’s absolutely nothing wrong with them. So when a plant undergoes an upgrade, reusing our mixers’ original propellers represents a big reduction in capital costs for customers. Together with energy savings from using much less horse power, we can help treatment plants on their way to becoming far more sustainable and ultimately, energy-neutral.

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