Water & Wastewater Treatment Magazine
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In the know Digging deeper: wastewater treatment 32 | FEBRUARY 2017 | WWT | www.wwtonline.co.uk 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 a™er 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. Switching from blowers to mixers may result in significant energy savings, especially outside of peak hours