Water & Wastewater Treatment

WWT February 2017

Water & Wastewater Treatment Magazine

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Page 30 of 43

www.wwtonline.co.uk | WWT | FEBRUARY 2017 | 31 In the know Digging deeper: wastewater treatment With energy efficient aeration a priority, is it time for a rethink on the way the industry sizes its blowers? 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 oƒen 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 W ith 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 - Rethinking air power HugH VaugHan UK & IRElAnd dIREcToR lAndIA 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 Submersible mixing means that many aerators may be larger than needed

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