<p>In my experience one of the biggest changes, I have seen in aeration efficiency is the advent of micro bubble diffusers. Any of you that run modelling packages will be familiar with the energy savings the models predict. What is the real world effect though? Whilst working on aeration efficiency on a standard plug flow activated sludge lane I monitored ammonia removal linearly down the lane and found the installation of microbubble diffusers enabled ammonia removal to commence earlier in the lane before carbonaceous oxidation had completed and after 40 mins hydraulic retention time the rate of ammonia removal made an exponential increase whereas in course bubble diffusion the rate of increase in ammonia removal was more diffuse and extended further down the lane. The connection with FGX3? This yeast extracted bioaugmentation product physically induces smaller bubbles. The driving force is the reduction of Inter facial tension (FGX3 amplifies the increase in Critical Miscelle Values - the measure of surfactant activity). In discussion with one process engineer the argument against the benefits of using FGX3 was stated as "Detergents are detrimental to aeration oxygen transfer". Well, yes ordinary surfactants can impact the benefits of producing smaller bubbles by making the interface stronger and more resistant to oxygen transfer. The small molecular chain proteins in FGX3, even at levels down to 1ppm or less, impact the interfacial tension of the bubbles and we see far superior oxygen transfer in practice - a contributed effect to the environmental acceleration of nutrient removal and easily observed in nitrification and increased Dissolved Oxygen levels as we have experienced over many projects. So apart from achieving DO targets earlier in a process the biochemical processing is more effective.</p><p>All process engineers are taught you need to maintain 2 mg/l of DO to achieve successful nitrification but microbubble diffusers are now enabling lower energy consumption and achieving successful nitrification at DO levels around 1 ppm without the nuisance of filamentous sludge bulking. No matter what oxygen transfer system you have in your process from Mechanical brush aerators to jet venturis and course bubble diffusers you will observe an energy drop using FGX3 and often a pay back off-setting the cost of the bioaugmentation product plus increased process improvement. This article caused me to look back on a good manual produced by the British Practice in Water Pollution Control and though published in 1987 it had a very good description:</p><p>The total resistance to the transfer of oxygen (R) can be considered as being the sum of individual resistances gas film resistance, interfacial resistance and liquid film resistance, and it’s said that the liquid film resistance is the most important<em> in fact all three are impacted by the increased CMC Induced by FGX3</em></p><p>R= Rg +Ri = Rl</p><p>In general terms, the addition of surface-active material to aqueous solutions alters the properties of that fluid with respect to bubbles. In a simple aqueous solution, bubbles tend to coalesce whereas when surface active agents are present coalescence occurs less readily. Normally the smaller the bubble size (i.e., the greater the interfacial area) the better is the mass transfer rate. This would suggest that detergents, which are invariably present in sewage, would enhance the transfer of oxygen in an activated -sludge aeration unit by reducing the bubble size. However, the effect of detergents on the mass transfer coefficient must also be considered. In reducing coalescence, surface-active agents immobilize the bubble surfaces, i.e., the surfaces become more ridged. Gas molecules thus tend to be reflected by the surface, rather than transmitted, resulting in a decrease in Kl, and the rate of mass transfer. Therefore, in assessing the effect of detergents, these two opposing effects must be considered.</p><p>In any activated-sludge system, it would seem likely that the variation in the precise value of Kla associated with the aeration system will be of a random nature depending on the amount of detergent in the incoming sewage."</p><p>In essence, the use of FGX3 decreases the randomness and predictably increases the rate of oxygen transfer because it increases the alpha factor the measure of the efficiency of the overall aeration system since measured properly a standard anionic detergent (3-5 mg/l) is added. Typically, the alpha factor increases 3-fold with hydraulic retention time and this increase is accelerated with FGX3 as well as the CMC values of any detergents present. In addition, cellular transmembrane biochemistry is also a major factor in increasing the efficiency of the biochemical process.</p><p> </p>
KR Expert - Ray Taylor
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