Very nice, concise and informative presentation as usual. However I strongly feel that is necessary to make an important correction. At minute 1:15 you say that the organics required for denitrification comes from the external recirculation from the bottom of secondary clarifiers. This is not true or at least is not significant because the soluble organics on the recirculation stream contains just the soluble-nonbiodegradable parts (same concentration as effluent BOD) which cannot be used biologically for denitrification. On the other hand the organics returned by external recirculation in the form of particulate matter (at an already degraded level corresponding to effective sludge age) cannot be so fast used by bacteria in the anoxic compartment which has a low volume and low hydraulic retention time. To use these organics the other bacteria should hydrolise and ferment them. The majority of usefull organics come just from the influent as soluble RBCOD (Rapid Biodegradable COD) or from external dosage (e.g. metanol, etanol etc). This concept is followed also in ASM or other models. Another observation is that will be nice to include and emphasise the term “facultative” heterotrophic bacteria in order to understand that the models consider that part of total heterotrophic bacteria are special because are able to solve his catabolic requirements switching from oxygen to nitrate (facultative OHO is part of total active OHO). Extremely important to note that if both molecules (O2 and NO3) are present, the bacteria will switch to the NO3 ONLY after finishing existing oxygen (by the virtue of bioenergetics). This is clearly predicted by the models and proved by real measurements. An important consequence of the above bacterial behaviour combined with the particular naming of the process itself - concept of SIMULTANEOUS nitrification / denitrification - is that many designers tend to wrongly consider that both process take place in the same volume with the maximum efficiency (as for e.g. in intermitent aeration in a same volume=active bacterial mass). The reality is that, depending on the oxygen concentration, the kinetics of one process complement the kinetics of the other - e.g. if for a particular conditions you have 30% eficiency for nitrification you may have just 70% for denitrification. That’s because maximum nitrification efficiency require an excess of oxygen (due to particular shape of Monod kinetics curve) but the denitrification require a complete absence of oxygen. For biofilms there are a lot of pros and cons on the multilayer model (e.g. why the nitrate produced by nitrifiers is difussed only to the inside of the biofilm and not to the outside as well ?) but probably the best thing to emphasise is that the process is very hard to be controlled because the biofilm thickness is continuously changed and strongly depends on the hard to be controlled or properly simulated hydraulic factors (e.g. water velocities, air scouring, water viscosity, local fluid patterns etc.). Even if I give few credits to the multilayer theory I prefer to think the nitirifcation / denitrification process in terms of anoxic pockets model.
Dear Dan thank you very much for your comment. We agree with your statement but we had to concentrate on the basics and simplify some facts to be able to fit everything into a 3 minute animation. We appreciate your input and please keep it up because it describes some things in more detail.
Thank you so much. Very informative and concise, just as it is needed. I'm new to wastewater treatment and have so many questions, yet the answers are the ones to look for. You have a new subscriber! Greetings from Banja Luka
