Activated Sludge Basin and Dissolved Oxygen

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Activated Sludge Basin and Dissolved Oxygen

QUESTION

I have an activated sludge basin that I can’t keep the DO levels up and its now starting to affect both my BOD and ammonia levels exiting the plant.  I’ve heard about other refineries using hydrogen peroxide and would like to know more.  I have a concern about how to safely use hydrogen peroxide and how this is going to work any better than if I use a temporary blower on the system.    Thanks for your help.  –  Ryan, Refinery Process Engineer

ANSWER

You know, Ryan, the recurring problem of intermittent periods of low dissolved oxygen levels is the number one area where hydrogen peroxide can provide immediate relief to a refinery.  But before we get into the specifics of this application, let’s talk about how USP Technologies’s full service supply scope can address  any safety concerns with using hydrogen peroxide:

1. Equipment – USP Technologies’s wastewater treatment programs include state-of-the-art turnkey peroxide bulk storage and handling systems, which eliminate the risk of operator exposure associated with handling of drums or totes.
2. Product – For our refinery customers, USP Technologies typically supplies “Refinery Grade” 27% hydrogen peroxide.     This product grade is a Class 1 Oxidizer (compared to a Class 2 Oxidizer rating for 50% hydrogen peroxide), and has inherent safety benefits to the higher concentrations of product.
3. Service – Our experienced Field Service Teams handle the equipment system installation and maintenance – so your folks don’t have to.  We also provide wastewater sampling and testing to assist with hydrogen peroxide dose rate optimization.  We also support all aspects of process hazard analysis (PHA), pre-start-up safety (PSSR), and management of change (MOC) reviews as part of the pre-installation process at the refinery.

Now, as far as the use of hydrogen peroxide for supplemental dissolved oxygen (DO), this is a treatment area we have a great deal of experience and many successfully operating systems.  As you know, the treatment efficiency of aerobic biological treatment processes depends on a number of factors including (but not limited to): Influent BOD/COD loading, F:M ratio, temperature, nutrient levels and dissolved oxygen (DO) concentrations.  Many refineries use hydrogen peroxide to supplement dissolved oxygen levels when oxygen limited conditions in their aeration basins result in poor BOD/COD removal, including ammonia levels when nitrifying bacteria do not have enough oxygen to convert ammonia to nitrate.  These conditions can be brought on by unexpected peaks or seasonal variations in influent BOD/COD loading and hot weather – which reduces the efficiency of oxygen transfer by mechanical aeration equipment (i.e., O2 solubility decreases as temperature increases).

When hydrogen peroxide is used to provide DO, it is metered in just upstream of the aeration basin of a biological treatment system to provide an immediate source of dissolved oxygen. The conversion of hydrogen peroxide to DO proceeds by the following reaction:

 2 H2O2 → O2 + 2 H2O

The decomposition of hydrogen peroxide occurs very rapidly due to the enzymatic decomposition of hydrogen peroxide.  Catalase enzyme is a natural decomposition catalyst for hydrogen peroxide, being produced by most aerobic organisms, and is found in all activated sludge mixed liquors.  Since the decomposition of hydrogen peroxide to DO is so rapid, the oxygen supplied by hydrogen peroxide is immediately available for uptake by the aerobic organisms.  Additionally, since hydrogen peroxide is a liquid, and infinitely soluble in water, the amount of DO provided is not limited by the mass transfer of oxygen from the gas phase to the liquid as is the case with mechanical aeration.  This last point is of significant importance to refinery wastewater treatment applications, when comparing the use of hydrogen peroxide to mechanical aeration for supplemental DO supply.  As you are aware, the temperatures of refinery activated sludge systems are often in excess of 95 F and many systems operate at temperatures above 100 F.  Henry’s law dictates that at these elevated temperatures it is even harder to transfer oxygen (gas phase) from a mechanical aeration device into the wastewater and since hydrogen peroxide is a liquid, this limitation does not exist.

 Ryan, I hope this helps answer your questions about how hydrogen peroxide can be applied to a refinery’s activated sludge system to safely and reliably provide supplemental dissolved oxygen.  Give our Applications Team a call at  (877) 346-4262 if you want to discuss this in more detail or want one of our technical team to visit your site for a technology fit assessment.

Sincerely,

USP Technologies Wastewater Technology Group