anaerobic vs anoxic 2

[ChaseKB]

Hi all,

I am currently researching treatment options for Nitrogen removal in a WW treatment plant. I came across a 4-stage treatment process called the Bardenpho process.

In it, there are seperate anaerobic and anoxic stages. Throughout my education these two terms have been interchangeable, but it seems they differ in some way.

The text I have says this: "The term 'anoxic' is used in preference to the term 'anaerobic'.. because the principal biochemical pathways are not anaerobic but only modifications of aerobic pathways"

Can anyone expand upon this?

Thanks,

Chase

 

[ChaseKB]

Note: Bardenpho is a 5-stage process, sorry

 

[bimr]

Anoxic denitrification is the process by which nitrate nitrogen is converted biolobically to nitrogen gas in the absence of oxygen. This process is also know as anaerobic denitrification.

per Metcalf & Eddy.


Anoxic denitrification refers specifically to a process for removal of nitrogen.

Anaerobic process is a generic term for biological processes that occur in the absence of oxygen.

 

[ChaseKB]

Yeah I actually came across that definition in the M&E book as well. But given that the Modified Bardenpho process depicts these graphically as seperate processes, I am still not 100% satisfied.

Denitrification occurs in both stages, the main difference being that the secondary Anoxic stage benefits from an internal recycle of nitrate from the tertiary aerobic (nitrification) stage.

 

[bimr]

Denitrification is an anoxic process because it occurs in the absence of dissolved oygen. The term anoxic refers to the availability of nitrate and absence of free oxygen in a reactor.

However, an anoxic process differs from an anaerobic process because it is not characterized by hydrogen sulfide and methane producing organisms.

I believe that the first stage of the Bardenpho process is a selector stage to promote growth of floc-forming bacteria and discourage growth of filaments in aeration zones, thus improving settling. An anoxic selector uses the nitrate produced in the aerobic nitrification process and returned to the selector via a recycle line.

Selectors may fall into 3 categories; aerobic, anoxic and anaerobic. However in the real work, selectors have a range of operational modes. An anaerobic selector receiving nitrate operates anoxically until nitrate is removed, and then becomes anaerobic.

http://www.rmwea.org/tech_papers/secondary_treatment/RMWEA Anoxci vs. Anaerobic Selectors.pdf

 

[kchayfie]

The simplified summary I was given when struggling with the same question goes something like this:

1) Aerobic - uses oxygen as oxygen (i.e. free oxygen)
2) Anoxic - like aerobic, only the oxygen has to be stolen from the nitrates first
3) Anaerobic - no oxygen anywhere (i.e. no free and none bound up in nitrates)

Admittedly this is a simplified view but it helped me to understand the purposes of the different stages in the process.

 

[ChaseKB]

Thanks bimr, and thank you kchayfie. Your summary cleared things up very well, and seems to fit nicely with my original text's definition.

 

[DoctorSludge]

I like to think of these zones like you measure them in the field.
Take an O2 meter - drop it in an aerated activated sludge tank - you get 2mg/l dissolved oxygen engineered minimum for the health of floc forming bacteria. This is fully aerobic. Not ideal for the floc formers - they actually enjoy 3-4 or even 5mg/l but the electricity cost is prohibitive so we add a bit to the volume of the tank and maintain the lower level. Happy accountants, happy floc formers.

Walk upstream, drop the meter into the selector tank ( high food to biomass ratio = few bacteria and lots of carbon food) the bacteria eat the food, breath in O2, breath out CO2, but heck this tank is not aerated. The meter drops to zero dissolved oxygen ANOXIC CONDITIONS and the bacteria are still growing on the carbon. Add a side stream containing nitrate (oxidized ammonia from the aerated tank above)and the bacteria that are 'passing out' for lack of oxygen grab it from a couple of passing NO3 molecules and the two Nitrogens fly off as a pair back to the atmosphere from whence they originated. In these anoxic conditions, strict anaerobic bacteria die, (methanogens,the oldest bacteria on the planet - archea bacteria - that learned to live before there was any oxygen in the atmosphere might as individuals live for a while but the population fails to grow and is washed out). These bacteria find any form of free oxygen fatally toxic given a few seconds or minutes. There are other bacteria (facultative types) that can switch between living in aerobic and anoxic conditions and some will operate in fully anaerobic conditions by having two metabolic pathways available - useful in other ways to the engineer but that is another story. Truely anaerobic conditions are required for methanogenic bacteria and this means no nitrate and no dissolved oxygen for happy methanogens.
It all comes down to being able to think like a bacterium!

 

[Erico44]

Agscan Inc.
Is anaerobic recovery of hydrous nitrogen practical at this time. Also, recovery of phosphorous would be helpful. My notion is at end point the remaining solids are seperated from the water, dehydrated and pulverized for sale as fertilizer. Water sterilization via x-radiation and filtering out microphage allows recovery of potable water to be sold back to local water supplier and/or irrigation district.
info@agscan.net