"Anoxic" is terminology which refers to biological denitrification. In anoxic zone, nitrate is electron acceptor instead of oxygen. In anaerobic zone, the biological phosphorus removal (BPR) is done by a group of microbes named PAOs (polyphosphate accumulating organisms). These microbes are capable of accumulating more phosphorus than non-PAOs organisms. Using the polyphosphate, PAOs are able to uptake fermentation products (acetate, VFAs) under anaerobic condition. However, if NO3- exists in the reactor, acetate and other fermentation products are preferentially used by denitrification organisms. Hence, PAOs cannot grows. Therefore, the important thing in biological phosphorus removal is to ensure that no nitrate enter the reactor.

If both denitrification and BPR are required, the BPR reactor will be located prior to Anoxic zone. RAS in settler is recirculated to BPR reactor while nitrate flow from aerobic zone is returned to anoxic zone.

Dear Carl Hanson,

Enhanced biological phosphorus removal works because Acinetobacter organisms, which are heterotrophic, have a metabolic quirk: they can absorb soluble BOD under anaerobic conditions and store it until they are in an aerobic environment,where they then metabolize it. (Most heterotrophic bacteria cannot transfer soluble BOD under anaerobic conditions.) So, in the right environment with the right type and amount of BOD—they prefer short-chain carbon compounds—Acinetobacter

Organisms will predominate.Enhanced biological phosphorus removal is a two-step process in which an anaerobic environment is followed by an aerobic one ( attached Figure ). In the anaerobic selector,

Acinetobacter organisms release phosphorus, thereby obtaining the energy to uptake readily biodegradable organics. This ability enables Acinetobacter organisms to become dominant. It also tends to result in orthophosphorus concentrations as high as40 mg/L. Phosphorus release typically occurs within 0.5 to 1 hour of HRT. When the mixed liquor enters the aerobic zone, Acinetobacter organisms grow new biomass and take up phosphorus—typically more than the amount they released in the anaerobic zone. EBPR effluent may contain less than 1.0 mg/L of soluble phosphorus.

Soluble BOD also drops from between 70 and 80 mg/L to 1.0 mg/L (attached fig).

In addition, some wastewater facilities have reported that operating in the EBPR mode provides superior sludge settling Acinetobacter organisms grow slowly, but faster than nitrifying bacteria. To avoid washout, the process’ overall MCRT should be between 2 and 3 days. Longer MCRTs (up to 40 to 60 days) do not hurt Acinetobacter organisms, but the RAS’ nitrate concentration Could prevent the first zone from being truly anaerobic. Ideally, an EBPR facility should avoid nitrification, but if both nitrification and BPR are necessary, then plant staff should take steps to eliminate interference from combined oxygen.

Thank you very much!

As much as I understand it, anaerobic and oxic zones mean more or less the same thing because in each case, oxygen is excluded so it means that only anaerobes and facultative aerobes can thrive.

Good points explained by Drs.Nhat Le .Baboo and Aloo.

Dear Carl,

Let me also put it this way: In Nitrification Denitrification and Biological Excess Phosphorus Removal (NDBEPR) systems, the meaning of Aerobic, Anoxic and Anaerobic is as folows:

Aerobic means there's dissolved oxygen (DO) and here you normally have degradation of biodegradable COD (organics) by Ordinary heterotrophic bacteria (OHOs) and also in this region you get the oxidation of Free and Saline ammonia (FSA i.e. NH3 and NH4+) by ammonia nitrifying organisms which convert FSA to nitrate (NO3-), nitrifiers in activated sludge systems are considered the slowest growing organisms and they fix/dictate the sludge age (solids retention time). 

Anoxic means there is no dissolved oxygen (no DO) but there is Nitrate. in this zone you get Denitrification taking place, i.e. Nitrate is reduced to Nitrogen gas by OHOs again which take the electrons of the organics and pass them to Nitrate and a terminal electron acceptor and it is reduced to nitrogen gas, that is in fact how we remove N in WWTPs by nitrification and Denitrification. 

Anaerobic part of the reactor volume/zone is where you have no DO and no Nitrate present and aiming to remove phosphorus. In this part of the reactor expecially if it is put before the anoxic and aerobic zone (e.g. UCT or JHB system) then the biodegradable COD of the incoming wastewater is hydrolysed to VFAs and taken by another set of organisms called ' Phosphorus accumulating organisms or PAOs' these then take the hydrolysed organics and release P and in the earobic zone of the reactor this P will be taken again and the difference is removal as you harvest the sludge in the reactor based on your sludge age. 

So in the Activated sludge system Anaerobic conditions mean 'no DO and No Nitrate' (both these electron acceptors are absent) but the reactor is not to an extent of Anaerobic condition like you will have with Anaerobic digesters, the redox potential is at least not at that degree that you even get the reduction of Sulphates if present....

obviously all these conditions are in one reactor that has been partitioned in different zones some aerated some not. there are various configurations based on the objectives of the WWTP.