To expound on Andre Visser's ...

To expound on Andre Visser's comments - 

1. Unless the wastewater is very cold, nitrification kinetics in activated sludge systems tends to be pretty fast. Conventional systems typically have an HRT around 3-5 hours, while extended aeration systems can have an HRT up to 35 or 40 hours. But that long HRT in extended aeration time is designed to resist washout events and help stabilize shock loads, not to give enough contact time for nitrification. The contactor in a contact-stabilization system can have an HRT as low as 30 minutes some times.  Or, think of an SBR, which can have relatively short aeration cycles.

2. WEF's MOP 8 has a table of common inhibitors if you think you have something inhibiting nitrification. There's other issues with industrial wastewaters that are relatively common, such as ammonia toxicity in landfill applications. 

3. There's a few different models for minimum SRT for nitrification - you can use a monod-type equation to figure out it.  One reference you can look at is the US EPA's 1993 Nitrogen Control Manual. But generally you want no less than 6 days SRT at 10C as the absolute minimum. Below that you'll likely get unstable nitrification and will see a lot of NO2 in your effluent instead of NO3 due to AOB/NOB metabolism dynamics.  

     a. Note that this ignores the impact of pH, DO concentration, etc - but there's corrections for those too. 

    b. Nitrifiers have a lower specific growth rate than heterotrophs, so this is almost always your limiting factor for SRT.

   c. As a rule of thumb, I usually define my operating SRT as no less than twice the design SRT. 

   d. Temperature is a really, really important factor. The growth rate of your AOB's (eg Nitrosonomas) tends to asymptote to zero below 6C. 

4. The vast majority of activated sludge processes are doing BOD treatment and nitrification in the same basin. You can't tell your system to stop nitrifying short of reducing the SRT to near-zero. 

5. You do need to remove most of the BOD prior to starting nitrification because heterotrophs will outcompete autotrophs (typically 90%+ of your biomass is heterotrophic) However, it doesn't need to be 100%. Nitrification will start before complete BOD treatment is achieved. I've heard that it typically starts around a TKN/BOD ratio of like 1/3, but I'd have to dig up a source on that. 

6. I'm not an expert on MBBR's, so I'll leave that comment to others. I will point out that autotrophs prefer an attached growth mechanism, which an MBBR obviously encourages.