Yes, aerating without RAS trigger good nitrification. Nitrifiers are dependent on organotrophs to reduce BOD to relatively low concentrations (

The attached growth system (for BOD removal) followed by activated sludge (for additional BOD removal and nitrification) is a classic system.  Ausra is correct in that relatively low BOD levels are needed in the activated sludge system, so that heterotrophic (BOD removing) organisms do not out-compete the more slow-growing autotrophic organisms required for nitrification.  RAS is provided in these systems so that a large activated microbial population can exist as needed.

See papers by myself and coauthors for FGR SGR designs and 10 years of operation at the Salmon Arm WWTP.  This facility has now operated for over 20 years.

Since you only have approx 15 NO3-N in the effluent and still 50 mg BOD/l, my firtst comment will be that the Trickling Filter is sligly owerloaded compared to the desired eflluent. If we assum a yield for Nitrification of 0.2 g VSS/g N nitrified your effluent will have a maximum of 8 mg Nitrifying VSS/l and with and expected nitrifying capacity of 0,5 g N/gVSS/d at ideal O2 and no NH4-N limitation. This ould give that your aeration tank will have to have at least 16 Hours retention time so the answer to me is no, just aeration will not do the job. More efficient will be to add some MBBR media. you efflunet contains 45 kg N4-N and the nitrifcation rate will be approx 0.5 g/m2/d due to the high Organic load remaing. This gives 90000 m2 of media. If you use i.e. our Fluopur mdia with 1500 m2/m3 and 30% filling degree you will have approx 200 m2 volume or 1/2 retetion time wich will be far more realistic.

Hello Kim

Thanks for your response. NO3 was only 3 mg/L and NH4 was 10 mg/L (150 kg). You must have taken the TKN value of 15 mg/L as NO3 concentration. Despite this your assessment was not wasted since it provides other researchers solutions for the scenario you had identified. Are you able to reassess your prediction based on the above values?

Kind regards

Selva

Hello Kim,

Simply aerating the effluent without a nitrification culture present would not reduce the ammonia concentration unless the effluent pH were high (unlikely).  You need a lower loading to the filter, a deeper Filter bed depth, or a treatment stage for nitrification.

Regards

Vernon "Smokey" Stack

Hello Selva and Vernon,

Kim mentions to add MBBR media during the aeration. This is biofilm carriers and in aerobic conditions it will contain nitrifiers.

Dear Vernon

The approach that I first demonstrated is that if a trickling filter is running and nitrifying there is always Nitrifying bacteria leaving with the effluent and if you give them Oxygen they will nitrify the effluent, but since there is only very few of them you will need a lot of time. What you then need to do is concentrate the bacteria. This you could do either by adding a clarifier and returning the bacteria back with your return activated sludge, but this will be complicated and still relative big volmes. Hence my suggestion is as Henrik correct commented is to add an MBBR stage since here by giving the bateria a place to grow (the Biofilm on the carrier) you will in a very simple way be able to up concentrate Biomass and hence get much higher volumetric removal rates. 

Pure aeration would be called ammonia Stripping and if we want all the Ammonia to be as NH3 and not NH4 we will need to be above pH 10.5 which will never be economical.

Dear Selva yes I see I had moved all figures one position. It will though not interferre with the conclusion - apparently we have a little denitrifcation going on, but you Nitrified influent will actually still be approx 15 - 20 mg/l so you will have approx the same amount of Nitrifiers, but now as TKN is 15 you in reality need to remove 5 times more Ammonia so teh volume for a olution with just aeration would be maybe double size (Not more because you will actually grow more nitrifiers, but 48 hour retetion is large volume. The MBBR volume would if calculate linear be 1000 m3, but if you make two volmes in series we will actually be able to reduce the volume to be between 600 and 800 m3, because the firt reator not will be NH4-N limited in the kinetics since now the concentration is 15 mg TKN instead of 3 mg/l.

Can you add height to the existing filter? What is the filter loading rate? What is the filter hydraulic loading rate (m³/m² plan area/d) and wetting rate (m³/m³/d)? What is the media specific surface area (m²/m³)? Whilst the BOD is high, much improvement can be made with additional recirculation flow around the filter. The other options are energy intensive and add extra process complications - optimise what you have. Do you have settlement before the filter? You could add another trickling filter in parallel and achieve

Hello Kim

Good answers. You concern is lack of nitrifier levels in the nitrifier tank (diffuser tank) which could rate limit hence the suggestion of nitrified effluent to be settled in a clarifier and RAS to boost nitrifier population to sustain high nitrification rate. A principle used in the extended activated sludge system? However, you prefer the MBBR option to sustain nitrifier population because of the compact nature and treatment efficiency.

If the MBBR option is used, given the predominantly nitrifying process, would the system likely to encounter low pH which could limit the rate? Can we assume at this stage of the treatment MBBR is capable of some degree of denitrification which could neutralise the acidity (because denitrification requires H+)? Can we expect decent denitrifier activity in an MBBR tank, purely from the viewpoint of sustaining pH rather than stripping NO3?

Hello Simon and Vernon

I agree, before considering other treatment options, one needs to consider fine tuning the existing TF to nitrify better. The next step may be any cost effective modification to the existing TF (e.g. increasing height). Simon, you did suggest a parallel TF. Why shouldn't it be in series since as you identified solids settling may be an issue from low loading? Has there been considerations to parallel vs series TF systems with regard to merits and demerits bearing in mind Kim and Henrik are promoting MBBR in series to the existing TF.

Dear Selva

I believe that the most efficient solution to gain some Denitrifcation will be to recirculate some of your Nitrified effluent back into the trickling filter.

Continuing Kim's last comment, reducing airflow in the TF could help.

Hi Selva,

I wouldn't try to denitrify in a trickling filter. Something that has been done successfully is creating a mixed liquor which receives nitrified effluent, crude wastewater  and no air (effectively an anoxic ASP). This will denitrify. I prefer filters in parallel as they will probably be more stable and potentially better settling. What is effluent standard to achieve?

Simon

Hi  Selva,

i would consider adding a denitrification filter after the trickling filter  Trying to denitrify in a trickling filter means having balancing aerobic and anoxic conditions in one bed.  Increased recirculation would only move the aerobic section further into the bed, and reduce the anoxic capacity.

Vernon Stack

 Hi Selva,

Sorry,  That response did not answer your question.  If the filter is lightly loaded, there will be some nitrification organisms in the effluent.  When the effluent is warm, providing holding time and aeration could achieve some nitrification.  Since the  number of nitrifiers can be small, I would check this possibility in the lab.  You might have to add alkalinity to maintain pH.

Vernon Stack

The BOD5 of 50 mg/L needs to be reduced before significant nitrification can be accomplished.  In aerobic systems microorganisms obtain more energy from BOD oxidation than from nitrification, so the BOD level must decrease before significant nitrifiers can be present in the system.  The reason RAS is necessary in an aerated post trickling filter basin, is so that a sufficient population of nitrifiers can be maintained to accomplish the treatment goal.  Nitrifiers grow relatively slowly, so a large mass of organisms is necessary.  As stated elsewhere, nitrification is pH and temperature sensitive.  In the USA upper midwest, solids retention time values of 30 days or more may be required in winter.  Alkalinity may be required if pH drops to neutral or lower.  The need for alkalinity addition could be reduced by incorporation of an anoxic zone in the tankage, with recycle of nitrified effluent.

Hi Selva, is the output you quote the value that you do achoieve or the value that you want to achieve?

Thanks Vernon, Michael and Simon. Since I have been away, I could not continue with your responses.

Simon, the output value I stated has already been achieved by the current system. What is required is nitrifying the current high NH4of 10 mg/L to NO3 (and ultimately denitrifying). In short a DIN (dissolved inorganic-N) level of

Selva,

If denitrification is to be considered in a trickling filter, it has to be a deep bed in which air is not permitted to circulate. It cannot be done by recirculating nitrified effluent back into a filter that has just achieved nitrification.

Vernon Stack

First, NH4  can not be reduced, but only oxidised ( to NO2 or NO3 for example).

Second, this is a biological oxidation : then it requires, besides CO2, autotrophic bacteria, which can utilize CO2, by deriving the necessary energy through the oxidation of N forms.

First, I am convinced that the author of the question used the word reduce in the meaning decrease. The chemists are convinced that reduction is reserved for the contrary process to the oxidation. But the first meaning of reduction (from Latin reduction) is decrease

Second, A lot of information about the removal of ammonium-nitrogen in aquaculture systems (RAS recalculating aquaculture system) are in the publication of James M. Ebeling e.g. Biofilter-Nitrification Design Overview. I hope, the author of the question will find answers for his doubts in these publications.

Yes thanks Korneliusz, I did refer in the context of decreasing ammoniacal-N by biological assimilation (i.e. bacterial NH4 conversion to organic/protein-N form) which is one of the key wastewater treatment processes involved in decreasing much of the wastewater ammoniacal-N rather than simply aerating and nitrifying. The ammoniacal-N assimilative process whose main C source is organic-C dominates wastewater treatment steps until F:M ratio drops substantially to enable nitrifying bacteria to function effectively using CO2 as the C-source. At high F:M ratios the dominant N removal process is NH4 assimilation.