Controlling ethanol dosage to an SBR

Hi,
I am setting up a model for an SBR with post denitrification. I want to dose ethanol during the anoxic phase and have included a Data Input Block to control the flow rate from the ethanol dosing unit. I have measured this flow rate from the ethanol dosing unit and it is correct with respect to value and time period when it is dosing (during the anoxic phase). I have set the COD content of the ethanol to 1450000 g/m3 (i.e. 1.45 kg COD/L ethanol). But I do not get any denitrification in the SBR (the nitrate concentration doesn’t decrease).

Nevertheless, the denitrification works if I skip the Input Data Block and instead let the ethanol be dosed constantly.
What could I possibly have done wrong with this set up?

Is it easier to control the ethanol dosing unit by skipping the Input Data Block and instead let it be controlled directly by the T2R2 phase in the SBR?

Thanks in advance,
/Fredrik

Hi, your layout looks ok - although the detailed settings of the models are evidently crucial here.
Please, have a look at the following threads too:
[list]
[li]"Nitrogen removal in SBR"[/li]
[li]"SBR & Desynchronization"[/li]
[/list]
you may find relevant information.
I believe the issue is that you [b]cannot dose anything[/b] to an SBR [b]outside of the "fill" phase[/b] (T1): during all other phases, any inflow is directed to the by-pass. On the other hand, if you replace the data input (which provides a dosage of ethanol during T2R2) by a controller which gives a constant dosage, you may indeed detect some denitrification: but this is because ethanol enter the SBR during T1.

If you really want to feed the SBR during other phases than the fill phase, you need to either modify the base model or consider one of the variants called CFCD or CFID (cf. the threads mentioned above).

Thanks for your answer Enrico,

I think a CFID could be a good choice in this case. I see one problem though; it seems like the Q_Waste is a constant flow, not an intermittent flow like in an SBR. Is it possible to mimic the SBR process and let the flow Q_Waste be equal to zero except for a short period at the end of the draw phase?

I built a model with a CFID instead of using an SBR and encountered another problem. When I used the predefined file ‘WEST.BODCOD.Month.Influent.txt’ the model worked just fine. But when I changed to my own data input file (with an intermittent inflow, as to an SBR) there was an error message: ‘Numerical error: RK4ASC: Stepsize underflow’. I have tried to change integrator but it didn’t help. Do you have any idea how to solve this problem? I don’t think there is any wrong with the input file; it works when I use it in an SBR.

Thanks in advance for your help,
/Fredrik

[quote]it seems like the Q_Waste is a constant flow, not an intermittent flow like in an SBR[/quote]
Yes you're right: in CFID (and CFCD), there's a continuous underflow equal to the Q_Waste parameter.

[quote]Is it possible to mimic the SBR process and let the flow Q_Waste be equal to zero except for a short period at the end of the draw phase?
[/quote]
Yes: but you need to modify either the base SBR so as to accept influent (i.e. not direct to by-pass) outside the fill phase; or the base CFID model so as to have no underflow but during a certain period.

Regarding the numerical error: what other integrator did you try? I would imagine RK4 (fixed step size) although very slow, should not give that problem. Indeed, I don't believe there's anything wrong in the data file as such, but there may be a local sudden change (maybe a coarser time step in your data file compared to the sample one; of large step-change in some quantity). Best would be that you carefully inspect the time point at which it happens.

Hi again,
And thanks for your answer.

I have now chosen to continue with an SBR model. I have modified the file wwtp.base.SBR.msl so as to permit adding of ethanol to the SBR during the reaction phase (T2), according to your recommendations in the thread “Nitrogen removal in SBR”.
After that I still had a problem with the model; there was no change of the volume in the SBR, despite I added large flow rates of ethanol. This problem was solved when I followed your recommendations and changed the file wwtp.base.SBR.msl again. This time by changing around line 226-241, from:
IF (independent.t - state.t_Begin < (interface.T1 + state.T2))
THEN state.ConversionTermPerComponent[Comp_Index]
To:
IF (independent.t - state.t_Begin < (interface.T1 + state.T2))
THEN state.FluxPerComponent[Comp_Index] +state.ConversionTermPerComponent[Comp_Index]

Now I can see that the volume changes in the SBR when the ethanol is added, it works fine. But another problem has occurred; there is no denitrification process. It seems like it is no content of ethanol in the ethanol dosage. I have altered the concentration of ethanol (C_dose) in an extreme way; from 1 g/m3 to 10^12 g/m3, but there is still no change in concentrations of either S_NO or S_S in the SBR. What could possibly be wrong?
Thanks in advance.

Best regards
Fredrik Stenström

Ok, now it gets too difficult to answer without actually taking a look at your project.
Can you please send the project (the whole folder) and model library (BlockLib.xml + Models subfolder) to mikebydhi@dhigroup.com ?

[quote]is it possible to mimic the SBR process and let the flow Q_Waste be equal to zero except for a short period at the end of the draw phase?[/quote]

The easiest way according to me is to add a timer in your layout that controls the Q_Waste (assuming that it is a manipulated variable and not a parameter).