ASM-AN model in WEST

We are using ASM-AN model for simulate Municipal WWTP which works with alternative cycles denitrification-nitrification. 

In the Guyer matrix of the model we changed the expression for the heterotrophic denitrification rates eliminating the terms:

1) NO2/(NO2+NO3) from the second step of the denitrification
2) NO3/(NO2+NO3) from the first step of the denitrification

We did this in order to avoid the excessive nitrite accumulation that is not observed in the our plant.
This acculation is too high because of the nitrate concentration is one order magnitude higher than the nitrite concentration in a municipal WWTP.

do you have any suggestions about this?

This is a very good question.


Other authors such as  B.J. Ni and  H.Q. Yu (An approach for modeling two-step denitrification in activated sludge systems
Chemical Engineering Science, 63 (2008), pp. 1449–1459) do not use such a “switching function”. If I recall correctly, then the term was inserted in order to incorporate the fact that the NOx component (NO2 or NO3) that was most available, would also be taken up fastest. So if a lot of nitrite is available this will be taken up preferentially. The K_NOx values indicate the affinity of the heterotrophs towards NO2 or NO3. Indeed, K_NO2 should be lower than K_NO3 because of the usual concentration ranges in a normal WWTP. So both terms (S_NO2/(S_NO2+S_NO3)) <-> S_NO2/(S_NO2+K_NO2))) indicate a somewhat different aspect of denitrification.

The (S_NO2/(S_NO2+S_NO3)) term will increase the reaction rate and as such decrease nitrite build-up. Omitting this term is according to me similar to increasing the K_NO2 term.

In conclusion, I am not so sure if the (S_NO2/(S_NO2+S_NO3)) terms should be omitted.