Activated sludge: pre-anoxic denitrification

Description

Pre-anoxic zone receiving the return sludge, ratio $R$, and the internal mixed liquor recycle, ratio $r$. Only the recycled fraction of the nitrate is available for denitrification.

Nitrate removed and effluent nitrate:

$NO_{3,rem} = NO_3 \frac{R + r}{1 + R + r}$

$NO_{3,e} = \frac{NO_3}{1 + R + r}$

Internal recycle required for a target effluent nitrate:

$r = \frac{NO_3}{NO_{3,e}} - 1 - R$

Specific denitrification rate at the liquid temperature and anoxic volume:

$TEDN_T = TEDN_{20} \, \theta^{(T-20)}$

$V_{anox} = \frac{Q \, NO_{3,rem}}{TEDN_T \, X_v}$

Anoxic fraction of the reactor and aerobic sludge age:

$f_{anox} = \frac{V_{anox}}{V}$

$\theta_{c,aer} = \theta_c (1 - f_{anox})$

The aerobic sludge age is compared with the minimum sludge age for nitrification, when given. Oxygen recovered: 2.86 g O₂/g N; alkalinity recovered: 3.57 g CaCO₃/g N denitrified.

Reference

VON SPERLING, M. Lodos ativados. Princípios do tratamento biológico de águas residuárias, v. 4. Belo Horizonte: DESA/UFMG.