It is important to maintain proper solids retention time (SRT) in anaerobic and anoxic tanks in order to obtain high nutrient removal. One drawback of modified UCT process is low MLSS in anaerobic and anoxic tanks, which makes it necessary to have large anaerobic and anoxic tanks.
A modified biological nutrient removal (BNR) with step feed has been developed as shown if Fig. 1, which allows higher MLSS in anaerobic and anoxic tanks than modified UCT. Approximately a half of raw wastewater is fed to the first anaerobic tank while the other half is fed to the second anoxic tank after the aerobic tank. Since the mixed liquor in membrane tank is recycled to the aerobic tank (#3), upstream of the second anoxic tank (#4), no pumping from aerobic tank to anoxic tank is required unlike in the modified UCT process. As a result, the overall pumping cost can be kept lower while nitrogen removal efficiency is higher.
Fig. 1. Process diagram of BNR process with step feed.
One significant advantage of this process is the MLSS in anaerobic and anoxic tank are maintained relatively higher than those in the modified UCT process. The relative MLSS in each tank as functions of sludge recycle rates can be calculated as follow.
Input¬†¬† ¬†=¬† ¬†Output
1st Anaerobic tank¬† : ¬† ¬† ¬† ¬†¬† ¬† ¬† ¬† ——— (1)
¬†¬†¬†¬†¬†¬†¬†¬†¬†¬† 2nd Anaerobic tank :¬†¬†¬†¬†¬†¬†¬† ¬† ¬† ¬† ——— (2)
¬†¬†¬†¬†¬†¬†¬†¬†¬†¬† 1st Aerobic tank¬†¬†¬†¬†¬† : ¬† ¬† ¬† ¬†——— (3)
Anoxic tank¬†¬†¬†¬†¬†¬†¬†¬†¬†¬†¬† : ¬†¬†¬†¬†¬†¬† ¬†¬†¬†¬†¬†¬†¬† ¬†¬†¬†——— (4)
Membrane tank¬†¬†¬†¬†¬† : ¬†¬†¬†¬†¬†¬†¬†¬† ¬† ¬† ——— (5)
Where Q is flow rate (m3/hr) and X is MLSS ( g/L ).
By solving the above equations, the MLSS in each tank are obtained as follow.
1st Anaerobic tank¬† : ¬†¬† ¬†¬†¬†¬† ¬†———————- (1′)
2nd Anaerobic tank : ¬†¬† ¬† ¬†¬†¬†¬† ¬† ¬†———————- (2′)
1st Aerobic tank¬†¬†¬†¬†¬† :¬† ¬†¬†¬†¬†¬†¬†¬†¬†¬† ———————- (3′)
Anoxic tank¬†¬†¬†¬†¬†¬†¬†¬†¬†¬†¬†¬† :¬†¬† ¬†¬† ¬†¬†¬†¬†¬†¬†¬†¬† ———————- (4′)
Membrane tank¬†¬†¬†¬†¬†¬† : ¬† ¬† ¬† ¬† ¬† ¬† ¬†¬† ———————- (5′)
For the condition shown in Fig. 1, MLSS ratio among the tanks is X1 : X2 : X3 : X4 : X5 : X6 = 0.64 : 0.64 : 0.86 : 0.80 : 0.80 : 1. It is noticeable that the relative MLSS in anaerobic and anoxic tanks are substantially higher in step feed process than those in the modified UCT process, where the relative MLSS in anaerobic and anoxic tank are only 0.36 and 0.53 respectively. Therefore, this step feed process can achieve better biological nutrient removal efficiency with same size tanks.
The relative MLSS in each tank can be calculated using the following spreadsheet calculator.
¬© Seong Hoon Yoon