Please justify your answer. In wastewater treatment, biological digestion of org
ID: 508408 • Letter: P
Question
Please justify your answer. In wastewater treatment, biological digestion of organic material is frequently used at the secondary treatment stage. One common method is to use an aerobic digester where the bacterial population (typical composition C_60 H_87 O_23 N_12 P) requires access to sufficient oxygen. A 35 degree C wash stream in a food processing plant contains around 1% glucose C_6 H_12 O_6 (by weight). It has been proposed that this wash stream should be sent to an existing aerobic reactor so that the bacteria can use the glucose as a 'food' source. (i) Calculate the biological oxygen demand (in mg O_2, per litre) for this wash stream. Compare this with the dissolved oxygen in the wash stream (~7 mg/L). (ii) As a plant operations engineer, do you think sending this wash stream to your aerobic reactor is a good thing to do?Explanation / Answer
(i) Glucose is oxidized by the bacteria as per the following equation
C6H12O6 + 6O2 ----- 6CO2 + 6H2O
As per the stoichiometry of the reaction, 1 mole (180 g) of glucose needs 6 moles (6 x 32 = 192 g) of dissolved oxygen. Glucose concentration in the given wash stream is 1 wt % (1 g in 100 mL), which will require 10.66 g dissolved Oxygen per Litre. Therefore, the theoretical biological oxygen demand for the wash stream is very high (10,660 mg/L) compared to the dissolved oxygen in the wash stream (7 mg/L)
(ii) As the oxygen demand is very high, it is not advisable to send the wash stream to the aerobic reactor. If aerobic digestion is attempted on this wash stream, the microbial process may eventually turn into an anerobic process, resulting in unwanted odour and inefficient treatment of the waste water.