A2. An activated sludge system will be upgraded to remove nitrogen species from the final effluent. Currently, the aeration tank can remove all degradable COD from the wastewater and perform full nitrification ( 100% efficiency). The effluent from the aeration tank contains nitrate at the concentration in nitrogen unit: 5mg/LNO3N. It is decided to use postdenitrification process which means adding a post anoxic tank after the aeration tank for denitrification. The denitrifying bacteria use the organic substrate (COD) for two purposes: the growth of their cells and the reduction of NO3to N2. Three types of organic substrates are available: methanol, acetate, and ethanol. The biomass' growth yield coefficient on the substrate is 0.25,0.40 and 0.36g VSS/g_COD respectively. (a) Choose two substrate types from the three options and calculate the required concentration (mg/L) in the anoxic tank to remove 90% of the nitrate. Assume the unit conversion is 1g _VSS \( =1.42 \mathrm{~g}_{\text {_ COD. }} \). [15 marks]