The answer is true. It is because the activated sludge
process made use of the aeration in which air being mixed with a substance
together with a bacteria and protozoa that is in a biological floc that will
help treat the wastewater.
Carbon dioxide is a byproduct of respiration while oxygen is a byproduct of photosynthesis.
Remember that a byproduct is a secondary product that is made in the synthesis of another. It is unintended and excreted.
