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.
My guess would be burning the waste
Although, I'm not sure about the entirety of this question, when reviewing claims and evidence, check your sources to make sure they are reliable.
Scientists start with general observation and then make a hypothesis
