This does not make sense to me.
Mark Watney needs water. He uses an iridium dish to decompose hydrazine into a gaseous mix of hydrogen and nitrogen (2:1 ratio) that he feeds into a hand made oven. Exact quote: "Soon I had short bursts of flame sputtering from the chimney." For some reason, hydrogen does not fully combust and contaminates the air. In the end there is seven times more the hydrogen in the air than oxygen (64% volume). It should have exploded when reached 4% concentration.
Also, there is a lot of nitrogen generated at the iridium dish, yet nitrogen is only at 22% in the end, even though part of hydrogen is utilized to produce water
One more thing, he produces hundreds of liters of water from CO2 (via regenerated O2), but it should have produced about 100kg of carbon - where did it go to? How does his oxidizer work? I haven't read beyond this point so I wonder if he had an idea to feed that carbon to his plants.