The correct answer is B. Dissolved oxygen.
Corrosion in concrete sewers is caused by the presence of dissolved oxygen in the wastewater. The oxygen reacts with the concrete, causing it to break down. This process is accelerated by the presence of acids and other chemicals in the wastewater.
Septic conditions do not cause corrosion in concrete sewers. In fact, septic systems are designed to prevent the buildup of oxygen in the wastewater. This is done by using anaerobic bacteria to break down the waste. Anaerobic bacteria do not require oxygen, so they do not produce any harmful byproducts that can corrode concrete.
Chlorine is sometimes used to disinfect wastewater. However, chlorine can also accelerate the corrosion of concrete. This is because chlorine reacts with the concrete to form calcium hypochlorite, which is a strong acid.
Nitrogen is not a major factor in the corrosion of concrete sewers. However, nitrogen can be converted into ammonia by bacteria in the wastewater. Ammonia is a base, and it can react with the concrete to form calcium ammonium nitrate, which is a salt. Salts can cause concrete to expand and crack, which can lead to corrosion.
In conclusion, the correct answer is B. Dissolved oxygen.