The presence of oxygen can be measured in-situ with a dissolved oxygen meter. A crude estimate of oxygen requirement is that of 6 mg of oxygen is required to oxidize 1 mg of nitrogen. Thus, it is important as a diagnostic tool to measure the dissolved oxygen in the aerobic portion of the system. Very low (<3mg/L) dissolved oxygen in the aerobic portion of the system should generally be addressed by adjusting those features that control the flow of air through the system. 

Sufficient oxygen and alkalinity are essential to the conversion of ammonium to nitrate (the precursor for denitrification). Low measurements can be a sign of a septic systems that is not removing enough nitrogen. 

In contrast to the need for dissolved oxygen in the aerobic phase of waste water nitrification, dissolved oxygen is actually counter-productive at the denitrification step. The majority of organisms that denitrify wastewater will not do so in the presence of available oxygen. Thus a measure of dissolved oxygen in the anoxic portion of the sustem > 1.0 mg/L probably inhibits denitrification. 

In conditions of low dissolved oxygen septic conditions can also ocu . Microbes known as sulfate-reducing bacteria use the sulfate ion (SO4-) that is naturally abundant in most waters as an oxygen source for respiration. The byproduct of this activity is hydrogen sulfide (H2S). This byproduct has a low solubility in the wastewater and a strong, offensive, rotten-egg odor. In addition to its odor, H2S can cause severe corrosion problems as well.