In the aerobic system microorganisms consume the organic waste as substrate and convert them into water and carbon di oxide.

You can look for cellular respiration if you are interested in knowing the whole pathway. In short, during aerobic respiration microorganisms use oxygen to breakdown the molecules and obtain energy. In water analysis, one of the processes that use this pathway is called BOD (Biochemical Oxygen Demand) and it measures the amount of oxygen consumed by the microorganisms in a sample of water, which can be related to the biodegradability when the BOD/COD(Chemical oxygen demand) ratio is obtained.

Depends on the microbes, substrate and growth conditions. The most common (natural) route is based on cellular respiration, as highlighted by Orlando above. But you really have to know the system before you can comment on the mechanism.

As a living matter, the microorganisms need sources for carbon, nitrogen and other nutrients as a requirement for reproduction, respiration and growth. . As a result, there will be an increase in the population leading to a greater number of them and consequently the process will be self generating and no extra resource will be needed. However, Some toxic pollutants may kill the microorganisms and reduce their population and again, such reduction stands as an evidence for the toxicity. For the last case you may consult the test for toxicity which employs the illuminating Bacteria that undergo a reduction in the illumination. Good luck

Most of the organic matters present in sewage are biodegradable hence; undergo biological decomposition by bacteria which can be divided into two types viz: (i) aerobic bacteria (ii) anaerobic bacteria and (iii) Facultative bacteria. The aerobic bacteria are those which flourish in the presence of free dissolved oxygen in wastewater and consume organic matter for their food and thereby oxidizing it to stable end products. The anaerobic bacteria flourish in the absence of free dissolved oxygen since they survive by utilizing the bounded molecular oxygen in compounds like nitrates (NO3) and Sulphates (SO4), etc., thereby reducing them to stable end products along with evolution of foul smelling gasses like H2S, CH4, etc. Facultative bacteria on the other hand can operate either as aerobically or as anaerobically. Hence, they can survive and cause decomposition of organic matter, either in the presence or absence of free dissolved oxygen in wastewater. In order words, if air or oxygen is available freely to the wastewater in dissolved form, then the biodegradable organic matter will undergo aerobic decomposition caused by aerobic bacteria as well as facultative bacteria–operating aerobically. These bacteria will then utilize the free oxygen as electron acceptor thereby causing the organic matter to stable and unobjectionable end products. The stable end products like nitrates, carbon (IV) oxide and sulphates are formed respectively for nitrogenous, carbonaceous and sulphurous organic matters. Water, heat, and additional bacteria will also be produced in this biological oxidation.

Please, have a look at the attached paper. You can find there the answers to your question.

Best regards

Vit