Biological Process

The first stage of the biological process is biomass destruction or biomass breakdown. In an aerobic digestion system, the food supply for the biomass decreases. As a result, the microorganisms begin consuming their own protoplasm to obtain energy.

Biomass requires 5 molecules of oxygen to oxidize its protoplasm. When the protoplasm is oxidized, carbon dioxide, water, and ammonium bicarbonate are reduced. During this period, the microorganisms feed on one another. As biomass destruction takes place, ammonia is generated within the system. This ammonia becomes highly toxic to the microorganisms, which is why such systems often produce foul odors.

To reduce ammonia, nitrification is required, which is the second stage of the biological process.

Nitrification occurs through nitrifying bacteria. These bacteria oxidize ammonia and combine it with 2 molecules of oxygen to convert it into nitrate, while reducing 2 molecules of water.

During this biomass destruction phase in nitrification, a total of 7 molecules are required to complete the process. The by-products are 5 molecules of carbon dioxide, 3 molecules of water, and 1 molecule of nitric acid.

As a result, nitric acid becomes highly toxic to the biomass because the pH decreases during this stage, causing the biomass to weaken.

If nitric acid needs to be removed from the process, it must be done through the denitrification process. Denitrification is the final stage of the biological system.

In a controlled aerobic digestion process, denitrification works in the absence of oxygen. During this stage, the biomass becomes inactive and 4 molecules of nitrate are oxidized. This helps reduce ammonia, alkalinity, and nitrogen gas.

When these three stages of the complete biological process are combined, it can be seen that biomass reacts with approximately 5.75 molecules of oxygen and produces 5 molecules of carbon dioxide, 3 molecules of water, and reduces 0.5 molecules of nitrogen gas.

It is very difficult to completely remove oxygen during denitrification. Therefore, when pH and alkalinity decrease, nitric acid can kill the biomass. For this reason, increasing alkalinity during this stage is essential to neutralize nitric acid and keep the nitrification process active.

— James Martin Adhikary