Question

Why did the environmental engineers cycle between oxic and anoxic growth for this reactor?

Answer 1

Solution:

Environmental engineers cycle between oxic and anoxic growth for this reactor because,

The biological nitrogen process involves two processes: nitrification and denitrification. Denitrification occurs almost exclusively under facultative anaerobic or microaerophilic conditions; however, aerobic denitrification can occur in aerated reactors. The nitrification is a strict aerobic process that involves the oxidation of ammonia (NH3) to nitrate (NO3−) by autotrophic bacteria. Firstly, ammonia is oxidized to nitrite (NO2−), by means of ammonia-oxidizing bacteria (AOB), and then nitrite is oxidized to nitrate by the nitrite-oxidizing bacteria (NOB). In the second step, named denitrification, nitrate is converted into a gaseous product, nitrous oxide (N2O) or molecular nitrogen (N2), which is finally eliminated into the atmosphere. Denitrification is an anoxic process performed by heterotrophic bacteria using nitrite and/or nitrate as the electron acceptor. In full denitrification, NO3− is reduced to NO2− and then to nitric oxide (NO), N2O, and finally to N2. Several biological processes have been proposed for nitrogen removal.

This process is a modification of a conventional activated sludge process where an anoxic reactor is located upstream of the aerobic reactor. This process with pre-anoxic configuration is commonly named anoxic/oxic (AN/OX) process. In the first reactor, denitrification is carried out using organic carbon from wastewater. For this, the process requires an internal recycle that carries nitrate, generated from ammonia by the nitrification process (aerobic reactor), to the anoxic reactor. The amount of nitrate removed in the anoxic reactor depends on both the recycle flow and availability of influent organic carbon.

That's why environmental engineers cycle between oxic and anoxic growth for this reactor.