Introduction
Investigation objectives:
Oxygen transfer, the process by which oxygen is transferred from a gaseous to the liquid phase, is vital in a variety of biological and hydrometallurgical processes, in particular wastewater treatment. The aim for this experiment is to evaluate how this oxygen mass transfer varies under a range of different operational conditions (E.g. Stirrer speed and oxygen flow rate), and formulate the optimal parameters for modelling a sewerage plant.

In wastewater treatment plants, diffused air system bubbles are distributed from diffusers at the base of the reactor, where the rising bubbles come in contact with the aqueous layer (BLA). When equilibrium is reached, the liquid is ‘saturated’ into the gas. This can be modelled using equation 1, as proposed by Merchuk et al(1990), where Cl is the gas concentration in the bulk liquid phase (ppm), t is time (sec), Cs is the saturation level of the gas in the liquid phase (ppm) and K is the overall mass transfer rate coefficient (sec-1). This can be further simplified to yield equation (2)

(1)
(2)

Both stirrer speed and oxygen flow rate significantly effect the mass transfer rate ox oxygen in a liquid. Altering these two parameters affects the size and flow of the bubbles which impact the surface area in contact with the liquid and hence the mass transfer. Higher stirrer speeds more efficiently break the air bubbles and consequently increase the superficial area of the bubbles, enhancing the oxygen transfer rate(Karimi et al 2013).
In order to measure the dissolved oxygen within each batch condition, Clark-type electrode probes will be used. These sensors are constructed with a silver anode and a gold cathode, which when submerged in an electrolyte (water) complete the electrolytic cell and serve as a reference electrode. The oxygen molecules present in the aqueous solution diffuse through the membrane resulting in a difference in the oxygen pressures. This produces a perceivable current that is recorded by the instrument.

Reference: http://www.ijehse.com/content/10/1/6 Results qualitative effect of stirrer speed
It was observed that with an increase in the stirrer speed(rpm), the K of the system also increased. This can be attributed to