Gas Infusion

Dissolving a gas into a liquid is a ‘mass transfer’ operation. Mass transfer requires two things to occur:

First, there must be a ‘driving force’. The driving force in a gas/liquid system is the difference between the amount of gas currently in the liquid, and the maximum amount of gas that that liquid can hold, or take into solution, also known as the solubility. The solubility of a gas in a liquid is governed by Henry’s Law and is unique to each gas/liquid system.

Second, there must be a means or pathway for the gas molecules to contact the liquid stream. This is also known as ‘interfacial surface area’.

Conventional methods of oxygenation of water, as illustrated below, are energy intensive processes. This is due to the fact that oxygen is only sparingly soluble in water. The solubility of atmospheric oxygen in water ranges from about 15 ppm (mg/l) at 0ºC to about 7 ppm at 35ºC under 1 atmosphere of pressure. Most of the critical conditions related to dissolved oxygen deficiency in biological operations, including bioremediation, occur during the summer months when temperatures are higher and solubility of oxygen is at a minimum. For this reason, it is customary to think of dissolved oxygen levels of about 6 to 8 ppm being the maximum available under critical conditions.

Because of this low solubility, there is very little ‘driving force’. In order to accomplish any mass transfer on a reasonable time scale, energy is expended to create interfacial surface area. Fine bubble diffusers, or chemical oxygen production compounds, release oxygen in the form of bubbles, usually in the range of 1 to 2 mm in diameter. These small bubbles create the interfacial surface area required for mass transfer.

Despite their small size, the vast majority of the oxygen (90 to 95%) created by these methods escapes from the water surface into the atmosphere. This escaping oxygen represents a high proportion of wasted energy and wasted money.

For example, the supply of oxygen to suspended biomass in wastewater treatment represents the largest single energy consumer in an activated sludge treatment facility. Recent studies indicate that the aeration system accounts for 50% to 90% of the total power demand. According to industry experts, only about 1% of all oxygen discharged from a fine bubble diffuser is absorbed per foot of tank depth. In a 10-foot deep tank, 90% of the applied oxygen escapes to the atmosphere. Along with the escaped oxygen and air are the noxious odors and VOC’s that often require scrubbing at further energy cost.

In any biological treatment process, the limited solubility of oxygen is of great importance because it governs the rate at which oxygen will be absorbed by the medium and therefore, the cost of oxygenation.

Before we discuss how Gas inFusionTM differs from these conventional means of oxygenation, we need to address the concept of how much dissolved gas a liquid can ‘hold’. Earlier we described ‘solubility’ as the maximum amount of gas a liquid can take into solution. This level of dissolved gas ‘saturation’ is also used extensively and is defined conventionally.