The most common flotation separators in onshore produced water use are horizontal IGF units. These units are generally multi-cell mechanical or hydraulic units. They have lower retention times than DGF and therefore are smaller units, but they generally have a much higher float recycle rate than DGF. IGF is efficient at elevated water temperatures, which are often produced from the wellhead. Without chemical addition flotation units should remove particles larger than 25 microns. With chemical addition to coagulate oil and solids, particles less than 10 microns may be removed.

In induced gas flotation, gas is introduced into water either mechanically or hydraulically. Mechanical distribution uses a cylinder and rotating impeller to suck gas from above the liquid level and discharge it at the lowest part of the cell. The result is a turbulent mixture of gas and liquid. Rising gas bubbles attach to oil and solids and float these particles to the surface where they are removed into a skimming launder as float recycle. Retention time is generally between 4 and 8 minutes depending on the method of inducing gas.

Hydraulic IGF units use a sparger device to inject gas bubbles at the lowest point of the flotation cell. Gas bubbles are often larger than those generated by mechanical means. The result is a system that is less turbulent with larger bubbles than mechanical units but provides more retention time for separation. Hydraulic IGF units have fewer moving parts than mechanical ones.

For both units, the scum (float material) is watery and needs further thickening or recycle. Subnatant from oil thickening can be recycled upstream of the IGF. Recovered float material may also be recycled to upstream separation equipment. Skimming devices exist to reduce water content of the float and lower recycle rates. Still float recycle can account for 3-15% of feed flow.

DGF is very effective for removing oil and solid particles. They operate by taking a recycle stream (15-20%) of treated water and dissolving gas into it under pressure. The recycle stream is combined with inlet water prior to the flotation cell. The reduction of pressure as the recycle stream combines with the inlet stream causes gas to come out of solution liberating tiny bubbles of gas. These bubbles attach and float particles to the surface where they are removed as float.

Gas bubbles produced by DGF are much smaller and provide denser flotation action than IGF allowing DGF to operate with much lower recycle float than IGF. Despite this advantage, DGF is rarely seen in produced water service. It requires more retention time and is sensitive to temperature since gas solubility reduces as temperature increases. DGF efficiency tends to reduce as water temperature increases. Given the adaptability of IGF to variations in flow and conditions and its relative small equipment size compared to DGF it is generally the preferred flotation solution for produced water. DGF is the preferred flotation solution for refinery waste water treatment.

It should be noted that demulsifying chemicals may be added in primary separation to aid removal of water from oil. These chemicals may have a negative effect on flotation cell performance. For this reason, some type of foaming agent may be needed to improve performance.