Removal of gas phase pollutants is an enormous subject with almost as many scrubbing liquids and absorbing solids, as there are pollutants. We will therefore address this in the form of a brief case study.

This can be applied either to effluent stream H₂S removal or to removal from fuel gas prior to combustion (prevention of SO₂ generation). Thus we can see this as part of an integrated pollution control approach rather than just an end of pipe solution. It also demonstrates how pollutants can often be recovered as by-products.

Since liquid effluents are also produced this is even more so. Hydrogen sulphide removal was originally developed to clean up town gas and from the start of this century the most common method was to pas the gas through boxes of iron oxide. The hydrogen sulphide was deposited as iron sulphide.

The recovery of sulphur from solids could only be achieved by roasting to generate sulphur dioxide for sulphuric acid manufacture (by roasting the "spent oxide").

After 1945 the need for recovery of good quality elemental sulphur lead to the development of a number of liquid phase processes which could be used to recover sulphur from hydrogen sulphide in gas streams with a H₂S content that is relatively low (as found in coal gas).

At the same time (or earlier there was also a need to remove higher concentration H₂S from effluent gas streams (and some natural gas deposits)- sometimes following an adsorptive pre-concentration step.

This resulted in various forms of the Claus process, which will be dealt with later. We will now consider the techniques used at relatively low hydrogen sulphide concentrations.