The bleed has a high cod and in certain cases will contain thiocyanate. Biological treatment will be required. An alternative is spray drying of the effluent followed by reduction of the solids with carbon monoxide to yield a mixture of sodium carbonate, sodium sulphide and hydrogen sulphide which can be returned to the process.

The disposal of collected pollutants or their products is always a consideration in air pollution control. When a chemical reaction is used this is particularly important.