Besides competitively-priced biological wastewater treatment, an adsorptive treatment process is required for substances that are particularly difficult to degrade. Large-molecule compounds have a particular tendency to adhere to HOK® Activated Lignite. From a technical point of view, adsorption processes like these can be set up using a suspension technique or as a fixed-bed filter, depending on the range of concentrations involved.

For processes using suspension techniques, stirred-tank reactors are used, and are set up as single or multi-level countercurrent cascades as required. The advantages of using the suspension technique include a large mass transfer and – thanks to the cascade – the ability to load the activated lignite with large quantities of harmful substances plus continuous plant operation.

In the fixed-bed option, which can be based on biological or adsorptive processes or on a combination of both by varying ventilation, the water to be treated flows through a solid HOK® Activated Lignite bed. If the water level is lowered below the HOK® Activated Lignite surface, the free HOK® Activated Lignite on the top captures odorants by adsorption. Targeted backwashing ensures that the filter will achieve a long service life and that the HOK® Activated Lignite is loaded with sufficient quantities of harmful substances.