The residual COD corresponds to the COD of the biologically treated wastewater in the course of secondary clarification and is thus a measure of the residual contamination of the cleaned water. The residual COD is composed of the COD of the dissolved and colloidal substances in the cleaned wastewater, as well as the COD of the suspended activated sludge residues, which drifts off with the cleaned water.

The COD of the dissolved substances in the treated wastewater corresponds to the non-biodegradable fraction of the dissolved substances in the feed. This COD value cannot be further reduced by biological cleaning with conventional activated sludge, since these substances cannot participate in the biochemical reaction or only to a very small extent. With the S::Select® process, it is possible to significantly reduce this COD value, and thus also the residual COD: The aerobic granular activated sludge (AGS) of the S::Select® process has a significantly higher sludge age as a conventional activated sludge, so that specialized, slow-growing microorganisms can establish themselves and convert even poorly degradable substances and thus significantly reduce the residual COD.

The COD values of the colloidal substances and of the suspended activated sludge are kept as low as possible with an operational management that is optimally tailored to the load. Furthermore, the design and the volume of the aeration tank and the clarifier play an important role. The S::Select® process enables also an additional significant reduction of these COD values: The granular sludge in the S::Select® process has very good settling properties with a sedimentation rate of more than 3 m/h, so that activated sludge, which drifts off with the clean water, can be almost completely avoided. Furthermore, the resulting granule-flake clusters behave like a filter carpet in the water that collects floating colloidal substances during the sedimentation process and transports them to the bottom of the secondary clarifier.


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