Filterable substances (suspended solids, non-settleable solids) cannot be retained by sedimentation in the existing secondary clarification. These are small particles, e.g. single bacteria or fragments of sludge flocs, which typically have a size of 0.5 - 2 microns and cause turbidity in the effluent of sewage treatment plants.

Filterable substances are measured in in mg/l using appropriate paper filters (0.45 μm). Typical values in the effluent of an activated sludge plant are 5 - 20 mg/l, very good values of <5 mg/l are found after a filtration stage.

Instead of filterable substances, turbidity can be measured as a substitute value. Turbidity is often measured in NTU (Nephelometric Turbidity Unit). Drinking water has 0.05 - 0.5 NTU, the wastewater plant effluent 2 - 20 NTU, and sewage 100 - 2000 NTU.


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