Figure 1: The ASAN system is relatively simple: a precise volume of activated sludge (7ml) was settled in an optical glass tube. The optical axis (OA) of an emitter/receiver system is arranged perpendicularly to the settling direction and positioned at an accurate distance from the water-air interface (dH = 10 ± 0.5 mm). The electrical intensity, I, throughout the receiver was automatically recorded. The control was formed from distilled water (2-3 μS/cm) and filtered using a 0.2 μm nitrocellulose filter. The intensity compared with black light was also measured and was found negligible (less than 1μA). (Colour code: blue, supernatant; brown, activated sludge; black, compacted activated sludge).