Test for assessing shear sensitivity of activated sludge flocs: a feasibility study

Sludge deflocculation can cause, like excessive growth of filamentous bacteria, activated sludge wastewater treatment failure. Yet, unlike the latter cause, there exists no widely accepted tool to assess the flocculation level of activated sludge and to predict sludge deflocculation. In this study, a test procedure is proposed to assess the sensitivity of activated sludge flocs to shear. The test consists in subjecting sludge sample to a shear treatment followed by a sludge volume measurement (SV30), with reference to a control. The ratio (%) obtained by dividing the SV30 of the sheared sample by the SV30 of the control is used to express the shear sensitivity. In a first series of experiments using two types of sludge, the test was shown to be able of ranging sludge samples in a correct order of shear sensitivity. Applying this so-called shear-SV test procedure to a series of sludge samples allowed to distinguish amorphous sludges (SV30 ratio<=100%) from normal sludges (SV30 ratio<100%). The test was shown to be repeatable and simple. It requires only basic laboratory equipment and implies minimum cost. It can be used as framework for the assessment of activated sludge shear sensitivity in practice.