Effect of the addition of precipitated ferric chloride on the morphology and it settling characteristics of activated sludge flocs

The activated sludge process is the most widely used biological wastewater treatment process. For a successful operation of this aerobic suspended growth process, the microorganisms must aggregate to form flocs that settle in the secondary settler. Poor plant operation or the increasingly stringent effluent quality requirements may lead in existing WWTPs to a failure to meet the effluent discharge legal requirements. A technically viable solution to this problem is to improve the suspended solid removal at the secondary settler. A low-cost option for short periods (eg., disturbances or emergency situations) would be to improve the settling properties of the sludge, via the addition of coagulants or ballasted sedimentation. The solid-liquid separation in the secondary settler is affected by the density, size and other characteristics of the flocs. For this reason, in this work, a fully automatic toolbox for recognizing activated sludge flocs and performing their morphological characterization based on digital image analysis and statistical processing was developed. This characterization included the determination of the equivalent diameter of the flocs, their area, perimeter, length and width, radius of gyration, reduced gyration radius, aspect ratio, form factor, roundness and fractal dimensions. The toolbox was used to characterize the activated sludge flocs from a full-scale urban WWTP, as well as the observed changes in their characteristics due to the addition of a dose of precipitated ferric chloride. A new methodology based on geometric properties (aspect ratio and floc size distribution) is proposed to discriminate the debris from small activated sludge flocs. The addition of the precipitate and the subsequent flocculation process produced larger flocs with more area and less perimeter, that is, larger, more compact, closed and regular flocs. The size and density of the flocs, as well as the hindered settling velocity, linearly increased with the addition of precipitated ferric chloride. Partial least squares (PLS) regression of all collected data revealed that the volumetric fraction of the sludge, the suspended solid concentration and the fractal dimension that relates area - perimeter were the variables most correlated with the hindered settling velocity.