Optimization of the polyelectrolyte dosage for dewatering sewage sludge suspensions by means of a new centrifugation analyser with an optoelectronic sensor

The flocculation of sewage sludge with poly(diallyldimethylammonium chloride) (PDADMAC) as well as cationic, poly[acrylamide-co-(N,N,N-trimethylammoniumethylacrylate chloride)] (PTAC), and anionic acrylamide copolymers, poly(acrylamide-co-acrylate) (PAAM/AA), was investigated by using a new method to evaluate the velocity of centrifugal separation and the packing behaviour of the sediments. By means of a centrifugation analyser with an optoelectronic measuring system the position of the centrate/sediment interface of the conditioned suspensions can be determined as a function of the separation time, which allows the polyelectrolyte dosage to be optimized for various rotational speeds. At high centrifugal accelerations it was found that using the anionic polyelectrolyte PAAM/AA does not result in better dewatering performance, whereas both polycations recorded high degrees of separation. Furthermore, the measurements with the high-molar-mass PTAC indicate a high resistance to centrifugation which correlates with floc stability. In contrast, addition of the low-molar-mass PDADMAC gives more fragile flocs, leading to faster collapse of the sludge cake in the compression zone of the centrifuge at lower gravitational forces. These experimental findings agree well with the much higher filtration capacity of sludges conditioned with PTAC which was measured by means of a pressure filtration device. Comparisons of the optimum polyelectrolyte concentrations obtained by pressure filtration or centrifugation at low gravitational forces with the zeta potential of the flocculated particles appear to be consistent with bridge formation for PTAC and with the charge patch mechanism for PDADMAC. The centrifugation experiments at high centrifugal accelerations show an optimum separation efficiency at relatively low degrees of coverage and a high level of agreement with the large-scale process. Received: 24 November 1998 Accepted in revised form: 12 February 1999