Influence of polyelectrolyte structure and type on the degree of flocculation and rheological behavior of industrial MBR sludge

Abstract

The influence of polyelectrolyte's type and structure on the rheological characteristics of industrial highly stable membrane bioreactor (MBR) sludge have been investigated and were related to its degree of flocculation and dewatering. The polyelectrolytes used for the conditioning of MBR sludge were polyacrylamides (PAMs) of varying charge density (CD), molecular weight (Mw) and molecular architecture, polydiallyldimethylammonium chloride (polyDADMAC) and polyamine. Rheological measurements showed that the MBR sludge with/without conditioning has a thixotropic (time-dependent) behavior with measurable yield stress. It exhibited a non-Newtonian shear thinning behavior and well represented by the Bingham model. The measurements of elastic (G′) and viscous (G″) moduli proved the viscoelastic solid-like behavior of the MBR sludge with/without conditioning (G′ > G″). Bingham yield stress, τo, viscosity, µ, and G′, were used for examining the rheological behavior of the conditioned MBR sludge. About 77–94% higher in τo, 86–96% in µ, and 76–92% in G′ were obtained by conditioning the sludge with different types of PAMs. These percentages vary depending on the type and structure of the PAMs. The highest τo, µ, and G′ was obtained by using a linear structure PAM, FO 4690 SSH. This behavior was in good agreement with flocs size, D50, zeta potential, ζ, and capillary suction time, CST. The results obtained by conditioning MBR sludge with different PAMs were compared with those obtained by conditioning with polyamine and polyDADMAC. Polyamine and polyDADMAC have raised the τo, µ and G′ of the MBR sludge by 52%, 84%, and 63%, respectively. Overall, MBR sludge conditioned with PAMs having a high CD in the case of high Mw (FO 4690 SSH, FO 4698 SSH) and low CD in the case of low Mw (FO 4498 XXR), resulted in greater τo, µ and G′ compare to those formed with polyamine and polyDADMAC. Therefore, PAMs are building a more compact network that highly reduces the trapped water within the sludge flocs.