On the free-settling test for estimating activated sludge floc density

Physical and hydrodynamic properties of flocs produced during biological hydrogen production

“…Although biohydrogen flocs examined here had similar fractal dimensions to other biological aggregates, they were much more dense than other biological aggregates, as evidenced by their lower porosities. Activated sludge flocs larger than 0.1 cm typically have a porosity greater than 0.95 (Li and Ganczarczyk, 1990;Lee et al, 1996; Yuan, 2002;Li et al, 2003). However, most biohydrogen flocs examined here had porosities that were less than 0.95.…”

“…However, most biohydrogen flocs examined here had porosities that were less than 0.95. As a result, biohydrogen reactor flocs settled faster than the similarly sized activated sludge aggregates (Lee et al, 1996;Li and Yuan, 2002). Highly permeable inorganic flocs have been found to settle 4 -8.3 times faster than predicted by Stokes' law due to flow through the aggregate (Johnson et al, 1996).…”

“…While Li and Ganczarczyk (1989) reported a relatively wide range of fractal dimensions of 1.4 -2.8 for particle aggregates generated in water and wastewater processes, most studies have found values of D > 2.1 for flocs produced in bioreactors. For example, the activated sludge collected from full-scale wastewater treatment plants had D = 2.24 for small flocs (Lee et al, 1996) and D = 2.26 for large flocs (Li and Yuan, 2002), and activated sludge flocs produced in laboratory reactors were found to range from D = 2.10 to D = 2.34 for sludge ages in the range of 5 -20 days . A somewhat lower value of D = 2.09 was reported for the bacterial flocs produced in a laboratory sequencing batch reactor treating synthetic wastewater (Li and Yuan, 2002).…”

“…The open structure of highly porous and fractal aggregates can allow intra-aggregate flow through the flocs, resulting in their settling velocity being appreciably faster than that predicted for impermeable spherical particles (Li and Ganczarczyk, 1992;Johnson et al, 1996;Li and Yuan, 2002). Biological flocs produced in activated sludge reactors are highly porous and fractal (Li and Ganczarczyk, 1990;Lee et al, 1996;Li and Yuan, 2002), but the flow through the floc is not large enough to affect their settling behavior. However, floc permeability can affect mass transport rates to bacteria inside the floc, even under conditions where it does not affect settling velocity (Logan and Hunt, 1988;Li and Ganczarczyk, 1992;Tsou et al, 2002;Li et al, 2003).…”

Modeling particle-size distribution dynamics in a shear-induced breakage process with an improved breakage kernel: Importance of the internal bonds

“…S3). As the fractal dimension decreased from 3.0, 2.5 to 2.0, according to the numerical simulation results, the mean size also reduced from 115 m, 39 m to 12 m. It is well known that fractal aggregates become more porous as they increase in size [7,23,[36][37][38]. Therefore, large aggregates with a lower fractal dimension would be much weaker in structure and hence more vulnerable to shear breakage.…”