Optimising activated sludge growth rate by intensifying hydrodynamic forces † GM Evans* and G Liu Department of Chemical Engineering, University of Newcastle, NSW, 2308, Australia Abstract: In this study a Couette device was used to examine the influence of hydrodynamic forces on the overall behaviour of activated sludge. The specific growth rate of activated sludge increased with increasing specific energy dissipation rate, reaching a maximum rate at an energy input of approxi- mately 40 m 2 s 3 , beyond which any further increases resulted in a reduction in the overall growth. At levels above about 250 m 2 s 3 the specific growth rate was negligible. Experimental growth rates in the range 0.04–0.20 h 1 were obtained, depending on the energy input levels and substrate (glucose) concentration. A theoretical model successfully predicted both the decrease in the floc radius and microbial activity with increasing energy input into the system, and when combined was able to provide a reasonable prediction for the optimum specific energy dissipation rate to maximise the specific growth rate. # 2003 Society of Chemical Industry Keywords: activated sludge; energy dissipation; microbial activity; specific growth rate; floc radius NOTATION A m Microbial activity (dimensionless) C S Mass concentration of substrate (kg m 3 ) COD Chemical oxygen demand (kg m 3 ) D S Effective diffusivity of substrate (m 2 s 1 ) G Shear rate (s 1 ) J S Mass flux of substrate (kg s 1 ) J S * Specific mass flux of substrate (s 1 ) k d Specific death rate of viable cells (s 1 ) K S Saturation constant (kg m 3 ) M E Molecular weight of enzymes (kg kmol 1 ) MLVSS Mixed liquid volatile suspended solids (kg m 3 ) OUR Oxygen uptake rate (kg m 3 s 1 ) r Radial coordinate measured from centre of floc (m) R Gas constant (kg m 2 s 2 K 1 ) R F Radius of floc (m) (R F ) crit Critical floc radius, given by eqn (14) (m) R 1 Inner cylinder radius of Couette device (m) R 2 Outer cylinder radius of Couette device (m) t Time (s) T Torque (kg m 2 s 2 ) T a Taylor number, defined by eqn (10) (dimensionless) T o Torque with no liquid present (kg m 2 s 2 ) V L Liquid volume inside Couette device (m 3 ) X V Mass of viable cells per volume (kg m 3 ) Y e Yield coefficient (dimensionless) e Specific energy dissipation rate (m 2 s 3 ) y Temperature (K) k Fraction of the initial cells, at MLVSSj t =0 , which are viable (dimensionless) m Specific growth rate (s 1 ) m m Maximum specific growth rate (s 1 ) n Kinematic viscosity (m 2 s 1 ) r F Density of floc (kg m 3 ) r L Density of liquid (kg m 3 ) j E Mass concentration of enzymes (kg m 3 ) O Rotational speed (s 1 ) INTRODUCTION The growth of cells relies on a number of processes, including transport of nutrients from the bulk medium into cells. The rate of transfer of nutrients is influenced by the amount of agitation applied to the suspension. Agitation of low to moderate intensity breaks up aggregates of cells (referred to as flocs), reducing their size and making the nutrients more readily available to cells. This results in an increase in the growth rates of the cells. According to this model, cell growth rate would continue to increase with increases in mixing intensity. However, this trend is not always consistent (Received 11 June 2002; revised version received 27 September 2002; accepted 2 October 2002) * Correspondence to: GM Evans, Department of Chemical Engineering, University of Newcastle, NSW, 2308, Australia E-mail: cggme@cc.newcastle.edu.au † Paper presented at the Process Innovation and Process Intensification Conference, 8–13 September 2002, Edinburgh, UK # 2003 Society of Chemical Industry. J Chem Technol Biotechnol 0268–2575/2003/$30.00 276 Journal of Chemical Technology and Biotechnology J Chem Technol Biotechnol 78:276–282 (online: 2003) DOI: 10.1002/jctb.769