Stability of membrane fouling particles under perikinetic and orthokinetic flow conditions

Stability of fouling particles against Brownian motion (perikinetic) and simple shear flow (orthokinetic) was investigated. The effect of the various parameters such as particle radius, Hamaker constant, salt (1-1 type) concentration, and surface potential on the perikinetic stability using DLVO theory was studied. Conditions at which particles are stable at salt concentration = seawater salt concentration (∼0.6 M) were identified. The calculations showed that typical fouling particles of the Hamaker constant of 1 × 10^-20 J and surface potential of -25 mV are not stable against Brownian motion at concentrations larger than 0.1 M. Under these conditions the particle size plays a minor role on the stability. Particles of surface potential <-40 mV are stable at salt concentration = seawater concentration. The type of fouling particles expressed by the Hamaker constant strongly affects its stability. Particles of 100 nm in radius, -25 mV in surface potential and a Hamaker constant of <6 × 10^-21 J are stable at salt concentration = seawater concentration. Above this value of the Hamaker constant, the fouling particles coagulate. The effect of particle radius, Hamaker constant, surface potential and shear rate represented by the two dimensionless parameters, C_A and C_R, on orthokinetic stability was also investigated. The investigation was made by solving the trajectory equations for the simple shear flow conditions. The results show that the coagulation efficiency goes through maxima as the ratio of the repulsive forces to the attraction forces is increased. Estimating the value of the Peclet number, the importance of the perikinetic to the orthokinetic coagulation can be determined.