Direct simulations of solid particles settling in a Newtonian liquid have been performed. The particles were given an attractive interaction potential of a square-well shape which made them aggregate and - as a result - settle faster. The lattice-Boltzmann method was used to solve the liquid flow in between the uniformly sized spherical particles. An immersed boundary method was applied to impose no-slip at the surfaces of the spheres that are free to move and rotate under the influence of net gravity, hydrodynamic forces, collisions and the interaction potential. Solids volume fractions were in the range 0.12-0.32, and Reynolds numbers (based on average superficial slip velocity) went up to order 50. Drag reduction due to aggregation has been correlated with average aggregate size. This correlation is a strong function of Reynolds number and solids volume fraction. (C) 2013 Elsevier Ltd. All rights reserved.