Suspensions of a microparticulate boron adsorbent can be effectively concentrated to relatively high concentrations of about 11 mass% by microfiltration in aerated submerged modules using hydrophilic capillary and hydrophobic polypropylene hollow fibre membranes. With the increasing trans-membrane pressure difference, the permeate flux increases significantly at suspension concentrations below 7 and 4 mass% for hydrophilic and hydrophobic membrane, respectively. The permeate flux is higher for the hydrophilic membrane than for the hydrophobic one. Concentration dependence of flux for the hydrophilic membrane shows only a slight decrease of the flux with increasing concentration of the suspension, up to 7 mass%, while the flux through the hydrophobic membrane decreases much in this interval. The permeate flux at the trans-membrane pressure of 40 kPa for suspensions with 7 mass% of the adsorbent is about 72 and 14 dm(3) m(-2) h(-1) for the hydrophilic and the hydrophobic membrane, respectively. Backwashing of the membrane with a reversed flow of permeate was tested in a module with a hydrophilic membrane and was found successful in keeping the membrane performance stable for a shorter term of operation.