The start-up of two biofilm infiltration-percolation aerated system (BIPAS) reactors, with calcareous and siliceous gravels as support materials, used for treating liquid piggery waste, was studied. The development of nitrifying activity and its evolution under the effects of different patterns of aeration or with different levels of influent alkalinity were analysed. During the initial phase, the reactors were regularly fed with 86 l m(-2) d(-1) of liquid lagoon piggery waste which contained 3.2 g l(-1) COD, and 1.08 g l(-1) TKN of which 1.02 g l(-1) was NH4+-N. Nitrite appeared in the two reactors 14 and 17 days after the initial piggery waste addition and accumulated up to 50 days, while NO3--N formation began after 2 months and immediately followed the first rest period at the beginning of the intermittent aeration pattern experiment Although nitrification was not initially influenced by the support material, it was move complete on the calcareous material with a total disappearance of NH4+-N and only NO3--N accumulation with the NH4+-N concentrations initially applied. Experiments with various influent soluble alkalinity levels demonstrated that this factor was strongly limiting with the siliceous material while nitrification in the calcareous material partly used CaCO3 dissolution as additional alkalinity. The distribution over the depth of the filters of organic biomass, of potential nitrifying and denitrifying activities and of the corresponding microflora is described There was stratification with depth, the surface horizons being more enriched in organic matter and more biologically active, even though the microflora developed in all horizons of the profile. The specific-activity rates were measured and varied from 0.5 to 11.7 ngN cell(-1) h(-1) for ammonia oxidizers; from 6.6 to 29.2 pgN cell(-1) h(-1) for nitrite oxidizers and from 28.8 to 96.3 pgN cell(-1) h(-1) for denitrifiers. These values could be overestimated, indicating the difficulty in counting microbial populations fixed on a support material. (C) 1996 Elsevier Science Ltd.