A laboratory-scale fixed bed reactor of five stages running serially, with free working volume of 61, and up- and down-flow successively, was used for treating low-strength wastewater. With this design was sought a greater passage of the liquid over the support of the biofilm, constituted of waste tyre rubber (disposable material). The objective was to demonstrate the efficiency of this system to purify domestic wastewaters of 1000 mg COD/1 of high microbiological load. Different hydraulic retention times (HRTs), 4, 2 and 1 days, and 12 and 8 h, were studied. For HRTs from 4 to 1 days, the efficiency of organic matter removal in the first stage reached the maximum value, and was maintained after passing the liquid through the rest of the stages. For 12 and 8 h, a greater variation of the removal efficiency was observed among all the stages; fundamentally, in the first three. BOD removal efficiencies greater than 60% were achieved: acceptable values for this primary treatment. The variations during the process in GOD, total and suspended solids, minerals and volatiles, volatile fatty acids, alkalinity, ammonia nitrogen and phosphorus, and faecal coliforms were determined. Good results were achieved for short HRTs, therefore, it would be possible to try a greater wastewater flow. The total and faecal coliform removal efficiencies varied from 99% for 4 days to 70% for 8 h HRT. This was considered acceptable taking into account the high microorganisms concentration of the wastewater used (1.4 x 10(10) MPN/100 ml).