Water hydraulic systems use water as a pressure medium and, thus, do not pose such adverse environmental impacts as oil hydraulics. Microbial deterioration of the pressure medium and biofouling of the surfaces restrict the applicability of the water-based technology. The potential of microbial growth control by UV-irradiation and filtration was studied in a pilot-scale water hydraulic system. The UV-irradiation (25 m Ws cm(-2)) of the pressure medium reduced the total viable counts of bacteria by 1-2 log(10) cfu cm(-3), whereas the total microbial cell numbers and the numbers of surface-attached microorganisms remained unaffected. Prefiltration (1.2 mum, absolute) of the pressure medium decreased the total microbial cell number in the water phase and retarded the attachment of bacteria. The filtration during the operation (2 mum, absolute) decreased the total numbers of microbial cells and the total viable counts in the pressure medium, and microbial attachment on the surfaces. Microbial attachment was not prevented by filtration. The microbial water quality obtained by pre- and on-line filtration of the pressure medium was sufficient to ensure the long-term operation of the water hydraulic system assuming that clean work practices are complied with in assembly and during the operation.