Nitrate often contaminates groundwater resources due to excessive use of fertilizers and uncontrolled on-land discharges of raw and treated wastewater and can therefore limit the direct use of groundwater for drinking water purposes. In order to investigate the possible application of a membrane bioreactor (MBR) for denitrification of groundwater, the performance of a pilot-scale MBR was tested as a function of hydraulic and biological parameters. For this purpose synthetic groundwater was prepared by taking lake water (Lago Maggiore, Italy) and adding known amounts of ethanol and sodium nitrate to study the nitrate removal capacity of the sludge, to search for an optimum C/N ratio and to measure filtering ability for micro-organisms through the membrane. The optimum C/N ratio was found at 2.2 gC/gN, resulting in an effluent nitrate concentration within the limits stated in EU Directive 98/83 and the US EPA for drinking water use. The effluent nitrite concentration was one order under the EU limit. The membrane module, Zenon ZW-10, was monitored and performed well except for a short stress episode due to low airflow, afterwards rapidly corrected and thus putting the membrane back to its previous stable behavior. Total bacterial count for the treated effluent was lower than influent water, and 100% removal was observed for both total coliforms and E. coli. Calorimetric thermograms related to heat dissipation due to biological denitrification (nitrate and nitrite consumption) and to substrate adaptation are discussed. A maximum nitrate removal rate close to 20 mgN-NO3- gVSS(-1) h(-1) was measured in the calorimetric tests.