Abstract: An apparatus for testing water quality for subsurface injection that provides for samples of infinite volume to be passed over a membrane filter at a constant pressure. The apparatus uses a positive displacement pump drawing suction on the water source and discharging to a membrane filter. Between the pump discharge and the filter is an oil/water separation vessel to allow for sampling of well production fluids before oil removal if necessary. Fitted to the oil separation vessel is an adjustable pressure regulator, also known as a back pressure regulator, for maintaining constant pressure over the filter. The filter discharge water is collected in a volume calibrated vessel. The vessel is connected to load cells, and a data collection device records weight as a function of time. By measuring the water density, a time/volume curve can be automatically plotted by the data collection device.