Using our portable in-line fluidic pretreatment apparatus we have adjusted the osmolality and pH of Swiss river samples automatically without any dilution or degradation of the samples. The samples will be tested by means of a cell-based water quality biosensor for detection of toxins. For cell-based biosensors, the osmolality and pH of the sample are critical parameters that must be precisely controlled to prevent cell lysis. Using forward osmosis (FO), we developed a device for a low-cost adjustment of these parameters to the standard values of a cell-culture medium (pH: 8.5, Osmolality: 330 ±10 mmol/kg).