Abstract: The subject invention is an optical sensor system for calculating the percentage of wastewater pretreat fluid within a fluid mixture. The system provides light from a light source to a transparent vessel containing a fluid mixture. The light source and a portion of the vessel are enclosed within a housing to prevent ambient light from polluting the system's measurements. Two photodiodes are placed within the housing to measure the characteristics of the light. One photodiode is placed in a location prior to the light reaching the vessel to measure the characteristics of light emanating from the light source and a second photodiode is placed in a location to measure the characteristics of the light after it has traversed through the vessel and fluid mixture. Using data collected by the photodiodes, a microprocessor can determine the characteristics of the fluid mixture, and more specifically, the percentage of a pretreat fluid within the fluid mixture.