Biofuel processing applications require concentration measurement of bio-ethanol during its evolution by algae in a proprietary chemical process to optimize the chemical parameters and the resulting yield of bio-ethanol. The production rate of bio-ethanol is initially very low with correspondingly minute concentration changes. This study presents a highly sensitive U-bend evanescent wave plastic optical fibre sensor for ethanol concentration measurement in water corresponding to the production rate limit of bio-ethanol by algae (cyanobacteria) i.e. 0.1-0.5g per liter per day or 0.0126%v/v-0.0633%v/v in terms of percent volume concentration of a solution or %volume/volume concentration. The sensor was fabricated using low cost, large core (1000μm diameter) plastic optical fibre (POF) and investigated in the broadband visible light spectrum (VIS) and specifically at 630nm wavelength. Real-time measurements show that the sensor provides a significant response in the range of 0.00633%v/v to 0.0633%v/v concentration of ethanol in water which corresponds to detection of change in refractive index as low as 10^-7. Moreover, sensitivity of sensor was derived for the above range as 0.6448 O.D/%v/v with 97.14% of linearity in results at 630nm wavelength, here O.D refers to optical density - unit of absorbance. It indicates that the proposed sensor represents a viable solution for real time and very low level concentration sensing in biofuel processing applications.