Rapid and Simple Determination of Oil and Urea Concentrations and Solids Content to Monitor Biodegradation Conditions of Wastewater Discharged from a Biodiesel Fuel Production Plant

To prepare and maintain the optimal biodegradation conditions of wastewater discharged from a biodiesel fuel (BDF) production process with alkali–catalysis transesterification, rapid and simple methods were investigated to measure oil (carbon source of microorganisms) and urea (nitrogen source) concentrations and solids content (indicator of growth inhibition of microorganism) of the wastewater. Two non-destructive methods were investigated. First, physical properties of the BDF wastewater such as electric conductivity (EC), specific gravity (Sp.Gr.) and Brix values were measured and the values were analysed with multiple liner regression (MLR). Only solids content could be predicted by measured physical properties. However, adjustment of the C/N ratio of the BDF wastewater was also necessary to prepare the optimal condition for the microorganism growth in the wastewater treatment process. Therefore, near infrared (NIR) spectroscopy was applied to simultaneous, non-destructive and rapid measurement of the constituents of the wastewater. The dominant absorptions caused by oil, urea and solids material in the wastewater were observed at 1718, 2154 and 2286 nm, respectively. These were used as the first wavelength to formulate calibration equations, a MLR analysis was carried out between the NIR spectral data and the values of conventional analyses, such as hexane extraction, enzymatic and oven-drying methods, in the calibration sample set (sample number, n = 50). To validate the calibration equations obtained, the predicted values of the oil, urea and solids in the validation sample set (n = 40), which was not used for formulating the calibration equations, were calculated using the calibration equations. Good agreement was observed between the values of the conventional analyses and the values predicted using NIR; the multiple correlation coefficients of determination (r²) for the validation equations for oil, solids and urea were 0.993, 0.877 and 0.960, respectively. Prediction of the solids content of the BDF wastewater using EC and Brix values is also possible with the handy EC and Brix meters which are inexpensive and easy to handle. However, these do not allow total management of the BDF production and BDF wastewater treatment processes. On the other hand, the NIR method is potentially suitable for automated process management of a BDF wastewater treatment system.