A sequential batch reactor (SBR) for aerobic treatment of dairy residues (activated sludge process) was monitored on-line by near infrared (NIR) spectroscopy. NIR spectra in the wavelength range of 900–1700 nm were acquired in situ with a transflectance probe. Measured in situ NIR spectra allowed the development of multivariate statistical process control models for supervision of the SBR process. The directly acquired spectra permitted characterisation of the different SBR operation stages and the detection of process disturbances. Calibration models based on partial least squares regression were developed for three process parameters: total solids (TS), total suspended solids (TSS) and chemical oxygen demand (COD). Model errors were estimated as root mean squared errors of cross-validation (RMSECV). The cross-validation errors obtained were 0.066 g TS L^–1, 0.075 g TSS L^–1 and 86.6 mg O₂L^–1 respectively. Results demonstrated the viability of NIR spectroscopy as an alternative method for monitoring and control of activated sludge wastewater treatment processes having advantages both in terms of efficient determination of process parameters (fast, in situ and requiring no toxic reagents) and in terms of process monitoring.

Keywords: near infrared spectroscopy, wastewater treatment, activated sludge processes, chemometrics, dairy industry