The potential of NIR spectroscopy to predict stability parameters in sewage sludge and derived compost

The aim of this study was to investigate the feasibility of using near-infrared (NIR) spectroscopy as an easy, inexpensive and rapid method to predict stability parameters in sewage sludge and compost derived there from. In the compost samples, predictions were successful (residual predictive deviation (RPD) higher than 2) for both CO2 accumulation after 8 days using alkali (NaOH) traps (CO2 NaOH 8d) and compost age. Predictions were moderately successful (RPD between 1.4 and 2) for water-soluble carbon (WSC), E2/E3 and E4/E6 ratios (ultraviolet–visible measurements in the water-soluble extract) and ash content. In sewage sludge, predictions were successful (RPD = 2.99) for ash content, moderately successful (RPD = 1.60) for WSC and CO2 NaOH 8d (RPD = 1.42), and unsuccessful (RPD < 1.4) for E2/E3 and E4/E6 ratios. The RPD values obtained for the predictions of properties provided by an automatic respirometer (maximum rates of O2 and CO2 and cumulative values after 8 and 30 days) in compost and sewage sludge ranged between 2.02–2.84 and 1.81–2.36, respectively. The low accuracy obtained for the prediction of the CO2 NaOH 8d in sewage sludge illustrated how high levels of respiration can affect the accuracy of the NaOH trap method. Global models combining the compost and sewage sludge samples were constructed and used for the prediction of data provided by the automatic respirometer, achieving RPD values higher than 3.5. This work demonstrates the potential of NIR spectroscopy for predicting certain parameters related to the stability of sewage sludge and its derived compost.