Composting Process Monitoring and Maturity Prediction by Spectroscopic Measurement and PLS Modelling

Abstract Recycling organic waste is an environmental issue that concerns both the energy perspective and global warming. Composting enables solid waste to be converted into compost, which is a valuable product that helps to store carbon in the soil. Among this waste, sewage sludge poses a problem because of the constant increase in the urban population. The high level of organic matter in sludge can be recovered by co-composting with green waste. Numerous chemical changes occur during the maturation process, transforming simple molecules into humic substances that are beneficial to soils. However, there are as yet no rapid, robust techniques for predicting the stability and maturity of compost during the process. Yet these days, it is necessary to measure a large number of biological (respirometric test, phytotoxicity test), physical (pH, temperature) and chemical (oxidation–reduction potential, carbon–nitrogen ratio C/N, cation exchange capacity) parameters in order to manage the evolution of compost. These measurements are expensive, time-consuming and rarely carried out on composting platforms. The C/N ratio of solid is one of the parameters that can be used to monitor compost during the various stages of production. A partial least squares regression only based on UV and fluorescence spectroscopic data and the pH of the compost water extract is used to predict compost maturation. C/N ratio reach a value near 10 and 4 for solid compost and water extracted organic matter respectively. The PLS prediction error is between 9.4 and 12% for solid organic matter and between 4.2 and 7% respectively for water-extracted organic matter, for two of the three composting conditions. This tool provides a simple and rapid estimation of compost maturity, and could be helpful for compost plant managers by enabling the final product to be qualified on the basis of a rapid spectroscopic indices and adapt their process for gaining time, space and limit the product's rejection.