Combining chemical sequential extractions with 3D fluorescence spectroscopy to characterize sludge organic matter

• Sludge organic matter bioaccessibility was revealed by chemical sequential extractions.
• Organic fractionation profile is characteristic of the sludge stabilization degree.
• Chemical complexity of organic fractions was revealed by fluorescence spectroscopy.
• Increase of complexity compounds fluorescence with decreasing accessibility.
• Combining extractions with fluorescence could reveal the sludge biodegradability.

The design and management of anaerobic digestion of sewage sludge (SS) require a relevant characterisation of the sludge organic matter (OM). Methods currently used are time-consuming and often insufficiently informative. A new method combining chemical sequential extractions (CSE) with 3D fluorescence spectroscopy was developed to provide a relevant SS characterisation to assess both OM bioaccessibility and complexity which govern SS biodegradability. CSE fractionates the sludge OM into 5 compartments of decreasing accessibility. First applied on three SS samples with different OM stability, fractionation profiles obtained were in accordance with the latter. 3D fluorescence spectroscopy revealed that the bioaccessible compartments were mainly constituted of simple and easily biodegradable OM while the unaccessible ones were largely made of complex and refractory OM. Then, primary, secondary and anaerobically digested sludge with different biodegradabilities were tested. Complexity revealed by 3D fluorescence spectroscopy was linked with biodegradability and chemical accessibility was correlated with sludge bioaccessibility.