Refractory organic matter content in sewage sludge: inaccessibility for hydrolysis or/and chemical resistance?

Sludge valorization through methane production is highly limited by the biodegradability of the organic matter. This may be related to the physical and chemical properties of the sludge organic matter that define its bioavailability, its bioaccessibility and its biodegradability. The objective is here to analyze the relationship between the physical state (floc) and the refractory content of a sludge. As depending on the sludge origin, experiments were made for sludge samples coming from plants running at very different sludge retention time. The range of the unbiodegradable COD content (CODU) is between 42 and 73%. The CODU remains mainly in the particulate form with a size distribution unchanged compared to the raw sludge before anaerobic digestion. Soft heating tests show that the major part of the biodegradable COD is easily solubilized whereas the refractory materials remain particulate. That means that the COD solubilisation potential of a given sludge can be used to indirectly get a fingerprint of its unbiodegradable COD content. Finally, the refractory particulate organic matter ensures the aggregated structure of flocculated sludge. Their components form a strongly cohesive porous matrix whose function could be assimilated to a skeleton entrapping the biodegradable matter.