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Thermal treatment applied in association with a biological system allows for a significant reduction in excess sludge production (~50%). In general, heat treatment is described as a sludge disintegration technique. This paper offers a thorough study on the impact of heat treatment, at temperatures below 100°C, on the solubilisation of the sludge COD and its biodegradability. Discontinuous heating experiments were performed on activated and digested sludge. At all temperatures tested the released COD for digested sludge was systematically higher than that for activated sludge (15 and 40%, respectively, at 95°C for 40 mm of contact time). For the first 30 mm, a 1st order kinetic, with respect to the residual COD, was systematically found. In the range of 40-95°C, digested sludge had a lower activation energy than activated sludge (26 kcal/mol compared to 70-160 kcal/mol). COD solubilisation is thus more positively influenced by temperature in the case of activated sludge. This may be due to the significant difference in the ratio of protein/carbohydrate in digested and activated sludge (1 -5 and 0.2-0.7, respectively). The increase in the COD/TKN ratio in the solubilised fraction after thermal treatment of activated sludge suggests a preferential solubilisation of proteins over carbohydrates. Respirometric tests performed on the solubilised COD showed that whatever the sludge origin, only 40-50% of released COD is biodegradable at a conventional hydraulic retention time (i.e. 24 h). Hence, heat treatment would act more through organic matter solubitisation rather than by a biodegradability increase. [ABSTRACT FROM AUTHOR]

Among the technologies aimed at reducing sludge production, the combination of thermal treatment at 95°C of sludge and the activated sludge process is a promising route. The feasibility of such a combined process is demonstrated (up to 60% sludge reduction) and the impacts of operating conditions on its efficiency are presented. Major emphasis was put on understanding the complex phenomena occurring within the thermal treatment: release and biodegradability of sludge organic matter, impact on the biological activity (decay, maintenance requirements, etc.). These effects were taken into account for the development of an ASM1-based model. Comparison between the modeling approach and experimental data (continuous and batch) showed that thermal treatment had three major issues partly explaining the reduction of sludge production: (i) a low release of organics; (ii) an immediate and reversible biological inactivation associated with additional maintenance energy requirements; and (iii) a potential inert production. [ABSTRACT FROM AUTHOR]

Presents results of physico-chemical and biological characterization of wastewater samples. Activated sludge; Biodegradability of physico-chemical fractions; Chemical oxygen demand; Coagulation; Hydrolysis; Respirometry.

A new respirometric method for COD fractionation was applied to various wastewaters collected on French treatment plants. Great variations were observed especially in the readily biodegradable fraction (RBCOD) ranging from 1 to 16% of total COD. Variability of the results among the origin can be explained by the reactions occurring in the sewers. In one of the towns studied, the wastewater samples collected directly in the sewer show COD characteristics very different from the sewage which reaches the treatment plant. By analysing the same wastewater after aerobic and anaerobic storage, it was proved that RBCOD respectively decreased or increased significantly. Seasonal fluctuations were also quantified, showing that RBCOD fraction decreased from 75 mg/L to -15 mg/L during the summer whereas the variation of solublefraction was less important. [ABSTRACT FROM AUTHOR]