Organic matter release in low temperature thermal treatment of biological sludge for reduction of excess sludge production

Abstract

Thermal treatment applied in association with a biological system allows for a significant reduction in excess sludge production (approximately 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 degrees 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 degrees C for 40 min of contact time). For the first 30 min, a 1st order kinetic, with respect to the residual COD, was systematically found. In the range of 40-95 degrees 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 solubilisation rather than by a biodegradability increase.

Assessment of physiological state of microorganisms in activated sludge with flow cytometry: application for monitoring sludge production minimization

“…These results are in agreement with previous studies which reported that matter and COD solubilization increased with temperature, but treatment time had little effect if it exceeds 30 min in a temperature range of 60-190°C [24]. Similarly, in another study [25], sludge COD solubilization rate dramatically decreased after 30 min of thermal treatment.…”

“…The relation between cell lysis and organic matter solubilization implies that the origin of the released organic matter during the thermal treatment was mainly intracellular. This hypothesis is in accordance with Paul et al [25] who concluded that a thermal treatment below 100°C allowed only partial floc sludge destruction. These treatment conditions should be sufficient to break the non-covalent links and modify the sludge structure, but are insufficient to allow a complete floc destruction.…”

Techno-economic evaluation of thermal treatment, ozonation and sonication for the reduction of wastewater biomass volume before aerobic or anaerobic digestion

“…This result confirmed the interest of the use of ozone pretreatment [22,28,31,50]. The values of TSS removal improvement (71% and 78.5%) are above the values proposed by Paul et al [31] (30%) to economically justify a process of sludge reduction furthermore above the results of Sievers et al [50] on full scale application who reached 20-35% and 19% after aerobic or anaerobic stabilisation and ozone treatment of 0.05 gO 3 gTSS −1 . Deleris et al [51] obtained comparable results (70% of reduction of sludge production) with lower ozone dose (ozonation on the recycling loop 0.05 gO 3 gVSS −1 ).…”

“…When sludge heating is to be performed in order to reduce sludge production, sludge solubilisation process would be necessary. For this purpose, two main temperature brackets are to be considered (from economic or efficiency point of view): temperatures either higher or lower than 150 • C [31]. The necessary temperatures to obtain solubilised sludge would be around 160-200 • C [6,7].…”

The viscoelastic characterisation of thermally-treated waste activated sludge

“…Paul et al (2006) [17] also reported that thermal treatment promotes cell lysis because extracellular polymeric substances (EPS) such as proteins, lipids and polysaccharides were released. Frølund et al…”