Kinetics of thermophilic batch anaerobic digestion of thermal hydrolysed waste activated sludge

INRA, UR50, Laboratoire de Biotechnologie de l’Environnement, Avenue des Etangs, Narbonne, F-11100, France
Biochemical Engineering Journal (Impact Factor: 2.47). 10/2009; 46(2):169-175. DOI: 10.1016/j.bej.2009.05.003


Enhancing waste activated sludge (WAS) anaerobic digestion by thermal pretreatment has become of high interest. However, thermal treatment has been mainly combined to mesophilic anaerobic digestion. This paper presents the combination of sludge thermal pretreatment (110, 165 and 220 °C) and batch thermophilic anaerobic digestion (55 °C). Optimal conditions of thermal pretreatment were shown to be 165 °C, involving a chemical oxygen demand (COD) and volatile solids (VS) solubilisation of 18 and 15% and a biodegradability increase from 47 to 61%. Treatments at 165 °C were carried out in electric and steam modes and no significant difference on the impact of heating mode on sludge anaerobic biodegradability was observed. Moreover, it may be recommended not to carry out successive batch experiments to assess thermophilic BMP of sludge as accumulation of volatile fatty acids (VFA), particularly propionate, and a decrease of VFA uptake rates may occur. However, thermal pretreatment at 165 °C allowed the decrease of propionate accumulation and an higher methane production.

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    • "It has been hypothesized that thermal pre-treatment can enhance the biological activities of some thermophilic bacterial populations; for this reason, it can be considered a valid pre-treatment step before thermophilic digestion [16] [17]. However, very few studies have addressed the combination of thermal pre-treatment and the thermophilic anaerobic digestion of sludge [1] [4] [16] [18]. "
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    ABSTRACT: Anaerobic digestion has been recognized as the most appropriate stabilization technology for approaching sludge reuse task, in part because of the methane production involved. The mesophilic anaerobic processing train can be upgraded by adding a pre-treatment step or by increasing the digestion temperature to thermophilic conditions. In this study, semi-continuous mesophilic (MAD) and thermophilic (TAD) anaerobic digesters were operated for 180 days at low (0.7–1.0 kg VS/m3 d), medium (1.4–1.8 kg VS/m3 d) and high (2.8–3.7 kg VS/m3 d) organic loading rates (OLRs). The impact of ultrasound and thermal hydrolysis pre-treatments on the mesophilic and thermophilic digestion efficiency, respectively, was assessed by performing parallel digestion tests. The increase of soluble COD (Chemical Oxygen Demand) and colloidal surface charge after the sludge pre-treatments suggested biopolymers solubilization and changes in surface floc properties. Thermal hydrolysis enhanced the release of lipids and long chain fatty acids, while ultrasounds application resulted in proteins being the main component of the released matter. Operating the digesters at OLRs between 0.7 and 1.4 kg VS/m3 d the methane conversion rate was not significantly affected by the temperature increase up to thermophilic conditions, whereas the integration of the pre-treatments accelerated the organic conversions, resulting in a noticeable methane gain (up to +51%). Conversely, at higher organic loads, the TAD yields were significantly higher with respect to the MAD ones, assuring the sustainable economic benefit of operating smaller anaerobic digesters to obtain higher methane productions. Nevertheless, the colloidal charge increase during thermophilic digestion impaired the sludge filterability much more rapidly than in mesophilic conditions.
    Full-text · Article · Jun 2015 · The Chemical Engineering Journal
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    • "Modifications to ADM1 model Fig. 1 representing the methane production rates for untreated and thermally pretreated WAS, gives relevant information concerning the COD conversion during anaerobic digestion of a WAS sample for which its organic matter structure was changed by thermal pretreatment. Indeed, the thermal pretreatment has been strongly studied as a pretreatment in order to make the organic matter more accessible to the anaerobic microorganisms and so to improve methane production rate [31] [34]. We observed that a low thermal pretreatment at 110 °C increases the methane production rate and a larger quantity of organic matter seems to be degraded during the first 4 days compared to the untreated WAS. "
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    ABSTRACT: A new model structure for the hydrolysis step is introduced in the IWA anaerobic digestion model no 1 (ADM1) in order to better represent the bioaccessibility of particulate organic matter. Two particulate organic matter fractions for waste activated sludge (WAS) samples were defined: a readily hydrolysable fraction (Xcr) and a slowly hydrolysable fraction (Xcs). These fractions were hydrolyzed according to a surface-limiting reaction. Batch anaerobic digestion test of untreated WAS was used to develop the model and calibrate the kinetic parameters and biomass concentrations. The validation was carried out with a similar substrate than the calibration but a thermal pretreatment was applied at two different conditions (110 °C and 220 °C). The behavior of thermophilic anaerobic digestion of WAS samples was effectively represented by the proposed model. No changes among kinetic parameter sets were done and the model is able to represent produced methane volume following the intrinsic changes of the WAS composition through the different thermal pretreatment conditions. Moreover, estimated parameters in 20 days of batch anaerobic digestion test were representative of continuous anaerobic digestion in a full-scale digester.
    Full-text · Article · Jul 2013 · The Chemical Engineering Journal
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    • "Because the thermal hydrolysis process holds the combined benefits of decreasing biosolids quantity and increasing biosolids acceptability, it is of great interest to the wastewater industry (Wilson and Novak, 2009). Nevertheless, few authors have investigated the effect of thermal hydrolysis on thermophilic digestion of waste activated sludge alone, and these studies are mostly carried out in batch systems (Bougrier et al., 2008; Liu et al., 2012; Mottet et al., 2009). There is however an upper limit of temperature (around 180– 200 °C) beyond which the effectiveness of THP not only starts to drop but rather results in the production of hardly degradable compounds. "
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    ABSTRACT: The performance of thermophilic digestion of waste activated sludge, either untreated or thermal pretreated, was evaluated through semi-continuous tests carried out at organic loading rates in the range of 1-3.7kgVS/m(3)d. Although the thermal pretreatment at T=134°C proved to be effective in solubilizing organic matter, no significant gain in organics degradation was observed. However, the digestion of pretreated sludge showed significant soluble COD removal (more than 55%) whereas no removal occurred in control reactors. The lower the initial sludge biodegradability, the higher the efficiency of thermal pretreated digestion was observed, in particular as regards higher biogas and methane production rates with respect to the parallel untreated sludge digestion. Heat balance of the combined thermal hydrolysis/thermophilic digestion process, applied on full-scale scenarios, showed positive values for direct combustion of methane. In case of combined heat and power generation, attractive electric energy recoveries were obtained, with a positive heat balance at high load.
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