Thermophilic anaerobic digestion in compact systems: investigations by modern microbiological techniques and mathematical simulation

Abstract

Thermophilic anaerobic digestion in compact systems can be an economical and ecological reasonable decentralised process technique, especially for rural areas. Thermophilic process conditions are important for a sufficient removal of pathogens. The high energy demand, however, can make such systems unfavourable in terms of energy costs. This is the case when low concentrated wastewater is treated or the system is operated at low ambient temperatures. In this paper we present experimental results of a compact thermophilic anaerobic system obtained with fluorescent in situ hybridisation (FISH) analysis and mathematical simulation. The system was operated with faecal sludge for a period of 135 days and with a model substrate consisting of forage and cellulose for a period of 60 days. The change in the microbial community due to the two different substrates treated could be well observed by the FISH analysis. The Anaerobic Digestion Model no. 1 (ADM1) was used to evaluate system performance at different temperature conditions. The model was extended to contribute to decreased methanogenic activity at lower temperatures and was used to calculate energy production. A model was developed to calculate the major parts of energy consumed by the digester itself at different temperature conditions. It was demonstrated by the simulation study that a reduction of the process temperature can lead to higher net energy yield. The simulation study additionally showed that the effect of temperature on the energy yield is higher when a substrate is treated with high protein content.

Modeling temperature variations in a pilot plant thermophilic anaerobic digester

“…These results might indicate that the temperature prediction should be improved. In this respect, previous reports [5,7] have shown that methane yield can decrease between 11.2 and 6.0%°C -1 when a temperature reduction takes place. This suggests that a minimization in the prediction error of the temperature variation could reduce the prediction error of the methane yield prediction.…”

“…That situation allows to infer that thermophilic anaerobic digestion (TAD) could be easily affected by environmental conditions. In this respect, two independent research groups [5,7] reported that, under TAD, methane yield decreases between 6 and 11.16%°C -1 when a temperature reduction takes place. This could be attributed to the strong dependence on temperature of the kinetic parameters of the reactions involved in anaerobic digestion.…”

“…Keywords Temperature prediction Á Mass and energy balances Á Overall heat transfer coefficient List of symbols A t , A he Tank and heat exchanger areas (m 2 ) Cp d , Cp i Specific heat of digestate and a stream i (subscript i refers streams 1, 2, 4 and 10) (J kg -1 K -1 ) h 1 , h 2 , h 4 Enthalpy of streams 1, 2, and 4 (J kg -1 ) h 5 , h 6 , h 7 Enthalpy of streams 5, 6, and 7 (J kg -1 ) h 8 , h 9 , h 10 Enthalpy of streams 8, 9, and 10 (J kg -1 ) h sat 5 Stream 5 saturation vapor enthalpy (J kg -1 ) l " w h ; l " w b Energy lost due to friction in heating and blowing recirculation (J kg -1 ) _ m 1 ; _ m 2 ; _ m 4 Mass flow of streams 1, 2, and 4 (kg s -1 ) _ m 5 ; _ m 6 ; _ m 8 Mass flow of streams 5, 6, and 8 (kg s -1 ) _ m 10 Mass flow of stream 10 (kg s -1 ) m d Mass (kg) of the digestate in the biodigester at a time t m d, 0 Initial mass in the biodigester (kg) M CH 4 ; M CO 2 Molecular weight of methane and carbon dioxide (kg kg mol -1 ) M H 2 O Molecular weight of water (kg kg mol -1 ) P nc Pressure at normal conditions (Pa) P 6 , P 7 Pressure of streams 6 and 7 (Pa) P 8 , P 9 Pressure of streams 8 Volumetric flow of stream (m 3 s -1 ) at normal conditions V d Volume (m 3 ) of the digestate in the biodigester at a time t x d , x 1,2, 10 Solids concentration of the digestate and the streams 1, 2 and 10 (kg kg -1 ) x CH 4 4 , x CO 2 4 Methane and carbon dioxide concentrations in stream 4 (kg kg -1 )…”

Simulation of the Anaerobic Process for the Biogas Production

“…Mikrobiologische Studien über die Population der Bakterien zeigen, dass die Vielfalt und Zusammensetzung der Population sehr komplex und variabel sind [31,32]. In der Praxis erhöht sich die Konzentration der Bakterien im Reaktor, wenn Gülle statt Abwasser als Substrat verwendet wird [28].…”

Co-digestion of food waste in a municipal wastewater treatment plant: Comparison of batch tests and full-scale experiences

“…This finding corresponds very well with results revealed by Kim et al (2003), who observed the highest methane production rate in co-digestion of sewage sludge with food waste in the range of 30-40% VS under mesophilic conditions. Reports on full-scale sludge co-digestion practices are scarce in the peer-reviewed literature and mainly focusing on technical requirements and economic efficiency (Björnsson et al, 2000;Dereli et al, 2010;Krupp et al, 2005;Lübken et al, 2007;Park et al, 2011). Mata-Alvarez et al (2014) assume that this is likely due to the low interest of the industrial sector to publish their results in the scientific literature.…”