Localized mixing of anaerobic plug flow reactors.

An innovative localized-mixing concept was tested in an Anaerobic Plug Flow Reactor (AnPFR) treating Food Waste (FW) mixed with municipal Wastewater (WW). The proposed concept consists of placing propellers along the shaft of the AnPFR at key points that represent the mid-region of each of the anaerobic digestion stages: hydrolysis, acidogenesis, and methanogenesis. First, the need for and efficiency of localized mixing (the new concept suggested by the authors) were investigated. While the main benefit of localized mixing is the reduction of energy demand associated with (conventional) uniform mixing (i.e., throughout the longitudinal axis), the system can also benefit from synergetic reactions in non-mixed zones. In fact, at a Total Solid (TS) content of 15% (Organic Loading Rate (OLR) of 4.2 g VS.L - 1.d - 1) and a Hydraulic Retention Time (HRT) of 28 days, the mixing pattern was sufficient to maintain stable operation, with high removal rates (up to 96% of solids) and high biogas generation (1128 ± 55 ml.g VSfed-1, of which 68.9% consisted of CH4); but when mixing was halted, the system's performance deteriorated. Second, the loading capacity of the locally-mixed AnPFR was investigated by subjecting it to different TS content (10%, 15%, 20%, and 22.5%, corresponding to OLRs of 2.8, 4.2, 6.3, and 7.9 g VS.L - 1.d - 1, respectively) while operating under the same HRT. It was found that the system can adequately sustain a feed with a maximum TS of 20% while achieving removal rates up to 92% for solids and a CH4 yield of 613 ml.g VSfed-1. The digester was simulated using computational fluid dynamics. The outputs revealed: (1) highest radial mixing at the center of the methanogenesis zone where the propeller is located and (2) low longitudinal mixing before and after the propeller of the methanogenesis stage, implying the presence of sedimentation zones that was visually verified. The former is assumed to favor better dispersion of inhibitors and improved stability, while the latter is expected to provide stagnant areas for enhanced biochemical synergies.

Anaerobic-aerobic sequential treatment: Temperature optimization and cost implications.

Traditionally, aeration units, used as a polishing stage after anaerobic digestion (AD) of wastes, are operated at ambient temperature. Yet, when effluent quality is the main design criterion, raising the temperature of the aeration stage can be justified by improved removal efficiencies. In this study, an anaerobic-aerobic sequential system (AASS) was operated to co-digest raw wastewater and food waste. The aerobic compartment was tested under psychrophilic and mesophilic temperatures. At the design loading rate of 2 gVS L-1 d-1, the anaerobic digester achieved removal efficiencies of 85 ± 2% of volatile solids (VS), 84 ± 3% of total chemical oxygen demand (CODT) and a biogas yield of 1,035 ± 30 mL gVSfed-1 (50% methane). The aerobic reactor achieved additional removal of 8% CODT and 7 % VS. By raising the temperature of the aerobic reactor to the mesophilic range, COD and solids concentrations of the effluent dropped to approximately half their values. This was accompanied by an increase in nitrification (from 68% to 91%) and denitrification (from 10% to 16%). The energy analysis showed that total energy consumption slightly increases (from 0.45 to 0.49 kWh kgCODfed-1) by raising the temperature of the aerobic reactor to mesophilic range. A preliminary evaluation of the sludge disposal cost, revealed a saving increase of 5-6% under mesophilic operation with respect to psychrophilic conditions. Implications: In order to cope with the globally increasing constraints on the disposal of urban wastes, efficient post-processing of effluents becomes a crucial requirement for the anaerobic digestion industry. In this context, the submitted manuscript shows that the quality of the effluent, of an anaerobic digester, treating food waste with raw wastewater, can be substantially improved by optimizing the aerobic polishing stage. Raising the temperature of the aerobic reactor to the mesophilic range resulted in a drop of solids and COD concentrations to approximately half their values. Equally important, the implications on operational costs were found to be favorable, compared to traditional psychrophilic aerobic post-treatment, when taking into consideration indirect sludge treatment costs and energy selling revenues.

Anaerobic digestion of food waste with aerobic post-treatment: Effect of fruit and vegetable content.

A mesophilic anaerobic digester, followed by a psychrophilic aerobic post-treatment, was used to treat food waste (FW) with different proportions of fruit and vegetable waste (FVW). Two types of FW were used: low fruit and vegetable mix (LFV, with 56.5% of FVW) and high fruit and vegetable mix (HFV, with 78.3% of FVW). The anaerobic digester fed with LFV failed at an organic loading rate of 1.6 g VS.L-1.d-1 (volatile fatty acid (VFA) = 6000 mg.L-1) due to high ammonia (reaching 3000 mg.L-1). It was shown that, in an unstable anaerobic environment, ammonia is highly correlated ( r2 = 0.77) with VFA and is negatively correlated with volatile solids, total solids, and chemical oxygen demand (COD) removal rates ( r2 = 0.88, r2 = 0.71, and r2 = 0.91, respectively). In contrast, the anaerobic digester fed with HFV exhibited a stable performance (VFA = 1243 mg.L-1), with limited ammonia accumulation (940 mg.L-1). Methane generation was affected by the FVW content and reached 531 ml CH4.g VS-1 (CH4 = 52%) with LFV feed and 478 ml CH4.g VS-1 (CH4 = 57.4%) with HFV. The overall TS, VS and COD removal rates (all ranging between 94% and 97%), were closely similar for LFV and HFV. Accordingly, the aerobic post-treatment seems to compensate for the reduced performance of the disturbed anaerobic system fed with LFV.

Correlation between system performance and bacterial composition under varied mixing intensity in thermophilic anaerobic digestion of food waste.

This study examines the stability and efficiency of thermophilic anaerobic digesters treating food waste under various mixing velocities (50-160 rpm). The results showed that high velocities (120 and 160 rpm) were harmful to the digestion process with 18-30% reduction in methane generation and 1.8 to 3.8 times increase in volatile fatty acids (VFA) concentrations, compared to mild mixing (50 and 80 rpm). Also, the removal rate of soluble COD dropped from 75 to 85% (at 50-80 rpm) to 20-59% (at 120-160 rpm). Similarly, interrupted mixing caused adverse impacts and led to near-failure conditions with excessive VFA accumulation (15.6 g l-1), negative removal rate of soluble COD and low methane generation (132 ml gVS-1). The best efficiency and stability were achieved under mild mixing (50 and 80 rpm). In particular, the 50 rpm stirring speed resulted in the highest methane generation (573 ml gVS-1). High-throughput sequencing of 16S rRNA genes revealed that the digesters were dominated by one bacterial genus (Petrotoga; phylym Thermotogae) at all mixing velocities except at 0 rpm, where the community was dominated by one bacterial genus (Anaerobaculum; phylum Synergistetes). The Petrotoga genus seems to have played a major role in the degradation of organic matter.

Performance optimization and validation of ADM1 simulations under anaerobic thermophilic conditions.

In this study, two experimental sets of data each involving two thermophilic anaerobic digesters treating food waste, were simulated using the Anaerobic Digestion Model No. 1 (ADM1). A sensitivity analysis was conducted, using both data sets of one digester, for parameter optimization based on five measured performance indicators: methane generation, pH, acetate, total COD, ammonia, and an equally weighted combination of the five indicators. The simulation results revealed that while optimization with respect to methane alone, a commonly adopted approach, succeeded in simulating methane experimental results, it predicted other intermediary outputs less accurately. On the other hand, the multi-objective optimization has the advantage of providing better results than methane optimization despite not capturing the intermediary output. The results from the parameter optimization were validated upon their independent application on the data sets of the second digester.

Population dynamics during startup of thermophilic anaerobic digesters: the mixing factor.

Two thermophilic digesters were inoculated with manure and started-up under mixed and stagnant conditions. The Archaea in the mixed digester (A) were dominated by hydrogenotrophic Methanobateriaceae (61%) with most of the methane being produced via syntrophic pathways. Methanosarcinales (35%) were the only acetoclastic methanogens present. Acetate dissipation seems to depend on balanced hydrogenotrophic-to-acetotrophic abundance, which in turn was statistically correlated to free ammonia levels. Relative abundance of bacterial community was associated with the loading rate. However, in the absence of mixing (digester B), the relationship between microbial composition and operating parameters was not discernible. This was attributed to the development of microenvironments where environmental conditions are significantly different from average measured parameters. The impact of microenvironments was accentuated by the use of a non-acclimated seed that lacks adequate propionate degraders. Failure to disperse the accumulated propionate, and other organics, created high concentration niches where competitive and inhibiting conditions developed and favored undesired genera, such as Halobacteria (65% in B). As a result, digester B experienced higher acid levels and lower allowable loading rate. Mixing was found necessary to dissipate potential inhibitors, and improve stability and loading capacity, particularly when a non-acclimated seed, often lacking balanced thermophilic microflora, is used.

Improving the stability of thermophilic anaerobic digesters treating SS-OFMSW through enrichment with compost and leachate seeds.

This paper examines the potential of improving the stability of thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste (SS-OFMSW) by adding leachate and compost during inoculation. For this purpose, two stable thermophilic digesters, A (control) and B (with added leachate and compost), were subjected to a sustained substrate shock by doubling the organic loading rate for one week. Feeding was suspended then gradually resumed to reach the pre-shock loading rate (2 gVS/l/d). Digester A failed, exhibiting excessive increase in acetate and a corresponding decrease in pH and methane generation, and lower COD and solids removal efficiencies. In contrast, digester B was able to restore its functionality with 90% recovery of pre-shock methane generation rate at stable pH, lower hydrogen levels, and reduced VFAs and ammonia accumulation.

Climate change and temperature rise: implications on food- and water-borne diseases.

This study attempts to quantify climate-induced increases in morbidity rates associated with food- and water-borne illnesses in the context of an urban coastal city, taking Beirut-Lebanon as a study area. A Poisson generalized linear model was developed to assess the impacts of temperature on the morbidity rate. The model was used with four climatic scenarios to simulate a broad spectrum of driving forces and potential social, economic and technologic evolutions. The correlation established in this study exhibits a decrease in the number of illnesses with increasing temperature until reaching a threshold of 19.2 °C, beyond which the number of morbidity cases increases with temperature. By 2050, the results show a substantial increase in food- and water-borne related morbidity of 16 to 28% that can reach up to 42% by the end of the century under A1FI (fossil fuel intensive development) or can be reversed to ~0% under B1 (lowest emissions trajectory), highlighting the need for early mitigation and adaptation measures.

Mixing effect on thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste.

This paper examines the effect of mixing on the performance of thermophilic anaerobic digestion of source-sorted organic fraction of municipal solid waste during the start-up phase and in the absence of an acclimated seed. For this purpose, two digesters were used under similar starting conditions and operated for 235 days with different mixing schemes. While both digesters exhibited a successful startup with comparable specific methane yield of 0.327 and 0.314 l CH4/g VS, continuous slow stirring improved stability by reducing average VFA accumulation from 2890 to 825 mg HAc/l, propionate content from 2073 to 488 mg/l, and VFA-to-alkalinity ratio from 0.32 to 0.07. As a result, the startup with slow mixing was faster and smoother accomplishing a higher loading capacity of 2.5 g VS/l/d in comparison to 1.9 g VS/l/d for non-mixing. Mixing equally improved microbial abundance from 6.6 to 10 g VSS/l and enhanced solids and soluble COD removal.