Dynamic simulation of cyclic batch anaerobic digestion of cattle manure

Dynamic simulation and optimization of anaerobic digestion processes using MATLAB

Time series-based modeling provides a fundamental understanding of process fluctuations in an anaerobic digestion process. However, such models are scarce in literature. In this work, a dynamic model was developed based on modified Hill's model using MATLAB, which can predict biomethane production with time series. This model can predict the biomethane production for both batch and continuous process, across substrates and at diverse conditions such as total solids, loading rate, and days of operation. The deviation between literature and the developed model was less than ± 7.6%, which shows the accuracy and robustness of this model. Moreover, statistical analysis showed there was no significant difference between literature and simulation, verifying the null hypothesis. Finding a steady and optimized loading rate was necessary to an industrial perspective, which usually requires extensive experimental data. With the developed model, a stable and optimal methane yield generating loading rate could be identified at minimal input.

Characterising and modelling free ammonia and ammonium inhibition in anaerobic systems

Inhibition by ammoniacal nitrogen, consisting of free ammonia (NH₃) and ammonium ion (NH₄⁺), has been widely investigated for anaerobic digestion. However, despite the large amount of research on the subject, ammoniacal nitrogen inhibition still threatens many anaerobic digesters. This paper presents (i) a method to reliably characterise ammoniacal nitrogen inhibition and (ii) a robust inhibition modelling approach. Results showed that NH₃ and NH₄⁺ inhibition need to be jointly determined, which can only be done by performing inhibition tests at various total ammoniacal nitrogen (TAN) concentrations and pH values. These test conditions were reliably achieved using the salts NH₄HCO₃ and NH₄Cl without pH adjustment, rather than by using NH₄Cl with pH adjustment. The use of only salts showed a lower pH change during the inhibition test (∼1.5 days), thereby decreasing the uncertainty in TAN speciation and strengthening the test and model outputs. A threshold inhibition function satisfactorily described (R² > 0.99) the joint inhibition of NH₃ and NH₄⁺ on three distinct inocula, and provided a better description of the inhibition testing results than a non-competitive inhibition function (R² ∼0.70). The key advantage of the proposed threshold inhibition function is its capacity to identify the inhibition lower limit (concentration where inhibition starts; KI_min) and upper limit (concentration where inhibition is complete; KI_max). The threshold inhibition function also identifies the 50% inhibition concentration (KI₅₀) at the midpoint of KI_min and KI_max. Finally, experimental and model results show that at pH 7.3-7.7 and TAN concentrations above 2000 mgN·L^-1, both NH₃ and NH₄⁺ contribute significantly to overall inhibition.

Anaerobic batch conversion of pine wood torrefaction condensate

Organic compound rich torrefaction condensate, owing to their high water content and acidic nature, have yet to be exploited for practical application. In this study, microbial conversion of torrefaction condensate from pine wood through anaerobic batch digestion (AD) to produce methane was evaluated. Torrefaction condensate exhibited high methane potentials in the range of 430-492mL/g volatile solids (VS) and 430-460mL/gVS under mesophilic and thermophilic conditions, respectively. Owing to the changes in the composition, the methane yields differed with the torrefaction condensates produced at different temperatures (225, 275 and 300°C), with a maximum of 492±18mL/gVS with the condensate produced at 300°C under mesophilic condition. The cyclic batch AD experiments showed that 0.1VS_substrate:VS_inoculum is optimum, whereas the higher substrate loading (0.2-0.5) resulted in a reversible inhibition of the methane production. The results suggest that torrefaction condensate could be practically valorized through AD.

Influence of initial pH on thermophilic anaerobic co-digestion of swine manure and maize stalk

The contradictions between the increasing energy demand and decreasing fossil fuels are making the use of renewable energy the key to the sustainable development of energy in the future. Biogas, a renewable clean energy, can be obtained by the anaerobic fermentation of manure waste and agricultural straw. This study examined the initial pH value had obvious effect on methane production and the process in the thermophilic anaerobic co-digestion. Five different initial pH levels with three different manure ratios were tested. All digesters in different initial pH showed a diverse methane production after 35 days. The VFA/alkalinity ratio of the optimum reaction condition for methanogens activity was in the range of 0.1-0.3 and the optimal condition that at the 70% dung ratio and initial pH 6.81, was expected to achieve maximum total biogas production (146.32 mL/g VS).

Microbial kinetic for In-Storage-Psychrophilic Anaerobic Digestion (ISPAD)

In-Storage-Psychrophilic-Anaerobic-Digestion (ISPAD) is a wastewater storage tank converted into an anaerobic digestion (AD) system by means of an airtight floating geo-membrane. For process optimization, ISPAD requires modelling with well-established microbial kinetics coefficients. The present objectives were to: obtain kinetics coefficients for the modelling of ISPAD; compare the prediction of the conventional and decomposition fitting approach, an innovative fitting technique used in other fields of science, and; obtain equations to predict the maximum growth rate (μmax) of microbial communities as a function of temperature. The method consisted in conducting specific Substrate Activity Tests (SAT) using ISPAD inoculum to monitor the rate of degradation of specific substrates at 8, 18 and 35 °C. Microbial kinetics coefficients were obtained by fitting the Monod equations to SAT. The statistical procedure of Least Square Error analysis was used to minimize the Sum of Squared Errors (SSE) between the measured ISPAD experimental data and the Monod equation values. Comparing both fitting methods, the decomposition approach gave higher correlation coefficient (R) for most kinetics values, as compared to the conventional approach. Tested to predict μmax with temperature, the Square Root equation better predicted temperature dependency of both acidogens and propionate degrading acetogens, while the Arrhenius equation better predicted that of methanogens and butyrate degrading acetogens. Increasing temperature from 18 to 35 °C did not affect butyrate degrading acetogens, likely because of their dominance, as demonstrated by microbial population estimation. The estimated ISPAD kinetics coefficients suggest a robust psychrophilic and mesophilic coexisting microbial community demonstrating acclimation to ambient temperature.

Treatment of screened dairy manure by upflow anaerobic fixed bed reactors packed with waste tyre rubber and a combination of waste tyre rubber and zeolite: effect of the hydraulic retention time

Two laboratory-scale anaerobic fixed bed reactors were evaluated while treating dairy manure at upflow mode and semicontinuous feeding. One reactor was packed with a combination of waste tyre rubber and zeolite (R1) while the other had only waste tyre rubber as a microorganism immobilization support (R2). Effluent quality improved when the hydraulic retention time (HRT) increased from 1.0 to 5.5 days. Higher COD, BOD5, total and volatile solids removal efficiencies were always achieved in the reactor R1. No clogging was observed during the operation period. Methane yield was also a function of the HRT and of the type of support used, and was 12.5% and 40% higher in reactor R1 than in R2 for HRTs of 5.5 and 1.0 days, respectively. The results obtained demonstrated that this type of reactor is capable of operating with dairy manure at a HRT 5 times lower than that used in a conventional reactor.

Preparation of activated carbons from cattle-manure compost by zinc chloride activation

Dried cattle-manure compost was pyrolyzed by a one-step process to obtain activated carbon using chemical activation by zinc chloride. The influence of activation parameters such as ZnCl(2) to cattle-manure compost (ZnCl(2)/CMC) ratio, activation temperature and retention time on the final products was investigated. The resultant activated carbons were characterized by nitrogen adsorption-desorption isotherms at 77 K. The results showed that the surface area and pore volume of activated carbons, which were estimated by BET and t-plot methods, were achieved as high as 2170 m(2)/g and 1.70 cm(3)/g in their highest value, respectively. Thermogravimetric analysis (TGA) was carried out to monitor the pyrolysis process of cattle-manure compost (CMC) and ZnCl(2) impregnated one (ZnCl(2)/CMC). The capabilities of phenol adsorption were also examined for the CMC carbons prepared with various treatments.

Mathematical modeling of non-ideal mixing continuous flow reactors for anaerobic digestion of cattle manure

Most conventional digesters used for animal wastewater treatment include continuously stirred-tank reactors. While imperfect mixing patterns are more common than ideal ones in real reactors, anaerobic digestion models often assume complete mixing conditions. Therefore, their applicability appears to be limited. In this study, a mathematical model for anaerobic digestion of cattle manure was developed to describe the dynamic behavior of non-ideal mixing continuous flow reactors. The microbial kinetic model includes an enzymatic hydrolysis step and four microbial growth steps, together with the effects of substrate inhibition, pH and thermodynamic considerations. The biokinetic expressions were linked to a simple two-region liquid mixing model, which considered the reactor volume in two separate sections, the flow-through and the retention regions. Deviations from an ideal completely mixed regime were represented by changing the relative volume of the flow-through region (a) and the ratio of the internal exchange flow rate to the feed flow rate (b). The effects of the hydraulic retention time, the composition of feed, the initial conditions of the reactor and the degree of mixing on process performance can be evaluated by the dynamic model. The simulation results under different conditions showed that deviations from the ideal mixing regime decreased the methane yield and resulted in a reduced performance of the anaerobic reactors. The evaluation of the impact of the characteristic mixing parameters (a) and (b) on the anaerobic digestion of cattle manure showed that both liquid mixing parameters had significant effects on reactor performance.