Existing Empirical Kinetic Models in Biochemical Methane Potential (BMP) Testing, Their Selection and Numerical Solution

Assessment of biochemical methane potential of dairy wastewater with different co-substrates and evaluation of different kinetic models

Dairy industry wastewater can be considered as an important source of pollution due to its high amounts and pollutant concentrations. Anaerobic treatment is seen as a suitable alternative over aerobic treatment which requires huge aeration systems. Biochemical methane potential (BMP) testing is a widely applied technique for estimating the performance of anaerobic digesters and still has no clear alternative. In the study, the biochemical methane potential change was investigated by mixing dairy wastewater with different co-substrates (cattle manure, chicken manure and slaughterhouse wastewater) at different rates. The highest biogas potential per gram of chemical oxygen demand added (CODadded) was determined as 574 mLbiogas in a mixture of 74% dairy wastewater + 2% chicken manure + 24% slaughterhouse wastewater inoculated with granular sludge. The highest methane potential was determined as 340 mLCH4 in the same co-substrate mixture inoculated with anaerobic sludge. In recent years, mathematical modeling offers an alternative to BMP tests and many different models are used for this purpose. In the study, six different mathematical models were used to simulate the BMP results, and the highest correlation coefficient in almost all mixtures ranged from 0.900 to 0.997 with the Modified Gompertz equation and Fitzhugh models.

Influence of Pre-Incubation of Inoculum with Biochar on Anaerobic Digestion Performance

The application of biochar as an additive to enhance the anaerobic digestion (AD) of biomass has been extensively studied from various perspectives. This study reported, for the first time, the influence of biochar incubation in the inoculum on the anaerobic fermentation of glucose in a batch-type reactor over 20 days. Three groups of inoculum with the same characteristics were pre-mixed once with biochar for different durations: 21 days (D21), 10 days (D10), and 0 days (D0). The BC was mixed in the inoculum at a concentration of 8.0 g/L. The proportion of the inoculum and substrate was adjusted to an inoculum-to-substrate ratio of 2.0 based on the volatile solids. The results of the experiment revealed that D21 had the highest cumulative methane yield, of 348.98 mL, compared to 322.66, 290.05, and 25.15 mL obtained from D10, D0, and the control, respectively. Three models-modified Gompertz, first-order, and Autoregressive Integrated Moving Average (ARIMA)-were used to interpret the biomethane production. All models showed promising fitting of the cumulative biomethane production, as indicated by high R2 and low RMSE values. Among these models, the ARIMA model exhibited the closest fit to the actual data. The biomethane production rate, derived from the modified Gompertz Model, increased as the incubation period increased, with D21 yielding the highest rate of 31.13 mL/gVS. This study suggests that the application of biochar in the anaerobic fermentation of glucose, particularly considering the short incubation period, holds significant potential for improving the overall performance of anaerobic digestion.

Anaerobic digestibility of aerobic granular sludge from continuous flow reactors: the role of granule size distribution

There is an increasing interest in integrating aerobic granular sludge (AGS) technology into wastewater industries. Several projects are being performed to cultivate the aerobic granules for continuous flow reactors (AGS-CFR), while there is a scarcity of those projects that investigate the bio-energy recovery from AGS-CFR. This research was designed to examine the digestibility of AGS-CFR. Beyond that, it aimed at defining the role of the granule size on their digestibility. For this purpose, a series of bio-methane potential (BMP) tests have been run at mesophilic conditions. The results showed that AGS-CFR has a lower methane potential (107.43 ± 4.30 NmL/g VS) compared to activated sludge. This may be the result of the high sludge age (30 days) of AGS-CFR. Additionally, the results revealed that the average size of granules is among the main factors that reduce their digestibility, but it does not inhibit it. It was noticed that granules of size >250 μm have a significantly lower methane yield than the smaller ones. Kinetically, it was noticed that the kinetic models with two hydrolysis rates fit well with the methane curve of AGS-CFR. Overall, this work showed that the average size of AGS-CFR characterizes its biodegradability, which in turn defines its methane yield.

Investigations into valorisation of trade wastewater for biomethane production

Biogas production from wastewater is one way that industrial sites can work towards the UN Sustainable Development Goals, while recovering a valuable resource. The objective of this study was to investigate the suitability of data collected by municipal wastewater service providers as a method of classifying and screening waste producers as potential sites for biogas resource recovery by anaerobic digestion. Industrial wastewater samples, including raw effluent and treated waste ready for discharge, were examined, and biomethane potential assays performed. Results of chemical analysis and lab-scale digestion were compared to historical service provider data, and patterns were observed. Biomethane yields of up to 357 mL/gVS and 287mL/gVS were achieved from raw and treated effluent respectively. Digestion at the top four prospects could produce over 4690 GJ of methane and save $47,000 in natural gas costs, offsetting 490 tonnes of CO₂ equivalent annually. These streams, from logistics, waste management, food and animal product businesses, combined high levels of degradable substrates and low levels of inhibitory components. While it is unlikely that this type of screening program can be completely accurate, certain parameters, including high sodium concentration, are applicable for discounting the potential for biogas production. This knowledge can be a valuable tool in the process of selecting sites for future resource recovery, therefore increasing the uptake of these processes, resulting in economic, environmental, and climate change mitigation benefits.

The advantages of co-digestion of vegetable oil industry by-products and sewage sludge: Biogas production potential, kinetic analysis and digestate valorisation

The production of edible vegetable oils generates considerable amounts of energy-rich waste, which is usually not utilised fully. Besides, inefficient management of such wastes can have a negative impact on the environment. On the other hand, this waste can also serve as a raw material for the production of high value-added products, such is biogas. The mono-digestion of seven different by-products and wastes from the vegetable oil industry was investigated in this study: Pumpkin seeds press cake (PSPC), grape seeds press cake (GSPC), olive mill pomace (OMP), coconut oil cake (CC), filtration additive (FA), spent bleaching earth (SBE) and sludge from a vegetable oil industry (SOI) wastewater treatment plant. In addition, co-digestion of these substrates was performed with municipal sewage sludge (SS). Besides inoculum, rumen fluid was added to the reactors to enhance biogas production. The biogas production potential of the tested substrates was monitored by measuring various parameters. A kinetic analysis was later carried out and a growth test was performed on the digestates to evaluate their potential for agricultural use. The highest biogas yields in the mono-digestion test were obtained with the substrates with the highest fat content: 1402, 1288, 830 and 750 mL of biogas/gVS for SOI, FA, PSPC and CC substrate, respectively. Co-digestion of SS with by-products of vegetable oil industry such as FA, SBE, CC, SOI and PSPC increased the biogas yields by 94.9%, 74.1%, 30.8%, 27.4% and 23.6% compared to SS mono-digestion. Furthermore, the data for mono-digestion of PSPC, GSPC, and FA, and co-digestion of SS with these substrates, CC and SBE, have not been found in the literature to date. The maximum methane content ranged from 61 to 74 vol%, while the chemical oxygen demand removal efficiency ranged from 42 to 78%. Relatively high fatty acids contents and ammonium concentrations were measured in the reactors. Kinetic analysis showed the best fit to the experimental data for the Cone kinetic model (R² > 0.98). The First order kinetic model, Monod, and the modified Gompertz model also exhibited high R² values. The digestates obtained from co-digestion proved to be excellent in the cress seeds growth test at digestate concentrations of 5-10 wt%, while higher concentrations had a toxic effect.

Anaerobic biogas formation from crops' agricultural residues - Modeling investigations

The cultivation of field crops necessarily produces large quantities of organic residues, which could be used for the production of biogas. However, this is only successfully possible if the operators of such biogas plants have sufficient expertise in running these plants, e.g. to avoid an overload of the biogas system. Against this background, the anaerobic degradability of various Egyptian agricultural residues is determined by laboratory testing; this includes both a determination of degradation behavior in batch tests and a feed analysis. The residues studied produce biogas yields ranging from 303 to 496 mL_N g_VS^-1. Co-digestion experiments demonstrate that hardly any interaction effects occur during the fermentation process for different mixtures of the investigated residues. Based on these findings, a model is developed to estimate biogas production using the investigated agricultural residues in continuous operation and to give recommendations for the optimal mode of operation.

Biochemical characterization and anaerobic degradability of flower wastes: Preliminary assessment and statistical interpretation towards energy recovery

The use of ornamental flowers and plants is widespread in several regions of the world, but the management of flower (or floral) waste (FW), classified as herbaceous biomasses, is scarcely addressed in the literature. However, climate change, population growth and the depletion of resources are expected to push towards the development of FW management strategies, according to principles of flexibility and integration of technologies. This study focuses on the characterization of ten different varieties of flowering plants, of which the wastes are of concern in the Pistoia Province (Italy). The possibility of recovering energy by means of anaerobic digestion is also preliminarily investigated. The interpretation of data through Principal Component Analyses proved to be effective to orientate the selection of technological solutions. The three main parts of each plant variety were analysed separately, showing that the biochemical composition of stems is statistically different from that of leaves and flowers, thus suggesting the viability of adopting different strategies to optimize material (value-added products) recovery from FW. Conversely, regarding biogas generation and energy recovery, the methane yield (in the range 82-330 NmLCH₄^.gVS^-1) is not significantly affected by the type of FW part, nor by the variety of flowering plant or by the use of pesticides during cultivation, whereas lower kinetics were observed for stems compared to leaves and flowers. In view of full-scale application, and depending on FW amounts locally produced, a careful evaluation is required, encompassing aspects of technical feasibility and economic expenses associated with FW parts separation.

Enhanced Production of Biogas Using Biochar-Sulfur Composite in the Methane Fermentation Process

The methane fermentation of organic waste is one way to minimize organic waste, which accounts for 77% of the global municipal waste stream. The use of biochar as an additive for methane fermentation has been shown to increase the production potential of biogas. Sulfur waste has a potential application to synergistic recycling in a form of composites with other materials including biochar. A composite product in the form of a mixture of biochar and molten sulfur has been proposed. In this experiment, additions of the sulfur−biochar composite (SBC) were tested to improve the fermentation process. The biochar was produced from apple chips under the temperature of 500 °C. The ground biochar and sulfur (<1 mm particle size) were mixed in the proportion of 40% biochar and 60% sulfur and heated to 140 °C for sulfur melting. After cooling, the solidified composite was ground. The SBC was added in the dose rate of 10% by dry mass of prepared artificial kitchen waste. Wet anaerobic digestion was carried out in the batch reactors under a temperature of 37 °C for 21 days. As an inoculum, the digestate from Bio-Wat Sp. z. o. o., Świdnica, Poland, was used. The results showed that released biogas reached 672 mL × gvs−1, and the yield was 4% higher than in the variant without the SBC. Kinetics study indicated that the biogas production constant rate reached 0.214 d−1 and was 4.4% higher than in the variant without the SBC.

Estimating the Methane Potential of Energy Crops: An Overview on Types of Data Sources and Their Limitations

As the anaerobic digestion of energy crops and crop residues becomes more widely applied for bioenergy production, planners and operators of biogas plants, and farmers who consider growing such crops, have a need for information on potential biogas and methane yields. A rich body of literature reports methane yields for a variety of such materials. These data have been obtained with different testing methods. This work elaborates an overview on the types of data source available and the methods that are commonly applied to determine the methane yield of an agricultural biomass, with a focus on European crops. Limitations regarding the transferability and generalisation of data are explored, and crop methane values presented across the literature are compared. Large variations were found for reported values, which can only partially be explained by the methods applied. Most notably, the intra-crop variation of methane yield (reported values for a single crop type) was higher than the inter-crop variation (variation between different crops). The pronounced differences in reported methane yields indicate that relying on results from individual assays of candidate materials is a high-risk approach for planning biogas operations, and the ranges of values such as those presented here are essential to provide a robust basis for estimation.