Kinetic study of biogas production from anaerobic digestion of vinasse waste

Rapid economic development has resulted in increased energy demand in all sectors, including industry, agriculture, and transportation, resulting in the depletion of fossil fuel reserves and environmental concerns. Sustainable renewable energy has been studied and developed, including solar power, wind, tides, waves, hydroelectric generation, hydrogen and fuel cells, geothermal power, biomass, and biogas.

Biomass has been recommended as an important renewable resource for generating biogas for electric power and heat that could replace fossil fuels for economic expansion in developing countries (Siddique & Wahid 2018). Anaerobic digestion (AD) has been applied widely for biogas production in wastewater treatment. The advantages of AD, in which organic matter is converted to energy via microbial action (Scarlat et al. 2018), include an alternative renewable energy source, greenhouse gas emission reduction, and sustainability of organic waste management (Parsaee et al. 2019). Both solid waste and wastewater have the potential to produce biogas via AD. Typically, wastewater used in AD should contain high concentrations of COD, if the process is to be economic.

Vinasse, a by-product of sugarcane processing, has high COD content, sometimes exceeding 100,000 mg/L (Seluy & Isla 2014). It cannot be discharged directly but it is time-consuming for aerobic digestion. Its treatment by an anaerobic process is much more efficient and cost-effective (Budiyono et al. 2013). Much research has been performed to enhance biogas production efficiency in AD; for example, by improving substrate composition (Liu et al. 2019), reaction temperature (Membere & Sallis 2018), pre-treatment of high-acidity waste, which affects the efficiency of microbial methane production (Widyarani et al. 2018), and improving bacterial nutrition (Bryant et al. 1971). It has been reported that:

• mesophilic temperature (35 °C) generated the highest biogas yield (Xu et al. 2018);

• anaerobic biofuel production could be speeded using a biocatalyst (Senko et al. 2019); and that,

• the presence of methanogenic archaea as a biocatalyst improved methane production from swine faeces (Zhu et al. 2011; Ziganshin et al. 2016).

Many studies of biocatalysts in anaerobic biofuel production have been reported. The presence of methanogenic archaea improved methane production from swine faeces and the anaerobic digestion of waste materials (Zhu et al. 2011). Involving microorganisms like crenarchaeota and euryarchaeota in the co-digestion of pig manure and maize straw has been shown to improve methane yield (Kong et al. 2019). Crenarchaeota and euryarchaeota are amongst the natural bacteria in Citadel BioCat + , which is used widely for AD systems in Europe. It has not been studied in tropical waste or conditions, however.

Several researchers have developed kinetic models of biogas production rates and organic biodegradation in anaerobic reactions (Shen & Zhu 2017). Tülay et al. (2019) applied a modified Gompertz equation to examine the effect of temperature on biogas production. Krzysztof & Monika (2015) used a kinetic model to study biogas production enhancement by adding enzymatic pre-treatment.

The aim of this research was enhancing biogas production using vinasse wastewater as the substrate and Citadel BioCat + as a methanogenic bacterial provider. The study was done batch-wise using online methane measurement. The parameters studied were biocatalyst concentration and temperature.

The vinasse wastewater, used as the biogas feedstock, came from an ethanol plant in Chacheongsao, Thailand. The microbial sludge used as the inoculum was collected from an anaerobic, covered lagoon in the same plant. The characteristic of the vinasse and microbial sludge are presented in Table 1.

The experiment was conducted in a 600 mL anaerobic reactor equipped with automatic methane potential (AMPTS II, bioprocess control, Sweden) as shown in Figure 1. 120 mL of vinasse wastewater, as substrate (S), and 270 mL of sludge, as inoculum (I), were added batchwise to the reactor with an approximate I:S ratio of 1.5 to 2.0, by weight (Budiyono et al. 2010; Budiyono et al. 2013). Some inoculum was added with the substrate to provide the microorganisms needed to start the reaction. A commercial biocatalyst (Citadel BioCat+, Citadel Environmental Solutions, UK), was added subsequently as a methanogenic bacteria provider. The effects of biocatalyst content (5 and 10 g) and reaction temparature (30 and 37 °C) on biogas production were investigated. Biogas production was measured daily for 14 days. The experimental AD design is given in Table 2.

Cumulative and daily biogas production are shown in Figure 2(a) and 2(b), respectively. The tests were carried out at two reaction temperatures, 30 and 37 °C, which are normal in the tropics. Increasing the temperature from 30 to 37 °C resulted in an increase in the cumulative volume of biogas produced from 44.06 to 74.02 mL/g-COD – see Figure 2(a). Daily biogas production was highest, in the absence of biocatalyst, on the trial's 8th day at 37 °C – Figure 2(b). Biogas production increased from the start up to the 10th day due to the exponential growth of the microorganism population, after which the production rate was on a plateau. In the 30 °C trial, on the other hand, methane production increased gradually from the start, the increase continuing throughout the study period. This demonstrated that the consumption rate of the carbon-containing substrate at this temperature was low, and it remained available to convert to biogas in AD (Budiyono et al. 2010). COD removal during the 30 °C trial was 27.9% and at 37 °C about 38.8%.

The influence of the amount of the biocatalyst on AD was clear from the trials as its addition enhanced biogas production at both reaction temperatures. As indicated in Table 3, COD removal performance improved greatly to give residual concentrations of 124,000, 75,000 and 72,000 mg-COD/L in the 37 °C trials, for 0, 5 and 10 g of biocatalyst, respectively, from the initial 202,460 mg/L concentration – that is, 38.8 and 64.4% COD removal in the 0 and 10 g biocatalyst trials, respectively. Although there was no significant difference in COD removal between using 5 and 10 g biocatalyst doses, the dosage did influence biogas production.

The results show that the reaction temperature and presence of biocatalyst are significant in biogas production. Control conditions, without biocatalyst, produced the lowest values of A, 59.66 and 94.14 mL/g-COD for 30 and 37 °C, respectively – that is, little biogas was generated in these conditions. Increasing the amount of used biocatalyst raised the value of A at 30 °C, but using 10 g instead of 5 g increased biogas generation only slightly at 37 °C, increasing A from 100.71 to 102.51.

The calculated value of the constant U – Table 4 – shows that digestion at 37 °C generated more methane than at 30 °C. When operating with 10 g biocatalyst, the values of U were 5.49 and 11.17 mL/g-COD/d at the 30 and 37 °C operating temperatures, respectively. AD biogas production increased until day 7 in the presence of biocatalyst and until day 8 without it, at 37 °C – see Figure 2(b). With the same amount of substrate, biogas generation continued throughout the 14-day study period both with 5 g of biocatalyst and in its absence at 30 °C. The AD could be terminated within 12 days, however, when 10 g of biocatalyst was used.

The value of λ was lower for the 37 °C operating conditions than for 30 °C. This indicates that the bacteria required less time to adapt and produce biogas, and that the temperature influenced the methanogenic bacteria's growth and metabolic activity significantly (Budiyono et al. 2010; Wang et al. 2019). This marked reduction in the time needed for biogas production has economic advantages.

It is clear from the results presented in Table 5 that a biocatalyst can reduce the time required for biogas generation at 37 °C. In this study, biogas production from vinasse wastewater in the presence of a biocatalyst resulted in A = 101.16 mL/g-COD, U = 11.59 mL/g-COD/d, and λ = 0.664 day. The biogas production calculated from the Gompertz model is validated against the experimental results in Figure 3 and the experimental results agreed well with the proposed kinetic model.

The values of demonstrating the biodegradation rate of vinasse were determined as shown in Table 6. The higher the value of k, the faster the biodegradation rate and, as can be seen, the condition comprising 10 g of biocatalysts at 37 °C yielded the highest value. In other words, the organic material in the vinasse wastewater was degraded faster than under other conditions.

Biogas production kinetics were investigated by conducting tests in a batch anaerobic reactor using vinasse wastewater as substrate. A biocatalyst – Citadel BioCat+ – and the operating temperature both affected the rate constant in the biogas production kinetic model. The biocatalyst affects cumulative biogas production significantly and a higher operating temperature (37 °C) increases biogas production, both with and without biocatalyst. Use of 10 g of the biocatalyst at 37 °C gave the highest biogas production potential (A) and maximum biogas production rate (U), as 102.51 mL/g-COD and 11.17 mL/g-COD/d, respectively. In the biodegradation kinetic model, 10 g of biocatalyst at 37 °C had the highest biodegradation rate constant, = 0.2836 day⁻¹. The biogas production and biodegradation rates were enhanced in this study compared to previous work.

This research was supported financially by Research and Technology Transfer Affairs, the Research Center for Environmental and Hazardous Substance Management (EHSM), Khon Kaen University and Center of Excellence on Hazardous Substance Management (HSM), Chulalongkorn University, Bangkok, Thailand. The authors would also like to express their attitude to Aqua Nishihara Corporation limited company, Thailand, for the instrument, laboratory and microbial enzyme products.

All relevant data are included in the paper or its Supplementary Information.