Characteristics of tofu whey degradation during self-sustaining batch anaerobic process for methane production
Graphical Abstract
Introduction
Tofu is a very popular process food from soybean in the east and south-east Asian countries. The consumption of tofu is on the increase, not only in Asia but also in Europe and America [1]. As bean curd, tofu is a coagulation product from protein in soymilk. Tofu whey as wastewater is a byproduct generated during the coagulation and tofu pressing processes. Approximately 9 kg of whey can be generated from 1 kg of soybean during tofu production [2], [3]; in other terms, 25 m3 whey can be produced from 1400 to 1600 kg of soybean [4]. Commonly, this high organic content wastewater is directly discharged into the environment (esp. river) which causes nasty odor and severe pollutions [3], [5], [6] to the surface and groundwater [3], [7], [8]. All of them have a serious impact on greenhouse gas emissions.
With the increase in tofu factories in many countries, the treatment of tofu whey becomes an important discussion point due to the pollutions caused by the wastewater. The idea of converting wastewater into biogas through anaerobic digestion is an appealing solution for overcoming this problem and producing valuable energy at the same time. Several research studies have been conducted to study the biogas production from tofu whey [9], [10], [11]. Although several continuous biogas production applications were found [12], [13], [14], batch process studies are still required. Since tofu whey is a protein-rich substrate that commonly needs acclimation [15], batch processing ensures the continuous process success of a start-up operation.
A protein-rich substrate with limited nitrogen concentration becomes an attractive raw material since the biogas process does not require added nutrients or neutralizing compounds. Besides carbon, the protein-rich substrate also contains nitrogen [16]. Both carbon and nitrogen are the main nutrient required by anaerobic microorganisms. Such a substrate also provides buffering capacity [17], due to the formation of ammonia during degradation. Buffering capacity is a term commonly used in anaerobic digestion discussions [16], [17], [18] to illustrate the VFA neutralization effect by ammonia generation. Since in protein degradation case, the ammonia which is generated neutralizes the VFA acidification and increases the pH, the term “VFA neutralization effect” is used in this study.
Biogas production from various substrates commonly requires added neutralizing compounds to control the pH level [19], [20], [21]. Several biogas productions have been successfully carried out without added neutralizing compounds, such as studies [22] using cattle slurry and maize silage mixture as substrate and [23] using a blend of vegetable waste. The studies of [24], [25], [26] for co-digestion of municipal solid waste and sewage sludge, co-digestion of waste activated sludge and Egeria densa, and single-digestion of waste activated sludge, were conducted without added nutrients. The experiments were done by Kafle et al. (2014) [27], using Chinese cabbage waste as the substrate; Koch et al. (2017) [28], using sewage sludge, maize, and food waste; and Koch et al. (2019) [29], using sewage sludge; were conducted either without added nutrients or neutralizing compounds.
In tropical areas, especially tropical rainforest climate, the year-round ambient temperatures create a mesophilic condition. Relatively stable warm temperatures can be maintained between around 24 °C and 27 °C throughout the year [30]. Mesophiles grow at moderate temperatures between 20 °C and 45 °C [31]. Therefore, biogas processes can be carried out without needing to control the temperature. An anaerobic process that needs only the input substrate without any physicochemical addition or intervention during the process will be called self-sustaining. Self-sustaining biogas production may become an appealing reason for converting tofu whey, a protein-rich substrate, into a valuable energy source. Since the process requires neither chemical additives nor temperature control, thus the operational costs could be reduced.
Such an application of biogas technology supports the 6th, 7th, and 13th of the UN’s 17 Sustainable Development Goals (SDGs), i.e., ensures availability and sustainable management of water and sanitation, access to affordable and clean energy, and combat climate change and its impact, respectively. Tofu whey can make water pollution if discharged directly into the environment. Anaerobic digestion can reduce BOD and COD, as well as raise the pH level. The effluent from the process can then be used for irrigation or microalgal cultivation. Biogas technology is environment-friendly, both in terms of the process and the energy generated. This process represents a mutual symbiosis between humans and microorganisms. We obtain the bioenergy freely from nature for sustainable consumption, while the microorganisms obtain freely available food for their survival. Self-sustaining biogas production from tofu whey reduces the global warming potential by capturing methane, originally emitted during this waste disposal. It also produces a clean alternative energy source since biogas combustion produces more H2O than CO2, compared to fossil fuel [32]. The application of this technology to the tofu industry provides a clean and affordable energy source for the local community.
The effect of pH on biogas production from tofu whey was studied by Syaichurrozi et al. [33] and Widyarani et al. [34], however, they obtained differing results. The study done by Syaichurrozi et al. showed the highest biogas production at a base substrate initial pH of 8 [33]. In contrast, Widyarani et al. showed the highest production at the acid tofu whey initial pH of 3.7, as the initial pH of tofu whey [34]. Syaichurrozi et al. [33] observed that tofu whey did not provide a VFA neutralization effect since the pH of digested whey decreased continually due to VFA accumulation produced from carbohydrate degradation. By the end of the process, pH levels could not reach 7 for all the initial pH variances. In contrast, the study done by Widyarani et al. [34] showed the VFA neutralization effect of tofu whey, which could increase pH level after the digestion process to more than 7 in the digester processing tofu whey without pH adjustment. Widyarani et al. [34] stated that ammonia as a product of protein hydrolysis contributes to process stability by creating a buffer. As a protein-rich substrate with more than 50% of organic compounds in protein [35], tofu whey should have a VFA neutralization effect. Therefore, an investigation is required to verify that the biogas production from tofu whey can be carried out in a self-sustaining manner.
To address the existing gaps in the body of literature, this research work verified the VFA neutralization effect of tofu whey by ammonia generation. The VFA neutralization effect and its effect on methane production during a self-sustaining batch anaerobic process were observed by comparing the digesters' performance, using tofu whey as a substrate, to the positive control digesters, which used a substrate without nitrogen content. Glucose was selected as the positive control to suit the characteristic of tofu whey which contains many simple organic compounds (more than 80% of the COD in the tofu whey is soluble [35]). According to Angelidaki et al. [36], glucose is a good model substrate for the acidogenic population, which utilizes the simple organic to become VFA.
Section snippets
Experimental setup
A batch experiment was carried out in a cylindrical reactor with 12.20 cm diameter and 17.36 cm height so that the total volume was 2.03 L. The reactor was equipped with a pressure gauge, biogas sampling pipe, and a liquid sampling pipe. The biogas sampling pipe was plugged with a rubber septum to prevent biogas from escaping from the digester. The working volume was designed at 1.20 L to ensure sufficient gas space, preventing overpressure that can cause digester leakage. The scheme of the
Results and discussions
The degradation characteristic of tofu whey compared to glucose during the self-sustaining process was observed by evaluating the trends of process parameters, i.e., VS and VFA concentrations, pH, specific methane production, and methane content. TAN and total alkalinity concentrations were used to confirm the process parameters. The biomethane potential was estimated using a modified Gompert model. The potential effects on energy saving and greenhouse gas mitigation were also evaluated.
Conclusions
Self-sustaining biogas production from tofu whey was successfully carried out in a laboratory batch experiment without any indication of process failure. Tofu whey provided a VFA neutralization effect and the ability to maintain pH levels between 6.8 and 8.4 during the process. The specific methane production obtained was 284.6 N mL/g VS, with a methane content of 69.2%; and requiring 23 days of processing time. The modified Gompert model could reasonably estimate the specific methane
Funding
This study was supported by “Beasiswa SDM Iptek (Keputusan Dirjen SDM Iptek No 2428.1, Tahun 2017)” and “Program Pendukung Beasiswa Saintek” of the Ministry of Research and Technology/National Research and Innovation Agency, of the Republic of Indonesia.
CRediT authorship contribution statement
Arini Wresta: Methodology, Data curation, Investigation, Formal analysis, Writing – original draft. Tonni Agustiono Kurniawan: Writing – review & editing. Sanggono Adisasmito: Conceptualization, Supervision. Neni Sintawardani: Conceptualization, Writing – review & editing, Supervision. Tjandra Setiadi: Conceptualization, Writing – review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that there are no potential conflicts of interest.
Acknowledgements
The authors would like to thank Dr. Antonius Indarto, Dr. Khoiruddin, and Dian Andriani, M. Eng. for all their help writing this article. This study was facilitated by the Doctoral Program of the Chemical Engineering Department of Bandung Institute of Technology. This study was also supported by Sekolah Pascasarjana ITB through “Program Peningkatan Kualitas Penulisan Jurnal Internasional bagi Mahasiswa Program Pascasarjana”.
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