Materials Today Chemistry
Effect of mixing ratios of natural inorganic additives in removing ammonia and sulfide in the liquid phase during anaerobic digestion of slaughterhouse waste
Graphical abstract
Introduction
Anaerobic digestion (AD) refers to the process in which substrates with a high organic content are treated in the absence of oxygen to produce biogas [1]. Generally, biogas is a mixture of carbon dioxide and methane (50%–70%) along with other compounds such as ammonia, hydrogen sulfide, and siloxane in trace amounts [2]. Since the world is turning to alternative sources of energy from commonly used fossil fuels, biogas is regarded among the alternative sources of energy for domestic uses such as cooking and lighting. Unlike fossil fuel sources, which endanger the environment and ecosystem at large due to the excessive production of polluting agents, the use of alternative sources of energy has gained much attention due to the little waste, which is accumulated in the environment. As a substitute for fossil fuels, the use of biogas is regarded as an environmentally friendly technology to eradicate the emission of greenhouse gases, hence reducing climate warming [3]. In practice, AD has been used to produce biogas from various organic wastes due to their availability and as a means of waste management [4]. Both domestic and industrial activities have been the major source of tons of organic waste discharged in the environment, for which the biogas technology can be considered as the best solution to overcome their detrimental effects on the environment [5]. Organic wastes such as domestic leftovers [6], cow dung [7], industrial waste [8], municipal solid waste [9], agro waste [10], and abattoir waste [11], to mention a few, are a good source to generate biogas due to their high carbon/nitrogen ratio. The higher C/N ensures the sustainability of microorganisms and a proper balance of nutrient ratio between carbon and nitrogen, in which a high carbon content favors the maximum production of methane gas [12]. For example, more than 50% of the total meat consumption in Tanzania is from beef cattle, which accumulates a lot of waste during meat processing [13]. This situation presents a need to utilize the particular waste as a means of resource recovery and waste management. Hence, batch reactors loaded with slaughterhouse waste were set for an experiment during this study. The selection of the substrate was based on its high composition of proteins and lipids mixed with its water content [14]. In that case, slaughterhouse waste is regarded as a potential substrate for methane production during the AD process. The breaking down of carbon-rich substrates by microorganisms during AD is sometimes associated with the emission of unwanted contaminants in the digester, which are considered toxic to microorganisms. Additionally, the excess production of other intermediate products such as siloxanes, halogens, ammonia, aromatics, hydrogen sulfide, and volatile organic compounds can result in digester failure due to the inhibition process, which particularly affects methanogens [15,16]. Since slaughterhouse waste is composed of animal fats and protein components of blood, the digestion process breaks down both cysteine and methionine, which results in the formation of sulfur [17]. Therefore, free ammonia is also accumulated in the aqueous phase because of a high nitrogen content resulting from protein decomposition, while long-chain fatty acids accumulate as a result of fat degradation [18]. The presence of both sulfur and ammonia in the liquid phase becomes inhibitory to methanogens during the AD process, and as a result, the quality of biogas produced decreases. The inhibition occurs as a result of competition between sulfur-reducing bacteria and methane-producing archaea by oxidizing molecular hydrogen at a high chemical oxygen demand (COD) value [19]. It is reported that the total ammonia nitrogen (TAN) in the range of 1500–5000 mg/L is inhibitory to microorganisms during biological treatments [20]. Nonetheless, the concentration of ammonia nitrogen (NH3–N) above 2 mg/L can be toxic to aquatic species at a certain range of pH, temperature, and other parameters [21]. Similarly, total sulfide () in the range of 100–800 mg/L is also reported to be inhibitory to methanogens as it suppresses methane production [22]. Several physical–chemical and biotechnological approaches such as ion exchange, struvite precipitation, membrane distillation, adsorption additives, C/N ratio adjustment, and nitrification–denitrification methods have been applied to lower the concentrations of sulfide and ammonia in the liquid phase during the AD process [23,24]. However, a physical–chemical method, which involves the use of various additives with high affinity to ammonia and sulfide, has been dominating over the other methods due to its high removal efficiency and affordability [25]. This method involves the adsorption process in which the surface of the adsorbent attracts adsorbate molecules due to intermolecular forces. The process eventually results in the formation of a new chemical bond at the adsorbent's surface as a result of chemical interaction between the surface of the adsorbent and the adsorbate [26]. Several adsorbents have been applied by researchers, for example, zeolites for the removal of both forms of ammonia (NH3 and ) and iron-rich materials such as iron pellets for hydrogen sulfide removal [27,28]. Furthermore, adsorption by activated carbon has been reported to have a relatively low adsorption capacity of about 6.079 mg/g in removing ammonia from aqueous solutions [29]. Generally, most of the materials that are already existing are industrially synthesized, which are readily expensive and unaffordable to apply for the removal of contaminants in the liquid phase. It is therefore recommended that economically feasible methods be employed for lowering both ammonia and sulfide inhibition in order to elevate the methane content produced during the AD process. In this study, affordable and readily available inorganic materials rich in hematite and aluminosilicate were employed in different stoichiometric amounts for both ammonia and sulfide removal in the liquid phase during AD of slaughterhouse waste. In addition, the effect of the adsorbent mixture on the methanogenesis process was also investigated by measuring the content of methane produced in relation to the accumulation of volatile fatty acids and alkalinity in the reactor. Moreover, the influence of calcination temperature on the physicochemical properties of the adsorbent materials was also examined. The selection of anthill and red rock soil samples was based on their elemental composition such as iron in the form of magnetite (Fe3O4) and hematite (Fe2O3) [28]. These iron contents are considered as remaining products of rocks that have undergone leaching and oxidizing practices during the weathering process. The presence of iron oxide in the red soil is a major breakthrough for the precipitation of sulfide into elemental sulfur [30]. Meanwhile, the high amount of alumina (Al2O3) and silica (SiO2) composing the aluminosilicate cage in anthill soil provides a means for removing ammonia/ammonium () from the substrate undergoing the AD process through ion exchange and adsorption processes [31].
Section snippets
Sample collection, pretreatment, and preparation
The substrate used in this experiment was a liquid mixture of blood and intestine discharged from a slaughterhouse waste point owned by Arusha Meat Company Limited, located in Arusha City, Tanzania. The sample was then stored in a fridge at 4 °C before characterization. The substrate stored was inoculated with 10% (w/v) of cow dung, which was obtained from cattle keepers. The mixture was then fed into a batch-reactor of 1 L capacity for biogas production. Adsorbent materials used in this study
Substrate characterization
The substrate mixture was characterized before being introduced into the reactor for analyzing the parameters such as TS, VS, COD, and BOD. The values of TS, VS, COD, and BOD in the raw substrate were found to be 0.93%, 78.09%, 11,025, and 1600, respectively. Characterization was also performed after 65 days of the digestion process where the values of TS, VS, COD, and BOD were found to be 3.59%, 68.12%, 1575, and 375 respectively. The VS removal efficiency at day 65 was 12.77%. The increase in
Conclusions
The present study investigated the effect of mixing ratios of powdered adsorbent materials processed from the anthill and red rock soil samples in removing ammonia and sulfide in the liquid phase during AD of slaughterhouse waste. Analysis of the adsorbent's physicochemical characteristics was performed using p-XRD, FT-IR, porosimetry, and FE-SEM techniques. The highest surface area (852.8 m2/g) and pore volume (0.75 cm3/g) were displayed by the red rock soil sample calcined at 900 °C. Among
Authors contribution
All authors contributed to the study conception and design. Material preparation, data collection, and analysis were performed by Eric Mutegoa, Nyemaga Masanje Malima, Askwar Hilonga, and Karoli Njau. The first draft of the manuscript was written by Eric Mutegoa, and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to acknowledge the African Centre of Excellence through WISE (Water Infrastructures and Sustainable Energy) ‒ Futures Project at The Nelson Mandela African Institution of Science and Technology (NM-AIST), Tanzania for financial support.
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