Enhanced bio-methane production from ammonium-rich waste using eggshell-and lignite-modified zeolite (ELMZ) as a bio-adsorbent during anaerobic digestion
Graphical abstract
Introduction
Along with the development of livestock industries, environmental problems such as soil contamination, eutrophication, and air pollution have increased rapidly due to the growing production of livestock wastes. Livestock waste is usually used as soil fertilizer, but repeated overload of livestock waste leads to the accumulation of nutrients and heavy metals, causing many environmental problems like soil and water contamination [1]. Anaerobic digestion is regarded as a desirable application due to its beneficial productivity with renewable energy recovery and efficient control on waste management. Furthermore, anaerobic digestion of livestock waste is also considered as one of the sustainable waste-to-energy technologies, which could convert livestock waste materials into methane as a valuable energy carrier [2].
However, the high concentration of ammonia released during the biodegradation of livestock wastes may seriously inhibit the anaerobic digestion process. Ammonia inhibition could lead to a decrease in methane production and accumulation of by-products, contributing to instability of the anaerobic digestion process [3]. Zeeman et al. [4] reported that ammonia inhibition occurs at an initial ammonium nitrogen concentration of 1700 mg/L in the thermophilic reactor, and Hobson and Shaw [5] have also reported that Methanobacterium formicicum could be inhibited at ammonium nitrogen concentration of 3290 mg/L.
Numerous practical solutions have been investigated for bio-methane recovery from ammonium-rich waste, such as adding ammonium-selective adsorbent, microorganism immobilization and ammoxidation [[6], [7], [8]]. Ammonia removal by adding ammonium-selective adsorbents has drawn a great attention owing to its non-toxicity, low cost and eco-friendly character [9]. Among numerous adsorbents, zeolite is commonly used because of its wide availability and low cost. However, the ion-exchange capacity of natural zeolite is usually quite low. Hence, modification of zeolite is considered as an attractive option, in order to increase the effective surface area and further enhance the adsorption capacity of zeolite. The adsorption behavior of the silicate-carbon modified zeolite has been reported to be effective to remove ammonium with adsorption capacity of 0.12 mg/g [10]. Meanwhile, Lei et al. [11] found that the microwave-treated zeolite could reach to adsorption capacity of 23.38 mg/g. However, due to the weak adsorption capacity of these modified zeolite, the development of more efficient adsorbent for ammonia removal is still needed.
Generally, cations (Na+, K+, Ca2+, and Mg2+) that can exchange with ammonium are widely used in the zeolite modification processes. These cations are crucial for the cell metabolism and growth, which also are trace elements important for stable process performance, e.g. by limiting the VFA accumulation [12]. Ca2+ has often an optimistic effect on the anaerobic digestion of organic wastes [13]. Therefore, natural materials rich in Ca2+ would be an eco-friendly and low-cost choice used as modifiers of adsorbents. To our best knowledge, there is no report focusing on the modification of adsorbent using calcium-rich natural material until now. Two kinds of natural material, eggshell and lignite, have been selected to modify zeolite in the present study.
Eggshell is commonly treated as waste in food industry and contains large quantities of calcium salt. In USA, 120,000 tons of waste eggshells are generated and disposed off annually [14]. The major component of eggshell is calcium carbonate. The dried eggshell contains 85–95% calcium carbonate, 1.4% magnesium carbonate, 0.8% phosphates, 4% organic matter as well as traces of sodium, potassium, zinc, manganese, iron and copper [15]. Therefore, the eggshell has an excellent potential to be used as an adsorbent material due to its high Ca2+ content and the presence of trace elements. On the other hand, lignite is widely allocated and abundantly available. More than 2600 billion tons of lignite accounts for about 24.4% total coal resources around the world [16]. However, because of high content of oxygen and water, low calorific value, and easy spontaneous combustion, the application of lignite has not been further investigated. Interestingly, the variety of cations in lignite, especially trace elements, may benefit for the growth of microorganisms [17]. Thus, lignite could work as a modifier to improve the ability of adsorption. The utilization of eggshell and lignite in suitable industrial application could enhance the methane production and deal with a waste problem. However, the application of both eggshell and lignite for the modification of nature zeolite has not been carried out yet.
Therefore, our aim was to develop a novel bio-adsorbent based on eggshell and lignite, with high ammonium-adsorption capacity to optimize anaerobic digestion of substrates giving rise to high concentrations ammonium. This study would provide a promising choice for a low cost and eco-friendly treatment of waste eggshell and lignite. This study was focus on (1) investigating the ammonium adsorption capacity of eggshell-and lignite-modified zeolite (ELMZ) adsorbents compared with the calcium chloride-modified zeolite and natural zeolite; (2) analyzing kinetics and thermal isotherms of the adsorption process; (3) evaluating the feasibility of modified adsorbent using ELMZ during anaerobic digestion.
Section snippets
Synthesis of modified zeolite
The process of integration of calcium chloride, eggshell only, ELMZ were prepared to modify the natural zeolite, respectively. For the CaCl2 modified zeolite, 8 g of natural zeolite powder, 1 g starch and 1 g CaCl2 were prepared. For the eggshell only modified zeolite, 8 g of natural zeolite powder, 1 g starch and 1 g eggshell were prepared. For the ELMZ, 8 g of natural zeolite powder, 1 g starch, 0.5 g eggshell and 0.5 g lignite were prepared. Firstly, the prepared materials were mixed to
NH4+-N adsorption by different modified zeolite
Fig. 1 showed the ammonium adsorption capacity of different adsorbents at the initial concentration of 4.1 g/L. The ELMZ exhibited the highest ammonium adsorption capacity of 55 mg/g, which was 5.9 folds higher than natural zeolite (9.2 mg/g), 1.4 folds of the zeolite modified by calcium chloride (40 mg/g), 1.1 folds of the eggshell-modified zeolite (52 mg/g), respectively. This result indicated that natural materials, eggshell and lignite, may substantially improve the adsorption efficiency of
Conclusions
In this study, a novel bio-adsorbent, eggshell-and lignite-modified zeolite (ELMZ) was developed for bio-methane recovery from ammonium-rich waste. ELMZ showed very high ammonium adsorption capacity. The abundant content of cations, especially the Ca2+, likely benefited the metabolic activity of microorganisms, which would contribute to high methane production during the anaerobic digestion system. Furthermore, the excellent performance achieved during the anaerobic digestion system, indicates
Acknowledgements
This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI [grant number JP17J00358].
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