Purpose
Aquaculture production has steadily increased over the past few decades to meet the growing demand driven by population growth. Recirculating aquaculture system (RAS) is a popular technology for fish farming, where water is continuously filtered and reused. The filtration process generates sludge, also called aquaculture waste (AW), comprised of fish feces, uneaten feeds and other metabolites. This sludge can create environmental pollution if not handled properly. Anaerobic digestion (AD) is a waste-to-energy technology that can convert AW into biogas. The digestate produced after AD is nutrient-rich and can be utilized for aquaponic production.
The performance of AD depends on several factors, including substrate characteristics, process conditions, and operational parameters. One critical factor is the carbon-to-nitrogen (C/N) ratio, which significantly influences AD efficiency. The optimal C/N ratio for AD is typically between 20 and 30. However, AW has a low C/N ratio due to its high protein content, which can limit biogas production. To maximize biogas yield, the C/N ratio of AW needs to be adjusted. Co-digesting AW with a high C/N feedstock can help achieve an optimal balance. Corn stover (CS), a high C/N substrate that is abundantly available in the U.S., can be used to enhance AD of AW. Therefore, the aim of this study was to find out the optimum mixing ratio that results in maximum biogas production.
What Did We Do?
In this study, AW was collected from the Aquaculture Research Lab at Purdue University, West Lafayette, IN, and CS was obtained from the Animal Sciences Research and Education Center at Purdue University. A batch experiment was conducted using 24 batch digesters, which were made of 1-L Corning polycarbonate square bottles. Ground CS was mixed with AW at seven different ratios (100:0, 90:10, 70:30, 50:50, 30:70, 10:90, and 0:100) as digester substrate. Digested slurry from a dairy farm manure biogas digester, which operated in mesophilic condition, was used as inoculum. The substrate-to-inoculum ratio was maintained at 1:3. Each digester was fed the feedstock containing 3.75 g volatile solids (VS) and the inoculum containing 11.25 g VS. A 1-L Tedlar bag was connected to each digester to collect biogas. All seven treatments and a blank, which contained only inoculum, were designed in triplicate and set up in an experimental chamber. The experiment was performed for 30 days under mesophilic conditions using two water baths to maintain a constant temperature. The volume and composition of biogas produced from each digester were measured daily and periodically, respectively. The biochemical methane potential for each mixing ratio was calculated by subtracting the biogas produced by the inoculum from the total biogas produced from a digester.
What Have We Learned?
The study showed that biogas production varied with different mixing ratios as shown in Figure 1. Biogas production was higher during the initial period and decreased as the digestion process progressed. Digesters with a higher proportion of AW took less time to produce 90% of the total biogas produced. The cumulative specific biogas production was highest (494.62 mL g−1 VS) for the 50:50 mixing ratio of CS and AW after 30 days of digestion. The methane concentration for all test groups ranged between 50.58% and 57.66%. The 50:50 ratio showed the highest cumulative methane yield (275.98 mL g−1 VS), which was 21.47% and 20.29% higher than the mono-digestion of CS and AW, respectively. The superior performance at this ratio can be attributed to a balanced C/N ratio.
Future Plans
This study is part of a USDA research project to develop a near-zero-pollution aquaculture production system. Future studies on AD for aquaculture production will focus on enhancing biogas yield from AW through various methods of substrate pretreatment and additive use. Additionally, the quality of the digestate from AD of AW will be evaluated for potential application of digestate in aquaponic production.
Authors
Presenting & corresponding author
Ji-Qin Ni, Professor, Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907, jiqin@purdue.edu
Additional authors
Rajesh Nandi, PhD student, Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907; Mohit Singh Rana, Postdoctoral Research Associate, Agricultural and Biological Engineering, Purdue University, West Lafayette, IN 47907
Additional Information
Mirzoyan, N., Tal, Y., Gross, A., 2010. Anaerobic digestion of sludge from intensive recirculating aquaculture systems: Review. Aquaculture 306, 1–6. https://doi.org/10.1016/j.aquaculture.2010.05.028
Acknowledgements
This research was supported by the intramural research program of the U.S. Department of Agriculture, National Institute of Food and Agriculture, Agriculture and Food Research Initiative grant no. 2023-68016-39718.
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