Growing Chlorella vulgaris on thermophilic anaerobic digestion swine manure for nutrient removal and biomass production
Introduction
Microalgae technology has attracted considerable attention nowadays because of its potential as high impact feedstock for production of biofuel, high value pigments, nutraceuticals and therapeutic compounds (Lowrey et al., 2015). However, the high cost of microalgae cultivation limits its commercial applications. Nutrients and water are important cost factors, accounting for 10–20% of the total cultivation cost (Singh et al., 2011). Many animal manures have nutrient compositions similar to classic microalgae culture media and were found to support the growth of some microalgal strains well (Zhou et al., 2014). In addition to large amounts of nitrogen and phosphorous, volatile fatty acids (VFAs) such as acetic acid, propionic acid and butyric acid were also found in animal manures (Hu et al., 2013), and considered as potential soluble organic carbon substrates for microalgae cultivation (Hu et al., 2012). There is an increasing interest in using animal manures to grow microalgae because manures provide low cost nutrients and water, and at the same time manure based algae production is a cost effective tool for manure waste management.
Recently, researchers have demonstrated the feasibility of growing microalgae on swine manure (Ji et al., 2014, Luo et al., 2016, Nam et al., 2017). However, there are some major issues with the use of animal manures for microalgae cultivation, including (1) high turbidity due to the presence of solid particles; (2) high ammonia concentration; (3) low available carbon sources, most of which are locked in the insoluble organic compounds; and (4) lack of high performance microalgal strains capable of adapting to the environment of animal manures (Zhou et al., 2014). Some of these issues could be addressed through diluting the manures 20–100 times with water (Zhou et al., 2014); however, this requires a large quantity of freshwater. In addition, researchers have attempted to convert organic materials to usable carbon sources through anaerobic digestion (Hu et al., 2012), remove NH4+-N through aeration and air stripping (Liao et al., 1995, Min et al., 2014), and separate solid particles using centrifugation to reduce the turbidity (Hjorth et al., 2008).
In the light of above discussion, the aim of this work was to investigate processes enabling fast growth of a selected microalgal strain on swine manure with minimal dilution. The specific objectives were (1) to determine whether the pretreatment methods of liquid swine manure (LSM) were feasible and efficient; (2) to investigate if C. vulgaris could grow well in minimally diluted pretreated anaerobic digestion swine manure (PADSM); (3) to understand how the growth of C. vulgaris could be affected by the bacteria in PADSM; and (4) to study the nutrient removal efficiency, biomass production, chemical composition, and fatty acid profiles of C. vulgaris grown on PADSM. It is hoped that the results of this study could provide a scientific basis and support for microalgae cultivation using minimally diluted animal manures in large scale.
Section snippets
Swine manure collection, pretreatment and analysis
Liquid swine manure (LSM) was collected from the University of Minnesota Southern Research and Outreach Center (Waseca, Minnesota), and anaerobically digested at 55 °C for 16 days with activated sludge for biogas production, resulting in anaerobic digestion swine manure (ADSM). Then, ammonia was stripped from the ADSM at 55 °C for 2 h under a vacuum of 25 inch Hg (84.7 kPa), the solid particles were removed using the methods of natural precipitation and centrifugation (10,000g, 10 min) to decrease the
Physicochemical analysis
The physicochemical characteristics of PADSM before and after autoclave were listed in Table 1. After pretreated with the method of this study, concentrations of TN and TP were 463.0 and 400.8 mg/L, 113.3 and 116.6 mg/L in PADSM and autoclaved PADSM, respectively. The TN-to-TP ratio (N/P) of PADSM and autoclaved PADSM were 4.1:1 and 3.4:1, respectively, which were similar with the optimal N/P ratio for algae growth (Min et al., 2014). In addition, concentrations of NH4+-N in PADSM were 255.4 and
Conclusions
It was concluded that (1) C. vulgaris (UTEX 2714) grew best in 3× PADSM; (2) C. vulgaris (UTEX 2714) was capable of completely depleting NH4+-N, TN, TP and COD from PADSM, particularly when it was cultivated with bacteria; and (3) C. vulgaris (UTEX 2714) grown on autoclaved PADSM could be used as animal feed, while oil from this strain cultivated in non-autoclaved PADSM could be used as a good-quality biodiesel resource. Thus, an integrated process of LSM pretreatment and microalgae cultivation
Acknowledgements
This manuscript was supported in part by the Jiangsu Overseas Research and Training Program for University Prominent Young and Middle-aged Teachers and Presidents, the Jiangsu Provincial Natural Science Foundation of China (no. BY2015065-10), and the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative Citizen Commission on Minnesota Resources (LCCMR) and University of Minnesota Center for Biorefining.
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2022, Science of the Total EnvironmentCitation Excerpt :Saranya and Shanthakumar (2020) indicated that after ozonation pretreatment for 90 min, the biomass concentration and cell density increased by 32.3 % and 13.0 % respectively compared with control (without ozonation) in the treatment of tannery effluent using microalga Nannochloropsis oculaa. However, pretreated swine digestate had to be diluted 3-fold to achieve good growth of C. vulgaris, because high organic loading still caused inhibition although TAN concentration was down to be suitable for microalgal growth after pretreatment of ammonia stripping and centrifugation (Deng et al., 2017). In order to better understand the physiological state of C. vulgaris grown in different pretreated digestate, the primary Chl fluorescence parameters of microalgae PSII were measured during cultivation (Fig. 3c and d).