Mechanism of contaminant removal by algae-bacteria symbiosis in a PBR system during the treatment of anaerobic digestion effluents
Introduction
Sustainable technologies for wastewater treatment display several advantages over traditional ones. For example, sustainable techniques are able to use wastewater as a resource for energy generation. For this reason, they have attracted the attention of experts in different areas (Huang et al., 2015). Biogas technology uses livestock and poultry manure to convert organic waste into bio-energy. However, they result in large amounts of anaerobic digestion effluents (ADEs). These effluents usually contain high levels of organic matter that is barely degradable, as well as inorganic salts (containing N and P) with an unbalanced C-N-P ratio. Thus, ADEs are very difficult to treat (Huang et al., 2015, Saidu et al., 2013). In this context, sustainable and efficient wastewater processes that are able to treat this type of residues are important in the field of environmental technology.
Past reports have indicated that microalgae are an excellent option for the biotreatment of wastewater since they efficiently remove inorganic salts, they grow rapidly, and display high adaptability (Foladori et al., 2018, Hernández et al., 2016, Rada-Ariza et al., 2017, Fan et al., 2017). Also, different systems have been developed where ABS are used in the PBRs, taking advantage of the synergistic effect between bacteria and algae. With ABS, it has been possible to degrade organic matter and other toxic and hazardous compounds present in wastewaters, as well as to remove inorganic salts (Xie et al., 2018). Shi (2013) treated beer wastewater with an ABS composed of spirulina and mycelium pellets of Flammulina velutipes. They showed that, with this system, the removal rate of COD, TN, and TP in the beer wastewater were 77.81%, 84.28%, and 50.88%, respectively. These values were higher as compared to those obtained in a system where only microalgae were present. These values were 70.59%, 70.17% and 37.99%, in the same order. Liu (2012) reported similar results when they compared the performance of an ABS with that of a system where only chlorella was used. On another study, Abou-Shanab et al. (2013) treated pig farm wastewater using six different microalgae (Ourococcus multisporus, Nitzschia cf. Pusilla, Chlamydomonas mesicana, Scenedesmus obliquus, Chlorella vulgaris, and Micractinium reisseri). These researchers determined that the highest TN and TP removal rates were observed in wastewaters treated with Micraactinium reisseri. In this case, TN and TP removal rates were 62% and 68%, respectively. In addition, . Elmasry et al. (2013) proved that a COD concentration of 1900 mg/L presented the best growth rate of Chlorella zofingiensis. Herein, the corresponding TN, TP, and COD removal rates reached 82.7%, 98.2%, and 79.8%, respectively. Their results also indicated that, after 24 h, phenol removal reached a 95%.
To the best of our knowledge, previous ABS research has been mainly focused on the treatment of municipal and industrial wastewaters. Thus, in order to determine potential new options for the sustainable remediation of ADEs, in the present investigation three different bioreactors were used. They included: (a) ABS (inlet water of unsterilized ADE); (b) single microalgae (inlet water of sterilized ADE); and (c) activated sludge bioreactors. These systems were used to compare their performances on the treatment of ADEs of livestock and poultry manure. We analyzed the effect of treatment conditions on COD, TN, AN, and TP. In addition, we determined the microbial diversity and explored the mechanism of ADEs degradation in the ABS systems.
Section snippets
Materials
The activated sludge used in the present experiments was obtained from the wastewater treatment plant of a factory located in Northern Shenyang in the Liaoning province. The inoculated activated sludge was transported on a 25-L airtight plastic container. During the transportation process, the temperature decreased to 20 °C; however, the activity was maintained. Later, cultivation was performed with the addition of a small amount of ADE, maintaining a temperature of 30 °C.
The microalga used in
Effect of treatment conditions on COD
Results for COD in ADEs treated with ABS (R1), sludge system (R2), and pure algae system (R3) are shown in Fig. 1. During the initial stage of the reaction (0 d), the initial COD concentrations in the R1, R2, and R3 systems were 1354.29 ± 45.23, 1354.29 ± 45.23, and 765.33 ± 34.17 mg/L, respectively. COD showed a decreasing trend in the first 10 days of ADEs treatment in the SBPBR. In addition, on day 3, the daily degradation rates in the R1 and R2 treatments reached the maximum values of
Conclusions
- (1)
A-SBPBR effectively removed contaminants present in the ADEs. After treatment with the A-SBPBR, the COD degradation rate in the ADEs was 73.78%. The outlet COD concentration was 355.09 ± 17.90 mg/L. The TN and AN degradation rates were 80.67% and 89.74%, respectively. The outlet concentrations of TN and AN were 83.87 ± 9.37 and 35.42 ± 2.65 mg/L, correspondingly. The degradation rate of TP was 95.39%, and the outlet TP concentration was 0.87 ± 0.29 mg/L.
- (2)
The data related to bacterial community
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.
Acknowledgements
This work was funded by the Tianyou Youth Talent Lift Program of Lanzhou Jiaotong University, Funds for Youth Science Foundation Project of Lanzhou Jiaotong University (2020018) and the National Natural Science Foundation of China (No. 51606090, No. 51866008).
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