Mechanism of contaminant removal by algae-bacteria symbiosis in a PBR system during the treatment of anaerobic digestion effluents

https://doi.org/10.1016/j.agwat.2020.106556Get rights and content

Highlights

  • A-SBPBR was used to treat anaerobic digestion effluents (ADEs).

  • The treatment effects of C, N, P in A-SBPBR.

  • Mechanism of ADEs degradation in the ABS.

  • A-SBPER treatment can effectively remove TN, AN and TP from ADEs.

  • The balance between bacteria and microalgae facilitates the treatment of ADEs.

Abstract

Large volumes of anaerobic digestion effluents (ADEs) are generated by intense livestock and poultry farm activities. If untreated, these effluents represent a threat to the environment. Previous data have indicated that microalgae are able to grow in photobioreactors (PBR) where high concentrations of inorganic salts are present. In the present research, algae-assisted sequencing batch photo-bio-reactor (A-SBPBR) with single micro-algae systems and single activated sludge systems were developed. We studied the effects of the treatments on different parameters including COD, TN, AN, and TP. We analyzed changes in bacterial community diversity using the high-throughput sequencing analysis. The degradation mechanism of the ADEs was studied by means of algal-bacterial symbiosis. Experimental results indicated that contaminants were efficiently removed from the ADEs when using the A-SBPBR. After treatment, the degradation rates of COD, TN, AN, and TP were 73.78%, 80.67%, 89.74%, and 95.39%, respectively. The outlet concentrations of COD, TN, AN, and TP were 355.09 ± 17.90, 83.97 ± 9.37, 35.42 ± 2.65 and 0.87 ± 29 mg/L, respectively. In the initial stage of the process (day 1), the most abundant bacteria present in the ABS (algal-bacterial symbiosis system) included proteobacteria, bacteroidetes, actinobacteria, acidobacteria and chloroflexi. Among these, those with the highest abundance values were proteobacteria and bacteroidetes. Abundance values were 24.33% and 19.97%, respectively. A-SBPBR contained various microorganisms including aerobic bacteria, fungus, and chlorella. In a reciprocal interaction, bacteria and microalgae were able to use each other´s metabolites. Under intermittent aeration conditions, organic C, N, and P present in the macromolecules were converted to smaller species. C was transformed to CO2 and organic C. In addition, N and P stayed as part of small molecules that were used by aerobic bacteria and fungus. When ABS operation reached stability, hydrolytic acidifying bacteria, which belonged to acidobacteria, were able to degrade different organic compounds including saccharides.

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).

References (33)

Cited by (0)

View full text