Diverse microbial communities contribute to the removal of various pollutants
Revisiting the history of biological wastewater treatment, with the removal of carbon, nitrogen, and phosphorus from wastewater, the key is to provide different microbial niches to enrich diverse functional microorganisms, such as heterotrophs, nitrifiers, denitrifiers, and polyphosphate accumulating organisms (PAOs). Figure 1 shows the redox potential distribution of typical reactions carried out by different types of functional microorganisms. Reactions for the biological nitrogen cycle usually occur at potentials ranging from 0.34 to 0.97 V, while reactions for anaerobic sulfate reduction and methanogenesis occur at potentials ranging from −0.22 to −0.14 V and −0.43 to −0.25 V, respectively. By suitable management, these functional microorganisms can carry out biological metabolisms sequentially in time or space, which can be applied to achieve successful wastewater purification9,10,11,12.
The dotted lines in the carbon cycle represent the aerobic reaction. The lower right corner shows an example of how these cycles are correlated with each other. Sulfate reduction/denitrification enables the simultaneous carbon removal and sulfate/nitrogen removal; the sulfammox process enables the simultaneous sulfate removal and nitrogen removal.
For the removal of organic carbon, anaerobic treatment shows a good example of how functional microorganisms cooperate with each other to achieve the conversion from organic carbon to renewable bioenergy methane (CH4) (Fig. 1). During anaerobic treatment, fermenting bacteria first degrade complex organic substrates such as protein and sugar into monomers which are subsequently utilized by acidogenic bacteria to produce acetate and hydrogen. Finally, methanogens consume acetate and hydrogen/carbon dioxide (CO2) to generate the end product CH4. Success to maintain the microbial population and the growth of these microorganisms is the primary cause of anaerobic system stability. In addition, in aerobic biological wastewater treatment processes, organic carbon is mainly degraded by heterotrophs to produce CO2 and synthesize biomass.
For typical municipal wastewater treatment, cultivating microbial communities through a serial of anaerobic, anoxic, and aerobic (A2O) reactors enable the enrichment of ammonia-oxidizing bacteria (AOB), nitrite-oxidizing bacteria (NOB), denitrifiers, and PAOs, resulting in the efficient removal of organic carbon, nitrogen, and phosphorus from wastewater13. In the anaerobic reactor, volatile fatty acids (VFAs) could be stored by PAOs with energy supplied from intracellular stored polyphosphate. In the anoxic reactor, denitrification will occur with organic carbon in wastewater as the electron donor, and recirculated oxidized nitrogen as the electron acceptor. In the aerobic reactor, phosphorus is uptaken by PAOs, ammonia is nitrified by nitrifiers, and also activities of heterotrophs will occur for organic carbon removal. Through activities of all these diverse microorganisms, wastewater can be efficiently treated.
When treating sulfate-containing wastewater, sulfate-reducing bacteria (SRB) which regulate the sulfidogenic bioprocess will become crucial microbes for sulfate removal (Fig. 1). During sulfate reduction, sulfate is first reduced to sulfite and then to sulfide by SRB. The reduction of sulfite to sulfide can be accomplished via the direct pathway in which sulfite is directly reduced to sulfide by receiving six electrons or another pathway that trithionate and thiosulfate are acted as intermediates14. Carbon sources as electron donors can be involved in the SRB metabolism. For example, some SRB metabolize organic compounds as electron donors through Acetyl CoA or a modified TCA pathway15. Many intermediate products originating from anaerobic fermentation/hydrolysis such as amino acids, sugars, long-chain fatty acids, and VFAs, can also be metabolized by SRB14. In this case, organic carbon can be simultaneously removed efficiently with sulfate.
Furthermore, the synergistic removal of contaminants may be completed by cooperative interactions, and the biological element cycle could be interlinked to each other (Fig. 1). It is well-known that denitrification can remove carbon and nitrogen simultaneously, sulfur-based denitrification can remove sulfur and nitrogen, and denitrifying PAOs can remove organic carbon, nitrogen, and phosphorus together. Rios-Del Toro et al.16 found that anaerobic denitrification and ammonium oxidation could be coupled with the reduction of sulfate in marine sediments (sulfammox). Free sulfide, elemental sulfur, and sphalerite were produced during the ammonium oxidation with the reduction of sulfate16. To achieve the niche development of sulfammox, it is obvious that certain concentrations of sulfate and ammonium should be present in wastewater. However, it remains difficult to connect specific microbes to these functions. Metagenomic analysis needs to be implemented to discover uncultivated functional microbes for further application. On the other hand, for the coupling of sulfate removal and CH4 production, conductive materials were reported to be able to alleviate the inhibition of sulfate on methanogenesis, which can enhance the diverse biogeochemical reactions. Liu et al.17 found that the addition of conductive materials could re-enrich syntrophic partners inactivated by sulfate. This new syntrophic community could efficiently produce CH4 in sulfate-containing environment. In this case, the proper addition of conductive materials in anaerobic systems is the key for achieving the coupling of CH4 production and sulfate removal. All these show that the interconnections between biogeochemical cycles such as carbon, nitrogen and sulfur would be potentially applied for developing novel environmental biotechnologies through the optimization of microbial niches (Fig. 1). Similar concepts could be developed for other biological element cycles.
Deciphering functions of known and unknown microorganisms
Biological treatment processes would be successfully functioned once that targeted microbial communities are enriched through microbial niches optimization. Therefore, the understanding of key microbial players is the fundamental step. With the application of novel molecular and bioinformatics techniques, more and more uncultured microbes and microbial functions have been and will be identified. For instance, the concept that nitrification is carried out by AOB and NOB sequentially has been accepted for more than a century and the known AOB and NOB are phylogenetically not closely related. However, some Nitrospira (NOB) species were found to possess all genes encoding enzymes necessary for ammonia oxidation via nitrite to nitrate, completely revising the picture of the nitrogen cycle18,19. The expression of genes during growth through ammonia oxidation to nitrate suggested that Nitrospira might be the key bacteria responsible for nitrification, and metabolic labor division in nitrification is not strictly required.
The niches of novel functional microbes may be different from ‘conventional’ microbes, thus investigating metabolic kinetics, diversity, and microbial interactions of these new microorganisms are crucial for developing novel wastewater treatment technologies based on the optimization of microbial niches.
Clarification of novel microbial metabolic mechanisms
Wastewater treatment processes can be improved through clarifying biological metabolic mechanisms. For example, interspecies hydrogen and formate transfer have been considered as the common pathways for syntrophic methanogenesis. Recently, it has been reported that some syntrophic bacteria and methanogens could exchange electrons directly by conductive pili or outer membrane cytochromes for syntrophic CH4 production20,21. Since electron carriers are not required during direct interspecies electron transfer (DIET), it was considered as a faster and potentially more energy-conserving pathway for CH4 production22. Therefore, DIET may be a crucial approach to improve the energy conversion from wastewater.
By discovery of this new microbial mechanism, several strategies have been proposed that can be potentially applied to achieve the stimulation of DIET so as to improve methanogenesis. The first one is the microbiology-based regulation. A high abundance of DIET-capable microorganisms often implies the good performance of DIET. Enriching DIET players by optimizing their niches can result in the dominance of DIET pathway in methanogenic systems. For example, the well-known bacteria with the DIET ability, Geobacter, which syntrophically consumes ethanol as the organic substrate for growth, could be enriched in an up-flow anaerobic sludge blanket reactor treating brewery wastewater21. In many cases, high CH4 production efficiency could be attributed to the high abundance of Geobacter. Conducting the pretreatment of ethanol-type fermentation may be a useful approach for cultivating Geobacter species23. Second, promoting the excretion of extracellular compounds and adjusting the syntrophic interaction could be also applied for better DIET performance24. Finally, applying conductive materials as electron conduits in methanogenic systems can provide a good external conductive environment for syntrophic partners with the DIET ability. In this case, electrons released from syntrophic bacteria can be directly transferred to methanogens via conductive materials without contacting closely, enhancing the efficiency of CH4 production25. In the wastewater treatment system, the addition of conductive materials could enhance the conductivity of anaerobic sludge, and stimulate the activity of respiratory chain and the extracellular electron transfer rate of syntrophic partners, thereby promoting the methanogenic efficiency26.
Strategies for emerging compounds removal through microbial niche tuning
Providing suitable niches for specific microbes can also enhance the removal of emerging compounds and alleviate their toxicity. Conventional AOB can remove EDCs due to their enzyme of ammonia monooxygenase, which can degrade certain types of micro-pollutants, and heterotrophs could be also responsible for the degradation of synthetic estrogen10. In addition, the recently discovered complete ammonia-oxidizing bacteria which could oxidize ammonia to nitrate via nitrite were also found to be able to degrade micro-pollutants27. Therefore, by tuning all these functional microorganisms, not only conventional pollutants will be removed efficiently, but also emerging compounds will be well controlled.
On the other hand, suitable microbial niches could be applied to alleviate the biological toxicity induced by emerging compounds. For example, the alternate operation of aerobic and extended anaerobic treatment resulted in the enhanced removal of endocrine activities and better control of biological toxicity4. Different redox situations of wastewater under aerobic and anaerobic conditions might be one of the reasons for promoting endocrine degradation4. In addition, the change of organic loading rate could lead to a niche variety of microbes as well, thus affecting the removal efficiencies4. Recently, it was confirmed that cysteine produced during the sulfate reduction could alleviate the nano-metal particle toxicity5. This shows that the microbial interactions during biological processes could be functioning diverse for achieving different purposes.
Microbial niche-based design of the wastewater treatment system
For wastewater treatment, if only organic carbon is removed, one aerobic reactor is adequate. While for organic carbon and nitrogen removal, anoxic combined with aerobic reactors would be applied. Furthermore, for organic carbon, nitrogen, and phosphorus removal, anaerobic, anoxic, and aerobic reactors would be adopted. With more types of pollutants removal, the numbers of biological reactors would be extended for wastewater treatment system design.
Besides biological reactors, the microbial niche nexus concept should be incorporated during wastewater treatment system design. For the upgradation of conventional WWTPs, novel microbial communities could be explored and utilized for solving new challenges, including emerging compounds removal. All these could be achieved through microbial niche optimization to enrich diverse microorganisms in the present WWTPs besides to build new infrastructure (Fig. 2). To achieve this purpose, it is essential to further explore the unrevealed biological processes or functions. For example, rare species in biological treatment processes should be paid attention to, which may act as the seed and would be dominant with varied environmental conditions28,29. In some cases, species with a low abundance may also contribute a lot to the key function of a microbial system. For instance, Pester et al.30 reported that Desulfosporosinus with only 0.006% of the total relative abundance was an important sulfate reducer in a peatland system.
Development of wastewater treatment plants based on infrastructure-based design or microbial niche-based design.
Source: Ecology - nature.com
