Biological Treatment of Water Contaminants: A New Insight

1. Introduction

Biological treatment of water contaminants can be performed by bacteria, algae or fungi, and the contaminants that these organisms can remove are very diverse and can range from metals [1] to plastics [2], which are present nowadays not only in marine environments [3] but also in freshwater streams [4]. Other highly contaminant materials which can be treated by microorganisms are textile dyes [5] and crude oil [6], and although accidents involving crude oil transportation ships have been highly reduced in recent decades [7], there are still continuous spillages of petroleum in refineries, harbors and oil transportation pipes and deposits. This situation makes petroleum n-alkanes one of the priority pollutants because of their presence in groundwaters [8].

Textile dyes are present in many rivers, especially in Asia, where production factories for textiles and clothes are located, contributing to harmful pollution that is difficult to reduce. Microorganisms can play an important role here, because the organic molecules which are present in textile dyes have been proven to be biodegradable [9].

Plastics and microplastics were originally detected in the sea, sometimes in huge amounts floating on the water, but later, they began to appear in freshwater streams all over the world, such as in Europe, North America and Asia [4]. Some bacteria seem to be able to degrade them, and an interesting challenge is the research focused on the biodegradation of different types of synthetic plastics, polyethylene, polypropylene, polystyrene, etc. [10,11].

The mechanism in which metals and metalloids can be reduced in wastewater by bacteria is related to the enhancement of phytoremediation by the biosorption of metals by plants or other co-processes [1,12]. Plant-associated bacteria can promote plant growth and resistance by controlling growth hormones and secondary metabolites and improving the antioxidant enzyme system. Some bacteria genera protect plants from toxic metals, and the final result of this association is the increase in plant growth and removal of toxic heavy metals or metalloids, such as lead, cadmium and arsenic [12].

Among the emerging pollutants which have been detected in water streams in low concentrations are pharmaceuticals, normally resistant to biological wastewater treatment due to their low biodegradability [8]. Although this is a difficult task, bacteria and fungi can reduce these contaminants in specific biological treatments or combined with advance oxidation processes (AOPs) [13].

Eutrophication is an extended concern affecting developing and developed countries, producing high and persistent changes in the qualities of natural streams, turbidity, green color and plant growth on the banks of rivers and lakes. Modern and sustainable processes for nitrification–denitrification like Anammox [14] and recently Feammox [15], in which the electron acceptor for N-NH₄⁺ oxidation to N-NO₂⁻ and N-NO₃⁻ is Fe(III), are being studied to improve nutrient removal.

Hydrogen production by microorganisms is one of the promising ways to achieve zero emission sources of energy and enables the reduction of carbon dioxide emissions as the necessary transition from fossil fuels to renewable energies. Hydrogen is produced by bacteria in dark fermentation (controlled anaerobic fermentation) or by other methods like microbial electrolysis, biophotolysis and photofermentation [16]. The state of the art in this topic is currently at the experimental level, and industrial implementation is still far off because of its imperfections and incomplete conclusions.

In this Special Issue, most of these topics are considered in high-quality articles published after a rigorous review process. Effective and sustainable processes are studied and described from the point of view of the different research areas, particularly civil, environmental, biological and chemical engineering, architecture, chemistry, biochemistry and ecology.

2. Overview of the Articles Published in This Special Issue

This Special Issue is focused on the latest research about biological treatment of water contaminants, especially advances in micropollutant removal and new proposals for xenobiotic elimination. Among the 11 articles enclosed in this SI (8 research articles and 3 reviews), 2 are on nitrification–denitrification (contributions 6 and 10), 3 are focused on persistent micropollutants (contributions 2, 8 and 11), 4 are on new advanced technologies in wastewater and landfill leachate treatment (1, 3, 4 and 7 contributions) and the other 2 articles cover hydrogen production (contribution 9) and the mechanism of crude oil biodegradation by bacteria (contribution 5).

In

Table 1. Contributions presented in this Special Issue.

nº	Area of Research	Topic	Type of Research	Country
1-	Civil and Environmental Engineering	Granulation improvement of activated granular sludge	Laboratory bioreactors	Korea
2-	Environmental Protection Agency	Biodegradation of CVO compounds and 1,4-dioxane in groundwater	Review	USA
3-	Engineering	Treatment of Slaughterhouse wastewater by UASB followed by a wetland	Pilot scale reactors	Colombia
4-	Architecture and Technology	Future trends of wetland treatment technology in China	Review	China
5-	Chemical Engineering	Mechanism of crude oil biodegradation by oleophilic bacteria	Laboratory bioreactors	Spain
6-	Engineering	Biological nitrate removal from groundwater using corn cobs	Pilot scale bioreactors	Panama
7-	Civil Engineering	Microalgae treatment for fly ash leachate from a landfill	Laboratory cultures	China
8-	Biochemistry-Engineering	River sediment bacteria for Trichloroethylene bioremediation	Field data and laboratory	USA-India
9-	Engineering	Enhancing bio-hydrogen and biochar production from an aquatic plant	Laboratory	Egypt
10-	Civil, Architectural and Environmental Engineering	Nitrogen removal using Feammox and nitrate-dependent Fe(II) oxidation	Laboratory	China
11-	Chemistry, Ecology and Biological Engineering	Fungal application in endocrine disruptors removal from wastewater	Review	Brazil-Portugal

, a summary of the 11 articles in this Special Issue is shown, including the area of research, the topic, the type of research and the original country of submission.

Contribution 1 is an article focused on the use of autoinducers for increasing the extracellular polymer production of activated granular sludge (AGS) in wastewater treatment. Among the autoinducers normally used, N-acyl-homoserine lactone (AHL)-related compounds can promote an increase in granular sludge size and sedimentation properties. In this manuscript, the authors use N-Octanoyl-L-homoserine lactone (C8-HSL), an AHL-type compound, which aids in increasing hydrophobicity and volume of the biomass of the granular sludge, with positive effects over granulation and settling.

The article presented in contribution 2 is a review covering the biodegradation of chlorinated volatile organic compounds (CVOCs) and 1,4-dioxane in groundwater by cometabolism. Normally, CVOCs are biodegraded by dechlorination, but in groundwater, concentrations can be too low for this process. The authors propose a deep characterization of groundwaters to implement this bioremediation method.

Contribution 3 is a laboratory study based on the use of a hybrid system, a UASB reactor followed by pilot horizontal flow wetland, for slaughterhouse wastewater treatment. Total COD removal reached 88% (initial COD value: 2200 mg/L) at 7.5 days of HRT, and this hybrid process is proposed by the authors to reach national regulations in the final concentrations of pollutants in wastewater.

In contribution 4, the authors present an in-depth review about constructed wetlands technology for domestic wastewater based on the study of articles published in China and English language articles in general, in order to understand the state of the art and to propose future trends. Chinese research is focused particularly on nutrient removal (nitrogen and phosphorous), and in general, emerging pollutants like heavy metal removal and greenhouse gas emission reduction are considered in English language articles. The future trends considered are treatments at low temperature, low carbon–nitrogen ratio and high load.

Contribution 5 is an article in which the authors propose the mechanism of crude oil biodegradation by bacteria attending the fixation of cells to the crude oil drops, mass transfer of petroleum alkanes to the cell and degradation kinetics. It is considered that alkanes should be oxidized to alcohols outside the cell and transported through the cell membrane to be further oxidized to aldehydes and carboxylic acids and finally integrated into the intermediary metabolism of the cell. Mass transfer and kinetic coefficients are calculated for three oleophilic bacteria (B. licheniformis, P. putida and P. glucanolyticus), and the oleotrophic potential of these microorganisms is analyzed and discussed.

In the work presented in contribution 6, a pilot-scale bioreactor is constructed for testing the denitrification (nitrate reduction) of groundwater with corn cobs used for a carbon source and biofilm formation. The results indicate high nitrate removal (90–99%), especially for the highest HRT employed (24 h). An increase in COD was observed using corn cobs because of carbon compounds released by this material and the bacteria growth that is supposed to occur on the surface of the corn cob. This vegetable is proposed to be used for in situ application as barriers to avoid groundwater contamination by nitrates.

In contribution 7, two algae species (C. vulgaris and S. obliquus) were used for the treatment of fly ash landfill leachate, obtaining 57–59% of nitrogen removal, by mixing a low percentage of fly ash leachate (10%) with the cultures of algae. Higher ratios of fly ash leachate in culture media decreased algae growth and effectivity in nitrogen reduction. A previous sterilization of fly ash leachate did not improve results, and the optimization of culture media conditions is studied in this article.

Contribution 8 is a manuscript in which the authors isolated 20 bacteria strains (Aeromonas, Bacillus and Gordonia genera) from river sediments (San Marcos River, TX, USA) and used them for Trichloroethylene (TCE) biodegradation. These bacteria (highest TCE degraders) were identified by 16S-rRNA sequencing, and after the optimization of biodegradation conditions, high efficiencies of TCE removal (40 mg/L) were obtained, between 77 and 88% in 5 days at 25–30 °C.

In contribution 9, the authors explore the use of Azolla filiculoides (freshwater plant) for bio-hydrogen production by anaerobic dark fermentation. This biomass is pretreated by alkali treatment, sterilization and ultrasonication to favor hydrolysis of the organic material. Bio-hydrogen production was measured in 60–108 mL-H₂/g VS, and the digestate (biomass after anaerobic fermentation) was concluded to be biochar after FT-IR, EDX, SEM and TEM analysis. This material is proposed to be applied for wastewater treatment.

Contribution 10 is a manuscript in which denitrification by the Feammox process is studied in a synergistic way with nitrate-dependent Fe(II) oxidation. Fe(II) is oxidized to Fe(III) by nitrate in an organic environment, and this nitrate is reduced to molecular nitrogen (Thermomonas and Acinetobacter). Fe(III) conducts ammonia oxidation to nitrite and nitrate by nitrification (Pseudomonas) with 89.4% of maximum removal of ammonia–nitrogen. The authors propose this nitrification–denitrification sustainable process in wastewater treatment.

The review presented in contribution 11 is focused on endocrine disruptor (ED) biodegradation by fungi, especially Trametes versicolor, a white rot fungus and laccase producer with high efficiency in ED removal. EDs are present in many aquatic environments because of their resistance to wastewater treatment and have negative effects on human health.

3. Future Trends

Biological treatment of water contaminants is a modern research field focused in the removal of nutrients, metals, xenobiotics or resistant compounds, petroleum and plastic biodegradation, hydrogen production and other topics in which natural or genetically modified microorganisms are involved. Biodegradation of emerging pollutants and pharmaceutics is a serious concern due to the detection of these contaminants in natural superficial and groundwaters [8].

In nutrient removal, optimization of the circular processes Anammox and Feammox is one of the most promising trends, in order to firstly improve nitrogen use and avoid nitrogen losses and secondly to assure available Fe(III) in the iron–nitrogen cycle of the Feammox process. Further research is needed about the mechanism of electron transfer in Feammox and the way to provide enough Fe(III) in solution as an electron acceptor. If these obstacles are overcome, these sustainable nutrient removal processes can be implemented safely at an industrial scale [14,15].

Textile dyes can be biodegraded because of their condensed aromatic structured molecules by white rot fungi or by bacteria, or in different consortia. Highly efficient processes can be designed for reaching very high removal percentages close to 100% in the reduction in dye concentration [9]. The study and application of these processes could mitigate the extremely harmful pollution produced by the textile industry and present in some natural streams in the regions in which clothes and textile materials are being produced.

For the implementation of metal and metalloid removal by plant-associated bacteria in bioremediation strategies, an in-depth investigation into the metal sequestration mechanism for enhancing biosorption is required. Characterization of the bacterial communities that are useful in removing different metals in different ecosystems by phytoremediation is needed because of the diversity in metals and metalloids and their variable concentrations [1]. Inhibitory tolerance of bacteria to the different pollutants is the starting point in the association with plants for the treatment of contaminated soils [12].

Oil-degrading bacteria are ubiquitous in nature, because there are many environments in which hydrocarbons are present, but in situ bioremediation of crude oil accidental spills has often not been successful [17], due to the low bioavailability of petroleum in water and the lack of nutrients, oxygen and low temperatures in the contaminated sites [18]. The current research in petroleum bioremediation is oriented to efficient proposals of treatment and to obtaining oil-degrading bacteria or consortia which degrade crude oil quickly and completely. Normally, petroleum degradation by oleophilic bacteria takes 3–4 weeks or even more, and reducing this time to remove the pollutant is one of the main objectives [19]. The microbial mechanism of alkane biodegradation is still being discussed currently, especially in relation to the internal or external enzymatic metabolism of the cell [20]. This concern needs to be clearly elucidated in future research.

In recent articles, a close relationship between bacteria candidates for synthetic plastic and high-molecular-weight n-alkane biodegradation has been discovered [19,21]. Because most synthetic plastics are obtained from crude oil and they are polymeric forms of n-alkenes, when the polymeric structure is broken, bacteria are exposed to n-alkanes, and the enzymatic system required has to be related to alkane hydroxylases (AlkH) or other alkane-degrading enzymes. The starting point for studying synthetic plastic biodegradation could be the biodegradation of alkanes present in petroleum and bacteria able to degrade these substrates, which are considered targets in plastic biodegradation. This issue could be an intriguing and interesting challenge in the future.