Integrative Strategies for Bioremediation of Environmental Contaminants, Volume 2: Avenues to a Cleaner Society

Integrative Strategies for Bioremediation of Environmental Contaminants, Volume Two: Avenues to a Cleaner Society focuses on the exploitation of various biological treatment technologies and their use to treat toxic contaminants present in industrial effluent to restore contaminated sites. The book includes coverage of combined treatments of microbes for reuse of wastewater and contaminated soil to successfully achieve eco-restoration, environment protection and sustainable development. In 14 chapters, this reference compiles current and advanced biotechnologies as well as future directions for research.

This is a valuable resource for researchers in microbiology, biotechnology, environmental engineering and environmental science, and all those who wish to broaden their knowledge in the field of applied microbiology to develop sustainable waste management.

• Provides comprehensive information on state-of-art applications of biochar, microbes and their synergistic use for wastewater/industrial effluent treatment and environment protection
• Summarizes current uses of biochar, microbes and dead biomass for dye decolorization, degradation, and removal of heavy metals which may play a key role in achieving a more productive and sustainable environment
• Explores various aspects of biological methods for contaminant removal for better insights into basic and advanced biotechnological applications

• Language: English
• Publisher: Academic Press
• Release date: Jul 29, 2023
• ISBN: 9780443140143

Book preview

Chapter 1: Bioaugmentation

an emerging ecofriendly technology for wastewater treatment

Anita V. Handore ¹ , Vinita S. Jagtap ² , Sharmila S. Ghangale ³ , Sharad R. Khandelwal ⁴ , Avinash D. Bholay ⁵ , Rajib Karmakar ⁶ , and Dilip V. Handore ⁷       ¹ Research and Development Department, Phytoelixir Pvt. Ltd., Nashik, Maharashtra, India      ² Medical Oncology Molecular Laboratory, Tata Memorial Hospital, Mumbai, Maharashtra, India      ³ Department of Biotechnology, C. K. Thakur Arts, Commerce and Science College, Mumbai, Maharashtra, India      ⁴ H.A.L. College of Science & Commerce, Nashik, Maharashtra, India      ⁵ Department of Microbiology, K.T.H.M. College, Nashik, Maharashtra, India      ⁶ Department of Agricultural Chemicals, B.C.K.V, Directorate of Research, Nadia, West Bengal, India      ⁷ Research and Development Department, Sigma Wineries Pvt. Ltd., Nashik, Maharashtra, India

Abstract

Bioaugmentation is the addition of specific and efficient pollutant biodegrading microorganisms in the polluted environment. In most of the biological and physicochemical wastewater treatments, highly difficult pollutants couldn't be biodegraded by microbes as they are recalcitrant to it and persist in the wastewater compromising the quality of water. To overcome such challenges, bioaugmentation strategy could be used as an effective, ecofriendly, and cost-effective solution. This chapter presents the overview of bioaugmentation strategy and its significance. It also focuses on various mechanisms and applications of bioaugmentation, as a tool which could be effectively used in different stressful circumstances. It also focuses its application against wastewater exclusively highlighting the microbiological aspects of bioaugmentation w.r.t. recalcitrant organic pollutants found in industrial wastewater. In addition, various limitations related to bioaugmentation strategies are also presented. Besides the crucial parameters affecting the biodegradation efficiency and potential new areas, like quorum sensing, nanotechnology, etc., utilized to exploit and improve the successful bioaugmentation of wastewater are also comprehensively discussed in this chapter.

Keywords

Bioaugmentation; Biodegradation; Ecofriendly; Quorum sensing; Wastewater

1. Introduction

Since last few decades, there is drastic progression in technological development. The resulted vigorous civilization together with industrialization, exhaustive usage of synthetic xenobiotics, heavy metals, typical pollutants including pesticides PAHs, that is, polycyclic aromatic hydrocarbons, petroleum products, organic dyes, chlorophenols, nitrophenols and their derivatives, etc., releases highly polluted wastewater causing environmental pollution and health hazards.

There is requirement to treat such polluted wastewater prior to its reuse or discharge into environment. Globally, legislations on pollutants discharge are being confined due to the awareness of pollutants and its consequences on human health and environment resulted in the development of different strategies for improving the efficiency of treatment plants.

In this context, there is a need of ecosustainable solution, leading to bioremediation of heavily polluted environment. These days application of several beneficial microbes has been emphasized as a workable biotechnological remedy, alleviating various environmental hazards and contamination due to their ability to biodegrade most of the hazardous and man-made pollutants (Charles and Anani, 2021).

In most of the biological and physicochemical wastewater treatments, extremely complex chemicals are not efficiently biodegraded by microbes as they are recalcitrant to biodegradation and subsequently persist in the wastewater compromising the water quality. To overcome such challenges, bioaugmentation strategy could be used as an effective, ecofriendly, and cost-effective solution. This process is the addition of specific and efficient pollutant biodegrading microorganisms in the polluted environment. In this, specific microbes having distinct ability to break down specific pollutants are used in place of the indigenous microbes which are not available in sufficient amount or unable to break down the chemicals. In bioaugmentation, microorganisms should not only possess degradation capacity w.r.t. particular pollutants of mobilized/immobilized inoculum state but also could survive in adverse environmental condition (Simarro et al., 2013). This process is influenced by the relationship between indigenous and exogenous microbial population (Charles and Anani, 2021).

Following are some important characters of microorganisms necessary for bioaugmentation.

1.1. Need of bioaugmentation

With the intention of establishment of bioreactors for meritorious treatment of chemical mixed wastes, it is necessary to maintain the important consortia of microbes having sufficient activity w.r.t degradation of pollutants in waste. Microbial populations which could be able to colonize such tough niches with preferred catabolic characters and support to develop particular inocula exploiting by process of bioaugmentation are required. In bioaugmentation, microbial strains could fail to grow or to be active in the bioreactor due to predation or competition with indigenous microbes/bacteriophages or due to failure to acclimatize environmental conditions. In such a dynamic context, the introduced microbes may not colonize or show any population dominance w.r.t variation in pH, temperatures, starvation periods, etc. Microbial degradation of pollutants could be increased by bio stimulation.

1.2. Significance of bioaugmentation

Basically, bioaugmentation works by increasing the level of active microorganisms within the treatment environment. These microbes work to transform the pollutants into less harmful chemicals, but the microbes could handle such transformation at certain level. If the ratio of active microbes to detrimental pollutant is not optimal, then the process efficiency will be prominently reduced resulting in the leftover of harmful compounds even after the treatment phase.

Bioaugmentation process is the targeted process in which measured as well as scientific approach could give opportunity for getting more from pushing the effectiveness w.r.t. cleaning of pollutant even optimizing the costs and results. Wastewater-activated sludge contains naturally existing microbes that could biodegrade numerous pollutants, but due to high toxicity, low water solubility, low bioavailability, high stability, and low biodegradability, some microbes show resistance toward biodegradation. In this situation, microbial metabolic enzymes may not be efficiently used as substrates. However, consortia of different microorganisms could help for their appropriate biodegradation. In numerous cases, recalcitrant chemicals might be new; therefore, microorganisms couldn't adapt themselves to use them as a substrate (Providenti et al., 1993). Bioaugmentation process can overcome such challenges, as this treatment could be tailored w.r.t. certain pollutant showing dominance in environment. Accordingly, this approach is effective to address both the increasing number of new pollutants and contaminant which are available at very high concentrations.

1.3. Factors affecting bioaugmentation efficiency

1.3.1. Selection of microorganisms for bioaugmentation consortium

In bioaugmentation process, it is reported that the microbial strain selection is an important aspect as successful bioaugmentation is predominantly influenced by the behavior of bioaugmentation enrichment culture, comprising metabolic adaptation, good survival, and retention in biosystems (Singer et al., 2005). In this way, effective application of bioaugmentation depends on the acclimatization of coexistence of bioaugmentation microbes to indigenous microbial community. The interactions of bioaugmentation microbes with operation conditions are very critical w.r.t. development and survival of bioaugmentation microorganisms. Also, the interactions of bioaugmentation microbes with native predating/competing microbes are significant factors affecting bioaugmentation.

1.3.2. Bioaugmentation dosage

Bioaugmentation plays a key role w.r.t. improving overall operational performance in biosystems. Mostly, amplified inoculum quantity could be used to avoid washout from the reactors. However, excessive inoculation of specific strains could break down the structure and function of indigenous microbial community. The selection of bioaugmentation dose is restricted by certain operational factors, such as seeding source, yield of parent reactor supplying bioaugmentation consortium, possibility of transferring of bioaugmentation biomass, as well as cost–benefit ratio. Usually, higher dose of bioaugmentation shows advantages to operation performance. However, regulated bioaugmentation dosage has been desired for economic benefit.

1.3.3. Bioaugmentation time

Bioaugmentation effects are found to be determined by timely bioaugmentation. It can be explained with the example that with the sudden oxytetracycline shock treatment, operation performance of five reactors, that is, R0, R1, to R4, in which, R1 has been bioaugmented before shock, R2 has been bioaugmented during the shock, R3 has been bioaugmented after shock, and R4 has been bioaugmented during and after shock. During the performance recovery tests, it was observed that R0, R1, R2, R3, and R4 required 45, 38, 38, 60, and 38 h, respectively, indicating that effects of bioaugmentation get affected by bioaugmentation time (Jin et al., 2014).

1.3.4. Bioaugmentation times and frequency

In spite of significant increase in operation performance, positive impacts of bioaugmentation could last for a less period. Consequently, quantification of bioaugmentation time has to be implemented for ensuring its success under severe stresses. It is reported that there is substantial reduction in the removal of p-nitrophenol on the initial day of bioaugmentation; however, similar shock intensity couldn't influence the removal performance in successive transient shock. Thus, bioaugmentation times and frequency should be considered. It is reported that the performance couldn't get influenced by any organic matter shock during long-term bioaugmentation application in Anammox sequencing batch reactor (SBR) (Martin-Hernandez et al., 2012; Tang et al., 2014).

1.3.5. Bioaugmentation interval

Optimal bioaugmentation interval between two bioaugmentation is very important w.r.t. economic cost. It is reported that the utmost effect of one-time bioaugmentation depends on dosage of bioaugmentation and decays due to losing of bioaugmented nitrifiers. Besides, the addition of bioaugmentation dosage by every day could result in long-term satisfactory performance. However, sufficient large bioaugmentation dosage is necessary. Besides successful bioaugmentation, total system management including the controlling the crucial factors like oxygen levels, pH, temperature, etc., is required.

1.4. Advantages of bioaugmentation

In several areas, bioaugmentation has been proved as beneficial. It increases the diversity and numbers of microbes for achieving the expected results. Numerous industrial waste plants face complications to achieve nitrification/toxic shocks. However, by regular addition of nitrifying microbes, appropriate population w.r.t. removal of ammonia could be maintained. Moreover, it could help to reduce odor. Due to strict environmental restrictions, various waste treatment plants face various compliances and challenge the capabilities of existing wastewater treatment plants. Sometime, bioaugmentation could also be the long-term solution due to the lack of capital investments of mechanical solutions. Moreover, other areas where bioaugmentation could be beneficial include odor reduction, grease and oil removal, rapid startup, and amended tolerance to toxic shocks. Besides, extensive research could endure exploration of new application areas.

Several areas where bioaugmentation has prevent beneficial are discussed below.

1.4.1. Amended solids settling

Solid removal is an important step in biological waste treatment commonly by settling in a clarifier or pond lagoon. Bacteria or other microbes naturally form a biopolymer to support in settling. Poisonous shocks and system alterations could be resulted in microbial population with minute biopolymer and poor settling properties. Although, the traditional method of addition of biological polymer or coagulants as settling aids is effective. Demand of polymer could be reduced greatly or eliminated by inoculating the system with organisms recognized as resistant to toxicity and outstanding flock formers.

1.4.2. Degradation of particular compounds

Low levels of specific compounds could be achieved by the use of particular microbes. Compounds like chlorinated aromatic hydrocarbons, phenols, etc., could also be reduced with bioaugmentation.

1.4.3. Improved nitrification

Numerous industrial waste plants have problems to achieve nitrification/toxic shock. However, due to frequent addition of nitrifying bacteria, proper microbial population for removal of ammonia could be retained.

1.4.4. Controlling level of fecal coliform

There are few microbial accelerators (MAs) which could control the Escherichia coli level in effluent or sewage without using bleaching powder or chlorine. There are some specific microorganisms that could synthesize normal strains to harmless strains.

1.4.5. Benefits to businesses

1.4.5.1. To optimize cost

Bioaugmentation is a cost-effective process than other wastewater treating methods. It costs only for sourcing/cultivating particular microbial culture.

1.4.5.2. Regulatory compliances

Bioaugmentation process is normally reinforced by regulators across different authorities. Regulators could apply various metrics as they could control viability of treatment process, including the level of risk which the harmful chemicals may be left over, stability and sustainability toward the environment, and the level of disruption which the treatment causes in the local community.

1.4.5.3. Social acceptance

In any businesses, there is a need to maintain a good association with local public by decreasing noise and environmental distraction along with the confirmation of safe and accountable wastewater disposal. Therefore, bioaugmentation could help to achieve the positive relationship.

1.4.5.4. Carbon footprint reduction

During bioaugmentation, there is no need to waste energy for heating the reactor or to give any treatment against environment nor the related businesses need any high levels of pressure for achieving the desired treatment results. It helps to optimize the cost as well as secure ongoing environmental sustainability and thereby reduces carbon footprint.

1.4.5.5. Efficient contaminant treatment

Bioaugmentation treatment could certainly play a key role in ecosustainable businesses. It is a very effective contaminant treatment w.r.t. scientific approach which could efficiently support the businesses for targeting environmental sustainability and stability in economic sense. However, it needs appropriate research and understanding.

1.4.5.6. Microbial selection criteria

The application of commercially useful products, potent microbial culture, indigenous-exogenous strains/tailor-made consortia have been proved as useful bioaugmentation options in which the main strategy has based on the isolation of natural environmental samples, selection of efficient microbial strains, selection followed by the use of mutagenic agents, and fermentation tests for single strains using traditional enrichment techniques. Adaptation w.r.t. scaling up is an extremely important phase for effective bioaugmentation. At this key stage, complexity of engineered wastewater biotreatment needs specific efforts.

Priming is predisposing the microbial population to future conditions carried out to perform a specific function. Accordingly, acclimation of microbial consortia must be considered in the process flow chart for predisposing them for better physicochemical conditions as well as to achieve efficient performance.

It is reported that microbes used in this process should be catabolically able to degrade the pollutant, even in the presence of other potentially inhibitory pollutants; they must survive and remain competitive after their introduction into the biosystem, and they should be compatible with the indigenous microbial populations. Therefore, the selection of microorganisms should be carefully carried out, because very few organisms may be appropriate for bioaugmentation, and it is not necessarily from the community needing bioaugmentation (Yu and Mohn, 2002). Besides, the selected microbes should not be closely related to human pathogens while using in field operation. It has been proposed that significant progress should be carried out by developing current information w.r.t. recent techniques in molecular biology, worldwide access to culture collections, etc., simultaneously, updating the information from accessible research supporting the effective way of overcoming barriers in bioaugmentation ((Singer et al., 2005) (Table 1.1).

Table 1.1

Adapted from Charles, O.A., Anani, A., 2021. Bio augmentation-A powerful biotechnological techniques for sustainable eco restoration of soil and groundwater contaminants. Microorg. Sustain. 1, 373–398. ISBN: 978-981-15-7447-4 (eBook). https://doi.org/10.1007/978-981-15-7447-4 and Alexis, N., Shaikh, A.R., Jesse, Z., 2016. Bioaugmentation-an emerging strategy of industrial wastewater treatment for reuse and discharge. Int. J. Environ. Res. Publ. Health. 13 (9), 1–20.

1.5. Bioaugmentation products

These days, various ready-to-use bioaugmentation products made up with MAs (bacterial/fungal strains) are available in market. These products have diverse applications in industrial, agricultural, and consumer field wastewater treatment and do not require any special storage. As soon as these are introduced into a polluted area, the microbes could immediately receive and being feeding and reproducing while attacking the material in the water causing the pollution.

Basically, the microorganisms are isolated from nature and mostly selected w.r.t. their accelerated reproduction rates as well as potential to perform particular function, for example, good floc forming abilities to augment settling/capability to degrade particular compound, etc. These products have been sold in various forms, for example, bran carrier and liquid products, dried organisms, etc. These natural products are particularly nonpathogenic and nontoxic that are pure and safe for plant, animal, human, aquatic, and marine life. These are compatible with the indigenous microbial populations which could work synergistically with microbes, increases their ability by creating a competitive environment. These products could efficiently stabilize the complex processes in effluent treatment plants (ETPs) and sewage treatment plants (STPs) and optimize efficacy in special reactors with/without activated sludge.

These products could be used in the fermenting and anoxidial process. It works aerobically/anaerobically with or without light and works in a wide range of temperature and pH value tolerance.

These could be efficiently used for food processing industries, fruit and pulp processors, beverages and bottling plants, milk, dairy and cheese processing plants, ice-cream plants, pharmaceutical industries, meat processing industries, paper and pulp mills, sugar industries, distilleries, wine and alcohol producing units, solvent extraction plants, paints and resin industries, town and municipalities, STPs, lagoons, lakes and ponds, city drains, zoo, textile units (using organic dyes), military bases, refineries, petrochemical industries, hotels and resorts, golf clubs (for their ponds), fish farming and aquaculture units, and any industry that produces organic wastewater/effluent.

1.6. Reasons for failures/limitations of bioaugmentation technologies

Although the concept of bioaugmentation w.r.t. wastewater treatment has been extensively studied with encouraging results at laboratory scale, till date success has not been turned to full-scale wastewater treatment. It was found that due to various biotic and abiotic stresses, number of exogenous microorganisms decreases very shortly after addition to a site. The reasons include deficient substrates, variations in temperature, pH, nutrient limitation, struggle between introduced microbes and indigenous microbes, phase infections, pollutant load shock, protozoa grazing, and some factors allied with quorum sensing (Alexis et al., 2016).

Till date, various reasons have been proposed by different researchers w.r.t. the fiasco of inocula used for bioaugmentation strategy (Alexis et al., 2016). The success of such strategy is influenced by the interactions between the inoculated microbes and new biological as well as nonbiological environments w.r.t. their survival, activity. During bioaugmentation, in spite of substantial increase in bioreactor performance, positive effects are found for a short duration after the inoculation of microorganisms.

According to few reports, the failures might be due to selecting bioaugmentation in the initial place, irrespective to the fact that indigenous microbial community may possess suitable catabolic genes for the degradation of targeted compounds. Therefore, the effectiveness of reported results w.r.t. bioaugmentation might frequently be questionable. As the microbes of similar genres are not equally fit for specific chores, and few might be competitive under a broad range of conditions, whereas remaining microbes may be suited for particular conditions.

In this context, species-level variation, even among the microbes exhibiting the identical catabolic characters, has been stated as a failure reason, alongside alterations at strain level w.r.t. tenacity in system followed by microbial inoculation leading to efficient bioaugmentation designs. Most often, the problems come across with regard to the limiting growth conditions resulted because of the low concentration of substrate; occurrence of inhibitory substances in the stream and even secreted by other microbes showing antagonistic effects like bacteriocin, antibiotics, poor biofilm formation, occurrence of bacteriophages, etc. (Fu et al., 2009). Generally, bioaugmentation has been correlated with population of consortia of bioaugmentation and disturbance of ecosystem equilibrium. In this context, hypotheses for bioaugmentation failure have been proposed as:

(i) Concentration of pollutant in targeted environment may be too low to support the metabolism of bioaugmentation bacteria.

(ii) Microbial inhibitors in environment may control the growth of bioaugmentation consortium.

(iii) Growth rate of bioaugmentation microbes may be slower than out rate due to predation/washout.

In fact, the failure rate of bioaugmentation may be high due to the complicated interactions of microbial inoculants with their growth environment (Alexis et al., 2016; Liu et al., 2015; Wu et al., 2005).

1.7. Promising strategies—mechanisms for efficient bioaugmentation

Before the application of bioaugmentation under actual conditions, study relating to the ability of bioaugmented microorganisms w.r.t. their growth efficiency in tested environment, inoculum size should be considered. During the bioaugmentation of groundwater, an inoculum of 10⁶–10⁷ cells/mL bacterial concentration is recommended. It is reported that the infections of bacteriophages could significantly affect the bioaugmented bacterial growth. Also, till date no sustainable strategies exist w.r.t. removal of bacteriophages from wastewater. However, before initiating a bioaugmentation, the ability of bacteria to grow in the new environment should be recognized. Various methods may be used to monitor microbial growth, including plating, the most probable number, polymerase chain reaction, that is, PCR, quantitative PCR, that is, qPCR, and microarray. During monitoring of groundwater treatment, if bacterial concentration falls down below 10⁶ cells/mL, its necessary to add new inocula for maintaining the bioaugmentation efficiency.

1.7.1. Improvement of detrimental conditions

In the anammox, that is, anaerobic ammonium oxidation system, bioaugmentation support increased in biomass with high speed by creating favorable environment. It also increased the amount of required microbes by exhibiting few growth factors necessary for functional microbes. In case of shock stress bioaugmentation-activated sludge system with particular strains of microbes, there is rapid reduction in toxicity on microbial community.

The rate of degradation toxic pollutants gets augmented by bioaugmentation in biosystems exposed to the shock of toxic substance. Consequently, indigenous bacterial culture along with other microbes get protected from toxic shocks, resulted in fast recovery of microbial activity. Bioaugmentation not only ensures optimum growth and activity of various microorganisms but also resulted in better sludge characteristics and better nitrification/denitrification (Zhang et al., 2017).

1.7.2. Bioaugmentation consortium and their retention, adaptation, and reactivation

During bioaugmentation, bacteria/other microbes go through significant criteria like persistent activity and compatibility before reflecting effects of bioaugmentation. Thereafter, steady treatment competency could be attained over long-term addition of bioaugmentation consortium. Successively, amount and activity of these microbes are found to enhance. Thus, the amplification of bioaugmentation consortium with improved activity is the principal cause behind better performance (Zhang et al., 2017).

1.7.3. Quorum-sensing effect

Biofilms, that is, structures where bacterial communities are attached through self-produced hydrated polymeric matrix formation is mediated by a quorum sensing. During this process, microbes like bacteria release chemical signals for bacteria–bacteria communication which leads to biofilm formation.

Such variation of whole bacterial community structures might be resulted from quorum-sensing ability of bioaugmentation consortium. Accordingly, in the biological systems, the bioaugmentation bacteria predominantly change the structure of entire bacterial community (Zhang et al., 2017). Besides, modulation of quorum-sensing could support the advancement of bioaugmentation w.r.t. wastewater treatment by regulating the microbial population dynamics in the bioreactors and help to open up the route for modified efficiency of bioaugmentation in the treatment of wastewater (Alexis et al., 2016).

1.7.4. Assessment of risk

It is found that sometime, stress is caused due to lack of nutrients, adverse environmental conditions such as competition between microbes, very less temperature, shock by toxic chemicals, etc. In this situation, it's become very hard to the bioaugmentation microbes for keeping a foothold, retentiveness, growth, and become dominant. In this context, it is required to carry out risk assessment with its optimization strategies. Thus, the detail assessment needs to be performed in order to reduce the bioaugmentation risk for optimization of bioaugmentation effects in future (Zhang et al., 2017).

1.7.5. Bioaugmentation and cell immobilization

Immobilization is the encapsulation of microorganisms allied with higher concentration of biomass. Their boosted cell survival could be used to overcome limitations/failure of bioaugmentation. It helps to protect against protozoa grazing, bacteriophage infections; increases biological/physical stability by reducing the challenges w.r.t. sudden and brief variations of temperature/pH; and protects from inhibitory effect of toxic substances including heavy metals and increase of shear stress (Alexis et al., 2016).

1.7.6. Utilization of gene transfer/genetically modified microorganisms

Bioaugmentation advancement could be carried out by using genetically modified microbes. Although, this method might be risky for environmental equilibrium of microorganisms. In this context, several countries are approving restrictive legislations against their widespread use in the environment (Alexis et al., 2016).

1.7.7. Plasmid-mediated bioaugmentation

In nature, microbes/bacteria could acquire new-fangled catabolic function by getting gene encoding catabolic enzyme from closely/distant-related microorganism or bacteria, through plasmids or transposons. In this context, bioaugmentation improvement could be carried out by horizontal gene transfer among microbes/bacteria. To carry out this process, there is need to use donor microbe or bacteria having plasmids of interest which could be mixed and cultured with recipient microbes like bacteria. Once the transfer occurs, the recipient microorganisms/bacteria become trans-conjugants by acquiring new catabolic biochemical processes.

However, sometime, this method fails due to incompetence of donors for persisting into the environment, incompetence of plasmid transfer in the recipient microbes like bacteria, less number of donor/recipient microbes, and decreased stability of plasmids once in the recipient microbe/bacteria. Therefore, understanding the genetic characters of recipient microbes including bacteria w.r.t. said biological events is essential to improve such plasmid-bioaugmentation approach for wastewater treatment. In this context, new tools such as metabolomics, metagenomics, transcriptomics, proteomics, etc., are needed for successful bioaugmentation (Alexis et al., 2016; Blakely, 2015; Thomas and Nielsen, 2005; McClure et al., 1989; Hausner et al., 2011; Top et al., 2002) (Table 1.2).

1.8. Applications of bioaugmentation

1.8.1. Removal of chlorinated/fluorinated compounds

Globally, there is extensive use of fluorinated as well as chlorinated solvents in both industry and home causing water contamination. In this regard, bioaugmentation has been proved as an important strategy for their removal. At laboratory scale, Comamonas testosteroni, Acinetobacter spp., etc., show the ability to biodegrade 4-fluoroaniline and 3-chloroaniline in synthetic wastewater medium added with activated sludge (Wang et al., 2013b).

Table 1.2

1.8.2. Lignin removal

Paper and pulp industry produces large amount of wastewater, that is, black liquor containing polysaccharides, lignin, resinous compounds, etc. During the wastewater treatment, natural biological treatment with activated sludge fails to reduce/eliminate these compounds efficiently due to the absence of microbes having ability to degrade lignin (Wu et al., 2005). Accordingly, selection and supplementation of such lignin-biodegraders into wastewater could provide remarkable strategy for the removal of specific pollutants originated from black liquor. It is reported that in a laboratory set-up consisting of an SBR, consortia of such lingo-cellulose-biodegrading microbes isolated from activated sludge in the SBR could enhance the reduction of lignin (>50%) (Zheng et al., 2013).

1.8.3. Pyridine and quinoline

Pyridine and quinoline solvents are used for paints, dyes, and wood treatment chemicals, thereby found in the industrial wastewater. It is reported that there is increase in biodegradation of quinoline and pyridine by using Bacillus spp., Burkholderia pickettii, Paracoccus spp., Pseudomonas spp., respectively (Tuo et al., 2012; Jianlong et al., 2002; Bai et al., 2009).

1.8.4. Synthetic dyes

Man-made dyes, which consisting azo-/anthraquinone-based molecules, are extensively used in textile/cosmetics industries. Basically, azo dyes are found to be the largest diverse group of dyes and are generally resistant to biodegradation with conventional activated sludge treatment (Pandey et al., 2007). It is reported that azo dye/Acid Orange-7 can be successfully removed with Shewanella sp. (Wang et al., 2012). Also at laboratory scale, it was found that the strain of the Sphingomonas spp., Sphingomonas xenophaga could successfully remove Bromoamine acid in bioaugmentation studies with synthetic wastewater medium (Qu et al., 2006, 2009).

1.8.5. Cyanides

It is reported that the use of Cryptococcus humicolus, that is, cyanide-degrading yeast and other unidentified cyanide degraders could remove ferric cyanide in wastewater at laboratory scale (Park et al., 2008).

1.8.6. Nicotine

Pollutants including nicotine compounds could be efficiently removed by using bioaugmentation strategy. Numerous bacterial species are showing ability to degrade nicotine, for example, Acinetobacter spp., Sphingomonas spp., Acinetobacter spp., etc. Besides, strain, Pseudomonas spp. HF-1, in an SBR could be used for the treatment of tobacco wastewater (Wang et al., 2011; Wang et al., 2013a; Wang et al., 2009).

1.8.7. Glycol ethers

Glycol ethers like ethylene glycol monobutyl ether and diethylene glycol monobutyl ether are polar solvents which are easily miscible with organic chemicals as well as water, thereby used in paints and cleaners. These compounds show toxicity in animal models. As they are recalcitrant to biodegradation, they are found to be accumulated in the environment. Strain of Serratia spp. shows potential to remove diethylene glycol monobutyl ether during bioaugmentation of contaminated wastewater both at laboratory as well as full-scale (Sitarek et al., 2012; Chen et al., 2016).

1.8.8. PAHs—polycyclic aromatic hydrocarbons

PAHs are primarily found in petroleum products and in the waste streams from various industrial processes like coal conversion and synthesis of organic chemicals. It is reported that the use of strains of Streptomyces spp. in a membrane bioreactor showed a noteworthy removal of naphthalene. Besides phenol, pyridine, quinoline, carbazole, dibenzofuran, dibenzothiophene, etc., present in the coking wastewater could be effectively removed by bioaugmentation with consortia of Paracoccus denitrificans, Pseudomonas sp, and Arthrobacter sp. both free and immobilized (Xu et al., 2015; Zhu et al., 2015; Shi et al., 2015).

Besides these applications, bioaugmentation is beneficial for the removal of enhanced biochemical oxygen demand (BOD), and it can be effectively used for the improvement of solid setting as it provides an overall healthier biomass. It can efficiently improve nitrification by regularly adding nitrifying bacteria. Moreover, other diverse areas where bioaugmentation strategy offers benefits include reduction of odor, removal of oil and grease, rapid system start-up, and improved tolerance to toxic shock. Moreover, extensive research has been continued for the exploration of new application areas for this progressing technology (Yanbo et al., 2021).

1.9. Future prospects of bioaugmentation for wastewater treatment

In upcoming era, it's necessary to fill up the crucial gaps in the addressed limited research for successful application of bioaugmentation. There is need to operate parallel studies with control and test process streams.

In this context, following are the future prospects:

• Two stages bioaugmentation reactor with first stage parent reactor for supplying bioaugmentation bacteria with timeliness, promptness, automaticity, etc., are necessary.

• Studies w.r.t. optimization of bioaugmentation parameters, for example, consortium, time of dosage, frequency, interval, interaction of bioaugmentation microbes with the indigenous species, quality of wastewater, configuration of reactor, and operation conditions are very critical for appropriate bioprocessing.

• Exact quantification of beneficial or adverse impacts of bioaugmentation on the bioprocess are still discreet. Hence, suitable model to evaluate its outcome should be developed.

• Further study related to real-time tracking molecular technique to monitor structure of microbial community, for example, analyzing the dominant microbes like bacteria in the bioaugmentation systems is needed.

• Technology w.r.t. sidestepping the loss of biomass during bioaugmentation process is urgently needed.

• Further studies related to the selection of proper bioaugmentation consortia, appropriate duration, and in what manner to employ bioaugmentation for successful adaptation under treatment stress are required.

• Progress related to understanding the degradation pathways and finding a suitable microbial species for the reduction of particular compound is needed.

• The synergistic effect of consortium as well as the collective action of a biosurfactant and a preadapted consortia could support the degradation of wastewaters having PAHs.

• Progresses in genetical study w.r.t. the development of suitable strains for targeting the xenobiotic compounds having limitation for elimination are needed.

• Detail understanding w.r.t. treatment process conditions, environmental factors, and location for introduction of the microbial strain for reducing its exposure to undesirable conditions might help to provide possible solutions.

• A close interaction between engineering and biotechnology approaches is needed for promoting the toxicity removal efficiency at higher level. These days, advent of membrane reactors in wastewater treatment plants and the capacity to keep the consortia in bioreactor have been considerably increased leading to improved performance as well as comparatively better quality of effluents. In this context, emerging areas of research will be helpful for developing innovative technologies for novel applications in bioaugmented