Production and characterization of bioflocculants from bacteria isolated from wastewater treatment plant in South Africa

Microbial strategies for lead remediation in agricultural soils and wastewater: mechanisms, applications, and future directions

High lead (Pb) levels in agricultural soil and wastewater threaten ecosystems and organism health. Microbial remediation is a cost-effective, efficient, and eco-friendly alternative to traditional physical or chemical methods for Pb remediation. Previous research indicates that micro-organisms employ various strategies to combat Pb pollution, including biosorption, bioprecipitation, biomineralization, and bioaccumulation. This study delves into recent advancements in Pb-remediation techniques utilizing bacteria, fungi, and microalgae, elucidating their detoxification pathways and the factors that influence Pb removal through specific case studies. It investigates how bacteria immobilize Pb by generating nanoparticles that convert dissolved lead (Pb-II) into less harmful forms to mitigate its adverse impacts. Furthermore, the current review explores the molecular-level mechanisms and genetic engineering techniques through which microbes develop resistance to Pb. We outline the challenges and potential avenues for research in microbial remediation of Pb-polluted habitats, exploring the interplay between Pb and micro-organisms and their potential in Pb removal.

Microbial remediation mechanisms and applications for lead-contaminated environments

High concentrations of lead (Pb) in agricultural soil and wastewater represent a severe threat to the ecosystem and health of living organisms. Among available removal techniques, microbial remediation has attracted much attention due to its lower cost, higher efficiency, and less impact on the environment; hence, it is an effective alternative to conventional physical or chemical Pb-remediation technologies. In the present review, recent advances on the Pb-remediation mechanisms of bacteria, fungi and microalgae have been reported, as well as their detoxification pathways. Based on the previous researches, microorganisms have various remediation mechanisms to cope with Pb pollution, which are basically categorized into biosorption, bioprecipitation, biomineralization, and bioaccumulations. This paper summarizes microbial Pb-remediation mechanisms, factors affecting Pb removal, and examples of each case are described in detail. We emphatically discuss the mechanisms of microbial immobilization of Pb, which can resist toxicity by synthesizing nanoparticles to convert dissolved Pb(II) into less toxic forms. The tolerance mechanisms of microbes to Pb are discussed at the molecular level as well. Finally, we conclude the research challenges and development prospects regarding the microbial remediation of Pb-polluted environment. The current review provides insight of interaction between lead and microbes and their potential applications for Pb removal.

Extracellular polymeric substances induced cell-surface interactions facilitate bacteria transport in saturated porous media

Bacteria often respond to dynamic soil environment through the secretion of extracellular polymeric substances (EPS). The EPS modifies cell surface properties and soil pore-scale hydration status, which in turn, influences bacteria transport in soil. However, the effect of soil particle size and EPS-mediated surface properties on bacterial transport in the soil is not well understood. In this study, the simultaneous impacts of EPS and collector size on Escherichia coli (E. coli) transport and deposition in a sand column were investigated. E. coli transport experiments were carried out under steady-state flow in saturated columns packed with quartz sand with different size ranges, including 0.300-0.425 mm (sand-I), 0.212-0.300 mm (sand-II), 0.106-0.150 mm (sand-III) and 0.075-0.106 mm (sand-IV). Bacterial retention increased with decreasing sand collector size, suggesting that straining played an important role in fine-textured media. Both experiment and simulation results showed a clear drop in the retention rate of the bacterial population with the presence of additional EPS (200 mg L-1) (EPS+). The inhibited retention of cells in sand columns under EPS+ scenario was likely attributed to enhanced bacteria hydrophilicity and electrostatic repulsion between cells and sand particles as well as reduced straining. Calculations of the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) interactions energies revealed that high repulsive energy barrier existed between bacterial cells and sand particles in EPS+ environment, primarily due to high repulsive electrostatic force and Lewis acid-base force, as well as low attractive Lifshitz-van der Waals force, which retarded bacterial population deposition. Steric stabilization of EPS would also prevent the approaching of cells close to the quartz surface and thereby hinder cell attachment. This study was the first to show that EPS reduced bacterial straining in saturated porous media. These findings provide new insight into the functional effects of extrinsic EPS on bacterial transport behavior in the saturated soil environment, e.g., aquifers.

Antibiotic resistance in wastewater treatment plants: understanding the problem and future perspectives

Antibiotics residues (AR), antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARG) are a new class of water contaminants, due to their adverse effects on aquatic ecosystems and human health. Contamination of water bodies occurs mainly by the excretion of antibiotics incompletely metabolized by humans and animals and is considered the main source of contamination of antibiotics in the environment. Given the imminent threat, the World Health Organization (WHO) has categorized the spread of antibiotics as one of the top three threats to public health in the twenty-first century. The Urban Wastewater Treatment Plants (UWWTP) bring together AR, ARB, ARG, making the understanding of this peculiar environment fundamental for the investigation of technologies aimed at combating the spread of bacterial resistance. Several methodologies have been employed focusing on reducing the ARB and ARG loads of the effluents, however the reactivation of these microorganisms after the treatment is widely reported. This work aims to elucidate the role of UWWTPs in the spread of bacterial resistance, as well as to report the efforts that have been made so far and future perspectives to combat this important global problem.

Valorization of glycerol/ethanol-rich wastewater to bioflocculants: recovery, properties, and performance

Microbial extracellular polymeric substances (EPS) were produced in two membrane bioreactors, each separately treating fresh and saline synthetic wastewater (consisting of glycerol and ethanol), with the purpose of applying them as sustainable bioflocculants. The reactors were operated under nitrogen-rich (COD/N ratios of 5 and 20) and limited (COD/N ratios of 60 and 100) conditions. Under both conditions, high COD removal efficiencies of 87-96% were achieved. However, nitrogen limitation enhanced EPS production, particularly the polysaccharide fraction. The maximum EPS recovery (g EPS-COD/g COD_influent) from the fresh wastewater was 54% and 36% recovery was obtained from the saline (30 g NaCl/L) wastewater. The biopolymers had molecular weights up to 2.1 MDa and anionic charge densities of 2.3-4.7 meq/g at pH 7. Using kaolin clay suspensions, high flocculation efficiencies of 85-92% turbidity removal were achieved at EPS dosages below 0.5 mg/g clay. Interestingly, EPS produced under saline conditions proved to be better flocculants in a saline environment than the corresponding freshwater EPS in the same environment. The results demonstrate the potential of glycerol/ethanol-rich wastewater, namely biodiesel/ethanol industrial wastewater, as suitable substrates to produce EPS as effective bioflocculants.

Heavy metal bioaccumulation and cation release by growing Bacillus cereus RC-1 under culture conditions

In an effort to explore the detoxifying mechanisms of B. cereus RC-1 under heavy metal stress, the bioaccumulation by growing cells under varying range of pH, culture time and initial metal concentration were investigated from a perspective of cation release. The maximum removal efficiencies were 16.7%, 38.3%, 81.4% and 40.3% for Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺, respectively, with initial concentrations of 10 mg/L at pH 7.0. In presence of Cu²⁺ or Zn²⁺, large quantities of cations were released into the medium in descending order of Na⁺>K⁺>Ca²⁺>Mg^2+, while bioremoval of the two essential metals Cd²⁺ and Pb²⁺ was accompanied with cellular Na⁺ and Mg²⁺ uptake from the medium^, respectively. The relative mean contributions of intracellular accumulation to the total removal were approximately 19.6% for Cu²⁺, 12.8% for Zn²⁺, 51.1% for Cd²⁺, and only 4.6% for Pb²⁺. Following exposure at high concentration, B. cereus RC-1 could keep intracellular Cd²⁺ concentrations constant, possibly by means of a Cd-efflux system whose activity coincided with uptake of Na⁺, and reduce intracellular Pb²⁺ concentration due to the effect of Mg²⁺ on limiting Pb²⁺ access to the cells. Cellular morphology, surface functional groups and intracellular trace elements were further investigated by SEM-EDX, TEM-EDX, FTIR and ICP-MS analysis. The phenomena that removal of Cd²⁺ and Pb²⁺ coincided with uptake of Na⁺ and Mg²⁺, respectively, inspires a novel research perspective towards the study of protective mechanism of bacterial cells against the toxicity of heavy metals.

Antibiotic resistance in wastewater treatment plants: Tackling the black box

Wastewater is among the most important reservoirs of antibiotic resistance in urban environments. The abundance of carbon sources and other nutrients, a variety of possible electron acceptors such as oxygen or nitrate, the presence of particles onto which bacteria can adsorb, or a fairly stable pH and temperature are examples of conditions favouring the remarkable diversity of microorganisms in this peculiar habitat. The wastewater microbiome brings together bacteria of environmental, human and animal origins, many harbouring antibiotic resistance genes (ARGs). Although numerous factors contribute, mostly in a complex interplay, for shaping this microbiome, the effect of specific potential selective pressures such as antimicrobial residues or metals, is supposedly determinant to dictate the fate of antibiotic resistant bacteria (ARB) and ARGs during wastewater treatment. This paper aims to enrich the discussion on the ecology of ARB&ARGs in urban wastewater treatment plants (UWTPs), intending to serve as a guide for wastewater engineers or other professionals, who may be interested in studying or optimizing the wastewater treatment for the removal of ARB&ARGs. Fitting this aim, the paper overviews and discusses: i) aspects of the complexity of the wastewater system and/or treatment that may affect the fate of ARB&ARGs; ii) methods that can be used to explore the resistome, meaning the whole ARB&ARGs, in wastewater habitats; and iii) some frequently asked questions for which are proposed addressing modes. The paper aims at contributing to explore how ARB&ARGs behave in UWTPs having in mind that each plant is a unique system that will probably need a specific procedure to maximize ARB&ARGs removal.

Study of optimization of wastewater contaminant removal along with extracellular polymeric substances (EPS) production by a thermotolerant Bacillus sp. ISTVK1 isolated from heat shocked sewage sludge

The present work involved study of wastewater contaminant removal along with EPS production by a thermotolerant bacterium Bacillus sp. ISTVK1, isolated from heat shocked sewage sludge. EPS production in basal and mineral medium containing 50% filter sterilized wastewater and 0.5% sucrose was found to be 0.83±0.12gL(-1) and 0.31±0.10gL(-1) culture, respectively. GC-MS analysis of EPS revealed the presence of β-d-glucose, α-d-galactose and β-d-arabinose. FT-IR spectrum confirmed the presence carbohydrates. Box-Behnken design was used to optimize process parameters for enhanced EPS production along with % COD reduction of wastewater. The optimised conditions when used in a 1.5L bioreactor showed EPS production of 1.67±0.06gL(-1) culture and 93.0±0.21 % COD removal.

Implications for public health demands alternatives to inorganic and synthetic flocculants: bioflocculants as important candidates

Chemical flocculants are generally used in drinking water and wastewater treatment due to their efficacy and cost effectiveness. However, the question of their toxicity to human health and environmental pollution has been a major concern. In this article, we review the application of some chemical flocculants utilized in water treatment, and bioflocculants as a potential alternative to these chemical flocculants. To the best of our knowledge, there is no report in the literature that provides an up‐to‐date review of the relevant literature on both chemical flocculants and bioflocculants in one paper. As a result, this review paper comprehensively discussed the various chemical flocculants used in water treatment, including their advantages and disadvantages. It also gave insights into bioflocculants production, challenges, various factors influencing their flocculating efficiency and their industrial applications, as well as future research directions including improvement of bioflocculants yields and flocculating activity, and production of cation‐independent bioflocculants. The molecular biology and synthesis of bioflocculants are also discussed.

Optimisation of bioflocculant production by a biofilm forming microorganism from poultry slaughterhouse wastewater for use in poultry wastewater treatment

Poultry slaughterhouse wastewater contains nutrients that are sufficient for microbial growth; moreover, the wastewater has microorganisms which can be harnessed to perform specific functions. Additionally, these microorganisms can grow either in planktonic (free floating) mode or sessile (attached) mode. This study focused on the optimisation of bioflocculant production by quantifying flocculation activity, determined using kaolin clay (4 g/L), by isolates prevalent in poultry slaughterhouse wastewater. Subsequent to their identification and characterisation, six bacterial strains were initially isolated from the poultry wastewater. Although all the isolated microorganisms produced bioflocculants under different conditions, i.e. pH and temperature, the strain that produced bioflocculants with a higher flocculation activity was isolate BF-3, a Comamonas sp., achieving a flocculation activity of 93.8% at 32.9 °C and pH 6.5. Fourier transform infrared spectroscopy (FTIR) analysis of the bioflocculant of the isolate, showed the presence of hydroxyl, carboxyl, alkane and amine functional groups, an indication that the bioflocculant was a protein constituent.

Production and characterization of a thermostable bioflocculant from Bacillus subtilis F9, isolated from wastewater sludge

A bacterium isolated from wastewater sludge, identified as Bacillus subtilis F9, was confirmed to produce bioflocculant with excellent flocculation activity. The effects of culture conditions such as initial pH, temperature, carbon source, nitrogen source, and inoculum size on bioflocculant production were studied here. The results indicated that 2.32g/L of purified bioflocculant could be extracted with the following optimized conditions: 20gL(-1) sucrose as the carbon source, 3.5gL(-1) peptone as the nitrogen source, an initial pH of 7.0, and a temperature of 40°C. The purified bioflocculant consisted of 10.1% protein and 88.3% sugar, including 38.4% neutral sugar, 2.86% uronic acid, and 2.1% amino sugar. The neutral sugar consisted of sucrose, glucose, lactose, galactose, and mannose at a molar ratio of 2.7:4.7:3.2:9.1:0.8. Elemental analysis of the purified bioflocculant revealed that the weight fractions of carbon, hydrogen, oxygen, nitrogen, and sulfur were 30.8%, 5.3%, 54.7%, 6.4%, and 2.9%, respectively. Furthermore, the purified bioflocculant was pH tolerant within the range of 2-8 and thermotolerant from 10°C to 100°C, with optimal activity at pH 7.0 and at a temperature of 40°C. The purified bioflocculant showed industrial potential for the treatment of drinking water. Considering these properties, especially its low molecular weight (5.3×10(4)Da), this bioflocculant with excellent solubility and favorable flocculation activity is particularly suited for flocculating small particles.

Extracellular polymeric substances of bacteria and their potential environmental applications

Biopolymers are considered a potential alternative to conventional chemical polymers because of their ease of biodegradability, high efficiency, non-toxicity and non-secondary pollution. Recently, extracellular polymeric substances (EPS, biopolymers produced by the microorganisms) have been recognised by many researchers as a potential flocculent for their applications in various water, wastewater and sludge treatment processes. In this context, literature information on EPS is widely dispersed and is very scarce. Thus, this review marginalizes various studies conducted so far about EPS nature-production-recovery, properties, environmental applications and moreover, critically examines future research needs and advanced application prospective of the EPS. One of the most important aspect of chemical composition and structural details of different moieties of EPS in terms of carbohydrates, proteins, extracellular DNA, lipid and surfactants and humic substances are described. These chemical characteristics of EPS in relation to formation and properties of microbial aggregates as well as degradation of EPS in the matrix (biomass, flocs etc) are analyzed. The important engineering properties (based on structural characteristics) such as adsorption, biodegradability, hydrophilicity/hydrophobicity of EPS matrix are also discussed in details. Different aspects of EPS production process such as bacterial strain maintenance; inoculum and factors affecting EPS production were presented. The important factors affecting EPS production include growth phase, carbon and nitrogen sources and their ratio, role of other nutrients (phosphorus, micronutrients/trace elements, and vitamins), impact of pH, temperature, metals, aerobic versus anaerobic conditions and pure and mixed culture. The production of EPS in high concentration with high productivity is essential due to economic reasons. Therefore, the knowledge about all the aspects of EPS production (listed above) is highly essential to formulate a logical and scientific basis for the research and industrial activities. One of the very important issues in the production/application/biodegradation of EPS is how the EPS is extracted from the matrix or a culture broth. Moreover, EPS matrix available in different forms (crude, loosely bound, tightly bound, slime, capsular and purified) can be used as a bioflocculant material. Several chemical and physical methods for the extraction of EPS (crude form or purified form) from different sources have been analyzed and reported. There is ample information available in the literature about various EPS extraction methods. Flocculability, dewaterability and biosorption ability are the very attractive engineering properties of the EPS matrix. Recent information on important aspects of these properties qualitatively as well as quantitatively has been described. Recent information on the mechanism of flocculation mediated by EPS is presented. Potential role of EPS in sludge dewatering and biosorption phenomenon has been discussed in details. Different factors influencing the EPS ability to flocculate and dewaterability of different suspensions have been included. The factors considered for the discussion are cations, different forms of EPS, concentration of EPS, protein and carbohydrate content of EPS, molecular weight of EPS, pH of the suspension, temperature etc. These factors were selected for the study based upon their role in the flocculation and dewatering mechanism as well the most recent available literature findings on these factors. For example, only recently it has been demonstrated that there is an optimum EPS concentration for sludge flocculation/dewatering. High or low concentration of EPS can lead to destabilization of flocs. Role of EPS in environmental applications such as water treatment, wastewater flocculation and settling, colour removal from wastewater, sludge dewatering, metal removal and recovery, removal of toxic organic compounds, landfill leachate treatment, soil remediation and reclamation has been presented based on the most recent available information. However, data available on environmental application of EPS are very limited. Investigations are required for exploring the potential of field applications of EPS. Finally, the limitations in the knowledge gap are outlined and the research needs as well as future perspectives are highlighted.

Characterization and application of bioflocculant prepared by Rhodococcus erythropolis using sludge and livestock wastewater as cheap culture media

A new bioflocculant was produced by culturing Rhodococcus erythropolis in a cheap medium. When culture pH was 7.0, inoculum size was 2 % (v/v), Na2HPO4 concentration was 0.5 g L(-1), and the ratio of sludge/livestock wastewater was 7:1 (v/v), a maximum flocculating rate of 87.6 % could be achieved. Among 13 different kinds of pretreatments for sludge, the optimal one was the thermal-alkaline pretreatment. Different from a bioflocculant produced in a standard medium, this bioflocculant was effective over a wide pH range from 2 to 12 with flocculating rates higher than 98 %. Approximately, 1.6 g L(-1) of crude bioflocculant could be harvested using cold ethanol for extraction. This bioflocculant showed color removal rates up to 80 % when applied to direct and disperse dye solutions, but only 23.0 % for reactive dye solutions. Infrared spectrum showed that the bioflocculant contained functional groups such as -OH, -NH2, and -CONH2. Components in the bioflocculant consisted of 91.2 % of polysaccharides, 7.6 % of proteins, and 1.2 % of DNA. When the bioflocculant and copper sulfate (CuSO4) were used together for decolorization in actual dye wastewater, the optimum decolorization conditions were specified by the response surface methodology as pH 11, bioflocculant dosage of 40 mg/L, and CuSO4 80 mg/L, under which a decolorization rate of 93.9 % could be reached.