Upgrading the hydrolytic potential of immobilized bacterial pretreatment to boost biogas production

Recent advancements on antibiotic bioremediation in wastewaters with a focus on algae: an overview

Antibiotic contamination from hospitals, animal husbandry, and municipal wastewater is graver than imagined, and it possess serious risks to the health of humans and animals, with the emergence of multidrug resistant bacteria; those affect the growth of higher plants too. Conventional wastewater treatment methods adopted today are inadequate for removing antibiotics from wastewater. Intuitively, the remediation process using mixed algae should be effective enough, for which algae-based remediation technologies have emerged as sustainable remedial methods. This review summarized the detection of antibiotics in field water in most countries; a comprehensive overview of algae-based technologies, algal adsorption, accumulation, biodegradation, photodegradation, hydrolysis, and the use of algae-bacteria consortia for the remediation of antibiotics in wastewaters in done. Green algae namely, Chlamydomonas sp., Chlorella sp., C. vulgaris, Spyrogira sp. Scenedesmus quadricauda, S. obliquus, S. dimorphus, Haematoccus pluvialis, and Nannochlopsis sp., had been reporting have 90-100% antibiotic removal efficiency. The integration of bioelectrochemical systems and genetically engineered prokaryotic algal species offer promising avenues for improving antibiotic removal in the future. Overall, this review highlights the need for tenacious research and development of algae-based technologies to reduce antibiotic contamination in aquatic environments, for holistic good.

Comparison of different sewage sludge pretreatment technologies for improving sludge solubilization and anaerobic digestion efficiency: A comprehensive review

Anaerobic digestion (AD) of sewage sludge reduces organic solids and produces methane, but the complex nature of sludge, especially the difficulty in solubilization, limits AD efficiency. Pretreatments, by destroying sludge structure and promoting disintegration and hydrolysis, are valuable strategies to enhance AD performance. There is a plethora of reviews on sludge pretreatments, however, quantitative comparisons from multiple perspectives across different pretreatments remain scarce. This review categorized various pretreatments into three groups: Physical (ultrasonic, microwave, thermal hydrolysis, electric decomposition, and high pressure homogenization), chemical (acid, alkali, Fenton, calcium peroxide, and ozone), and biological (microaeration, exogenous bacteria, and exogenous hydrolase) pretreatments. The optimal conditions of various pretreatments and their impacts on enhancing AD efficiency were summarized; the effects of different pretreatments on microbial community in the AD system were comprehensively compared. The quantitative comparison based on dissolution degree of COD (DDCOD) indicted that the sludge solubilization performance is in the order of physical, chemical, and biological pretreatments, although with each below 40 % DDCOD. Biological pretreatment, particularly microaeration and exogenous bacteria, excel in AD enhancement. Pretreatments alter microbial ecology, favoring Firmicutes and Methanosaeta (acetotrophic methanogens) over Proteobacteria and Methanobacterium (hydrogenotrophic methanogens). Most pretreatments have unfavorable energy and economic outcomes, with electric decomposition and microaeration being exceptions. On the basis of the overview of the above pretreatments, a full energy and economy assessment for sewage sludge treatment was suggested. Finally, challenges associated with sludge pretreatments and AD were analyzed, and future research directions were proposed. This review may broaden comprehension of sludge pretreatments and AD, and provide an objective basis for the selection of sludge pretreatment technologies.

Stimulating effect of hydrostatic pressure on ultrasonic sewage sludge treatment for COD solubilization and methane production

The performance of ultrasonic treatment was assessed under different hydrostatic pressures for two different waste activated sludges. The impact of pressurized sonication was evaluated based on the degree of disintegration (DD_COD) and the specific methane yield (SMY). An enhancement of DD_COD was observed at an intermediate pressure level (max. 53% at 1.0 bar), but at higher pressure levels (up to 3.5 bar), the enhancement was not as pronounced as for the intermediate ranges (max. 11%). In contrast to DD_COD, SMY increased with increasing pressure for both sludge samples tested (max 17% at 3.5 bar) so that SMY did not show a notable correlation with DD_COD. A positive energy balance (max. 167%) considering the energy input for ultrasonication and the additional methane generated was only achieved in samples treated under elevated hydrostatic pressure. Since this can be achieved with negligible effort, the enhancement can be considered as "methane for free".

Current advances and future outlook on pretreatment techniques to enhance biosolids disintegration and anaerobic digestion: A critical review

Waste activated sludge (biosolids) treatment is intensely a major problem around the globe. Anaerobic treatment is indeed a fundamental and most popular approach to convert organic wastes into bioenergy, which could be used as a carbon-neutral renewable and clean energy thus eradicating pathogens and eliminating odor. Due to the sheer intricate biosolid matrix (such as exopolymeric substances) and rigid cell structure, hydrolysis becomes a rate-limiting phase. Numerous different pretreatment strategies were proposed to hasten this rate-limiting hydrolysis and enhance the productivity of anaerobic digestion. This study discusses an overview of previous scientific advances in pretreatment options for enhancing biogas production. In addition, the limitations addressed along with the effects of inhibitors in biosolids towards biogas production and strategies to overcome discussed. This review elaborated the cost analysis of various pretreatment methods towards the scale-up process. This review abridges the existing research on augmenting AD efficacy by recognizing the associated knowledge gaps and suggesting future research.

Waste-to-energy nexus: A sustainable development

An upsurge in global population due to speedy urbanization and industrialization is facing significant challenges such as rising energy-demand, enormous waste-generation and environmental deterioration. The waste-to-energy nexus based on the 5R principle (Reduce, Reuse, Recycle, Recovery, and Restore) is of paramount importance in solving these Gordian knots. This review essentially concentrates on latest advancements in the field of 'simultaneous waste reduction and energy production' technologies. The waste-to-energy approaches (thermal and biochemical) for energy production from the agricultural residues are comprehensively discussed in terms environmental, techno-economic, and policy analysis. The review will assess the loopholes in order to come up with more sophisticated technologies that are not only eco-friendly and cost-effective, but also socially viable. The waste-to-energy nexus as a paradigm for sustainable development of restoring waste is critically discussed considering future advancement plans and agendas of the policy-makers.

Sustainable environmental management and related biofuel technologies

Environmental sustainability criteria and rising energy demands, exhaustion of conventional resources of energy followed by environmental degradation due to abrupt climate changes have shifted the attention of scientists to seek renewable sources of green and clean energy for sustainable development. Bioenergy is an excellent alternative since it can be applied for several energy-requirements after utilizing suitable conversion methodology. This review elucidates all aspects of biofuels (bioethanol, biodiesel, and butanol) and their sustainability criteria. The principal focus is on the latest developments in biofuel production chiefly stressing on the role of nanotechnology. A plethora of investigations regarding the emerging techniques for process improvement like integration methods, less energy-intensive distillation techniques, and bioengineering of microorganisms are discussed. This can assist in making biofuel-production in a real-world market more economically and environmentally viable.

Enzymatic pretreatment to enhance anaerobic bioconversion of high strength wastewater to biogas: A review

Oil and grease, carbohydrate, protein, and lignin are the main constituents of high strength wastewaters such as dairy wastewater, cheese whey wastewater, distillery wastewater, pulp and paper mill wastewater, and slaughterhouse wastewaters. These constituents have contributed to various operational problems faced by the high-rate anaerobic bioreactor (HRAB). During the hydrolysis stage of anaerobic digestion (AD), these constituents can be hydrolyzed. Since hydrolysis is known to be the rate-limiting step of AD, the overall AD can be enhanced by improving the hydrolysis stage. This can be done by introducing pretreatment that targets the degradation of these constituents. This review mainly focuses on the biological pretreatment on various high-strength wastewaters by using different types of enzymes namely lipase, amylase, protease, and ligninolytic enzymes which are responsible for catalyzing the degradation of oil and grease, carbohydrate, protein, and lignin respectively. This review provides a summary of enzymatic systems involved in enhancing the hydrolysis stage and consequently improve biogas production. The results show that the use of enzymes improves the biogas production in the range of 7 to 76%. Though these improvements are highly dependent on the operating conditions of pretreatment and the types of substrates. Therefore, the critical parameters that would affect the effectiveness of pretreatment are also discussed. This review paper will serve as a useful piece of information to those industries that face difficulties in treating their high-strength wastewaters for the appropriate process, equipment selection, and design of an anaerobic enzymatic system. However, more intensive studies on the optimum operating conditions of pretreatment in a larger-scale and synergistic effects between enzymes are necessary to make the enzymatic pretreatment economically feasible.

Feasibility analysis of homogenizer coupled solar photo Fenton process for waste activated sludge reduction

In this study, an attempt has been made to reduce the sludge using novel homogenizer coupled solar photo Fenton (HPF) process. At an optimum pH of 3 and Fe²⁺ to H₂O₂ dosage of 1:6, PF process yielded 63.7% solids reduction at a time interval of 45 min. Coupling of homogenizers with photo Fenton (PF) process effectively enhanced treatment efficiency. When homogenizer (specific energy - 1150.694 kJ/kg TS) was coupled with PF, a sharp increase in solid reduction 73.5% and decrease in reaction time (20 min) were observed. Cost benefit analysis revealed the efficiency of HPF process and achieved a net cost of 15.59 USD whereas PF achieved a negative net cost of -82.69 USD. Based on the above study it can be concluded that coupling of homogenizers with PF not only increased its efficiency but also make it field applicable.

Sodium thiosulphate induced immobilized bacterial disintegration of sludge: An energy efficient and cost effective platform for sludge management and biomethanation

The present study aimed to gain better insights into profitable biomethanation through sodium thiosulphate induced immobilized protease secreting bacterial disintegration (STS-IPBD) of sludge. STS disperse the flocs at 0.08 g/g SS of dosage and assists the subsequent bacterial disintegration. Immobilization of bacteria increases the hydrolytic activity of cells towards effective liquefaction of sludge. A higher liquefaction of 22% was accomplished for STS-IPBD when compared to immobilized protease secreting bacterial disintegration (IPBD alone). The kinetic parameters of Line Weaver Burk plot analysis revealed a maximal specific growth rate (µmax) of 0.320 h^-1 for immobilized cells when compared to suspended free cells showing the benefit of immobilization. Floc dispersion and immobilization of bacteria imparts a major role in biomethanation as the methane generation (0.32 gCOD/g COD) was higher in STS-IPBD sample. The cost analysis showed that STS - IPBD was a feasible process with net profit of 2.6 USD/Ton of sludge.

Extraction of consortium of hydrolytic enzymes from waste activated sludge using ultrasonication and stirring with surfactants

In the present study, the consortium of hydrolytic enzymes namely protease, α-amylase, lipase, cellulase and α-glucosidase were extracted from sludge flocs of municipal returned waste activated sludge (MRWAS) and different proportion of mixed sludge namely (MRWAS) and pulp and paper sludge using ultrasonication and stirring with TX100 (Triton X100) and AOT (Dioctyl sodium sulphosuccinate). Ultrasonication with specific energy of 27,027kJ/kg TS with duration 10min was optimized to get maximum activity of enzymes. Mixed sludge with ratio (55:75) had yielded more enzymes activity than the municipal returned waste activated sludge. Further, enzymes extraction efficiency by stirring using TX100, AOT and ultrasonication combined with TX00 and AOT methods were investigated in an optimized mixed sludge ratio (55:75) with varying dosage and stirring or sonication time. In stirring method, the optimum dosage and time of (1% v/v, 60min) and (2% v/v, 180min) respectively were obtained for TX100 and AOT. In ultrasonication method, the optimum dosage of TX100 (1% v/v) and AOT (2% v/v) were obtained at an optimized specific energy of 27,027kJ/kg for 10min. Among the extraction methods, ultrasonication combined with TX100 method exhibited maximum activity of protease, α-amylase, cellulase, lipase and α-glucosidase and these were predicted to be respectively 43.6, 54.4, 34.7, 23, 12.5Units/g VSS. It was concluded that ultrasonication combined with TX100 method is more suitable as it requires a short time and minimum dosage adequate to extract maximum activity of consortium enzymes from sludge flocs, which is essential for the enzymes to be recovered.

Functionality of surfactants in waste-activated sludge treatment: A review

Proper treatment of waste-activated sludge (WAS) involves three pivotal processes, dewatering, anaerobic digestion, and pollutants removal, which need to be re-assessed urgently. Although many traditional sludge treatments have been developed, it is prudent to enhance the efficiency of sludge treatment using multifunctional, flexible, and environmentally friendly surfactants. With regard to sludge dewatering, surfactants can weaken the binding interaction between sludge flocs and promote the dissolution of extracellular polymeric substances (EPSs), resulting in the release of bound water. Using surfactants in anaerobic digestion promotes the release of enzymes trapped in sludge and improves the activity of enzymes during hydrolysis. Owing to their characteristic encapsulation of hydrophobes into self-assembled aggregates (micelles), surfactants can form host-guest complexes with polycyclic aromatic hydrocarbons (PAHs). Additionally, surfactants can enhance the desorption of heavy metals and prevent the emergence of heavy metal residue. This review summarizes the current surfactant-based sludge treatment technologies according to their roles in sludge disposal solutions. Then, possible mechanisms of surfactants in sludge dewatering, anaerobic digestion, and the removal of organic pollutants and heavy metals are analysed systemically. Finally, changes to sludge treatment via the aid of surfactants are highlighted. This review presents the comprehensive advances in the use of surfactants in WAS reduction, recycling, and risk relief, underscoring their roles in increasing economic efficiency and ensuring environmental quality.

Impact of mild alkali dosage on immobilized Exiguobacterium spp. mediated cost and energy efficient sludge disintegration

Approaches to (extracellular polymeric substance) EPS removal were studied with major aim to enhance the biodegradability and sludge solubilization. In this study, a novel approach of entrapment of bacterial strain was carried out to achieve long term activity of protease secreting bacteria Exiguobacterium sp. A mild treatment of potassium hydroxide (KOH) was applied to remove EPS which was followed by entrapment under the biological pretreatment. The efficiency of Exiguobacterium was predicted through dissolvable organic and suspended solids (SS) reduction. The maximum dissolvable organic matter released was 2300mg/L with the solubilization of 23% which was obtained for sludge without EPS (SWOE). For dissolvable organic release, SWOE showed higher final methane production of 232mL/g COD at the production rate of 16.2mL/g COD.d. The SWOE pretreatment was found to be cost effective and less energy intensive beneficial in terms of energy and cost (43.9KWh and -8.2USD) when compared to sludge with EPS (SWE) pretreatment (-177.6KWh and -91.23USD).