Influence of deflocculation on microwave disintegration and anaerobic biodegradability of waste activated sludge

On the potential of microwave heating to convert waste into added-value chemicals and materials: a review

Microwave (MW) heating represents a superior alternative to conventional heating techniques due to its unique ability for rapid, selective, uniform and volumetric heating. However, challenges such as temperature non-uniformity, especially in certain materials and processing conditions, can limit its widespread application. Nevertheless, this heating method can enhance the physicochemical properties and performance of materials produced, making it a vital tool in sustainable material processing to produce valuable porous carbons for CO2 capture, essential for climate change mitigation through carbon capture, utilization and storage (CCUS) strategies. MW heating significantly reduces processing time, energy consumption and operational costs, providing an efficient, green and cost-effective processing option. In this review we explore the use of MW heating in converting biomass, plastics and materials such as tyres and electronic waste into biofuels, bioenergy and hydrogen through biological and thermochemical processes. We highlight MW heating for producing solid adsorbents such as activated carbons from waste, their role in carbon capture and regeneration after CO₂ exposure. We also examine the principles of MW heating, its unique processing advantages and diverse applications across fields. In addition, we emphasize the life cycle assessment (LCA) of MW-assisted treatment for biomass and plastics while addressing the limitations of MW processing in material applications.This article is part of the discussion meeting issue 'Microwave science in sustainability'.

Recovery of phosphorus from chemical-enhanced phosphorus removal sludge: Influence of sodium sulfide dosage on phosphorus fractionation, sludge dewaterability, and struvite product

Despite the potential of sodium sulfide (Na2S) for phosphorus (P) recovery from iron-phosphate waste, the underlying mechanism regarding its impact on P conversion and product quality has not been well addressed. In this study, the effects of Na2S addition on P release and recovery from a chemical-enhanced phosphorus removal (CEPR) sludge during anaerobic fermentation were systematically investigated. The results revealed that the effective mobilization of P bound to Fe (Fe-P) by Na2S dominated the massive P release from the CEPR sludge, while the organic P (OP) release was not significantly enhanced during anaerobic fermentation. Due to the rapid reaction of Na2S with Fe-P and the prevention of Fe(II)-P precipitation by excess S2-, the Fe-P was decreased by 9.7%, 15.2% and 24.9% at S:Fe molar ratios of 0.3, 0.5 and 1, respectively. After anaerobic fermentation, the released P mainly existed as soluble phosphate (SP), P bound to Ca (Ca-P) and P bound to Al (Al-P). The nitrogen and P contents in the fermentation supernatant significantly increased with higher S:Fe ratios, facilitating the efficient recovery of P as high-purity struvite. However, the increased Na2S dosage deteriorated the sludge dewaterability because of the dissolution of hydrophilic extracellular polymeric substances and the looser secondary structure of proteins. Comprehensively considering the P recovery, sludge dewaterability and economic cost, the optimal Na2S dosage was determined at the S:Fe ratio of 0.3. These findings provide novel insights into the role of Na2S in P recovery as struvite from CEPR sludge.

Combined sodium citrate and ultrasonic pretreatment of waste activated sludge for cost effective production of biogas

This study aimed to pretreat the waste activated sludge (WAS) by ultrasonication in an energy efficient way by combining sodium citrate with ultrasonic pretreatment at 0.03 g/g suspended solids (SS) of dosage. The ultrasonic pretreatment was done at various (20-200 W) power levels, sludge concentration (7 to 30 g/L), sodium citrate dosages (0.01 to 0.2 g/g SS). An elevated COD solubilization of 26.07 ± 0.6 % was achieved by combined pretreatment at a treatment time of 10 min, ultrasonic power level of 160 W when compared to individual ultrasonic pretreatment (18.6 ± 0.5 %). A higher biomethane yield of 0.26 ± 0.009 L/g COD was achieved in sodium citrate combined ultrasonic pretreatment (SCUP) than ultrasonic pretreatment (UP) 0.145 ± 0.006 L/g COD. Almost 50% of the energy can be saved through SCUP when compared to UP. Future study evaluating SCUP in continuous mode anaerobic digestion is vital.

Thiosulfate pretreatment enhancing short-chain fatty acids production from anaerobic fermentation of waste activated sludge: Performance, metabolic activity and microbial community

A novel strategy based on thiosulfate pretreatment for enhancing short-chain fatty acids (SCFAs) from anaerobic fermentation (AF) of waste activated sludge (WAS) was proposed in this study. The results showed that the maximal SCFA yield increased from 206.1 ± 4.7 to 1097.9 ± 17.2 mg COD/L with thiosulfate dosage increasing from 0 to 1000 mg S/L, and sulfur species contribution results revealed that thiosulfate was the leading contributor to improve SCFA yield. Mechanism exploration disclosed that thiosulfate addition largely improved WAS disintegration, due to thiosulfate serving as a cation binder for removing organic-binding cations, especially Ca²⁺ and Mg²⁺, dispersing the extracellular polymeric substance (EPS) structure and further entering into the intracellularly by stimulated carrier protein SoxYZ and subsequently caused cell lysis. Typical enzyme activities and related functional gene abundances indicated that both hydrolysis and acidogenesis were remarkably enhanced while methanogenesis was substantially suppressed, which were further strengthened by the enriched hydrolytic bacteria (e.g. C10-SB1A) and acidogenic bacteria (e.g. Aminicenantales) but severely reduced methanogens (e.g. Methanolates and Methanospirillum). Economic analysis confirmed that thiosulfate pretreatment was a cost-effective and efficient strategy. The findings obtained in this work provide a new thought for recovering resource through thiosulfate-assisted WAS AF for sustainable development.

Performance and mechanisms of citric acid improving biotransformation of waste activated sludge into short-chain fatty acids

Numerous effective chemical strategies have been explored for short-chain fatty acids (SCFAs) production from waste activated sludge (WAS), but many technologies have been questioned due to the chemical residues. This study proposed a citric acid (CA) treatment strategy for improving SCFAs production from WAS. The optimum SCFAs yield reached 384.4 mg COD/g VSS with 0.08 g CA/g TSS addition. Meanwhile, CA biodegradation occurred and its contribution to the yield of total SCFAs, especially acetic acid, cannot be ignored. Intensive exploration indicated the sludge decomposition, the biodegradability of fermentation substrates, as well as the abundance of fermenting microorganisms were definitely enhanced in the existence of CA. The optimization of SCFAs production techniques based on this study deserved further study. This study comprehensively revealed the performance and mechanisms of CA enhancing biotransformation of WAS into SCFAs and the findings promotes the research of carbon resource recovery from sludge.

Augmentation in polyhydroxybutyrate and biogas production from waste activated sludge through mild sonication induced thermo-fenton disintegration

In this study, an innovative approach was developed to enhance the hydrolysis through phase-separated pretreatment by removing exopolymeric substances via mild sonication followed by thermo-Fenton disintegration. The exopolymeric substances fragmentation was enhanced at the sonic specific energy input of 2.58 kJ/kg total solids. After exopolymeric substance removal, the disintegration of biomass by thermo-Fenton yield the solubilization of 29.8 % at Fe²⁺:H₂O₂ dosage and temperature of 0.009:0.036 g/g suspended solids and 80 °C as compared to thermo-Fenton alone disintegration. The polyhydroxybutyrate content of 93.1 % was accumulated by Bacillus aryabhattai at the optimum time of 42 h, while providing 70 % (v/v) pre-treated supernatant as a carbon source under nutrient-limiting condition. Moreover, the biogas generation of 0.187 L/g chemical oxygen demand was achieved using settled pretreated sludge. The pretreated sludge sample thus served as a carbon source for polyhydroxybutyrate producers as well as substrate for biogas production.

Phase separated pretreatment strategies for enhanced waste activated sludge disintegration in anaerobic digestion: An outlook and recent trends

A significant ecological problem was developed on disposing the enormous amounts of waste activated sludge (WAS) produced by traditional wastewater treatment. There have been various attempts recently originated to develop innovative methods for substantial sludge treatment. The most frequently used approach for treating sludge to produces methane and reduces sludge is anaerobic treatment. The hydrolysis phase in WAS limits the breakdown of complex macrobiotic compounds. The presence of extracellular polymeric substances (EPS) in biomass prevents the substrate from being hydrolyzed. Enhancing substrate hydrolysis involves removal of EPS preceded by phase separated pretreatment. Hence, a critical assessment of various phase separated pretreatment that has a remarkable effect on the anaerobic digestion process was documented in detail. Moreover, the economic viability and energy requirement of this treatment process was also discussed. Perspectives and recommendations for methane production were also provided to attain effectual sludge management.

Breakthrough in hydrolysis of waste biomass by physico-chemical pretreatment processes for efficient anaerobic digestion

Anaerobic digestion (AD) is the most comprehended process to stabilise the waste biomass efficiently and to obtain bioenergy. The AD starts with the hydrolysis process, where the major liability is the action of inhibitors during the hydrolysis process. The biomass pretreatment preceding anaerobic digestion is obligatory to improve feedstock biodegradability for enhanced biogas generation. It can be prevailed by the application of various pretreatment processes. This review explains the major inhibiting compounds and their formation during hydrolysis that affect the efficiency of anaerobic digestion and the benefits of the physico-chemical pretreatment (PCP) method for enhancing hydrolysis in the digestion of waste biomass. The synergistic effect of PCP on macromolecular release, liquefaction and biodegradability were presented. The feasibility of the pretreatment process was evaluated in terms of energy and cost assessment for pilot scale implementation. The outcome of this review reveals that the physico-chemical process is one of the best pretreatment methods to enhance anaerobic digestion by optimising various parameters and increasing the solubilization by about 90%. The thermochemical pretreatment at lower temperature (<100) increases the net energy yield. The solubilization of waste biomass in terms of macromolecular release and liquefaction cannot describe the pretreatment potential. The effectiveness of pretreatment was evaluated by the substrate pre-treatment followed by anaerobic digestibility of pretreated substrate.

Surfactant induced microwave disintegration for enhanced biohydrogen production from macroalgae biomass: Thermodynamics and energetics

This research work aimed about the enhanced bio-hydrogen production from marine macro algal biomass (Ulva reticulate) through surfactant induced microwave disintegration (SIMD). Microwave disintegration (MD) was performed by varying the power from 90 to 630 W and time from 0 to 40 min. The maximum chemical oxygen demand (COD) solubilisation of 27.9% was achieved for MD at the optimal power (40%). A surfactant, ammonium dodecyl sulphate (ADS) is introduced in optimal power of MD which enhanced the solubilisation to 34.2% at 0.0035 g ADS/g TS dosage. The combined SIMD pretreatment significantly reduce the treatment time and increases the COD solubilisation when compared to MD. Maximum hydrogen yield of 54.9 mL H₂ /g COD was observed for SIMD than other samples. In energy analysis, it was identified that SIMD was energy efficient process compared to others since SIMD achieved energy ratio of 1.04 which is higher than MD (0.38).

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.

A review on energy and cost effective phase separated pretreatment of biosolids

Extracellular Polymeric Substances (EPS) existent in anaerobic sludge proves to be a barrier for sludge liquefaction and biomass lysis efficiency. Hence EPS deaggregation heightens the surface area for the subsequent pretreatment thereby uplifting the sludge disintegration and biomethanation rate. This review documents the role of EPS and its components which inhibits sludge hydrolysis and also the various phase separated pretreatment methods available with its disintegration mechanism to enhance the biomass lysis and methane production rate. It also illustrates the effects of phase separated pretreatment on the sludge disintegration rate which embodies two phases-floc disruption and cell lysis accompanied by their computation through biomethane potential assay and fermentation analysis comprehensively. Additionally, energy balance study and cost analysis requisite for successful implementation of a proposed phase separated pretreatment on a pilot scale level and their challenges are also reviewed. Overall this paper documents the potency of phase separated pretreatment for full scale approach.

Pulse Electric Field Technology for Wastewater and Biomass Residues’ Improved Valorization

Development and adoption of more efficient and robust technologies for reuse of wastewater embedded resources, in particular materials and energy, is becoming an unavoidable necessity. Among many emerging technologies in the sector of wastewater treatment residuals valorization, Pulsed Electric Field (PEF) processes have shown interesting potential, although they have not yet entered the sector’s mainstream as a consolidated commercial technology, as in other industrial applications, such as the food, medical, and bio-based industries. PEF is a non-thermal technology suitable to biological applications, involving gentle cell disintegration and enhanced cell membrane permeability and as such applicable to disinfection, sterilization, and to those processes that benefit from an enhanced extraction of organic compounds from biological matter, such as anaerobic digestion, biological processes for recovery of nutrients, and biorefinery of cell-embedded compounds. PEF technology applications in wastewater/biomass residues management are reported and advantages, drawbacks, and barriers of the technology are discussed in this paper.

A comprehensive review of the recent development and challenges of a solar-assisted biodigester system

The extensive use of fossil fuels and the environmental effect of their combustion products have attracted researchers to look into renewable energy sources. In addition, global mass production of waste has motivated communities to recycle and reuse the waste in a sustainable way to lower landfill waste and associated problems. The development of waste to energy (WtE) technology including the production of bioenergy, e.g. biogas produced from various waste through Anaerobic Digestion (AD), is considered one of the potential measures to achieve the sustainable development goals of the United Nations (UN). Therefore, this study reviews the most recent studies from relevant academic literature on WtE technology (particularly AD technology) for biogas production and the application of a solar-assisted biodigester (SAB) system aimed at improving performance. In addition, socio-economic factors, challenges, and perspectives have been reported. From the analysis of different technologies, further work on effective low-cost technologies is recommended, especially using SAB system upgrading and leveraging the opportunities of this system. The study found that the performance of the AD system is affected by a variety of factors and that different approaches can be applied to improve performance. It has also been found that solar energy systems efficiently raise the biogas digester temperature and through this, they maximize the biogas yield under optimum conditions. The study revealed that the solar-assisted AD system produces less pollution and improves performance compared to the conventional AD system.

Comparison of Pre-treatment Technologies to Improve Sewage Sludge Biomethanization

This research study evaluates various pre-treatments to improve sewage sludge solubilization prior to treatment by mesophilic anaerobic digestion. Microwave, thermal, and sonication pre-treatments were compared as these pre-treatments are the most commonly used for this purpose. The solubilization of sewage sludge was evaluated through the variation in soluble total organic carbon (sTOC, mg/L) and soluble total nitrogen (sTN, mg/L). Thermal and microwave pre-treatments increased sTOC/VS by 19.2% and 83.4% (VS, total volatile solids), respectively, after applying lower specific energy through (20 kJ/g TS, approximately) (TS, total solids) unlike the sonication pre-treatment, which required 136 kJ/g TS. Although sTN content did not increase significantly with the pre-treatments with respect to sTOC, both showed proportional trends. Sonication pre-treatments allowed the highest increase in volatile fatty acids (VFA) with respect to the raw sewage sludge (15% ∆VFA/sTOC). Methane production with and without pre-treatment was also evaluated. Methane production increased by 95% after applying sonication pre-treatment compared to the methane production of raw sewage sludge. Thermal and microwave pre-treatments entailed lower improvements (29% and 20%, respectively). Economically, thermal pre-treatments were the most viable alternative at real scale. Graphical abstract.

Effect of hydrodynamic disintegration on the solubilisation and bioavailability of thickened excess sludge

The main objective of the study was the verification whether conducting the hydrodynamic disintegration (HD) of thickened excess sludge (TES) before the anaerobic hydrolysis (AH) can cause an increase in the efficiency of the hydrolysis process, and therefore a reduction in its duration, or allow for complete omission of the stage before the anaerobic digestion (AD). For this purpose, the HD (conducted in five levels of energy density (E_L): 140, 280, 420, 560 and 700 kJ/L) of TES was carried out, and then all sludges (before and after disintegration) were subjected to the AH. The obtained results confirmed that the process of HD can be an effective method of increasing the solubilisation and bioavailability of TES. In the process of HD, the maximum increase in ΔVFA (308-428 mg VFA/L), was reported when E_L was increased from 140 to 280 kJ/L (the solubilisation degree increased from approximately 2 to 8%). The obtained results also showed that the ΔSTN and ΔSTP were related to solubilisation degree. The most intensive increase in the ΔSTN was determined for solubilisation degree in a range of 15-20%. In the case of ΔSTP, constant intensity of release of the compounds to the sludge liquid was observed. The obtained results also confirmed that conducting the process of AH of disintegrated TES proved to change the SCOD value when contrasted with the value of this indicator at the start of the experiment (before hydrolysis): (i) the E_L equal to 140 and 280 kJ/L allowed for a higher SCOD value; (ii) at E_L higher or equal to 560 kJ/L it caused a decrease in the SCOD value.

Effect of citric acid on extracellular polymeric substances disruption and cell lysis in the waste activated sludge by pH regulation

This paper investigated the effects of citric acid (CA) on extracellular polymer destruction and cell lysis in sludge at different initial pH by measuring capillary suction time (CST), extracellular polymeric substances (EPS) and intracellular bound water. The results indicated that under CA concentration at 0.05 g/g suspended solids (SS) and initial pH 4, the CST value decreased from 175.5 s to 112.3 s, slime extracellular polymeric substances (S-EPS) and loosely bound EPS (LB-EPS) content respectively to increase from 4.92 to 41.43, 2.27 to 5.49 mg/g volatile suspended solids (Vss), while tightly bound EPS (TB-EPS) content to decrease from 12.35 to 5.01 mg/g (Vss), which suggested CA could disrupt outer EPS effectively. Intracellular bound water content decreased from 1.23 g/g to 0.41 g/g dry solid (DS). As a result, CA could release intracellular bound water effectively, thereby improving sludge dewatering degree.

Improving biogas production performance of dairy activated sludge via ultrasound disruption prior to microwave disintegration

In this study, ultrasound disruption was employed to enhance the efficiency of microwave disintegration of dairy sludge. Results revealed that ultrasound specific energy input of 1,500 kJ/kg TS was found to be optimum with limited cell lysis at the end of the disruption phase. Biodegradability study suggested an enhancement in suspended solids reduction (16%) and biogas production (180 mL/gVS) in floc disrupted (deflocculated) samples when compared to sole microwave pretreatment (8.3% and 140 mL/gVS, respectively). Energy assessment to attain the 15% optimum solubilization revealed a positive net production of 26 kWh per kg sludge in deflocculated samples compared to 18 kWh in flocculated (sole microwave) samples. Thus, ultrasound disruption prior to microwave disintegration of dairy sludge was considered to be a feasible pretreatment technique.

Microwave-Assisted One-Step Conversion of Wood Wastes into Levulinic Acid

This study aimed to evaluate the use of softwood and hardwood waste for the production of levulinic acid by one-stage conversion using microwave radiation combined with acid catalysis. The analysis demonstrated that the type and concentration of the acid used, the concentration of biomass in the reaction mixture and pressure value had the greatest impact on the yield of levulinic acid. The highest efficiency of carbohydrate conversion to levulinic acid, regardless of the type of raw material, was achieved using a pressure of 225 PSI and sulfuric acid as a catalyst. Maximum yield from biomass, ca. 16.5% for cherry wood chips and ca. 25% for pine chips, was obtained using sulfuric acid at a concentration of 1% v/v and 2% v/v, respectively, for the following process parameters: Exposure time 20 min, biomass concentration 3.3%, and the pressure of 225 PSI. The ratio of actual yield to theoretical yield was high: 64.7% ± 4.5% for pine chips and 43.4% ± 1.0% for cherry wood chips. High efficiency of the presented method of biomass conversion to levulinic acid indicates the possibility of its use for waste management in the wood processing industry. High concentration of levulinic acid in the post-reaction mixture allows for cost-effective extraction and purification of the compound.

Evaluation of photocatalytic thin film pretreatment on anaerobic degradability of exopolymer extracted biosolids for biofuel generation

This study reports the result of sodium citrate induced exopolymer extraction on the photocatalytic thin film (TiO₂) pretreatment efficiency of waste activated sludge (WAS). TiO₂ is immobilized through DC spluttering method followed by annealing process. The exopolymer removal of 94.2% by sodium citrate (0.05 g/g SS) promotes better disintegration. This TiO₂ thin film efficiently extricate the intracellular components of exopolymer extracted sludge at 50 min increasing the solubilization to 19.33%. As a result, the exopolymer extracted sludge shows high methane generation (0.24 gCOD/gCOD) than the other (pretreated sludge without exopolymer removal - 0.12 gCOD/gCOD and raw sludge without treatment - 0.075 gCOD/gCOD). The methane generated in sodium citrate induced TiO₂ thin film pretreated sludge is 398.99 kWh. In cost analysis, it gives net cost of -57.46 USD/ton of sludge. In addition, the proposed method also accounts 51.3% of solid reduction.

Microwave-assisted pretreatment of maize distillery stillage with the use of dilute sulfuric acid in the production of cellulosic ethanol

This study aimed to evaluate the effectiveness of microwave-assisted pretreatment in production of cellulosic bioethanol from maize distillery stillage. High glucose concentration (104.4 ± 0.4 mg/g dry weight) and the highest yield of enzymatic cellulose hydrolysis (75.8 ± 0.9%) were obtained for microwave pretreatment characterized by the following parameters: 300 W, 54 PSI, 15 min. Such pre-treatment parameters allowed the obtaining of not only a high concentration of glucose, but also a low concentration of fermentation inhibitors, i.e. 5-hydroxymethylfurfural (6.8 ± 0.4 mg/g of DW) and furfural (6.0 ± 1.2 mg/g dry weight). Optimal dose of yeast, Saccharomyces cerevisiae strain Ethanol Red which gave a high attenuation was 2 g/L of cellulose fermentation medium. Detoxification of cellulose hydrolysates with activated carbon enabled achieving high fermentation yield (approximately 77% of the theoretical yield). Microwave processing can be an effective pretreatment method in production of cellulosic ethanol from maize distillery stillage, but this process requires careful selection of parameters.

Novel insights into scalability of biosurfactant combined microwave disintegration of sludge at alkali pH for achieving profitable bioenergy recovery and net profit

In the present study, a novel alkali rhamnolipid combined microwave disintegration (ARMD) was employed to achieve net energy production, increased liquefaction and to increase the amenability of sludge towards biomethanation. Additionally, biosurfactant rhamnolipid under alkali conditions enhances the liquefaction at alkali pH of 10 with a maximal liquefaction of 55% with reduced energy consumption (1620 kJ/kg TS) than RMD (45.7% and 3240 kJ/kg TS specific energy) and MD (33.7% and 6480 kJ/kg TS specific energy). A higher biomethane production of 379 mL/g COD was achieved for ARMD when compared to RMD (329 mL/g COD) and MD (239 mL/g COD). The scalable studies imply that the ARMD demands input energy of -282.27 kWh. A net yield of (0.39 USD/ton) was probably achieved via novel ARMD technique indicating its suitability at large scale execution when compared to RMD (net cost -31.34 USD/ton) and MD (-84.23 net cost USD/ton), respectively.

Pretreatment of macroalgal Laminaria japonica by combined microwave-acid method for biohydrogen production

Suitable pretreatment can effectively enhance the fermentative hydrogen production from algae biomass. In this study, combined microwave-acid pretreatment was applied to disintegrate the biomass of macroalgae L. japonica, and dark fermentation in batch mode was conducted for hydrogen production. The results showed that combining microwave pretreatment at 140 °C and 2450 MHz with 1% H2SO4 for 15 min could effectively disrupt macroalgal cells and release the organic matters, and soluble chemical oxygen demand (SCOD) concentration increased by 1.92-fold and achieved 5.12 g/L. During the fermentation process, both polysaccharides and proteins were consumed. Hydrogen production process was dominated by acetate-type fermentation, and the dominance of genus Clostridium contributed to more efficient hydrogen production. After the pretreatment, hydrogen yield increased from 15 mL/g TSadded to 28 mL/g TSadded, and energy conversion efficiency increased from 9.5% to 23.8%. Combined microwave-acid pretreatment is potential in enhancing hydrogen production from the biomass of L. japonica.

Combining microwave irradiation with sodium citrate addition improves the pre-treatment on anaerobic digestion of excess sewage sludge

This study investigated the synergistic effect of sodium citrate (SC; Na₃C₃H₅O(COO)₃) and microwave (MW) treatment on the efficiency of the anaerobic digestion of excess sewage sludge. In terms of the methane yield, an increase of the digestion's efficiency was observed. Taking into account the cost for the MW energy supplied to the system, the optimum treatment conditions were a MW energy input of 20 MJ/kg TS and a SC concentration of 0.11 g/g TS, obtaining a methane yield of 218.88 ml/g VS, i.e., an increase of 147.7% compared to the control. MW treatment was found to break the sludge structure, thereby improving the release of extracellular polymeric substances (EPS) and volatile fatty acids (VFAs). The treatment of sodium citrate further strengthened the breakage of loosely bound extracellular polymeric substances (LB-EPS) and tightly bound extracellular polymeric substances (TB-EPS). The increased VFA content stressed the improved digestion by this pretreatment. Furthermore, the preliminary economic analysis showed that at this point in the research, only operational but no financial gains were achieved.

The rate of denitrification using hydrodynamically disintegrated excess sludge as an organic carbon source

This study investigates the potential of hydrodynamically disintegrated excess activated sludge when used as a supplementary carbon source for denitrification. Two objectives constituted this study: (i) to analyse the denitrification rate by using excess sludge subjected to hydrodynamic disintegration (HD), performed at different energy densities, as an organic carbon source, and (ii) to analyse the impact of hydrolysis of disintegrated sludge on the denitrification rate. Nitrate reduction tests were conducted to assess the denitrification rate for the following sources of organic carbon: thickened excess sludge disintegrated at three levels of energy density (70, 140 and 210 kJ/L), acetic acid solution and municipal wastewater after mechanical treatment. It was found that the HD of excess sludge conducted at different levels of energy density led to dissolved organic compounds characterised by various properties as donors of H⁺ in the denitrification process. The susceptibility of disintegrated sludge to anaerobic hydrolysis decreased along with the increasing energy density. The obtained organic carbon contributed to a lower increase in the denitrification rate in comparison to that when disintegrated sludge not subjected to hydrolysis was applied.

Profitable ultrasonic assisted microwave disintegration of sludge biomass: Modelling of biomethanation and energy parameter analysis

In this study, microwave irradiation has been employed to disintegrate the sludge biomass profitably by deagglomerating the sludge using a mechanical device, ultrasonicator. The outcomes of the study revealed that a specific energy input of 3.5 kJ/kg TS was found to be optimum for deagglomeration with limited cell lysis. A higher suspended solids (SS) reduction and biomass lysis efficiency of about 22.5% and 33.2% was achieved through ultrasonic assisted microwave disintegration (UMWD) when compared to microwave disintegration - MWD (15% and 20.9%). The results of biochemical methane potential (BMP) test were used to estimate biodegradability of samples. Among the samples subjected to BMP, UMWD showed better amenability towards anaerobic digestion with higher methane production potential of 0.3 L/g COD representing enhanced liquefaction potential of disaggregated sludge biomass. Economic analysis of the proposed method of sludge biomass pretreatment showed a net profit of 2.67 USD/Ton respectively.

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).

Effect of deflocculation on photo induced thin layer titanium dioxide disintegration of dairy waste activated sludge for cost and energy efficient methane production

In the present study, the deflocculated sludge was disintegrated through thin layer immobilized titanium dioxide (TiO₂) as photocatalyst under solar irradiation. The deflocculation of sludge was carried out by 0.05g/g SS of sodium citrate aiming to facilitate more surface area for subsequent TiO₂ mediated disintegration. The proposed mode of disintegration was investigated by varying TiO₂ dosage, pH and time. The maximum COD solubilization of 18.4% was obtained in the optimum 0.4g/L of TiO₂ dosage with 5.5 pH and exposure time of 40min. Anaerobic assay of disintegrated samples confirms the role of deflocculation as methane yield was found to be higher in deflocculated (235.6mL/gVS) than the flocculated sludge (146.8mL/gVS). Moreover, the proposed method (Net cost for control - Net cost for deflocculation) saves sludge management cost of about $132 with 53.8% of suspended solids (SS) reduction.

Immobilized and MgSO4 induced cost effective bacterial disintegration of waste activated sludge for effective anaerobic digestion

In this study, an attempt was made to disintegrate waste activated sludge (WAS) in a cost-effective way. During the first phase of this study, effective break down of extracellular polymeric substance (EPS) was performed by deflocculating WAS with 0.1 g/g SS of MgSO₄. Deflocculation rate was 92% with discharge rate of extractable EPS at 185 mg/L. In the second phase, effective bacterial cell disintegration was obtained at 36 h post treatment. Maximum solubilization of deflocculated sludge was approximately 21%, which was higher than that of flocculated sludge (14.2%) or the control (4.5%). Biodegradability studies were assessed through kinetic analysis by non-linear regression modeling. Results revealed that the deflocculated sludge had higher methane generation (at about 235.8 mL/gVs) compared to flocculated sludge (at 146.1 mL/gVs) or the control (at 34.8 mL/gVs). Cost assessment of the present work revealed that the net yield for each ton of the deflocculated sludge was about 32.99 USD.

Effect of deflocculation on the efficiency of sludge reduction by Fenton process

A novel approach to improve the efficiency of Fenton treatment for sludge reduction through the implication of a deflocculating agent citric acid, for the exclusion of extracellular polymeric substances (EPS) from waste-activated sludge (WAS), was investigated. Deflocculation was achieved with 0.06 g/g suspended solids (SS) of citric acid dosage. Fenton optimization studies using response surface methodology (RSM) revealed that 0.5 and 0.0055 g/g SS were the optimal dosages of H2O2 and Fe(2+). The addition of a cation-binding agent set the pH value of sludge to 5 which did not affect the Fenton efficiency. The results presented in this study shows the advantage of deflocculating the sludge as SS and volatile suspended solids (VSS) reductions were found to be higher in the deflocculated (53 and 63 %, respectively) than in the flocculated (22 and 34 %, respectively) sludges. Kinetic investigation of the treatment showed that the rate of the reaction was four times higher in the deflocculated sludge than control. The methodology reported in this manuscript was successfully applied to a real case were the deflocculated mediated Fenton process reduced the sludge disposal cost from 297.8 to 61.9 US dollars/ton of sludge.

Improving the biogas production performance of municipal waste activated sludge via disperser induced microwave disintegration

In this study, the influence of disperser induced microwave pretreatment was investigated to analyze the proficiency of floc disruption on subsequent disintegration and biodegradability process. Initially, the flocs in the sludge was disrupted through disperser at a specific energy input of 25.3kJ/kgTS. The upshot of the microwave disintegration presents that the solids reduction and solubilization of floc disrupted (disperser induced microwave pretreated) sludge was found to be 17.33% and 22% relatively greater than that achieved in microwave pretreated (9.3% and 16%) sludge alone. The biodegradability analysis, affords an evaluation of parameter confidence and correlation determination. The eventual biodegradability of microwave pretreated, and floc disrupted sludges were computed to be 0.15(gCOD/gCOD) and 0.28(gCOD/gCOD), respectively. An economic assessment of this study offers a positive net profit of about 104.8USD/ton of sludge in floc disrupted sample.

Enhancement of sludge anaerobic biodegradability by combined microwave-H2O2 pretreatment in acidic conditions

The aim of this study was to increase the sludge disintegration and reduce the cost of microwave (MW) pretreatment. Thermodynamic analysis of MW hydrolysis revealed the best fit with a first-order kinetic model at a specific energy of 18,600 kJ/kg total solids (TS). Combining H2O2 with MW resulted in a significant increment in solubilization from 30 to 50 % at 18,600 kJ/kg TS. The pH of H2O2-assisted MW-pretreated sludge (MW + H2O2) was in the alkaline range (pH 9-10), and it made the sludge unfavorable for subsequent anaerobic digestion and inhibits methane production. In order to nullify the alkaline effect caused by the MW + H2O2 combination, the addition of acid was considered for pH adjustment. H2O2-assisted MW-pretreated sludge in acidic conditions (MW + H2O2 + acid) showed a maximum methane production of 323 mL/g volatile solids (VS) than others during anaerobic biodegradability. A cost analysis of this study reveals that MW + H2O2 + acid was the most economical method with a net profit of 59.90 €/t of sludge.

Effect of chemo-mechanical disintegration on sludge anaerobic digestion for enhanced biogas production

The effect of combined surfactant-dispersion pretreatment on dairy waste activated sludge (WAS) reduction in anaerobic digesters was investigated. The experiments were performed with surfactant, Sodium dodecyl sulfate (SDS) in the range of 0.01 to 0.1 g/g suspended solids (SS) and disperser with rpm of 5000-25,000. The COD (chemical oxygen demand) solubilization, suspended solids reduction, and biogas generation increased for an energy input of 7377 kJ/kg total solids (TS) (12,000 rpm, 0.04 g/g SS, and 30 min) and were found to be 38, 32, and 75 %, higher than that of control. The pretreated sludge improved the performance of semicontinuous anaerobic digesters of 4 L working volume operated at four different SRTs (sludge retention time). SRT of 15 days was found to be appropriate showing 49 and 51 % reduction in SS and volatile solids (VS), respectively. The methane yield of the pretreated sample was observed to be 50 mL/g VS removed which was observed to be comparatively higher than the control (12 mL/g VS removed) at optimal SRT of 15 days. To the best of the authors' knowledge, this study is the first to be reported and not yet been documented in literature.

Combined thermo-chemo-sonic disintegration of waste activated sludge for biogas production

In the present study, there was an investigation about the impact of a new combined thermo-chemo-sonic disintegration of waste activated sludge (WAS) on biodegradability. The outcome of sludge disintegration reveals that maximum Suspended Solids (SS) reduction and Chemical Oxygen Demand (COD) solubilization effectuated at a specific energy input of 5290.5kJ/kgTS, and was found to be 20%, 16.4%, 15% and 27%, 22%, and 20%, respectively for the three alkalis (NaOH, KOH, and Ca(OH)2). The conversion coefficient of the Volatile Suspended Solids (VSS) to product Soluble COD (SCOD), calculated by nonlinear regression modeling, was found to be 0.5530gSCOD/gVSS, 0.4587gSCOD/gVSS, and 0.4195gSCOD/gVSS for NaOH, KOH, and Ca(OH)2, respectively. In the biodegradability studies, the parameter evaluation provides an estimate of parameter uncertainty and correlation, and elucidates that there is no significant difference in biodegradability (0.413gCOD/gCOD, 0.367gCOD/gCOD, and 0.342gCOD/gCOD) for three alkalis (NaOH, KOH, and Ca(OH)2).