Ultrasound assisted biogas production from co-digestion of wastewater sludges and agricultural wastes: Comparison with microwave pre-treatment

Optimization of ultrasonication and alkalinization as pretreatment methods for leachate co-digested with food waste toward maximum synergistic effects

Anaerobic co-digestion (AcoD) of food waste (FW) and landfill leachate has shown promising results in enhancing the methane yield. However, leachate includes toxic and refractory compounds that may impact the decomposition process. In this research, co-digested leachate was pretreated using ultrasonication and alkalinization to manipulate its characteristics toward improved synergism with FW. Experimental optimization was conducted through biochemical methane potential (BMP) assays to identify the optimum operating conditions of the pretreatment methods. The study evaluated the synergistic effects of co-digestion with raw and pretreated leachate on enhancing the performance in terms of feedstock solubilization and methane production. The BMP test demonstrated that alkalinization and ultrasonication improved the total methane generation by 35% and 27%, respectively, yielding around 397 and 375 mL CH4 per g of volatile solids. Moreover, ultrasonication and alkalinization enhanced the synergistic effects by 28% and 36%, respectively, compared to co-digestion with untreated leachate. Optimization by response surface methodology revealed that maximum performance could be achieved with leachate sonication at 212 W for 37.5 min or augmenting 788 g NaOH per kg of volatile solids. Kinetic and statistical models were derived to simulate and assess the impacts of the pretreatment parameters on the AcoD process. The results indicated that the ultrasonication energy had a higher influence on total solubility and methane production than alkaline dosage. Additionally, energy efficiency analyses were performed to examine the overall viability of the examined management approach and found that alkalinization increased the net energy efficiency by 23%, whereas ultrasonication was inefficient within the examined laboratory conditions despite the improved performance. The findings support an integrated organic waste management system where separated FW is co-treated with landfill leachate.

Improved methane production from ultrasonically-pretreated secondary sedimentation tank sludge and new model proposal: Time series (ARIMA)

Secondary sedimentation tank sludge (SSTS) is a severe type of waste that needs to be treated to prevent human health risks. Anaerobic digestion (AD) alone can be ineffective for treating SSTS due to low substrate biodegradability. This study proposes the use of ultrasonic pretreatment (UP) on SSTS for 0 - 60 min. UP effect on SSTS was analyzed through the actual specific energy input, nominal energy input, and soluble chemical oxygen demand. The highest cumulative methane yield (CMY) of 241.56 mL/g VS obtained during the AD of the pretreated substrate (i.e., 30 min) was 58.4% higher than that of untreated SSTS. The resulting CMY curves were subsequently simulated via the modified Logistic and Gompertz, Artificial Neural Network, Auto-Regressive Integrated Moving-Average (ARIMA) models. As a novel proposal for predicting CMYs, the ARIMA model is recommended due to its simplicity and better performance than sigmoidal models (R2 = 0.9975 - 0.9988).

Insights into the recent advances of agro-industrial waste valorization for sustainable biogas production

Recent years have seen a transition to a sustainable circular economy model that uses agro-industrial waste biomass waste to produce energy while reducing trash and greenhouse gas emissions. Biogas production from lignocellulosic biomass (LCB) is an alternative option in the hunt for clean and renewable fuels. Different approaches are employed to transform the LCB to biogas, including pretreatment, anaerobic digestion (AD), and biogas upgradation to biomethane. To maintain process stability and improve AD performance, machine learning (ML) tools are being applied in real-time monitoring, predicting, and optimizing the biogas production process. An environmental life cycle assessment approach for biogas production systems is essential to calculate greenhouse gas emissions. The current review presents a detailed overview of the utilization of agro-waste for sustainable biogas production. Different methods of waste biomass processing and valorization are discussed that contribute towards developing an efficient agro-waste to biogas-based circular economy.

A review on the hydration properties of dietary fibers derived from food waste and their interactions with other ingredients: opportunities and challenges for their application in the food industry

Dietary fiber (DF) significantly affects the quality attributes of food matrices. Depending on its chemical composition, molecular structure, and degree of hydration, the behavior of DF may differ. Numerous reports confirm that incorporating DF derived from food waste into food products has significant effects on textural, sensory, rheological, and antimicrobial properties. Additionally, the characteristics of DF, modification techniques (chemical, enzymatic, mechanical, thermal), and processing conditions (temperature, pH, ionic strength), as well as the presence of other components, can profoundly affect the functionalities of DF. This review aims to describe the interactions between DF and water, focusing on the effects of free water, freezing-bound water, and unfreezing-bound water on the hydration capacity of both soluble and insoluble DF. The review also explores how the structural, functional, and environmental properties of DF contribute to its hydration capacity. It becomes evident that the interactions between DF and water, and their effects on the rheological properties of food matrices, are complex and multifaceted subjects, offering both opportunities and challenges for further exploration. Utilizing DF extracted from food waste exhibits promise as a sustainable and viable strategy for the food industry to create nutritious and high-value-added products, while concurrently reducing reliance on primary virgin resources.

Environment friendly emerging techniques for the treatment of waste biomass: a focus on microwave and ultrasonication processes

In the recent past, an increasing interest is mostly observed in using microwave and ultrasonic irradiation to aid the biological conversion of waste materials into value-added products. This study is focused on various individual impacts of microwaves and ultrasonic waves for the treatment of biomass before the synthesis of value-added products. Following, a comprehensive review of the mechanisms governing microwaves and ultrasonication as the treatment methods, their effects on biomass disruption, solubilization of organic matter, modification of the crystalline structure, enzymatic hydrolysis and production of reducing sugars was performed. However, based on the lab-scale experiments evaluated, microwaves and ultrasonication were studied to be economically and energetically ineffective despite their beneficial effects on the waste biomass. This article reviews some of the difficulties associated with using microwaves and ultrasonic irradiation for the efficient processing of waste biomasses and identified some potential directions for future study.

Strategies for energy conversion from sludge to methane through pretreatment coupled anaerobic digestion: Potential energy loss or gain

Anaerobic digestion (AD) of wasted activated sludge from wastewater plants is recognized as an effective method to reclaim energy in the form of methane. AD performance has been enhanced by coupling various pretreatments that impact energy conversion from sludge. This paper mainly reviewed the development of pretreatments based on different technologies reported in recent years and evaluated their energy benefit. Significant increases in methane yield are generally obtained in AD with pretreatments demanding energy input, including thermal- and ultrasound-based methods. However, these energy-intense pretreatments usually gained negative energy benefit that the increase in methane yield consumed extra energy input. The unbalanced relationship counts against the goal of energy reclamation from sludge. Combined pretreatment consisting of multiple technologies normally outcompetes the single pretreatment, and the combination of energy-intense methods and chemicals potentially reduces energy input and simultaneously ensure high methane yield. For determining whether the energy reclamation from sludge via AD contribute to mitigating global warming, integrating greenhouse gas emission into the evaluation system of pretreated AD is further warranted.

Characteristics of Solidified Carbon Dioxide and Perspectives for Its Sustainable Application in Sewage Sludge Management

Appropriate management is necessary to mitigate the environmental impacts of wastewater sludge. One lesser-known technology concerns the use of solidified CO₂ for dewatering, sanitization, and digestion improvement. Solidified CO₂ is a normal byproduct of natural gas treatment processes and can also be produced by dedicated biogas upgrading technologies. The way solidified CO₂ is sourced is fully in line with the principles of the circular economy and carbon dioxide mitigation. The aim of this review is to summarize the current state of knowledge on the production and application of solid CO₂ in the pretreatment and management of sewage sludge. Using solidified CO₂ for sludge conditioning causes effective lysis of microbial cells, which destroys activated sludge flocs, promotes biomass fragmentation, facilitates efficient dispersion of molecular associations, modifies cell morphology, and denatures macromolecules. Solidified CO₂ can be used as an attractive tool to sanitize and dewater sludge and as a pretreatment technology to improve methane digestion and fermentative hydrogen production. Furthermore, it can also be incorporated into a closed CO₂ cycle of biogas production-biogas upgrading-solidified CO₂ production-sludge disintegration-digestion-biogas production. This feature not only bolsters the technology's capacity to improve the performance and cost-effectiveness of digestion processes, but can also help reduce atmospheric CO₂ emissions, a crucial advantage in terms of environment protection. This new approach to solidified CO₂ generation and application largely counteracts previous limitations, which are mainly related to the low cost-effectiveness of the production process.

Comparison of different pretreatment methods on phosphorus release and recovery as struvite from excess sludge

Recovering phosphorus (P) from excess sludge of wastewater treatment plants (WWTPs) has attracted considerable attention. An efficient P release method is undoubtedly critical for a satisfactory recovery performance. In this study, the effectiveness of three sludge pretreatment methods, i.e. anaerobic digestion (AD), EDTA-anaerobic digestion (EA) and ultrasound combined with EA (U-EA), on P release and struvite recovery from excess sludge was investigated. The results showed that different pretreatment methods resulted in the different characteristics of P release and recovery. For P release, the highest P release rate (57.14% of sludge total phosphorus, TP) was achieved by U-EA pretreatment, followed by EA and AD. Furthermore, U-EA was beneficial for sludge disintegration and reduction, by which the mixed liquor suspended solids (MLVSS) reduction rate reached 42.00% at a specific energy of 110,000 kJ/kg TS. For the P recovery (in the form of struvite), there was only a little difference in the optimal conditions and P recovery rate (89.29-94.49% of TP in the supernatant). AD pretreatment was beneficial for the purity of products and achieved the highest struvite purity (85.14%), followed by EA (80.95%) and U-EA (77.56%). In summary, the highest recovery rate of TP from excess sludge (53.50% of sludge TP) and struvite yield (26.10 mg/gSS) was obtained by U-EA.

Dispersion assisted pretreatment for enhanced anaerobic biodegradability and biogas recovery -strategies and applications

Disperser assisted homogenization is a promising mechanical based disintegration process to improve the substrate biodegradability and biogas recovery from biomass. During dispersion, the extent of liquefaction relies on the dispersion parameters and biomass properties. Hence, assessment of the optimal parameters varies with type of disperser and biomass. Dispersion assisted homogenization of some biomass such as sludge is not only studied in lab scale but also investigated in full scale plants providing positive outcome. For instance, the large-scale investigation of disperser homogenization has attained nearly 40-50 percent increment in bioenergy recovery. However, research gaps in terms of energy and cost efficiency still exists. This review paper outlines the impact of disperser parameters, its efficiency in biomass disintegration and biogas recovery. It has been proposed to combine homogenization process in the bioenergy generation to investigate the energy and cost efficiency of the entire process.

Effect of ultrasonic and electrokinetic post-treatments on methane yield and viscosity of agricultural digestate

The impact of post-treatment of digestate prior to its recirculation to the digester has been evaluated with industrial-scale ultrasonication and electrokinetic treatment units. Residual methane yields of untreated digestate samples from four biogas plants varied between 99 and 134 ml/g of volatile solids (after 97 days of digestion). At the tested conditions (1.90-7.60 kWh/t for ultrasonication and 4.37-6.06 kWh/t for electrokinetic disintegration), no significant increase in methane yields were obtained. Increases in maximum methane production rates of up to 42% were observed in samples from one of the biogas plants, resulting in net energy gains between 1.06 and 7.04 kWh/t (after 26 and 66 days of digestion). Both treatments significantly reduced digestate viscosities, especially of highly viscous samples. Lower viscosities could reduce the energy demand for pumping and stirring in the digester, enabling the potential for increased organic loading and increased methane production.

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.

Overview of pretreatment technologies on vegetable, fruit and flower market wastes disintegration and bioenergy potential: Indian scenario

Disposal of segregated organic fractions of centralized wholesale market wastes (i.e. vegetable, fruit and flower markets waste) in dumpsites/landfills are not only a serious issue but also underutilizes the huge potency of these organic wastes. Anaerobic digestion (AD) is a promising technology for converting organic wastes into methane, as a carbon-neutral alternative to conventional fuels. The major challenges related to the AD process are poor biodegradation of wastes and buffering capacity within the anaerobic digester that lowers the biogas yield. To accelerate biodegradation and to enhance the process efficacy of anaerobic digestion, several pretreatment technologies (mechanical, thermal, biological, chemical and combined pre-treatments) for organic wastes prior to the AD process were developed. This review article presents a comprehensive analysis of research updates in pretreatment techniques for vegetable, fruit and flower markets wastes for enhancing biogas yields during the AD process. The technological aspects of the pretreatment process are described and their efficiency comparison with the resultant process yields and environmental benefits are also discussed. The challenges and technical issues associated with each pretreatment and future research directions for overcoming the field implementation issues are also proposed.

A review on anaerobic digestion with focus on the role of biomass co-digestion, modelling and optimisation on biogas production and enhancement

The status, recent trends and future perspectives in modelling and optimisation of anaerobic co-digestion is investigated. Areas that can be focused on and those which need further research towards enhancing biogas production are pointed out. Co-digestion, modelling and optimisation of anaerobic digestion as well as techno-economic aspects are reviewed in this paper. It was noted that co-digestion requires more research into a variety of bio-resources and their specific blend proportions. Modelling and optimisation of co-digestion with substrate seasonal fluctuations has not been addressed in previous studies. Controlling key process factors including temperature, pH, and carbon to nitrogen ratio is critical in improving biogas yield. Biogas hybridisation is yet to be explored in depth. The majority of researches are focused on mono-digestion, feedstock co-digestion, modelling, and optimisation of anaerobic digestion needs significant further investigations. A multi-objective approach taking all technical and economic parameters in the modelling and optimization is essential.

How to assess the efficiency of the agro waste disintegration process

The objective of the study was to verify whether the method of determining the efficiency of sewage sludge disintegration, i.e. the disintegration degree (DD), can also be used to assess agro-waste disintegration. The following types of agro waste were tested: remains of fruits, sugar beet pulp and sugar beet pulp in the form of pellets. It was shown that DD as used for sewage sludge can also be a useful tool in assessing the disintegration efficiency of agro waste, although it requires the following modifications: (a) a methodology of chemical hydrolysis for each type of agro waste in order to determine the total amount of soluble COD (SCOD) that can be released from the sample and (b) possible changes in the SCOD value that may occur in the sample left in ambient conditions for a time period corresponding to the duration of the disintegration process. DD of agro waste determined according to the formula adopted for sewage sludge resulted in a considerable overestimation of the value in comparison to the formula proposed by the authors, i.e. DD determined for an energy density of 35 kJ L^-1 was higher by 55.9 ± 21.5%, and for an energy density of 140 kJ L^-1 it was higher by as much as 73.8 ± 28.1%. Taking into account the differences in the determined disintegration degree values that result from the methodology of conducting the chemical hydrolysis, it is recommended, in order to assess the efficiency of disintegration of agro waste, to use the efficiency of organic compound release.

Insight into the recent advances of microwave pretreatment technologies for the conversion of lignocellulosic biomass into sustainable biofuel

The utilization of renewable lignocellulosic biomasses for bioenergy synthesis is believed to facilitate competitive commercialization and realize affordable clean energy sources in the future. Among the pathways for biomass pretreatment methods that enhance the efficiency of the whole biofuel production process, the combined microwave irradiation and physicochemical approach is found to provide many economic and environmental benefits. Several studies on microwave-based pretreatment technologies for biomass conversion have been conducted in recent years. Although some reviews are available, most did not comprehensively analyze microwave-physicochemical pretreatment techniques for biomass conversion. The study of these techniques is crucial for sustainable biofuel generation. Therefore, the biomass pretreatment process that combines the physicochemical method with microwave-assisted irradiation is reviewed in this paper. The effects of this pretreatment process on lignocellulosic structure and the ratio of achieved components were also discussed in detail. Pretreatment processes for biomass conversion were substantially affected by temperature, irradiation time, initial feedstock components, catalyst loading, and microwave power. Consequently, neoteric technologies utilizing high efficiency-based green and sustainable solutions should receive further focus. In addition, methodologies for quantifying and evaluating effects and relevant trade-offs should be develop to facilitate the take-off of the biofuel industry with clean and sustainable goals.

Enhanced biogas production from anaerobic digestion of wastewater from the fruit juice industry by sonolysis: experiments and modelling

The aim of this study is to investigate the use of ultrasound pretreatment as potential technique to solubilize organic matter and fermentation of fruit juice effluents in anaerobic batch reactor. The efficacy of ultrasound pretreatment has been assessed at low frequency of 20 kHz and at different sonication times (20, 40 and 60 min). Compared with control, the amount of biogas produced increased by 47, 57 and 60% for sonication times of 20, 40 and 60 min, respectively. Methane content of the produced biogas was about 59% in the control and 64% in the case of effluent subjected to ultrasonication for 60 min. After 20 days of anaerobic digestion of the fruit juice effluents, the efficiency of chemical oxygen demand (COD) increased by 9, 31 and 35% with respect to control for sonication times of 20, 40 and 60 min, respectively, corresponding to total sugars uptake efficiency of about 35, 51 and 54%, respectively. The modified Gompertz equation was used to describe the cumulative biogas production. A good agreement was found between simulated and experimental data.

Sugarcane Bagasse as a Co-Substrate with Oil-Refinery Biological Sludge for Biogas Production Using Batch Mesophilic Anaerobic Co-Digestion Technology: Effect of Carbon/Nitrogen Ratio

Man-made organic waste leads to the rapid proliferation of pollution around the globe. Effective bio-waste management can help to reduce the adverse effects of organic waste while contributing to the circular economy at the same time. The toxic oily-biological sludge generated from oil refineries’ wastewater treatment plants is a potential source for biogas energy recovery via anaerobic digestion. However, the oily-biological sludge’s carbon/nitrogen (C/N) ratio is lower than the ideal 20–30 ratio required by anaerobic digestion technology for biogas production. Sugarcane bagasse can be digested as a high C/N co-substrate while the oily-biological sludge acts as a substrate and inoculum to improve biogas production. In this study, the best C/N with co-substrate volatile solids (VS)/inoculum VS ratios for the co-digestion process of mixtures were determined empirically through batch experiments at temperatures of 35–37 °C, pH (6–8) and 60 rpm mixing. The raw materials were pre-treated mechanically and thermo-chemically to further enhance the digestibility. The best condition for the sugarcane bagasse delignification process was 1% (w/v) sodium hydroxide, 1:10 solid-liquid ratio, at 100 °C, and 150 rpm for 1 h. The results from a 33-day batch anaerobic digestion experiment indicate that the production of biogas and methane yield were concurrent with the increasing C/N and co-substrate VS/inoculum VS ratios. The total biogas yields from C/N 20.0 with co-substrate VS/inoculum VS 0.06 and C/N 30.0 with co-substrate VS/inoculum VS 0.18 ratios were 2777.0 and 9268.0 mL, respectively, including a methane yield of 980.0 and 3009.3 mL, respectively. The biogas and methane yield from C/N 30.0 were higher than the biogas and methane yields from C/N 20.0 by 70.04 and 67.44%, respectively. The highest biogas and methane yields corresponded with the highest C/N with co-substrate VS/inoculum VS ratios (30.0 and 0.18), being 200.6 mL/g VSremoved and 65.1 mL CH4/g VSremoved, respectively.

The Effect of Static Magnetic Field on Methanogenesis in the Anaerobic Digestion of Municipal Sewage Sludge

The present study aimed to determine the effect of a 17.6 mT static magnetic field (SMF) on the efficiency of anaerobic digestion (AD) of municipal sewage sludge (MSS). The SMF had a significant impact on methane (CH4) production efficiency, the levels of fermentation rate (ηFMSS) vs. removal rate (ηVS), and the structure of the anaerobic bacteria consortium, but it did not affect cumulative biogas production. The highest CH4 yield (431 ± 22 dm3CH4/kgVS) and the highest methane content in the biogas (66.1% ± 1.9%) were found in the variant in which the SMF exposure time was 144 min/day. This variant also produced the highest ηFMSS and ηVS values, reaching 73.8% ± 2.3% and ηVS 36.9% ± 1.6%, respectively. Longer anaerobic sludge retention time in the SMF area significantly decreased AD efficiency and caused a significant reduction in the number of methanogens in the anaerobic bacteria community. The lowest values were observed for SMF exposure time of 432 min/day, which produced only 54.8 ± 1.9% CH4 in the biogas. A pronounced reduction was recorded in the Archaea (ARC915) and Methanosaeta (MX825) populations in the anaerobic sludge, i.e., to 20% ± 11% and 6% ± 2%, respectively.

Effect of Low-Temperature Conditioning of Excess Dairy Sewage Sludge with the Use of Solidified Carbon Dioxide on the Efficiency of Methane Fermentation

This study aimed to determine the effect of the low-temperature conditioning of excess dairy sewage sludge using solidified carbon dioxide on the efficiency of methane fermentation. An increase in the solidified carbon dioxide to excess dairy sewage sludge volumetric ratio above 0.3 had no significant effect on chemical oxygen demand concentration in the dissolved phase. The highest chemical oxygen demand values, ranging from 490.6 ± 12.9 to 510.5 ± 28.5 mg·dm−3, were determined at solidified carbon dioxide to excess dairy sewage sludge ratio ranging from 0.3 to 0.5. The low-temperature conditioning caused ammonia nitrogen concentration to increase from 155.2 ± 10.2 to 185.9 ± 11.1 mg·dm−3 and orthophosphates concentration to increase from 198.5 ± 23.1 to 300.6 ± 35.9 mg·dm−3 in the dissolved phase. The highest unitary amount of biogas, reaching 630.2 ± 45.5 cm3·g o.d.m.−1, was produced in the variant with the solidified carbon dioxide to excess dairy sewage sludge volumetric ratio of 0.3. Methane content of the biogas produced was at 68.7 ± 1.5%. Increased solidified carbon dioxide dose did not lead to any significant changes in biogas and methane production. The efficiency of biogas production from unconditioned excess dairy sewage sludge was lower by 43.0 ± 3.2%. The analysis demonstrated that the low-temperature conditioning is an energetic viable technology aiding the methane fermentation process.

Enhancement on enzymolysis of pigskin with ultrasonic assistance

The fur is hard to decompose during the fermentation process of diseased swine carcasses. In order to enhance the enzymolysis of pigskin, the ultrasonic was proposed to use during the process of the enzymatic hydrolysis. The response surface optimization experiments were carried out with the DH (degree of hydrolysis) as the response value and the optimum conditions for enzymatic hydrolysis were determined. Based the optimum conditions, orthogonal experiments were carried out with ultrasonic frequency, power and time as variables, and optimal ultrasonic parameters were obtained. Without the assistance of ultrasonic, the descending order of influence factors on DH was, temperature＞SC(Substrate concentration)＞RES(The ratio of enzyme to substrate)＞pH. Moreover, the DH value is of 10.42% under the following optimal conditions: RES is of 16,006 U/g, the temperature is of 48.92°C, the SC is of 59.76 g/L and pH is of 10.43. Frequency has the greatest effect on DH, followed by power, and finally time. The optimum hydrolysis time is of 5 h, and the DH is of 22.94% were obtained under the following optimum ultrasonic pretreatment conditions: frequency combination is of (20,40,40), power is of 600 W and time is of 25 min. Comparing with the group without ultrasonic pretreatment, the DH for the ultrasonic assistance increased by 4%, the hydrolysis time was shorten by 3 h, and the total amino acids increased by 15.98%.

Biomethane recovery from olive mill residues through anaerobic digestion: A review of the state of the art technology

Residues from production of olive oil are generated yearly in great amounts, both in liquid and solid forms. Different waste treatment systems were proposed in literature, to minimize environmental pollution while raising the energy recovery. Anaerobic digestion is one of the available routes to recover energy from waste via production of biogas while reducing organic load and pollutants to the environment. The use of farming and agro industrial wastes as co-substrate in anaerobic digestion can induce benefits related to the simultaneous treatment of different wastes. In particular, co-digestion can significantly enhance the process stability as well as the bio-methane generation. This work aims at reviewing the latest achievements in anaerobic digestion of olive mill residues, focusing on the aspects that can mostly favor the process, principally from a technical but also from an economical point of view. For the mono-digestion processes, methane yields up to 419 L_CH4 kg_VS^-1 were reported for olive mill wastewaters (Calabrò et al. 2018), while a production of 740 L_CH4 kg_VS^-1 was achieved when digesting olive mill solid waste together with olive mill wastewater and milk whey (Battista et al. 2015). An increase up to 143% in the methane yield was also reported when the feedstock was subjected to a 5 days aeration before digesting it in a semi-continuous stirred tank reactor (González-González and Cuadros 2015).

Anaerobic Co-Digestion of Wastewater Sludge: A Review of Potential Co-Substrates and Operating Factors for Improved Methane Yield

Anaerobic digestion has been widely employed in waste treatment for its ability to capture methane gas released as a product during the digestion. Certain wastes, however, cannot be easily digested due to their low nutrient level insufficient for anaerobic digestion, thus co-digestion is a viable option. Numerous studies have shown that using co-substrates in anaerobic digestion systems improve methane yields as positive synergisms are established in the digestion medium, and the supply of missing nutrients are introduced by the co-substrates. Nevertheless, large-scale implementation of co-digestion technology is limited by inherent process limitations and operational concerns. This review summarizes the results from numerous laboratory, pilot, and full-scale anaerobic co-digestion (ACD) studies of wastewater sludge with the co-substrates of organic fraction of municipal solid waste, food waste, crude glycerol, agricultural waste, and fat, oil and grease. The critical factors that influence the ACD operation are also discussed. The ultimate aim of this review is to identify the best potential co-substrate for wastewater sludge anaerobic co-digestion and provide a recommendation for future reference. By adding co-substrates, a gain ranging from 13 to 176% in the methane yield was accomplished compared to the mono-digestions.

Waste-activated sludge disruption by dry ice: bench scale study and evaluation of heat phase transformations

The freezing process consists of dissipating heat from the product until the final temperature is lower than the temperature of crystallisation of that product. Freezing can be used for numerous applications, including for disruption of waste-activated sludge (WAS). The aim of this study was to calculate the estimated amount of heat conveyed between the solidified carbon dioxide and the WAS, in the following ratios: 0.25:1; 0.5:1; 0.75:1 and 1:1. In heat of phase transformations, dry ice sublimation, water solidification, the amount of heat transferred by other substances and heat transferred from the sludge (dry sludge) were taken into account during the process of WAS freezing. Heat changes on the surface of WAS were registered using a thermovision camera. The effectiveness of WAS disintegration was confirmed by several biochemical parameters such as soluble chemical oxygen demand (increase over 14 times), degree of disintegration (48%), proteins (increase over 5 times), carbohydrates (increase almost 7 times), RNA (increase by 2.23 mg L^-1), ammonia nitrogen (increase over 23 times), phosphates (increase almost 27 times) and turbidity (increased over 7 times). It was found that dry ice pretreatment of WAS can be an intriguing alternative for the conventional methods used.

Improvement of the thermophilic anaerobic digestion and hygienisation of waste activated sludge by synergistic pretreatment

Hybrid disintegration of waste activated sludge (WAS) before the thermophilic anaerobic stabilization of WAS contributes to the intensification of organic compounds decomposition and increases the effectiveness of the anaerobic stabilization process compared to the fermentation of raw WAS. This article investigates the influence of a chemical-thermal pretreatment procedure with the use of NaOH and freezing by the dry ice on WAS. We found that the hybrid pretreatment of WAS causes higher concentration of released organics in the liquid phase (represented here as a change in soluble chemical oxygen demand - SCOD value) in comparison to these disintegration techniques used separately. The use of disintegrated WAS (WASD) as an additional material in the digester chambers impacts (varying on its proportion added), the generation of biogas and its yield. The recorded amount of the produced biogas and biogas yield after 21 days of fermentation increased by 26.6% and 2.7%, respectively (in comparison to blank sample). In addition, it was observed that the hybrid process before anaerobic stabilization contributes to a higher hygienisation of the digested sludge.

Disintegration of Wastewater Activated Sludge (WAS) for Improved Biogas Production

Due to rapid urbanization, the number of wastewater treatment plants (WWTP) has increased, and so has the associated waste generated by them. Sustainable management of this waste can lead to the creation of energy-rich biogas via fermentation processes. This review presents recent advances in the anaerobic digestion processes that have led to greater biogas production. Disintegration techniques for enhancing the fermentation of waste activated sludge can be apportioned into biological, physical and chemical means, which are included in this review; they were mainly compared and contrasted in terms of the ensuing biogas yield. It was found that ultrasonic- and microwave-assisted disintegration provides the highest biogas yield (>500%) although they tend to be the most energy demanding processes (>10,000 kJ kg−1 total solids).

Cell Disruption of Chaetoceros calcitrans by Microwave and Ultrasound in Lipid Extraction

Downstream processing, such as cell disruption and extraction, constitutes a key step in microalgal-based industrial bioprocesses, mainly due to high costs and environmental impact. In this context, extraction technologies need to be improved, including the use of nonconventional cell disruption techniques suitable for scale-up, such as microwave and ultrasound. Therefore, this study aimed at investigating the effects of different methods of cell disruption (microwave and ultrasound) on lipid extraction from biomass of the diatom Chaetoceros calcitrans cultured in mixotrophic conditions in a medium with natural sea water and residual glycerol, with different treatment times. Both techniques applied to the biomass were efficient; that is, the results were 24.6 ± 1.3% lipids (ultrasound for 5 min) and 24.2 ± 0.9% lipids (microwave for 40 s), with no significant differences between them (p≥0.05). Likewise, there was no significant difference regarding the chemical disruption with hydrochloric acid 2 M as control (24.2 ± 1.0%). The ultrasound method consumed less energy than the microwave method. Both cell disruption methods applied to the biomass resulted in changes in the fatty acid profiles, that is, percentages of saturated fatty acids increased from 7.7% (control) to 16.6% (microwave) and 15.5% (ultrasound), whereas polyunsaturated ones increased from 12.8% (control) to 22.8% (microwave) and 21.8% (ultrasound). Concerning monounsaturated fatty acids, percentages decreased from 79.5% (control) to 60.6% (microwave) and 62.7% (ultrasound).

Emerging technologies for the pretreatment of lignocellulosic biomass

Pretreatment of lignocellulosic biomass to overcome its intrinsic recalcitrant nature prior to the production of valuable chemicals has been studied for nearly 200 years. Research has targeted eco-friendly, economical and time-effective solutions, together with a simplified large-scale operational approach. Commonly used pretreatment methods, such as chemical, physico-chemical and biological techniques are still insufficient to meet optimal industrial production requirements in a sustainable way. Recently, advances in applied chemistry approaches conducted under extreme and non-classical conditions has led to possible commercial solutions in the marketplace (e.g. High hydrostatic pressure, High pressure homogenizer, Microwave, Ultrasound technologies). These new industrial technologies are promising candidates as sustainable green pretreatment solutions for lignocellulosic biomass utilization in a large scale biorefinery. This article reviews the application of selected emerging technologies such as ionizing and non-ionizing radiation, pulsed electrical field, ultrasound and high pressure as promising technologies in the valorization of lignocellulosic biomass.

Cook Your Samples: The Application of Microwave Irradiation in Speeding Up Biological Processes

Classic and conventional procedures in molecular cloning are inherent compositions in modern molecular biological experiments and are frequently involved in daily laboratory activities. They take up the majority of the total time input in spite of the availability of well-designed specialized commercial kits. A similar situation is also in the field of biotechnology. Fortunately, microwave/ultrasonic irradiation has been found to be capable of speeding up these processes, such as proteolysis in sample preparation for proteomics research, and digestion, ligation, (de)phosphorylation of DNA with the corresponding enzymes, even the introduction of DNA samples to recipient cells, and biotransformation (e.g., the production of biodiesel). Microwave/ultrasonic irradiation, when used solely or in combination with other existing operations, makes it possible to finish these time-consuming processes in as short as 1 min with comparable or even improved efficiency, and there is no need of reagent upgradation. The adoption of irradiation is ideal because it eliminates any possible side effects of the chemicals used as performance enhancer(s) that will inevitably make the system more complicated at least. More notably, the needed irradiation in the laboratory can be generated by a common microwave oven or ultrasonic cleaner. Taken together, microwave/ultrasonic irradiation provides an accessible method to make the procedures mentioned above time- and cost- efficient. In this article, we reviewed the relevant literature and discussed the experiment and mechanism details.