Low strength ultrasonication positively affects the methanogenic granules toward higher AD performance. Part I: physico-chemical characteristics

Ultrasonic Disintegration of Municipal Sludge: Fundamental Mechanisms, Process Intensification and Industrial Sono-Reactors

Sludge disintegration is an environmental and industrial challenge that requires intensive research and technological development. Sludge has a complex structure with a high yield of various chemical and biological compounds. Anaerobic digestion is the most commonly used process for sludge disintegration to produce biogas, detoxify sludge, and generate biosolids that can be used in agriculture . Biological cell lysis is the rate-limiting cell lysis. This review discusses the application of sonolysis as a sludge pretreatment for enhanced anaerobic digestion via three combined processes: thermal destruction, hydrochemical shear forces, and radical oxidation. The mechanistic pathways of sono-pretreatment to enhance biogas, sludge-enhanced dewatering, activation of filamentous bacteria, oxidation of organic pollutants, release of heavy metals, reduction of bulking and foaming sludge, and boosting ammonia-oxidizing bacterial activity are discussed in this review. This article also discusses the use of ultrasound in sludge disintegration, highlighting its potential in conjunction with Fenton and cation-binding agents, and reviews common large-scale sonoreactors available on the market..

Effect of ultrasonic application during KOH pretreatment and anaerobic process on digestion performance of wheat straw

The digester performance was enhanced by ultrasonic application during pretreatment and the anaerobic digestion (AD) process. Two setups (with and without ultrasonic application) were applied during pretreatment and AD, to untreated and potassium hydroxide (KOH) pretreated wheat straw. The results confirmed that the ultrasonic application enhanced the hydrolysis process during pretreatment. The highest total volatile fatty acid (TVFA) (3012 ± 18 mg L⁻¹) production and overall lignin, hemicellulose, and cellulose (LHC) reductions (22.4 ± 0.5%) were obtained from ultrasonic assisted KOH pretreated (KOH_U) samples, after 36 hours of pretreatment. Similarly, the SEM analysis showed obvious disruption in the outer structure of KOH_U samples. However, the ultrasonic application during AD showed significant improvement in biodegradation rate, biogas and biomethane production. Obvious reduction in sonication time (76%) along with enhanced biogas (570 ± 9 mL gm⁻¹ VS) and biomethane (306 ± 12 mL gm⁻¹ VS) production were observed from KOH pretreated digesters, when ultrasonication was applied during AD. Moreover, the biodegradation rate reached up to 76% along with highest total solid (TS) and volatile solid (VS) reductions from digesters with single KOH pretreatment and ultrasonic influence during AD. Finally, the digester effluent ranged in between the stable values, confirming the completion of the AD process. These results suggested that ultrasonic application was more effective when applied during AD due to the higher liquid to solid ratio. The reduction in energy input can be beneficial for commercial applications and to recognize the better stage for ultrasonic application for enhanced biomethane yield.

The effect of ultrasonic application during KOH pretreatment and anaerobic digestion of wheat straw.

Intensifying the biogas production from food waste using ultrasound: Understanding into effect of operating parameters

The present work depicts the novel approach of using ultrasound (US) induced cavitation for feedstock pretreatment with an objective of improving the anaerobic digestion (AD) process for biogas production. Initially the critical analysis of literature based on US for improving the biogas production has been presented briefly followed by study on use of US in pretreatment of food waste (FW) as well as during AD to quantify the intensification in the biogas production. Effect of different operating parameters for pretreatment such as irradiation time (over the range 2-14 min), power density (0.2-1 W/mL), duty cycle (20-80%) and substrate loading (3-11%w/v) has been investigated. Highest increase in soluble chemical oxygen demand (sCOD) with final value as 18500 mg/L (±20) (increase of 61.5%) was obtained at optimum treatment conditions of 10 min as irradiation time, 0.4 W/mL as power density, 60% as duty cycle and 7% w/v as the substrate loading. Pretreated FW was further subjected to low intensity US assisted AD process. Parameters optimized for biogas production were power (over the range of 100-200 W), irradiation time (5-15 min) and duty cycle (10-60%). Increased biogas production (almost two times) with 80% sCOD removal after 15 days was obtained at best conditions of 200 W as the US power (0.04 W/mL as power density), 5 min as the US irradiation time and 10% of US duty cycle while AD process without US resulted in only 48% sCOD removal within same time period. The work demonstrated the effective application of US in the pretreatment of feedstock and subsequent AD process giving much higher yield of biogas in shorter time.

Physicochemical characteristics of microbial content in a modified anaerobic inclining-baffled reactor (MAI-BR) treating recycled paper mill effluent (RPME)

Microbial content formed in bioreactors plays a significant role in the anaerobic process. Therefore, the physicochemical characteristics of microbial content in a modified anaerobic inclining-baffled reactor (MAI-BR) treating recycled paper mill effluent (RPME) were investigated using Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric (TG), and derivative thermogravimetric (DTG) analyses, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), Brunauer-Emmett-Teller (BET), and surface area analyzer. FTIR spectra revealed that the microbial content had stronger characteristic peaks corresponding to alcohols, water, lipids carbohydrates, proteins, and mineral compounds. Calcite, muscovite, and lepidolite were the prevalent mineral phases found by XRD analysis. The elemental of these minerals like C, Ca, N, O, and Si was confirmed by XPS results. The microbial content samples from each compartment showed similar thermal behavior. SEM images showed that straight rod-shaped and Methanosaeta-like microorganisms were predominant, whereas C, O, and Ca were noticed by EDS on the surface of granules. The BET surface areas and pores of granules are found to decline throughout the reactor's compartment, where Compartment 1 had the largest values. Thus, the findings of this study establish further understanding of the physicochemical properties of microbial content formed in MAI-BR during the RPME treatment.

Recent advances on biogranules formation in dark hydrogen fermentation system: Mechanism of formation and microbial characteristics

Hydrogen producing granules (HPGs) are most promising biological methods used to treat organic rich wastes and generate clean hydrogen energy. This review provides information regarding types of immobilization, supporting materials and microbiome involved on HPG formation and its performances. In this review, importance has been given to three kinds of immobilization techniques such as adsorption, encapsulation, and entrapment. The HPG, characteristics and types of organic and inorganic supporting materials followed for enhancing hydrogen yield were also discussed. This review also considers the applications of HPG for sustainable and high rate hydrogen production. A detailed discussion on insight of key mechanism for HPGs formation and its performances for stable operation of high rate hydrogen production system are also provided.

Ultrasound-assisted biological conversion of biomass and waste materials to biofuels: A review

Ultrasound irradiation has been gaining increasing interests over the years to assist biological conversion of lignocellulosic biomass and waste materials to biofuels. As such, this study reviewed the different effects of sonication on pre-treatment of lignocellulosic biomass and waste materials prior to biofuel production. The mechanisms of ultrasound irradiation as a pre-treatment technique were initially described and the impacts of sonication on disruption of lignocellulosic materials, alteration of the crystalline lattice structure of cellulose molecules, solubilisation of organic matter, reducing sugar production and enzymatic hydrolysis were then reviewed. Subsequently, the influences of ultrasound irradiation on bio-methane, bio-hydrogen and bio-ethanol production were re-evaluated, with most studies reporting enhanced biofuel production from anaerobic digestion or fermentation processes. Nonetheless, despite its positive impacts on biofuel production, sonication was found to be energetically inefficient based on the lab-scale studies reviewed. To conclude, this study reviewed some of the challenges of ultrasound irradiation for enhanced biofuel production while outlining some areas for further research.

Low-strength ultrasonication positively affects methanogenic granules toward higher AD performance: Hydrolytic enzyme excretions

In our previous studies, enhanced methane (CH₄) production using low-strength ultrasonication was achieved and the results were evidenced by physico-chemical and molecular biological approaches. As a final continuation study, the effects of low-strength ultrasonication on the activities of hydrolytic enzymes (amylase, cellulase, and protease) were investigated on methanogenic granules given that hydrolysis regulates the whole anaerobic digestion (AD) process. Up to 213% enhanced hydrolytic enzyme activities were observed, and they seem to be highly related to the enhanced CH₄ production. However, the effects of ultrasonication on the distribution (liquid- and solid-phases) of hydrolytic enzymes were negligible. Enzymatic activation by low-strength ultrasonication was hypothetically caused by acoustic streaming, presumably enabling to overcome the masking effect, substrate inhibition and spatial constraint.

Low-strength ultrasonication positively affects methanogenic granules toward higher AD performance: Implications from microbial community shift

To elucidate the enhanced methane yield from organic wastes, the effects of low-strength ultrasonication on the microbial community structures in upflow anaerobic sludge blanket reactors were for the first time analyzed using pyrosequencing. Interestingly, a more even microbial community was observed in the ultrasonicated granules than in the control, which could compensate for the decreased richness and resulted in comparable (archaea) or even higher (bacteria) diversity. The ultrasonicated granules contained higher levels of δ-Proteobacteria, of which many are reportedly potential syntrophs, as well as methanogenic genera Methanosaeta, Methanotorris, and Methanococcus. The increased presence of syntrophic bacteria with their methanogenic partners was discussed with respect to hydrogen flux; their selective proliferation seems to be responsible for the enhanced anaerobic performance. This study is the first research shedding light on the novel function of low-strength ultrasound shifting the microbial structure towards better biogas production performance, and will facilitate application of low-strength ultrasound to other bioprocesses.

Example study for granular bioreactor stratification: Three-dimensional evaluation of a sulfate-reducing granular bioreactor

Recently, sulfate-reducing granular sludge has been developed for application in sulfate-laden water and wastewater treatment. However, little is known about biomass stratification and its effects on the bioprocesses inside the granular bioreactor. A comprehensive investigation followed by a verification trial was therefore conducted in the present work. The investigation focused on the performance of each sludge layer, the internal hydrodynamics and microbial community structures along the height of the reactor. The reactor substratum (the section below baffle 1) was identified as the main acidification zone based on microbial analysis and reactor performance. Two baffle installations increased mixing intensity but at the same time introduced dead zones. Computational fluid dynamics simulation was employed to visualize the internal hydrodynamics. The 16S rRNA gene of the organisms further revealed that more diverse communities of sulfate-reducing bacteria (SRB) and acidogens were detected in the reactor substratum than in the superstratum (the section above baffle 1). The findings of this study shed light on biomass stratification in an SRB granular bioreactor to aid in the design and optimization of such reactors.

The anaerobic digestion of biologically and physicochemically pretreated oily wastewater

To enhance the degradation of oily wastewater and its biogas production, a biological-physicochemical pretreatment was introduced prior to the anaerobic digestion system. The digestion thereafter proceeded more efficiently due to the inoculation by oil degrading bacteria (Bacillus). A 2-stage pre-mixing is more effective than directly mixing. The effects on the methane production were also investigated by pre-treatment with ultrasonic (US) treatment, combined with citric acid (CA) addition. US pre-treatment was found to improve the initial methane production, and CA pre-treatment could maintain this improvement during the whole digestion stage. Pre-mixing Bacillus at 9 wt.% inoculation, combined with US for 10 min and a CA concentration of 500 mg/L provided the optimum conditions. The most effective enhancement of methane yield was 1100.46 ml/g VS, exceeding that of the control by 280%. The change of coenobium shape and fatty acid content further proved that such pretreatment of oily wastewater can facilitate digestion.

Application of low-strength ultrasonication to the continuous anaerobic digestion processes: UASBr and dry digester

In this study, the positive effects of low-strength ultrasonication (LS-ultrasonication) on the anaerobic digestion (AD) performance were investigated by continuously operating an upflow anaerobic sludge blanket reactor (UASBr) and a dry digester. In the ultrasonicated UASBr system (1 s per min, 0.05 W/mL), ultrasonication enhanced the CH₄ production by 38% and 19% in an ambient and a mesophilic condition, respectively. In addition, a different sludge yield and a changed electron flow were observed after ultrasonication. In the ultrasonicated dry digestion system (2 s per 30 s, 0.0025 W/mL), a 40% increase in the production of CH₄ was observed after lowering the total solid content of the reactor from 12% to 10%, implying that a high solid content diminished the ultrasonic stimulation effect. Moreover, the ultrasonication strength itself appeared to be a more crucial factor than the ultrasonication density during the application of LS-ultrasonication in the AD system.