enhanced anaerobic digestion by ultrasonic pretreatment of organic residues for energy production

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Journal of Cleaner Production xxx (2014) 1e6

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Enhanced anaerobic digestion by ultrasonic pretreatment of organicresidues for energy production

Alessandra Cesaro a,*, Silvana Velten b, Vincenzo Belgiorno a, Kerstin Kuchta c

a Sanitary and Environmental Engineering Division (SEED), Department of Civil Engineering, University of Salerno, via Giovanni Paolo II, 84084 Fisciano, SA,ItalybUltrawaves GmbH, Kasernenstrasse 12, D-21073 Hamburg, GermanycHamburg University of Technology (TUHH), Institute of Environmental Technology and Energy Economics, Harburger Schlossstrasse 36, 21079 Hamburg,Germany

a r t i c l e i n f o

Article history:Received 11 September 2013Received in revised form10 March 2014Accepted 10 March 2014Available online xxx

Keywords:Anaerobic digestionClean energyProteinSubstrate compositionUltrasound

* Corresponding author. Tel.: þ39 089 96 4181; faxE-mail addresses: [email protected] (A. Cesaro

(S. Velten), [email protected] (V. Belgiorno), kuchta

http://dx.doi.org/10.1016/j.jclepro.2014.03.0300959-6526/� 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Cesaro, Aproduction, Journal of Cleaner Production (2

a b s t r a c t

This study is aimed at assessing the improvement of anaerobic digestion yields produced by the ultra-sonic pretreatment of solid organic substrates.

The possibility of enhancing methane production from anaerobic digestion of organic residues isparticularly interesting. To this end, application of pretreatments represents one of the most suitableoptions. Among pretreatments, sonolysis arises as a novel technology, whose effectiveness is related tothe occurrence of cavitation phenomena. They promote the solubilisation of organic matter, thusincreasing the amount of substrate that can be converted into methane.

The application of ultrasound as pretreatment of sewage sludge for biological processing has beenextensively studied and full-scale facilities have already been established.

However, ultrasonic pretreatment of solid organic substrates has not yet been actioned as its effec-tiveness is highly influenced by process conditions.

Experimental results show that ultrasonic energy inputs in the range 31e93 W h/L enhanced biogasproduction from differently composed substrates up to 71%. The highest increase was found forlignocellulosic-based materials and was related to improvements in solubilisation. Conversely, a lowerbiogas enhancement, in the range 3e23%, was found for protein-rich substrates. In this case, any relevantvariation in soluble COD was observed after ultrasonic pretreatment.

Thus, dependent upon the specific substrate composition, the relationship between solubilisation andanaerobic biodegradability was found to differ significantly and this evidence represents a key point forscaling up of the combined ultrasonic/anaerobic digestion process.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In Europe, energy consumption increased by 3.3% in 2010,compared to 2009 (EU, 2012). Most of this energy came from crudeoil and petroleum products, which accounted for 35% of the totalenergy consumed in 2010. A slight reduction in the use of theseproducts was observed with reference to the previous year, as aresult of European strategies directed towards the promotion ofsustainable energy production.

In this context, anaerobic digestion represents an interestingoption for the conversion of biomass to energy (Arafat et al., in

: þ39 089 96 9620.), [email protected]@tuhh.de (K. Kuchta).

., et al., Enhanced anaerobic014), http://dx.doi.org/10.101

press; Menikpura et al., 2013). The process consists of the biolog-ical degradation of organic matter in the absence of oxygen, tocreate a methane-rich biogas that can be used to produce energy.

The substrates characterised by high level of biodegradablematerials are the most suitable for anaerobic processes (Banks andZhang, 2010).

Anaerobic digestion is a well-established technology for therecovery of clean energy from solid organic waste (Cesaro et al.,2010) and the current interest is directed towards process optimi-sation in order to maximise the methane production (Siles et al.,2013).

Several studies (Carlsson et al., 2012) suggest that ultrasound(US) application can positively affect anaerobic digestion throughthe occurrence of acoustic cavitation phenomena. These phenom-ena promote the physical disintegration of organic matter or the

digestion by ultrasonic pretreatment of organic residues for energy6/j.jclepro.2014.03.030

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Fig. 1. Protigrain� composition.

Table 1US treatment conditions.

Energy [W h/L] Specific energy [kJ/kg TS]

OFMSW digestate DDGS DDGS digestate

31 2102 � 21 2835 � 63 2264 � 462 4219 � 40 5630 � 122 4581 � 5993 6291 � 95 8493 � 252 6858 � 46

A. Cesaro et al. / Journal of Cleaner Production xxx (2014) 1e62

extraction of substances (Neis et al., 2001) along with theenhancement of enzymatic activity (Banduch, 2011; Xie et al.,2009): as a result, an increase in biogas production can be pursued.

The ultrasound-improved anaerobic digestion has been largelyapplied for sludge treatment (El-Hadj et al., 2007; Naddeo et al.,2009; Tiehm et al., 2001) and full-scale applications have alreadybeen tested (Hogan et al., 2004; Neis et al., 2012).

However, the spread of this technology to differently composedorganic substrates has been limited, mainly due to the variability ofits yield (Pilli et al., 2011). Indeed, several factors influence theextent of the sonolysis effects: the predominance of each oneduring the sonication as well as their role on the disintegrationmechanisms still need to be clarified, especially when ultrasound isapplied to solid organic residues.

This work is aimed at evaluating the effectiveness of theultrasound-improved biodegradability of different organicmatrices, in order to assess the influence of substrate compositionon the combined process.

To this end, both OFMSW (Organic Fraction of Municipal SolidWaste) digestate and DDGS (Dried Distilled Grains with Solubles)were investigated.

OFMSW digestate originates from the anaerobic treatment ofOFMSW and is characterised by a high biogas potential due to thepresence of residual and undigested volatile solids (Menardo et al.,2011). The anaerobic processing of OFMSW digestate represents aninteresting option (Fantozzi and Buratti, 2011) to take advantage ofits residual energetic potential and to reduce the amount to handleafter the treatment. However, the organic matter contained inOFMSW digestate is lignocellulosic-based (Hartmann et al., 2000)and hence not readily degradable by bacteria. Therefore, a suitablepretreatment is required to improve its biological degradability.

DDGS is a bio-ethanol fermentation by-product, characterisedby a prevailing protein content. Its anaerobic treatment is beingregarded as a valuable strategy to produce methane (Taheripouret al., 2010), then used to produce energy. This could reduce en-ergy consumption of bio-ethanol production process, thus makingit economically competitive with petroleum fuel production(Gonela and Zhang, 2014). Therefore, in this study, both raw DDGSand its residue from anaerobic digestion were investigated.

Experimental activity was carried out under the hypothesis thatanaerobic digestion yield improvement is directly related to thesolubilisation promoted by ultrasound, which determines an in-crease in the amount of readily digestible materials. Therefore,anaerobic biodegradability tests followed the evaluation of organicmatter solubilisation provided by ultrasonic treatments performedunder different operating conditions.

2. Materials and methods

2.1. Substrate and inoculum

Experimental work was carried out using the organic fraction ofmunicipal solid waste (OFMSW) digestate, as lignocellulosic ma-terial and dried distilled grainwith solubles (DDGS), as protein-richsubstrate.

The digestate was sampled at the OFMSW anaerobic digestionplant in Luebeck (Germany) and was characterised by a total solidcontent (TS) of 2.5% (�0.01).

The DDGS was produced by the Südzucker bio-ethanol plant inZeitz (Saxony-Anhalt, Germany), where it is sold as Protigrain�, acertified animal feed for cattle, pigs and poultry.

The DDGS showed a TS content of 93.00% (�0.02) and a proteincontent as reported in Fig. 1.

The digested DDGS was collected from a bench scale anaerobicfermenter using Protigrain� solution as feeding material. The

Please cite this article in press as: Cesaro, A., et al., Enhanced anaerobicproduction, Journal of Cleaner Production (2014), http://dx.doi.org/10.101

related anaerobic process was operated with a loading organic rateof 5 g VS/(L d) and a retention time of 7 d; the resulting digestatewas characterised by a dry matter content of 1.69% (�0.01).

The optimal condition for ultrasonic treatment requires a sub-stratewith a TS value ranging between 5 and 10%. Therefore the drymatter content of both OFMSW and DDGS digestates was adjustedby centrifugation at 10,000 rpm for 15 min.

Conversely, raw DDGS was milled and used to prepare a 5% TSsolution.

The inoculum for biological tests was thickened digested sludge(Radnige and Clarke, 2007), sampled at the municipal wastewatertreatment plant in Hamburg (Germany) and incubated at 35 �C for5 d ahead of experimental start up. The incubation step wasnecessary to reduce sludge own gas production. The inoculum wascharacterised by an average total solid (TS) content of 3.1% (�0.2)and a volatile solid (VS) content of 62.1% TS (�3.2).

2.2. Experimental set up

2.2.1. Ultrasonic unitUltrasound technology was applied using the low frequency

(20 kHz) sonication unit developed by Ultrawaves GmbH(Hamburg, Germany).

The system consisted of a sonotrode coupled to a generator,characterised by 1 kW nominal power. The power released to thesample was displayed on an energy counter attached to the powersource and it was recorded every 15 s, in order to calculate energyinput. The instrument amplitude was set to 100%, according tocompany recommendation and samples were sonicated for 5, 10and 15 min. The volume of substrate treated in each test was800 mL. Therefore, average energy inputs of approximately 30, 60and 90 W h/L were provided.

Each test was performed in a 1000 mL cylindrical beaker withthe US sonotrode placed at the centre and immersed up to 2 cm.The substrate was continuously stirred so as to avoid precipitationof solids to the bottom of the reactor.


Fig. 2. Disintegration degree of pretreated OFMSW digestate for increasing energyinputs.

Fig. 3. Variation of both organic carbon and nitrogen in OFMSW digestate solublefraction for increasing ultrasonic energy values.

A. Cesaro et al. / Journal of Cleaner Production xxx (2014) 1e6 3

The specific energy input to the samples is summarised inTable 1 and was calculated as the ratio between energy supplied(kW) and initial TS content of the mixture.

2.3. Analytical set up

TS and VS were determined following Standard Methods(AWWA, APHA, WEF, 1998).

The soluble fraction of investigated samples, obtained aftercentrifugation (14,000 rpm, 15 min) and filtration (<0.45 mm), wasanalysed in terms of COD, DOC and Nitrogen content. These pa-rameters were determined according to Standard Methods(AWWA, APHA, WEF, 1998) procedure, Multi N/C 2000 (Analy-tikjena, Jena, Germany) and Kjeldahl Method, respectively.

The disintegration degree was calculated according to thefollowing equation:

DDCOD ¼ ½ðCODUS � COD0Þ=ðCODNaOH � COD0Þ�$100 (1)

where: CODUS is the soluble COD of the sonicated sample; COD0 isthe soluble COD of the untreated one; CODNaOH is the soluble CODin the reference sample supernatant obtained with 1 M NaOHdigestion, as referenced in Muller (2001).

Anaerobic digestion tests were carried out, in triplicate, ac-cording to the methodology recommended in the German guide-line VDI 4630. Each test run was repeated twice.

For the biological tests, the Eudiometer systemwas used. It wascomposed of 500 mL reactors, each one connected to a Eudiometertube, which had been previously filled with a block solution, con-sisting of 200 g NaCl/L and 5 g/L of citric acid; few drops of methyl-orange were also added. Biogas produced in each reactor wasreleased in the tube, so that its volume could be measured dailythrough solution displacement. Biogas volume was calculated un-der normal conditions (273 K and 1013 hPa), according to the VDI4630 procedure. Biogas was sampled from suitable rubber septaand its composition was analysed by a GC-TCD, model HP 6890(Hewlett Packard), which allowed detection of both methane andcarbon dioxide.

pH was measured by pH meter (WTW, Germany) before andafter each biological test.

3. Results and discussion

3.1. Sonolysis effects on OFMSW digestate

Sonolysis of the OFMSW digestate determined an improvementin the degree of the organic matter disintegration: DDCOD rangedbetween 22 and 53% for increasing energy inputs (Fig. 2).

The increase in disintegration degree can be related to thedisruption of microbial cells as referenced by Neis et al. (2001) aswell as to the extraction of undigested lignocellulosic-based ma-terials into the solvent media (Garcia et al., 2011).

As a result of disintegration provided by ultrasound, the initialsoluble COD (sCOD) value of 4124 (�20)mg/L rose to 9085 (�2)mg/L.

Indeed, the increased soluble COD was consistent with theenhanced concentration of organic carbon and nitrogen in samplesoluble fractions (Fig. 3). The former was related to the solubilisa-tion of undigested lignocellulosic material, which represented arelevant source of carbon (Bhattacharyya et al., 2008), as well as tothe rupture of microbial cells. This also determined the release ofnitrogen, as reported in several studies dealing with ultrasonicsludge pretreatment (Neis et al., 2001; Bougrier et al., 2005).

In all samples, volatile solid content remained almost constant,suggesting that the amount of organic matter was not mineralised


by ultrasound application but remained available for conversioninto biogas. This outcome is particularly interesting as it indicatesthat ultrasonic energy promotes the simplification of organicstructures, enhancing their availability to anaerobic microorgan-isms rather than their conversion into inorganic products.

These results differ slightly from those obtained for raw OFMSWin a previous study (Cesaro and Belgiorno, 2013). In that case, theapplication of higher energy inputs determined lower solubilisa-tion as well as the occurrence of mineralisation phenomena. Thisevidence, however, should be related to both the composition ofthe substrate and the different operational conditions. In particular,the latter did not permit the occurrence of a homogeneous collapseof cavitational bubbles, so that the ultrasonic effects were notadequately distributed across the whole sample.

The enhanced solubilisation affected positively anaerobicbiodegradability of sonicated OFMSW digestate samples.

At the end of the anaerobic digestion tests, specific biogas vol-umes increased up to 71%, as shown in Fig. 4a. Moreover, biogascomposition analysis highlighted an increase in methane contentwith both ultrasonic energy inputs and digestion time (Fig. 4b). Thisoutcome was consistent with the increased amount of solubleorganic carbon promoted by sonolysis and it is particularly inter-esting for its technical implications. Methane increase indicatesthat, due to sonolysis, not only an improvement in hydrolysis rate


Fig. 4. Effects of increasing energy inputs on the cumulative biogas production fromOFMSW digestate (a.) and its methane content (b.).


can be achieved, but also an effective increase in the energy re-covery from the surplus biogas can be pursued.

The methaneecarbon dioxide ratio was almost constant andfixed on an average value of 1.9 � 0.1, which is comparable withscientific literature (Rasi et al., 2007).

However the recorded methane content (33e43%) was rela-tively low if compared with the more frequently ones detected inbiogas produced from the anaerobic treatment of other organicsubstrates.

The reason is that OFMSW digestate is a partially degradedsubstrate; therefore a considerable amount of carbon hasalready been converted into biogas. The gaseous product ob-tained from further anaerobic processing of this substrate rep-resents only its residual potential, as also highlighted inscientific literature. Menardo et al. (2011) found that themethane proportion of biogas produced by different digestatesamples in batch trials was always lower than 50%, with aminimum value of 13%.

It can be concluded that an increase in ultrasonic energy inputdetermined an improvement of organic material solubilisation,which was found to be in direct correlation with the enhancedbiogas production from sonicated OFMSWdigestate samples. Thesefindings are congruent with the ones obtained by ultrasonic pre-treatment of both mixed sludge (Chu et al., 2002) and waste acti-vated sludge (Wang et al., 1999), whose composition is comparablewith that of OFMSW digestate.


3.2. Sonolysis effects on protein-rich substrates

The characterisation of untreated and sonicated protein-richsamples with regard to main chemical and physical parameters issummarised in Table 2.

Data show that volatile solid content was almost constant forboth DDGS and DDGS digestate, indicating the absence of miner-alisation phenomena, as already observed for OFMSW digestate.

However, sCOD slightly decreased for raw DDGS while itincreased for its partially degraded form. In the latter case, adisintegration degree level up to 47% was found.

The analysis of the soluble fraction of DDGS digestate sampleshighlighted a direct correlation between both dissolved organiccarbon and total nitrogen and sCOD. This evidence suggested thatthe application of ultrasound resulted in disintegration mecha-nisms which promoted release of both organic carbon and nitrogenin the solvent phase, with a consequent increase in sCOD, as alreadydiscussed for OFMSW digestate.

Conversely, no relationship could be established among thesame parameters for raw DDGS, as shown in Table 2. According tothese data, sonolysis proved to be ineffective in increasing organicmatter solubilisation, both in terms of organic carbon and nitro-gen. Similarly, no qualitative variation of solubilised compoundswas observed, as sCOD values were found to oscillate within avery narrow range when increasing ultrasonic inputs wereapplied.

Although solubilisation did not enhance, DDGS anaerobicbiodegradability was observed to increase for improving sonolysisextent (Fig. 5).

Specific biogas volumes of DDGS were enhanced by ultrasonicenergy, but the increase was not as remarkable as the one observedfor OFMSW digestate, which reached 71% because of the enhancedsolubilisation.

Methane content remained constant, at an average value of65.1% (with a standard deviation of 1.0). This evidence wasconsistent with chemical and physical test results as, due tosonolysis, the amount of carbon that could be converted to biogaswithin the anaerobic process changed little.

These outcomes suggested that the ultrasound promoted theoccurrence of mechanisms not necessarily leading to the disinte-gration and solubilisation of organic matter. On the contrary, it isreasonable to assume that they determined a change of the organicsubstrate structure.

DDGS is composed principally of proteins, consisting of ami-noacid chains. These chains are linked together to form a complexfolded structure which determines the native configuration. Thealteration of the native structure of a protein, resulting in itsunfolding, is a phenomenon documented in food chemistry and istermed denaturation.

Environmental conditions, such as changes in temperature, pH,exposure to shear forces, contact with organic solvents andchemical denaturants, may induce destabilisation and cause thedenaturation of the protein (Schiffter, 2011). Therefore, the effectsof the cavitation phenomena provided by ultrasound, such as high-intensity shock waves, microjets, shear forces and turbulence(Mason and Peters, 2002), could promote similar effects. Denatur-ation usually results in the loss of biological function and/or specificproperties of the protein itself, such as the reduction of solubility(Chandrapala et al., 2012) and the increase in digestibility (Ljøkjelet al., 2000; Sagum and Arcot, 2000), as found in this investigation.

The de-agglomeration of the protein structure through unfold-ing makes it more susceptible to microorganism attack. The samephenomenon can influence solubility by allowing the exposure ofhydrophobic side chains rather than hydrophilic ones. Thus, theincrease in biogas volumes could be related to the occurrence of


Table 2Chemical and physical parameters of DDGS and DDGS digestate for increasing US energy values.

Energy input [W h/L] DDGS DDGS digestate

VS [%TS] sCOD [mg/L] DOC [g/L] sTKN [mg/L] VS [%TS] sCOD [mg/L] DOC [g/L] sTKN [mg/L]

0 92.3 � 0.5 16,744 � 269 5.88 � 0.16 341 � 6 75.9 � 0.1 10,170 � 18 3.44 � 0.04 1463 � 15131 92.1 � 0.2 16,317 � 69 5.94 � 0.06 361 � 16 73.8 � 0.1 23,055 � 134 7.00 � 0.02 2413 � 5262 92.5 � 0.4 14,319 � 67 5.98 � 0.03 317 � 1 75.9 � 0.2 26,210 � 198 8.17 � 0.00 2864 � 893 92.8 � 0.1 16,198 � 168 6.10 � 0.10 351 � 5 75.4 � 0.1 27,950 � 35 9.04 � 0.00 2760 � 115

Fig. 5. Soluble COD (sCOD) and cumulative biogas production of DDGS for increasingUS energy values.

A. Cesaro et al. / Journal of Cleaner Production xxx (2014) 1e6 5

denaturation phenomena, which could also explain the absence ofsolubilisation.

The extent of denaturation phenomena depends on bothdenaturating conditions and the original protein configuration.Under the same operating conditions, effects on the investigatedraw protein-rich substrate proved to be different from ones ob-tained on its partially degraded form.

Despite enhancement in sCOD provided by sonolysis on DDGSdigestate, cumulative biogas volumes decreased slightly forincreasing energy inputs.

Biogas production was observed to increase during the tests(Fig. 6) whilst its composition remained stable over time: detectedaverage content of methane and carbon dioxide was 60.8% (�1.6)and 26.1% (�1.2), respectively. pH values did not show significantvariation during the batch trials, rising from 7.42 � 0.13 at thetesting outset to 7.67 � 0.02 at the testing conclusion.

Fig. 6. Biogas production for untreated and sonicated DDGS digestate samples.


These results indicate that the anaerobic process was notsignificantly affected by sonolysis. Solubilised compounds wereneither bio-available within the anaerobic digestion process, norwas any evidence of ammonia inhibition phenomena recognised:even assuming that the nitrogen was completely present asammonia, the detected concentration values were lower than theinhibitory ones (Chen et al., 2008; Khalid et al., 2011).

These results are reasonably related to the prevailing occurrenceof sonolysis effects on protein residues of DDGS digestate.

In DDGS digestate, proteins have already undergone a biologicaldegradation process, which necessarily results in alteration of theirnative configuration. Thus, presence of both folded and unfoldedproteins could be expected in DDGS digestate. Interaction betweennative and unfolded states of proteins has been recognised tradi-tionally as the explanation of larger-molecular-weight aggregates,reasonably difficult to biodegrade as pointed out in studies dealingwith protein denaturation phenomena (Schiffter, 2011).

It is evident that the original protein configuration played a keyrole in the definition of ultrasound effects, both in terms of sol-ubilisation and anaerobic biodegradability. However, these effectswere found to be highly unpredictable, thus limiting process scaleup.

3.3. Scale up remarks

This study showed that sonolysis can improve anaerobicdigestion of both OFMSW and DDGS. However, results are stronglyaffected by the composition of the organic substrate.

With regard to OFMSW digestate, specific energy inputs in therange 31e93 W h/L allowed a disintegration degree improvementvarying between 22 and 53% aswell as an increase in specific biogasvolumes between 46 and 71%. The increase in biogas productionfrom sonicated OFMSW digestate was dependent on the disinte-gration provided by ultrasound, which is in direct relation with theultrasonic energy input. Moreover, during digestion time, increasedultrasonic energy supply further enhanced methane content.

These outcomes are consistent with scientific literature (Nickeland Neis, 2007; Pilli et al., 2011), such that a full-scale installationinvolving partial recirculation of sonicated OFMSW digestate couldresult in attractive incomes from additional biogas produced andfurther facilitate the reduction of fossil fuel energy production.

In this context, sonolysis can be considered a renewable-liketechnology.

Full-scale energy balances showed that 7 kW net generationfrom pretreatment of sludge destined to anaerobic digestion can beachieved by using 1 kW of ultrasonic energy (Barber, 2005). Withregard to OFMSW treatment, further processing of the data dis-cussed in this paper is currently ongoing in order to extrapolateresults obtained to a full-scale plant treating OFMSW.

Different considerations arose for protein-rich substrates.When increasing ultrasonic energy inputs were applied, an

improvement in anaerobic biodegradability ranging between 3 and23% was found for raw DDGS, whereas no variation in biogas pro-duction was observed for DDGS digestate.



The specific methane potential of untreated DDGS was found tobe relatively high, corresponding to 373 L/kg TS. If applied to rawDDGS, sonolysis could enhance this potential, though any increasewould be as remarkable as the one obtained, under the sameoperating conditions, for OFMSW digestate. Moreover, as the ul-trasonic action is not selective, sonolysis of raw substrate coulddetermine the oxidation of readily bio-available substances, withthe potential loss of part of the supplied energy.

From this, the assessment of the technical and economic feasi-bility of the ultrasound-improved anaerobic digestion process forprotein-rich substrates is untimely at present and further researchis required to provide an in-depth analysis of the mechanismsresponsible for anaerobic biodegradability enhancement aftersonolysis.

4. Conclusions

Ultrasound-improved anaerobic digestion of organic substratesstates as an interesting renewable-like technology for energy pro-duction. However, the effectiveness of process is highly influencedby the substrate composition, which was observed to affect bothultrasound-induced solubilisation and anaerobic degradability.

The application of ultrasound to OFMSW digestate permits boththe simplification of undigested residues and disruption of micro-bial cells, which are transferred to the solvent phase, thus becomingavailable within the anaerobic process. This results in increasingmethane recovery for energy production as well as in reducing theamount of digestate to be handled at treatment end.

Conversely, for protein-rich substrates, experimental resultsshowed the absence of a direct correlation between disintegrationdegree and energy inputs as well as between solubilisation andbiogas production. These outcomes suggest the occurrence of re-action mechanisms that need to be further investigated in order toassess the technical and economic feasibility of the process on full-scale applications.

OFMSW anaerobic digestion yields can be enhanced by feedingthe reactor with sonicated digestate. To this end, a full-scaleinstallation should involve the recirculation of part of the diges-tate pretreated by ultrasound. Such standing also promotessonolysis application for the upgrade of existing plants usingalready present recirculation systems to ensure substrate mixing inthe anaerobic digesters.

Thus, the application of sonolysis in full-scale facilities forOFMSWanaerobic digestion optimisation represents a valid option,whose economic feasibility assessment is currently ongoing.

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enhanced anaerobic digestion by ultrasonic pretreatment of organic residues for energy production

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