Effect of ultrasonication on anaerobic degradability of solid waste digestate

Waste Management xxx (2015) xxx–xxx

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Effect of ultrasonication on anaerobic degradability of solid waste digestate M.R. Boni, E. D’Amato, A. Polettini, R. Pomi, A. Rossi ⇑ Dept. Civil and Environmental Engineering, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy

a r t i c l e

i n f o

Article history: Received 3 August 2015 Revised 23 October 2015 Accepted 29 October 2015 Available online xxxx Keywords: Sonication Hydrolysis Anaerobic digestion Biomethane Digestate treatment Lignocellulosic waste

a b s t r a c t This paper evaluates the effect of ultrasonication on anaerobic biodegradability of lignocellulosic residues. While ultrasonication has been commonly applied as a pre-treatment of the feed substrate, in the present study a non-conventional process configuration based on recirculation of sonicated digestate to the biological reactor was evaluated at the lab-scale. Sonication tests were carried out at different applied energies ranging between 500 and 50,000 kJ/kg TS. Batch anaerobic digestion tests were performed on samples prepared by mixing sonicated and untreated substrate at two different ratios (25:75 and 75:25 w/w). The results showed that when applied as a post-treatment of digestate, ultrasonication can positively affect the yield of anaerobic digestion, mainly due to the dissolution effect of complex organic molecules that have not been hydrolyzed by biological degradation. A good correlation was found between the CH4 production yield and the amount of soluble organic matter at the start of digestion tests. The maximum gain in biogas production was 30% compared to that attained with the unsonicated substrate, which was tentatively related to the type and concentration of the metabolic products. Ó 2015 Published by Elsevier Ltd.

1. Introduction The renewed interest in anaerobic digestion (AD) of biodegradable organic wastes stems from its ability to attain waste stabilization, at the same time exploiting the energy content of the waste. Moreover, the AD digestate, composed of partially stabilized organic matter, anaerobic biomass, inorganic matter and nutrients, may be suitable for utilization as an organic fertilizer or soil conditioner (Mata-Alvarez et al., 2014). Thus, AD contributes to concomitantly meet the targets of materials and energy recovery from wastes, along with the reduction of biodegradable waste landfilling. A variety of organic residues has been considered for biogas production, including pig, poultry and cow manure, cheese whey, maize silage, straw, residues from food preparation, olive mill residues, food and kitchen waste, as well as sludge from wastewater treatment plants. Although some of these wastes have been used, either individually or in co-digestion, for full-scale biogas production and although AD is a well-established technology, the identification of appropriate strategies aimed at further improving the overall process performance and stability is still a matter of concern. In particular, the maximization of both waste stabilization and conversion into biogas, as well as the

⇑ Corresponding author.

improvement in digestate quality, appear as key targets to increase the competitiveness of the process. To this regard, one of the simplest strategies to improve the overall process yield is considered to be co-digestion of different residues. The advantages of codigestion can be ascribed, for example, to the control of the organic load, the reduction of seasonal variability of individual waste streams, the supply of nutrients and active biomass, as well as the dilution of contaminants and inhibiting compounds. However, co-digestion may not be suitable for substrates containing high amounts of cellulose, hemicellulose and lignin (Zheng et al., 2014), which are all highly resistant to microbial degradation due to their structural and chemical properties. For such materials, dedicated more severe treatments are required to enhance the availability of substrate constituents to the biomass (Shah et al., 2015). Specifically, in order to convert lignocellulosic substrate into biogas, carbohydrates should be made available through the hydrolysis of the original complex molecules. However, the activity of hydrolytic microorganisms is limited by several factors, including operating parameters (e.g. pH and temperature) and substrate characteristics (e.g. lignin and hemicelluloses content; cellulose crystallinity and degree of polymerization; specific surface area and particle size distribution; cell wall thickness) (Hendriks and Zeeman, 2009). In order to enhance the hydrolysis of recalcitrant lignocellulosic substrates, pre/post-processing is required (Behera et al., 2014), which relies on various mechanisms (Carlsson et al.,

E-mail address: [email protected] (A. Rossi). http://dx.doi.org/10.1016/j.wasman.2015.10.031 0956-053X/Ó 2015 Published by Elsevier Ltd.

Please cite this article in press as: Boni, M.R., et al. Effect of ultrasonication on anaerobic degradability of solid waste digestate. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.10.031

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2012): physico-mechanical (milling, grinding, ultrasonication), physico-chemical (steam explosion, wet oxidation, etc.), chemical (alkaline or acidic treatment, chemical oxidation and organic solvents treatment) or biological. So far, according to a review by Mata-Alvarez et al. (2014), research activities have been mainly focused on mechanical pre-treatments, followed by thermal and chemical pre-treatments. Among the investigated mechanical methods, ultrasonication (US) has been extensively applied for wastewater sludge processing as a means to produce particulate matter disintegration (Weemaes and Verstraete, 2005), enhance biodegradability (Khanal et al., 2007), minimize sludge production (Onyeche et al., 2002), reduce retention time (Tiehm et al., 1997) and increase the CH4 yield (Barber, 2005). However, more recently, some authors have applied US to other substrates, such as manure (Castrillón et al., 2011; Elbeshbishy et al., 2011), food waste (Elbeshbishy and Nakhla, 2011; Jiang et al., 2014), agricultural wastes (Fernández-Cegrí et al., 2012), distillery residues (Sangave and Pandit, 2006) and by-products from bio-ethanol production (Cesaro et al., 2014). The application of US to a liquid produces molecules oscillation around the average equilibrium position, resulting in a variation of their average relative distance. The increase in the relative distance among the molecules may turn into liquid breakdown and voids (cavitation bubbles) generation. If US intensities above 10 W/cm2 are applied, transient cavitation bubbles are formed, reaching a radius size which is twice the initial value and collapsing violently upon compression (Santos and Lodeiro, 2009). Two phenomena result from cavitation bubbles collapse, namely the formation of high mechanical stresses causing both particulate matter disintegration and cells lysis, and the production of highly reactive radicals (such as hydroxyl radicals and hydrogen peroxide) potentially able to oxidize complex molecules (Badday et al., 2012; Luo et al., 2014). Cavitation is known to be affected by a number of factors, including operating conditions (e.g., treatment duration and temperature, US frequency, power input), physico-chemical properties of the substrate (e.g., solids and lignin content, viscosity) as well as system design (e.g., reactor configuration, diameter and position of the transducer) (Gogate et al., 2011). The abovementioned factors may be interdependent, so that their mutual interactions become relevant, which also complicates the prediction of their overall effect on process kinetics and yield. Consequently, the comparison of results of different studies often shows inconsistencies due to the different experimental conditions adopted (e.g., substrate properties, power input and transducer design). Despite so far US has been widely investigated as a method for substrate pre-treatment (Khanal et al., 2007; Pilli et al., 2011; Tyagi et al., 2014), a limited number of studies has been carried out to evaluate the ability of US to increase the biodegradability/bioavail ability of recalcitrant organic compounds remaining in the digestate downstream of AD. Such a process scheme represents an innovative interesting option that, applying US to the sole hardly degradable fraction remaining after the digestion process, allows to exploit the residual energetic potential of the digestate and to increase the biological stability of the final residues, making US post-treatment of digestate technically and economically more attractive compared to its application ahead of AD. Literature studies on this topic for the digestate from the organic fraction of municipal solid waste are currently limited to a single publication (Cesaro et al., 2014), therefore there are numerous aspects involved that still deserve further investigation. Even the mechanisms of substrate dissolution upon sonication may differ depending on whether the treatment is applied as a pre-processing or as a post-processing step, since – as already mentioned above – the nature of the species involved changes even drastically as a result of AD.

In order to make a step forward to understanding the effect of US as a post-treatment on the biodegradability of food waste, in the present study a non-conventional process configuration based on sonicated digestate recirculation to the biological reactor was evaluated at the lab scale. The main aim of the present work was to evaluate the effect of sonication on both digestate properties and AD process performance, in terms of residual CH4 production yield and kinetics. 2. Materials and methods 2.1. Substrate composition The experimental campaign was carried out on a digestate sample (referred to as the substrate), collected at the outlet of a full-scale AD plant located in Central Italy, treating a mixture of organic wastes of a food industry, with high lignocellulose and fibers contents, and activated sludge from a wastewater treatment plant. Substrate samples were stored under controlled conditions at 4 °C and characterized for pH, Total Solids (TS), Volatile Solids (VS), Chemical Oxygen Demand (COD), N-NH3 and metals content, which were determined according to APHA et al. (2005). The Total Organic Carbon (TOC) concentration was measured using a Shimadzu TOC analyzer equipped with a dedicated module for the analysis of solid samples. Dissolved organic carbon (DOC) and soluble COD (sCOD) were analyzed in the liquid phase after sample centrifugation at 5000 rpm for 15 min and subsequent filtration through a glass microfiber filter (1.2 lm pore size). Carbohydrates were analyzed using the colorimetric phenol–sulfuric acid method using glucose as the standard (Dubois et al., 1956). The metal content was determined using an atomic absorption spectrometer (Perkin Elmer Model 3030B) after sample digestion according to APHA et al. (2005). The same analytical procedures were also adopted for the characterization of the mixtures subjected to the AD process (see Section 2.2.2). The substrate properties are summarized in Table 1, where the average values and related standard deviations of three replicate measures are reported. 2.2. Experimental set-up 2.2.1. US treatment A lab-scale ultrasound generator (model VCX 750, Sonics, USA) was used for the US treatment of the substrate. The US frequency and maximum power input were 20 kHz and 750 W, respectively.

Table 1 Substrate properties. Parameter

Unit of measure

Average

Std. deviation

pH TS VS TOC DOC COD sCOD Carbohydrates Soluble carbohydrates N-NH3 Fe Mn Mo Co Ni Acetate Propionate

– g/l g/l g/l g/l g/l g/l g/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l

7.6 23.7 16.3 8.2 0.5 24.6 1.5 3.2 63.6 610.4 195 3.5 4.3 0.2 0.6 124.0 60.6

0.1 1.5 1.0 0.4 0.04 1.2 0.3 0.4 15 35.7 6.74 0.09 0.20 0.05 0.02 12 22

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The choice of a 20-kHz apparatus was motivated by the fact that low frequencies are known to generate larger cavitation bubbles which upon collapse cause strong shear forces in the medium (Tiehm et al., 2001). Sonication tests were carried out using a 1 l cylindrical Pyrex vessel with a working volume of 800 ml; the US probe was placed centrally inside the vessel and immersed into the substrate at a depth of 2 cm. The specific energy (Es), expressed as the ratio between the sonication energy supplied and the initial mass of TS, was varied in the range 500–50,000 kJ/kg TS, while keeping the oscillation amplitude at a fixed value. According to the previous definition, Es can be calculated as:

Es ¼

Pt V  TS

where P is the US power, t the sonication time, V the volume of the sonicated substrate and TS the TS concentration of the substrate. The US tests were conducted at a sonication density and intensity of 0.05 W/ml and 8.15 W/cm2, respectively. The digestate temperature was controlled at 25 °C through a cooling bath. In order to evaluate the effects of the US treatment, the following parameters were analyzed on the sonicated samples: TOC, DOC, COD, sCOD, TS, VS and pH. The degree of solubilization was evaluated as the variation of the soluble fraction (s) of TOC, COD and carbohydrates as a function of either the initial particulate fraction or the initial soluble fraction, giving rise to the two following indices Ip and Is (Carlsson et al., 2012):

sX us  sX 0 sX us  sX 0 Ip ¼  100 Is ¼  100 X 0  sX 0 sX 0

ð1Þ

where X is the concentration of the parameter of concern (TOC, COD, carbohydrates) before (X0) and after (Xus) the sonication process. 2.2.2. Anaerobic digestion tests Anaerobic batch reactors were used to evaluate the effect of sonication on the yield and kinetics of AD of the treated substrate. On the basis of the results of the US treatment described in the previous section, a subset of sonicated samples was selected for the AD tests; these included the substrates sonicated at 6000, 10,000, 20,000 and 50,000 kJ/kg TS. The experiments were carried out in stirred batch glass reactors (total volume = 1 l, working volume = 0.8 l) under mesophilic conditions (T = 37 ± 2 °C). Anaerobic tests were performed on samples prepared by mixing the sonicated (Sus) and untreated substrate (S) at two different ratios (Sus:S = 25:75 or 75:25 w/w). For reference purposes, a control test on untreated substrate (100% S) was also carried out. Since US has been shown to negatively affect biomass activity (Cesaro et al., 2012), no run with 100% Sus was conducted. All runs were conducted in duplicate, and the result reported as average values. The details of the experimental runs are reported in Table 2. The biogas volume was monitored daily using a eudiometer, connected to each reactor, which was initially filled with a NaCl-saturated solution acidified with HCl to pH = 2 to prevent

Table 2 Summary of the anaerobic tests performed. Run

Sus:S (w/w)

Specific energy (kJ/kg TS)

Control 6000_25Sus 6000_75Sus 10000_75Sus 20000_25Sus 20000_75Sus 50000_25Sus 50000_75Sus

0:100 25:75 75:25 75:25 25:75 75:25 25:75 75:25

– 6000 6000 10,000 20,000 20,000 50,000 50,000

gas dissolution. Gas measurement was made using the volume displacement principle, expressing the measured volume at standard temperature and pressure conditions (T = 0 °C, p = 1 atm). The biogas was periodically sampled from the eudiometers with a 25-ml gastight syringe and analyzed through a gas chromatograph (Model 3600 CX, VARIAN) equipped with a thermal conductivity detector and 2-m stainless-steel packed column (ShinCarbon ST) with an inner diameter of 1 mm. The operation temperatures of the injector and the detector were 100 and 130 °C, respectively, with helium as the carrier gas. The oven temperature was initially set at 80 °C and subsequently increased to 100 °C at 2 °C min1. The digestate was periodically sampled and the concentration of volatile fatty acids, VFAs (acetic [Hac], propionic [Hpr], butyric [Hbu], iso-butiric [HIbu], valeric [Hval], isovaleric [HIval], hexanoic [Hhex], isohexanoic [HIhex], heptanoic [Hhep]) was determined in 1.2-lm filtered and HCl-acidified (pH = 2) liquid effluent (1 ll) with a gas chromatograph equipped with a flame ionization detector (FID) and a 30-m capillary column (TRB-WAX) with an inner diameter of 0.53 mm. The temperature of the detector and the injector was 270 and 250 °C, respectively. The oven temperature was initially set at 60 °C, which was held for 3 min, subsequently increased to 180 °C at a rate of 10 °C min1 and finally increased to 220 °C at a rate of 30 °C min1 and held for 2 min. All the analytical determinations were performed in triplicate. 2.3. Kinetic model Both the modified Gompertz equation and the logistic bacterial growth model were used to fit the experimental data and estimate the kinetics of CH4 production (Zwietering et al., 1990). Numerical fitting of the experimental data was performed using TableCurve 2D v. 5.01Ó. Comparing the models, the best fit of CH4 production data was obtained using the logistic model for all the investigated runs:

M¼n

P 1 þ exp½4RmðktÞ þ 2 P

o

ð2Þ

where M (Nl CH4/kg VS) is the CH4 production at time t (d), P (Nl CH4/kg VS) is the maximum CH4 production yield, Rm (Nl CH4/kg VS d) is the maximum CH4 production rate and k is the lag phase duration (d). 3. Results and discussion 3.1. Effect of sonication energy on digestate composition and solubilization In terms of digestate composition, sonication did not result in significant modifications of the total content of substrate constituents in the whole set of the investigated energy values. Upon inspection of Fig. 1a, both the TS and VS concentrations remained unchanged after sonication, thus suggesting that, under the investigated conditions, ultrasound did not result in mineralization or volatilization of organic matter, as also observed by other investigators (Cesaro et al., 2014). Conversely, when applying US as a pretreatment of food waste before AD, some although slight reduction in TS and VS concentrations (12% and 14%, respectively) at a specific energy of 15,000 kJ/kg TS was observed (Elbeshbishy et al., 2012). The effect of US on substrate solubilization can easily be observed from the results depicted in Fig. 1b–d. The effect of sonication on the dissolution of the original particulate material was, as expected, related to the sonication energy applied. It can also be noted that the gain in solubilization of carbohydrates, TOC

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Fig. 1. Influence of Es on: (a) total contents of TS, VS, TOC and carbohydrates; (b) DOC, sCOD and soluble carbohydrates; (c) Ip; (d) Is.

and COD in all cases levelled off at Es = 20,000 kJ/kg TS (see Fig. 1b). According to Eq. (1), substrate solubilization was evaluated in terms of carbohydrates, TOC and COD dissolution. The inspection of the Is and Ip values as a function of the specific energy applied reveals that a significant increase in the soluble fraction of the parameters of concern could be attained upon US. The Ip values varied over the following ranges at the different sonication energies applied: 4.1–20.1%, 2.8–22.1% and 3.0–21.3% for TOC, COD and carbohydrates (see Fig. 1c); the corresponding ranges for Is were: 64–289%, 55–280%, 117–1335% (see Fig. 1d). The large numerical difference between the two indices is caused by the lower content of the soluble organic fraction compared to the particulate matter for the substrate under investigation, which results in largely different denominators in the two expressions of Eq. (1). The increase in Ip and Is for TOC and COD (see Fig. 1c and d) was particularly significant for Es in the range 6000–20,000 kJ/kg TS; conversely, at higher Es values no significant gain in solubilization of organic matter was observed. According to Khanal et al. (2007), the behavior observed at Es > 20,000 kJ/kg TS can likely be ascribed to the depletion of the easily disintegrable organic particles and/or to the depletion of dissolved gases promoting cavitation bubbles formation. Although the increase in the degree of carbohydrates solubilization (when evaluated with reference to the initial amount in solution [Is]) was more appreciable than that observed for TOC and COD (see Fig. 1d), the absolute values attained for the soluble concentrations were notably lower than both TOC and COD. For the investigated range of specific energies tested, the ratio between soluble

carbohydrates and DOC concentrations, equal to 4.8% in the initial substrate, ranged from 7.8% to 13.5% at Es of 500 and 50,000 kJ/kg TS, respectively. In other words, the applied US treatment was able to produce only a partial breakdown of the original complex structures, but not the full liberation of simple, easily biodegradable monomers. However, it is expected that the observed effect of organic matter solubilization upon US can still have implications on the yield of the subsequent AD process (see Section 3.2 for further details). On the basis of the experimental results of substrate solubilization shown in Fig. 1b–d, an attempt was made at comparing the US performance attained under the investigated conditions with that observed by other authors. Unfortunately, a direct comparison of numerical values for the solubilization yield (expressed as Is, Ip or sXus/sX0) from different studies cannot be made – unless due care is taken – due to the following reasons: (1) authors often use different indices to express the degree of dissolution, and the parameters of substrate composition required to convert one index into another are frequently missing; (2) the origin and composition of the investigated substrate are largely different among the different studies due to both geographical/seasonal constraints and type of pre-processing applied to the waste (if any); (3) the sonication conditions vary widely in the existing literature and are in some cases not fully detailed, so that only seldom is it possible to compare the process performance at the same (or at least close) experimental conditions; (4) the results are in some cases compared at fixed values of operating parameters of US which have a marginal influence on the process performance compared to other more significant

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variables, the value of which is unspecified. It should also be noted that, given that the purpose of US is to hydrolyze the original particulate matter contained in the waste, it would appear to be intrinsically more meaningful if the degree of solubilization were quantified with regard to the initial particulate fraction; as a result, in the author’s opinion, among the mentioned parameters Ip is believed to be more representative to quantify the effect of sonication on the release of soluble compounds from the initial substrate. Of course, a more accurate quantification of the degree of organic matter dissolution may be obtained by correcting the denominator of Ip in Eq. (1) for the maximum amount of particulate matter that is expected to be chemically solubilized (Cesaro et al., 2014; Naddeo et al., 2009; Bougrier et al., 2005). However, when considering a given substrate type, the Ip values calculated according to Eq. (1) can still be usefully adopted to compare the process performance under different operating conditions. Among the literature studies focusing on untreated food waste, the Ip values (either directly provided by the authors or derived by calculation from the experimental data) are as follows: Ip(COD) = 0.2–26.3% and Ip(carbohydrates) = 5.7–22.9% at Es = 350–23,000 kJ/kg TS (Elbeshbishy et al., 2012); Ip(COD) = 2.0–7.0% at Es = 22,500– 67,500 kJ/kg TS (Gadhe et al., 2014); Ip(COD) = 8.4% and Ip(carbohydrates) = 3.6% at Es = 5000 kJ/kg TS (Elbeshbishy and Nakhla, 2011). The ranges attained in the present study (Ip(COD) = 2.8–22.1; Ip(carbohydrates) = 3.0–21.3%) are thus numerically comparable with those obtained by other investigators. However, it should also be mentioned that the absolute value of Ip is affected by the relative contents of the soluble and particulate matter, so that at a constant sXus/sX0 ratio Ip decreases with increasing the particulate/total matter ratio. In our study, where a pre-digested organic waste was investigated, the soluble matter content before the US treatment was considerably lower than that of other studies which focused on untreated food waste (sCOD/COD = 6% as opposed to values of 48–53% for the previously mentioned studies). Furthermore, in our work the waste subjected to sonication derived from an AD process that had likely already removed a significant portion of the easily degradable substances, and also the sonication density adopted (0.05 W/ml) was considerably lower than that used in the studies quoted above, which varied over the range 0.15– 1.5 W/ml. On the basis of such considerations, the results obtained in our study point out the good performance of the sonication treatment in terms of dissolution of the particulate matter. Although direct measurements were not available neither for our work nor for the previous investigations, it is tempting to hypothesize that the higher efficiency attained in the present study is related to the smaller particle size of the waste, likely due to the effect of the AD process applied upstream of sonication. To this regard, further investigation on this issue is currently underway

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to evaluate the influence of size and shape of the solid particles on the US process. 3.2. Anaerobic digestion tests The effect of digestate sonication on the AD process was assessed in terms of CH4 production yield and kinetics, as well as evolution of digestate composition over time. 3.2.1. Methane production Fig. 2a and b show the specific cumulative CH4 yields, expressed per unit of initial VS content in the reactors, for the Sus:S = 25:75 and Sus:S = 75:25 mixtures along with the theoretical production curves derived by interpolation of the experimental data according to Eq. (2). The values of the kinetic parameters (Rm, k, and P) estimated by data fitting are reported in Table 3. Considering the CH4 production yield, at a Sus:S ratio of 25:75 (see Fig. 2a), a maximum of 184.0 Nl CH4/kg VS was attained for the substrate sonicated at 6000 kJ/kg TS, while the lowest production yield (158.5 Nl CH4/kg VS) was observed at a US energy of 50,000 kJ/kg TS. Considering that for the control test a yield of 169.4 Nl CH4/kg VS was attained, at a Sus:S ratio of 25:75 no significant effect of Es was evidenced. Similar results were also attained in terms of process kinetics, with Rm and k varying over relatively narrow ranges, specifically 18.2–21.4 Nl CH4/kg VSd and 5.9– 6.5 days. A tentative hypothesis explaining these results may be that the substrate to active biomass ratio for the 25:75 Sus:S mixtures was not suitable, negatively affecting the biogas production yield. At a 75:25 Sus:S ratio (see Fig. 2b), no relevant effect of sonicated substrate addition was detected at a sonication energy of 6000 kJ/kg TS, with a specific CH4 production of 180.6 Nl CH4/kg VS; conversely, when the substrate sonicated at higher Es values was added to the mixture, a significant increase in the specific CH4 production was attained, with a maximum of 212.5 Nl CH4/kg VS being displayed for the substrate sonicated at 50,000 kJ/kg TS, which corresponded to an increase in total biogas production by 30.1%. In this case, however, the onset of the metabolic pathways appeared to be delayed, as evident from the lag phase duration (k = 8.1 d as compared to 5.0 d of the control run). The delay produced by sonication on biogas generation is likely related to an effect of the treatment on the biomass acclimation time, which may have been caused by the fact that US is not selective toward different particulate substances and can thus also produce biomass cell lysis. This may also be confirmed by the fact that a clear correlation was observed between the lag phase duration (see Table 3) and the sonication energy applied.

Fig. 2. Specific cumulative CH4 production fitted with the logistic model (Eq. (2)) for Sus:S = 25:75 (a) and Sus:S = 75:25 (b). The results of control runs are also reported.

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Table 3 Estimated parameters of the modified logistic equation describing CH4 production. Run

Control 6000_25Sus 6000_75Sus 10000_75Sus 20000_25Sus 20000_75Sus 50000_25Sus 50000_75Sus

Parameter P Nl/kg VS

Rm Nl/(dkgVS)

k d

r2 –

169.4 184.0 180.6 200.8 167.1 201.9 158.5 212.5

17.4 21.4 17.8 27.2 19.2 26.6 18.2 25.8

5.0 5.9 6.5 6.9 6.0 7.7 6.5 8.1

0.999 0.998 0.998 0.999 0.999 0.999 0.998 1.000

Fig. 3. Final cumulative CH4 production as a function of the initial content of soluble organic matter of the mixture (Sus:S = 75:25).

The observed increase in biogas production caused by sonication well compares with the values typically reported in the literature for similar substrates. In particular, the following gains in biogas production are reported: 26% at Es = 5000 kJ/kg TS for a mixture of food waste and anaerobic sludge (Elbeshbishy and Nakhla, 2011); 24% at Es = 90,700 kJ/kg TS for the sonicated organic fraction of MSW mixed with sewage sludge (Cesaro et al., 2012); 16% at Es = 15,000 kJ/kg TS for the sonicated organic fraction of MSW mixed with anaerobic sludge (Cesaro and Belgiorno, 2013); in a single study by Cesaro et al. (2014), focusing on sonicated digestate from AD of the organic fraction of MSW, a consistently higher (71%) biogas production increase was attained for a sonication energy of 6300 kJ/kg TS.

When interpreting the observed CH4 production yields in light of the effects produced by sonication, it is clear that a linear correlation (r2 = 0.90; see Fig. 3) existed with the concentration of soluble organic matter of the mixture at the start of the AD experiments, thus confirming the findings of previous work on AD of sonicated waste activated sludge (Wang et al., 1998). The relatively low slope of the correlation line in Fig. 3 also indicates that, for a significant improvement in total biogas production to be attained, a high initial DOC concentration would be required. On the basis of the experimental results, unless unreasonably high sonication energies were applied, a DOC concentration in excess of 2000 mg/l would hardly be obtained, which can thus be taken as the upper threshold of the process performance under the investigated conditions; as a result, the expected maximum gain in CH4 production is calculated to be 30%. As for biogas composition, for the control test and the runs performed on the samples sonicated at 6000 kJ/kg TS, the volumetric CH4 content of the biogas varied from 50% at the start of the process to 85% at the end of the tests; for the mixtures containing the substrate sonicated at 10,000 and 20,000 kJ/kg TS, a similar CH4 content varying from 40% to 85% was measured, while for the run 50000_75Sus the value spanned from 20% to 85%. The lower initial CH4 content in the biogas is in agreement with the previously mentioned effect of sonication on the acclimation period of the biomass at increasing sonication energies. It is however worth mentioning that, although the substrate used in the present study derived already from a digestion process, the detected CH4 content of biogas was significantly higher than would be expected for the biogas produced from a second digestion stage (see e.g. Menardo et al., 2011). A mass balance of carbon for the different digestion tests showed that the total amount of TOC consumed till the end of the experiments ranged from 23% to 36% of the initial value, and the fraction of initial carbon finally converted to CH4 was 15–21%, corresponding to 280–374 Nl CH4/kg TOC. 3.2.2. Metabolite production and substrate degradation The analysis of VFA production provides useful information on the evolution of the AD process. The initial VFA content, as well as the VFA speciation, is known to affect the CH4 generation yield (Buyukkamaci and Filibeli, 2004; Franke-Whittle et al., 2014; Kondusamy and Kalamdhad, 2014; Zhang et al., 2014). In the present study, however, the applied US conditions did not produce any significant increase in the initial VFA content, with the exception of mixture 50000_75Sus. However, the effect of substrate sonication on VFA evolution was still evident during the digestion process, with an increase in the total VFA (TVFA) concentration over time for the mixtures containing the substrate sonicated at 20,000 and

Fig. 4. TVFA concentration (as acetic acid) as a function of digestion time: Sus:S = 25:75 (a) and Sus:S = 75:25 (b). The results of control runs are also reported.

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Fig. 5. VFA distribution and specific cumulative CH4 production as a function of digestion time for control and Sus:S = 75:25.

50,000 kJ/kg TS irrespective of the adopted Sus:S ratio, with the exception of the Sus:S = 75:25 containing the substrate sonicated at 20,000 kJ/kg TS (see Fig. 4). As expected, for all the investigated mixtures the plateau in the cumulative CH4 production was reached when VFA consumption attained completion (see Fig. 5). It may however be inferred that additional metabolites, including alcohols produced as a result of solventogenesis, should also be investigated in the future in order to better understand the differences observed in CH4 production. For the mixtures containing Sus at Es > 6000 kJ/kg TS, the observed gain in total CH4 production was accompanied by higher TVFA concentrations at both the investigated Sus:S ratios, confirming the

results attained by other investigators (Jiang et al., 2013; Quarmby et al., 1999; Wang et al., 1998) who found similar results for different substrates, although their analysis was limited to the hydrolysis and acidogenesis phases. In addition to an increased TVFA production (see Fig. 4), the use of digestate sonicated at Es > 6000 kJ/kg TS was also observed to result in a different distribution of the individual investigated VFAs. In particular, as shown in Fig. 5, higher sonication energies were found to give rise to the production of highermolecular-weight metabolites. This effect was particularly evident for the mixture containing the substrate sonicated at Es = 50,000 kJ/kg TS: while for the control run and for the one containing the substrate treated at Es = 6000 kJ/kg TS the prevalent VFAs included

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Fig. 6. Profiles of DOC over time for Sus:S = 25:75 (a) and Sus:S = 75:25 (b). The results of control runs are also reported.

acetate and propionate, a progressive increase in the number and amount of metabolic products was evident with increasing the sonication energy. The presence of C P 4 VFAs may be explained considering the probable increased amounts of more complex substances released upon sonication at increasing sonication energies. For all runs, the pH values during digestion ranged between 7.0 and 8.0, indicating the stability of the digestion process. While the adoption of a high organic load and the presence of inhibitory factors are widely reported to potentially cause an unbalance between acid production and consumption with an accumulation of VFAs (Palacio-Barco et al., 2010; Siegert and Banks, 2005; Wu et al., 2015; Xu et al., 2014) no similar phenomena were observed during the experiments and, as mentioned above, the VFAs in the system could be completely degraded till the end the process. During the AD tests, the evolution of soluble organic matter was also monitored. The DOC concentration profile (Fig. 6a and b) was similar for the two investigated Sus:S ratios, with the maximum value being reached within five days from the start of the tests. The initial increase in the concentration of soluble organic matter is of course the result of hydrolytic reactions by the fermentative biomass, which thus provided an additional contribution to the particulate matter dissolution already produced by sonication. The highest DOC concentrations during the AD tests were attained for the Sus:S = 75:25 mixture, in particular at the highest Es values adopted during US, mainly as a result of the larger degree of organic matter dissolution produced by the treatment itself.

4. Conclusions The present study provided interesting indications about the effects of US treatment on AD of food waste and sewage sludge digestate. In particular, the main findings of the investigation may be summarized as follows:  The increase in soluble organic matter upon sonication was particularly significant for Es in the range 6000–20,000 kJ/kg TS, while the incremental gain for higher energies was not appreciable.  Under the investigated treatment conditions, US did not induce the mineralization of organic matter (which would be unfavorable for the following AD process), but rather modified its distribution between the particulate and the soluble fraction.  Sonication enhanced the hydrolysis of the particulate fraction during the AD process, improving the process yield and kinetics. The CH4 production yield was not significantly affected by sonication at a Sus:S ratio of 25:75, while was improved when increasing the Sus:S ratio (with a maximum production of

212.5 Nl CH4/kg VS for a sonication energy of 50,000 kJ/kg TS, corresponding to a 25% gain with respect to the unsonicated substrate).  The improved biogas production at higher sonication energies was caused by a presumable modification of the metabolic pathways, as evident from the analysis of the nature and concentration of the metabolites produced. In particular, higher energies were found to result in larger concentrations of total VFAs, as well as in increased proportions of higher-molecularweight metabolites. While US may represent a potentially valid option for the treatment of residual organic substrates containing recalcitrant substances in view of AD, the results of the present study also suggest that additional efforts should be devoted to improving the degree of involvement of complex organic matter in the sonication process. This may be achieved through a systematic investigation of the individual and joint influence of the relevant parameters of the US process, which is still missing in the literature. In particular, potentially important parameters that may significantly affect the sonication performance include the sonication density, the solids content as well as the size and shape characteristics of the solid particles. With regard to this last issue, the feasibility of combining sonication with other physical/mechanical processing methods may also be assessed with a view to improving the degree of substrate solubilization. Another relevant issue to be evaluated concerns the energetic evaluation of US in combination with AD, in order to define the economic feasibility in view of its full-scale application. Although further data on this topic is needed, in the authors’ opinion the energetic profile of US in combination with AD is expected to be favorable only if the former is applied exclusively to the recalcitrant portion of the substrate, therefore being adopted as a posttreatment rather than as a pre-treatment. References APHA, AWWA, WEF, 2005. Standard methods for the examination of water and wastewater, 21st ed. American Public Health Association, Baltimore. Badday, A.S., Abdullah, A.Z., Lee, K.T., Khayoon, M.S., 2012. Intensification of biodiesel production via ultrasonic-assisted process: a critical review on fundamentals and recent development. Renew. Sustain. Energy Rev. 16, 4574–4587. http://dx.doi.org/10.1016/j.rser.2012.04.057. Barber, W.P., 2005. The effects of ultrasound on sludge digestion. Water Environ. J. 19, 2–7. http://dx.doi.org/10.1111/j.1747-6593.2005.tb00542.x. Behera, S., Arora, R., Nandhagopal, N., Kumar, S., 2014. Importance of chemical pretreatment for bioconversion of lignocellulosic biomass. Renew. Sustain. Energy Rev. 36, 91–106. http://dx.doi.org/10.1016/j.rser.2014.04.047. Bougrier, C., Carrère, H., Delgenès, J.P., 2005. Solubilisation of waste-activated sludge by ultrasonic treatment. Chem. Eng. J. 106, 163–169. http://dx.doi.org/ 10.1016/j.cej.2004.11.013.

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