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

Ultrasonics - Sonochemistry 59 (2019) 104755

Contents lists available at ScienceDirect

Ultrasonics - Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Intensifying the biogas production from food waste using ultrasound: Understanding into effect of operating parameters Saurabh M. Joshi, Parag R. Gogate

T

⁎

Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400019, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Low intensity ultrasound Biogas production Food waste Cavitation Process intensification

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.

1. Introduction Today, global demand of energy has increased tremendously with industrialization and rising population. Over 84% of the total energy supply is dependent on non-renewable energy sources like coal, oil and natural gas, causing major economic problems for nations and also environmental hazards [1]. Researchers are looking for alternatives which can replace or reduce dependency on these sources and one of the potential alternatives available is biofuels production from sustainable feedstocks such as agricultural waste, food industry waste, energy crops and organic fraction of municipal solid waste. There is no food against fuel issue as these feedstocks do not compete with food production chain like first generation feedstocks i.e. corn and sugarcane [1]. Production of biofuels from the sustainable sources also helps in environmental protection based on the utilization of waste and reduction in the toxic emissions from subsequent applications. Sustainable feedstocks are mainly made up of cellulose, hemicellulose and lignin with the interaction mainly governed by lignin content resulting in formation of complex and recalcitrant structure. In

⁎

order to make them susceptible for use in any processing, the disruption of complex structure is required which would mean better utilization in subsequent chemical or biological processes. Pretreatment with different processes like milling, acid hydrolysis, steam explosion, biological treatment, cavitation etc. are some of the important approaches for pretreatment focusing on lignin removal and breaking the complex structure. After pretreatment, the cellulose and hemicellulose are effectively broken-down into monomeric sugars and are further utilized in biological processes for biofuel production [2]. Anaerobic digestion (AD) is the biological process typically employed for production of biofuels mainly biogas. 1.1. Anaerobic digestion for biogas production Anaerobic digestion (AD) is a microbial process employed in absence of oxygen resulting into breakdown of biodegradable material into value added products such as biogas, bioethanol, secondary metabolites, composts etc. AD can handle diverse, non-sterile and complex feedstocks with a variety of microorganisms giving the desired products

Corresponding author. E-mail address: [email protected] (P.R. Gogate).

https://doi.org/10.1016/j.ultsonch.2019.104755 Received 2 June 2019; Received in revised form 25 August 2019; Accepted 25 August 2019 Available online 26 August 2019 1350-4177/ © 2019 Elsevier B.V. All rights reserved.

Ultrasonics - Sonochemistry 59 (2019) 104755

S.M. Joshi and P.R. Gogate

inhibitors to AD process. The pretreatment can also break open the complex structure based on effective removal of the lignin, the binding element in the structure. Cavitation as pretreatment can be very effective in achieving these changes in the feedstock. Cavitational reactors produce microscale cavities which grow and implode rapidly causing disruption of feedstock present in the vicinity giving lower particle size and also removal of lignin [10]. Generally speaking, ultrasound (US) induced cavitation is known to produce intense cavitation dependent on intensity and frequency of irradiation source which also affects the size of the nuclei generated [11]. US can be divided into different categories as power ultrasound (two specific range of frequency as 20–100 kHz and 200 kHz−1 MHz suiting the specific applications) and diagnostic ultrasound (> 1MHz) whereas depending upon application intensity, it can be sub divided as high intensity (10–1000 W/cm2) or low intensity (< 1 W/cm2) [12]. Typically application of high intensity US will be useful for disruption of food waste whereas low intensity US will be useful for AD of the treated food waste. Cavitation employed can also be in combination with chemical treatments like alkali treatment especially in the pretreatment stage to enhance the rates of processing and reduce treatment cost [13]. Cavitation can also play a significant role in reduction of time required by biological processes with increase in solubilisation of organic matter. In the present work, only ultrasound induced cavitation has been considered in the analysis of literature and experiments involving conversion of food waste to biogas.

[3]. Biogas is one of the best alternatives available to obtain clean energy with very less emissions from waste based on subsequent conversion to electricity or heat or even as a transportation fuel [1]. A typical conversion process from waste to biogas is a four step process with first step as hydrolysis where hydrolytic microbes producing exo-enzymes disintegrate feedstock resulting into production of simple sugars, fatty acids and amino acids. Subsequent step 2 is the acidogenesis step responsible for production of alcohols, short chain organic acids and organic-nitrogen compounds from the monomers generated in hydrolysis step in the presence of hydrogen and carbon dioxide [4]. Step 3 of the process is acetogenesis step in which acetic acid is produced from reduction of hydrogen and carbon dioxide with help of homoacetogenic microorganisms. Final step is called as methanation step, in which hydrogenotrophic methanogens convert carbon dioxide to methane while acetotrophic methanogens convert acetic acid and hydrogen to methane [5]. The overall yield of biogas mainly depends on feed material, temperature, efficacy of microorganisms and type of reactor used in the process. Biogas consists mainly of methane (CH4) typically 40–75% and carbon dioxide (CO2) over the range of 25–55% as the major components with carbon monoxide (CO) and hydrogen sulfide (H2S) in lesser proportions [6]. The present work has focused on utilization of food waste as a sustainable feedstock for biogas production. 1.2. Food waste: a potential feedstock

2. Analysis of application of ultrasound for intensified biogas production

Population growth with economic development is one of the key factors contributing to generation of food waste in significant proportions. FW is one of the abundantly available feedstocks in highly populated countries like India and it accounts for 30–50% share in overall municipal solid waste (MSW) generated. Annually, food waste of 400 billion Rs. (INR) value is generated in the form of cooked food or raw food (waste grains, vegetables and fruits) [7]. FW being organically rich and containing diverse source of nutrients with carbohydrates, fats and proteins can be an excellent source for the biogas production. AD is one of the suitable processes to produce energy in the form of biogas from FW. AD has disadvantages with requirement of complex reactor configuration for better contact, slow rates of digestion due to the complex nature of feedstock and also stability of the microbial culture. Improper development of microbial culture during the biogas production results in biogas with lesser content of methane and hence requires stringent maintenance of reactor parameters for better performance [3]. In recent years, researchers are focusing on enhancing the bioactivity of anaerobic cultures leading to enhanced utilization capacity with different methods like feedstock pretreatment, use of engineered microbial strains, exposure of microbial strains to growth enhancing elements etc. Feedstock pretreatment is one of the easy approaches and provides easy access of substrates to microbes resulting in increased microbial growth and product generation. To increase the biogas producing potential of FW, application of pretreatment processes such as mechanical, chemical, thermal or biological is required. Cavitation is one of the promising upcoming technologies that can be used in pretreatment of raw materials. The current work focuses on literature analysis and actual application of cavitation in pretreatment of FW and also subsequently in the actual production of biogas from FW.

2.1. Ultrasound in food waste pretreatment Application of US as a pretreatment technique can significantly affect feedstock parameters like surface area, particle size, organic matter solubilisation, crystallinity and degree of polymerization. US when applied on FW causes solubilisation of organic matter, increasing the soluble COD (sCOD). Study performed by Bundhoo [14] reported 159% increase in sCOD when food waste was subjected to US at 20 kHz frequency and 500 W power for 30 min. Another study on food waste treatment at US energy density of 480 W/L for 15 min reported 1.7 fold increase in the sCOD [15]. Study performed by Ormaechae et al. [16] on co-digestion of food waste with cattle manure and glycerin reported increased COD (8%)(g/kg) and subsequent biogas production with US treatment. Cesaro et al. [17] performed a study on sonolysis of food waste and reported optimum parameters of US as ultrasonic density of 0.4 W/mL and irradiation time of 60 min giving 71% increase in sCOD. Another study which was performed as two stage digestion process of converting food waste for biogas production reported that application of US as pretreatment resulted in 9% and 29% increase in sCOD and volatile fatty acids (VFA) respectively resulting in increased biogas production subsequently. 2.2. Ultrasound treatment in anaerobic digestion Low intensity US can also be applied in AD to increase product yield and reduce digestion time based on the ability of ultrasound to stimulate enzymatic activity of microbes as well as enhance the mass transfer rates [18]. Indeed, US assisted AD has been reported to be successful at laboratory and pilot scale with many studies reporting enhanced biological activity. Xu et al. [19] performed a study using online ultrasonic equipment to disintegrate sludge in anaerobic membrane reactor and reported improvement in reactor performance with 51.3% higher consumption of volatile solids (VS). In another study, the effect of low intensity US on anaerobic digestion process has been quantified in terms of the variation in TTC-Dehydrogenase activity (TTC-DHA) and also the content of co-enzyme F420 factor was estimated [20]. Application of low intensity US at 0.2 W/cm2 with frequency of 35 kHz was

1.3. Pretreatment technology-cavitation Processing feedstock to yield particle size reduction and increased surface area, is one of the important objectives of pretreatment, which can enhance the effect of hydrolysis stage with increased contact between substrate and microbes [8]. Particle size in range of 2.5–8 mm is beneficial for proper substrate-microbe interaction giving increased biogas production [9]. The pretreatment can also reduce crystallinity and degree of polymerization of cellulose that can give lesser production of furfural and hydroxymethyl furfural (HMF), which act as 2

Ultrasonics - Sonochemistry 59 (2019) 104755

S.M. Joshi and P.R. Gogate

reported to result in significant increase in the biological activity (F420 as 0.04 µmol/g VSS) but when the intensity was increased above 0.4 W/ cm2, the activity was found to decrease drastically [20]. Another study reported that when US was employed for 2 s on time per 30 s at 0.0025 W/mL power density, 40% increased methane yield was obtained. Application of low intensity US to biogas production process was reported to be beneficial with increased production of methane by 38 and 19% in ambient and mesophilic condition respectively [21]. Continuous application of ultrasound at a power density of 0.05 W/mL for 1 s on time per 1 min in upflow anaerobic sludge blanket reactor (UASBr), was reported to yield 43% increased methane production. Morphological study of cells applied with low intensity US demonstrated presence of numerous caters and cracks with 37% higher permeability and 2.3 folds higher specific area as compared to non US treated cells [22]. Physical effects due to US was also reported to provide favorable conditions for transportation of nutrients and substrate to the cell structure. Another study also reported that application of low intensity US results in activation of intracellular calcium channels with 3 times more production of Ca2+ which further helps in shortening of log phase in the growth profile of microorganisms [23]. Studies reported on application of US in feedstock pretreatment and biogas production indicate that US can be employed as a potential pretreatment process leading to increased solubilisation of organic matter and can also act as an intensifying source in actual biogas production stage with induced favorable modifications in microbes along with better mass transfer effects resulting in increased biogas production in lesser time period. In the current work, optimization of food waste pretreatment with US to increase solubilisation of feedstock has been performed based on monitoring of the sCOD. Further anaerobic digestion process in the presence of low intensity US is also studied and effect of operating parameters on the extent of biogas production, has been elucidated.

Fig. 1. Schematic representation of setup based on US horn for FW treatment.

3.2. Experimental setup 3.2.1. Pretreatment of FW US treatment was performed using US horn procured from Dakshin Pvt. Ltd., Mumbai (11 mm diameter tip with material of construction as titanium) operating at 20 kHz frequency and supplied power of 120 W (the actual transfer efficiency as per calorimetric measurements is about 10%). Schematic representation of the experimental setup is shown in Fig. 1. Pretreatment of FW was studied in details with variation of parameters like time of US irradiation (min), US density (W/ mL), US duty cycle (%) and substrate loading (% w/v). 100 mL mixture of FW was typically pretreated in a glass reactor in different experiments involving variation of US irradiation time in the range of 2–14 min, US density in the range of 0.2–1 W/mL, US duty cycle in the range of 20–80% (20% duty cycle means ultrasound will be on for 2 s and off for 8 s) and substrate loading in the range of 3–11% w/v. sCOD was measured at desired time intervals for monitoring the progress of the treatment.

3. Materials and methods Experimental work has been divided into two parts:

3.2.2. US assisted biogas production AD was performed in 500 mL gas bottle and biogas production was measured with the help of water displacement setup as shown in Fig. 2. Pretreated FW mixed with SS was charged in the gas bottle and flushed with nitrogen gas initially for 5 min. US was applied during anaerobic process of biogas production using US bath of 5 L capacity equipped with four transducers operating at 25 kHz frequency and power output of 200 W (Dakshin Pvt Ltd., Mumbai). During the experiment, gas bottle was kept at the zone of maximum cavitational activity. Parameters varied during the study were US power, US irradiation time and US duty cycle. US power was varied over the range of 100–200 W, US irradiation time over the range of 5–15 min and US duty cycle over the range of 10–60%. Parameter range was decided based on earlier study performed by Subhedar and Gogate on US assisted AD [24]. Gas bottles were subjected to US irradiation after every 8 h [25]. Bottles with US and without US treatment were thoroughly shaken every day for proper mixing of the contents. Monitoring of process was performed for 15 days with adjustment of pH to 7.5 and ambient temperature was around 35 °C observed during studies. Analysis of biogas formation with composition was performed on daily basis. Soluble COD (sCOD) was also estimated at the start and end of process after 15 days, which enabled the calculation of the sCOD reduction.

1. Pretreatment of food waste (FW) with ultrasound (US) for organic matter solubilisation. 2. Application of low intensity US in AD for biogas production.

3.1. Materials Food waste was collected from university canteen of Institute of Chemical Technology (ICT), Mumbai campus and its main contents were wheat, gram flour and rice with some content of fruit peels and vegetable waste. FW was crushed and homogenized to a thick slurry with a mini-blender and stored at 4 °C in refrigerator until further use. Sludge was obtained from sewage treatment plant (SS) in Mumbai, India. Characteristics of sludge and FW are mentioned in Table 1. Supernatant from US pretreated FW (USFW) with sCOD of 18500 mg/L ( ± 20) was subsequently used in the studies of biogas production.

Table 1 Characterization of substrates used in experiment work. Parameters

Sewage sludge (SS)

Food waste (FW)

Ultrasonicated FW

pH Total Solids (TS) (%) Volatile solids (VS) (% TS) sCOD (mg/L)

7.1 ± 0.2 1.0 ± 0.1 67 ± 0.2

6.2 ± 0.2 42 ± 3 65 ± 3

5.8 ± 0.2 7.5 ± 0.2 90 ± 0.2

145 ± 20

11450 ± 20

18500 ± 20

3.3. Analysis Composition of biogas produced was analyzed with the help of Gas Chromatography (GC) unit equipped with TCD detector, DB column- 3m-long (Hayesep) and with carrier gas as hydrogen. Total solids (TS) 3

Ultrasonics - Sonochemistry 59 (2019) 104755

S.M. Joshi and P.R. Gogate

Fig. 2. Schematic representation of setup based on US bath for biogas production.

density for optimum time period can be effective depending on the contents of feedstock to be treated. US irradiation for longer time period may also result in excessive cavitation giving degradation of solubilized contents and hence the optimum observed in the current work is explained. In study performed by Jiang et al. [15], US irradiation for 15 min was reported to be optimum with increased solubilisation of organic matter by 1.6–1.7 folds. FW containing complex materials like vegetable waste or fruit waste that are rich in lignin content require more time or combination of pretreatment techniques for solubilisation of the organic matter [24]. It is important to note the changes in sCOD are dependent on the type of raw material and hence detailed investigation along the method followed in the current work is important.

and volatile solids (VS) were analyzed using standard methods [26]. sCOD, which is the soluble COD (without contribution from solids), was quantified using the EPA approved reactor digestion method for analysis of samples in range of 0–1500 mg/L with color measurements based on HACH DR/2000 spectrophotometer. Standard error of ± 20 mg/L was observed in the obtained values of sCOD.

4. Results and discussion 4.1. Effect of US on solubilisation of organic matter in the FW in the pretreatment 4.1.1. Effect of US irradiation time on sCOD Effect of US irradiation time on the changes in sCOD was studied by varying time in the range of 2–14 min keeping US density constant at 0.4 W/mL, US duty cycle at 60% and substrate loading at 7% w/v. It was observed as per results shown in Fig. 3 that sCOD was the highest for the case of 10 min of US irradiation with 18500 mg/L as the actual sCOD, corresponding to a 61.5% increase from initial value of 11450 mg/L. Negligible difference in sCOD values was observed at 10 and 12 min (61.4% increase) as the US irradiation time but further increase in US time to 14 min actually decreased sCOD to 17200 mg/L (50.2% increase as compared to initial). Similar study performed by Cesaro et al. [17] on FW pretreatment reported 60 min as optimum US irradiation time with 71% increased solubilisation quantified in terms of sCOD. The optimum period of US irradiation depends upon complexity of feedstock to be treated and typically US with optimum

4.1.2. Effect of US power density on sCOD Effect of US density on the changes in sCOD was studied by varying power density in range of 0.2–1.0 W/mL keeping US irradiation time constant at 10 min, US duty cycle at 60% and substrate loading at 7% w/v with initial sCOD of 11450 mg/L. US density is the power dissipated into system and affects the cavitational intensity (20 W of US power dissipated in 100 mL is represented as 0.2 W/mL). As per the results reported in Fig. 4 much higher sCOD increase to a final value of 18,500 (61.5% increase) and 18650 mg/L (62.8% increase) was obtained at 0.4 and 0.6 W/mL as the power density respectively. Further increase in US power to 1 W/mL led to decrease in sCOD value to 17800 mg/L (55.5% as the actual increase compared to initial). Significantly higher power dissipation causes degradation of organic compounds and may also result in cushioning effect giving lower cavitational effects [13]. Elbeshbishy and Nakhala [27] reported a 9% increase in sCOD when FW was treated with US at 20 kHz and 500 W

100 90

100 80

80

Increase in sCOD (%)

Increase in sCOD (%)

90 70 60 50 40 30 20

70 60 50 40 30 20

10

10 0

0 0

2

4

6

8

10

12

14

0.2

16

0.4

0.6

US density (W/mL)

US irradiation time (min)

Fig. 3. Effect of US irradiation time on sCOD of FW.

Fig. 4. Effect of US density on sCOD of FW. 4

1

Ultrasonics - Sonochemistry 59 (2019) 104755

100

100

90

90

80

80

70

Increase in sCOD (%)

Increase in sCOD (%)

S.M. Joshi and P.R. Gogate

60 50 40 30

70 60 50 40 30

20

20 10

10

0 20

40

60

80

0

Duty Cycle (%)

3

5

7

9

11

Substrate Loading (% w/v)

Fig. 5. Effect of US duty cycle on sCOD of FW.

Fig. 6. Effect of substrate loading on sCOD of FW.

for every 1 s after 59 s of rest period. Study performed by Bougrier et al. [28] reported 15% increase in sCOD when waste activated sludge was subjected to specific ultrasonic energy of 9350 kJ/kg TS. It can be thus established that US at different densities has been applied on various feedstocks like food waste, waste newspaper, sugarcane baggase, wheat straw etc. and found to be an efficient pretreatment technique giving increased biodegradability or enhanced solubilisation [19]. It is important to note that the optimum power density is different for each starting material making a detailed study into effect of power important as demonstrated in the current work.

w/v (13200 mg/L) have been considered to understand the effect of substrate loading on the changes in sCOD keeping US irradiation time constant at 10 min, US density at 0.4 W/mL and US duty cycle at 60%. The obtained results have been depicted in Fig. 6 and sCOD was found to increase from 11450 mg/L to the highest value of 18500 mg/L (61.5% increase) for the case of 7% w/v substrate loading. Further increase in substrate loading decreased the effect of cavitation affecting solubilisation of organic matter and sCOD change was found to decrease to 38% with 17200 mg/L as the final sCOD at 9% w/v substrate loading. Further increase in substrate loading to 11% w/v decreased the extent of solubilisation to only 8% (14250 mg/L as the final value). Increased substrate loading beyond optimum dampens the US effect in the medium attributed to reduced total water content of system resulting in lesser cavity generation. Study performed by Jiang et al. [15] on FW have reported similar 2.6 and 2.7 fold increase in sCOD at 40 and 100 g/L total solids respectively using US energy density of 480 W/ L for 15 min. Similar study on US assisted FW pretreatment for organic matter solubilisation, reported the optimum substrate loading of 9% w/ v beyond which positive effects were not obtained. The observed differences in the trends for effect of substrate loading can be attributed to the fact that FW contents are diverse and vary from system to system which further have effect on water absorption from dispersion medium as well as the degree of effect on the cavitation [17]. It is thus clearly demonstrated that detailed study into effect of substrate loading is important to obtain the optimum value highlighting the importance of the current work.

4.1.3. Effect of US duty cycle on sCOD US duty cycle is one of the important parameters for optimization of process efficiency as it decides the intensity of cavitational effects and also helps in proper maintenance of US equipment. ON-OFF time for the ultrasound varies with the duty cycle, for example at 20% duty cycle, the ultrasound will be ON for 2 s and OFF for 8 s in every 10 s cycle. US duty cycle was varied over the range of 20–80% keeping US irradiation time constant at 10 min, US density at 0.4 W/mL and substrate loading at 7% w/v with initial COD of 11450 mg/L. It can be observed from the results given in Fig. 5, that duty cycle of 60% gave highest sCOD changes with increased value of 18500 mg/L resulting in more solubilisation of organic matter while at 80% duty cycle the value of sCOD was found to decrease to 17400 mg/L (only 52% increase compared to initial) due to cushioning effect of cavitation giving decrease in solubilisation of organic matter. Duty cycle at 20 and 40% resulted in 20.5 (13800 mg/L) and 35% (15500 mg/L) sCOD solubilisation, which was much lower as compared to optimum case. Study performed by Subhedar and Gogate [13] on US assisted delignification of paper waste established 70% duty cycle as the optimum with 70 min as US irradiation time and 100 W as power. Application of US in continuous mode causes wear and tear of equipment and also affects the efficiency of process making pulsed operation important. Pan et al. [29] reported antioxidant yield of 24 and 22% from pomegranate peel using continuous US and pulsed US respectively and negligible difference in yield made pulsed US to be more sustainable. The optimum duty cycle also depends on specific system and should be established using studies as mentioned in the current work, clearly highlighting the importance of such detailed study.

4.2. US application during AD process for biogas production 4.2.1. Effect of US irradiation power Irradiation power of US bath was varied in the range of 100–200 W keeping irradiation time constant at 5 min with a constant duty cycle of 10%. It can be observed from the results given in Fig. 7 that biogas production was found to increase with an increase in power and was found to be highest at irradiation power of 200 W i.e. 0.04 W/mL as the power density giving 14800 mg/L (80%) sCOD removal. The sCOD removal at 100 W, 120 W and 160 W was observed as 11470 mg/L (62%), 12395 mg/L (67%) and 13320 mg/L (72%) respectively. Increase in biogas production and higher sCOD removal with an increase in power can be attributed to the fact that application of US leads to increased cell wall porosity causing higher transfer of nutrients from fermentation media to microbial cells [24], which is expected to be higher at higher power dissipation into medium. A study was performed by Cho et al. [25] on US irradiation of methanogenic granules at constant optimum US density of 0.05 W/mL and US irradiation time of 5 min revealed 43% increase in biogas production. Xie et al. [20] performed a study on enhancement of anaerobic sludge activity using low

4.1.4. Effect of substrate loading on sCOD Substrate loading is generally monitored to control density of slurry as cavitation effect decreases with increased density of slurry. Depending on the type of feedstock, substrate loading is generally kept below 10% w/v so that pretreated feedstock can be further utilized for fermentation processes [17]. In current study, different substrate loadings as 3%w/v (8900 mg/L as the initial sCOD), 5% w/v (10,200 mg/L), 7% w/v (11450 mg/L), 9% w/v (12400 mg/L) and 11% 5

Ultrasonics - Sonochemistry 59 (2019) 104755

S.M. Joshi and P.R. Gogate 4500.00

Another study reported that US irradiation time of 10 min with US intensity of 0.2 W/cm2resulted in 91.5% COD removal, which was the best result [20]. Study performed by Cho et al. [33] on up flow anaerobic sludge blanket reactor (UASBr) reported that US irradiation time of 1 s at irradiation interval of 1 min during the entire process at 0.05 W/mL resulted in 213% enhanced activity of hydrolytic enzymes acting on methanogenic granules. Study performed on US assisted enhanced ethanol production reported optimum US irradiation time for yeast cells to be 10 min with duty cycle of 20% and 0.03 W/mL as the power density [24]. Study reported by Wang et al. [34] on laccase production also established that US irradiation time of more than 5 min significantly decreased production of laccase. It can be noted from reported studies that each microbial strain will respond differently to US irradiation depending upon its unique cellular structure and properties pertaining to them, thus making detailed study on effect of irradiation time important.

Cumulative methane yield (mL)

4000.00 3500.00 3000.00 100W

2500.00

120W 2000.00

160W

1500.00

200W Without US

1000.00 500.00 0.00 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16

Time (days)

Fig. 7. Effect of US power on cumulative methane yield.

power US and reported 0.2 W/cm2 to be optimum US intensity. Subhedar and Gogate [24] reported US density of 0.03 W/mL to be optimum for enhanced ethanol production from waste newspaper with 1.8 times higher ethanol production as compared to control. Different microorganisms respond differently to US irradiation and at optimum power highest production of desired product can be obtained due to the maximum activity enhancement and it is important to note that any excess power may also lead to decreased production due to cell disruption by excess US energy input [30–32]. Thus establishing the optimum power for specific system in question is important requirement.

4.2.3. Effect of US duty cycle Duty cycle is also one of the important factors when employing US as it helps in maintenance of the equipment with efficient working. In current study, duty cycle was optimized by studying the effects at 10, 20, 40 and 60% keeping constant US irradiation power and irradiation time at 200 W and 5 min respectively. For 10% duty cycle the ON period is 1 s and OFF time is 9 s; so in time of 5 min, US irradiation was ON for 6 s per min i.e. 30 s for 5 min operation. It can be observed from the results depicted in Fig. 9 that the highest biogas production with 14800 mg/L (80%) as the sCOD reduction was obtained at 10% duty cycle. sCOD removal at 20, 40 and 60%was reported to be 13,690 (74%), 9620 mg/L (52%) and 5550 mg/L (30%)respectively. It can be observed that biogas production is found to decrease with an increase in US duty cycle and can be due to the fact that continuous operation of US for prolonged period within the cycle results in generation of excess heat and mechanical shear which result in detrimental effects on microbial cells. At 60% duty cycle, the negative effect on the microbes is at peak leading to worst results even as compared to that obtained in the absence of ultrasound. Performance of transducers is also affected due to continuous operations causing erosion and hence it important to operate at optimum levels of duty cycle. Study performed by Cho et al. [33] have reported an optimum duty cycle of 1.6% for the maximum activities of hydrolytic enzymes (amylase, cellulase, and protease). Optimized duty cycle of 20% was reported for ethanol production studies [24,35] whereas in the current study 10% duty cycle was found to be optimum. The differences in the optimum duty cycle are attributed to the differences of microbial response to US and hence specific studies are required for the system in question, clearly confirming the importance of the current detailed study into effect of duty cycle.

4.2.2. Effect of US irradiation time To study effect of US irradiation time on production of biogas, the gas bottles were subjected to US irradiation after 8 h of digestion time for different time period varying in range of 5, 10 and 15 min keeping other parameters at constant values like US power as 200 W and duty cycle as 10%. It can be observed from the data given in Fig. 8 that biogas produced was found to be significantly higher at US irradiation time of 5 min and found to be decreased if irradiation was used for longer time period. US application for 5 min resulted in 14800 mg/L (80%) sCOD removal while US for 10 min and 15 min resulted in 13135 mg/Land 11470 mg/L sCOD removal respectively. It appears that 5 min ultrasound application is the best condition though still lower time were not evaluated in the study as per generally reported investigations for application of ultrasound for biogas production. Definitely application for extended time beyond the optimum seems to be detrimental as prolonged US exposure to microbial cells results in detrimental changes based on the excess mechanical shear and shock waves effects generated by US. Study performed by Cho et al. [25] reported US irradiation time of 5 min to be optimum with continuous US application at 0.05 W/mL power density giving 72% COD removal.

4500.00

4500.00

4000.00

Cumulative methane yield (mL)

Cumulative methane yield (mL)

4000.00 3500.00 3000.00 2500.00

5 min

2000.00

10 min 15 min

1500.00

Without US

1000.00

3500.00 3000.00 10%

2500.00

20% 2000.00

40% 60%

1500.00

Without US

1000.00 500.00

500.00 0.00

0.00

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16

0

Time (days)

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16

Time (days)

Fig. 8. Effect of US irradiation time on cumulative methane yield.

Fig. 9. Effect of US duty cycle on cumulative methane yield. 6

Ultrasonics - Sonochemistry 59 (2019) 104755

S.M. Joshi and P.R. Gogate

Cumulative methane yield (mL)

4500.00

of 0.4 W/mL, duty cycle of 60%, US irradiation time of 10 min and substrate loading of 7% w/v giving 61.5% increase in sCOD. Comparison with literature also established that the optimized parameters vary depending on the composition of FW to be treated and may require intense US conditions when treating complex waste to get similar intensification. US when applied at lower US density in the AD resulted in two fold increase in the production of biogas due to increased permeability and mass transfer through cell wall of microbial cells. Optimized parameters for low strength US application in AD were US power of 200 W (0.04 W/mL), US irradiation time of 5 min and US duty cycle of 10% applied after every 8 h that resulted in 80% sCOD removal as compared to 48% sCOD removal when performed without US within same time period of 15 days. Overall it can be concluded from results of the current study that US has a vast potential of application in feedstock pretreatment and biogas production processes giving significant intensification benefits.

4000.00 3500.00 3000.00 2500.00 Conventional

2000.00

US assisted 1500.00 1000.00 500.00 0.00 0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15 16

Time (days)

Fig. 10. Comparison of US assisted process with non-sonicated process in terms of cumulative methane yield.

Acknowledgement

Table 2 Comparison of methane production from control (without US) and US assisted process. Condition

US irradiation Power (W)

US irradiation time (min)

US duty cycle (%)

COD removal efficiency (%)

Without US US assisted process

200

5

10

48 80

160 120 100 200 200 200 200 200

5 5 5 10 15 5 5 5

10 10 10 10 10 20 40 60

62 67 62 71 62 74 52 30

Authors would like to acknowledge the funding of University Grants Commission, Government of India, New Delhi, India under UGC-BSR scheme of providing Ph.D. fellowship to SMJ. References [1] C. Sawatdeenarunat, K.C. Surendra, D. Takara, H. Oechsner, S.K. Khanal, Anaerobic digestion of lignocellulosic biomass: challenges and opportunities, Bioresour. Technol. 178 (2015) 178–186. [2] S.M. Joshi, P.R. Gogate, Intensified Synthesis of Bioethanol from Sustainable Biomass, in: L. Singh, V. Kalia (Eds.), Waste Biomass Manag. – A Holist. Approach, Springer International Publishing AG, 2017, pp. 251–287. [3] R. Kadam, N.L. Panwar, Recent advancement in biogas enrichment and its applications, Renew. Sustain. Energy Rev. 73 (2017) 892–903. [4] V.A. Vavilin, S.V. Rytov, L.Y. Lokshina, A description of hydrolysis kinetics in anaerobic degradation of particulate organic matter, Bioresour. Technol. 56 (1996) 229–237. [5] R.J. Patinvoh, O.A. Osadolor, K. Chandolias, I. Sárvári Horváth, M.J. Taherzadeh, Innovative pretreatment strategies for biogas production, Bioresour. Technol. 224 (2017) 13–24. [6] I.V. Yentekakis, G. Goula, Biogas management: advanced utilization for production of renewable energy and added-value chemicals, Front. Environ. Sci. 5 (2017) 1–18. [7] P. Kumar, A. Hussain, S.K. Dubey, Methane formation from food waste by anaerobic digestion, Biomass Convers. Biorefinery. 6 (2016) 271–280. [8] C. Zhang, G. Xiao, L. Peng, H. Su, T. Tan, The anaerobic co-digestion of food waste and cattle manure, Bioresour. Technol. 129 (2013) 170–176. [9] F.O. Agyeman, W. Tao, Anaerobic co-digestion of food waste and dairy manure: Effects offood waste particle size and organic loading rate, J. Environ. Manage. 133 (2014) 268–274. [10] P.R. Gogate, A.M. Kabadi, A review of applications of cavitation in biochemical engineering/biotechnology, Biochem. Eng. J. 44 (2009) 60–72. [11] P.R. Gogate, Cavitation: an auxiliary technique in wastewater treatment schemes, Adv. Environ. Res. 6 (2002) 335–358. [12] K.S. Ojha, T.J. Mason, C.P. O’Donnell, J.P. Kerry, B.K. Tiwari, Ultrasound technology for food fermentation applications, Ultrason. Sonochem. 34 (2017) 410–417. [13] P.B. Subhedar, P.R. Gogate, Alkaline and ultrasound assisted alkaline pretreatment for intensification of delignification process from sustainable raw-material, Ultrason. Sonochem. 21 (2014) 216–225. [14] M.A.Z. Bundhoo, Effects of microwave and ultrasound irradiations on dark fermentative bio-hydrogen production from food and yard wastes, Int. J. Hydrogen Energy. 42 (2017) 4040–4050. [15] J. Jiang, C. Gong, J. Wang, S. Tian, Y. Zhang, Effects of ultrasound pre-treatment on the amount of dissolved organic matter extracted from food waste, Bioresour. Technol. 155 (2014) 266–271. [16] P. Ormaechea, L. Castrillón, E. Marañón, Y. Fernández-Nava, L. Negral, L. Megido, Influence of the ultrasound pretreatment on anaerobic digestion of cattle manure, food waste and crude glycerine, Environ. Technol. 38 (2017) 682–686. [17] A. Cesaro, V. Naddeo, V. Amodio, V. Belgiorno, Enhanced biogas production from anaerobic codigestion of solid waste by sonolysis, Ultrason. Sonochem. 19 (2012) 596–600. [18] J. Mata-Alvarez, S. Macé, P. Llabrés, Anaerobic digestion of organic solid wastes. An overview of research achievements and perspectives, Bioresour. Technol. 74 (2000) 3–16. [19] Q.-Q. Zhang, R.-C. Jin, The application of low-intensity ultrasound irradiation in biological wastewater treatment: a review, Crit. Rev. Environ. Sci. Technol. 45 (2015) 2728–2761. [20] B. Xie, H. Liu, Y. Yan, Improvement of the activity of anaerobic sludge by lowintensity ultrasound, J. Environ. Manage. 90 (2009) 260–264.

4.3. Comparison of conventional approach and US assisted approach Comparison of biogas production process without US was performed with US assisted process using optimum parameters as 200 W (0.04 W/ mL) US irradiation power, 5 min US irradiation time and 10% duty cycle with application of US irradiation after every 8 h. The control batch without US was thoroughly shaken after every 8 h so as to mix the contents. It was observed from the results given in Fig. 10 that at the end of 15 days there was almost two fold increase in biogas production in US assisted process also corresponding to 80% as the actual obtained sCOD reduction. AD without US resulted in only 8880 mg/L (48% of the initial value) sCOD removal (Table 2). It can be attributed to the fact that US assisted production results in higher mass transfer of nutrients across cell walls resulting in higher production of biogas within shorter period of time resulting in net of 80% sCOD removal within 15 days. The other contributing effects of ultrasound include overall increase in surface area, enzymatic activity and enhanced cell wall permeability. Increased enzymatic activity can be due to activation of self-defense mechanisms and changes in physical characteristics. Increased permeability also enhances the uptake of soluble H2 from medium, balancing the osmotic pressure in medium and enrichment of desired microbes [25]. Similar results of enhanced COD consumption and biogas production from methanogenic granules when irradiated with US have been reported in literature [20,25,33]. 5. Conclusions US was established to be beneficial in solubilisation of organic matter present in the food waste (FW) and further application during AD resulted in enhanced biogas production from pretreated FW. Optimized parameters obtained for pretreatment of FW were US power 7

Ultrasonics - Sonochemistry 59 (2019) 104755

S.M. Joshi and P.R. Gogate

[28] C. Bougrier, C. Albasi, J.P. Delgenès, H. Carrère, Effect of ultrasonic, thermal and ozone pre-treatment on waste activated sludge solubilisation and anaerobic biodegradability, Chem. Eng. Process. 45 (2006) 711–718. [29] Z. Pan, W. Qu, H. Ma, G.G. Atungulu, T.H. McHugh, Continuous and pulsed ultrasound-assisted extractions of antioxidants from pomegranate peel, Ultrason. Sonochem. 19 (2012) 365–372. [30] N. Sainz, J.L. Lopez, J.A. Perez, Y. Chisti, Effects of ultrasound on culture of Aspergillus terreus, J. Chem. Technol. Biotechnol. 83 (2008) 593–600. [31] Y. Zhao, W. T-Ang, J. Xing, J. Zhang, J. Chen, Applications of ultrasound to enhance mycophenolic acid production, Ultrasound Med. Biol. 38 (2012) 1582–1588. [32] Y. Chisti, Sonobioreactors: using ultrasound for enhanced microbial productivity, Trends Biotechnol. 21 (2003) 89–93. [33] S.K. Cho, Y.M. Yun, S.G. Shin, Low-strength ultrasonication positively affects methanogenic granules toward higher AD performance: hydrolytic enzyme excretions, Ultrason. Sonochem. 36 (2017) 168–172. [34] W. Feng, M. An-Zhou, G. Chen, Z. Guo-Qiang, L. Chun-Zhao, Ultrasound-intensified laccase production from Trametes versicolor, Ultrason. Sonochem. 20 (2013) 118–124. [35] M. Shaheen, M. Choi, W. Ang, Y. Zhao, J. Xing, R. Yang, J. Xing, J. Zhang, J. Chen, Application of low-intensity pulsed ultrasound to increase bio-ethanol production, Renew. Energy 57 (2013) 462–468.

[21] S.K. Cho, D.H. Kim, I.S. Jeong, H.S. Shin, S.E. Oh, Application of low-strength ultrasonication to the continuous anaerobic digestion processes: UASBr and dry digester, Bioresour. Technol. 141 (2013) 167–173. [22] S.K. Cho, Y.H. Hwang, D.H. Kim, I.S. Jeong, H.S. Shin, S.E. Oh, Low strength ultrasonication positively affects the methanogenic granules toward higher AD performance. Part I: Physico-chemical characteristics, Bioresour. Technol. 136 (2013) 66–72. [23] W. Bochu, S. Lanchun, Z. Jing, Y. Yuanyuan, Y. Yanhong, The influence of Ca2+ on the proliferation of S. cerevisiae and low ultrasonic on the concentration of Ca2+ in the S. cerevisiae cells, Colloid. Surf. B Biointerf. 32 (2003) 35–42. [24] P.B. Subhedar, P.R. Gogate, Ultrasound-assisted bioethanol production from waste newspaper, Ultrason. Sonochem. 27 (2015) 37–45. [25] S.K. Cho, D.H. Kim, M.H. Kim, H.S. Shin, S.E. Oh, Enhanced activity of methanogenic granules by low-strength ultrasonication, Bioresour. Technol. 120 (2012) 84–88. [26] Standard Methods for the Examination of Water and Wastewater, in: 20th ed., American Public Health Association/American Water Works Association/Water Environment Federation, Washington, DC, 1998. [27] E. Elbeshbishy, G. Nakhla, Comparative study of the effect of ultrasonication on the anaerobic biodegradability of food waste in single and two-stage systems, Bioresour. Technol. 102 (2011) 6449–6457.

8