Ultrasound assisted biogas production from co

Ultrasonics - Sonochemistry 40 (2018) 193–200

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

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

MARK

⁎

B. Aylin Alagöz , Orhan Yenigün, Ayşen Erdinçler Boğaziçi University, Institute of Environmental Sciences, Bebek, İstanbul, Turkey

A R T I C L E I N F O

A B S T R A C T

Keywords: Agro-wastes Anaerobic co-digestion Grape/olive pomace Microwave Ultrasonication Wastewater sludge disintegration

This study investigates the effect of ultrasonication and microwave sludge disintegration/pre-treatment techniques on the anaerobic co-digestion efficiency of wastewater sludges with olive and grape pomaces. The effects of both co-digestion and sludge pre-treatment techniques were evaluated in terms of the organic removal efficiency and the biogas production. The “co-digestion” of wastewater sludge with both types of pomaces was revealed to be a much more efficient way for the biogas production compared to the single (mono) sludge digestion. The ultrasonication and microwave pre-treatments applied to the sludge samples caused to a further increase in biogas and methane yields. Based on applied specific energies, ultrasonication pre-treatment was found much more effective than microwave irradiation. The specific energy applied in microwave pre-treatment (87,000 kj/kgTS) was almost 9 times higher than that of used in ultrasonication (10,000 kj/kgTS), resulting only 10–15% increases in biogas/methane yield. Co-digestion of winery and olive industry residues with pre-treated wastewater sludges appears to be a suitable technique for waste management and energy production.

1. Introduction Anaerobic digestion (AD) systems generally have good records in treating a wide spectrum of waste streams in agriculture, such as animal wastes (cattle, chicken, pig, sheep, etc. manures), industrial wastes (slaughterhouse wastes, blood, fish wastes etc.), agricultural crops and residues (corn, corn silage, wheat straw, barley pomace, grass, clover leaves, bagasse, etc.) and domestic wastes [1]. The total global market potential by 2020 has been estimated to become nearly ten times the current energy production from biogas of over 90 PJ/year [2]. In Turkey, the industrial application of AD technology emerged in the late 1980 s. Today, there are approximately 70–80 industrial plants that use this technology, mainly in the food industry. In anaerobic digestion; biological hydrolysis has been mostly identified as the rate-limiting step [3,4]. Sludge disintegration methods have been investigated as pre-treatment processes to eliminate this rate limiting step (hydrolysis) and increase the efficiency of digestion. Sludge disintegration speeds up/increases the solubility of the organics in sludge, by destroying the floc structure and disrupting the sludge cells leading to a release of intracellular materials (cell lysate), and an accelerated and enhanced biogas/methane production [5–8]. The application of pre-treatment methods successfully minimizes sludge production in the AD process while improving the sludge anaerobic degradability [8]. Ultrasonication disintegration (US)

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Corresponding author. E-mail address: [email protected] (B. Aylin Alagöz).

http://dx.doi.org/10.1016/j.ultsonch.2017.05.014 Received 21 December 2016; Received in revised form 27 April 2017; Accepted 9 May 2017 Available online 11 May 2017 1350-4177/ © 2017 Elsevier B.V. All rights reserved.

[9–14], microwave irradiation (MW) [15–17], ozone oxidation [18,19], mechanical disintegration [20], alkali treatment [21,22], thermal treatment [23] and biological hydrolysis with enzyme usage [24] are investigated by many researchers for the purpose of disintegrating wastewater sludges (WAS). The main fraction of WAS consists of a polymeric network formed by extracellular polymeric substances (EPS) and microbial cells that are resistant to direct anaerobic degradation since cell walls and EPS present physical and chemical barriers [25,26]. Higher WAS floc and cell destruction and release of EPS and intracellular materials into the soluble phase through US and MW pretreatment could increase the rates and biogas production. Recent studies showed that US and MW pretreatments applied to WAS were effective in solubilization of organic matter leading to an increased biogas/methane production [5,13,27,28]. There are two important mechanisms in ultrasonic pre-treatment: cavitation which benefits at “low frequencies” and chemical reactions at “high frequencies” involving OH%, HO2·, and H radicals [29]. The low frequency ultrasound technology, which is widely used in food processing/ fermentation industry [30], has been started to be used for the purpose of wastewater sludge pre-treatment. In contrast to chemical digestion of wastewater sludge, ultrasonic sludge pre-treatment does not necessitate the addition of chemicals. However, ultrasound treatment could be influenced by sonication density and solids concentration leading to an


B. Aylin Alagöz et al.

improvement in the characteristics of WAS by reducing particle size and increasing the soluble chemical oxygen demand (sCOD) concentration. Higher the sonication power employed, the more sludge particles are disrupted. Taken as a whole, the application of US pretreatment to WAS prior to anaerobic digestion could enhance biogas production by 24% to 140% and 10% to 45% for the batch and continuous AD systems, respectively [31]. Although biogas production increases with the energy input; applied specific energies are usually kept in the range from 1000 to 16,000 kJ/kgTS considering the energy consumption [6]. The studies on MW irradiation also indicate that MW pretreatment enhances sludge disintegration and hydrolysis, increases the rate/ extent of AD of WAS and inactivates the microbial population leading to an efficient stabilization [16,17,32,33]. MW are part of the electromagnetic spectrum and are considered to be radiation ranging in a frequency from 300 MHz to 300 GHz, which corresponds to a wavelength range of 1 m down to 1 mm. Materials like biomass, capable of absorbing microwave radiation, can be heated by the molecular dipole rotation. The application of MW irradiation for WAS pre-treatment either causes the acceleration of ions colliding with other molecules or causes dipoles to rotate and line up rapidly with an alternating electric field resulting in a change in structure of proteins of microorganisms [34–36]. In the MW pretreatment, absorption of MW energy by water and organic components in sludges causes to solubilization of sludge solids. In literature, the most common temperatures used in MW pretreatment are in the range of 60 to 180 °C [15]. Under boiling temperatures with municipal sludges, both batch and continuous flow AD yielded 20 to 30% higher biogas and methane yields compared to controls [33,17]. In a recent study using a bench-scale industrial MW unit at 175 °C, achieved higher biogas production and dewaterability. The biogas/methane production and dewaterability of pretreated municipal sludge were improved by 31% and 75%, respectively [35]. Water, energy and food (WEF) security is recently being recognized as one of the most important global issues. There is a very strong and complex relationship between these systems. In recent years, the popularity of biofuel production from energy crops jeopardizes food and water security. The use of crop residuals, instead of crops, as a source of energy contributes to food safety while creating an alternative solution to waste disposal problem. Every year, a huge amount of agro-wastes is generated from worldwide agricultural activities. In Turkey, 65 Mtons of agricultural wastes from cultivation and 160 Mtons of animal manure from stockbreeding are produced annually and the total utilized agricultural land of Turkey is over 38,500 thousand hectares [37,38]. Among these agricultural activities, grape and olive cultivation is one of the most important agricultural sectors in all of the Mediterranean countries. Olive and grape residuals, due to their availability and high amount of organic matter content, have recently started to be considered as potential energy sources for AD systems for the production of biogas, which mainly contains methane and carbon dioxide [39,40]. Agro-wastes can be digested either alone or through co-digestion with another biomass. In addition to the production of renewable energy, controlled anaerobic digestion/co-digestion of agricultural biomass reduces emissions of greenhouse gases, nitrogen and odor from manure management, and intensifies the recycling of nutrients within agriculture practices [41]. This study focuses on the effects of “ultrasonication” and “microwave” pre-treatments on the biogas production from the co-digestion of wastewater sludge and agricultural residues, olive pomace (OP) and grape pomace (GP).

Table 1 Characteristics of sludge samples used in the study. Parameter

Unit

Inoculum

WAS

TS VS MLSS MLVSS COD sCOD TKN NH4+ Alkalinity (as CaCO3) pH Total Coliform Fecal Coliform

mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L – cfu/100 mL cfu/100 mL

45210 35360 37250 24000 38470 5154 870 294.00 5302.5 7.3 2*106 2.2*104

20200 12385 15480 9570 16750 927 980 51.75 1155 6.72 1.4 × 107 4.7 × 106

2. Materials and methods 2.1. Substrates Seed used in the experiments was an anaerobically digested sludge (inoculum) obtained from a mesophilically operated anaerobic digester of a yeast factory in Turkey. Wastewater treatment plant (WWTP) sludge samples were taken from the return activated sludge stream of one of the advanced biological wastewater treatment plants in Istanbul, Turkey. The characteristics of sludge samples used in the study are presented in Table 1. The co-digestion studies were performed by using WAS and olive and grape pomaces. Prior to AD, the pomaces were analyzed for their total solid (TS) and volatile solid (VS) contents. Olive pomace contained 38.20% TS of which 61.44% was volatile. Grape pomace had 40.76% TS of which 88.52% was volatile.

2.2. Sludge pre-treatment optimization studies In the study, to increase the solubility of organics and speed up the hydrolysis step, mechanical (ultrasonication) and thermal (microwave) disintegration methods were applied to WAS samples. Disintegration degrees were calculated according to Müller and Pelletier’s method as given in the relationship below [42]:

DDCOD = (sCOD−sCOD0)/(sCOD NaOH−sCOD0) × 100 where; DDCOD : Disintegration Degree sCOD : sCOD value of the supernatant of the disintegrated sludge, mg/L sCOD0 : sCOD of the supernatant of the original sludge, mg/L sCODNaOH: sCOD value of the sludge disintegrated with 1 mol/L NaOH kept in room temperature, 20 ± 1 °C, for 24 h. Viscosity measurements were carried out by using a digital viscometer (Brookfield RVDV-I PRIME). Dewaterability of sludge samples was determined in terms of the capillary suction time (CST) by using CST apparatus (Triton Electronics Ltd, England). Particle size analyses were conducted by employing a particle size analyzer (Malvern Mastersizer 2000 with the wet dispersion unit of Hydro2000MU). The Mastersizer 2000 is used to measure the aggregate size in the range of 0.02–2000 µm. Water was used as dispersant liquid and its refractive index was 1.33. Sludge samples had refractive index of 1.5. The pump/stirrer speed was kept at 600 rpm, which is the minimum pump/stirrer speed available to minimize damage to sludge particles. Particles size distribution of each sample was measured three times by the instrument (standard deviation: 0.1–3.4%). The mean size (d 0.5) is used to denote floc size. 194



2.2.1. Ultrasonic pre-treatment studies The sludge samples were mechanically pre-treated by using an ultrasonic homogeniser (Bandelin-Sonopuls HD 3400). The ultrasonic homogenizer is equipped with a generator (GM 3400), an ultrasonic converter (UW 3400), a booster horn (SH 3425) and a probe (VS 200 T). The ultrasonic unit has a constant frequency of 20 kHz and a maximum energy-output of 400 W. The acoustical power entering the ultrasonication system can be determined by different methods including calorimetric dosimetry, acoustical dosimetry, chemical dosimetry and electrical power measurement [43–46]. In this study, calorimetry was applied to find the actual power dissipated into 400 mL sludge sample. The amount of power was calculated by using the equation below (deionized water was used as a solvent) [44].

Power =

Table 2 The content of the reactors.

⎛ dT ⎞ ⎜ ⎟ . C .M ⎝ dt ⎠ p

R1 R2 R3 R4 R5 R6 R7 R8

Inoculum Inoculum + WAS Inoculum + WAS + OP Inoculum + Ultrasonically Pre-treated WAS + OP Inoculum + Microwave Pre-treated WAS + OP Inoculum + WAS + GP Inoculum + Ultrasonically Pre-treated WAS + GP Inoculum + Microwave Pre-treated WAS + GP

2.3. Anaerobic co-digestion studies Anaerobic co-digestion studies were performed in eight replicate 2.5 L batch reactors, each having 1600 mL active volume. The reactors were operated at mesophilic conditions (37 °C) for a hydraulic retention time of 30 days. The inoculum to substrate ratios (ISR) in the reactors were adjusted to 1/1. The contents of the reactors are given in Table 2. In reactors R4 and R7, the sludge samples were ultrasonically pretreated by Bandelin-Sonopuls HD 3400 operated at the frequency of 20 kHz and a supplied power of 200 W with a specific energy of 10,000 kJ/kg TS. Microwave disintegration was applied to the sludge samples in reactors R5 and R8 for 10 min at 175 °C and 30 bar (with a specific energy of 87,000 kJ/kgTS), based on the results of the optimization part and a preliminary MW optimization study previously held by the authors for different pressures, temperatures and application times [48]. After the MW pre-treatment, samples were removed from the heating source, and cooled to room temperature in closed vessels to avoid evaporation of organics before AD. Analyses for the mixtures of substrates to evaluate the degree of waste stabilization were performed in duplicates according to the Standard Methods [49]. The averages of the measurements were used in the data for evaluations. The amount of biogas production in the reactors was measured continuously by using Miligascounters (Ritter MGC-1). The gas compositions (CH4 and CO2) were determined with a Gas Chromatograph (GC HP 5890).

P: power dissipated in the system (W)

dT ) dt

: change in temperature at a certain time interval (°C/s) Cp: heat capacity of water (4.18 J/g) M: mass of water (g) The calorimetric method involves the measurement of the temperature (T) rise versus time (t) with 30 s intervals using a thermocouple immersed in 400 mL of deionized water. The value of dT/dt was estimated from the slope of the curves fitting the best polynomial/ straight line or the tangent at time zero. After the calibration study, 400 mL of WAS sample was sonicated at different power inputs of 60, 100, 200 and 300 W at different sonication times to calculate the specific energies (SE) applied to the sludge samples. The specific energy (SE) is defined as the energy supplied per unit of mass of sludge solid (as TS) to evaluate the solubilization and disintegration performance of the sludge [47,5]. SE is a function of ultrasonic power, ultrasonic duration, volume of sonicated sludge and TS concentration, and was calculated by using the equation below [5]:

SE =

Content

190 °C and two different duration times of 10 and 20 min were applied at the pressure of 30 bar.

Where,

(

Reactors

2.4. Statistical analyses

Pxt VxTS 0

In the evaluation of results, standard statistical procedures were used, including standard deviation (for more than duplicate data points) and absolute difference between main and duplicate data points. To determine whether any of the differences between the means are statistically significant, One-way ANOVA and Tukey-HSD Test were used. ANOVA was carried out with a confidence level of 95%. Error bars in the figures represent standard deviations of duplicate measurements.

where, SE : specific energy input in kWs/kg TS (kJ/kg TS) P : ultrasonic power in kW t : ultrasonic duration in seconds V : volume of sonicated sludge in liters TS0 : total solids concentration in kg/L Actual powers, specific energies, sCOD concentrations and DDCOD were then calculated and the optimum operational conditions for ultrasonication were determined.

3. Results and discussions

2.2.2. Microwave pre-treatment studies Microwave disintegration was applied to the sludge samples as thermal pre-treatment by using a Berghof Speedwave MWS-3+ Microwave Digestion System having 12 Teflon vessels with a capacity of 60 mL. The system works with a power supply of 230 V, a magnetron frequency of 2.45 GHz and with a power output of 1450 W. The maximum temperature of the instrument is 300 °C and the pressure measurement range is between 0–150 bar. In MW optimization study, the temperatures of 100, 150, 175 and

Ultrasonic disintegration was applied to WAS samples at different nominal power outputs of 60, 100, 200 and 300 W (42, 49.6, 101.7 and 113.3 W respectively, based on calorimetry) to find the most appropriate conditions. For this process, 400 mL of wastewater sludge samples were brought to ambient temperature (20 ± 2 ⁰C) before the application of ultrasonication. In the calibration process of ultrasonication, heat capacity of sludge was calculated to be 4.15 J/g/°C through using heat capacity of water (Cpwater = 4.186 J/g/°C) and solids in sludge (Cpwater = 1.95 J/g/°C)

3.1. Ultrasound optimization

195



Table 3 Effect of ultrasonication applied at different powers on disintegration and solubility of sludge solids. SE (kJ/kg TS)

Ultrasonic Power (Watt)

Nominal

Actual

Nominal

Actual

10000

7000 4960 5085 3776

60 100 200 300

42 49.6 101.7 113.3

sCOD (mg/L)

Table 5 Effect of MW pre-treatment on disintegration and solubility of sludge samples at different temperatures and application times.

DD (%)

702 1506 2850 2790

6.30 15.60 17.80 16.40

with the equation presented below [50]:

Cpsludge = Water content of sludge × Cpwater + Water content of solids

Conditions

sCOD (mg/L)

DD (%)

No pre-treatment 100 °C 10 min 100 °C 20 min 150 °C 10 min 150 °C 20 min 175 °C 10 min 175 °C 20 min 190 °C 10 min 190 °C 20 min

122 1125 2219 3248 3498 4100 3810 3415 2562

– 10.40 21.75 32.42 35.01 41.25 38.25 34.15 25.30

⁰C for 10 and 20 min under 30 bar pressure. The results are presented in Table 5. At the end of the experimental runs, 175 ⁰C and 10 min (with 87,000 kJ/kgTS) were found to be the optimum operational conditions for MW disintegration, regarding the energy efficiency. The solubility, dewaterability, viscosity and the particle size analyses of sludge samples for this optimum condition are given in Table 6.

× Cpsolids Cpsludge = 0.998 × 4.186J/g/° C+ 0.012 × 1.95J/g/° C= 4.15J/g/°C The calculated heat capacity of 4.15 J/g/°C was quite similar to the value given in literature which is 4.18 J/g/°C. Therefore, Cpsludge was accepted as 4.18 J/g/°C. Table 3 shows the effect of ultrasonication applied at different power outputs on disintegration and solubility of sludge solid (at 10,000 kJ/kg TS with 70% amplitude). The optimum power for the rest of the experimental study was selected to be 200 W since it was more effective in disintegrating and solubilizing organic solids in sludge. As a following step, different specific energies of 5000, 10,000, 15,000, 25,000 and 50,000 kJ/kgTS were applied to sludge samples at 200 W and 70% amplitude conditions. The consequent changes in sludge characteristics are given in Table 4. It was observed that increasing the specific energy during the ultrasonic pre-treatment caused a decrease in particle size and viscosity of sludge samples and increases the solubility of organics. Menardo et al. [51] and Neves et al. [52] focused on thermal and ultrasonic pre-treatments applied to agricultural wastes. In parallel with the findings of this study, they indicated that ultrasonic pre-treatment could decrease the particle size of sludge samples to a range of 0.2 to 5 µm leading to higher methane production. It was found that the increasing specific energies led to a rise in CST values indicating deteriorated dewaterability of sludge samples. However, the obtained specific energy is directly proportional to the applied sonication time and higher energies require longer sonication time. As a result of the optimization studies, the most energy efficient operational conditions for ultrasonication were selected to be 200 W (101.7 W, as actual power) and 70% amplitude to achieve a specific energy of 10,000 kJ/kg TS (5085 kJ/kg TS, as calculated after calibration).

3.3. Anaerobic co-digestion Initially, the effect of different TS contents on the cumulative biogas production from the anaerobic mono-digestion of OP and GP were evaluated. The pomace substrates and inoculum were digested at mesophilic condition (37 °C) for 30 days in 2500 mL reactors each having 1600 mL working volume. The reactors were operated at four different TS concentrations of 6.5, 7, 8 and 10%. ISR of the reactors were adjusted to 1:1. The results of this study revealed that the highest biogas production was obtained at a TS content of 6.5% (Fig. 1). Therefore, TS contents of the reactors used in the rest of the study were adjusted to 6.5% in accordance with the findings of the study on olive pomace digestion by Tekin and Dalgıç [53]. The overall anaerobic co-digestion efficiency in the study was then evaluated in terms of efficiencies of organic removal and biogas production. 3.3.1. Organic removal In all reactors pH was initially adjusted to around 7–7.5, which is a suitable condition favoring methanogenic activity. The initial and final alkalinity concentrations in the reactors were in the range of the accepted limits and ranged between 2300–3450 mg CaCO3/L and 3000–5800 mg CaCO3/L, respectively [54]. Due to high initial alkalinity concentrations of the agricultural substrates, the alkalinity concentrations in the co-digestion reactors were higher than the mono sludge digestion reactor. TS and VS contents and removal efficiencies of the reactors are presented in Fig. 2. The removal of organics indicated an efficient stabilization. The organic content of all the reactors decreased through to the end of digestion as expected. However, the highest organic removal efficiencies were obtained in co-digestion reactors containing

3.2. Optimization of microwave disintegration In this study, MW pre-treatment was optimized by applying MW irradiation to sludge samples at temperatures of 100, 150, 175 and 190

Table 4 Effect of ultrasonication at different specific energies on sludge characteristics (at 200 W power and 70% amplitude). SE (kJ/kg TS)

0 5000 10000 15000 25000 50000

sCOD (mg/L)

135 1650 2850 2906 3064 7063

DD (%)

– 15.5 17.80 19.7 30.3 82.1

CST (sec)

52 1296 1345 1449 1528 1652

Viscosity (Mpas)

34 28 24 23 20 18

Particle Size Surface Weighted Mean D[3,2]

Volume Weighted Mean D[4,3]

d (0.1) µm

d (0.5) µm

d (0.9) µm

36.400 22.100 14.240 6.133 4.620 3.419

81.931 42.821 36.648 32.407 35.480 38.744

18.763 18.360 10.478 2.657 2.074 1.176

63.037 39.676 30.820 19.303 16.264 11.677

145.369 73.045 60.472 59.722 65.453 59.639

196



Table 6 Effect of 10 min MW pre-treatment on sludge characteristics at 175 °C. Sample

sCOD (mg/ L)

Cumulative Biogas Production (ml)

Untreated Sludge 10 min MW Pretreated Sludge

122 4100

DD (%)

– 41.25

CST (sec)

52 437

Viscosity (Mpas)

34 31

Particle Size Surface Weighted Mean D [3,2]

Volume Weighted Mean D [4,3]

d (0.1) µm

d (0.5) µm

d (0.9) µm

36.400 86.094

81.931 173.504

18.763 50.102

63.037 147.851

145.369 331.816

The results also showed that both olive and grape pomaces were found to be suitable crop residuals for biogas production with AD process. The biogas production efficiency of grape pomace was found to be higher than olive pomace. The results of ANOVA applied for the evaluation of biogas production data showed that the differences between the mean values obtained in the reactors found statistically significant with confidence level of 95%. Additionally the groups obtained by Tukey HSD Test reveals that the most close reactor pairs on the basis of biogas productions were pretreatment applied reactors, being R4-R7, containing ultrasonicated WAS, and R5-R8, containing MW irradiated WAS. The obtained higher production of biogas can be attributed to the synergistic effect of co- sludge pre-treatment and co-digestion. Recently published review reports that ultrasonication pretreatment in sludge digestion improved the biogas production by 24 to 138% in batch mesophilic AD while this improvement was 45–79% when the sludge was pre-treated with microwave [8]. Accordingly; Mata-Alvarez et al. [1] showed that anaerobic co-digestion can increase CH4 production by 50–200% depending on the operating conditions and co-substrates used. In addition to the biogas production (mL), the composition of biogas is an important parameter in determining the performance of an anaerobic digestion process. As shown in Fig. 4, the methane contents of the biogas produced in the reactors ranged between 54–65%. The maximum methane composition of 65% was obtained in between 12th and 20th days of the digestion in Reactors R6 and R7, being followed by reactor R8 (64%). Fig. 5 shows the biogas and methane yields of the reactors as a function of time. The biogas and methane yields obtained from the codigestion of WAS with both grape and olive pomaces followed a similar trend. The maximum biogas yields were achieved with co-digestion of these pomaces with microwave pre-treated WAS. In co-digestion process, addition of OP and GP to WAS enhanced the biogas yield by 31 and 40% respectively, due to improved C/N ratio of the mixtures. Biogas yields were further increased about 11–14% and 30–32% by applying ultrasonic and MW pre-treatments to WAS samples prior to AD, respectively. In this study, the maximum biogas yield of 0.18 L/gVSremoved and methane yield of 0.11 L/gVSremoved were obtained from the co-digestion of microwave pre-treated WAS and grape pomace (R8). Ultrasonic pretreatment of WAS, in the co-digestion of OP and GP, also produced slightly lower biogas and methane yields. Our results were quite comparable with the reported values in the literature. Amon and his colleagues [57] studied the anaerobic digestion of energy crop, maize and dairy cattle manure, and effects on biogas production. The highest biogas yields obtained in their experiments were between 0.108–0.268 L/gVS removed. Fantozzi and Buratti [58] also reported a biochemical methane potential of olive pomace as 110 NL/kgVS using a mesophilic batch reactor. When the performance of US and MW pre-treatments were compared based on the applied specific energies; ultrasonication appeared to be much more effective than MW irradiation. Specific energy applied to sludge samples were 10,000 kJ/kgTS and 87,000 kj/kgTS for US and MW pre-treatments respectively. Although applied energy was almost 9

6000 5000 4000 3000 OP 2000

GP

1000 0 6.5

7 8 TS Content (%)

10

Fig. 1. Effect of substrate TS concentrations on the biogas production from anaerobic mon-digestion of OP and GP.

pretreated WAS which is known to have low carbon to nitrogen (C/N) ratio. Co-substrates provided various nutrients and balanced the C/N ratio of the reactor content [40]. TS and VS removal rates were found to be slightly higher in the reactors containing grape pomace than the ones having olive pomace. In anaerobic digestion process, initial concentrations and further accumulation of ammonia nitrogen is an important parameter to be appropriately monitored. WAS, olive and grape pomaces are all organic materials that contain various forms of organic nitrogen which will be converted to ammonium during anaerobic degradation. If high levels of ammonium nitrogen are present at elevated pH values, free ammonia nitrogen is produced which has severe inhibitory and even toxic effects on methanogenic archaea. In the study, NH4+-N concentrations were analyzed at the beginning and at the end of the experiments. The final ammonium nitrogen concentrations in the digesters were found to be in the range between 118–405 mg/L which were sufficiently lower than the reported inhibitory levels in the literature [55]. If pre-treatments at high temperature (> 175 °C) were applied for a longer time than necessary, nitrogenous organic materials become less biodegradable and inhibitory concentration of ammonia can be formed [56]. In the anaerobic digesters, no ammonia inhibition case was reported. 3.3.2. Biogas production The gas analyses covered a wide range of parameters; cumulative biogas production, biogas composition (in terms of methane and carbon dioxide content), biogas and specific methane yields. The cumulative biogas productions in the reactors are shown in Fig. 3. The first few days of the digestion period are critical since the maximum substrate utilization is generally expected to occur in 5 to 7 days. It is clearly seen that starting from the first day of the AD process; both sludge pre-treatment techniques speeded up the hydrolysis step resulting in higher biogas productions (Fig. 3). This rapid increase in the gas productions can be explained with the increased solubility of the sludge organics. The results indicated that microwave pre-treatment (at SE of 87,000 kj/kgTS) of sludge was performed slightly better than ultrasonic pre-treatment (at SE of 10,000 kj/kgTS) in terms of biogas production. Among the reactors, the highest cumulative biogas production of 6351 mL was observed at reactor (R8) containing grape pomace and microwave pre-treated WAS. 197



Fig. 2. TS and VS concentrations and removal efficiencies in the reactors.

Cumulative Biogas Production (ml)

7000 6000 5000

R1 R2

4000

R3 3000

R4 R5

2000

R6 R7

1000

R8 0 0

1

4

5

6

7

8

11 12 14 18 19 20 22 25 26 30 Time (day)

Fig. 3. Cumulative biogas productions in the reactors.

of the ultrasonicators limits the scale at which they may be operated. However, with the arrival of scale-up and affordable equipment, ultrasonication has become a method of sludge pre-treatment. There are many large scale ultrasound applications used for industrial and environmental purposes. This should also be taken into consideration in the selection of sludge pre-treatment options.

times higher in MW irradiation, the biogas yield was just about 6–15% higher than that of ultrasonication. Another important drawback of MW application is its application at large scale plants. In this study, the MW irradiation was applied to small size samples in small size vessels under pressure. This fact limits the use of MW irradiation in large scale applications. In the same way, the size 198



Methane Content of the Biogas (%)

70 60 50 40 30 20 10 0 5

9

12

15

19

24

30

Time (days) R1

R2

R3

R4

R5

R6

R7

R8

Fig. 4. Methane contents of the biogas in the reactors.

0.25

Gas Yield

0.20 0.15 0.10 0.05 0.00 R1

R2

R3

Ultimate Biogas Yield (L/g VS removed)

R4 R5 Reactors

R6

R7

R8

Ultimate Methane Yield (L CH4/g VS removed)

Fig. 5. Biogas and methane yields in the reactors.

4. Conclusions

by using crop residuals, rather than energy crops, provides an environmental-friendly solution for the disposal of WAS and the agricultural wastes.

This study investigated the effects of ultrasonic and microwave sludge pre-treatments on the biogas production through the anaerobic co-digestion of wastewater sludges (WAS) with olive pomace (OP) and grape pomace (GP), as agricultural wastes. The co-digestion of WAS with OP and GP was found to be much more efficient way of biogas production, compared to the single (mono) sludge digestion. The results of the batch anaerobic digestion studies clearly demonstrated that both of OP and GP were appropriate energy crop residuals for biogas production, while the GP had higher biogas/ methane production potential. The daily biogas production increased approximately by 1.3-fold with the co-digestion of GP and WAS. The ultrasonication and microwave sludge pre-treatments speeded up the rate limiting “hydrolysis” step and enhanced the anaerobic biodegradability of organics in sludge samples leading to an increase in biogas and methane yields. Ultrasonication was found to be more effective sludge pre-treatment method than microwave irradiation based on applied specific energies. Although the specific energy applied in microwave was almost 9 times higher than that of ultrasonication; microwave pre-treatment led to only 10–15% increase in the biogas/ methane yields. In the frame of water, energy and food nexus; biogas production from the co-digestion of pretreated WAS and residues of winery and olive industries appears to be an appropriate technique for waste management and energy harnessing. Additionally, biogas production

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