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Modeling the Effect of Sonication on the Anaerobic Digestion of Biosolids

Modeling the Effect of Sonication on the Anaerobic Digestion of Biosolids
Modeling the Effect of Sonication on the Anaerobic Digestion of Biosolids

Energy Fuels2010,24,4703–4711:DOI:10.1021/ef901255k

Published on Web06/15/2010

Modeling the Effect of Sonication on the Anaerobic Digestion of Biosolids?Saad Aldin,?Elsayed Elbeshbishy,§George Nakhla,?and Madhumita B.Ray*,??Department of Chemical and Biochemical Engineering and§Department of Civil and Environmental Engineering, The University of Western Ontario,London,Ontario N6A5B9,Canada

Received October30,2009.Revised Manuscript Received May19,2010

Ultrasound treatment of wastewater sludge prior to anaerobic digestion disrupts the flocs and causes lysis

of the bacterial cells,releasing both inter-and intracellular materials.Primary and waste-activated sludge

(WAS)were treated with different ultrasonic intensities,varying sonication time and amplitude at a

constant frequency.Results showed that the gas production,volatile fatty acids,ratio of soluble chemical

oxygen demand to total chemical oxygen demand,and soluble protein increased,while the particulate

protein and particle size of the sludge decreased,with sonication time.An empirical model was developed

to determine the economic viability of ultrasound based on electrical energy input and energy obtained

from enhanced methane production.It has been found that ultrasonic pretreatment is only economically

viable for primary sludge at low sonication doses.The Anaerobic Digestion Model No.1(ADM1)was

applied to the batch anaerobic digestion for sonicated and non-sonicated sludge.In almost all cases,the

model successfully simulated the experimental trends.

Introduction

Biosolids produced during wastewater treatment are one of the most abundant renewable energy resources.1Within the agricultural sector in the European Union only,about1500 million tons of biosolids are produced each year.2Consump-tion of biosolids for energy production has increased signifi-cantly in recent years.Biosolids can be converted to energy either directly by combustion,where the main energy output is heat and electricity,or an energy carrier that can be used as fuel for vehicles.3

Energy from biosolids can be produced by biological (fermentation)or non-biological(thermo-chemical)pro-cesses.4Biological processes consume less energy than non-biological processes in the production of a variety of gaseous and liquid energy carriers.3Anaerobic digestion is the most commonly applied process for the stabilization of biosolids. Mass reduction,methane production,and improved dewater-ability of sludge are the most important advantages of anae-robic digestion.On account of carbon removal in the form of methane and carbon dioxide,the end product shows a sub-stantially better biological stability than the unfermented material.A disadvantage of the fermentation technique is the slow degradation rate of biosolids.Conventional residence times in anaerobic digesters are about20-40days,requiring large digesters.Sludge hydrolysis has been considered as the rate-limiting step of anaerobic digestion.5The biodegradabil-ity of biosolids can be improved by sludge pretreatment methods that enhance the solubilization of solids.

Various mechanical disintegration methods have been ap-plied for the pretreatment of biosolids to enhance the rate and extent of anaerobic digestion.Sonication is a method for the breakup of microbial cells to extract intracellular material.6 When the ultrasound wave(>20kHz)propagates in an aqueous medium,such as primary sludge(PS)and waste-activated sludge(WAS),it generates a repeating pattern of compressions and rarefactions in the medium.The rarefac-tions are regions of low pressure(excessively large negative pressure),in which liquid or slurry is torn apart.Microbubbles are formed in the rarefaction regions.As the wave fronts propagate,microbubbles oscillate under the influence of positive pressure,thereby growing to an unstable size before they violently collapse.The collapsing of the bubbles often results in localized temperatures up to5000K and pressures up to180MPa.7The sudden and violent collapse of huge numbers of microbubbles generates powerful hydro-mechanical shear forces and forms a high-speed liquid microjet that impacts the surfaces of the bulk liquid constituents surround-ing the bubbles.8The collapsing bubbles,microjet of liquid, disrupt adjacent bacterial cells by extreme shear forces,rup-turing the cell wall and membranes.Several studies have reported the benefits of ultrasound as a pretreatment method for sludge prior to anaerobic digestion,such as improved

?This paper has been designated for the Bioenergy and Green Engineering special section.

*To whom correspondence should be addressed.Telephone:519-661-2111ext.81273.Fax:519-661-3498.E-mail:mray@eng.uwo.ca.

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dewaterability,solubilization,rapid hydrolysis rate,and en-hanced biogas production.9-11Ultrasound has also been tested for the enhancement of full-scale digesters.12 Mathematical anaerobic digestion models(ADMs)have been extensively investigated and developed during the last 3decades.13As one of the most sophisticated and complex ADM,the International Water Association(IWA)anaerobic digestion model1(ADM1)is the integrated anaerobic model developed by the IWA task group for modeling of anaerobic digestion processes.14It consists of a number of processes to simulate all possible reactions occurring in anaerobic sludge digestion,including not only biological reactions,such as disintegration and hydrolysis of suspended solids,uptake (growth),and decay of microorganisms,but also physico-chemical reactions,including ion association/dissociation and liquid-gas transfer.In total,19processes,24compo-nents,and56relative stoichiometric and kinetic parameters were assumed for biological processes,and also,additional processes and parameters were determined for physico-chemical processes.The steady-state ADM1had been used with good success for approximately2years on a wide range of full-scale wastewater treatment facilities.15-17However, ADM1has a critical disadvantage that many parameters are difficult or impossible to measure.18,19Batstone et al.15 used the ADM1to evaluate two industrial treatment ap-plications.The first was the assessment of acid addition for pH decrease and avoidance of calcium carbonate(CaCO3) precipitation in a paper mill fed upflow anaerobic sludge blanket(UASB).The simulation work found,with a high degree of confidence,that acid dosing was neither econom-ical for pH control nor had any real effect on the CaCO3 levels present in the reactor.A specific calcium carbo-nate precipitation equation was added to the ADM1to undertake this study.The second study was an assessment of the benefits of thermophilic(as opposed to mesophilic) operation for reduced ammonia inhibition,improved sta-bility,and gas production in a solid digester at a gelatin production facility.Here,it was predicted that thermophilic operation could not attain either goal to a satisfactory extent.In addition to demonstrating the application of the ADM1to the two systems,they assessed the predictions generated in the case studies in terms of quality and utility. Johnson et al.16have reported in their study that a number of modifications were necessary to allow it to be used in the context of municipal wastewater treatment.It was found that the use of the model was greatly simplified if used in conjunction with a larger plant simulator to assist in the feed fractionation.It was also found that a better fit to actual operating data was achieved if some of the slowly biodegradable particulate fraction was partitioned into ADM particulate fractions other than the composite frac-tion X c.Blumensaat et al.17successfully implemented a process model to simulate the dynamic behavior of a pilot-scale process for anaerobic two-stage(thermophilic/ mesophilic)digestion of combined PS and WAS.

While most of the studies based on ultrasound energy have focused on the dewaterability and solubilization of chemical oxygen demand(COD),there is a definite paucity on infor-mation related to the impact of sonication on other biosolid characteristics,like odors precursors,such as proteins,as well as anaerobic biodegradability.Furthermore,a simplification of the complex ADM and its application to simulate pre-treated sludges enhance both process understanding and practical application.This study focuses on assessing the impact of sonication on various protein fractions and their anaerobic biodegradability as well as developing simple pre-dictive empirical models that could be of significant practical use.The effect of ultrasound pretreatment on particle disin-tegration was also evaluated.Thereafter,the effect of sonica-tion pretreatment on the anaerobic digestion coefficients and gas production was determined using ADM1.An empirical model was developed to assess the economical viability of ultrasound based on electrical energy input versus energy obtained from methane gas produced.

Materials and Methods

Experimental https://www.doczj.com/doc/a17479195.html,b-scale ultrasonic treatments were applied to PS and WAS.A total of800mL of sludge samples from the Adelaide wastewater plant,London,Ontario,Canada, were sonicated for1,5,10,20,40,and60min.A20kHz,500W ultrasonic generator(model VC-500from Sonic and Materials, Newtown,CT)with a standard probe(titanium alloy Ti-6Al-4V; diameter,1in.;length,53/4in.)was used for this purpose,while3/4 of the probe submerged in the sample during sonication.The amplitude was set to100%,and the sonication pulse was set to2s on and3s off,whereas the cooling water bath was used to control the sludge temperature,which remained constant at27(3°C during the experiments.All samples were placed on a magnetic stirrer with a speed of350(50rpm during sonication.

Analytical Methods.Sludge parameters,such as COD,biolo-gical oxygen demand(BOD),total suspended solid(TSS), volatile suspended solid(VSS),volatile fatty acid(VFA),parti-cle size distribution(PSD),lipids,ammonia,and total,soluble, and bound proteins,were measured in triplicate for both PS and WAS after each sonication time.All sludge parameters were analyzed according to the standard methods.20However,

(9)Hwang,K.Y.U.;Shin,E.B.;Choi,H.B.A mechanical pretreat-ment of waste activated sludge for improvement of anaerobic digestion system.Water Sci.Technol.1997,36(12),213–220.

(10)Tiehm, A.;Nickel,K.;Neis,U.The use of ultrasound to accelerate the anaerobic digestion of sewage sludge.Water Sci.Technol. 1997,36(11),121–128.

(11)Cao,X.Q.Enhanced Sludge Decomposition by Ultrasound;The Research Centre for Sustainable Environmental Biotechnology,Beijing Institute of Civil Engineering and Architecture:Beijing,China,2004. (12)Brown,J.P.Ultrasonic Solids Treatment Yields Better Digestion, Biosolids Technical Bulletin;Water Environment Research Foundation (WERF),Alexandria,V A,2004.

(13)Gavala,H.N.;Angelidaki,I.;Ahring, B.K.Kinetics and modeling of anaerobic digestion process.Adv.Biochem.Eng.Biotechnol. 2003,81,57–93.

(14)Batstone,D.J.;Keller,J.;Angelidaki,I.;Kalyuzhnyi,S.V.; Pavlostathis,S.G.;Rozzi.,A.;Sanders,W.T.M.;Siegrist,H.;Vavilin, V.A.Anaerobic digestion model no.1.IWA task group for mathema-tical modelling of anaerobic digestion processes1-77.Water Sci. Technol.2002,45(10),65–73.

(15)Batstone,D.J.;Keller,J.Industrial applications of the IWA anaerobic digestion model no.1(ADM1).Water Sci.Technol.2003,47 (12),199–206.

(16)Johnson,B.A.R.;Shang,Y.Applications and limitations of ADM1in municipal wastewater solids treatment.Water Sci.Technol. 2006,54(4),77–82.

(17)Blumensaat,F.;Keller,J.Modelling of two-stage anaerobic digestion using the IWA anaerobic digestion model no.1(ADM1). Water Res.2004,39,171–183.

(18)Vanrolleghem,P.A.;Spanjers,H.;Petersen,B.;Ginestet,P.; Takacs,I.Estimating(combination of)activated sludge model no.1 parameters and components by respirometry.Water Sci.Technol.1999, 39(1),195–214.

(19)Choi,D.Modeling for optimization of activated sludge process and parameter estimation using artificial intelligence.Ph.D.Thesis, Korea Advanced Institute of Science and Technology,Republic of Korea,2000.

(20)American Public Health Association(APHA).Standard Meth-ods for the Examination of Water and Wastewater;APHA:Washington, D.C.,1998.

particulate,soluble,and bound proteins were analyzed by Lowry et al.21Protein was determined by the micro-bicinchoninic acid protein assay(Pierce,Rockford,IL),which was modified by Lowry et al.21using a standard solution of bovine serum albumin. Cell protein was calculated as the difference between particulate and bound proteins.The concentrations of VFA were measured from the filtrate after passing the sludge through a0.45μm filter using a gas chromatograph(Varian8500,Varian Inc., Toronto,Ontario,Canada)with a flame ionization detector (FID)equipped with a fused silica column(30m?0.32mm), with a1μm film thickness.Helium was used as the carrier gas at a flow rate of5mL/min.The temperatures of the column and detector were110and250°C,respectively.

The PSD and specific surface area(SSA)were determined by a Malvern Mastersizer2000(version5.22)laser beam diffrac-tion granulometer(Malvern Instruments Ltd.,Malvern,U.K.). Table1displays the full characteristics of the sludges used in this study.

Batch Anaerobic Digestion.Batch anaerobic biodegradability studies were conducted to evaluate the effect of sonication on biodegradability and gas production.A total of90mL of seed or anaerobic culture(VSS of12000mg/L)obtained from a full-scale anaerobic digester at St.Marys,Water Pollution Control Plant(Ontario,Canada),was mixed with110mL of PS in 250mL bottles capped with Teflon septum and incubated in a rotary shaker(MaxQ4000,incubator and refrigerated shaker, Thermo Scientific,Fremont,CA)at37°C and180rpm.Simi-larly,140mL of WAS and60mL of seed were added together in 250mL bottles capped with Teflon septum.All bottles were sealed after purging the headspace with nitrogen to eliminate the presence of oxygen/air.A total of23bottles were used for each type of sludge;2bottles were set as blanks;and the rest were used for sonicated and non-sonicated samples,3for each sonication time.The volumes of substrate(PS and WAS)and seed (anaerobic digester sludge)were determined on the basis of a food(as COD)to microorganisms(as VSS)ratio of4.For the blank,the substrate volume was replaced by distilled water. The total gas volume was measured by releasing the gas pressure in the bottles using appropriately sized glass syringes (Perfektum,Popper and Sons,Inc.,New York)in the5-100mL range and equilibrating with the ambient pressure,as recom-mended by Owen et al.22Biogas composition was determined by a gas chromatograph(model310,SRI Instruments,Torrance, CA)equipped with a thermal conductivity detector(TCD)and a molecular sieve column(Molesieve5A,mesh80/100,182.88?0.3175cm).The temperatures of the column and the TCD were 90and105°C,respectively.Argon was used as the carrier gas at a flow rate of30mL/min.The experiment continued until the increase in methane over a period of1day was only1%of the prior total volume.

Specific Energy Input.Specific energy input is defined as the energy input per unit mass of sludge(as TSS)to achieve a certain degree of disintegration.The specific energy input is a function of the ultrasonic power(P in kW),sonication time(t in seconds), volume of sonicated sludge(V in L),and TSS concentration (TSS in g/L)and can be calculated using the following equation:23 specific energyekJ=g of TSST?

Pt

V3TSS

For PS,sonication times of1,5,10,20,40,and60min correspond to specific energies of0.5,2.3,4.8,9.1,17.6,and24.6kJ/g of TSS, while for WAS,the sonication times of1,5,10,20,40,and60min correspond to specific energies of0.7,3.2,6.8,12.2,24.9,and 32.9kJ/g of TSS.Furthermore,the specific energy calculated is based on the actual power drawn by the device and does not reflect the efficiency of power transmission to the sludges.

Anaerobic Modeling.As mentioned earlier,ADM1is a com-plex model involving many input parameters;for example,the decay of microorganisms and the regeneration cycle are strongly interrelated,and the COD content assumed in ADM1is rather complex.The decay processes of all microorganisms result in the production of carbohydrate,protein,and lipid,which can be used as substrates after disintegration and hydrolysis,and this regeneration approach makes model analysis more complex.24 Inhibition and gas transfer are also complex steps in the model. In this study,inhibition because of pH,NH3,etc.was ignored for simplicity,and the simulation results were compared to the total methane production.The ADM1stoichiometric matrix used was the one presented by Batstone et al.14and Gal?et al.25 The stoichiometric matrix contains the components,stoichio-metric coefficients,and reaction rates.14

Statistical Analysis.The student t test was used to test the hypothesis of equality at a95%confidence level.The null hypothesis was defined to be no different between the two groups tested versus the alternative hypothesis if there is a statistical difference between the two groups.

Results and Discussion

COD.Because of sonication,average SCOD increased from2175to7405mg/L and from720to5070mg/L after60 min of sonication(~25kJ/g of TSS for PS and~33kJ/g of TSS for WAS)for PS and WAS,respectively.As expected, TCOD remained constant with less than10%variation during sonication and averaged42180(2629and21350( 1809for PS and WAS,respectively.Figure1shows that the maximum SCOD/TCOD ratio for treated sludge increased from5.3to18%and from3.3to27%after60min(~25kJ/g of TSS for PS and~33kJ/g of TSS for WAS)of pretreatment for PS and WAS,respectively.It is important to mention that,although the SCOD/TCOD ratios between PS and WAS were different,after60min of sonication(12.7and 23.7%for PS and WAS,respectively),the SCOD released was comparable at5230and4350mg/L for PS and WAS, respectively.The increase in the SCOD/TCOD ratio is due to the release of extracellular polymeric substances(EPS), i.e.,polysaccharides,proteins,etc.,which are embedded in the floc matrix26and disintegrated because of sonication.

Table1.Characteristics of the PS and WAS Used in the Experiment parameter(mg/L)PS WAS TCOD40760(225022060(1530 SCOD2175(140720(25 TSS31500(200222380(1967 VSS27840(187615740(1034 lipid4930(1931647(98 VFA1064(721778(180 total protein2694(1871478(79 bound protein571(34350(22 soluble protein242(18121(11 SBOD450(30105(17 ammonia436(28322(18

(21)Lowry,O.H.;Rosebrough,N.J.;Farr,A.L.;Randall,R.J. Protein measurement with the folin phenol reagent.J.Biol.Chem.1951, 193(2),265–275.

(22)Owen,W.F.;Stuckey,D.C.;Healy,J.B.;Young.,L.Y.; McCarty,P.L.Bioassay for monitoring biochemical methane potential and anaerobic toxicity.Water Res.1979,13,485–492.

(23)Bougrier,C.;Carrere,H.;Delgenes,J.P.Solubilization of waste-activated sludge by ultrasonic treatment.Chem.Eng.J.2005,106(2), 163–169.

(24)Gujer,W.;Henze,M.;Mino,T.;Loosdrecht,M.Activated sludge model no.3.Water Sci.Technol.1999,39(1),183–193.

(25)Gal?,A.;Benabdallah,T.;Astals,S.;Mata-Alvarez,J.Modified version of ADM1model for agro-waste application.Bioresour.Technol. 2009,100(11),2783–2790.

(26)Drews,A.;Vocksa,M.;Iversena,V.;Lesjean,B.;Kraume,M. Influence of unsteady membrane bioreactor operation on EPS forma-tion and filtration resistance.Desalination2006,192,1–9.

This shows that sonication influences the solubilization of particulate COD as the ratio increases with sonication;however,the final SCOD/TCOD ratios suggest that the majority of the particulate matter was not solubilized.

BOD.Because of sonication,soluble biological oxygen demand (SBOD)increased from 450to 1032mg/L and from 105to 975mg/L after 60min of sonication (~25kJ/g of TSS for PS and ~33kJ/g of TSS for WAS)for PS and WAS,respectively.The initial rate of SBOD release of 207mg/L per kJ/g of TSS for WAS was higher than the 52mg/L per kJ/g of TSS observed for PS.Figure 2shows the ratio of SBOD/TCOD for both PS and WAS as a function of the specific energy;the SBOD/TCOD ratio increased because of the sonication of the organic matter by 1.4%(582mg of SBOD/L)and 3.9%(870mg of SBOD/L),that is,from 1.1to 2.5%and from 0.5to 4.4%after 60min,for PS and WAS,respectively.Furthermore,it is noticeable that the SBOD/TCOD ratio almost reaches a plateau after 20min of sonication (4.35and 2.47%for PS and WAS,respectively).This increase in biodegradable organic matter is an indica-tion of the potential enhancement of sludge digestion.10,27-29

Protein.Dimock and Morgenroth 30divided proteins in wastewater and sludge into three fractions:soluble,bound/labile (loosely attached with the cells),and tightly bound fractions (within the bacterial cells).Labile proteins are thought to become readily bioavailable,giving rise to higher odor potential.31Particulate (cell tbound),soluble,and bound protein were monitored in this study.

An increase in soluble protein from 242to 1335mg/L and from 121to 956mg/L was observed for PS and WAS,respectively,simultaneous to a decrease in the particulate protein from 2694to 884mg/L and from 1478to 876mg/L and a decrease in bound protein from 571to 452mg/L and from 350to 163mg/L during sonication.Particulate protein,which is the cellular and extracellular protein loosely bound

to the cell disintegrated,was mostly converted into soluble protein.It was observed that the overall protein (total protein tsoluble protein)remained constant at 2476(331mg/L for specific energy in the range of 0-25kJ/g of TSS for PS and 1802(117mg/L for specific energy in the range of 0-33kJ/g of TSS for WAS.Panels a -c of Figure 3show the variation of particulate,soluble,and bound protein per TCOD with specific energy,while panels a and b of Figure 4depict the variation of bound protein per milligram of VSS with specific energy for PS and WAS,respectively.After 60min of sonication,the bound protein/TCOD dec-reased from 1.4to 1.1%and from 1.6to 0.8%,

particulate

Figure 2.SBOD/TCOD ratio as a function of the specific energy for PS and

WAS.

Figure 3.(a)Particulate protein/TCOD,(b)bound protein/TCOD,and (c)soluble protein/TCOD as a function of the specific energy for PS and

WAS.

Figure 1.SCOD/TCOD ratio as a function of the specific energy for PS and WAS.(27)Tiehm,A.;Nickel,K.;Zellhorn,M.;Neis,U.Ultrasonic waste activated sludge disintegration for improving anaerobic stabilization.Water Res.2009,35(8),2003–2009.

(28)Neis,U.;Nikel,K.;Tiehm,A.Enhancement of anaerobic sludge disintegration by ultrasonic disintegration.Water Sci.Technol.2000,42,73–80.

(29)Hogan,F.;Mormede,S.;Clark,P.;Crane,M.Ultrasonic sludge treatment for enhanced anaerobic digestion.Water Sci.Technol.2004,50(9),25–32.

(30)Dimock,R.;Morgenroth,E.The influence of particle size on microbial hydrolysis of protein particles in activated sludge.Water Res.2006,40,2064–2074.

(31)Higgins,M.;Glindemann,D.;Novak,J.T.;Murthy,S.N.;Gerwin,S.;Forbes,R.Standardized biosolids incubation,headspace odor measurement and odor production consumption cycles.Proceed-ings of the Water Environment Federation and AWWA Odors and Air Emissions Conference ,Bellevue,WA,2004.

protein/TCOD decreased from 6.6to 2.2%and from 6.7to 4%,and soluble protein/TCOD increased from 0.6to 3.3%and from 0.6to 4.4%,for PS and WAS,respectively.While this marginal decrease in bound protein may reflect the beneficial impact of ultrasound on the odor precursors in biosolids,there are two major concerns,namely,the cost of energy and equally important is the anaerobic biodegrad-ability.After anaerobic digestion,the percentage reductions in bound protein for sludges sonicated at 0,1,5,10,20,40,and 60min were 63,62,48,54,52,45,and 40%for PS and 73,70,62,57,53,48,and 44%for WAS,as depicted in panels a and b of Figure 4.The t test method was conducted to compare the digested bound protein in both PS and WAS.The null hypothesis,i.e.,there are no differences between bound protein in both sludges,has been accepted on the basis of the calculated t value (2.23)and p value of 0.68at a 95%confidence level.Thus,it can be concluded that there was no statistically significant difference in the digested bound proteins between PS and WAS samples,with the PS being in the range of 23.2-28.6versus 23.8-31.9mg of protein/g of VSS for the WAS.The results suggest that there is no enhancement in the final degradation of bound protein after digestion in both sludges because of sonication.

Figure 5depicts the cell protein released,calculated as particulate protein minus the bound protein,as a function of the specific energy for both PS and WAS.It is clear from Figure 5that low sonication times in the range of 0-5min have no effect on the microbial cells,as reflected by the initial lag phase on the curves.After 5min of sonication (~2.3kJ/g of TSS for PS and 3.2kJ/g of TSS for WAS),a slow increase in WAS cell protein release relative to the fast increase in PS was experienced,with WAS increasing from 20to 415mg/L versus from 70to 1690mg/L in the case of PS,clearly emphasizing that microbial cells in the WAS are harder to

rupture than the microbial cells in PS.Moreover,sonication energy of less than 5kJ/g of TSS appears to have no effect on microbial cells in both types of sludges.

VFA.VFA increased from 1065to 1795mg/L and from 1778to 2932mg/L after 60min of sonication for PS and WAS,respectively.Figure 6depicts that the VFA/TCOD ratio increases with an increasing specific energy;the ratio distinctly increased in the case of WAS but not as much as in the case of PS.The VFA/TCOD ratio for treated sludge increased from 2.6to 4.4%and from 8.1to 13.3%after 60min of pretreatment for PS and WAS,respectively.The observed increase in VFA is similar to the observation by Appels et al.,32who sonicated WAS at 3.8-4.85%solids (38.1-48.5g of dry solid/kg)at 1.25kJ/g of TS and observed an increase in VFA from 94.4to 565mg/L.The ratio of VFA released (730and 1154mg/L)to SCOD released (5230and 4350mg/L)was 14and 27%for PS and WAS,respectively.The increase in VFA is probably due to the oxidation of larger hydrocarbons by the hydroxyl radicals produced during the explosion of cavitation bubbles.32However,the observation that VFA increased by about 1150mg/L in WAS and only 730mg/L in PS coupled with the much higher cell protein destruction in the PS relative to the WAS (Figure 5)indicates that a possible microbial role cannot be ruled out.

PSD.The PSDs by volume fraction as a function of the sonication time are shown in Table 2for PS and WAS.The results clearly show the change in particle size.In the case of PS,the median (d 50)of the particle size decreased by 78%from 59.4to 18.3μm after 60min of sonication and,in the case of WAS,by 89%from 106.6to 20.2μm.

Because

Figure 4.Bound protein/mg of VSS as a function of the specific energy for (a)PS and (b)WAS during anaerobic

digestion.

Figure 5.Cell proteins released as a function of the specific energy for PS and

WAS.

Figure 6.VFA/TCOD as a function of the specific energy for PS and WAS.

(32)Appels,L.;Dewil,R.;Baeyens,J.;Degeve,J.Ultrasonically enhanced anaerobic digestion of waste activated sludge.Int.J.Sustain-able Eng.2008,1(2),94–104.

hydrolysis is predominantly dependent upon surface area rather than volume,33,34the hydrolysis rate decreased when the biomass concentration is high,because mass-transfer limitations were observed as a result of limited surface area,35 and hence,the impact of sonication on SSA was examined. Figure7shows the SSA(m2/g)as a function of the specific energy.It is interesting to note that no significant effect on SSA was experienced after5min(3.2kJ/g of TSS)of

sonication in the case of WAS because it reaches a plateau of~0.62m2/g.This finding suggested that,with sonication pretreatment,particle size can only be reduced to a certain level in the case of WAS.The PS,however,reaches the same SSA of0.62m2/g between5and20min of sonication;an increase(from0.62to~1.4m2/g)in SSA was experienced after40min of sonication(~17kJ/g of TSS).

Methane Production.Panels a and b of Figure8show the cumulative milliliters of CH4per gram of COD added pro-duced from PS and WAS,respectively.The PS results, however,exhibited an interesting pattern;the maximum rate of methane production remained constant at3.65(0.21 mL/h for low sonication times of0-10min but decrea-sed significantly with higher sonication times to1.99and 1.66mL/h at40and60min.This decrease of the maximum methane production rate at sonication times of40and60min corresponding to specific energies of17.6and24.6kJ/g of TSS is consistent with the sharp decrease in biomass(as reflected by cell protein release)from338to1691mg/L after 10min of sonication,depicted in Figure5.This pattern was not observed in the case of WAS,where the maximum rate of methane production increased from1.66mL/h in the untreated sample to2.15,2.28,2.18,2.08,and2.12mL/h for sonication times of1,5,10,20,40,and60,respectively.The results show that there is not significant change in the maximum rate after 1min of sonication,because the average(standard deviation was2.17(0.07.Table3displays the percentage increase in ultimate methane(CH4)production in PS and WAS;28and 25%enhancement in methane production were observed after 5min of sonication for PS and WAS,respectively.The methane production did not increase significantly with further increases in sonication time in the case of WAS;after5min,the percentage increase in methane production varied randomly between24and26%.However,the percentage increase in methane production reached38%in the case of PS at60min of sonication because of higher COD solubilization. Anaerobic Modeling.In this study,the ADM1steady-state equations were modified and rearranged to simulate the anaerobic digestion in batch reactors.AQUASIM2.1was used to solve the dynamic differential and algebraic system of equations.Total protein,lipids,carbohydrates,and VFA were the model inputs,with protein based on C5H7NO2,lipid

Table2.Median,10th Percentile,and90th Percentile for the Volume

Fraction of the PS and WAS

specific energy(kJ/g of TSS)d10d50d90

PS

017.759.4126.3 0.58.126.279.8 2.3 5.819.956.5 4.8 5.520.979.8 9.1 4.823.476.6 17.6 1.518.5

24.6 1.513.3

WAS

0.023.7107.0297.2

0.79.840.5135.3

3.2 6.220.878.7

6.8 6.116.13

7.3

12.2 6.516.434.8

24.9 6.415.932.1

32.9 5.112.2

28.9

Figure8.Methane yield of the untreated and treated(a)PS and(b)

WAS over the digestion

time.

Figure7.SSA(m2/g)as a function of the specific energy.Table3.Percentage of CH4Increase in PS and WAS as a Function of

the Sonication Time

percentage of CH4increase(%) sonication time(min)PS WAS 11616

52825

102925

203125

403624

603826

(33)Miller,W.P.;Baharuddin,M.K.Particle size of interrill-eroded sediments from highly weathered soils.Soil Sci.Soc.Am.1987,51,1610–1615.

(34)Vavilin,V.A.;Rytow,S.V.;Lokshina,L.Y.Modelling hydro-gen partial pressure change as a result of competition between the butyric and propionic groups of acidogenic bacteria.Bioresour.Technol. 1995,54,171–177.

(35)Munch,E.;Keller,J.;Lant,P.;Newell,R.Mathematical model-ing of prefermenters;I.Model development and verification.Wat.Res 1999,No.12,2757–2768.

based on C 57H 104O 6,36carbohydrate (estimated from the particulate COD mass balance),acetate,propionate,and butyrate.14Table 4presents the ADM1input sludge char-acterization at different sonication times.The model para-meters (reaction coefficients)and degraders were set to the default values suggested by the ADM1technical report.14The various components of biomass,i.e.,sugar degraders,amino acid degraders,long-chain fatty acid degraders,vale-rate and butyrate degraders,propionate degraders,acetate degraders,and hydrogen degraders,were set at 200mg of COD/L following the recommendation of Jeong et al.,37who evaluated the sensitivities of the kinetic and stoichiometric ADM1parameters in predicting anaerobic glucose digestion and concluded that biomass was closely associated with the Monod maximum specific uptake rate,in which values could not be independently determined and verified.The typical variations of methane production with time at different sonication intensities obtained from the batch experiments were used to optimize the model parameters,namely,k m_c4

(valerate and butyrate)and k m_ac (acetate)using the auto-mated parameter estimation feature available in AQUASIM.The default values of k m_c4and k m_ac were changed on the basis of the estimation results from 0.833to 1.092h -1and from 0.333to 1.154h -1,respectively,for PS and from 0.333to 0.526h -1for k m_ac in the case of WAS.It must be noted that the ADM1report has indicated that variations of 30and 300%in k m_c4and k m_ac from the default values are acceptable.Panels a and b of Figure 9show a comparison of the ADM1predicted and measured methane yield,expressed in milliliters of CH 4per gram of COD added.It is evident that the model results with optimized parameters showed good agree-ment with the experimental data for methane production,with average percentage error of 13,11,15,and 20%for PS and 9,3,4,and 3%for WAS at sonication times of 0,1,5,and 40min,respectively.Furthermore,the spread of the data on both sides of the diagonal line confirms that the ADM1does not system-atically over-or underpredict the experimental data.Table 5compares the model and measured concentrations of VFA for both PS and WAS.It is evident from Table 5that,in general,the ADM1predictions for the individual volatile acids and the overall VFA are well within the range of experimental data,i.e.,average (standard deviation.Table 6displays the ADM1parameter variations at different sonication times.As expected and described in the ADM1technical report,acetic acid is

Table 4.PS and WAS Characterization for ADM1

sonication time

description

01510204060PS (mg of COD/L)

X_pr proteins 1585816144161381445112557101839770X_li lipids

14192142001383112060117401085210703X_ch carbohydrate 882495901080010988121021267313170S_ac acetic acid 330393534544650670835S_pro propionic acid 237272323340377382384S_bu butyric acid 497514535523534555576WAS (mg of COD/L)

X_pr proteins 11689117361172910406921988058024X_li lipids

4743480549624617435442993916X_ch carbohydrate 5281508453714461454643414141S_ac acetic acid 719108512211346170117781912S_pro propionic acid 498547701681673633688S_bu

butyric acid

562

553

664

665

664

665

682

Figure 9.Predicted and measured methane yields for the untreated and treated (a)PS and (b)WAS.

(36)Jeppsson,U.Investigation of Anaerobic Digestion Alternatives for Henriksdal’s WWTP ;Department of Industrial Electrical Engineering and Automation,Lund University:Lund,Sweden,2007.

(37)Jeong,H.-S.;Suh,C.-W.;Lim,J.-L.;Lee,S.-H.;Shin,H.-S.Analysis and application of ADM1for anaerobic methane production.Bioprocess Biosyst.Eng.2005,27,81–89.

the most sensitive parameter in the dynamic system.14As the ace-tic acid concentration increased with sonication time,the simu-lated reaction coefficient of acetic acid decreased.This result is not intuitive because the reaction rates of the VFA are influenced by both the reaction rate constant and the concentration of the VFA in the sludge.As can be seen in Table6,the acetic acid concentration increased with an increasing sonication time for both PS and WAS.However,the decrease in the rate constant is only about30-34%for both sludges,which also corresponds to the increased CH4yield in both cases(about26-38%).

Economic Viability of Ultrasound.An empirical model was developed to illustrate the relationship between the CH4 increase and specific energy for PS and WAS.The model was then used to verify the economical viability of ultrasonic pretreatment.Panels a and b of Figure10show the empirical model for PS and WAS,respectively.The results are pre-sented in Table7,which shows the specific energy input per ton of TCOD,as well as the value of the methane produced on the basis of power and natural gas costs of$0.07/kWh and $0.28/m3,respectively.It must be asserted that the sonication

Table5.Predicted and Measured Concentration(Average(Standard Deviation)of Acetic,Butyric,and Propionic Acids and VFA after Anaerobic

Digestion

E100%(kJ/g of TSS)input experimental output ADM output input experimental output ADM output

Acetic Acid(mg of COD/L)Butyric Acid(mg of COD/L)

PS

0.0330(29.713(1.311237(28.543(4.342 0.5392(31.456(5.115272(29.951(5.153 2.3533(37.347(3.342323(42.0124(11.2111 4.8543(48.964(5.154340(27.2181(14.5129 9.1649(51.867(6.763377(26.4278(22.3289 17.6669(66.668(7.565382(34.4350(31.5320 24.6835(91.773(6.50384(42.3352(24.7323

WAS

0.0719(71.8205(20.5175562(50.6505(60.6498 0.71085(79.7487(48.7428553(44.3488(53.6467 3.21221(85.5548(49.3502664(46.5647(84.1621 6.81346(107.7567(45.4594665(59.8643(51.4628 12.21701(177.8966(77.3987664(53.1589(41.2577 24.91778(195.5987(89.31071665(66.5645(58.0633 32.91702(153.2998(69.91010612(67.3544(59.9588

Propionic Acid(mg of COD/L)VFA(mg of COD/L)

PS

0.0497(59.7209(20.91981065(127.8265(26.5251 0.5514(56.6261(26.12771178(129.6369(33.2346 2.3535(69.5323(29.03231391(180.8493(34.5471 4.8523(41.9354(28.33401406(112.5599(47.9514 9.1534(37.4386(30.93771559(109.1731(73.1729 17.6555(49.9396(35.63801606(144.5814(89.6755 24.6576(63.3396(27.73401795(197.4821(73.9664

WAS

0.0498(49.8473(42.64601778(213.41183(106.51133 0.7547(49.2450(36.04522185(240.41425(114.01347 3.2701(49.1599(41.96622586(336.21794(125.61785 6.8681(54.5625(56.26632692(215.41835(165.11885 12.2673(67.3659(52.76623038(212.72214(175.12226 24.9633(69.6594(59.46133076(276.82225(222.52317 32.9618(55.6591(65.05802932(322.52133(234.72178

Table6.PS and WAS Model Parameters at Different Sonication Times

sonication time

parameter01510204060

PS k hyd_CH carbohydrate0.25

no change

k hyd_PR proteins0.2

k hyd_LI lipids0.1

k m_su sugar0.03

k m_aa amino acid0.05

k m_fa LCFA0.006

k m_c4butyric and valeric acid 1.092 1.0010.9140.8850.8420.769 k m_pro propionic acid0.013no change

k m_ac acetic acid 1.154 1.1570.9920.9960.8300.803 k m_h2hydrogen0.035no change

WAS k hyd_CH carbohydrate0.25

no change

k hyd_PR proteins0.2

k hyd_LI lipids0.1

k m_su sugar0.03

k m_aa amino acid0.05

k m_fa LCFA0.006

k m_c4butyric and valeric acid0.833

k m_pro propionic acid0.013

k m_ac acetic acid0.5260.4280.4220.4010.3560.3540.344 k m_h2hydrogen0.035no change

power used in the economic evaluation is the “real”power drawn by the sonicator and not the “actual”power trans-mitted to the liquid sludge because no information was available on the efficiency of the ultrasonic generator.It is evident that ultrasonic pretreatment is not economically viable for high specific energy.However,it is economically viable for PS at low sonication doses of 0.1,0.5,and 1kJ/g of TSS (values in bold).The empirical model can be used to estimate the increase in methane production for different sludges using different values of specific energy.

Conclusions

The effect of pretreatment of both PS and WAS using ultrasound can be summarized as follows:(1)After 60min of sonication corresponding to specific energy of ~25kJ/g of TSS for PS and ~33kJ/g of TSS for WAS,the SCOD/TCOD ratio increased from 5.5to 18%and from 3.3to 27%,the SBOD/TCOD ratio increased from 1.1to 2.5%and from 0.5to 4.4%,the VFA/TCOD ratio increased from 2.6to 4.4%and from 8.1to 13.3%,the bound protein/TCOD ratio decreased from 1.4to 1.1%and from 1.6to 0.8%,the total protein/TCOD ratio decreased from 6.6to 2.2%and from

6.7to 4%,and the soluble protein/TCOD ratio increased from 0.6to 3.3%and from 0.6to 4.3%,while total methane production increased by 28and 25%after 5min of sonication,for PS and WAS,respectively.(2)The effect of sonication on digested bound protein was not statistically significant for both PS and WAS samples at 95%confidence.(3)Although there is an increase in sludge surface area with sonication,no significant effect on SSA was found after 5min (3.2kJ/g of TSS)of sonication in the case of WAS,but for PS,SSA increased by 8times after 40min of sonication (~17kJ/g of TSS).(4)The ADM1predicted well both the methane production and VFA concentrations.The simulated rate constants for acetic acid and butyric acid uptake decreased by 30-34%with sonication.(5)Ultrasound is neither eco-nomical for biogas enhancement,despite the high solubiliza-tion of COD,nor effective in enhancing the biodegradability of bound proteins.However,at a low sonication energy of 0.1,0.5,and 1kJ/g of TSS,the process is economical for PS only.

Acknowledgment.We acknowledge the Natural Sciences and Engineering Research Council (NSERC),Ottawa,Ontario,Canada,and Trojan Technologies,Inc.,London,Ontario,Canada,for their financial support for this

project.

Figure 10.Increase in volume of methane produced in treated (a)PS and (b)WAS.

Table 7.Specific Energy,Power,and Methane Energy Per Ton of COD Using the Empirical Model

PS TCOD 40765mg/L

WAS TCOD 22058mg/L

SE (kJ/g of TSS)

power $/ton of COD in

CH 4$m 3/ton of COD

power $/ton of COD in

CH 4$m 3/ton of COD

0.1 3.638.52 6.70 5.100.518.1328.3433.5016.74136.2539.9466.9923.42272.5050.24133.9929.264145.0157.66267.9833.4410362.5263.28669.9436.5615

543.77

64.68

1004.91

37.34

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