anaerobic digestion of co-mingled municipal solid waste and sewage sludge

6
8) Pergamon PH: S0273-1223(98)00438-7 Waf. Sci. Vol. 38,No. 2, pp. 127-132.1998. IAWQ iC> 1998 Published by Elsevier Science LId. Printed in Great Britain. Allrights reserved 0273-1223198 $19'00+ 0'00 ANAEROBIC DIGESTION OF CO-MINGLED MUNICIPAL SOLID WASTE AND SEWAGE SLUDGE N. Harnzawi, K. J. Kennedy and D. D. McLean Departmentof ChemicalEngineering, University of Ottawa, Ottawa, Ontario, CanadaKIN 6N5 ABSTRACf This study evaluated the technical feasibility of the anaerobic co-digestion process in the context of typical North American solid waste. Using biological activity tests, an optimal mixture was identified with 25% organic fraction of municipal solid waste (OFMSW) and 75% sewage sludge (65% raw primary sludge (RAW), 35% thickened WAS OW AS» based on biogas production. Also. based on the rate of biogas production. the most anaerobically biodegradable components of the OFMSW were paper and grass. The TWAS and the newspaper were found to be the least biodegradablecomponents. Lab-scale testing indicated that alkaline pretreatmentincreasedthe biodegradability of the sewage sludgclOFMSWmixture the most, as compared to the untreated control. Thermochemically pretreated feedstocks inhibited anaerobic biodegradabilityas compared to the control. whereasthe anaerobic biodegradability of thermally pretreated feed was not found to be significantlydifferent from that of the control. Empirical models were developed based on alkaline dose, feed total solids concentrationand particle size for biogas production and removal of TS and YS. All three experimentalfactors were found to be significant with respect to the response variables studied. @ 1998 Published by Elsevier Science Ltd. All rights reserved KEYWORDS Municipal solid waste; sewage sludge; simulation; thermal; alkaline; thermochemical. INTRODUCI10N As a solution to the problems of waste management, anaerobic digestion possesses the optimal combination of volume reduction, probability of success and potential for resource and energy recovery. In the past, applications of anaerobic processes treating high-solids streams were unsuccessful, however, recent developments have made it possible to handle wastes with solids levels as high as 40%. With these advances, attention is now being drawn to the development of innovative high-solids anaerobic processes. The anaerobic co-digestion of sewage sludge and the organic fraction of municipal solid waste (OFMSW) has been implemented in Europe as an alternative to conventional anaerobic sludge digestion, and is currently breaking into the North American market. In this study, the technical feasibility of this process using typical North American waste was evaluated, considering the Regional Municipality of Ottawa- Carleton (RMOC), in Canada, as a case study. Municipal solid waste (MSW) consists of all the solid and semi-solid materials discarded by a community including food scraps, containers, packaging, yard trimmings and miscellaneous inorganic wastes from 127

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8) Pergamon

PH:S0273-1223(98)00438-7

Waf.Sci. T~ch. Vol. 38,No.2, pp. 127-132.1998.IAWQ

iC> 1998 Published byElsevier Science LId.Printed inGreat Britain. Allrights reserved

0273-1223198 $19'00+ 0'00

ANAEROBIC DIGESTION OFCO-MINGLED MUNICIPAL SOLID WASTEAND SEWAGE SLUDGE

N. Harnzawi, K. J. Kennedy and D. D. McLeanDepartmentofChemicalEngineering, University ofOttawa, Ottawa, Ontario,CanadaKIN 6N5

ABSTRACf

This study evaluated the technical feasibility of the anaerobicco-digestion process in the context of typicalNorth American solid waste. Using biological activity tests, an optimal mixture was identified with 25%organic fraction of municipal solid waste (OFMSW) and 75% sewage sludge (65% raw primary sludge(RAW), 35% thickened WAS OWAS» based on biogas production. Also. based on the rate of biogasproduction. the most anaerobically biodegradablecomponents of the OFMSW were paper and grass. TheTWAS and the newspaper were found to be the least biodegradablecomponents.Lab-scale testing indicatedthat alkaline pretreatment increased the biodegradability of the sewage sludgclOFMSWmixture the most, ascompared to the untreated control. Thermochemically pretreated feedstocks inhibited anaerobicbiodegradabilityas compared to the control. whereas the anaerobic biodegradability of thermally pretreatedfeed was not found to be significantlydifferent from that of the control. Empirical models were developedbased on alkaline dose, feed total solids concentrationand particle size for biogas productionand removal ofTS and YS. All three experimentalfactors were found to be significantwith respect to the response variablesstudied. @ 1998Published by ElsevierScienceLtd. All rights reserved

KEYWORDS

Municipal solid waste; sewage sludge; simulation; thermal; alkaline; thermochemical.

INTRODUCI10N

As a solution to the problems of waste management, anaerobic digestion possesses the optimal combinationof volume reduction, probability of success and potential for resource and energy recovery. In the past,applications of anaerobic processes treating high-solids streams were unsuccessful, however, recentdevelopments have made it possible to handle wastes with solids levels as high as 40%. With theseadvances, attention is now being drawn to the development of innovative high-solids anaerobic processes.The anaerobic co-digestion of sewage sludge and the organic fraction of municipal solid waste (OFMSW)has been implemented in Europe as an alternative to conventional anaerobic sludge digestion, and iscurrently breaking into the North American market. In this study, the technical feasibility of this processusing typical North American waste was evaluated, considering the Regional Municipality of Ottawa­Carleton (RMOC), in Canada, as a case study.Municipal solid waste (MSW) consists of all the solid and semi-solid materials discarded by a communityincluding food scraps, containers, packaging, yard trimmings and miscellaneous inorganic wastes from

127

128 N. HAMZAWI et at.

residential, commercial, institutional and industrial sources (EPA, 1995). In 1995, 174,000 tons of MSWwere collected by the RMOC (RMOC, 1995). Also, the local wastewater treatment plant treats on average4.34 million litres of wastewater from the RMOC each day (RMOC, 1995). In the process, 30 tons ofbiosolids (BIOS) are produced daily. The biosolids are currently used as a soil amendment and as acomponent in final cover material at the local landfill; the remainder is landfilled. The sludge treatmentprocess and the disposal of BIOS are currently the plant's most costly operations. Accordingly, cost-effectiveand environmentally sound management of MSW and sewage sludge has been identified as a high priorityissue. The concept of integrated solid waste management is being implemented by the RMOC and othermunicipalities as they plan for the future. Future plans typically include reduced waste production,composting and recovery of waste for recycling.

The raw primary sludge (RAW)/thickened WAS (TWAS) sludge mixture fed to the anaerobic digesters atthe wastewater treatment plant in combination with OFMSW collected by the RMOC could be a richersource of organic materials and hence conducive to effective anaerobic biodegradation. Anaerobic co­digestion of a RAWITWAS sewage sludge and OFMSW mixture would take advantage of the inherentmicrobial population in the sewage sludge to digest the OFMSW as well as the sludge itself. Ultimately, thiswould allow for resource recovery through the production of a nutrient-rich compost, and energy recoverythrough methane biogas production. As such, a feasibility study of such a process as a long term solidsmanagement alternative for the RMOC is warranted.

The overall purpose of this research was to evaluate the technical feasibility of anaerobic co-digestion ofOFMSW and the RAWITWAS sludge mixture currently fed to the anaerobic digesters at the wastewatertreatment plant. Specifically, the goals of the research were:

1. To evaluate the performance of the high-solids anaerobic co-digestion process using different ratiosof OFMSW and sewage sludge (RAWITWAS);

2. To evaluate the contribution to biodegradability of each of the components of the OFMSW andRAWITWAS sludge mixture;

3. To quantify the effects of alkaline, thermal and thermochemical pretreatments on anaerobicbiodegradability of this mixture.

MATERIALS AND METHODS

Measurements of biogas production, biogas methane concentration, pH, volatile fatty acid (VFA)concentrations, TS and VS were carried out in accordance with Standard Methods (Greenberg et al., 1992).The sewage sludge used was a 65:35 mixture by volume of RAW and TWAS from the local treatment plant.The OFMSW was a simulated mixture based on a waste audit of RMOC MSW (RMOC, 1992). It consistedof 20% yard waste (50% leavesl50% grass), 40% paper waste (50% white paper/50% newspaper) and 40%food waste (50% lettuce/50% tomatoes), by volume. The mixture was then passed through appropriatesieves to obtain the desired particle size as outlined in Harnzawi et al. (1998). To identify an optimal ratio ofOFMSW to sludge, several mixtures were prepared and the performance of the reactors evaluated based onbiogas production. Also, given the highly variable composition of OFMSW, a detailed study of thecontributions to biodegradability of each of the individual mixture components was performed.

The next stage of the investigation involved characterization of the effects of key feed properties, namelytotal solids (TS) and particle size, on common indicators of process performance and metabolic activity:biogas production, biogas methane content and removal of TS and volatile solids (VS). Based on a centralcomposite experimental design, these effects were quantified through the development of empirical modelsand are listed in Table I. Mixtures were prepared in batch reactors (one litre glass bottles), 150 ml of whichwere inoculum (anaerobic seed with 2.4% VS), 2.4 g of sodium bicarbonate and the total working volume ofthe reactor was 800 mI. All batch tests were conducted at 37°C in a heated shaker operated at 100 rpm.

The enhancement of solubilization of particulate organics is of significant engineering consequence.Accordingly, the effects of alkaline, thermal and thermochemical pretreatments were investigated in the final

Anaerobicco-digestionprocess 129

stage of the research. Alkaline pretreatment consisted of a 12-hour hydrolysis with 185 meq/l of sodiumhydroxide. Thermal pretreatment involved autoclaving the feed at l30°C for one hour at 1 atm. The sodiumhydroxide addition followed by the thermal pretreatment consisted of the thermochemical pretreatment.Continuous reactors had a working volume of 20 I and were operated at 37°C at a 20 day HRT. Theeffectiveness of these pretreatments was studied in both batch and continuous reactors. For the experimentsin the batch reactors. empirical models were built, as previously discussed, to elucidate the effects of totalsolids content of the feed, its particle size and alkaline dosage (where applicable) on the above-mentionedprocess performance indicators.

Table 1. Operating conditions for batch tests

• Could not use optimizedrun COnditionS becauseofphYSIcal constraintson setting operatingconditions

Control and Thermal Thermochemical and Alkaline

ParticleSize Total Solids ParticleSize Total Solids Alkaline DoseX"mm X,,% X"mm X,,% X"meqlL

2.00 10.0 2.00 10.0 SO6.35 10.0 6.35 10.0 502.00 20.0 2.00 20.0 506.35 20.0 6.35 20.0 500.85 15.0 2.00 10.0 1508.00 15.0 6.35 10.0 1504.00 7.9 2.00 20.0 1504.00 22.1 6.35 20.0 1504.00 15.0 4.00 15.0 164.00 15.0 4.00 15.0 1844.00 15.0 4.00 6.6 1004.00 15.0 4.00 23.4 100

0.85 15.0 1008.00 15.0 1004.00 15.0 1004.00 15.0 1004.00 15.0 1004.00 15.0 1004.00 15.0 1004.00 15.0 IDa

. . ..

RESULTS AND DISCUSSION

As shown in Figure Ia, all mixtures performed significantly better than the control which consisted of theinoculum and distilled water. The mixture with 25% OFMSW exerted the highest, and thereby optimal.cumulative biogas production. Accordingly. the OFMSW and sewage sludge were combined in a 25:75volumetric ratio, respectively. in all ensuing experiments. The contribution to biodegradability of each of thecomponents of the OFMSW/sewage sludge mixture was also investigated. From the cumulative biogasproduction profile shown in Figure Ib, there appear to be two distinct rises in biogas production (0 to 100hours. and 400 to 500 hours) representing two distinct regions of biological activity. These two regions aremost likely based on biodegradability of two different classes of substrates: readily biodegradable, andmoderate to difficult to biodegrade. Moreover. the performance of the individual components did notcumulatively reflect the mixture results, indicating that the microbial population may have acclimated to thecomplex mixture differently than when biodegrading a "pure" component.

The next stage of the experiments involved characterization of the effects of key feed properties. namelytotal solids and particle size. on commonly used indicators of metabolic activity. that is. biogas production.biogas methane content. TS and VS removal. Based on a central composite experimental design. theseeffects were quantified through the development of empirical second order polynomials in terms of coded

130 N. HAMZAWI et al.

values of particle size, TS and alkaline dose describing the experimental system. For each indicator ofmetabolic activity, the model was of the form:

where

E(Y) is the expected value of the response variable,e; ~I' ~2' ~3' ~It' ~22' ~33' ~12' ~13' ~23 are the model parameters,XI is the coded particle size = [2*PS - (PS1o + PShi)]/[PShi - PS1o]'X2 is the coded TS concentration=[2*TS - (TSlo+TShi)]/[TShi - TS1ol, andX3 is the coded alkaline dose =[2*Alk - (Alkto+ Alkhi)]/[Alkhi - Alklo]'

a) Overall/or Varying Amountso/OFMSW

t:2000 .,----------------------,

~ a Q Q t~ t~ t~ ta ta :lOOO

TIme,h

1000800

b) Breakdown ofConmbutions 0/IndividualComponents

t; 9000,..----------------------,..IE 8000

g 7000

~ 6000

! 5000

:: 4000

E3000

12000

:I 1000

~ 0uo 200 400 600

TIme, hours__lei...

-+-RAW-.-Paper -.-N_paper -Laltuce__lWAS -+-Mix -+-Conlllll

Figure I.Cumulative biogas production mixtures ofsewage sludge and OFMSW.

Anaerobic co-digestion process 131

Table 2. Summary of parameter estimates with 95% confidence limits and measures of model adequacy

Factor2 corresponds to codedTSconcemranon

Parameter Est/mates WIth 95% Confldenee Um/faPretrNtment Parameter GllsProd. GIISProd.IVSII GIISProd.IVSr VS RemovlIl TS RemovlIl Methllne Cone.

L UgVS_ UgVS_ % % %

Control lla 4.29 %.31 0.58 %.04 1.05 %.04 53.14 %2.50 33.51 :t 1.06 61.71 %1.56

Il,' -o.35:t .26 -0.04 %.03 -0.07 %.03 - - -\3t 1.n% .30 0.07 t .04 -o.16t .04 13.tl3t2.42 8.56t 1.03 -Il" -0.35 %.21 -o.04:t .03 - -3.72 t 1.64 -2.45:t .70 -\322 - - 0.08t .04 - - -1l'2 -0.63 %.52 -0.06 %.05 - -4.16:t 3.14 -3.02 t 1.33 -R2 0.973 0.907 0.950 0.959 0.983 -F.. 2.27 4.14 0.94 1.94 2.95 -

FOl_.... 9.12 9.12 9.01 9.01 9.01 -Thennal lla 3.51 t .38 0.49:t .04 0.93% .10 55.54 t 2.41 34.89% 1.22 59.35% 3.04

Il, 0.44 t .29 0.06:t .03 0.13:t .07 - - -132 1.39t .34 - -0.28 %.07 13.23 t 2.33 9.5H 1.18 -\3" - - O.06t .05 -4.29t 1.58 -3.17t .80 -1322 0.50t .38 0.07t .04 0.15:t .08 - - -1l'2 - - 0.13% .09 -4.37 t 3.02 -7.04 t 1.53 -R2 0.934 0.750 0.960 0.966 0.985 -F.. 1.84 1.33 1.44 2.04 1.34 -

F....... 9.01 8.94 9.28 9.01 9.28 -00o Factor I corresponds to codedpanicle IIZe ...

Pllrameter GII.tProd. GIISProd.IVSII GIISProd.IVS, VS RemovlIl TSRemoval Mettl.Cone.L UgVS_ UgVS_ % % %

Thennochem/clIl lla 2.61 t .07 0.36t .02 1.11 t .14 24.49 t 1.14 37.46 t 1.42 58.84 t .87Il, 0.38 t .07 0.04 t .01 0.18t .12 - - 1.0St.98132 1.02 t .07 - -0.14 t .13 3.97:t .90 4.64 t 1.12 -~- -o.17:t .07 -0.02 t .01 0.27t .13 -5.42 t .90 -8.57 t 1.12 -\3" - - - -1.71 t .73 -2.21 t .91 -1322 0.38:t .07 0.05 t .01 0.17t .12 - - -13» - - - -2.07 t .87 -2.10 t 1.08 -\3.2 0.19 %.08 0.02:t .02 - -1.33 %1.08 - -\31S - - - - - -Il.. - - - -3.09 t 1.17 -3.69% 1.46 -R2 0.988 0.879 0.712 0.959 0.964 0.218F.. 4.77 3.99 2.65 1.80 1.22 1.42

Fos ww 4.77 4.74 crt 4.82 crt 4.64Allulltne lla 4.03 t .18 0.48 %.04 0.54:t .07 83.76:t 2.08 51.32 t 1.84 58.87t 1.80

II, 0.54 %.14 0.04:t .03 0.06:t .05 - - -132 1.35% .14 - -0.06 t .05 3.nt 1.97 4.68t 1.74 -~ 0.18 %.14 - 0.15 %.05 -11.43% 1.97 -8.39 t 1.74 -\3" -0.15 %.12 - - - - -\322 0.34% .13 0.05 t.04 0.08:t .05 - - -13.. - - 0.09 t .05 -4.93 t 1.90 -3.10 t 1.68 -1l'2 0.48 t.17 0.05 t .04 0.08 t .06 - - -\311 - - - - - -132, - - - - -3.10 t2.27 -R2 0.976 0.571 0.840 0.918 0.906 -F.. 5.01 1.48 5.01 2.72 1.61 -

Foo • • 5.05 5.05 5.05 5.05 5.05 -000 Factor3 corresponds to codedalkahnedose

132 N. HAMZAWI et al.

Table 2 summarizes the parameter estimates obtained by least-squaresanalysis and their corresponding95%confidence intervals. The R2 values are included and indicate that all but two models accounted for over70% of the variability in the data. Fifteen of the 19 remaining models explained more than 90% of thevariability in the data. None of the factors studied had a significant effect on the biogas methaneconcentration as indicated by the control in Table 2. Based on the control results of the models, both theparticle size and TS of the feed were found to significantly affect digester performance. Optima wereobserved at small particle sizes (0.85 mm) and high TS concentrations (22.1%) maximizing the surface-to­volume ratio and substrate concentration. Hence, the enhancementof solubilizationof particulate organics isof significant engineering consequence.

From Table 2, all pretreatments showed overall optima at large particle sizes (8.0 mm) which is in contrastwith the control where the optima were at small particle sizes. This may have been due to the methanogens'inability to handle the resultant increased hydrolytic activity induced by feed pretreatment. Of the threepretreatments,die alkaline pretreatmenthad the best overall effect on the anaerobicco-digestion of OFMSWand sewage sludge. In the 20 I continuous reactors, the alkaline pretreated reactor displayed the highest rateof biogas production (and hence organics removal rate) whereas the thermal, control and thermochemicalpretreated mixtures displayed comparable biogas production rates. Relative to the control, increases inbiogas production for the alkaline, thermal and thermochemical pretreatments were 31%, 6% and -5%,respectively.Corresponding increases in TS removals were 32%, -22% and -25%, respectively; and 21%,­19%and 34% were the respective VS removals.

CONCLUSIONS

Based on the results of this study, the applicationof anaerobicco-digestionof sewage sludge and the organicfraction of municipal solid waste appears to be technically feasible within a North American context. Toenhance the operation of the process, it was also found that alkaline pretreatment significantly increases thebiodegradabilityof the waste mixture. Overall, optimal conditions for the control and thermal pretreatmentwere at small particle sizes (0.8 mm) and high TS concentrations(22.1%). For thermochemical and alkalinepretreatments, the optimal conditions were at large particle sizes (8.0 mm), high TS concentrations (23.4%)and high alkaline doses (150 meq/l). New process configurations and advanced reactors can now bedesigned based on these findings and the empirical modelsdeveloped in the study.

ACKNOWLEDGEMENTS

Financial support for this research was provided by the Regional Municipality of Ottawa-Carleton, theNatural Sciences and Engineering Research Council and the NationalResearch Council Canada.

REFERENCES

EPA(1995). Characterization of Municipal SolidWaste in the United States: /994 Update, U.S.Government Printing Office.Greenberg, A. E., Clesceri, L. S. and Baton, A. D. (1992). Standard Methods for the Examination ofWater and Wastewater, 18th

Edition, American PublicHealthAssociation.Hamzawi, N. and Kennedy, K. 1. (1998). Technical feasibility of anaerobic co-digestion of sewagesludgeand municipal solid

waste. Environmental Technology (inpress).RMOC (1992). Waste Composition Study: Description of the Waste Stream and Program Implications, DSM Environmental

Services, Ottawa, Canada.RMOC (1995). /995 Operating and Monitoring Report, Reportby the Environment and Transportation Department, Regional

Municipality of Ottawa-Carleton.