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Optimization of Solid State Anaerobic Digestion by inoculum recirculation: The caseof an existing Mechanical Biological Treatment plant

Francesco Di Maria ⇑, Alessio Sordi, Caterina MicaleDepartment of Industrial Engineering, Via G. Duranti 1/A-4, 06125 Perugia, Italy

a r t i c l e i n f o

Article history:Received 25 July 2011Received in revised form 29 December 2011Accepted 30 December 2011Available online 28 January 2012

Keywords:Anaerobic DigestionBiogasOrganic fraction of municipal solid wasteSolid stateWaste to energy

a b s t r a c t

The energetic exploitation of the organic fraction of municipal solid waste treated in an existing Mechan-ical Biological Treatment plant was found to be successful by the Solid State Anaerobic Digestion. Theamount of inoculum used per tonne of waste for starting the anaerobic process was shown to have a rel-evant effect on both biogas and biomethane production. For a waste-to-inoculum ratio ranging from 1:1to 1:3 (w/w), the energy production increased from about 100 kWh/tonne to about 380 kWh/tonne ofwaste organic fraction. Consequently, the investment costs also rise, going from about 180 € to more than370 €/tonne of treated municipal solid waste organic fraction. The economic optimization analysisshowed that the waste-to-inoculum ratio that minimized the treatment cost, ranged from 1:1.5 to1:2.5, being strongly influenced by the plant size (i.e. electrical efficiency) and by the Green Certificatevalue. Considering the cost of producing electrical energy, the optimum ratio was 1:2, leading to a costof 0.26 €/kWh, during the investment period, and of 0.14 €/kWh after the investment period.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Biodegradable residue and waste represent an important sourceof renewable energy that can contribute in a sound way to reducingfossil fuel consumption [1–4]. Among the various biodegradablesources, a correct management of the organic fraction of municipalsolid waste (OF MSW) can constitute an important economic andenvironmentally sound energetic potential for many countries.

The overall production of the OF MSW in Italy ranges from 10 to12 million tonnes per year [5], representing about 30–40% of thetotal production of MSW. Different strategies have been pursuedover the years for reducing the environmental impact of this frac-tion. The main one has been rotting of the organic material [6–8]before its reuse or final disposal.

Another strategy is source segregation (SS) of the OF MSW thatcan be treated using a biological aerobic process (i.e. composting)[6–8] to produce a high quality natural fertilizer exploitable inagriculture. Another common strategy for managing the OF MSWnot collected by SS, which is largely diffused in areas that lackWtE plants, consists in mechanically sorting the Non-Differenti-ated Waste (NDW) to separate the MSW with a higher OF (i.e.OFMS) content from the remaining fraction that has a higher con-tent of dry material.

Before being disposed of in sanitary landfills, the OFMS can be aer-obically processed to reduce its mass and stabilize the organic matter,

usually expressed as Volatile Solids (VS) content (% of TS). This stabil-ization is aimed at reducing both the production of Green House Gas(GHG) emissions and leachate pollutants, as well as the final disposalcost (i.e. weight reduction), when disposed of in landfills [1].

All these operations are generally performed in dedicated MBTplants in which both the mechanical sorting and the biological aer-obic stabilization/composting treatment are usually present. Forthese reasons, MBT facilities are places in which a huge amountof OFMS is processed.

A large number of these MBT plants operating in the differentItalian waste management areas [5] have been built since theend of the 1980s, when both the amount and the composition ofthe MSW were quite different from the present waste. This has re-sulted in the need for some modifications both in plant manage-ment and operating conditions. In particular, the increasedamount of OFMS causes an increase in the Organic Load Rate(OLR) (kgVS/m3 day) at which the aerobic biological section works.This phenomenon causes two main negative effects:

(1) Increase in energy consumption as a consequence of theincreased need for process air.

(2) Lower reduction of the VS content of the OFMS, leading to alower stabilization level of the material at the biological sec-tion outlet.

For these reasons, an important tool for upgrading existing MBTplants is the implementation of systems and processes that canlead to further reduction and stabilization of the organic contentof the OFMS along with reduction of demand for process energy.

0306-2619/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.doi:10.1016/j.apenergy.2011.12.093

⇑ Corresponding author. Tel.: +39 0755853738; fax: +39 0755853736.E-mail address: fdm@unipg.it (F. Di Maria).

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Upgrading MBT to Mechanical Biological Stabilate [9] or bio-drying [10] to enhance the production of Solid Refined Fuel (SRF)can be an effective solution for the aforementioned problems. Nev-ertheless, a complete energetic and environmental benefit can beachieved only if SRF is exploited as fuel in co-combustion in dedi-cated facilities (e.g. cement factory kilns or thermoelectric powerplants). According to several studies [11–14], another interestingsolution in areas that lack WtE and co-combustion plants, is Anaer-obic Digestion (AD) of the OF MSW. This process has the potentialof producing a high quality biogas that can generate up to morethan 200 kWh of electrical energy per tonne of treated waste.

Generally, the most common AD facilities are based on wet,semi-dry and dry technologies working with a TS content rangingfrom% 10 to 25% [15]. These technologies have some significanttechnical and economic drawbacks when applied in the field ofMSW. That is, the need for a extensive pre-treatment of the waste[16,17] and for management of the large amount of sludge andliquid discharged by the processes (i.e. Waste Water Treatment)before their final disposal.

The Solid State Anaerobic Digestion (SSAD) process is an inter-esting solution that appears to reduce the aforesaid problems, as

well as matching in a rather satisfactory way the features of theexisting MBT facilities [11,18,19]. The concept of Solid Stateconsists of treating a material that can maintain its shape when ar-ranged in an open heap, a condition generally achieved when TSare greater than 25% w/w.

The full scale SSAD process is generally performed inside con-crete, static, batch biocells (Fig. 1) in which all the main processparameters can be controlled. The OF is usually treated under mes-ophilic conditions (35 �C ± 2�) for about 28 days. As a consequenceof the batch load, each new process is inoculated with a givenamount of digestate arising from the previous AD process. Thepre-treatments required are very similar to those currently usedin existing MBT plants for treating the OF MSW in the aerobicbiological section. They consist of light shredding and mixing, ifnecessary, with shredded wood to achieve the required level ofporosity. The percolate, represented by the liquid fraction pro-duced during the anaerobic phase, is generally not more than10% by weight of the treated waste [14]. These aspects make SSADvery interesting as a possible solution for upgrading existing MBTplants. The use of the SSAD as a biological pre-treatment, prior tothe aerobic section of the MBT, can contribute significantly to

Nomenclature

AD Anaerobic DigestionBMY BioMethane Yield (Nl/kgVS)BY Biogas Yield (Nl/kgVS)CHP Combined Heat and PowerCOD chemical oxygen demand (mg/l)EEP electrical energy price (€/MWh)GC Green Certificate (€/MWh)GHG Green House Gassesg efficiency (%)I inoculumIn investment (€)ICE Internal Combustion EngineLHV Low Heating Value (kWh/Nm3)MBT Mechanical Biological TreatmentMSW municipal solid wasteNB NonBiodegradable Fraction (%)NDW NonDifferentiated WasteOF organic fraction (household, greenery and kitchen

waste)OLR Organic Load Rate (kgVS/m3 day)O&M operating and maintenance (€/year)

ORP oxidation reduction potential (mV)SS source segregatedSRF solid recovered fuelSSAD Solid State Anaerobic DigestionTN Total Nitrogen (g/kgTS (mg/l))TOC Total Organic Carbon (g/kgTS)TS total solids (g/kg)TP Total Phosphorous (g/kgTS (mg/l))VS Volatile Solids (g/kgTS)

SubscriptsComp composting basinSS source segregatedel electricalin inletOF organic fractionout outletMS mechanical sortingnet net

Fig. 1. Single biocell reactor scheme of the SSAD.

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reducing the OLR since the VS of the OFMS is partially convertedinto biogas, with positive effects on the whole stabilization andmass reduction process. Furthermore, the renewable energy gener-ated from the biogas produced can contribute to increasing theplant’s energetic efficiency, making the whole process more sus-tainable and environmentally sound.

Unfortunately, when operating with full scale SSAD, some prob-lems have not been completely solved and further investigation isneeded. A relevant open question is represented by the BiogasYield (BY) and the Biomethane Yield (BMY), along with process sta-bility. In fact, as is known [20,21], when operating with a high TSconcentration, some inhibition phenomenon can occur, causing areduction in BY and BMY (i.e. Nl/kgVS), when compared to otherwet or dry processes, together with process instability. A possiblesolution for managing these problems is increasing the amountof inoculum (I) per tonne of OFMS, exploited for the AD start up.Anyway, this practice significantly affects the Investment (In)and, partially, the Operating & Maintenance (O&M) costs becauseof an increase in the volume required by the SSAD reactors (Fig. 1).

In the present study, the application of SSAD for upgrading anexisting MBT plant was analyzed. An energetic and economic eval-uation of the optimization of the AD process by means of the recir-culated amount of inoculum was also performed. The data used in

the analysis came from several experimental tests performed onthe SSAD of the OF MSW.

2. Description of MBT plant

The MBT facility (Figs. 2 and 3) operates in a waste managementarea in Italy that lacks WtE and co-combustion facilities. It cur-rently treats about 67,000 (2010) tonnes of NDW per year (Table2), along with about 6000 tonnes of OFSS (2010), arising fromsource segregation of MSW.

The plant consists of three main treatment sections:

(1) An Inlet section.(2) A mechanical sorting section (Fig. 2).(3) A biological treatment section (Fig. 3).

The NDW is unloaded directly from the collection lorries intothe bunkers of the inlet section. Using an orange peel crane bridge,the NDW is moved to a conveyor that takes it to the bag opener ofsection 2). After the bag opener (Fig. 2), it is sieved in a trommellwith 100 mm diameter holes, which splits the NDWin into twomain streams, the OFMS and the dry fraction. The MSW fractionwith a high organic content, OFMS (about 33,000 t/year), passes

Fig. 2. Diagram of the mechanical sorting section of the MBT plant.

Fig. 3. Aerobic stabilization/composting basin.

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through the sieve holes onto a conveyer belt, which transports it tothe biological treatment section (Fig. 3). Before this section, thereare three metal separation systems for steel and aluminum recov-ery. The biological treatment section consists of a stirred, continu-ous flow, aerobic stabilization/composting facility with a concretebasin straddled by a mobile crane bridge equipped with screws. Inthis section the rapidly biodegradable OFMS is reduced by biologi-cal oxidation, to reduce its reactivity (i.e. Green House Gasses(GHG) and leachate pollutant production) once disposed of in alandfill. The air required for the aerobic process is supplied by elec-tric fans through the aerated floor of the basin.

The ferrous metal content of the dry fraction exiting the trom-mel is separated from the remaining fraction by a magnetic beltsystem. After the ferrous metal separation section, the dry fractioncan be further selected with an air classifier that separates out thelighter fraction with a higher Low Heating Value (LHV) (Fig. 2) forthe production of SRF suitable for co-combustion. Currently theresidual dry fraction is generally disposed of in landfills.

The OFSS,in is unloaded from the collection lorries directly into adedicated inlet and pre-treatment section, before being processedin a dedicated area of the composting basin (Fig. 3).

The MBT plant started working in 1994, when both the quality(i.e. composition) and quantity of the waste (Fig. 4) were quite dif-ferent from the current waste. This has required some significantchanges in plant operating conditions, such as increase in the dailyworking shift, the overall increase in energy consumption andsome effects on different process efficiency. In particular, stabiliza-tion efficiency of the aerobic basin (Fig. 3) has been affected by theincrease in the amount of waste treated, together with the amountof energy consumed as a consequence of the increased amount ofprocess air supplied by the fans.

All these aspects show that the plant needs some upgrading inorder to be able to continue operating in an efficient and soundway in the current and near future waste management scenarios.

Considering that the rapidly biodegradable fraction of the OFMS

contains a rather high biochemical energy content [13,14,16–21], itcan be exploited in a SSAD process, before entering the aerobicbasin. This will lead to the production of renewable energy able

to increase the energetic efficiency and to reduce the environmen-tal impact of the MBT plant considered. Furthermore, the OFMS atthe SSAD outlet has features suitable for being processed in theexisting aerobic treatment section, without requiring any furtherpre-treatments, along with a reduction in the VS content.

3. Methods and assumptions

The energetic and economic analysis of upgrading the existingMBT plant with a new SSAD section was performed on the basisof several experimental runs aimed at evaluating the effect of theOFMS-to-inoculum OFMS/I (w/w) ratio, on the BY and BMY.

The OFMS/I ratio significantly affected both the revenues arisingfrom the renewable electrical energy production and the SSADfacility investment and treatment cost. For this reason, on the basisof the experimental runs and on the basis of previous economicanalyses performed by the authors [14], different scenarios havebeen analyzed.

3.1. Experimental apparatus

The experimental tests were performed in a 100-l, cylindrical,gastight, batch and static reactor (Fig. 5) able to treat about 30–40 kg of OFMS and I mixture, designed for reproducing the operat-ing conditions similar to those used in full scale SSAD biocells. Theprocess temperature was maintained at mesophilic conditions(35 �C ± 2�) by the aid of a thermal band and a Resistance Temper-ature Detector positioned inside the reactor. The gas produced dur-ing the process was piped to a demoisturizing device and then to athermal gas flow meter (±0.01% FS).

The percolate produced during the process was collected in adedicated space at the bottom of the reactor, able to maintain thesolid phase (i.e. OFMS and I) physically separated from the liquidphase. A timed circulation pump was used to spread a pre-deter-mined amount of percolate on top of the mixture being treated.

3.2. Samples and runs

OFMS samples were withdrawn from the existing MBT plant.One sample was used for the anaerobic runs, whereas three differ-ent samples, withdrawn at different times of the year, were usedfor determining the mean features of the OFMS, in terms of TSand VS, used in the optimization analysis. The samples were main-tained at �15 �C. Then, before each AD run, they were defrostedand maintained at ambient temperature for 48 h.

Table 1The energetic, economic and technical parameters.

Parameter Value Unit Reference

gel,net 34–38(1) % [14,22]LHVCH4 9.70 kWh/Nm3

In 81.7(2) €/tonne [14,22]CHP 1010 €/kWe [14]CHP O&M 40 €/MWhPercolate disposal 3.50(2) €/tonne [14]Investment period 10 YearEEP 75 €/MWh [23]GC 70-120-170 €/MWh

For ICE with less than 850 kWe, 34%, then 38%.Total tonnes of waste processed in the SSAD plant, OFMS + I.

Table 2Quantity and quality of MSW treated by the MBT plant.

Parameter Class Value Unit

Total amount NDWin 67,350 tonne/yearOFDF,in 6000 tonne/yearOFMS 32,894 tonne/year

Mean composition of OFMS NB 21.85 % w/wOrganic & Green 58.45 % w/wPaper & Cardboard 8.60 % w/wTextile 2.90 % w/w<20 mm 8.20 % w/w

Fig. 4. Amount of NDW treated by the MBT plant and trend of SS in the wastemanagement area vs years.

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The inoculums used for activating the AD runs were came fromprevious laboratory mesophilic OF MSW AD tests. The preservationof the inoculums and bacterial activity among the runs, was main-tained among the runs, were maintained at mesophilic conditionsby adding about 1% w/w saccharose every time the biogas produc-tion became almost null. The percolate exploited in the runs camefrom the same AD laboratory tests.

Seven different SSAD runs were performed, exploiting differentOFMS/I ratios, ranging from 1:1 to 1:3 w/w.

The biogas produced during each run was sampled and ana-lyzed using a portable bio-gas analyzer.

3.3. Analysis and characterization

The main waste streams treated by the MBT facility were eval-uated by determining the mass balance of the waste entering andexiting the plant in the three-year period 2008–2010. The meancomposition of the OFMS was determined by several manual sort-ing operations, to determine the %w/w the Non BiodegradableFraction (NB), the Organic & Greenery, the Paper & Cardboard,the Textiles and the fines (<20 mm size).

Both the OFMS and the I used in the runs were characterized byphysical and chemical analysis. The mean concentrations of the TSand VS of the OFMS processed in the MBT facility were determinedon the basis of three samples, withdrawn from the aerobic stabiliza-tion basin inlet, at different times of the year. The TS were determinedby heating at 105 �C for 24 h. The VS were determined by heating theTS at 550� for 24 h. Total Organic Carbon (TOC) was determined usingthe Springer and Klee wet dichromate oxidation method, while TotalNitrogen (TN) was determined using the Kjeldahl method. TotalPhosphorous (TP) was determined by extraction with a 0.5 M NaH-CO3 solution at pH 8.5 and measured using a spectrophotometer.

Also the percolate inserted in each run was characterized bothphysically and chemically.

The biogas composition was determined by means of its CH4

(±1%) CO2 (±1%) content (v/v), exploiting infra-red sensors,whereas the H2S (±2%) and O2 (±2%) content (v/v) were determinedby means of electrochemical cells. Other biogas components notinvestigated in this study were included in a balance term. Further-more, the contribution of the inoculums to the BY run was evalu-ated and subtracted to the final results.

3.4. Energy and economic assumptions

The possible different operating conditions with respect tothose evaluated in the experimental runs were evaluated usingthe following main steps:

(1) OFMS/I (w/w) ratio expressed as means of the kgVSOF,MS/kgVSI during the evaluation of BY and BMY.

(2) Calculation of two linear functions able to fit the experimen-tal values of the BY and BMY for each kgVSOF,MS/kgVSI ratioused in the runs.

(3) Representing the quality of the OFMS treated by the plant bythe mean TS and VS concentrations of the three sampleswithdrawn at different times of the year.

(4) Assuming to use an I with the same TS and VS concentrationas that used in the experimental runs.

On the basis of the previous assumptions, five different operat-ing conditions with OFMS/I ratios ranging from 1:1 to 1:3 (w/w), atan interval of 0.5, were used.

The economic analysis was conducted using the data availablein Refs. [1,14,22,23], and referring the specific In and O&M coststo the total amount of waste processed in the full scale SSAD bio-cells (i.e. OFMS + I). The Combined Heat and Power (CHP). In andO&M costs, including those for the voltage transformer and electri-cal grid connection, were determined on the basis of both the elec-trical power of the CHP and the producible electrical energy.Furthermore, the net electrical efficiency of the CHP was assumedto be 34% for electrical power less than 850 kW, and 38%, for elec-trical power equal to or more than 850 kW [14,22].

The different economic scenarios were simulated by using threedifferent values for the Green Certificate (GC), assuming 10 yearsas the investment time (Table 1). In effect, as plant size increases,the specific investment costs (€/tonne) tend to decrease. Anywayconsidering the size of these plants, as shown in a previous study[14], the variation in cost is quite low. For this reason this aspectwas disregarded in the analysis.

4. Main results

The mean amount of waste treated by the MBT facility (Table 2)showed that the OFMS stream rate was about 32,894 tonnes per year.About 22% w/w of this waste stream was the NB fraction, consistingof the components that are biologically inert such as metal, glass,stones and plastic. The Organic & Greenery fraction was 58% w/wand consisted of kitchen, vegetable, fruit and yard trimming waste.Paper & Cardboard were about 9% w/w, Textiles about 3% w/w andthe fine fraction, consisting of other non-distinguishable componentswith a size less than 20 mm, was about 8% w/w. This means that thecomponents useful for the AD process (i.e. OF and <20 mm fraction)were about 65–70% w/w of the whole amount of OFMS.

The chemical and physical characterization of the Inoculum andthe OFMS used in the runs (Table 3) showed a TS content greaterthan 35% w/w, VS greater than 85% w/w of TS and a TOC greaterthan 40% w/w of TS. The C/N ratio ranged from 23 to 26, with thatof I being higher, and TP ranging from 6% to 15% w/w of TS, withthat of I being higher. The higher values of the C/N ratio and theTP for I instead of for the OFMS is a consequence of the compositionof the OF MSW used in laboratory for generating the I.

The percolate spread during the runs had a TN concentrationcomparable to that of the treated mixture (Table 4). The otherparameters, such as TS, COD and P were significantly lower.

Fig. 5. Experimental apparatus of the SSAD.

Table 3Characterization of the Inoculum and OFMS used in the runs.

Parameter Inoculum OFMS Unit

TS 386 ± 5 403 ± 4 g/kgVS 866 ± 3 926 ± 3 g/kgTSpH 6.67 ± 0.3 5.17 ± 0.2TOC 403 ± 1.5 446 ± 1 g/kgTSC/N 26.5 ± 0.05 23.2 ± 0.05TP 14.8 ± 0.5 6.1 ± 0.3 g/kgTS as P2O5

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The runs showed that the higher was the amount of I per unit ofOFMS, the higher was the amount of BY and BMY producible foreach kgVS of the OFMS (Fig. 6). This is in agreement with the reduc-tion in the inhibition caused when working with high TS values[20]. In fact, for an OFMS/I ratio of around 0.7, the BY was about350 Nl/kgVS and the BMY was about 180 Nl/kgVS. When theOFMS/I value was around 0.2, the BY reached about 750 Nl/kgVS

and the BMY was about 400 Nl/kgVS. The mean methane concen-tration in the biogas was always more than 50%, which is a suitablelevel for exploitation as fuel in Internal Combustion Engines (ICE)for electrical energy production.

Fitting the experimental results (Fig. 6) into linear Eqs. (1) and(2), the expected BY and BMY were calculated when operating withOFMS/I ratios different from the tested ones.

BY ¼ �846:41 � OFMS=Iþ 923:6 ð1Þ

BMY ¼ �578:61 � OFMS=Iþ 556:48 ð2Þ

In agreement with the experimental results and Eqs. (1) and (2),as I increases, at constant kgVSOF,MS, the increase in BMY, whichinfluences the amount of recoverable renewable energy, is lowerthan the increase in BY. This is coherent with the biochemistry ofthe AD process. In effect, a methane concentration greater than40–45% (v/v) in the biogas during SSAD was generally achievedafter a period of time ranging from 2 to 7 days (Fig. 7), dependingon many factors such as waste characteristics, process conditions(temperature, pH, etc.) and on the amount and quality of the OFMS

and I used [11,18]. In any case, keeping the other parameters con-stant, the more I used per unit mass of OFMS, the shorter was thetransient period, generally.

This means that the increase in biogas methane content waslimited with respect to the production of Biogas (BY), due to theOFM/I ratio.

As the OFMS/I (w/w) ratio increased, the specific investmentcost, related to the tonnes of OFMS treated, increased (Fig. 8a) butnot linearly. A similar shaped curve shows the amount of electricalenergy produced per each tonne of OFMS. As the OFMS/I (w/w) in-creases, the amount of electrical energy producible by the SSADplant increases, but also in this case, it shows an asymptotic trend.

There are two main reasons for this phenomenon:

(1) Divergence of BY and BMY as OFMS/I increases.(2) Increase in gel,net of the CHP, as OFMS/I increases.

The first phenomenon leads to a lower increase in the BMY, re-lated to that of BY, as OFMS/I rises, always causing less of an in-crease in the electrical energy produced as the amount of OFMS

treated in the SSAD facility increases.The second phenomenon is due to the increase of gel,net as the

CHP size becomes greater than 850 kW. This condition is achievedwhen the OFMS/I ratio is higher than 1:1. From the economic pointof view, these two aspects cause a linear increase in the SSAD In

Fig. 6. BY and BMY for different OFMS/I ratios expressed as kgVSOF,MS/kgVSI.

Fig. 7. Methane % v/v for different OFMS/I ratios, expressed as kgVSOF,MS/kgVSI, vsdays of SSAD treatment.

Fig. 8. Specific Investment and energy production (a) and electrical energy costs during and after the investment period (b) vs OFMS/I ratios (w/w).

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and O&M costs, whereas the CHP In and O&M coasts are deter-mined by the amount of energy able to be produced.

The interaction of these two different phenomenon causes aminimum value of the cost of the electrical energy producible bythe plant (Fig. 8b). Both during the investment period (i.e. the first10 years) and later, the minimum is achieved at an OFMS/I (w/w) ra-tio of 1:2. During the first 10 years, the energy production costsrange from about 0.26 to 0.36 €/kWh, becoming about 0.14–0.25 €/kWh after the 10th year, depending on the OFMS/I ratio. Alsoincluding the revenue arising from the electrical energy price (EEP)and the GC (Fig. 9) in the technical and economic analysis, it ispossible to note the presence of a clearly definite minimum valueof the costs of the SSAD treatment per tonne of OFMS. The optimumOFMS/I ratio corresponding to the minimum treatment cost €/tonneof OFMS increases with the GC value. For a GC of 70 €/MWh, the opti-mum OFMS/I ratio is about 1:1.5 w/w, becoming 1:2 w/w for a GC va-lue of 170 €/MWh. In particular, for a GC of 170 €/MWh, for an OFMS/I ratio ranging from 1:1.5 to 1:2.5 w/w, revenues are higher thancosts. In terms of overall investment, an increase in the OFMS/I ratiofrom 1:1.5 to 1:2 w/w corresponds to an increase of about13,00,000 €, whereas O&M and revenues increase, respectively, byabout 260,000 €/year and from about 320,000 to 540,000 €/year.

Overall, the investment costs range from a minimum value ofabout 60,00,000 up to 120,00,000 € at which O&M costs corre-spond, respectively, to about 800,000 and 20,00,000 €/year. Theoverall electrical energy production ranges from about 3000 to12,500 MWh per year, depending on the OFMS/I ratio.

5. Conclusion

The Solid State Anaerobic Digestion process appears to be asuitable solution for treating the organic fraction of municipal solidwaste emerging from the mechanical sorting section of the MBTfacility examined. Along with the possibility of recovering renew-able energy from this waste fraction, there is also the possibilityof increasing the efficiency of the stabilization section of the MBTby reducing the amount of organic load to be treated.

The experimental runs showed the positive effects that the mountof inoculum used per unit of mass of mechanically sorted organicfraction has both on the biogas and on the biomethane yield. Fromthe economic point of view, the optimal size of the SSAD plant isone that is able to process about 33,000 tonnes per year of mechani-cally sorted organic fraction, with an inoculum ratio ranging from1:1.5 to 1:2.5 w/w, depending on both the Green Certificate valueand the biomethane yield. Anyway, an increase in the amount ofinoculum used causes a significant increase in the investment costs.This can be an important limiting factor to consider when optimizingthe SSAD reactor size and, as a consequence, the amount of inoculumto be used. The amount of renewable electrical energy recoverable foreach tonne of treated waste ranges from about 100 kWh, whenexploiting a waste-to-inoculum ratio of 1:1 w/w, to about380 kWh, when exploiting a ratio of 1:3 w/w (Tables 3 and 5).

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Fig. 9. SSAD specific treatment costs including revenues from GC and EEP vs OFMS/Iratios (w/w).

Table 4Percolate characterization.

Parameter Value Unit

TS 9.100 ± 0.2 g/lpH 8.780 ± 0.3ORP �78.70 ± 0.3 mVCOD 3850 ± 8.5 mg/lTN 3395 ± 7.5 mg/lTP 75.60 ± 3.8 mg/l as P2O5

Table 5TS and VS concentrations of OFMS.

Sample OFMS

TS (g/kg) VS (g/kgTS)

1 500.0 471.12 274.6 705.93 378.3 790.5

Average 384.3 632.1

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