Semi-dry thermophilic anaerobic digestion of the organic fractionof municipal solid waste: focusing on the start-up phase

D. Bolzonella a, L. Innocenti b, P. Pavan b,*, P. Traverso b, F. Cecchi a

a Department of Science and Technology, University of Verona, Strada Le Grazie, I-37134 Verona, Italyb Department of Environmental Sciences, University of Venice, Dorsoduro 2137, I-30123 Venice, Italy

Abstract

The paper concerns the results of a pilot-scale study of the simulation of the start-up phase of the thermophilic semi-dry an-

aerobic digestion of the organic fraction of municipal solid wastes. The aim of the study was to aid and shorten the start-up phase of

the full-scale plant (500 t/d) in Verona––Ca’ del Bue, where the semi-dry anaerobic digestion process is being used. The substrate

used in the experimentation was the mechanically sorted organic fraction of municipal solid waste (MS-OFMSW) enriched with the

putrescent fraction from the source sorted OFMSW in order to simulate the substrate which is dealt with in the Verona plant. The

results of the pilot scale study agreed with literature data and previous work of the authors: it showed a specific gas production of

0.23 m3/kgTVSfeed and a gas production rate of 2.1 m3/m3 d when operating at a specific organic loading rate of 0.135 kgTVSfeed/

kgTVSreactor d. No problems regarding process stability were encountered in the gradual acclimation of the biomass. The design

organic loading rate of 9 kgTVSfeed/m3reactor d was reached in about 30 days, during which the total solids content in the feed was

increased. Only a partial comparison with the full scale start-up, which is now in progress, is possible: this shows an initial general

concordance with the results found in previous work.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Anaerobic digestion; Organic solid wastes; Semi-dry process; Thermophilic

1. Introduction

The need for processes in the field of conservation of

resources has become more than clear in recent years,

thus interest in anaerobic digestion processes has in-

creased. Anaerobic digestion of organic wastes is now a

reliable technology, as confirmed by the growth of full-scale applications in Europe in the last decade (Mata-

Alvarez et al., 2000). At present, some one million

tonnes of organic wastes (wet weight) per year are di-

gested worldwide. These wastes are converted to biogas

and stabilised residual matter (Verstraete et al., 2000).

Furthermore, the reduction of carbon dioxide emissions,

resulting from the application of anaerobic digestion

processes, especially when coupled with post-compo-sting processes, is of great interest from an environ-

mental point of view (K€uubler and Rumphorst, 1999;

Verstraete et al., 2000). According to De Baere (2000),

the capacity evolution rate of the full scale application

of anaerobic digestion processes was around 30,000 tons

per year during the period 1990–1995, while the rate of

increase averaged 150,000 ton per year for the period

1996–2000. An increase of 200,000 tons per year is ex-

pected in 2001 and 2002. The number of new plants for

those same periods rose from 2.4 to 7.2 plants per year.The average capacity, originally at 24,420 tons per plant,

increased to an averages of 50,000 tons per year in recent

years as large grey or mixed waste digestion projects

were planned. Plants were initially operated only at the

mesophilic range of temperature, the first thermophilic

plants coming on line in 1992 and 1993. The capacity of

mesophilic operations increased to 350,000 tons from

1994 to 1999, while thermophilic capacity increased to280,000 tons. That was an increase of 70,000 and 56,000

tons per year, respectively. Thermophilic operation was

developed later but has been established as a reliable

and acceptable option for digestion (Mata-Alvarez et al.,

2000). Between 1990 and 1993, more wet digestion

plants (<10% total solids (TS) in the feed) were con-

structed; since then, dry digestion (>20% TS in the feed)

Bioresource Technology 86 (2003) 123–129

*Corresponding author. Fax: +39-0422-326498.

E-mail address: pavan@unive.it (P. Pavan).

0960-8524/03/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0960-8524 (02 )00161-X

has prevailed. So far, no clear trends have been observedfor these two processes and they are both successfully

being used in the new plants.

In addition, a number of research activities has been

carried out on the topic of two-phase and single-phase

digestion but, as yet, two-phase digestion has not been

able to demonstrate the advantages claimed by its sup-

porters. Only 10.6% of the available capacity is catered

for by two-phase digestion systems (De Baere, 2000).In such a scenario, the new Verona plant (northern

Italy) is an example of particular application of the

semi-dry process: the total solids in the feed range from

10% to 20% TS. This is the first full-scale application of

this process in Italy after several years of bench and

pilot-scale tests. This process, patented in 1991, has re-

cently been applied to a plant with a 500 t/d capacity, to

treat the larger part of the solid wastes produced in theVerona area (Cozzolino et al., 1992; Farneti et al., 1999).

The anaerobic digestion section of the plant, started

in 1999–2000, works with four digesters, 2000 m3-con-

tinuously stirred tank reactors (CSTR), operating in

semi-dry conditions (TSreactor ffi 120–150 g/kg) in the

thermophilic range of temperature (Farneti et al., 1999).

The process has been studied, at pilot level, for more

than 10 years by the research group of the Universitiesof Venice and Verona. An inclusive set of information

about the optimisation of the process, the operational

conditions to be applied and the obtainable yields is

available in the literature (Cecchi et al., 1990; Cozzolino

et al., 1992; Farneti et al., 1999; Pavan et al., 2000).

During recent years, the composition of the organic

fraction of the municipal solid wastes (OFMSW) pro-

duced in the municipality of Verona has changed sig-nificantly (see Table 1), owing to an increase in the use

of plastic and cardboard packaging. Thus, a final check

of the process, using the new characteristics of the

wastes (substrates), was needed to transfer the correct

information on full-scale.

This paper, then, concerns the pilot-scale simulation

of the digester start-up and steady-state performance

with this ‘new’ kind of substrate, reporting the datacollected during a nine-month experimentation, repro-

ducing all the conditions that could occur during thestart-up phase.

2. Methods

The study considered firstly a start-up phase in me-

sophilic conditions (37 �C). The feed was characterised

by a low TS content. The TS content in the feed was then

increased to get the final steady-state condition with 20%

TS in the feed. The pilot-scale digester used in the ex-

perimentation was a 3 m3-mechanically stirred CSTRreactor which was electrically heated and automati-

cally controlled using a microprocessor to maintain the

working temperature within �1 �C. After reaching

steady state conditions, the temperature was increased to

the thermophilic range (55 �C). The substrates during thestart-up phase were firstly, sewage sludge originating

from the wastewater treatment plant of Treviso, and then

the municipal organic wastes. These were a blend of twotypes of substrates, so as to reproduce the characteristics

of Verona wastes (see feed characteristics in Table 2): the

first, the mechanically sorted organic fraction of muni-

cipal solid wastes (MS-OFMSW), was provided by the

San Giorgio di Nogaro (Udine, northern Italy) sorting

plant (TS � 600 g/kg, TVS 50% TS) and the second

was collected daily from the municipality of Treviso

(Northern Italy) as source sorted waste (SS-OFMSW)mainly from supermarkets and vegetable markets (TS �150 g/kg, TVS 80–90% TS). These substrates were mixed

together on a mass balance basis, mill-shredded (5 mm

blades) and fed to a 1 m3-feedstock tank before being fed

to the digester. Analyses were carried out according to

the Standard Methods (APHA, 1992), slightly modified

to suit the substrates used in the experimentation.

3. Results and discussion

3.1. Mesophilic start-up phase

The operating temperature in the start-up phase was

within the mesophilic range (37 �C). A mixture of pri-

mary and secondary sludge from a civil wastewater

treatment plant was used to fill 50% of the digester

volume and then diluted with wastewater to make up thefinal working volume. The mesophilic range of temper-

ature was reached within 24 h. In the first 8 days no feed

was applied to the digester: in these conditions, the

volatile fatty acids (VFA) content dropped from 800 to

200 mg/l within ten days (Fig. 1b). A second acclimation

period from day 10 to 40, followed when the digester

was fed with sewage sludge at a hydraulic retention time

(HRT) of 15 days and an OLR < 1 kgTVS/m3reactor d.

The VFA content remained almost constant, but the pH

was quite low, e.g., <7 (see Fig. 1a). More stable con-

ditions were reached after day 40, when the pH in-

Table 1

Characteristics of the MSW collected in Verona area (350,000 inhabi-

tants), percentage on wet weight basis

Fractions Verona municipality

1989 1998

Cellulosic matter 24 30

Organic 37 27

Plastics 8 15

Glass and inerts 7.5 9

Metals 3.5 4

Other 20 15

Total 100 100

124 D. Bolzonella et al. / Bioresource Technology 86 (2003) 123–129

creased to 7.5 and the VFA dropped under 100 mg/l;

these levels were maintained during the acclimation pe-

riod, until day 80 (Fig. 1b). Acetic and propionic acids

predominated, while the other acids were virtually ab-

sent. The value of the specific gas production (SGP) was

about 0.15 m3/kgTVSfeed. Previous experiences of the

authors (Cecchi and Traverso, 1986), using an analo-

gous substrate (sewage sludge), showed that higher

yields (SGP of 0.35 m3/kgTVSfeed) could be achieved

applying an OLR of 1.6 kgTVS/m3 d and a solid recyclein the reactor, reaching a solid retention time of 33 days.

The same was observed by other authors who reported

SGP values in the range 0.25–0.50 m3/kgTVSfeed

(Speece, 1988) when treating mixtures of sewage sludge

(primary and secondary).

From day 87 to 119, the digester was fed with a blend

of SS and MS-OFMSW, with 20% TS content. The ap-

plied OLR was 6 kgTVS/m3reactor d to test the possibility

of a fast start-up of the process. In fact, the digester

initially showed stable conditions, but evidence of pro-

cess upset due to an organic overloading was observed

after day 100, when the VFA concentration rose to 2500

mg/l and then to 3500 mg/l (see Fig. 2b). The amount of

all the short chain volatile fatty acids (C2–C6) increased,

in particular acetic and propionic acids. The main cause

for this failure could be ascribed to the low content ofbiomass in the reactor: the digester TS content was

always below 50 g/kg (see Fig. 2a). This value was very

Fig. 1. Profiles of pH (a) and VFA (b) during the start-up phase.

Table 2

Average values in thermophilic steady-state conditions at pilot scale

Parameter Average Min Max Std. dev.

Operational conditions T (�C) 54.9 54.5 55.2 0.2

HRT (d) 13.5 12.8 14.2 0.4

OLR (kgTVS/m3r d) 9.2 8.3 10.2 0.5

Feed TS (g/kg) 201.4 194.4 209.8 4.3

TVS (g/kg) 124.3 113.3 137.0 5.9

TCOD (gO2/kg) 114.1 95.1 135.4 11.0

SCOD (gO2/kg) 7.1 4.0 12.1 2.4

pH 6.5 5.8 8.0 0.4

VFA (mg/l) 3389 601 5376 1254

Lactic acid (mg/l) 1373 675 1729 288

TKN (g/kg) 14.1 11.4 16.4 1.6

Digester TS (g/kg) 201.9 186.1 209.1 4.8

TVS (g/kg) 91.4 74.9 102.8 8.0

STS (g/l) 15.9 8.2 28.2 6.5

SVS (g/l) 9.9 3.9 17.9 4.1

TCOD (gO2/kg) 85.9 76.1 99.9 7.1

SCOD (gO2/kg) 3.6 2.5 5.5 1.0

pH 7.4 7.1 7.7 0.1

TA(4) (gCaCO3/l) 5.5 2.7 7.0 1.2

VFA (mg/l) 2067 1219 3007 468

Lactic acid (mg/l) 710 478 862 125

NH3 (mg/l) 439.7 342.3 485.8 38.6

Biogas GPR (m3/m3 d) 2.1 1.7 2.4 0.2

SGP (m3/kgTVSfeed) 0.23 0.18 0.28 0.03

CH4 (%) 68.7 62.0 76.0 4.3

HRT: hydraulic retention time; OLR: organic loading rate; TS: total solids; TVS: total volatile solids; TCOD: total chemical oxygen demand; SCOD:

soluble chemical oxygen demand; VFA: volatile fatty acids; TKN: total kjeldhal nitrogen; STS: soluble total solids; SVS: soluble volatile solids;

TA(4): total alkalinity at pH 4; GPR: gas production rate; SGP: specific gas production. All parameters are expressed on wet weight basis.

D. Bolzonella et al. / Bioresource Technology 86 (2003) 123–129 125

low if compared with the usual solid contents in a semi-

dry process; generally in the range 100–150 g/kg. Theseconsiderations can be supported by a previous study

concerning the mesophilic application of a semi-dry

process (Cecchi et al., 1990), where a start-up of the

process was performed using MS-OFMSW as substrate.In that case, an OLR of 4.1 kgTVS/m3 d was applied for

the first 30 days, and the VFA concentration reached

about 200mg/l. After that period, the OLRwas increased

up to 6.8 kgTVS/m3 d, without any problem of reactor

stability (VFA always <200 mg/l), but in that experi-

mentation the digester TS content was about 150 g/kg.

Cecchi et al. (1990) reported that the final biogas yield in

those conditions was 0.23 m3/kgTVS for the SGP.On the basis of the results and considerations ob-

tained in the first phase, the feeding was stopped and the

second part of the start-up programme was commenced.

This was based on the gradual increase of the OLR

while maintaining the HRT constant, diluting the waste

with water before feeding: see the feed concentration

profile in Fig. 3a. The starting condition was an OLR

equal to 10% of the final OLR (9.5 kgTVS/m3 d). Thiswas reached after 5 HRT (days 120–195). This approach

allowed a gradual acclimation of the biomass, guaran-

teeing also a progressive increase of solids content in the

digester (see Fig. 3b). More stable operational condi-

tions were achieved, even though a higher content of

VFA was observed (about 2000 mg/l, see Fig. 3c) in

comparison with previous works. This aspect must be

compared with the results related to the previous expe-riences in the mesophilic range (Cecchi et al., 1990),

where an average concentration of VFA of about 3200

mg/l was observed. The pH value of 7 was the same of

the previous study and the CO2 content in the biogas

was basically the same in the two studies (about 50% of

the biogas).

Fig. 2. Trends of the digester solid content (a) and VFA concentration

(b) during the start-up phase.

Fig. 3. Profiles of TS and TVS in the feed (a), TS and TVS in the reactor (b), VFA (c) and pH (d), during the period of gradual increase of the OLR.

126 D. Bolzonella et al. / Bioresource Technology 86 (2003) 123–129

3.2. Moving from mesophilic to thermophilic conditions

The following step of the start-up phase was to change

the reactor temperature from the mesophilic (37 �C)to the thermophilic range (55 �C). According to Cecchi

et al. (1993), the digester loading was stopped for a week

(days 184–192, see Fig. 3a and b) to prevent the process

from becoming unstable during the transient conditions.

The change of temperature was performed abruptly andthe thermophilic temperature was soon reached (48 h).

Because the feeding was stopped, overloading was

avoided and VFA did not build up; in fact, the VFA

production was offset by VFA conversion into biogas.

3.3. Thermophilic steady-state phase

Steady state conditions were reached about 60 days

after getting to the design OLR, 9.5 kgTVS/m3 d. In Fig.

4a–f the trends of the parameters are shown. After day

224 the process could be considered to be in steady-state

conditions. This can be seen from the trends of the pHand the alkalinity (Fig. 4a and b). The latter, even if

decreasing in the last period, showed a decrease in the

difference between the alkalinity value determined at pH

6 (TA6) and that determined at pH 4 (TA4). This in-

dicated a reduction in the acid content of the digester.

This evidence was also confirmed in the plot of VFA

concentration, which dropped from about 3000 to 2000

mg/l, a typical value for steady-state conditions of asemi-dry process (Fig. 4c). The ammonia concentration

in the digester also showed a change in the same period,

moving up to about 450 mgN/l. In fact, changing from

mesophilic to thermophilic conditions usually results in

an increase in organic matter degradation, and conse-

quently in proteins (Poggi-Varaldo et al., 1997); this

leads to an increase in the buffer capacity of the system.

Furthermore, the CO2 content in the biogas passed fromthe 40–50% of the start-up phase to the typical 30% of

steady-state conditions. All these trends concord with the

behaviour previously observed by Cecchi et al. (1993),

Fig. 4. Profiles of pH (a), alkalinity (b), VFA (c), CO2% in the biogas (d), GPR (e) and SGP (f) in thermophilic steady state conditions.

D. Bolzonella et al. / Bioresource Technology 86 (2003) 123–129 127

even though, in that case, a lower level of VFA wasfound (about 400 mg/l). This difference can be ascribed

to the substrate characteristics: in this experimentation a

mixture of SS- and MS-OFMSW was used, whereas in

the past only MS-OFMSW was adopted for the digester

feeding. This suggests that the evolution of substrate

characteristics in recent years towards a more ‘‘biode-

gradable’’ mixture (substrate), due to the increase in the

separate collection of organic wastes could be an im-portant factor for the stability and process control of the

digesters. In particular, when treating substrates with a

high biodegradability, the digester can be up-set by an

increase in VFA concentration in the medium (Pavan

et al., 2000; Lissens et al., 2001). Therefore, particular

attention has to be paid to the monitoring of the process

stability.

The average results of the steady-state conditions arereported in Table 2: SGP and gas production rate

(GPR) values were not very high, 0.23 m3/kgTVS and

2.1 m3/m3 d, respectively, if compared with other results

obtained in previous studies with similar blends of

substrates (i.e., 0.32–0.5 kgTVS/m3 d and 3.1–6.2 m3/

m3 d found in Pavan et al., 2000). A comparison of this

data with the results reported in other works can be seen

in Table 3. These can probably be explained by the poorbiodegradability of the substrate fed to the digester. In

fact, the specific loading rate (SOLR), expressed as the

ratio of the TVS content in the feed to the TVS content

in the reactor was low (<0.1 kgTVSfeed/kgTVSreactor d).

In the plot of Fig. 5, several operative conditions of the

semi-dry process are compared in terms of SOLR versus

GPR. These were obtained using SS or MS-OFMSW as

sole substrate or the blend of MS and SS-OFMSW. Ingeneral, substrates with the higher biodegradability

show high values of both the SOLR and GPR para-

meters, while substrates with low biodegradability show

low values of those parameters.

3.4. The full-scale application of the thermophilic semi-

dry process

It is possible to compare these results with the ones

obtained in the full scale plant of the municipality of

Verona in the first operational phases: in fact, in 2000,Verona-plant was in the start-up phase, and a first

pseudo-stationary condition was reached. The OLR

applied was very low (1.2 kgTVSfeed/m3 d) if compared

to the design options of the process (9 kgTVSfeed/m3 d).

This gave a SOLR of 0.135 kgTVSfeed/kgTVSreactor d but

a low GPR. The results obtained from this pilot-scale

study are plotted together with those from previous

work and the full scale ones in Fig. 5: about this last, the

yields are low because of the low SOLR applied, but

they are in complete agreement with the trend found inprevious studies of the authors.

Besides the digestion performance in terms of organic

matter stabilisation and biogas production, the reli-

ability of the semi-dry process is also supported from the

point of view of energy production. The design figures

for the full-scale application at the Verona plant suggest

the possibility of producing 26,200–35,400 Nm3/d. This

means a daily production of 50–70 MW of electric en-ergy and 85–115 MW of thermal energy to be compared

to a request of some 20 MW for the heating of incoming

streams, considering an external temperature of 15 �C.

4. Conclusions

On the basis of the results obtained in the experi-

mental work the following conclusions can be drawn:

Table 3

SGP and GPR values in thermophilic anaerobic digestion of MS-OFMSW

Reference SGP (m3/kgVSfeed) GPR (m3/m3reactor d) OLR (kgVS/m3

reactor d) TS in the feed (%)

This study 0.23 2.1 9.2 20

Cecchi et al. (1991) 0.26–0.40 2.5–4.1 5.9–13.5 16–22

Mata-Alvarez et al. (1993) 0.32–0.37 3.1–6.1 9.7–17.8 18–25

Vallini et al. (1993) 0.30 4.1 13.5 22

Pavan et al. (2000) 0.32 3.1 9.7 25

Scherer et al. (2000) 0.22 5.7 7.6 16

Fig. 5. SOLR vs. GPR in different studies concerning the thermophilic

anaerobic digestion of MS-OFMSW and comparison with the results

of the present study.

128 D. Bolzonella et al. / Bioresource Technology 86 (2003) 123–129

– the mesophilic start-up of the pilot-scale digester wassuccessfully carried out directly feeding the OFMSW

and increasing the OLR to the final design values;

– the change from a mesophilic (37 �C) to a thermo-

philic (55 �C) environment in the reactor can easily

be performed in a short time with interruption of

feeding for a few days. So, the possibility to have a

short start-up phase to reach the thermophilic condi-

tions with a mesophilic inoculum has been confirmedalso at full scale;

– the lower yields obtained in terms of biogas produc-

tion in thermophilic steady state conditions (0.23

m3/kgTVS) can be considered appropriate due to

the low SOLR applied (0.1 kgTVSfeed/kgTVSreactor d);

– low full scale yields, compared to pilot-scale yields,

can be explained by very low OLR applied (1.2

kgTVS/m3 d), instead of 9.5 kgTVS/m3 d (design fig-ure);

– the stability parameters during the whole experiment,

also considering the full scale application, are in com-

plete agreement with the previous study, demonstrat-

ing an easy transfer of the know-how obtained in the

pilot test to the real application.

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