DOI: 10.3303/CET2292004 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 20 January 2022; Revised: 3 March 2022; Accepted: 28 April 2022 
Please cite this article as: Lucian M., Merzari F., Messineo A., Volpe M., 2022, Hydrothermal Carbonization of Sludge Residues via Carborem 
C700 Industrial Scale Continuous Operating Plant, Chemical Engineering Transactions, 92, 19-24  DOI:10.3303/CET2292004 
  

  CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 92, 2022 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Rubens Maciel Filho, Eliseo Ranzi, Leonardo Tognotti 
Copyright © 2022, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-90-7; ISSN 2283-9216 

Hydrothermal Carbonization of Sludge Residues via 

Carborem C700 industrial Scale Continuous Operating Plant 

Michela Lucian*a , Fabio Merzaria, Antonio Messineob , Maurizio Volpea,b 

aCARBOREM, piazza Manifattura 1 – 38068 Rovereto (TN), Italy 
bUniversità di Enna, KORE, Facoltà di Ingegneria e Architettura, viale delle Olimpiadi snc, 94100 Enna, Italy 
michela.lucian@carborem.com 

This study describes the main features of a novel industrial-scale continuous operating hydrothermal 

carbonization (HTC) developed by Carborem srl named C700. The C700 industrial-scale HTC plant is a highly 

compact, modular and containerized system capable of treating more than 5 kton per year of sludge. The results 

of HTC operated with C700 plant at 190 °C and 1 h of residence time of three different high moisture content 

residues (sewage sludge, agro-industrial sludge, and organic fraction of municipal solid waste digestate) 

demonstrate the efficiency and versatility of the process. C700 system is capable of promoting dewaterability of 

sludge residues, producing solid residues (hydrochars) upon centrifugation of HTC slurries with improved 

phosphorus content. Results show that the system allows concentrating P into the solid residue up to 3.6 wt% 

on a dry basis and leads to hydrochars with a total solid content, upon centrifugation of 34.6, 38.6 and 41.0 wt% 

for sewage and agro-industrial sludge and organic fraction of municipal solid waste digestate, respectively. 

Moreover, while hydrochars show good potential as soil fertilizers, when excess heavy metals like copper and 

zinc are removed, recirculation of the high carbon content liquid residue to the anaerobic digestion system offers 

the opportunity to enhance biogas production in the waste management plant thus lowering the total external 

energy demand of the process.  

1. Introduction 

Due to the growing attention to the environmental risks associated with the use of digested sludge in agriculture 

practices and the corresponding more stringent legislations, sludge managements operation are facing 

increasing costs for sludge treatments and disposal (Wang et al., 2019). Moreover, the global increase in 

anthropogenic pressure causing a rapid increase in sludge production is pushing the development of new 

strategies and technologies, in full agreement with a circular economy approach, for the reduction and the safe 

treatment of such residues. Between the new technologies for the treatments of such as high moisture residues 

as sewage sludge (Merzari et al., 2020; Volpe et al., 2020), organic fraction of municipal solid waste (Pawlak-

Kruczek et al., 2019), agro-waste (Lin et al., 2021) and agro-industrial residues (Benavente et al., 2015), 

hydrothermal carbonization (HTC) demonstrated to be one of the most promising. HTC is a wet thermochemical 

process carried out at sub-critical water conditions where high moisture organic residues are treated at 

temperatures between 180-250 °C and corresponding saturated water pressure of 10-50 bars.  During HTC, 

water acts as a catalyst promoting hydrolyzation, dehydration, and decarboxylation reactions (Kruse and 

Dahmen, 2015) leading to a solid residue showing an enhanced degree of carbonization (Miliotti et al., 2017), 

high energy density, hydrophobicity (Sharma et al., 2020) and adsorption properties toward organic water 

contaminants (Saha et al., 2020). HTC process produces also a considerable amount of an aqueous liquid 

residue, with increased organic carbon content, suitable for biogas production when recirculated in an anaerobic 

digestion plant (Villamil et al., 2019). 

Despite the high and growing attention for HTC technology and its recognized efficiency and economic viability 

for upgrading wet waste biomass effluents (Miliotti et al., 2020),  just few studies have focused on up-scaling of 

the process (Child, 2014; Kruse et al., 2013) and even less are the reported industrial scale applications for 

forestry residues (Hitzl et al., 2015) and agro-industrial digested sludge (Lucian et al., 2021). 

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The aim of the present study is to report the main features of a novel continuous operating HTC industrial scale 

developed by Carborem srl named C700 capable of treating up to 5 kton/year (corresponding to 0.7 ton/h) of 

biomass waste effluents with a solid content ranging between 2-10 wt% at temperature as high as 220 °C and 

pressure up to 25 bars. Main properties of HTC solid and liquid residues produced by C700 demonstrate its 

high performance and versatility enhancing dewatering properties of hydrochars. HTC thus reduces the volumes 

of waste to dispose while also offering the opportunity of producing a valuable product such as an organic 

fertilizer with high P content.  

2. Materials and methods 

2.1 Materials 

Three different waste effluents were treated in the continuous HTC plant: sewage sludge digestate (SSD), 

organic fraction of municipal solid waste digestate (OFD) and agro-industrial digestate (AWD).  All the substrates 

investigated in this study were treated as soon as received at the waste management plant for at least a 48 h 

in continuously operating mode.  The produced HTC residues (slurries) were collected in a 11.3 m3 tank before 

centrifugation and/or analytical characterization. A portion of each feedstock and corresponding slurry were 

carefully sampled and stored in sealed plastic containers kept at 4 °C before analytical characterizations.  

2.2 Feedstock and HTC residues analytical characterization 

The total solid content of the feedstock and corresponding slurries were measured by the moisture analyzer 

DAB 100-3 60. The total nitrogen was determined following the CNR IRSA 6 Q64 Vol.3 1985 method. The pH 

of hydrochars was measured according to the CNR IRSA 1 Q 64 Vol 3 1985 + APAT CNR IRSA 2060 Man 29 

2003 method, while the pH value of the liquid/slurry samples was measured with a portable LLG-pH meter 5. 

The total chemical oxygen demand (COD) of the HTC liquids was determined following the ISO 15705:2002 

standard method. The heavy metal (Cd, Hg, Ni, Pb, Cu, Zn) and phosphorus concentrations of all samples (raw 

materials and HTC residues) were evaluated by means of an ICP 8300 DV Optima Perkin Elmer spectrometer 

equipped with a high-performance SCD detector using EPA 6010D: 2018 + EPA 3051A: 2007 standard 

methods. Leaching of the samples was carried out via microwave-assisted digestion using a 10 wt% nitric and 

hydrochloric solution in water at a temperature of 175 °C. All analyses were performed at least in duplicate, 

obtaining a maximum deviation less than 5%. Total organic carbon (TOC) determination of the samples was 

carried out by a TOC analyzer (model PRIMACS SNC-100) following the UNI EN 15936:2012 standard method.  

 

2.3 C700 Continuous operating Industrial plant: main design features and operation 

Figure 1 reports a schematic representation of the C700 HTC plant. The digestate collected from a storage tank 

is fed into the system via a pump soon after a first mechanical removal of coarse material. The raw digestate is 

preheated through a heat exchanger that exchanges with the hot  HTC slurry coming from the reactor. The 

preheated feedstock, continuously fed into the reactor, is warmed up to reaction temperature by means of hot 

diathermic oil flowing through the outer jacket of the reactor.  The valve on the HTC slurry pipe regulates the 

flow rate leaving the reactor, while maintaining the pressure into the HTC reactor. The produced liquid solid 

mixture, called HTC slurry (SL), is sent into a centrifuge that separates a liquid aqueous phase rich in carbon 

that is recirculated to the anaerobic digestion system of the plant, and a residue high in solid content (about 40 

wt%) named hydrochar (HC).  

C700 plant is composed of a feedstock storage tank of about 10.0 m3, a 11.3 m3 HTC slurry storage tank, a 

diathermic oil boiler fed with provided by the municipal network and / or biogas produced by the anaerobic 

digester of the waste management plant, and the reactor system located into a container (Figure 2a-b). The 

diathermic oil boiler, when continuously operated at 190 °C with a residence time of 1 h, consumes about 8.5 

±0.3 Nm3 of methane per ton of digested sludge treated while the overall electricity consumption, for the same 

amount of treated sludge, is about 4.6 ±0.1 kWh.  Figure 2b shows the 0.7 ton/h jacketed C700 HTC reactor.     

 

  

20



 
Figure 1: Schematic representation of the continuously operating C700 HTC plant. 

 

 
 

Figure 2: (a) C700 plant container; (b) C700 HTC jacketed reactor vessel  

 

The customized reaction system container, 8.0 m long, 2.0 m large and a maximum height of 3.5 m houses the 

reactor vessel, the pumping and heating exchanging system and the PLC control system. The PLC can be also 

operated remotely. A dedicated software, developed for the plant, allows real-time monitoring and recording of 

all process parameters and data regarding energy consumption and productivity. The diathermic oil boiler and 

its piping are located in a separate cabinet close to the reactor container. 

 

2.4 C700 HTC experiments and slurries recovery 

In this study SSD, OFD and AWD feedstock were carbonized at 190 °C and 1 hour using C700 plant operating 

in a continuous mode at 0.7 ton/h reaction rate. The produced HTC slurries (SL) were centrifuged at 2800 rpm 

by a CBB Decanter model CD 50 SIV industrial centrifuge (Figure 3) working at a flow of 7 ton/h of SL. The HTC 

slurry materials were named SSD-SL, OFD-SL and AID-SL while centrifuged materials (hydrochars) were 

named SSD-HC, OFD-HC and AID-HC respectively. A sample of at least 5 kg of each hydrochar was stored in 

sealed plastic containers and kept at 4 °C before further treatments and characterization. 

During the HTC process, a portion of the solid fraction of the starting biomass is converted into a gas that is 

mainly composed of CO2 and a liquid aqueous phase. As a consequence, the amount of total solid (TS) found 

in the HTC slurries, before centrifugation, was lower than solid in the starting sludge. Eq. (1) and (2) are used 

to compute the solid (MYS) and the liquid (MYl) mass yields obtained during HTC: 
 

𝑀𝑌𝑠(%) =
𝑴𝑺−𝑺𝑳 
𝑴𝑺−𝑫𝑺 

 ∙ 100  (1) 

21



𝑀𝑌𝑙(%) =
𝑴𝒍−𝑺𝑳 𝒅𝒃
𝑴𝑺−𝑫𝑺 𝒅𝒃

 ∙ 100  (2) 

where MS-SL and Ml-SL are the dry basis masses of the solid and liquid phase in the slurries obtained during the 

HTC process respectively and the MS-DS is the mass of the solid in the digestate on a dry basis. 

The mass yield of the gas phase Mg-HC is computed by the difference:   

𝑀𝑌𝑔(%) = 100 − 𝑀𝑌𝑙 − 𝑀𝑌𝑠  (3) 

 

 
 

Figure 3: industrial centrifuge at Mezzocorona waste management plant for HTC slurry treatment 

3. Results and discussion 

3.1 Feedstock properties and characterization 

Table 1 shows the properties in terms of pH, total solids (TS), total organic carbon (TOC), total nitrogen (TKN), 

phosphorus (P) and heavy metals concentration of the feedstock before HTC. SSD and AWD raw materials 

show similar characteristics in terms of pH, TOC and P content except for the TS, the OFD sample shows a 

significantly higher pH, TS and TOC but a considerably lower, as expected, P and heavy metals content. 

Table 1:  Raw materials properties, pH, total solids TS, total organic carbon TOC, total nitrogen TKN, 

phosphorus P, and heavy metal concentrations (all values are determined on dry basis) 

Sample pH 

(U.pH) 

TS 

(wt%) 

TOC 

(wt%) 

TKN 

(wt%) 

P  

(wt%) 

Cd 

(ppm) 

Cu 

(ppm) 

Hg 

(ppm) 

Ni 

(ppm) 

Pb 

(ppm) 

Zn 

(ppm) 

SSD 7.5 

(±0.1) 

1.5 

(±0.1) 

22.1 

(±0.3) 

4.3 

(±0.2) 

2.7 

(±0.1) 

2.24 

(±0.1) 

257 

(±4.0) 

0.36 

(±0.02) 

44.0 

(±2.0) 

28.1 

(±0.1) 

810 

(±4.0) 

OFD 8.5 

(±0.1) 

7.8 

(±0.2) 

38.5 

(±0.6) 

5.1 

(±0.2) 

0.07 

(±0.01) 

0.26 

(±0.1) 

4.0   

(±0.1) 

< 0.20 

(±0.0) 

3.2 

(±0.1) 

0.2 

(±0.05) 

71.0 

(±1.0) 

AWD 7.0 

(±0.1) 

5.1 

(±0.1) 

26.0 

(±0.4) 

3.6 

(±0.1) 

2.3 

(±0.1) 

< 0.12 

(±0.01) 

440 

(±0.5) 

< 0.20 

(±0.0)  

17.2 

(±0.1) 

6.4 

(±0.1) 

232 

(±1.5) 

Table 2 reports the pH of HTC slurries and mass yields. The pH values of slurries are similar to the pH of the 

raw biomass for all the samples investigated, dropping of about 0.5 units maximum. Mass yields data show that 

between 54.8 and 63 wt% of solid mass is retained in the slurries after HTC, a lesser amount of mass yield is 

22



obtained in the gas phase (averagely between 4.4 and 5.6 wt%) while the remaining is moved into the liquid 

phase (averagely between 31.8 and 40.8 wt%). 

Table 2:  pH, solid, gas and liquid mass yields, of SSD, OFD and AWD slurries  

Sample pH MYS 

(wt%) 

MYg* 

(wt%) 

MYL 

(wt%) 

SSD-SL 7.0 

(±0.1) 

63.1 

(±2.0) 

5.1 

(±0.4) 

31.8 

(±1.6) 

OFD-SL 8.0 

(±0.1) 

54.8 

(±1.1) 

4.4 

(±0.2) 

40.8 

(±0.9) 

AWD-SL  7.0 

(±0.1) 

57.6 

(±1.3) 

5.6 

(±0.1) 

36.8 

(±1.2) 

(* computed by difference) 

Table 3 reports the data of TS, TOC, TKN, P and heavy metals concentration. The results demonstrate that 

HTC is capable of substantially increase the dewaterability of the sludge leading to TS values, upon 

centrifugation of 34.6, 42.0 and 38.6 wt% for SSD, OFD and AWD hydrochars. Notably the P recovery into 

hydrochars was 75, 78 and 85 wt% for SSD, OFD and AWD samples respectively when compared to the 

corresponding starting materials. P segregation into hydrochar during HTC has been already reported for 

sewage sludge (Volpe et al., 2020) and food waste (Idowu et al., 2017). 

Table 3:  TS, TOC, TKN, P and Heavy metal concentration in SSD, OFD and AWD hydrochars obtained after 

centrifugation of the corresponding slurries (all data computed on a dry basis). 

Sample TS 

(wt%) 

TOC 

(wt%) 

TKN 

(wt%) 

P  

(wt%) 

Cd 

(ppm) 

Cu 

(ppm) 

Hg 

(ppm) 

Ni 

(ppm) 

Pb 

(ppm) 

Zn 

(ppm) 

SSD-HC 34.6 

(±0.1) 

23.0 

(±0.5) 

3.3 

(±0.2) 

3.2 

(±0.1) 

1.25 

(±0.1) 

340 

(±5.0) 

0.36 

(±0.02) 

49.0 

(±2.5) 

44.1 

(±1.0) 

1050 

(±4.0) 

OFD-HC 41.0 

(±0.1) 

29.6 

(±0.2) 

4.4 

(±0.2) 

0.10 

(±0.1) 

1.05 

(±0.1) 

19   

(±0.1) 

< 0.20 

(±0.0) 

14.7 

(±0.1) 

1.0 

(±0.05) 

265 

(±3.0) 

AWD-HC  38.6 

(±0.2) 

23.6 

(±0.3) 

1.9 

(±0.1) 

3.4 

(±0.1) 

< 0.12 

(±0.0) 

630 

(±5.0) 

0.32 

(±0.1)  

10.3 

(±0.1) 

19.4 

(±0.2) 

450 

(±3.5) 

 

Together with P concentration, hydrochars show a considerable increase in heavy metals (in particular Cu and 

Zn for SSD-HC and AWD-HC residues). Hydrochars containing high levels of heavy metal need to be adequately 

treated before their possible use as soil amendment (Lucian et al., 2021). HTC liquid residues showed a 

chemical oxygen demand of 9550, 15560 and 9880 mgO2/L for SSD, OFD and AWD respectively. TOC values 

of starting materials and hydrochars demonstrate that relevant amount of organic carbon is moved into the 

aqueous liquid residues that can be recirculated into the anaerobic digestion system of the plant to produce 

biogas to cover part of C700 energy needs.  

4. Conclusions 

This study reports the main feature of a novel continuous operating industrial scale HTC plant developed by 

Carborem srl. C700 is a compact containerized HTC system that can be easily integrated in existing waste 

management plant capable of treating up to 5 kton/year (0.7 ton/h) of sludge with a TS between 2 and 10 wt%. 

C700 system, even at relatively mild condition, 190 °C and 1 h of residence time, promotes dewaterability of 

sludge residues, producing hydrochars with improved phosphorus content. Results show that the system is able 

to segregate up to 85 wt% of the initial P content into the HTC solid residues. The process enhances 

dewaterability of the slurry obtaining hydrochars with a total solid content, upon centrifugation, of 34.6, 38.6 and 

41.0 wt% for sewage sludge, agro-industrial sludge and organic fraction of municipal solid waste digestate, 

respectively. HTC liquid aqueous residues can be recirculated into the anaerobic digestion system of the waste 

management increasing biogas production to be used for the energy needs of C700. 

 

 Acknowledgments  

This research has been carried out under the research industrial project “Upfield-HTC”, funded by the 

Autonomous Province of Trento (L.P. 6/99).  

23



Authors gratefully acknowledge Ecoopera Soc. Coop. and Consorzio Lavoro Ambiente Soc. Coop. for the 

technical support at the waste water treatment plant, where the C-700 is located. In particular, authors 

acknowledge Maurizio and Antonio Schönsberg, Roberto Munter for their needful help in plant management. 

References  

Benavente, V., Calabuig, E., Fullana, A., 2015. Upgrading of moist agro-industrial wastes by hydrothermal 

carbonization. J. Anal. Appl. Pyrolysis 113, 89–98. https://doi.org/10.1016/j.jaap.2014.11.004 

Child, M., 2014. INDUSTRIAL-SCALE HYDROTHERMAL CARBONIZATION OF WASTE SLUDGE 

MATERIALS FOR FUEL PRODUCTION. Lappeenranta – Lahti University of Technology LU. 

Hitzl, M., Corma, A., Pomares, F., Renz, M., 2015. The hydrothermal carbonization (HTC) plant as a decentral 

biorefinery for wet biomass. Catal. Today 257, 154–159. https://doi.org/10.1016/j.cattod.2014.09.024 

Idowu, I., Li, L., Flora, J.R.V., Pellechia, P.J., Darko, S.A., Ro, K.S., Berge, N.D., 2017. Hydrothermal 

carbonization of food waste for nutrient recovery and reuse. Waste Manag. 69, 480–491. 

https://doi.org/10.1016/j.wasman.2017.08.051 

Kruse, A., Baris, D., Tröger, N., Wieczorek, P., 2013. Scale-Up in Hydrothermal Carbonization, in: Sustainable 

Carbon Materials from Hydrothermal Processes. pp. 341–353. https://doi.org/10.1002/9781118622179.ch9 

Kruse, A., Dahmen, N., 2015. Water - A magic solvent for biomass conversion. J. Supercrit. Fluids 96, 36–45. 

https://doi.org/10.1016/j.supflu.2014.09.038 

Lin, J., Mariuzza, D., Volpe, M., Fiori, L., Ceylan, S., Goldfarb, L., 2021. Integrated thermochemical conversion 

process for valorizing mixed agricultural and dairy waste to nutrient-enriched biochars and biofuels. 

Bioresour. Technol. 328, 124765. https://doi.org/10.1016/j.biortech.2021.124765 

Lucian, M., Merzari, F., Gubert, M., Messineo, A., Volpe, M., 2021. Industrial-Scale Hydrothermal Carbonization 

of Agro-Industrial Digested Sludge: Filterability Enhancement and Phosphorus Recovery. Sustainability 13, 

9343. https://doi.org/10.3390/su13169343 

Merzari, F., Goldfarb, J., Andreottola, G., Mimmo, T., Volpe, M., Fiori, L., 2020. Hydrothermal carbonization as 

a strategy for sewage sludge management: Influence of process withdrawal point on hydrochar properties. 

Energies 13. https://doi.org/10.3390/en13112890 

Miliotti, E., Casini, D., Lotti, G., Bettucci, L., Pennazzi, S., Rizzo, A.M., Chiaramonti, D., 2017. Valorization of 

solid residues from anaerobic digestion through thermal and hydrothermal carbonization process. 25th Eur. 

Biomass Conf. Exhib. 1063–1069. https://doi.org/10.5071/25thEUBCE2017-3DO.3.4 

Miliotti, E., Casini, D., Rosi, L., Lotti, G., Rizzo, A.M., Chiaramonti, D., 2020. Lab-scale pyrolysis and 

hydrothermal carbonization of biomass digestate: Characterization of solid products and compliance with 

biochar standards. Biomass and Bioenergy 139, 105593. https://doi.org/10.1016/j.biombioe.2020.105593 

Pawlak-Kruczek, H., Sieradzka, M., Mlonka-Mędrala, A., Baranowski, M., Serafin-Tkaczuk, M., Magdziarz, A., 

Niedźwiecki, Ł., 2019. Structural and energetic properties of hydrochars obtained from agricultural and 

municipal solid waste digestates, in: ECOS 2019 - Proceedings of the 32nd International Conference on 

Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems. 

Saha, N., Volpe, M., Fiori, L., Volpe, R., Messineo, A., Reza, M.T., 2020. Cationic dye adsorption on hydrochars 

of winery and citrus juice industries residues: Performance, mechanism, and thermodynamics. Energies 13, 

1–16. https://doi.org/10.3390/en13184686 

Sharma, H.B., Sarmah, A.K., Dubey, B., 2020. Hydrothermal carbonization of renewable waste biomass for 

solid biofuel production: A discussion on process mechanism, the influence of process parameters, 

environmental performance and fuel properties of hydrochar. Renew. Sustain. Energy Rev. 123, 109761. 

https://doi.org/https://doi.org/10.1016/j.rser.2020.109761 

Villamil, J.A., Mohedano, A.F., Rodriguez, J.J., De la Rubia, M.A., 2019. Anaerobic co-digestion of the aqueous 

phase from hydrothermally treated waste activated sludge with primary sewage sludge. A kinetic study. J. 

Environ. Manage. 231, 726–733. https://doi.org/10.1016/j.jenvman.2018.10.031 

Volpe, M., Fiori, L., Merzari, F., Messineo, A., Andreottola, G., 2020. Hydrothermal carbonization as an efficient 

tool for sewage sludge valorization and phosphorous recovery. Chem. Eng. Trans. 80, 199–204. 

https://doi.org/10.3303/CET2080034 

Wang, L., Chang, Y., Li, A., 2019. Hydrothermal carbonization for energy-efficient processing of sewage sludge: 

A review. Renew. Sustain. Energy Rev. 108, 423–440. https://doi.org/10.1016/j.rser.2019.04.011 

 

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