© Springer Nature Switzerland AG 2019

Liselotte Schebek, Christoph Herrmann and Felipe Cerdas (eds.)Progress in Life Cycle AssessmentSustainable Production, Life Cycle Engineering and Managementhttps://doi.org/10.1007/978-3-319-92237-9_7

Hydrothermal Carbonization (HTC) of Sewage Sludge: GHG Emissions of Various Hydrochar Applications

Fabian Gievers^{1, 2}  , Achim Loewen¹ and Michael Nelles^{2, 3}

(1)

Faculty of Resource Management, HAWK-University of Applied Sciences and Arts, Rudolf-Diesel-Straße 12, 37075 Göttingen, Germany

(2)

Faculty of Waste and Resource Management, University of Rostock, Justus-von-Liebig-Weg 6, 18059 Rostock, Germany

(3)

DBFZ-German Biomass Research Centre, Torgauer Str. 116, 04347 Leipzig, Germany

 

 

Fabian Gievers

Email: fabian.gievers@hawk.de

Abstract

Sewage sludge contains valuable nutrients like phosphorus (P) as well as a whole series of harmful substances. Therefore, conditioning should be designed to remove those pollutants. In Germany sewage sludge is treated mainly at thermal facilities such as sewage sludge mono-incineration plants, cement plants or coal fired power plants. However, ecological impacts of new treatment methods like hydrothermal carbonization (HTC) remain unknown. In the study presented in this paper, the complete life cycles of the carbonization process of sewage sludge (5% dry matter) with associated auxiliary flows (e.g. electricity and naturals gas) and different applications of the produced char were modelled. In order to identify the environmentally most promising and sustainable application, four different scenarios of hydrochar utilization as fuel or fertilizer were analyzed. The resulting global warming potentials (GWP) after ReCiPe midpoint methodology were calculated. Results show that the best scenario in environmental terms has savings of 0.074 kg CO₂ eq/kg. The highest emissions were observed for the agricultural use of hydrochar as a substitute for NPK-fertilizer with 0.025 kg CO₂ eq/kg, which even outnumbers the emissions of the benchmark process chain of sewage sludge mono-incineration (0.013 kg CO₂ eq/kg). Results underline the sustainability of hydrothermal carbonization of sewage sludge as compared to sewage sludge mono-incineration.

Keywords

Life cycle assessmentHydrothermal carbonizationSewage sludgeHydrocharGlobal warming potentialSustainable sludge management

1 Introduction

The growing demand for fertilizers due to a growing world population increases the pressure on limited natural nutrient resources. In particular, the supply routes of phosphorus, a non-renewable resource that could be depleted in 50 to 100 years, need to be reconsidered (Cordell et al. 2009; Sartorius et al. 2011). Recycling of materials with high phosphorus content must therefore be improved in order to secure the livelihoods of future generations. Sewage sludge is an important natural and locally available phosphorus source (Cordell and White 2011; Schoumans et al. 2015; Klinglmair et al. 2015). Existing treatment methods such as co-combustion in the cement industry or co-incineration in lignite-fired power plants reduce the P concentration in the ash and thus make phosphorus extraction more difficult and expensive. An ideal technology should offer maximum P-recovery rates, removal and destruction of potentially hazardous substances such as heavy metals, organic micropollutants and pathogens, good fertilising properties of the product, a good profitability and low environmental risks. (Egle et al. 2016; Leinweber et al. 2018). Therefore the Hydrothermal Carbonization (HTC) of sewage sludge is investigated as a promising approach for implementing a circular economy for nutrients (Brookman et al. 2016; Heilmann et al. 2014; Zhao et al. 2017) and a sustainable energy generation (Stucki et al. 2015; Libra et al. 2011; Titirici et al. 2007) with simultaneous decrease of pathogens and other organic pollutants (Vom Eyser et al. 2015, 2016; Weiner et al. 2013). HTC is a thermochemical process in aqueous phase under saturated pressure and temperatures between 160 and 250 ℃. Typically, over several hours biomass is converted into a valuable solid coal (hydrochar), partially dissolved fractions in the aqueous phase and a small amount of gases (usually CO₂) (Berge et al. 2011; Bergius 1932; Kruse et al. 2013; Funke and Ziegler 2010). In recent years, there has been a growing interest in industrial applications of HTC as a waste treatment method and in the usability of the produced hydrochar (Hoekman et al. 2013; Buttmann 2011; Stucki et al. 2015). To evaluate the sustainability of this new approach, life cycle assessment (LCA) studies of HTC of different biomass feedstocks were performed (Owsianiak et al. 2016; Benavente et al. 2017; Liu et al. 2017; Stucki et al. 2015; Berge et al. 2015). In this study, an LCA of HTC of sewage sludge digestate and four different hydrochar applications were carried out an the results were compared to the usual process chain of sewage sludge mono-incineration with subsequent ash landfilling. The main goal was to identify the best utilization for hydrochar from sewage sludge characterised by the lowest CO₂ footprint in comparison to the benchmark process of mono-incineration.

2 Methods

2.1 Material and Energy Flows

A model was set up for the HTC process and used to examine the carbonization of sewage sludge with a dry matter (DM) content of 5% (organic solids content in dry matter (oDM): 48%) after anaerobic digestion in an existing wastewater treatment plant (WWTP) (Table 1).

Table 1

Model parameters for sewage sludge and carbonization conditions

Parameter	Unit	Value
Sewage Sludge
Dry matter (DM)	(%)	5
Organic dry matter (DM)	(% of DM)	48
Specific heat capacity	(kJ kg−1 K−1)	4.58
Sludge input temperature	(℃)	38
Hydrothermal Carbonization
Carbonization temperature	(℃)	220
Duration	(h)	4

• Hydrothermal Carbonization Parameters:

The process parameters of the HTC were set to the following values in the model (Table 1):

• Material flows:

Every sewage sludge comes with a certain load of inorganics and heavy metals. While organic compounds react during the process, heavy metals cannot be destroyed and accumulate in the solid fraction (Yue et al. 2016). Since their accumulation has a toxic risk potential, the concentrations of heavy metals have to be carefully observed (Libra et al. 2011). Beside inorganic hazardous substance sewage sludge contain organic pollutants such as polychlorinated dibenzo-dioxins (PCDD) and polychlorinated dibenzo-furans (PCDF), polychlorinated biphenyl (PCB), pharmaceuticals or pesticides. These substances can be degraded or reformed, regenerated and accumulated in certain fractions through hydrothermal process conditions (Weiner et al. 2013; Tirler and Basso 2013). Therefore, hydrothermal carbonsiation is a good way to treat sewage sludge in order to destroy harmfull organics while saving the availability of nutrients, such as N, K and P. In this study flows and accumulation of heavy metals, organic pollutants and valuable nutrients were modelled to identify the best utilization of hydrochar from sewage sludge. Carbonization of sewage sluge with very high amounts of water mainly results in process water, which contains soluable organics, nutrients and inorganics. In the model, the treatment of the generated process liquid in a WWTP, characterized through investigated concentrations of C, N and P, was implemented.

• Energy flows:

The thermochemical modelling of the energy flows of the carbonization process was carried out with a mathematical modelling approach. Initially, the specific heat capacities of the individual parts of the sewage sludge (water, organic and inorganic substances) were used to determine the energy requirements for the carbonization process. The results were confirmed with literature data. Other energy flows, such as the electricity required for the dewatering process, were also taken from the literature. The aggregated processes of system extension to include emission credits have been taken from the above-mentioned databases. The data for the benchmark process of mono-incineration was taken from generic data by econinvent.

2.2 Life Cycle Assessment

The LCA was conducted in accordance with the requirements of the ISO standard 14044:2006 (DIN EN ISO 14044 2006).

• Functional Unit:

The main function of the hydrothermal carbonization of sewage sludge is to stabilise the sludge produced. In order to compare the HTC of sewage sludge with mono-incineration, the functional unit was defined as follows:”Treatment of 1 kg of sewage sludge from anaerobic digestion with a dry matter content of 5%”.

• System Boundaries:

The boundary of the system includes the carbonization of digested sewage sludge, possible transportation and storage, power and heat generation and char application. For comparison purposes the benchmark process of sewage sludge mono-incineration was also investigated. The examined carbonization of sewage sludge covered the construction and decommissioning of the HTC-plant, the actual carbonization process of the sludge, linked energy and equipment provisions as well as the separation, storage and transportation of hydrochar by truck to the respective location considered in the four different scenarios. The energy for the HTC and the filter press for separating hydrochar from process water were provided as electricity for pumping the sludge and natural gas combustion for providing the necessary process-heat. Treatment of HTC process water was performed in a WWTP on side, characterized by C, N and P-content. The utilization of hydrochar as fuel or fertilizer was compared to combustion and application of fossil-based products: NPK-fertilizer, peat, municipal solid waste (MSW) and lignite. Therefore, the avoided burden approach was performed to consider the emissions of processes replaced by the HTC process chains. Substitution of lignite and municipal solid waste (MSW) was balanced by the energy content of the hydrochar for co-combustion in power plants using existing incineration capacities. For avoided NPK-fertilizer application the nutrients content of hydrochar was taken into account. Credits for peat in horticulture were calculated as substitution by weight. Altogether, four different utilization paths were analyzed and compared regarding emissions of appropriate benchmark processes (Fig. 1).

Fig. 1

Different ulitizations scenarios of hydrochar with system boundaries of process chains and benchmark scenario

• Geographic scope:

Although sustainable nutrient recycling is not bound to a specific region, Germany served as a model region for the assessment. All data concerning hydrochar and sewage sludge characterization were taken from different former studies located in Germany. The background processes such as electricity mix or natural gas supply, which were taken from the database of GaBi, were also based on german backround processes. If background data were not available for Germany, either European or Swiss data were used.

• Modeling Framework:

Since HTC is not yet a market-penetrating technology for treating sewage sludge, and the production and use of hydrochar as a fuel or substrate is unlikely to result in any structural changes in the near future, an attributional approach was applied in the assessment for the foreground system of HTC. In cases of processes with substitution of commodities, credits were accounted.

• LCA-software:

The product systems were modeled in the LCA software GaBi 8.1 (thinkstep AG, Leinfelden-Echterdingen, Germany).

• Life Cycle Inventory (LCI) data:

The LCI datasets provided by GaBi and ecoinvent (v3.3) (Wernet et al. 2016) were used as data background. Some data concerning the HTC-Process and some auxiliary flows were either estimated, calculated or taken from literature. For example, process parameters of HTC-plant were first determined based on thermodynamic calculations and then reconciled with data of two pilot scale HTC plants.

• Life Cycle Impact Assessment (LCIA):

Life Cycle Impact Assessment (LCIA) was performed using the ReCiPe midpoint methodlogy 2016, as implemented in GaBi TS. In this paper the focus was on gobal warming potential (GWP) measured in kg CO₂ eq (excl. biogenic carbon) with a Hierarchist (H) perspective, which is based on the most common policy principles and uses a medium time frame of 100 years (Huijbregts et al. 2017).

• Assumptions and limitations:

Due to the lack of data from HTC plants and hydrochar users on an industrial scale, the material flows (including transport routes and the weighting and use of modifications from generic data) were determined on the basis of reasonable assumptions and data from pilot plants and literature. In addition, the geographical scope only includes energy data from Germany and there were only four scenarios modelled for the use of hydrochar.

3 Results and Discussion

• Benchmark Process:

The benchmark process of sewage sludge digestate mono-incineration was based on a process from the generic ecoinvent database: [Jungbluth, N., treatment of digester sludge, municipal incineration, future, CH, Substitution, consequential, long-term, ecoinvent database version 3.3]. The functional unit of the process refers to the mono-incineration of 1 kg of wet sludge with 95% water and is therefore comparable to the results of the HTC model. For the mono-combustion of sewage sludge, 0.013 kg CO₂-equivalent/kg of sewage sludge were determined as benchmark emissions. In this process, all relevant flows were considered, in particular the dewatering of the sludge and all auxiliary flows associated with the sludge treatment. Credits were giving for energetic output of the incineration process (electricity mix of Germany). As the current state of commercial HTC has not reached market penetration, a process with future energy demands and emission reductions has been chosen for the benchmark process.

• Emissions associated with HTC:

The distribution of emissions for the entire HTC process was analyzed to identify the processes with the highest environmental impact (Fig. 2). The generation of electricity for running the HTC plant and auxiliary processes lead to the main part of total CO₂ eq emissions (63%). In addition to power demand, the heat supply by natural gas resulted in 28% of the emissions. These results underline the importance of the water content of sewage sludge for optimizing the environmental performance of HTC plants. The energy consumption for heating up the carbonization process and the energy for pumping increases with the water content of the feedstock (Owsianiak et al. 2016). Furthermore, a broad spectrum of energy consumption of dewatering technologies can be observed, also for the same technology (Yoshida et al. 2013). Therefore, further investigations in large-scale HTC plants should determine the optimal process parameters for dewatering before or after the carbonization process. One option would be the firing of HTC reactors with biogas from the wastewater treatment plant to replace natural gas. A further energetic optimisation of the HTC would be to increase the dry matter content of sewage sludge before carbonization. The third relevant source of CO₂ eq emissions of HTC is the treatment of the process water (9%). By changing the process water treatment from aerobic to anaerobic, emissions could be further reduced (Wirth et al. 2012). Overall, the treatment of sewage sludge with HTC resulted in emissions of 0.051 kg CO₂ equivalent per kg of sewage sludge (Fig. 3).

Fig. 2

Distribution of GHG emissions for HTC process

Fig. 3

Greenhousegas emissions of examined scenarios

• Hydrochar utilization scenarios:

The net emissions derived from HTC, application of the hydrochar and credits are distributed as follows (Fig. 3): The first scenario of agricultural use of hydrochar has total emissions of 0.025 kg CO₂ equivalent per kg of sewage sludge. The credits are the lowest of all scenarios due to the relatively low nitrogen content of hydrochars. By substituting NPK fertilizers in agriculture, the greatest greenhouse gas emissions could be saved by the avoided production of artificial nitrogen fertilizers. In addition, the agricultural use of hydrochar causes the highest emissions of all application-related CO₂ emissions. However, in comparision to the HTC emissions and the credits these emissions are relatively low and mainly derive from transportation, storage and handling activities. The second scenario leads to a saving of 0.0024 kg CO₂ equivalent per kg of sewage sludge, mainly from credits for the substitution of peat in commercial horticulture. For the third scenario of co-firing of hydrochar in waste incineration overall savings of 0.015 kg CO₂ eq per kg of sewage sludge were observed. These savings mainly depend on the substitution of fossil-based fractions of municipal solid waste such as plastics. Finally, co-firing of hydrochar in lignite power plants leads to the highest savings of 0.074 kg CO₂ eq per kg of sewage sludge.

4 Conclusion

The LCA results of HTC of sewage sludge showed that substituting fossil based fuel (lignite and parts of MSW) with hydrochar had the highest potential to reduce global warming potential (GWP) of sewage sludge treatment. In comparison to the benchmark process of mono-incineration, it is even possible to achieve negative emissions. In general, the use of hydrochar for energy purposes had a higher GHG saving potential than their use in agriculture or horticulture. An improvement in the process design of sludge treatment with HTC would be the implementation of phosphorus extraction, which could enable phosphorus recycling even in the case of energetic use of hydrochar and therefore be helpful to boost the hydrothermal carbonization of sewage sludge. However, HTC process optimization should also take other LCIA categories into account. In addition, improvements in LCIA methodology are needed in order to evaluate benefits of the sludge treatment process, such as pathogen reduction and recycling of organic matter and nutrients back to agricultural or horticultural soil. Overall, the results show that there is a more sustainable alternative to the mono-incineration of sewage sludge.

Acknowledgements

This publication is a result of a project funded with federal state resources from the “Niedersächsisches Vorab”.

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