Elsevier

Bioresource Technology

Volume 177, February 2015, Pages 318-327
Bioresource Technology

Hydrothermal carbonisation of sewage sludge: Effect of process conditions on product characteristics and methane production

https://doi.org/10.1016/j.biortech.2014.11.096Get rights and content

Highlights

  • Effects of temperature and time on product characteristics were investigated.

  • High temperature and shorter time favoured hydrochar carbon and energy properties.

  • Response surface methodology was used to optimise sewage sludge carbonisation.

  • Estimated methane yields decreased as reaction temperature and time increased.

Abstract

Hydrothermal carbonisation of primary sewage sludge was carried out using a batch reactor. The effect of temperature and reaction time on the characteristics of solid (hydrochar), liquid and gas products, and the conditions leading to optimal hydrochar characteristics were investigated. The amount of carbon retained in hydrochars decreased as temperature and time increased with carbon retentions of 64–77% at 140 and 160 °C, and 50–62% at 180 and 200 °C. Increasing temperature and treatment time increased the energy content of the hydrochar from 17 to 19 MJ/kg but reduced its energy yield from 88% to 68%. Maillard reaction products were identified in the liquid fractions following carbonisations at 180 and 200 °C. Theoretical estimates of the methane yields resulting from the anaerobic digestion of the liquid by-products are also presented and optimal reaction conditions to maximise these identified.

Introduction

It is estimated that between 0.6 and 1 billion tons of human faeces are generated each year (Sobsey, 2006), a figure which is set to increase because of the predicted growth in population. In the developing countries, over 90% of sewage is discharged untreated (Langergraber and Muellegger, 2005). Faecal contamination of water sources causes almost 4 billion cases of diarrhoea globally each year, killing nearly 2.2 million children the under age of five (WHO/UNICEF/WSSCC, 2000). In those regions of the world where sewage treatment is routinely undertaken management of sewage sludge continues to present environmental and health challenges.

The prospects for recovering energy from sewage and waste sludges have recently received renewed interest (Goto et al., 2004, Mumme et al., 2011, Zhao et al., 2014). Hydrothermal carbonisation (HTC) is as an effective method for converting wet biomass at relatively mild reaction temperatures into a coal-like material commonly referred to as ‘hydrochar’ along with aqueous products and gases-primarily CO2 (Funke and Ziegler, 2010, Libra et al., 2011). When applied to sewage sludge, HTC results both in sanitisation of the sludge and in its stabilisation (Catallo and Comeaux, 2008). The hydrochar, which is typically the main product, has H/C and O/C ratios comparable to to that of low-grade coal, but a higher calorific value than such coals (Demir et al., 2008) and can therefore serve as a potential fuel source. Alternatively, hydrochar is a carbon-rich compound that can be added to soils as a conditioner (Libra et al., 2011) and as it is slow to oxidise provides a means for sequestering carbon that would otherwise be released into the atmosphere as greenhouse gases (Titirici et al., 2007). Other options for the solids produced include biofuel production by transesterification (Levine et al., 2013), VFA extraction (Wood et al., 2013, Kaushik et al., 2014), and gasification for syngas production (Castello et al., 2014). These options require different ratios of O/C in the hydrochars generated which requires the tailoring of conditions during carbonisation in order to achieve them.

The aqueous products from HTC contain organic compounds such as furans, phenols, acetic acid, levulinic acid, and other soluble organic compounds (Goto et al., 2004, Berge et al., 2011, Wang et al., 2012). The discharge of wastewaters rich in such organic compounds would pose serious environmental challenges to receiving waters (Arora and Saxena, 2005) and therefore some means of treating such wastes is essential. The formation of difficult to treat coloured organic compounds through the Maillard reaction during hydrothermal treatment (HT) of sludges has been reported (Penaud et al., 1999). However, the conditions leading to the production of Maillard products have not yet been fully elucidated.

The majority of studies into HTC have to date focused on the chemistry and mechanisms of the process, and how it can be used to produce various solid products for carbon sequestration when applied to land or as an energy source (Titirici et al., 2007, Funke and Ziegler, 2010, Libra et al., 2011) rather than viewing HTC as an alternative waste treatment process. The application of HTC to achieve volume reduction and sterilisation of animal and municipal wastewater solids has been reported (Catallo and Comeaux, 2008), as have those for evaluating product characteristics (Berge et al., 2011). However, the effects of the operating conditions on product characteristics have not been fully investigated.

The potential to produce methane from the liquid products separated from the hydrochar following HTC of biomass has not been previously studied and reported. Hence, a more comprehensive study into methane production from HTC liquid product is required in order to identify the optimum conditions for highest yields. In the work described here, the effects of process temperature and reaction times during the HTC of primary sewage sludge were investigated. Results are presented of hydrochar characteristics in terms of energy contents, physical and chemical properties, wastewater quality indicators of the liquid products and of the nature of dissolved organic compounds, composition of gaseous products, and finally of the potential for methane production from the liquid products.

Section snippets

Primary sewage sludge

Primary sewage sludge (PSS), was collected from Wanlip Sewage Treatment Works (Leicestershire, UK) in an enclosed container. PSS primarily comprises faecal matter removed by settlement. The PSS typically contained 4.3% (wt.) solids as received. The PSS was stirred manually before use as the solids had a tendency to settle. The physical and chemical characteristics associated with the PSS feedstock are shown in Table 1.

Experimental design

Response surface methodology (RSM) by means of a central composite rotatable

Results and discussion

A series of randomised HTC runs were carried out to investigate the effects of reaction temperature and reaction time on the physical, chemical and thermal characteristics of the hydrochars produced, as well as those of the liquid- and gas-phase products.

Conclusions

Reaction conditions exert a significant effect on the characteristics of all the products obtained by hydrothermally carbonising primary sewage sludge. Models were developed here which could aid in the identification of reaction conditions to tailor such products for specific end uses. Hydrothermal carbonisation at 180 °C for 60 min and 200 °C for 30 min resulted in hydrochars having optimal characteristics and also for obtaining optimum carbon recovery in the liquid by-product. These reaction

Acknowledgements

This research was part of Gates Foundation: “Reinventing the Toilet Challenge”. The authors are thankful to Geoffrey Russell for his help with collection of primary sewage sludge.

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