Abstract
Nowadays the sludge treatment is recognized as a priority challenge to the wastewater industry due to the increasing volumes produced and tighter environmental controls for its safe disposal. The most cost-effective process for sewage sludge is the anaerobic digestion but raw digestate still contains high levels of organic matter that can be transformed into an energy carrier by using processes like Hydrothermal Carbonization (HTC). In this work, the influence of solid loading (2.5, 5.0, 10.0, 15.0, 17.5, 20.0, 25.0 and 30.0% solids w/w) on the composition of hydrochar and process water was studied, together with an evaluation of product yields, solubilisation of organic carbon and biomethane potential of process waters from HTC processing (250 °C, 30- minute reaction time). Hydrochar yields ranged from 64 to 88%wt, whereas the concentration of soluble organic carbon increased from 2.6 g/L in the raw digestate to a maximum of 72.3 g/L in the process water following HTC at the highest solid loading. Furthermore, process modelling with Aspen Plus shows that the integration of AD with HTC to wastewater treatment works provides a significant positive energy balance when process water and hydrochar are considered as fuel sources for cogeneration.
Highlights
Sewage sludge (SS) is produced as part of routine operations at wastewater treatment works (WWTWs) and its management is still an important global issue due to the large amounts generated on a daily basis [1]
These values were slightly lower compared with the study carried out by Zabaleta et al [12], who reported mass losses between 2.3 and 7.1% when food waste was under Hydrothermal carbonization (HTC) processing at different solid loadings and different temperatures (180e200 C); mass losses were attributed to the production of gaseous components, mainly CO2 [12]
The process waters were influenced by the solid loading increasing the concentration of carbon, nitrogen and phosphorus compounds, which increased the potential for resource recovery from sewage digestate
Summary
Sewage sludge (SS) is produced as part of routine operations at wastewater treatment works (WWTWs) and its management is still an important global issue due to the large amounts generated on a daily basis [1]. In the UK, 1.4 million tonnes of sewage sludge (dry weight) are produced annually and around 75% of that undergoes anaerobic digestion (AD) [2]. The resulting sewage digestate is still rich in organic matter and it has the potential to be used as a feedstock for the production of solid energy carriers [1,3e7]. HTC products include process waters rich in organic compounds suitable for anaerobic digestion and a charcoal like material (hydrochar) that can be used either as a solid fuel or as a soil amender [1,6,8,10]. Hydrochars often have a higher energy density than the feedstock due to deoxygenation [8] and process waters tend to concentrate soluble organic matter and nutrients like nitrogen and phosphorus compounds [11]. The specific hydrochar and process water characteristics are highly dependent on the choice of feedstock and process conditions [9,12]
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