Research Article pubs.acs.org/journal/ascecg
Continuous Anaerobic Degradation of Liquid Condensate from Steam-Derived Hydrothermal Carbonization of Sewage Sludge Benjamin Wirth*,†,‡ and M. Toufiq Reza†,§ †
Leibniz Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany Technische Universität Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany § Department of Mechanical Engineering, Ohio University, 1 Ohio University, Athens, Ohio 45701, United States ‡
ABSTRACT: Hydrothermal carbonization (HTC) at an industrial level most likely will use steam as heating media. Excess steam will yield a condensate rich in water-steam volatile organic compounds. This experimental study investigated the use of HTC condensate as the sole substrate for anaerobic digestion (AD). Process performance at 37 °C (mesophilic) and 55 °C (thermophilic) was determined with two identical anaerobic filters (26 L each). Both reactors were continuously fed for five weeks at a constant organic loading rate of 0.8 gCOD L−1 d−1. After a start-up of 15 days, both reactors yielded up to 0.22 LCH4,STP gCOD−1. Increased temperature had slightly negative impacts on phenol removal and COD degradation. Macro- and micronutrients were not found in sufficient concentrations in the HTC condensate and could cause deficiencies during long-term operation. Presumably, co-digestion of the HTC condensate together with its corresponding process liquor could alleviate these problems and therefore seems to be an easy and beneficial solution. KEYWORDS: Anaerobic digestion, Biogas, Liquid byproducts, Nutrients, Phenol, Steam, Waste biomass
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INTRODUCTION Hydrothermal carbonization (HTC) of biomass has gained more and more attention in recent years as an intermediate treatment to widen the field of application of highly wet biomass. Hydrochar, the solid product of HTC, is interesting to use as a solid fuel as it can be mechanically dewatered to a high extent, pelletized, and transported at lower cost due to increased energy density.1−3 Furthermore, hydrochar is discussed as a base material for activated carbon materials and catalysts, energy storage, and soil amendment.1,4 Most HTC research focused on maximizing energy yield in terms of solid hydrochar and often overlooked its byproducts. Liquid water acts both as reaction medium and catalyst during HTC, as it shows maximum ionic strength around 180− 280 °C, helps dissolve the feedstock, and facilitates liquid state reactions.1 HTC process liquor is therefore an inevitable byproduct, and around 15% of the feedstock carbon remains in the liquid state primarily as acetic and formic acid.5 Effective anaerobic digestion (AD) of the process liquor to generate methane and degrade part of the organic load was reported elsewhere.6−11 Lab-scale batch HTC experiments often apply surface heating, while industrial-scale uses hot steam generated onsite with natural gas, part of the hydrochar, or biogas along with excess heat from nearby processes.12 Excess steam is often applied to maintain a steady reaction temperature, which is condensed afterward in a flash tank.12 As a result, various polar compounds formed during HTC are found in the condensate. Furthermore, toxic compounds like phenol, furfural, and their derivatives were observed in concentrations of up to few grams © XXXX American Chemical Society
per liter in HTC process liquor and most probably concentrate further in the resulting condensate.13,14 The overall aim of this study was to determine the process performance of HTC condensate AD at thermophilic (55 °C) and mesophilic (37 °C) conditions. The major focus was hereby put on process stability in terms of methane productivity and potential inhibition. AD performance was compared with AD of HTC liquor in order to recommend the feasibility of mixing both liquid streams prior to AD.
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EXPERIMENTAL SECTION
Feedstock Acquisition. The HTC condensate used in this study was obtained from an HTC experiment as reported in a previous study.15 A 3 m3 demonstration-scale batch-operated HTC plant at Leonberg, Germany, was used to process 754 kg of digested sewage sludge at 200 °C for 6 h. Excess steam was condensed throughout the experiment. Approximately 100 L of condensate was transferred to a barrel made of HDPE, transported to the Leibniz Institute for Agricultural Engineering Potsdam (Germany), and stored at 4 °C to avoid aerobic degradation. AD Experimental Setup and Procedure. This study was carried out using the same experimental setup for a previous study15 using two identical anaerobic filter (AF) systems with a liquid working volume of 26 L each connected to a liquid buffer tank of an additional 15 L in size. Both reactors already contained its liquid working medium from the previous experiment with a total solids (TS) content of 1.8%wt and a volatile solids (VS) content of 54.0%TS (1.0%wt). Levels of total Received: December 2, 2015 Revised: January 14, 2016
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DOI: 10.1021/acssuschemeng.5b01607 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
Table 1. Comparison of Chemical Characteristics of HTC Condensate and Corresponding HTC Process Liquor Using Sewage Sludge as Feedstock parameter [unit]a
HTC condensate
HTC process liquor15
4.3 0.1 77.7 12.7 3.7 0.1 0.2 1.4 124.8
