Waste Management 36 (2015) 241–255 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Co-gasification of sewage sludge and woody biomass in a fixed-bed downdraft gasifier: Toxicity assessment of solid residues Le Rong a, Thawatchai Maneerung a, Jingwen Charmaine Ng b, Koon Gee Neoh b,⇑, Boon Huat Bay c, Yen Wah Tong b, Yanjun Dai d, Chi-Hwa Wang b,⇑ a NUS Environmental Research Institute, National University of Singapore, Create Way, Create Tower #15-02, Singapore 138602, Singapore Department of Chemical and Biomolecular Engineering, National University of Singapore, Engineering Drive 4, Singapore 117585, Singapore Department of Anatomy, Yong Loo Lin School of Medicine, National University Health System, Medical Drive, Singapore 117597, Singapore d School of Mechanical Engineering, Shanghai Jiaotong University, Shanghai 200240, China b c a r t i c l e i n f o Article history: Received August 2014 Accepted 28 November 2014 Available online 19 December 2014 Keywords: Gasification Sewage sludge Bottom ash Downdraft gasifier Toxicity assessment a b s t r a c t As the demand for fossil fuels and biofuels increases, the volume of ash generated will correspondingly increase Even though ash disposal is now strictly regulated in many countries, the increasing volume of ash puts pressure on landfill sites with regard to cost, capacity and maintenance In addition, the probability of environmental pollution from leakage of bottom ash leachate also increases The main aim of this research is to investigate the toxicity of bottom ash, which is an unavoidable solid residue arising from biomass gasification, on human cells in vitro Two human cell lines i.e HepG2 (liver cell) and MRC-5 (lung fibroblast) were used to study the toxicity of the bottom ash as the toxins in the bottom ash may enter blood circulation by drinking the contaminated water or eating the food grown in bottom ash-contaminated water/soil and the toxic compounds may be carried all over the human body including to important organs such as lung, liver, kidney, and heart It was found that the bottom ash extract has a high basicity (pH = 9.8–12.2) and a high ionic strength, due to the presence of alkali and alkaline earth metals e.g K, Na, Ca and Mg Moreover, it also contains concentrations of heavy metals (e.g Zn, Co, Cu, Fe, Mn, Ni and Mo) and non-toxic organic compounds Although human beings require these trace elements, excessive levels can be damaging to the body From the analyses of cell viability (using MTS assay) and morphology (using fluorescence microscope), the high toxicity of the gasification bottom ash extract could be related to effects of high ionic strength, heavy metals or a combination of these two effects Therefore, our results suggest that the improper disposal of the bottom ash wastes arising from gasification can create potential risks to human health and, thus, it has become a matter of urgency to find alternative options for the disposal of bottom ash wastes Ó 2014 Elsevier Ltd All rights reserved Introduction Thermal processes such as incineration and gasification can be considered as sustainable technology which is mostly used for the solid waste treatment It is sustainable both in terms of waste volume reduction and a source of renewable energy However, burning of solid waste in incinerators leads to serious negative environmental and human health effects, due to emission of massive amount of toxic gases (Beylot and Villeneuve, 2013; Roy et al., 2011) resulting from the complete combustion process inside the incinerators Gasification is a process that slowly converts large ⇑ Corresponding authors Tel.: +65 65162176; fax: +65 67791936 (K.G Neoh) Tel.: +65 65165079; fax: +65 67791936 (C.H Wang) E-mail addresses: chenkg@nus.edu.sg (K.G Neoh), chewch@nus.edu.sg (C.-H Wang) http://dx.doi.org/10.1016/j.wasman.2014.11.026 0956-053X/Ó 2014 Elsevier Ltd All rights reserved molecules of organic or fossil based carbonaceous materials into smaller molecular and finally, into gaseous product which mainly consists of carbon monoxide, hydrogen and carbon dioxide (Ruiz et al., 2013; Simone et al., 2012) This is achieved by chemically reacting carbonaceous materials at high temperatures (>700 °C) with a controlled amount of oxygen and/or steam to avoid complete combustion (Samolada and Zabaniotou, 2014; Zhang et al., 2013) Therefore, gasification not only provides a cost-effective and environmental friendly way of discharging solid waste, but also produces syngas as a clean energy fuel, offering an alternative clean process for recovering energy from the waste Apart from the gaseous product, gasification also generates solid residues as unavoidable by-products, which amounts varying from 10% to 30% of the original feedstock mass, depending on efficiency of the gasifier and feedstock materials (Sabbas et al., 2003; Belgiorno et al., 2003) Depending on the carbon content of the 242 L Rong et al / Waste Management 36 (2015) 241–255 residue, it can be classified as char (unreformed carbon) or bottom ash (primarily minerals and metals with minimal carbon) Char is usually re-introduced into the gasifier to generate more energy On the other hand, bottom ash is generally disposed in landfills (Travar et al., 2009) or re-utilized as a filling material for construction applications (Forteza et al., 2004) Although bottom ash is classified as nonhazardous waste, fresh bottom ash contains various inorganic compounds, consisting mainly of oxides, hydroxides and alkali salts, which cause a high pH value of the bottom ash (Rocca et al., 2012) and trace amounts of heavy metals Moreover, the fresh bottom ash may also contain organic compounds arising from incomplete combustion of solid wastes (Liu et al., 2008) As a result, the fresh bottom ash arising from solid waste gasification can have toxic properties related to trace elements, organic contaminants and alkalinity, or a combination of these factors The improper disposal of bottom ash wastes may cause all types of pollution i.e air, soil, and water (Sivula et al 2012a; Sivula et al 2012b; Liu et al 2008) As the demand for bioenergy production increases, ash and residue volumes will increase Even though the ash disposal is now strictly regulated in many countries, the rapid increase of ash results in the high pressure on landfill sites regarding cost, capacity and maintenance Moreover, ash leachate may leak or spill from the landfill sites For example, in the Kingston Fossil Plant (Tennessee, USA) spill in 2008, 1.1 billion gallons (4,200,000 m3) of fly ash slurry was spilled from an on-site landfill, covering more than 300 acres of surrounding land and water (Dewan, 2008) Therefore, the chance of environmental pollution by ash leachate is ever increasing The ash contaminants can accumulate inside plants and animals, which can harm or kill them Up to date the effect of emissions from the gasification of solid wastes such as sewage sludge on the human health has seldom been addressed (Kabir and Kumar, 2012; Pa et al 2011), especially with regard to the effect of solid residues, because their effect may last for a long period, such as 100–1000 years However, it is quite difficult to draw firm conclusions on human toxicity, because even the best emissions data is incomplete and the true impacts of most chemicals and mixtures of chemicals are poorly understood From those points of view, the main purpose of this work is to investigate the toxicity of bottom ash arising from biomass and/or solid waste gasification The main reason is that the bottom ash arising from gasification is usually disposed in the landfills, and the leaching of toxic compounds by rain or surface water can cause severe pollution to environment and possibly endanger human health By drinking the contaminated water or eating the food grown in bottom ash-contaminated water/soil, toxic compounds may be carried all over the human body to important organs such as the lung, liver, kidney and heart Moreover, understanding of toxic properties of bottom ash wastes generated from gasification is necessary for establishing environmentally and economically benign solid wastes management In this work, the extracts from the bottom ashes arising from gasification processes of (i) pure woodchip (which is represented by PWG in the following) and (ii) 20 wt.% sewage sludge and 80 wt.% woodchips (which is represented by 20SWCG in the following) (all samples were collected from our present work) were characterized by determination of elements (i.e heavy metals, alkaline earth metals and alkali metals), polycyclic aromatic hydrocarbons (PAHs) and other organic compounds concentrations Two types of human cell lines, i.e HepG2 (liver cell) and MRC-5 (lung cell), were used to examine toxicity of the bottom ash leachates The liver cell line, HepG2 was selected as it possesses a number of characteristic enzyme pathways of human hepatocytes and is regarded as a ‘‘gold standard’’ in toxicology (Kang et al 2010; Babich et al 1988) MRC lung fibroblasts were chosen as the lungs are major organs in the respiratory system and as a large volume of blood passes through the pulmonary circulation, toxic substances which may exert deleterious effects are easily conveyed to the lungs The viability and morphology of HepG2 and MRC-5 cells were used to analyze the toxic effects on those human cells Materials and methods 2.1 Collection of sewage sludge and bottom ash samples from gasification Sewage sludge samples were obtained from the Ulu Pandan Water Reclamation Plant, under the Public Utilities Board (PUB), the National Water Agency of Singapore The sewage sludge sample from different batches was mixed, dried and ground into powders In the present study, woody biomass with an average length of 35 mm, width of 10 mm, and thickness of 2.5 mm or its mixture with the above sewage sludge sample was used as the feedstock for gasification in a 10 kW fixed bed downdraft gasifier (from All Power Labs, USA) The air flow rate investigated was from 4.0 to 10.0 L/s The temperatures in pyrolysis and reduction zones were 550–800 °C and 650–900 °C, respectively The main processes taking place in this downdraft gasifier can be described as follows:  Drying À removal of moisture from the feedstock BiomassðwetÞ þ heat ! BiomassðdryÞ þ H2 OðgÞ  Pyrolysis À thermal breakdown the feedstock into tar and charcoal BiomassðdryÞ þ heat ! Ccharcoal þ tar  Combustion and tar cracking À burning of charcoal and tar to provide heat for the rest of the processes and the thermal cracking of a portion of the tar into CO and H2 (syngas) Ccharcoal ; tar þ O2 ðfrom airÞ ! CO2 ðgÞ þ H2 OðgÞ þ heat tar þ heat ! CO þ H2  Reduction À reaction of combustion products and charcoal to produce syngas CO2 þ Ccharcoal þ heat ! 2CO H2 O þ Ccharcoal þ heat ! CO þ H2 Within the gasifier the feedstock flowed downwards by gravity and air as a gasifying (or oxidizing) gas was introduced into the combustion zone of the reactor Bottom ash samples were collected from the ash pit at the bottom of the fixed bed downdraft gasifier after the gasification of pure woodchip (PWG) and 20% sludge and 80% woodchip (20SWCG) However, due to the structure of the gasifier, some small char particles formed in the reduction zone may fall into the ash pit and subsequently be mixed with the bottom ash As char can be recycled rather than landfilled in actual gasification processes, they were removed via sieving before the toxicity test 2.2 Extraction of soluble toxic substances from samples Fig shows the process for solute extraction and toxicity test g of sample of either sewage sludge or sieved bottom ash was weighed and placed in a 50 mL tube, and mixed with 40 mL of deionized (DI) water, resulting in a liquid-to-solid (L/S) ratio of 5.0 The mixture was then vortexed for min, followed by overnight static leaching for 12 h, and centrifuged at 15,000 rpm for 10 The supernatant was transferred into a separate tube and L Rong et al / Waste Management 36 (2015) 241–255 243 Fig Schematic drawing of extraction process and in vitro toxicity test the above centrifugation step was repeated twice to remove as much particles as possible from the extracted solution The collected supernatant was then filtered using a filter with a pore size of 0.2 lm (Minisart, Sartorius AG, Germany) This not only removes the tiny particles originally in the sample but also helps to sterilize the exaction solution by removing almost all of the bacteria Based on experience, the filtration method is a more effective method for disinfection than UV irradiation of the sample powders Since the bottom ash extract was highly basic (as discussed below), a neutralization step was adopted to avoid possible denaturation of proteins in the cell culture medium This was done by gradually adding 0.3 M nitric acid into the extract until its pH value reached 8, close to the pH value of the cell culture medium used, during which no formation of any precipitates was observed This implies that the amount of dissolved metals in the leachate did not change substantially, which is consistent with the findings by Meima and Comans (1997, 1999) The neutralization step was not needed for extraction of sewage sludge since the solution had a pH value of about 8.4 After the neutralization, the extract was freeze-dried and the solute powder was then re-dissolved in cell culture medium to the original volume (30 mL) before freeze-drying The asobtained solution, with designated solute concentration of C0, was diluted with fresh medium to prepare extract solutions of different concentrations, such as C/C0 = 0.5, 0.25, etc 2.3 Characterizations of the bottom ashes and their extractions 2.3.1 Scanning electron microscopy (SEM) analysis Morphology of the bottom ashes arising from PWG and 20SWCG gasification was investigated by Scanning Electron Microscopy (SEM, JEOL2872), carried out using an electron beam of 15 kV with magnifications in the range of 2000–5000Â 2.3.2 X-ray diffraction analysis Crystalline phases of bottom ashes arising from PWG and 20SWCG gasification were characterized by powder X-ray diffraction using SHIMADZU XRD-600 diffractometer with Cu Ka radiation (k = 0.154 nm), operated at 40 kV and 30 mA Data was collected at 0.02° with s per step, in the 2h range of 20–50° 2.3.3 Elemental analysis The elemental analyzer (Elementar Vario Micro Cube) was used to determine the content of carbon (C), hydrogen (H), nitrogen (N) and sulfur (S) in the feedstock materials The percentage by mass of C, H, N and S was determined and the oxygen (O) mass percent was calculated by difference 2.3.4 Inductively Coupled Plasma (ICP) analysis Inductively coupled plasma analysis optical emission spectroscopy (ICP-OES) was employed for the detection of trace metals present in the feedstock, bottom ash product and its respective cell culture medium extracts Dual-view Optima 5300 DV ICP-OES system was used, which has a minimum detection level of 0.1 ppm Each sample was digested with HNO3/HCl and topped up to 10 ml with H2O before being analyzed The metals chosen for detection were Ti, Ag, Zn, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Al, Hg, As and Mo for bottom ashes and their cell culture medium extracts 2.3.5 pH value and conductivity measurement The pH value of each extract solution was measured using a Lab 850 pH meter (SCHOTT Instruments GmbH, Mainz, Germany) The conductivity of solution was measured using a Thermo Scientific™ Orion™ 3-Star Plus Conductivity Benchtop Meter (Thermo Fisher Scientific, MA, USA) Each measurement was repeated with three samples 2.3.6 Gas chromatography–mass spectrometry (GC–MS) The analysis of PAHs in the extracts was carried out using gas chromatography coupled with mass spectrometry (GC–MS) analysis (Shimadzu GCMS-QP2010 Plus with AOC-5000 auto injector with an Agilent J&W DB-5 ms column) Bottom ash samples were directly extracted using acetonitrile (ACN), with the same method as the cell culture extractions A mixture of methanol (MeOH) and dichloromethane (DCM) in volume ratio 1:1 was used to further extract organic compounds from the aqueous cell culture extracts The following conditions were used for the GC–MS analysis: Constant flow of helium carrier gas at 45 cm/s; inlet at 300 °C, 40 psi until 0.2 and purge flow 30 ml/min at 0.75 min; oven temperature between 55 °C to 320 °C at 25 °C/min and held min; and a scan range of 50–350 g/mol A standard solution, M-610A, containing a mixture of the 16 priority PAHs (AccuStandard, New Haven, USA) (including Acenaphthene, Anthracene, Benzo(a)anthracene, Fluoranthene, Naphthalene, Phenanthrene, Benzo(a)pyrene, Benzo(b)fluoranthene, Benzo(ghi)perylene, Acenaphthylene, Benzo(k)fluoranthene, Dibenzo(a,h)anthracene, Indeno(1,2,3-cd)pyrene, Chrysene, Fluorene and Pyrene) was used as calibration standard in GC–MS analysis Post-run analysis of chemical components of each sample was carried out using the NIST library in the system 244 L Rong et al / Waste Management 36 (2015) 241–255 Char Bottom ash (b) (a) Fig SEM photographs of (a) char and (b) bottom ash arising from gasification process 2.4 Culture of MRC-5 lung fibroblasts and HepG2 cells in 96-well plate MRC-5 lung fibroblasts (from ATCC, USA) cultured in a cell culture flask, were harvested using trypsin and re-suspended in medTable Elemental analysis of bottom ashes obtained from gasification of PWG and 20SWCG Samples Bottom Bottom Bottom Bottom ash ash ash ash Elemental composition (weight %) from from from from PWG > 500 lm PWG < 500 lm 20SWCG > 500 lm 20SWCG < 500 lm C H N S 68.96 26.03 53.91 19.92 2.16 0.94 1.93 1.09 1.25