Influence of pyrolysis temperature on physical and chemical properties of biochar made from sewage sludge
Introduction
Sewage sludge is one type of biomass residue resulting from wastewater treatment plants, and is composed of organic compounds, macro and micronutrients, trace elements, micro organisms and micro pollutants [1], [2]. More than 25 million of sewage sludge (moisture content of approximate 80%) has been annually produced in China [3], it is crucial to treat sewage sludge with an economically and environmentally acceptable manner. Agricultural utilization might recover nutrients contained in sewage sludge, incineration can recover energy, and both of them have large treatment capacity [4], [5]. However, the subsequent environmental contamination limits their extensive applications. For instance, noxious viruses, pathogens, ova and heavy metals might present hazards for plants and animals, and further jeopardize human health when sewage sludge is direct applied in soils [4]. The releases of dioxin, NOx, SO2, and heavy metals during sewage sludge incineration also result to serious air pollutions [5]. Pyrolysis is an alternative choice to sewage sludge disposal due to some specific advantages, for instance, the pyrolysis temperature may decompose organic pollutants and kill pathogens, oxygen-limited atmosphere restrains the generation of pollutants released in the incineration process [6], [7].
Syngas, bio-oil and solid residue are the three different pyrolysis products. Syngas and bio-oil can be used as fuel or industrial chemicals [8], [9], and the solid residue (biochar) has been regarded as a very promising soil amendment since the remarkable discovery of “terra preta” soils in Amazonia [10]. The biochar has significant effect on soil sustainable fertility because the former could improve soil organisms, soil water retention [11], [12], [13] and soil aggregation and aeration, reduce acidity, enhance nutrient retention and availability [14], [15], and abate the bioavailability of some detrimental substances, such as heavy metals [16], [17], pesticide residues and organic contaminants [18], [19].
Extensive studies [17], [20], [21], [22] have shown that the effects of biochar on improving soil fertility depend on its physical and chemical properties, such as being rich in aromatic carbon and difficult to decompose chemically and biologically, having high porosity, being alkaline and containing plentiful nutrients. Among the pyrolysis conditions, such as temperature, residence time and heating rate, temperature has the largest effect on physical and chemical properties of biochar. The previous researches on biochar derived from cottonseed hulls indicated that elevating temperature increased the specific surface area [17], and could reduce biochar acidity [20], [21]. However, a higher CEC (cation exchange capacity) was observed in biochars derived from poultry litter, peanut hulls and pine chips at a lower pyrolysis temperature [22].
As shown above, most of the researches were about biochars derived from agricultural and forestry residuals. However, still some attention was paid on sewage sludge biochar, considering about its great nutrient elements contents. For example, Méndez et al. investigated pH, BET surface area, porosity, CEC, EC (electrical conductivity), and heavy metal content of sewage sludge biochar at two pyrolysis temperatures of 400 and 600 °C [23]. Hossain et al. also investigated the influence of temperature on production of wastewater sludge biochar and evaluated the properties required for agronomic applications in a temperature range of 300–700 °C [24]. Apart from biochars derived from various biomass residuals, a strong interest point of sewage sludge biochar was its heavy metal toxicity. Various leaching methods, such as TCLP (toxicity characteristic leaching procedure), BCR (Community Bureau of Reference) sequential extraction procedure were used to test its environmental toxicity, after all, a great percentage of heavy metals originally contained in sewage sludge still remained in biochar [3], [23], [24], [25], [26]. Nevertheless, the investigation on sewage sludge biochar was still scarce, it was necessary to conduct more researches to understand the effect of temperature on biochar physical and chemical properties, especially to evaluate the leaching toxicity of heavy metal presented in biochar. In this study, the pyrolysis experiments were conducted using sewage sludge to investigate the influence of temperature on biochar yield, pH, constituents, pore structure, dissolved salts content, nutrients content and heavy metal content. Additional, (diethylenetriaminepentaacetic acid) DTPA leaching was applied to analyze the availability of trace nutrient elements. TCLP was applied to analyze the leaching toxicity of heavy metals in the biochar, in order to determine the biological safety of biochar application as a soil amendment.
Section snippets
Sewage sludge materials
A digested sewage sludge sample was collected from the dewatering room in an urban wastewater treatment plant with improved anaerobic–anoxic–oxic process treatment in Guangzhou, China, and the main properties of the sewage sludge sample are listed in Table 1. The sewage sludge sample was first separated from other impurities, such as glass fragments and plastic bags, and then dried in a convection laboratory oven at room temperature until it qualified for pulverization. The sewage sludge sample
Properties of sludge sample and biochars
As shown in Table 1, with the temperature rising from 300 to 700 °C, the biochar yield decreased gradually from 83.3 to 65.0%. The decrease in biochar yield was predominately attributed to the more organic materials decomposing at the higher temperatures. The sewage sludge sample as well as biochar could be categorized into volatile matter, ash and fixed carbon according to proximate analysis. Their contents were 39.7, 55.7, and 4.6%, respectively in the sewage sludge sample. As expected, the
Conclusions
The biochar yield decreased with the rise in temperature. 700 °C appeared to be the proper temperature for biochar production in terms of stronger alkalinity, more developed pore structure, fewer dissolved salts, and higher contents of nutrients except nitrogen. Biochars had lower contents of available trace nutrient elements compared to sewage sludge. The leaching toxicity of heavy metals in the biochar was lower than that in the sewage sludge although the pyrolysis process intensified the
Acknowledgments
This work is financially supported by National 973 Project of China (2011CB201501), Projects of International Cooperation and Exchanges NSFC (51161140330), Knowledge Innovation Program of the Chinese Academy of Sciences (NKSCX2-EW-G-1-5), and The Program of Guangdong Province – Chinese Academy of Sciences Strategic Cooperation (2010A090100035).
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