Characterization of sewage sludge-derived biochars from different feedstocks and pyrolysis temperatures
Introduction
Biochar refers to the carbon-rich material of pyrolysis of biomass from a wide variety of biomass residues (e.g. wood chips, compost and animal manure) in an oxygen-free or low oxygen environment [1], [2]. It is widely used as a soil amendment for agricultural application to improve physical properties of soils such as air permeability, water and nutrient retention capacity and surface drainage.
Recently attention has been given to biochar derived from sewage sludge because of its potential for soil amelioration and the potential additional benefits for sewage sludge treatment [3], [4]. Sewage sludge is an organic waste which usually contains high levels of nitrogen and phosphorous as well as significant concentrations of micronutrients. Currently, with the substantive construction of Waste Water Treatment Plants (WWTPs) in China, more than 25 million tons of sewage sludge (with a moisture content of approximately 80%) from WWTPs has been annually produced. Management of the increasing volume of sewage sludge has been one of the prime environmental issues in China in recent years. The application of wastewater sludge-derived biochar (SDBC) to land has been reported to significantly increase the soil electrical conductivity, phosphorus and nitrogen content [5]. It has also been recently reported that biochar can effectively remove heavy metals and organic pollutants from contaminated soil and waters [6], [7], [8].
Biochar properties can be significantly influenced by feedstock source along with pyrolysis conditions such as temperature, residence time and activation treatment [2], [8], [9], [10], [11]. It is these differences in physicochemical properties that govern the specific interactions between biochar and a wide range of organic and inorganic compounds in the environment. For example, biochars produced from crop residues (e.g. rye, maize), manures and seaweed are generally finer and less robust (i.e. lower mechanical strength). The latter are also nutrient-rich, and therefore, more readily degradable by microbial communities in the environment [12]. The ability of high temperature biochars are limited for nutrient cation retention capacity of soil due to their decrease in surface acidity and Cation Exchange Capacity (CEC), compared with low-temperature biochars [13]. The manure-based biochars may mineralize and release nutrients more rapidly in soil than the plant-based biochars rich in condensed aromatic structures [9], [13]. Additionally, the concentration of extractable phosphorus was found increased in tropical soils amended with a variety of charred materials [14], [15].
The chemical properties of the biochar produced from wastewater sludge has been recently reported to be significantly influenced by pyrolytic temperature [16]. However, literature is limited on the relationship between the characteristics of SDBC with its different sources of sludge. Little work has been conducted to determine the effects of sludge which has received no anaerobic digestion on biochar production. For sewage sludge to be considered as feedstocks for biochar production, a rigorous study should be carried out to assess the biochar characteristics and the potential risks of bioaccumulation that may be associated with its subsequent applications (e.g. to land).
The present work has been undertaken to evaluate the influence of sources of feedstock and pyrolysis temperatures on the surface characteristics of SDBC and to determine whether heavy metal limits are exceeded for specific application rates. Our objectives were to prepare biochars with the optimal surface characteristics while minimize their environmental risk.
Section snippets
SDBC sample preparation
The sewage feedstocks for the preparation of the SDBC used in this study were sourced from three different WWTPs, Xilang WWTP (XL), Liede WWTP (LD), and Datansha WWTP (DTS), all located in Guangzhou, China. In common, the sewage sludge was belt filtered or centrifuged for dewatering without any anaerobic digestion pretreatment in China and therefore contained large amounts of organic matter.
Each time about 100 g sludge from different feedstock was introduced to prepare SDBC by pyrolysis in a
Biochar yield rates
For the raw sludge containing high moisture of water, the process of pyrolysis may be divided into two phases, evaporation and carbonization. The energy for the two processes was calculated to be 2.47 MJ kg−1 and 0.62 MJ kg−1 on the wet basis of raw sludge, respectively. After the water vapor has been emitted in the temperature between 100 and 127 °C, the dried biosolids may continue to be decomposed from 127 to 219 °C and followed by a slow pyrolysis process when the temperature increased to 550 °C.
Conclusions
The conversion of sewage sludge to biochar offers a solution for minimizing waste volume and producing potentially valuable biochar adsorbents. Environmental risk of SDBCs can be minimized and managed by controlling the source of feedstock and pyrolysis temperature. For those WWTPs with pure sewage influent, the wastewater sludge may be more suitable for preparing biochars with more uniform charge distribution. Compared with higher temperatures, controlling the pyrolysis temperature to 300 °C
Acknowledgments
This study was financially supported by the Natural Science Foundation of China (Nos. 41171374 and 41225004) and the National Environmental Protection Department Public Benefit Research Foundation (No. 201109020) and the Fundamental Research Funds for the Central Universities.
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