Environ. Sci. Technol. 2008, 42, 7417–7423
Ceramsite Made with Water and Wastewater Sludge and its Characteristics Affected by SiO2 and Al2O3 GUOREN XU,* JINLONG ZOU, AND GUIBAI LI State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
Received May 25, 2008. Revised manuscript received July 28, 2008. Accepted August 5, 2008.
To solve the disposal problems of residual sludges, wastewater treatment sludge (WWTS) and drinking-water treatment sludge (DWTS) were tested as components for production of ceramsite. SiO2 and Al2O3 were the major acidic oxides in WWTS and DWTS, so their effect on characteristics of ceramsite was also investigated to optimize the process. Results show that WWTS and DWTS can be utilized as resources for producing ceramsite with optimal contents of SiO2 and Al2O3 ranging 14-26% and 22.5-45%, respectively. Ceramsite within the optimal SiO2 and Al2O3 contents ranges was characterized using thermal analysis, X-ray diffraction (XRD), morphological structures analyses, and compressive strength measurements. Significant weight loss below 600 °C is through the release of structural water and gases. Bloating and crystallization in ceramsite above 900 °C are caused by the oxidation and volatilization of inorganic substances. Higher strength ceramsite with less Na-Ca feldspars and amorphous silica and more densified surfaces can be obtained at 18% e Al2O3 e 26% and 30% e SiO2 e 45%, while porous ceramsite with complex crystalline phases and lower strength can be obtained at 14% e Al2O3 < 18% and 22.5% e SiO2 < 30%. This revolutionary technology of utilization of WWTS and DWTS can produce high performance ceramsite, in accordance with the concept of sustainable development.
Introduction The disposal of wastewater treatment sludge (WWTS) from wastewater treatment plants (WWTPs) and drinking-water treatment sludge (DWTS) from drinking water treatment plants (DWTPs) is currently one of the most important environmental issues. WWTS is a mixture of biosolids generated in the treatment of the organic substances of municipal sewage along with the inorganic substances such as sand and metal oxides (1, 2). DWTS mainly consists of organic and inorganic compounds in solid, liquid, and gaseous forms, with variable physical, chemical, and biological characteristics (3, 4). Concerns have been increased about the amounts and qualities of WWTS and DWTS in recently years. Traditional options for WWTS disposal are landfilling, composting, combustion/incineration, pyrolysis, and land application * Corresponding author phone: +86-451-86282559; fax: +86-45186282559; e-mail:
[email protected]. 10.1021/es801446h CCC: $40.75
Published on Web 08/30/2008
2008 American Chemical Society
following anaerobic and aerobic digestion, etc. (5-8). Typical practices for disposal of DWTS are landfilling, recycling, regeneration, reuse, and mechanical sludge treatment (3, 4). Moreover, in some countries, DWTS from DWTPs is most commonly disposed of through discharge to municipal sewers and flows directly into surface water (4, 9). However, the traditional options for WWTS and DWTS management are more difficult to be acceptable for environmental problems such as atmospheric contamination, and soil contamination, etc. Alternative options need to be explored to solve their handling in a more environmentally sound manner. Today, some wastes with high contents of water, inorganic/organic matter, and pathogenic agents have been postulated as a precursor of materials that can be successfully employed in several environmental applications (10-12). Converting waste into useful products can alleviate the problems of disposal and offer a new reserve for depleting resources. For example, many inorganic/organic wastes, such as steelworks slag, mining residues, slag, paper sludge, fly ash, and even municipal solid waste have been widely tested as alternatives to produce clay, shale, fuel, glass ceramics, adsorbent, activated carbon, and cement, etc. (10-17). So, the conversion of WWTS and DWTS into useful resources or materials is of great interest and must be intensely investigated. Research has been carried out on the reuse of WWTS for production of innovative aggregates or ceramsite, to be both used as construction materials or filter media (19-21). Our investigations have successfully examined the feasibility of using clay and WWTS to produce ceramsite (21, 22). Valuable information about the chemical speciation of heavy metals in ceramsite and their potential environmental risks is also obtained. Results show that heavy metals are properly stabilized in ceramsite and cannot be easily released into the environment again to cause secondary pollution (23, 24). To avoid more consumption of clay and protect the earth surface environment, searching other materials for replacing clay in ceramsite production can be prompted to achieve the sustainable development of natural resources as well as the protection of environment. DWTS is one of the best substitutes for clay because its major components are similar to those of clay. From the viewpoint of natural resources recovery and conservation, utilization of DWTS as a substitute for clay for production of ceramsite is of great interest and significance. The development of production of artificial lightweight ceramsite provides the potential application in significant quantities that can resolve the management problems of both DWTS and WWTS. To the best of our knowledge, there is no study investigating both DWTS and WWTS for production of ceramsite. The compositional and structural variations of ceramsite obtained in previous studies were attributed to many factors, such as sintering temperature, sintering time, and ratios of WWTS/clay, etc. (22), but, no studies have been conducted to investigate the impact of the specific and important constituents on the characteristics of ceramsite, such as SiO2 and Al2O3. The major acidic oxides (Al2O3 is considered as acidic oxide) in DWTS and WWTS may strongly affect the bloating behavior and crystals formation of the ceramsite during the heat treatment process. To validate this hypothesis, the present study has been conducted to (i) utilize DWTS and WWTS for production of ceramsite; (ii) investigate the effect of SiO2 and Al2O3 on the physical characteristics (bulk density, particle density, water adsorption, and porosity) of ceramsite; (iii) characterize the ceramsite within the optimal contents ranges of SiO2 and Al2O3 by thermal analysis, VOL. 42, NO. 19, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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morphological structures analysis, XRD and compressive strength; and (iv) analyze the sintering mechanisms, as well as to establish effective parameters for evaluation.
Experimental Section Materials. WWTS used in this study was obtained from the Wen-chang Wastewater Treatment Plant, Harbin, China, which has a design capacity of 1.0 × 106 m3 d-1. Dewatering of WWTS was conducted with a belt press filter, and cationic polymeric flocculants were used for enhanced dewatering. The sludge cake generated from the activated sludge process is approximately 1.6 × 105 kg d-1 in wet weight with 24% solids, which is directly landfilled. DWTS was collected from the chemical coagulation/flocculation unit of the third drinking-water treatment plant in Harbin, China. The coagulant was aluminum sulfate (Al2(SO4)3). The WWTS and DWTS were treated by air-dry method and were ground into sizes below 100 µm that are sufficiently fine to be mixed homogeneously. The components of DWTS were similar to those of clay, which indicate that DWTS can be tested as a substitute for clay for production of ceramsite (Supporting Information, Tables SI1 and SI2). WWTS and DWTS differed strongly in terms of their components (Supporting Information, Tables SI2 and SI3). The ceramsite was made of DWTS, WWTS, and water glass (sodium silicate Na2O · (SiO2)x · (H2O)y). The modulus of water glass used in the study was 3.2. All the used oxides (SiO2, Al2O3, Fe2O3, CaO, and MgO) with particle sizes below 10 µm were of the highest purity and of analytical grade. Methods. The raw materials were mixed and pelletized to particle sizes of 5-8 mm and left in a room at a temperature of about 20 °C for 3 days and then the samples were dried at 110 °C in a blast roaster for 24 h. Sample heating started at 20 °C and continued at a rate of 8 °C/min in a muffle furnace; then the samples were soaked at 200, 600, and 800 °C for 10 min and at 1000 °C for 35 min, and finally naturally cooled until they reached room temperature. Bulk density, which includes all voids and spaces in the volume, particle density, which is also the apparent specific gravity of the aggregates includes all intraparticle voids, water absorption determined from the weight differences between the sintered and water saturated samples (immersed in water for 1 h), and porosity ((1 - bulk density/particle density) × 100%), were analyzed according to the methods reported in previous study (21). To achieve statistical soundness, at least three replicates were carried out for each sample. The thermal behaviors of samples were examined by simultaneous thermodifferential and thermogravimetric analyses (DTATGA) (ZRY-2P, China), while the samples were heated at a rate of 8 °C/ min from 20 to 1080 °C in air. Samples weighed from 4 to 10 mg in mass, and they were put into a Pt-Rh crucible with 20 taps. All curves were evaluated using the TA-instruments software. The second derivative differential thermal curve was used for determination of peak temperature. Powder XRD patterns of ceramsite were recorded on a D/max-γ β X-ray diffractometer with 50mA and 40Kv, Cu KR radiation (Japan). SEM analyses were conducted using a S-570 scanning electron microscope (Japan) at an accelerating voltage of 20 kV. Major components of materials were analyzed using a Philips PW 4400 XR spectrometer (X-ray fluorescence-XRF, Netherlands). Compressive strength of ceramsite was analyzed using an INSTRON 5569 automatic material testing machine (USA).
Results and Discussion Matrix and Single Factor Experiments for Production of Ceramsite. For studying the effect of parameters for making ceramsite, four control factors were selected to optimize the process: DWTS/WWTS ratios, water glass/(DWTS + WWTS) ratios, sintering temperature, and sintering time. Moreover, 7418
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an L9 orthogonal array was used to determine the effect of the four control factors on physical characteristics of ceramsite. For each factor, three levels were chosen to cover the wide variational region on the basis of the results obtained from our previous investigations (21, 22). These factors and their levels are shown in Table SI4 (Supporting Information). The L9 orthogonal array (Supporting Information, Table SI5) consists of nine experimental runs corresponding to the nine rows, four columns, and the experiment results. The final analysis shows that most significant factor is the DWTS/ WWTS ratio and the second one is the sintering temperature. Tests on the effect of single above-mentioned factors on the characteristics of ceramsite were then conducted and are shown in Table SI6 (Supporting Information) and Figure 1. Results show that DWTS and WWTS are good raw materials for making ceramsite with desired density, water adsorption, and porosity. In this study, the principle for the selection of each factor is that the ceramsite with lower water adsorption (
