Environmental Friendly Activated Carbon Processes for Removing Water Pollutants

中国环境学会  2011年 06月21日

  Wei-chi Ying[1], Bingjing Li, Liuya Huang, Wenxin Jiang, Wei Zhang, Qigang Chang  
  School of Resources and Environmental Engineering
  East China University of Science and Technology and Science, Shanghai 200237, China


  Abstract:
  Granular activated carbon (GAC) adsorption treatment for removing water pollutants has five environmental friendly characteristics: 1) capable of removing the target pollutant totally, 2) taking up less space, 3) no residual chemicals or toxic intermediates, 4) less costly to construct and to operate, and 5) extended service likely due to the possible biological activated carbon (BAC) capability, catalytic reactions and reactivation. The paper recommends steps to be taken for developing a cost effective GAC treatment process with a focus on the capacity indicator method for identifying high potential GACs and the improved micro column rapid breakthrough method for selecting the best carbon, verifying the treatment effectiveness, and estimating the adsorption treatment cost in a small fraction of time required by the conventional breakthrough methods. The recommended steps are: 1) understand the fundamentals of carbon adsorption technology, 2) select the efficient methods for conducting the adsorption isotherm and breakthrough experiments, 3) perform efficient treatability studies to choose the best GAC and to define the optimal process scheme, 4) establish BAC function to enable long term organic and total cyanide removal capability without carbon replacement, 5) take advantage of the carbon’s catalytic reduction of free chlorine, and 6) reactivate the spent carbon.
  Key words: Environmental friendly water treatment process, capacity indicator method of carbon selection, micro column rapid breakthrough method, capacity utilization rate, biological activated carbon


  摘要:颗粒活性炭吸附技术去除水中污染物有五个环境友好特性:1)能够完全去除目标污染物,2)占地小,3)无残留有毒化学品或中间产物,4)建造、运行费用低廉,5)达到生物活性炭的功能后、应用其催化功能、或经由再生可以长期使用。本文推荐有利于形成经济有效的活性炭处理工艺的有效步骤,重点介绍吸附性能指标方法选优质活性炭,及改良式的微型快速穿透实验方法可用于较短时间内取得选炭和验证处理效益的数据。建立高效益环境友好活性炭水处理系统的步骤如下:1)掌握活性炭吸附技术的基本原理,2)选用适当的方法进行必要的吸附容量实验及穿透实验,3)进行高效处理性实验,从而选择最佳炭型及吸附处理工艺,4)建立生物活性炭功能,使活性炭能够长期去除有机物和总氰,5)利用活性炭对余氯的催化还原能力,6)进行活性再生。
  关键词:环境友好水处理工艺,吸附容量指标,微型快速穿透,活性炭容量利用率,生物活性炭


  Introduction
  Activated carbon is a powerful adsorbent; it is very effective for removing a wide variety of organic pollutants from aqueous influents. In the U.S. and many other developed countries, activated carbon adsorption technology is often selected for water and wastewater treatment to meet stringent effluent discharge limits. However, its application potential is still undeveloped in China for three reasons: 1) the concept of using either granular activated carbon (GAC) or powder activated carbon (PAC) is not generally understood, 2) simple and effective methods for carbon selection and adsorption studies are not commonly available(1,2,3), and 3) commercial carbon regeneration service is unknown to most users.
  Relative to the biodegradation and chemical destruction methods, GAC for advanced treatment of wastewaters has at least five environmental friendly advantages: 1) capable of removing the pollutants totally to meet the stringent discharge limits (i.e., 1 ppb or lower), 2) taking up less space because additional unit of the high loading GAC adsorber can be added in the future when the need is present, 3) no residual treatment chemicals or toxic intermediates, 4) less costly to construct and to operate, and 5) extended service likely due to the possible biological activated carbon (BAC) capability, catalytic reactions (such as for free chlorine removal) and spent GAC regeneration.
  This paper presents the recommended steps to be taken for developing a cost effective GAC treatment process with a focus on conducting the efficient treatability study that utilizes the four adsorptive capacity indicators (phenol, iodine, methylene blue and tannic acid numbers) method for identifying high potential GACs and the improved micro column rapid breakthrough (MCRB) method for selecting the best carbon, verifying the treatment effectiveness, and estimating the adsorption treatment cost in a small fraction of time required by the conventional breakthrough methods.


  Fundamentals of Carbon Adsorption Treatment Technology


  Activated carbon is employed for aqueous phase adsorption applications either as PAC in mixed reactors or as GAC in adsorbers. The fundamentals and applicability of the two carbon adsorption technologies are quite different. A certain dose (g/L) of PAC is introduced to a mixed reactor to achieve the desired degree of pollutant removal, while GAC in well designed adsorbers can remove them totally to meet very stringent discharge limits for these compounds.
  For all potential applications of adsorption technology, a series of carbon isotherm experiments are first conducted for some candidate carbons in the feasibility study stage; the resulting adsorptive capacities and the final residual (equilibrium) concentrations are correlated by the Freundlich adsorption isotherm model to allow selection of few high potential carbons to be evaluated in the treatability study stage in a series of adsorption breakthrough experiments to select the best carbon, to verify the treatment effectiveness, and to estimate the adsorption treatment cost based on the observed capacity utilization rate for carbon in the adsorber under actual treatment conditions. Table 1 presents calculation methods for the important adsorption study parameters; Table 2 presents the estimated PAC and GAC required for treating a wastewater(4,5). It is clear that GAC treatment technology is more efficient in an application which requires a very high degree of target compound removal. Indeed, GAC treatment process has been successfully employed for meeting effluent discharge limits of less than 1 ppb(6,7).
  Figure 1 presents relationships of capacity utilization rates of different adsorption breakthrough curves and the carbon bed change criteria; Figure 2 shows GAC columns of three sizes(micro, mini and small) that can be employed in the lab to obtain the desired breakthrough curves(8,9). For situations requiring a high degree of target compound removal and/or displaying early breakthrough (Curves C and D), two large adsorbers in series mode of operation is often the best treatment process since GAC in the leading adsorber can be heavily loaded to achieve a high utilization rate (at a higher C1 than would be possible using only one adsorber), while the polishing treatment of the second adsorber ensures a high quality (C2) effluent(1,5).


  Select the best methods for conducting the adsorption isotherm and breakthrough experiments


  The GAC’s adsorptive capacity for a specific water pollutant is dependent on both the surface chemistry and the pore structure (i.e., surface area and volume vs. pore size distribution) of its particles, which are governed by the GAC’s raw material and also the activation method as illustrated in Figures 3. Because of the high cost and long time required to obtain the desired pore size distribution data, several adsorptive capacity indicators have been proposed for selection of high potential GAC for a specific application(1,3,4).
  We have proposed the 4-indicator method of carbon selection consisting of phenol, iodine, methylene blue, and tannic acid numbers to cover water pollutants of different molecular weight fractions; a high phenol number indicates the GAC has a large internal surface area of very small micropores (diameter < 1 nm) and a low surface acidity which enhances the adsorption of polar organic compounds, while high values of iodine number, methylene blue number, and tannic acid number indicate abundant small micropores (diameter <1.5 nm), large micropores (diameter = 1.5-2.8 nm), and larger pores (diameter > 2.8 nm), respectively (8,9,10).  A table of the four capacity indicators for popular commercial activated carbons is a useful database for identify few GACs to be evaluated in a treatability study. This method has been successfully employed to select high potential GAC for removing a wide variety of such water pollutants as phenol, dichlorophenols, nitrobenzene, gasoline ingredients benzene, toluene, ethylbenzene, xylene (BTEX) and methyl-tert-butyl ether (MTBE), red dye X3B, chloroform, trichloroethylene (TCE), and biphenyl(10,11). 
  To enable completing the essential breakthrough experiment in a small fraction (1-5%) of time required by using small or mini conventional carbon columns in an ordinary environmental research laboratory, we have modified the Calgon’s HPMC(12) and USEPA’s RSSCT(13,14) methods with the use of less costly test equipments and simplified procedure to result in an more efficient MCRB method. The validity and the benefits of this improved method were demonstrated by the results of many isotherm and breakthrough experiments for the four indicator compounds, such as Figure 4 for phenol, and many typical water pollutants(8,15). Depending on the nature of the pollutants, sample availability, type and scale of the study, the most effective breakthrough experiments may employ small (>100 g), mini (>5 g) traditional or pressurized micro (<2 g) carbon columns, as illustrated in Figure 2, to select the best carbon, to verify the treatment effectiveness, and to estimate the adsorption treatment cost based on the observed capacity utilization rate for carbon in the adsorber.


  Perform efficient treatability studies: a case of TCE removal from groundwater


  An efficient treatability study to define the best treatment process scheme for removing the target pollutant(s) from an influent stream involves two steps: 1) conduct adsorption isotherm experiments or employ a table of the 4 capacity indicators to identify few high potential GACs and 2) conduct the necessary breakthrough experiments, preferably using the efficient MCRB method, to obtain a valid breakthrough curve for each of the GACs under conditions simulating the full scale treatment of the influent stream.
  A case of employing four Chinese GACs of different raw materials (coal, coconut shell, fruit nut/shell and bamboo of Shanghai Activated Carbon Company) and three popular US GACs (coconut based Norit GCN830 and coal based Calgon F300 and Calgon F400) to conduct the efficient GAC adsorption treatability study for removing TCE is presented to illustrate the procedures of selecting the best GAC and defining the optimal scheme(16). Table 3 presents the four capacity indicators of the 7 GACs, and Figure 5 presents their capacities for TCE in well water. Figure 6 presents the MCRB curves of six GACs in removing TCE from tap water (solid lines) and well water (broken lines), and Table 4 summarizes the TCE removal data of the MCRB runs. The following can be deduced from the experimental data:  
  1. The poor mass transfer condition of the filled bottles employed for the isotherm experiments required a long batch contact time of 48 hours to ensure that the measured adsorptive capacity of GAC for TCE.
  2. The GACs’ adsorptive capacities for TCE were in the same order as their phenol numbers and that their capacity utilization rates (availability) were indicated by their tannic acid numbers.
  3. GACs’ capacities for TCE in pure water were reduced by competitive adsorption of other organic constituents of the sample; small organic compounds present in tap water were more competitive than the NOM in the higher TOC well water sample.
  4. The MCRB data confirmed the GACs’ available TCE capacities and that the serial bed mode of treatment is essential for the cost effective GAC adsorption process because most of the capacity can be utilized for removal of the pollutant with the high allowable effluent concentration of the first bed. 
  5. The GAC made from toxicant free, low cost and renewable bamboo is attractive because its relatively high TCE capacity is readily available in actual adsorption treatment.


  Establish BAC function to enable long term organic removal without carbon replacement


  Although BAC has never achieved its originally purpose as an alternative to the biological secondary treatment processes (activated sludge & trickling filter),it has many applications for removing POPs present in biotreated effluent or trace level of organic contaminants of groundwater because of long term organic removal capability without the need of GAG replacement(17).
  Figure 7 presents the BAC study demonstrating the long term removal capability of removing sulfolane from water, and Figure 8 presents the successful full size application of BAC capability in removing sulfolane from the Lathrop (CA, USA) groundwater(7). Our recent BAC research demonstrated the very high organic BTEX removal capacity of a small BAC column (as illustrated in Figure 9 for removal of benzene) and confirmed that the DO requirements were well below that of the conventional aerobic biological WWT noted earlier(17,18). Although different GACs produced about the same effluent quality (in COD/TOC); the degree of regeneration were different. Larger pore of coal carbon was better regenerated than the smaller pore coconut carbon(18). Figure 10 presents the long term organic and total cyanide (TCN) removal capability of the BAC in treating the Shanghai Coking Plant biotreated effluent as a case of application for advanced treatment(19). 


  Taking advantage of the carbon’s catalytic reduction of free chlorine


  GAC has been commonly employed for removing free chlorine due to its catalytic reduction capability. Most spent GAC from commercial carbon dechlorination systems are still useful for the intended purpose; Figure 11 shows the spent coconut and fruit GACs after 1 year of service continue to have high capacities for removing free chlorine by adsorption (2h) and catalytic reduction (difference between 5 and 2 hr) (20).
  Reactivate the spent carbon
  Without the benefits of BAC and the GAC’s catalytic properties, the GAC treatment cost can be significantly reduced with cost effective spent carbon regeneration. Commercial spent carbon reactivation services, which includes picking up the spent GAC, reactivating it in a large efficient furnace and supplying the regenerated GAC for adsorption service (as illustrated in Figure 12) will likely to be the most cost effective way of carbon regeneration to achieve multiple reuses of GAC.  


  Conclusions
  The recommended steps that will lead to a cost effective environmental friendly GAC treatment process for removing water pollutants are: 1) understand the fundamentals of carbon adsorption technology, 2) select the efficient methods for conducting the adsorption isotherm and breakthrough experiments, 3) perform efficient treatability studies to choose the best GAC and define the optimal process scheme, 4) establish BAC capability to enable long term organic and TCN removal without carbon replacement, 5) take advantage of the carbon’s catalytic properties to extend the service period of a GAC system (e.g., free chlorine removal in a GAC dechlorinator), and 6) reactivate the spent carbon to reduce the carbon cost.


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  [1]. Corresponding author, visiting professor, e-mail: wcying@ecust.edu.cn , 011+86-21-64252978

 
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