Jun 1, 2000 - The coal-based carbon usually has a structure somewhere .... TABLE 1: Iodine numbers and percentage geosmin remaining after treatment at 15 .... In Umgeni Water's experience, consumer complaints usually start when .... It was realised that if it was assumed that the same trend existed for geosmin, ...
Presented at WISA 2000, Sun City, South Africa, 28 May – 1 June 2000
POWDERED ACTIVATED CARBON: CAN THIS BE EFFECTIVELY ASSESSED IN THE LABORATORY? S D FREESE, D J NOZAIC, R A SMITH and D L TROLLIP
Umgeni Water, P O Box 9, Pietermaritzburg, 3200
ABSTRACT Powdered activated carbon (PAC) is widely used in the water treatment industry for the removal of a variety of organic contaminants. Traditionally tests, such as Freundlich isotherms and iodine, methylene blue and tannin number tests have been used to assess PAC in the laboratory. Attempts to draw correlations between these indicator parameters and the adsorption properties of PAC are described, but these were generally found to be inadequate in assessing PAC for treatment applications. A modified jar test procedure was developed which accounts for a large number of variables, including the natural organic matter in the water, the contact time and the effect of other treatment chemicals such as coagulants, oxidants and lime. Using this test it was possible to accurately determine the optimum PAC dose required for geosmin removal at a water works. Based on the findings of other researchers, the data obtained from this test can be used to determine the most suitable type of PAC and dose for use in a particular application. Using this test, it is possible to effectively assess PAC in the laboratory. _____________________________
KEY WORDS Powdered activated carbon, geosmin, methylisoborneol, taste and odour compounds, Isotherm tests.
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INTRODUCTION
Deteriorating water quality in South Africa, due largely to inadequate sanitation, is giving rise to eutrophication with the concomitant increase in algogenic taste and odour compounds, as well as the possibility of algal toxins, such as microcystin. Eutrophication, together with runoff from agricultural lands and discharges of industrial effluents to water courses aggravates the situation by increasing the levels of organic contaminants in the water, which in turn results in the formation of increased amounts of disinfection-by-products (DBP). Industrial discharges can further complicate the situation by containing odorous and sometimes toxic organic compounds. These pollutants can result in a water that is not only unpalatable, but may also pose a health risk. Traditional treatment processes are usually inadequate in removing many of these micropollutants and more advanced processes such as ozonation and activated carbon treatment are required. The use of activated carbon is expanding rapidly in the potable water treatment industry. Initially activated carbon was principally used in water treatment for the removal of taste and
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odour compounds. However, with increasingly stringent limits for organic contaminants, including pesticides, being imposed by the United States Environmental Protection Agency, the European Economic Community and other statutory bodies, activated carbon use is becoming much more wide spread. Umgeni Water uses PAC primarily for the removal of the two algogenic taste and odour compounds, namely geosmin and 2-methylisoborneol (2-MIB), which are due mainly to the presence of two cyanobacteria or blue-green alga genera, Microcystis and Anabaena. It has been pointed out however, that only circumstantial evidence exists to suggest that geosmin is produced by Microcystis aeruginosa, the predominant species in the majority of taste and odour incidents that have occurred in South Africa (Wnorowski and Scott, 1992). There is however scientific evidence to prove that geosmin is produced by Anabaena circinalis, the species of Anabaena that usually gives rise to taste and odour problems in the Umgeni Water operational area (Bowmer et al, 1992). For laboratory testing of PAC, isotherm tests are usually undertaken in which the reduction in target compounds at different PAC concentrations is determined after a specific contact time, but these are rather laborious, the adsorption models used are often complicated and they generally give only an estimate of the suitability of the carbon for the intended application. A number of other tests has been developed over the years for the purpose of assessing the “acceptable” activity level of a carbon and these include isotherm-type tests such as iodine number, methylene blue number, phenol value, molasses number and tannin number. These tests are in some cases more rapid than the standard isotherm tests, but are often meaningless in determining the suitability of a carbon in meeting the treatment objectives. An investigation was carried out to determine whether there is any correlation between the traditional adsorptive capacity indicators and the ability of PAC to remove geosmin in the presence of natural organic matter (NOM), or alternatively to devise a reliable and rapid test method for meaningful assessment of PAC in terms of a particular treatment objective.
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ACTIVATED CARBON: PAC VERSUS GAC
Both powdered and granular activated carbon (PAC and GAC) can be used for the adsorption of dissolved organic compounds, colour, taste and odour compounds and DBP precursors. The primary characteristic that differentiates PAC and GAC is particle size, commercially available PAC typically having between 65 and 90% of the particles passing through a 325 mesh (± 45 µm) sieve (Water Quality and Treatment, 1990). PAC is generally used in controlling seasonal or sporadic incidents and can be added in at the influent, the rapid mix section, the flocculation basin influent or at the filter influent (Water Quality and Treatment, 1990). Umgeni Water employs PAC at a number of its water treatment works where it is added together with coagulant at the rapid mixing stage and then settled out in the pulsator clarifiers. Due to the low coagulant doses employed, Umgeni Water experience has shown that PAC doses exceeding approximately 20 mg/l eventually result in carry over to the filters, causing these to block prematurely. However, it is reported in the literature that doses of up to 50 mg/l PAC can be required in some cases (Water Quality and Treatment, 1990), but these can be tolerated where high coagulant dosages are used. GAC is generally more expensive than PAC treatment in that a large capital outlay is required, but it is also more effective than PAC in removing organic compounds, since GAC is usually preceded by pre-treatment which reduces the load on the carbon. In spite of this PAC has a number of advantages over GAC, the main ones being the low capital cost of PAC and the ability to apply it only when it is needed (Najm et al, 1990). This is particularly S D FREESE
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important in a country like South Africa where outbreaks of taste and odour compounds are usually seasonal and intermittent. The disadvantages of using PAC become evident if one requires carbon over an extended period of time. PAC is not regenerated in most cases and therefore becomes very costly when used for long periods. It also provides a lower rate of NOM removal than GAC, creates more sludge disposal problems and difficulties are often experienced in removing the PAC particles from the water (Sontheimer, 1976).
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PHYSIOCHEMICAL PROPERTIES OF PAC
Activated carbon is produced from a variety of raw materials, although coal, wood and coconut are most commonly used in the manufacture of PAC for potable water treatment applications. The raw material first undergoes carbonisation or pyrolysis in which it is heated to a temperature below 700 °C in the absence of air to form a char. The material is then activated using oxidising gases such as steam, carbon dioxide, air and oxygen or using chemicals at temperatures of up to 1000 °C (Sanks, 1978; Water Purification Works Design, 1997). Activation can give rise to surface areas in excess of 2 000 m2/g, although for potable water applications, activated carbon with a surface area in the region of 500 to 1 500 m2/g is generally used. The physical properties of the carbon are dependent upon the raw material, as well as both the method and extent of activation used. In general, if using similar methods and degrees of activation, activated carbon made from coconut tends to have a dense structure consisting of large graphite plates situated close together, with only a few larger pores, while wood-based activated carbon has an open structure with smaller graphite plates and many more larger pores. The coal-based carbon usually has a structure somewhere between that of coconut- and wood-based carbons (Greenbank, 1992). It is the high degree of porosity and the large surface area that accounts for the adsorptive properties of activated carbon and by changing the activating conditions, the size and number of pores can be controlled to produce a carbon suited to a particular application (Sanks, 1978). The pores in activated carbon are generally divided into two classes depending on size, these being the micropores and the macropores, but the ranges quoted in the literature vary between 10 to >100 nm for the macropores (Chemviron, 1974; Gregg and Sing, 1982; Sanks, 1978; Water Purification Works Design, 1997). The micropores are responsible for most of the surface area providing PAC with its adsorptive properties and in water grade carbons more than 70% of the of the available surface area is attributed to pores having a radius of less than 5 nm. Generally the external surface area of a typical water treatment PAC is insignificant compared to the surface area contained within the pores and therefore reducing the particle size, for example by grinding, will have a negligible effect on the total surface area (Sanks, 1978). In fact, the same is true of GAC and once ground, the same tests used to assess PAC can be used to assess GAC as well. The physical adsorption of organic compounds onto PAC occurs via several transport mechanisms, which take place in a series of steps each of which can affect the rate of removal (Water Quality and Treatment, 1990), these being: Bulk solution transport: the adsorbates are transported from the bulk solution to the boundary layer of the water surrounding the PAC particle. This occurs either by diffusion or by mixing. Film diffusion transport: the adsorbates are transported by molecular diffusion through the boundary layer of water surrounding the PAC particles when water is flowing past them. This is influenced by the rate of flow past the particle. S D FREESE
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Pore transport: the absorbates are moved through the pores to available adsorption sites. Adsorption: the adsorption bond is formed at available sites, with physical adsorption occurring rapidly. If chemical adsorption occurs, this is much slower and this may become the rate determining step.
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LABORATORY ASSESSMENT OF PAC
A number of different test procedures has been used in the past to assess PAC in terms of its suitability for a particular treatment application. One of the most popular methods has been adsorption equilibria or isotherm tests. The Freundlich isotherm test is an example of such a test and this shows the relationship between the residual concentration of a target compound against the loading of the compound on the carbon, sometimes referred to as the liquid phase concentration versus the solid phase concentration (Chemviron, 1998). However, accurately determining the effect of background organic matter on adsorption of the target compound, or compounds, complicates these tests. Some researchers use the Equivalent Background Model Compound (EBC) model; (Gillogly et al, 1998; Najm et al, 1991), while others employ Fictive Components (FC) (Crittenden et al, 1985), but in both cases fairly exhaustive experimental results are required as well as complicated mathematical manipulation of the models. A number of standard liquid phase isotherm tests are currently used for the assessment of PAC, including iodine number, methylene blue number, phenol number and tannin number. The various tests used in this investigation are described below. Freundlich isotherm tests Prior to isotherm tests being conducted, the PAC was ground in a laboratory ball mill until at least 95% of the original PAC sample passed through a 325 mesh (45 µm) sieve. The ground carbon was dried at 150 °C to constant weight. Five different weights of the dried, ground carbon were placed into each of 5 one litre Pyrex bottles. Each bottle was then filled with exactly one litre of a representative water sample containing geosmin and/or 2-MIB. A blank sample containing the water sample with geosmin and/or 2-MIB, but no PAC was also prepared. The five samples and blank were placed on a mechanical stirrer and shaken for 24 hours at 22-24°C. The water was filtered through Whatman No. 3 filter paper under vacuum and the geosmin and 2-MIB present in the filtrate was determined as described below. Iodine number: The iodine number is defined as the milligrams of iodine adsorbed by one gram of carbon when the iodine concentration is 0,02 N (American Society of Testing and Materials [ASTM] D4607). This is a good quality control parameter to use when comparing different production batches of PAC, but in determining whether a PAC is suitable for a certain treatment objective, the value of this test is limited. The reason for this is that iodine is a small molecule that is well adsorbed and the test is conducted at high iodine concentrations, resulting in a loading that is much higher than that encountered in practice. For example, the iodine number specified for Chemviron Fitrasorb 400 (a GAC) is 1050 mg/g which is equivalent to a weight loading of 105% w/w, while the typical loading achieved in most liquid phase applications is less than 20% w/w (Chemviron, 1998). Methylene blue number: This procedure determines the capacity of an activated carbon to decolorise the aromatic dye, methylene blue and is also a measure of adsorption capacity. Two different types of methylene blue tests can be used. The first is the Chemviron Carbon method (TM-11) (Chemviron, 1998) and is similar to the iodine number. It involves adding a measured amount of activated carbon to a standard methylene blue solution. The methylene blue number is determined from the reduction in colour and is quoted in milligrams per gram. The other test procedure, which is the CEFIC Test Method (European Council of Chemical Manufacturers’ Federations, 1986), involves the addition of a standard methylene blue S D FREESE
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solution to a sample of activated carbon until no further colour reduction occurs and the figure is then quoted in millilitres per gram (Chemviron, 1998). Whichever procedure is used, the methylene blue number, like the iodine number, provides only an indication of the adsorption potential of the carbon and usually has only limited value in assessing the PAC in terms of operational performance. The Chemviron TM-11 method was used in this investigation. Phenol number: There are three different phenol number tests. There is an isotherm method which is contained in the German Standard DIN 19603 and is defined as the adsorption of phenol (in % w/w) on the activated carbon required to reduce the phenol concentration from 10 mg/l to 1 mg/l. There are also two American Water Works Association (AWWA) methods, one for PAC and the other for GAC. The PAC (AWWA B600-90) method is also an isotherm test, similar to the DIN method, except that it is carried out at a much higher phenol concentration, the test requiring that the phenol concentration be reduced from 200 mg/l to 20 mg/l (Chemviron, 1998). In both tests, the lower the phenol number the better the adsorption potential of the carbon. As with the iodine number and methylene blue number tests, it is difficult to translate the phenol number into plant performance. Not only is the loading of phenol during the test much higher than say that for a compound such as geosmin, but the phenol number value is affected by pH. Phenol number tests were not conducted in this study. Tannin number: This test is an AWWA method (AWWA B600-78, revised in ANSI/AWWA B600-90, 1991) and the tannin number is defined as the concentration of activated carbon in milligrams per litre required to reduce the standard tannic acid concentration from 20 mg/l to 2 mg/l. Another disadvantage with isotherm tests is that although they can provide an estimation of the PAC consumption and indicate whether a compound is adsorbable or not, they do not give certain important design information, such as the required contact time. It was on account of this and the fact that the isotherm tests described here were not found to correlate well with full scale operation, that a practical test was devised which was capable of accounting for factors such as the background organics, the contact time, treatment process and the effect of the other process chemicals on the PAC’s potential for adsorption of the taste and odour compounds such as geosmin and 2-MIB. Jar test procedure for measurement of Geosmin adsorption potential: The geosmin adsorption potential was determined using a modified jar test procedure. A slurry of the PAC was prepared (0,08%) and the required volume of this was then added to 800 ml raw water from a potable water treatment works which had been spiked to contain 250 ng/l geosmin (this is for cases where no geosmin is present in the water). Carbon concentrations of 3, 6, 9, 12 and 15 mg/l were used and a control containing no carbon was also prepared. Chemical addition to the water was kept as close as possible to that being used on the plant at the time of sample collection. The same coagulant and dose as being used at the plant was added to each jar and chlorine, lime and bentonite were added if these were being added on the plant at the same concentrations as being used on the plant. The carbon was added to the water while mixing at 40 rpm and a contact time of 20 minutes was allowed. Thereafter the mixing speed was increased to 300 rpm and lime, if required, was added. 30 seconds after the addition of the lime, chlorine was added and after another 30 seconds the coagulant was added. Stirring at 300 rpm continued for 2 minutes after the addition of the coagulant. Thereafter the mixing speed was reduced to 40 rpm and stirring continued for 2 hours. The water was then filtered through Rundfilter M&N filter paper (Whatman No. 1 equivalent) and analysed for geosmin. These conditions are adapted where necessary in order to more closely simulate plant conditions. S D FREESE
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Geosmin and 2-MIB Analysis: The geosmin and 2-MIB were extracted from the water using solid-phase membrane filtration through C19 membrane filters. The geosmin and 2-MIB were then eluted from the membrane using dichloromethane and the extract concentrated under vacuum on a rotary evaporator, to produce a final concentrate solution of 1 ml. The geosmin and 2-MIB concentrations were determined on a Hewlett-Packard 5890/5970 gas chromatograph-mass selective detector according to a South African National Accreditation Services (SANAS) accredited procedure. The ash and moisture content of the PAC were also determined, although there was no correlation between these parameters and the suitability of the carbon for a particular application. However, a high moisture content means that one is purchasing and transporting a high proportion of water and a very high ash content could be an indication of a low grade PAC. Ash content: The ash content was analysed using the ASTM D 2866-83 (reapproved 1988) “Standard Test Method for Total Ash Content of Activated Carbon”, 1988. Moisture content: Moisture content was analysed using the ASTM D 2867-83 (Reapproved 1988) “Standard Test Methods for Moisture in Activated Carbon”, 1988.
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RESULTS AND DISCUSSION
5.1
Isotherm Tests
The AWWA standard for PAC specifies a minimum iodine number of 500 mg/g (AWWA, 1991) and this standard has also been adopted by Umgeni Water. Numerous assessments of PAC have been carried out by Umgeni Water over the past 10 years for tender purposes. Originally, PAC selection was based almost entirely on this iodine number standard, although the ash and moisture content were also taken into account. It was noted that although there is a weak trend linking iodine number and PAC adsorption capacity for taste and odour compounds, it was in fact inadequate in selecting the most suitable PAC for these applications. A PAC with a low iodine number (
