POWDERED ACTIVATED CARBON: CAN THIS BE EFFECTIVELY ASSESSED IN THE LABORATORY?

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1 Presented at WISA 2000, Sun City, South Africa, 28 May 1 June 2000 POWDERED ACTIVATED CARBON: CAN THIS BE EFFECTIVELY ASSESSED IN THE LABORATORY?, 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. 1 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

2 2 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. 2 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

3 3 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). 3 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 m 2 /g, although for potable water applications, activated carbon with a surface area in the region of 500 to m 2 /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 <1 and 100 nm for micropores and anything from >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.

4 4 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. 4 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 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

5 5 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 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.

6 6 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 (reapproved 1988) Standard Test Method for Total Ash Content of Activated Carbon, Moisture content: Moisture content was analysed using the ASTM D (Reapproved 1988) Standard Test Methods for Moisture in Activated Carbon, 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 (<500 mg/g) is almost always unsuitable for taste and odour removal. A reasonably high iodine number, usually around 700 mg/g and higher, but at least more than 500 mg/g is required before a carbon is found to provide good removal of compounds such as geosmin and 2-MIB. However, a high iodine number is not necessarily an indication that a PAC will be effective in adsorbing the target compounds. The set of results displayed in Table 1 highlights the inadequacy of the iodine number in assessing a suitable activated carbon. The samples are ranked from best to worst in terms of adsorption potential for geosmin as determined from the modified jar test procedure which is discussed below. These results are graphically depicted in Figure 1, which indicates that although there is a general decrease in the iodine number as the adsorption potential of a PAC for geosmin deteriorates, a high iodine number is not a guarantee of effective geosmin removal (sample 23) and conversely, a PAC with a relatively low iodine number can still give rise to effective geosmin removal (sample 8).

7 7 TABLE 1: Iodine numbers and percentage geosmin remaining after treatment at 15 mg/l for different samples of PAC. PAC Iodine No. mg I 2 /g %Geosmin remaining PAC Iodine No. mg I 2 /g %Geosmin remaining Iodine Number mg/g Iodine No % Geosmin Remaining % Geosmin Remaining 0 PAC FIGURE 1: Iodine number and % geosmin remaining after treatment with 15 mg/l PAC for different samples of PAC. Other indicator tests such as the methylene blue and tannin number tests were also investigated to determine whether these might more suitable in assessing the ability of a PAC to remove taste and odour compounds. Good correlation was observed between the iodine number and methylene blue number results as can be seen in Figure 2 which shows iodine number and methylene blue number results for 46 different PACs. This indicates that both tests generally give similar data and that in both cases, this data is not always of much value in terms of assessing full scale performance of the PAC.

8 8 Iodine No. mg /g Iodine No MB No Methylene Blue No.g/100 g PAC FIGURE 2: Iodine numbers and methylene blue numbers for different PAC samples. Tannin number analyses were conducted on 11 different samples of PAC, together with iodine numbers, methylene blue numbers and the results of the modified jar test procedure designed to assess geosmin adsorption potential. This data appears in Figure 3 and again indicates the poor correlation between geosmin adsorption potential of a PAC and the iodine and methylene blue numbers. A far stronger relationship was observed between tannin number and geosmin adsorption potential. The lower the tannin number, the greater the quantity of tannic acid adsorbed by the PAC and the lower the percentage geosmin remaining after treatment with PAC i.e. the better the geosmin adsorption by that PAC. Other researchers have made similar observations. Simpson and MacLeod (1991) found a strong inverse relationship between tannin number and the effectiveness of a carbon in removing geosmin and 2-MIB i.e. the lower the tannin number the better the removal of these two taste and odour compounds. It is interesting to note that the tannic acid molecule, which has a molecular mass of more than 1000, is much larger than either geosmin or 2-MIB (182 and 168 respectively) (Lalezary et al, 1986), so one would expect adsorption to occur in a different size range of pores. In spite of the fairly good agreement between the tannin number and the ability of the PAC adsorb geosmin, the tannin number tests is not always reliable in determining the best PAC for this application. As can be seen in Figure 3, PACs with relatively poor geosmin adsorption potential sometimes have fairly low tannin numbers. In addition to this, the tannin number fails to account for the background organics, the contact time, or the effect of other chemicals used in the treatment process, such as coagulants, chlorine and lime. The correlation between tannin number and target compounds other than geosmin and 2-MIB would need to be evaluated before deciding whether this test could be used to assess a PAC for removal of such compounds.

9 9 Iodine No. Iodine/Tannin No Tannin No. MB No. %Geosmin remaining PAC ranked for Geosmin removal MB No. / %Geosmin Remaining FIGURE 3: Iodine, methylene blue and tannin numbers together with percentage geosmin remaining at 15 mg/l for 11 samples of PAC. Freundlich isotherm tests proved to be ineffective in assessing PAC. It was found that the concentrations of geosmin and 2-MIB were in many cases close to the detection limits (5 ng/l) at the higher PAC concentrations and that small differences in the concentration values could result in fairly significant differences in the 1 /n and k Freundlich parameters. Often PAC samples which according to the Freundlich tests had a fairly good adsorption potential for geosmin, did not prove to be particularly efficient in removing this contaminant when used on the plant. 5.2 Modified Jar Test Procedure The modified jar test for assessment of PAC was developed to simulate as closely as possible the conditions on the plant. This test takes into account a number of variables, including the effect of the background organics and other water treatment chemicals on PAC adsorption as well as the influence of the contact time and mixing energy. During refinement of this test, a standard contact time of half an hour was initially used, but correlation between laboratory and full scale results was sometimes poor. It was subsequently realised that a contact time similar to that being used on the plant, needed to be employed, since the kinetics of the adsorption reaction are so important when assessing a PAC for treatment applications. This is suspected of being one of the reasons why many of the isotherm tests, such as the iodine and methylene blue number tests and Freundlich tests are often inadequate in assessing PAC. All these isotherm tests are conducted under equilibrium conditions, which means that the kinetics of the reaction are not accounted for. It became obvious from these tests that two different activated carbon samples might both have similar total adsorption capacities for a contaminant such as geosmin, but one sample may adsorb the geosmin at a much greater rate than the other. This has important implications in water treatment, where a PAC may only be in contact with the water for as little as two hours. A carbon may have a high capacity for adsorption of a contaminant, but if the rate of adsorption is slow, it may not be suitable for the removal of this contaminant under normal water treatment conditions. Obviously the isotherm tests could be modified to simulate the contact time used on the plant, but since the modified test has proved so successful and because it is so easy to conduct, this is considered by the authors to be a better option.

10 10 A case study demonstrates the usefulness of this modified jar test procedure. In December 1998, an algal bloom in the Umzinto Dam on the South Coast of KwaZulu/Natal resulted in a geosmin incident at the Umzinto Water Works, a small water treatment plant operated by Umgeni Water. The geosmin concentrations in the raw water were relatively low (50 to 100 ng/l), but high enough to result in consumer complaints and so PAC addition was implemented at the head of the works. Despite the fact that the geosmin concentration was relatively low, a PAC dose of around 20 to 25 mg/l was needed to reduce the geosmin to levels where it no longer caused a nuisance. These concentrations of PAC were far higher than one would normally have expected under the conditions and so the modified jar test was used in order to check whether the correct PAC dose was being used. The results of the tests conducted on the Umzinto water appear in Table 2 and clearly indicate that a PAC dose of 20 mg/l was needed before the geosmin concentration was reduced to below 20 ng/l. The human threshold odour concentrations for geosmin and 2-MIB can vary over a fairly wide range of concentrations, depending on the water temperature and residual chlorine levels (Persson, 1980), but in the literature threshold figures of 10 ng/l (Krasner et al, 1983), 15 ng/l (Persson, 1979) and ng/l (Bowmer et al, 1992) are quoted. In Umgeni Water s experience, consumer complaints usually start when the geosmin concentration reaches approximately 20 ng/l and therefore a geosmin concentration of less than 20 ng/l in the final treated water is aimed at during occurrences of this contaminant. The laboratory tests accurately confirmed the relatively high PAC dose required for adequate geosmin removal. TABLE 2: Modified jar test results for Umzinto Water Works water. PAC Concentration Geosmin ng/l One of the biggest advantages of the modified jar test procedure is that apart from accounting for variables such as the background organic content of the water and the contact time, it also allows for assessment of the effect of treatment chemicals, such as chlorine, lime, coagulant and ozone on the adsorption properties of the PAC. Lalezary and her co-workers (1986) also developed a modified jar test procedure in order to better evaluate PAC in the laboratory and their studies revealed that the effect of coagulants and chlorine on adsorption did not follow the patterns which they anticipated. The presence of coagulants or chlorine or both enhanced the removal of taste and odour causing organics, whereas Lalezary and co-workers had expected a decrease in adsorption. Effects such as these are not possible to evaluate using the traditional PAC assessment tests such as the iodine number or tannin number. In addition to this, the modified jar test procedure is particularly simple to conduct. One needs only a jar stirrer apparatus and equipment which is generally found in all routine analytical laboratories. One could argue that the analysis of taste and odour compounds requires expensive and sophisticated analytical instrumentation such as a gas chromatograph

11 11 (GC) and mass spectrometer (MS), but the test can be successfully used without the need to analyse these compounds. Since consumer complaints are derived from the smell of the water and at these levels the effect is purely aesthetic, posing no health dangers, one needs only to determine the PAC dose required to reduce the odour of the water to below the human threshold concentration and this can be done using sense of smell. The modified jar test is carried out as described above, using increasing quantities of PAC. The beakers containing the water are then brought to the sniffer starting with the water treated with the highest PAC concentration. It is preferable to use a heated smell bell, but not necessary. The correct PAC dose is the lowest dose at which geosmin is still undetectable. The carbon dose required to remove a taste and odour problem, or any organic contamination problem depends on both the adsorptive capacity of the carbon and on the background organic matter present in the water. The adsorptive capacity is in turn dependent upon the concentration of the contaminant, and this can vary widely. In order to fully characterise such a system, a large amount of data is required, the collection of which can be time consuming and the analysis, especially in the case of compounds such as geosmin and 2-MIB, can be complicated and expensive. However recent studies by Knappe and co-workers (1998) have shown that a single isotherm may be adequate in providing enough information to properly assess an activated carbon. They showed that trace compounds, such as atrazine, exhibit a unique relationship with NOM and that if the initial concentration of the trace compound is lower than a certain value, then the percent removal of this compound at equilibrium for a given carbon concentration in a particular water is independent of the initial concentration of the trace compound. In the case of atrazine, Knappe and co-workers (1998) found that these conditions held if the concentration of the atrazine was less than 50 µg/l. Gillogly and co-workers (1999) investigated this trend further to assess its applications for 2- MIB adsorption for a number of activated carbons and natural waters. They found that if they plotted the equilibrium data as percent 2-MIB remaining (C e /C o x 100%) in solution against the carbon dose, rather than the usual isotherm plot of the concentration of 2-MIB adsorbed onto the carbon (q e ) against the 2-MIB concentration in solution at equilibrium (C e ), the data plotted as a single line, provided that the initial concentrations of 2-MIB were sufficiently low. Initial 2-MIB concentrations varying between 45 ng/l and 178 µg/l were investigated and confirmed Knappe and co-workers (1998) findings. It was shown (Gillogly et al, 1999) that for a given carbon/water system, a particular carbon dose will remove a fixed amount of the 2-MIB present and that it should therefore be possible to predict the minimum amount of carbon required to remove a 2-MIB odour problem by analysing a single bottle-point isotherm. This tests determines the minimum amount of carbon (i.e. under equilibrium conditions), and therefore higher carbon concentrations will be required for the same amount of contaminant if equilibrium conditions are not achieved. It was realised that if it was assumed that the same trend existed for geosmin, which is highly likely since it has been shown to exist for both atrazine and 2-MIB, and the results of the modified jar test were plotted as described above (i.e. percentage geosmin remaining in solution against the PAC concentration), then it should be possible to determine the best type of PAC and the optimal PAC concentration to use for a particular geosmin removal in a given natural water, provided that the geosmin concentration is not excessively high. The equation which describes this relationship has been calculated as being the following: C = Co m K C Co n Where: C = solution concentration of geosmin after test

12 12 Co = initial concentration of geosmin m = mass of PAC K = a constant 1 n = a constant If a log probability curve is plotted, the equation becomes: C 1 C log 1 + log m = log + log K Co n Co The greater the adsorption of geosmin by the PAC, the smaller C/C 0 becomes and log (1- C/C 0 ) approaches zero. The equation therefore approaches linearity Percentage Geosmin in solution % PAC PAC 2 PAC PAC mg/l FIGURE 4: PAC concentration as a function of the percentage geosmin remaining in solution for different PAC samples assessed using the modified jar stirrer test. It can be seen from the data plotted in Figure 4 below, that as the percentage geosmin remaining gets smaller, so the curve approaches linearity. Extrapolation of the straight line portion of the graphs reveals that for different geosmin removal requirements, different PACs are better. For example, assuming that the concentration of geosmin in the water is 400 ng/l, then one would require a PAC dose that reduced the geosmin in the water to less than 5% (i.e. less than 20 ng/l) and under these conditions, PAC 1 and PAC 2 would be best at a concentration of approximately 22 mg/l. If the initial geosmin concentration was only 100 ng/l, achieving a final geosmin concentration of less than 20 ng/l would require that the percentage geosmin remaining in solution is less than 20%. In this case PAC 3 would perform the best, a PAC dose of 8 mg/l being required. The PAC 2 and PAC 3 doses required to achieve the same geosmin reduction would be approximately 9 and 10 mg/l respectively. 6 CONCLUSIONS The modified jar stirrer procedure described in this paper has been found to be a simple and reliable test for the assessment of different PACs for taste and odour compound removal for water treatment applications This modified test can be used to accurately determine the optimum PAC dose and the most effective PAC for a particular application and it takes into account a wide number of variables, including the effect of NOM, contact time and the effect

13 13 of other water treatment chemicals. Using this test it is possible to effectively assess PAC in the laboratory. 7 ACKNOWLEDGEMENTS Umgeni Water are gratefully acknowledged for enabling this work to be conducted. Martin Pryor is thanked for assistance given with the mathematical equations and Rand Water are thanked for providing information regarding the tannin number test. 8 REFERENCES 1. American Society of Testing and Materials (ASTM) D , Standard Test for Determination of Iodine Number of Activated Carbon, American Society of Testing and Materials (ASTM) D (Reapproved 1988), Standard Test Method for Total Ash Content of Activated Carbon, American Society of Testing and Materials (ASTM) D (Reapproved 1988), Standard Test Methods for Moisture in Activated Carbon, American Water Works Association (AWWA), Standard for Powdered Activated Carbon ANSI/AWWA B600-90, USA, Bowmer, K H; Padovan, A; Oliver, R L; Korth, W; and Ganf, G G; Physiology of geosmin production by Anabaena circinalis isolated from the Murrumbidgee River, Australia, Wat. Sci. Tech., 25(2), pp , Chemviron Booklet, Basic concepts of adsorption on activated carbon, Printed in Belgium, Chemvrion Booklet, Characterisation of activated carbon, Printed in Belgium, October, Crittenden, J C; Luft, P; and Hand, D W; Prediction of multicomponent adsorption equilibria in background mixtures of unknown composition, Water Research, 19(12), pp , DIN 19603; Activated carbon for water treatment, technical conditions of delivery, Germany, European Council of Chemical Manufacturers Federations, (CEFIC), Test Methods for Activated Carbon, Belgium, Gillogly, T E T; Snoeyink, V L; Elarde, J R; Wilson, C M; and Royal, E P; C-MIB adsorption on PAC in natural water, J.AWWA, 90(1), pp , Gillogly, T E T; Snoeyink, V L; Newcombe, G; and Elarde, J R; A simplified method to determine the powdered activated carbon dose required to remove methylisoborneol, Wat. Sci. & Technol., 40(6), pp 59-64, 1999.

14 Greenbank, A I; Effects of starting material on activated carbon characteristics and performance in water treatment, Poster presented at the Brewing Congress of the Americas (BCOA), St Louis, MO, Gregg, S J; and Sing, K S W; Adsorption, surface area and porosity, Academic Press, London, Knappe, D R U; Matsui, Y; Snoeyink, V L; Roche, P; Prados, M J; and Boubigot, M-M; Predicting the capacity of powdered activated carbon for trace organic compounds in natural waters, Environ. Sci. & Technol., 32(11), pp , Krasner, S W ; Hwang, C J; and MacGuire, M J; A standard method for quantification of earthy-musty odorants in water, sediments and algal cultures, Wat. Sci.Technol., 15(6/7), pp , Lalezary, S; Pibazari, M; and McGuire, M J; Oxidation of five earthy-musty taste and odour compounds, J.AWWA, 78(3), pp 62-69, Najm, I N; Snoeyink, V L; Suidan, M T; Lee, C H; and Richard, Y; Effect of particle size and background natural organics on the adsorption efficiency of PAC, Research & Technology, J.AWWA, 82(1), pp 65-72, Najm, I N; Snoeyink, V L; and Richard, Y; Effect of initial concentration of SOC in natural water on its adsorption by activated carbon, J.AWWA, 83(8), pp 57-63, Persson, P -E; Notes on muddy odor: IV Sensory properties of geosmin in water, Aqua Fennica, 9, pp 57-66, Persson, P -E; Sensory properties and analysis of two muddy odour compounds, geosmin and 2-methylisoborneol, in water and fish, Wat. Res., 14, pp , Sanks, R L; Water Treatment Plant Design for the Practicing Engineer, Pub. Butterworth, Simpson, K M R; and MacLeod, B W; Comparison of various powder activated carbons for the removal of geosmin and 2-methylisoborneol in selected water conditions, Proc. Annual Conf. AWWA, Orlando, Florida, pp , Sontheimer, H; The Use of Powdered Activated Carbon, Translation of Reports of Special Problems of Water Technology, vol. 9, Adsorption, U S Environmental Protection Agency, Report EPA-600/ , December, Water Purification Works Design, Editor F A van Vuuren, Pub. Beria Printers, Water Quality and Treatment. A Handbook of Community Water Supplies, 4 th Edition, Edited by F W Pontius, Pub. McGraw-Hill, Wnorowski, A U; and Scott, W E; Incidence of off-flavours in South African waters, Wat. Sci. Tech., 25(2), pp , 1992.