A serious and increasingly frequent problem that water utilities face is the occurrence

Transcription

1 filtration A serious and increasingly frequent problem that water utilities face is the occurrence of tastes and odors in drinking water. One of the most common and problematic types of tastes and odors is the musty/earthy type caused by 2-methylisoborneol (MIB) and trans-1,10-dimethyl-trans-9-decalol (geosmin). Common taste and odor treatment methods such as powdered activated carbon, chlorine oxidation, and potassium BY ROBERT NERENBERG BRUCE E. RITTMANN WILLIAM J. SOUCIE permanganate oxidation are largely ineffective for reducing these compounds. Fortunately, ozonation followed by biofiltration can provide a well-suited and effective treatment combination. Ozonation provides at least partial removal of MIB and geosmin, but it also creates undesirable biological instability. Subsequent biofiltration can remove residual MIB and geosmin while at the same time removing the biological instability. A preliminary investigation at a full-scale treatment plant indicates that the ozonation/biofiltration tandem can effectively reduce MIB to below threshold concentrations and that biofiltration plays an important role in this reduction. ozone/biofiltration for removing MIB AND GEOSMIN he presence of objectionable tastes and odors in drinking water is a serious and increasingly frequent problem for water utilities across the United T States and the world. A survey of water consumers showed that more than 15% of respondents used bottled water or a point-of-use treatment device because of unpleasant tastes and odors in their tap water. 1 In one survey of more than 800 utilities in the United States and Canada, 43% reported taste and odor events lasting more than one week, and 16% reported that they had experienced serious taste and odor problems. 2 The same report showed that utilities spent on average 4.5% of their total budgets on taste and odor control. Tastes and odors are the main basis by which most consumers judge the safety of their tap water; therefore, inadequate taste and odor control can create perceived health problems. 3 One of the most common and problematic types of tastes and odors is the musty/earthy type caused by the microbial metabolites 2-methylisoborneol (MIB) and trans-1,10-dimethyl-trans-9-decalol (geosmin). The chemical structures of MIB and geosmin are provided in Figure 1. The presence of MIB and geosmin in raw water sources is a major concern because of their extremely low threshold con- NERENBERG ET AL PEER-REVIEWED JOURNAL AWWA DECEMBER

2 TABLE 1 TABLE 2 MIB Geosmin ng/l ng/l Reference 4 22 Ito et al Young et al Rahesh et al 7 *MIB 2-methylisoborneol Examples of odor threshold concentrations for MIB* and geosmin Typical water quality at Central Lake County Joint Action Water Agency Parameter Influent Effluent ph Alkalinity mg/l as CaCO Total organic carbon mg/l Total chlorine mg/l 0.8 centration and their persistence through conventional water treatment processes. The threshold concentration of a taste and odor compound is the lowest concentration at which it is perceptible to humans. A characteristic feature of MIB and geosmin is that they have threshold concentrations in the nanogram-per-litre (i.e., parts-per-trillion) range. 4 7 Some examples of threshold odor concentrations for MIB and geosmin are provided in Table 1. H 3 C TASTE AND ODOR IN THE GREAT LAKES REGION Although problems with tastes and odors caused by MIB and geosmin commonly occur in reservoirs and eutrophic surface waters, 8 they were FIGURE 1 not a significant problem in oligotrophic Great Lakes waters before the 1990s. In recent years, however, many utilities throughout the Great Lakes region have experienced intense musty/earthy tastes and odors, attributable to MIB, during the summer and OH fall. Although scientific information regarding this occurrence is limited, this taste and odor problem seems to be an indirect consequence of zebra mussel colonization of lake shorelines. 9 CH Filter-feeding zebra mussels lower lake 3 turbidity, enhancing light penetration and increasing productivity of benthic algae in shallow areas. Benthic, filamentous green algae grow in early summer, experience heavy growth in midsummer, and begin to die off by mid to late summer as water temperatures rise. The die-off of the green algae may supply nutrients that support growth of geosmin and MIB-producing cyanobacteria or actinomycetes. 9 Because neither the zebra mussel colonization nor the taste- and odor-producing microorganisms in the lake can be controlled, the earthy/musty odors are likely to remain a challenge to water providers. In response to the large number of utilities affected by MIB and geosmin and the difficulty encountered in treating it, the AWWA Research Foundation organized a taste and odor workshop in July 1998 to address mitigation strategies for extraordinarily high odorant (e.g., MIB and geosmin) levels in the Great Lakes and other regions. 10 Specifically, the conference addressed the apparent failure of powdered activated carbon (PAC), used by most utilities for taste and odor management, to eliminate the high levels of MIB and geosmin. Numerous utilities along the Great Lakes participated in the workshop, including those from Chicago, Ill.; Detroit, Mich.; Buffalo and Rochester, N.Y.; Toledo, Ohio; Windsor, Ont.; Milwaukee, Wis.; and Toronto, Ont. A good example of the severity of the problem is in the Chicago metropolitan area, the largest urban area on the Great Lakes. In this area alone, the MIB problem currently affects approximately 23 treatment plants 11 drawing water from Lake Michigan, including two of the world s largest treatment facilities (in Chicago). Taste and odor problems affect more than 6 million water customers in this area. Nearly all of the affected utilities use conventional treatment processes (coagulation, flocculation, and filtration), either plain sand or anthracitecapped filters, and PAC for seasonal taste and odor control. The incidence of the taste and odor problems is Chemical structures of MIB and geosmin MIB MIB 2-methylisoborneol CH 3 CH 3 CH 3 OH CH 3 Geosmin 86 DECEMBER 2000 JOURNAL AWWA PEER-REVIEWED NERENBERG ET AL

3 evidenced by the increase in the number of musty/earthy taste- and odor-related complaints for the city of Chicago (Figure 2). The large increase in the number of musty/earthy taste and odor complaints beginning in 1993 coincides with the colonization of the shorelines by zebra mussels. 9 The difficulty that utilities encounter in removing MIB using PAC is illustrated by Figure 3, which provides the monthly average influent MIB and effluent MIB concentrations and PAC dosage from Chicago s Jardine Water Purification Plant for 1997, a year with high MIB concentrations. The figure shows that despite relatively high applied PAC doses, minimal MIB removals were achieved. For example, during August 1997, the monthly average influent MIB concentration was 90 ng/l and the average effluent concentration was 64 ng/l, despite an average PAC application rate of 6.5 mg/l. This is an average removal of less than 30%. Although monthly average operational data cannot be used to determine precise PAC removal efficiencies, they illustrate the limitations of high levels of PAC in reducing MIB to below threshold concentrations. The Lake Bluff plant has three medium-frequency ozonators, each with a capacity of 230 kg/d. Although few of the other 23 plants in the Chicago area measured MIB and geosmin analytically, most of them had similar experiences with high applied PAC doses (up to 12 mg/l) and high numbers of complaints related to musty/earthy taste and odor. Only one treatment facility in Chicago s North Shore area was unaffected by taste and odor complaints the Central Lake County Joint Action Water Association (CLC- JAWA) plant in Lake Bluff, Ill. This plant used ozonation and biologically active granular activated carbon (GAC). The fate of MIB in this plant is described later. FIGURE 2 Number of Complaints 1,800 1,600 1,400 1,200 1, Musty/earthy taste and odor complaints for the Chicago, Ill., water supply system Year LIMITATIONS OF CONVENTIONAL METHODS FOR REMOVING MIB AND GEOSMIN The methods most commonly used to combat taste and odor are PAC, chlorine oxidation, and potassium permanganate (KMnO 4 ) oxidation. All of these methods have significant limitations for removing MIB and geosmin, as discussed later. PAC. PAC is the most commonly used method for removing seasonal tastes and odors. However, the effectiveness of PAC for MIB and geosmin is lower than that for other taste- and odor-producing organics; 12 furthermore, the presence of natural organic matter (NOM) and oxidants such as chlorine or chloramines can significantly further reduce its effectiveness For NERENBERG ET AL PEER-REVIEWED JOURNAL AWWA DECEMBER

4 FIGURE 3 PAC mg/l Mar. PAC usage and influent and effluent MIB concentrations at Chicago s Jardine water purification plant in 1997 PAC Intake MIB Outlet MIB Apr. May June example, Gillogly et al 13 performed batch tests with five common types of PAC with MIB-spiked Lake Michigan water (1.8 mg/l total organic carbon [TOC]). They determined that for a 4-h contact time, mg/l PAC was required to reduce MIB from 50 to 5 ng/l, and mg/l was required to reduce MIB from 100 to 5 ng/l. The presence of chlorine products, commonly applied at the intake or head of the plant for zebra mussel control or for disinfectant contact time, further increases the required amounts of PAC. 14 PAC dosages that exceed 12 mg/l are greater than what most conventional plants can apply. July Aug. Sept. Oct. Nov. Dec. Oxidation by chlorine and chlorine products. Chlorine (Cl 2 ), chloramines, and chlorine dioxide (ClO 2 ) are effective for removing some types of tastes and odors but are not effective in removing geosmin and MIB For example, studies by 80 Lalezary et al 16 showed that doses 70 of Cl 2 and ClO 2 as high as 20 mg/l 60 as Cl 2 had removal efficiencies below 50 60% for geosmin and 35% for MIB. 40 For doses below 5 mg/l, removals were below 50% for geosmin and 30 25% for MIB. In some cases, chlorine residuals may enhance the musty/earthy odors, and in others, 0 they may temporarily mask it. 18 Oxidation by KMnO 4. KMnO 4 is an oxidant used in water treatment processes, primarily to prevent the growth of algae and slime in the intake pipes and treatment processes. Permanganate has low removal efficiencies for MIB and geosmin. For example, Lalezary et al 16 showed that 20 mg/l KMnO 4 removed less than 10% of geosmin and MIB. Glaze et al 17 showed that 3 mg/l of KMnO 4 removed only 15% of geosmin and 13% of MIB. MIB ng/l OZONATION FOR REMOVING MIB AND GEOSMIN Ozonation and other advanced-oxidation processes are less commonly used but can be effective methods for removing MIB and geosmin. Ozone (O 3 ) is a strong oxidant used to disinfect surface water, remove tastes and FIGURE 4 Schematic of the Lake Bluff treatment plant Raw water supply Pumping and distribution Ozonation Rapid mix Flocculation Clearwell Sedimentation Filtration 88 DECEMBER 2000 JOURNAL AWWA PEER-REVIEWED NERENBERG ET AL

5 odors, enhance coagulation, and provide other benefits. O 3 is being used in the United States with increasing frequency to comply with disinfection by-product regulations and to inactivate Cryptosporidium. O 3 destroys MIB and geosmin if the dose is high enough. 17,19 23 Lundgren et al 19 ozonated a surface water having TOC of 9 mg/l with dosages of 1.5, 7, and 20 mg/l of O 3 for 10 min in the laboratory. MIB and geosmin were effectively removed (greater than 95%) at the 7-mg/L level while only partially removed (75 and 45%, respectively) at the 1.5-mg/L level more typically used. Glaze et al 17 showed 78% destruction of MIB and 89% destruction of geosmin with an ozonation dose of 4 mg/l on bench-scale tests of Colorado River water. Removals dropped to 40 and 38%, respectively, with a 2-mg/L O 3 dose. Raw water for the Lake Bluff plant is obtained from Lake Michigan through an intake located 915 m offshore. One of the most common...types of tastes and odors is the musty/earthy BIOFILTRATION FOR REMOVING MIB AND GEOSMIN Biofilm treatment, or biofiltration, is being used with increasing frequency in North America, where drinking water is traditionally treated exclusively by physicochemical processes. Although biofilm processes are common treatment technologies for wastewaters, 29 biofilm processes used in drinking water treatment are special because the environment is oligotrophic, meaning that the concentration of growth substrates is extremely low. As a result, biofilm attachment is the only practical means for maintaining a high density of active bacteria. Biofilm processes used in water treatment include slow sand filtration, large-granule filters, fluidized-bed contactors, and rapid filters (sand, GAC, or other media). 5,30 The most popular application of biofilm water treatment in North America consists of retrofitting a conventional rapid filter to perform particle removal and biological treatment, creating a hybrid biofilter. 30 Biofiltration can decrease the potential for bacterial regrowth, reduce chlorinated disinfection by-products formed during secondary disinfection, reduce Cl 2 requirements, and decrease corrosion potential. 27 Additionally, biofiltration can remove taste- and odor-producing compounds and other micropollutants that create health and aesthetic concerns. 5,19,31 35 Biofiltration is a relatively new process that requires numerous areas of research to fully understand its behavior and to develop adequate design criteria. Some variables type caused by the microbial metabolites MIB and geosmin. Although O 3 can at least partially destroy MIB and geosmin, it also reacts with natural organic substances to produce low-molecular-weight oxygenated by-products such as aldehydes, ketones, and carboxylic acids that are much more biodegradable than their precursors. 24,25 These compounds create biological instability that can lead to biological regrowth in the distribution system and the need to apply much greater Cl 2 doses during distribution. 17,26,27 For this reason, a process to remove the instability, such as biological filters, must follow ozonation. 28 that require research include the effects of media, contact time, backwashing, prechlorination, and temperature. Potential concerns regarding biofiltration include turbidity and particle removal, head loss buildup, filter run length, startup time, creation of soluble microbial products, and the presence of pathogens. 36 Ozonation and biofiltration provide a particularly well-suited combination to treat taste and odor problems. Ozonation alone can destroy, at least partially, MIB and geosmin. Biofilm treatment following ozonation stabilizes the water by significantly reducing the concentration of these highly degradable ozonation products. Additionally, the higher instability created by ozonated water is likely to increase biomass in the filter and thereby enhance the biofilter s ability to degrade micropollutants such as taste- and odor-producing compounds. 35,37,38 BIODEGRADATION POTENTIAL FOR MIB AND GEOSMIN The chemical structures of MIB and geosmin suggest that they ought to be biodegraded. 5 Geosmin is an alicyclic alcohol that can be oxidized by a series of dehydrogenase and monooxygenase reactions that lead to central metabolic intermediates and mineralization. MIB is a NERENBERG ET AL PEER-REVIEWED JOURNAL AWWA DECEMBER

6 TABLE 3 TABLE 4 Sampling dates and conditions EBCT Ozone Raw Water For 6-ft/4-ft Event Sampling Dose Temperature Plant Flow Bed Depth* Number Date Time mg/l o C m 3 /d (mgd) min 1 8/3/98 8:30 a.m ,000 (25.9) 14.4/ /10/98 8:30 a.m ,000 (18.5) 20.2/ /17/98 9:00 a.m ,500 (23.9) 15.6/ /24/98 8:30 a.m ,600 (25.0) 14.9/9.9 *The empty bed contact time (EBCT) is provided for the total bed of 6 ft (1.83 m) and for the granular activated carbon bed of 4 ft (1.22 m) only. Fate of MIB* concentrations in Lake Bluff samples MIB Concentrations ng/l Narayan and Nunez 41 performed oxidative manometric studies on known geosmin-degrading bacteria and other bacteria enriched from a soil sample and showed Bacillus strains were most effective at oxidizing geosmin. Yagi et al 42 studied MIB and geosmin removal in several full-scale treatment facilities in Japan and found that slow sand (biological) filtration was most effective for MIB and geosmin removal; with influent concentrations of 25 and 69 ng/l, MIB removals ranged from 88 to 100%, and with an influent geosmin concentration of 360 ng/l, the geosmin removal was 98%. Ashitani et al 33 found that geosmin was removed to near odor threshold levels and MIB was reduced to a lesser extent in a rapid sand filter in a conventional water treatment plant with no prechlorination. The greatest MIB reduction reported by Ashitani 33 was from 120 ng/l (filter influent) to 55 ng/l (filter effluent). The greatest geosmin reduction was from 240 ng/l (filter influent) to 0 ng/l (filter effluent). Lundgren et al 19 used biologically active slow sand filters to remove geosmin and MIB. Using a raw river water containing 8 9 mg/l TOC, MIB and geosmin concentrations of 50 ng/l were passed Percent MIB Removal Attributable to: Date Raw Ozone Settled Filtered Ozonation Settling Filtration Overall 8/3/ D ND 8/10/ (<5) 36 NA** (29 64) D ND 8/17/ D ND *MIB 2-methylisoborneol Musty/earthy odor detected by treatment staff Sample not analyzed Musty/earthy odor not detected by staff **Value not applicable (29 64%) is based on concentrations of 5 to 0 ng/l for ND Note: MIB was not detected at any location on 8/24/98. The most popular application of biofilm water treatment in North America consists of retrofitting a conventional rapid filter to perform particle removal and biological treatment, creating a hybrid biofilter. bicyclic compound that probably follows monooxygenation steps leading to ring cleavage and mineralization. Field and laboratory studies have proved that geosmin and MIB are biodegraded under conditions relevant to biofiltration. Field biodegradation experience. Field experience began in the 1960s with Silvey and Roach, 39 who found that musty/earthy odors intensified during and after cyanobacteria and actinomycete blooms. The metabolites and products of bacterial decay appeared to stimulate the growth of gram-positive bacteria, corresponding with a decrease in taste and odor in the water source. This finding suggests that gram-positive bacteria were able to use organic material released by the cyanobacteria and actinomycetes as a primary substrate and to remove the musty/earthy odor as a secondary substrate. 5 Hoehn 40 isolated and cultured a gram-positive Bacillus strain that could be added to reservoirs to remove musty/earthy tastes and odors. through a 0.5-m sand filter at a rate of 25 mm/h, resulting in a greater than 95% removal for both compounds. Laboratory biodegradation studies. Numerous laboratory studies have documented biodegradation of MIB and geosmin in various types of biofilters. Namkung and Rittmann 32 confirmed that bacteria isolated from lake water were able to degrade geosmin and MIB in a biofilm reactor. The biofilm was developed in a plug-flow column reactor filled with 3-mm glass beads using 1 mg/l of peat fulvic acid as a primary substrate. With input con- 90 DECEMBER 2000 JOURNAL AWWA PEER-REVIEWED NERENBERG ET AL

7 The Lake Bluff plant (below) has nine 60-m 2 biologically active granular activated carbon (GAC) filters (left). At the time of the study, the GAC had been in service for six years without replacement or regeneration. centrations of 100 µg/l (several orders of magnitude higher than the threshold odor concentration), geosmin and MIB were removed at 55 and 44%, respectively. The researchers suggested that greater fractional removals could be obtained by lowering the influent concentration, increasing primary substrate removal, increasing biofilm surface area, improving mass transfer kinetics, and improving microorganism retention. Yagi et al 43 studied MIB and geosmin removals on a biologically active carbon filter seeded with Bacillus cereus. With a contact time of 2.4 min and a loading rate of 8.3 cm/min, they found that 56 58% of MIB and 72% of geosmin were removed biologically. Hattori 19 examined the ability of bacteria in river water to remove spiked amounts of geosmin and MIB. Batch tests were conducted with a honeycomb tube biofilter having a 2-h contact time. Geosmin was reduced from approximately 90 to approximately 15 ng/l, whereas MIB was reduced from approximately 160 to approximately 60 ng/l. Huck et al 34 conducted an experiment to determine the kinetic parameters for removing two taste and odor compounds geosmin and 2,4,6-trichloroanisole in a bench-scale biofilter. The bioreactor showed small removals compared with the control reactor. The authors state that the biofilm reactor had been switched from several weeks of exposure to high-toc river water to the synthetic solution with odorous compounds; furthermore, the experiments were conducted over a short period of time because of scheduling constraints. Earlier studies performed after several weeks of exposure to synthetic water showed significant removals compared with the control bioreactor. Terauchi et al 44 performed pilot-scale tests on a biofilter with porous ceramic media, with a contact time of 12.7 min and a loading of 7.1 m/h (2.9 gpm/sq ft). With average influent MIB concentrations ranging from 50 to 800 ng/l, removals of natural MIB in lake water ranged from 60 to 80%. With lake water spiked with reagent MIB at concentrations of 219 ng/l and 893 mg/l, the average removals were 83 and 73%, respectively. In all cases, the removals were stable despite abrupt changes in influent concentration. Using batch experiments, Izaguirre et al 45 studied conditions enhancing MIB degradation and isolated and identified the responsible bacteria. MIB supported growth as a sole carbon source at mg/l, and at 10 µg/l, MIB was degraded in sterile lake water inoculated with washed bacteria. PRELIMINARY FIELD RESULTS FROM LAKE BLUFF O 3 BIOFILTRATION PLANT An excellent example of the O 3 -biofiltration tandem occurs at the CLCJAWA water treatment facility in Lake Bluff, Ill. This plant, located approximately 32 km (20 mi) north of Chicago, draws its water from southwest Lake Michigan. As with its neighboring treatment facilities, the Lake Bluff source water contains musty/earthy Although scientific information... is limited, musty/earthy taste and odor problems seem to be an indirect consequence of zebra mussel colonization of lake shorelines. NERENBERG ET AL PEER-REVIEWED JOURNAL AWWA DECEMBER

8 TABLE 5 Summary of ozonation studies Water Quality Ozone Ozone Initial Percent TOC* Alkalinity Dose Dose/ MIB MIB mg/l ph mg/l as CaCO 3 mg/l TOC ng/l Removal Source Lundgren et al > > Hattori Glaze et al Ferguson et al Koch et al This study *TOC total organic carbon, MIB 2-methylisoborneol No data reported Typical odors in late summer and fall; however, its finished water is free of musty/earthy odors, as indicated by staff odor tests and a lack of customer complaints, while nearby plants are receiving numerous complaints. A survey found that the CLCJAWA was the only Chicago North Shore facility not experiencing musty/earthy odor complaints. 11 The Lake Bluff treatment facility has a capacity of 142,000 m 3 /d (37.5 mgd) and consists of ozonation, rapid mixing, flocculation, sedimentation, and filtration with biologically active granular activated carbon. Figure 4 provides a schematic of the treatment plant. The absence of such odors in Lake Bluff s treated water appears to be related to its advanced treatment process, particularly the O 3 /biological filtration tandem; however, this cause and effect had not been confirmed, nor were the roles of O 3 and biological filtration known. In order to investigate the role of ozonation and biofiltration at Lake Bluff, the authors conducted a field campaign at the treatment facility in August 1998 to confirm the presence of MIB in the raw water and track its removal through the treatment process. The average filter run during July to September 1998 was 54 h. The typical empty bed contact time from July to September 1998, based on an average flow of 82,600 m 3 /d (22 mgd), was 17 min (based on 1.83 m [6 ft] of total bed height) or 11 min (based on 1.22 m [4 ft] of GAC only). The typical plant influent water quality is shown in Table 2. The GAC had been in service without regeneration for 6 years; therefore, activated carbon adsorption was unlikely to have played a significant role in MIB removal. Water samples were collected once a week during August 1998 from the plant influent, postozonation, postsedimentation, and postfiltration. The sampling dates and conditions are summarized in Table 3. A solid phase microextraction* (SPME) technique 46 was used to analyze for MIB and geosmin. This new method was chosen because it is less expensive and easier to perform than traditional methods, such as closed-loop stripping, liquid liquid extraction, steam distillation, and purge and trap. 47,48 The manufacturers* claim that the method, when used with gas chromatography/mass spectrometry Ozonation and biofiltration provide a particularly well-suited combination to treat taste and odor problems. (GC/MS), is able to detect MIB and geosmin at 1 ng/l (i.e., 1 part per trillion). This study followed the same method except for using a gas chromatography/ flame ionization detector instead of GC/MS, with triplicate runs for each sample. The response was linear from 0 to 100 ng/l. As a backup to the SPME analysis, CLCJAWA s odor *Supelco, Bellafonte, Pa. 92 DECEMBER 2000 JOURNAL AWWA PEER-REVIEWED NERENBERG ET AL

9 TABLE 6 Summary of biofiltration studies Raw Water Loading Contact Total Organic Initial Biological Rate Time Carbon Concentration Removal Reactor/Media m/h min mg/l C ng/l % Source Slow sand filter * * * Yagi et Slow sand filter >95 Lundgren et 19 CMBR with glass beads Namkung and Rittmann Bench GAC biofilter * ( ) Yagi et al Honeycomb tube biofilter Hattori Rapid sand biofilter * Ashitani et al 33 Pilot biofilter with porous * Terauchi et al 44 granular ceramic Rapid GAC biofilter with 7.7 (8/3/98) 14.4/9.6** This study preozonation 5.5 (8/10/98) 20.2/ (8/17/98) 15.6/ *Data not reported Estimated based on reactor volume and loading rate and assuming a porosity of 0.4 CMBR completely mixed biofilm reactor Estimated based on reactor volume and feed flow rate **For a total bed depth of 1.83 m (6 ft) for the GAC bed of 1.22 m (4 ft) only panel determined whether the raw and finished waters had detectable musty/earthy odors. CLCJAWA s standard operating procedure for odor includes a panel of three or more staff members who blindly sample odors from stoppered 250-mL Erlenmeyer flasks. The samples and blanks are heated in a water bath to 60 o C and are sampled in an area free of background odors. If an odor is detected in a sample but not in the blank it is considered a positive detection. Raw and treated water samples are tested. The treated water samples are collected after filtration but before chlorination to avoid potential Cl 2 masking effects. The authors assumed that the odor detection level was around 5 ng/l, which is the target level for the city of Chicago and is consistent with other studies on MIB threshold odor levels. Results of the MIB assays are shown in Table 4. The raw water MIB concentration varied from a high of 43 ng/l (8/17) to nondetect (8/24). Removals by ozonation varied from 36% of total MIB (8/10) to 65% (8/17). The lowest removal corresponded with the lowest O 3 dose (1.3 mg/l), whereas the highest percentage removed was during the date with the highest O 3 dose (1.6 mg/l). No appreciable change in concentration occurred during settling, and in all cases the filtered water concentration was below the detection limit. SPME-determined removals by biofiltration ranged from 26 (8/17) to 46% (8/3), and removal up to 64% may have occurred on 8/10. On 8/10, the raw water MIB concentration was 14 ng/l, and the ozonated water concentration was 9 ng/l. The settled and filtered water concentrations were not measured by SPME that day; however, the filtered water presumably contained less than 5 ng/l, because no odor was detected in the filtered water by Lake Bluff staff on this date. Comparison with other work using O 3. Table 5 compares the CLCJAWA results with others work using O 3 to remove MIB. The conditions of the authors investigation were most similar to those of Glaze et al 17 and Ferguson et al 21 in terms of ph, alkalinity, and O 3 TOC ratio. In each of these cases, the ph was approximately 8.3, the alkalinity was approximately 130 mg CaCO 3 /L, and the initial MIB concentration was 100 ng/l. In the authors study, the ph was 8.4, the alkalinity was 111 mg Biofiltration can decrease the potential for bacterial regrowth, reduce chlorinated disinfection by-products,... reduce chlorine requirements, and decrease corrosion potential. CaCO 3 /L, and the O 3 TOC ratios were 0.66 to The MIB removal was 40% in Glaze s study and 78% in Ferguson s 21 study. In the authors study, the MIB removal was 36 to 54% for the O 3 TOC ratio of 0.66 and 65% for the O 3 TOC ratio of Therefore, the authors results are consistent with prior results in showing the ability of O 3 to achieve partial removal of MIB. Comparison with other work using biofiltration. Table 6 compares the previous field and laboratory results on NERENBERG ET AL PEER-REVIEWED JOURNAL AWWA DECEMBER

10 MIB degradation with the results of this study. Reported percentage removals range from 0 to 100%, with slow sand filters giving the highest percentage removals. The percentage removals from this field study, 26 to 64%, are similar to those reported by others for biofiltration systems. The results from the literature cannot necessarily be directly compared with the results of this study because of differing reactor configurations and conditions; nevertheless, they indicate that these results are consistent with prior work on MIB biodegradation. FUTURE RESEARCH NEEDS The study documents that the significant potential of ozonation plus biofiltration for MIB removal can be achieved in practice. Although ozonation for MIB removal is fairly well studied, information on the fundamental processes of MIB degradation during biofiltration, particularly the influence of ozonation on this process, is lacking. It has been hypothesized that biodegradation of MIB during biofiltration is carried out through a secondary-use mechanism. 5,32 In secondary use, bacteria meet their energy and carbon-source needs by metabolizing other, more readily available compounds. NOM and ozonation by-products are the key growth-supporting substrates, called primary substrates. Because of its very low concentration and/or transient presence, the secondary substrate contributes negligibly to supporting the bacteria that degrade it. If MIB is degraded by secondary use, preozonation could have a beneficial effect by increasing the bacteria s primary substrate. Then, ozonation can significantly increase the total biofilm accumulation. Whether that added biomass is capable of degrading MIB is not known and is a key research need. The biodegradation kinetics for MIB and geosmin and the effects of mass transfer resistance in the biofilm need to be addressed. Accurate estimates of the intrinsic Monod parameters are necessary to make broad assessments of biodegradation potential in any biofilter setting. Greater biofilm accumulation will have a beneficial effect for MIB and geosmin when the biofilm is not already deep or diffusion is limited. 30 Faster intrinsic biodegradation kinetics tend to limit the diffusion of the biofilm. REFERENCES 1. Manwaring, J.F.; Zdep, S.M.; & Sayre, I.M. Public Attitudes Toward Water Utilities. Jour. AWWA, 78:6:34 (June 1986). 2. Suffet, I.H. et al. AWWA Taste and Odor Survey. Jour. AWWA, 88:4:168 (Apr. 1996). 3. McGuire, M. Off-flavor as the Consumer s Measure of Drinking Water Safety. Water Sci. & Technol., 31:11:1 (1995). 4. Ito, T. et al. The Relationship Between Concentration and Sensory Properties of 2-Methylisoborneol and Geosmin in Drinking Water. Water Sci. & Technol. 20:8/9:11 (1988). 5. Rittmann, B.E.; Gantzer, C.J.; & Montiel, A. Biological Treatment to Control Taste and Odor Compounds in Drinking Water. Advances in Taste and Odor Treatment and Control. AWWA Res. Fdn., Denver (1995). 6. Young, W.F. et al. Taste and Odor Threshold Concentrations of Potential Potable Water Contaminants. Water Res., 30:2:331 (1996). 7. Rashash, M.C.; Dietrich, A.M.; & Hoehn, R.C. FPA of Selected Odorous Compounds. Jour. AWWA, 89:4:131 (Apr. 1997). 8. Mallevialle, J. & Suffett, I.H., editors. Sources of Taste and Odor in Drinking Water. Identification and Treatment of Tastes and Odors in Drinking Water. AWWA Res. Fdn., Denver (1987). 9. Vogel, J.C. et al. Chicago s Suddenly Appearing, High-intensity, Longduration, Musty-earthy, Taste and Odor Incidents: Why Chicago? Why Now? Proc AWWA WQTC, Chicago. 10. Taste and Odor Current Issues Workshop. Workshop Binder. July 23 24, 1998, AWWA Res. Fdn. Taste and Odor Current Issues Workshop, Chicago. 11. West Shore Water Producers Assn Odor Survey. Wilmette Water Plant, Wilmette, Ill. (unpubl). 12. Lalezary, S.; Pirbazari, M.; & McGuire, M.J. Evaluating Activated Carbons for Removing Low Concentrations of Taste- and Odor-producing Organics. Jour. AWWA, 78:11:76 (Nov. 1986). 13. Gillogly, E.T. et al. 14C-MIB Adsorption on PAC in Natural Water. Jour. AWWA, 90:1:98 (Jan. 1998). 14. Gillogly, T. et al. Effect of Chlorine on PAC s Ability to Adsorb Mib. Jour. AWWA, 90:2:107 (Feb. 1998). 15. Lalezary-Craig, S. et al. Optimizing the Removal of Geosmin and 2- Methylisoborneol by Powdered Activated Carbon. Jour. AWWA, 80:3:73 (Mar. 1988). 16. Lalezary, S.; Pirbazari, M.; & McGuire, M.J. Oxidation of Five Earthy-musty Taste and Odor Compounds. Jour. AWWA, 78:3:63 (Mar. 1986). 17. Glaze, W.H.; Schep, R.; & Chauney, W. Evaluating Oxidants for the Removal of Model Taste and Odor Compounds From a Municipal Water Supply. Jour. AWWA, 82:5:79 (May 1990). 18. Duguet, J.P. & Mallevialle, J. Oxidation Processes. Advances in Taste and Odor Treatment and Control. AWWA Res. Fdn., Denver (1995). 19. Lundgren, B.V. et al. Formation and Removal of Off-flavor. Water Sci. & Technol., 20:8/9:245 (1988). 20. Hattori, K. Water Treatment Systems and Technology for the Removal of Odor. Water Sci. & Technol., 20:8/9:237 (1988). 21. Ferguson, D.W. et al. Comparing Peroxone and Ozone for Controlling Taste and Odor Compounds, Disinfection By-products, and Microorganisms. Jour. AWWA, 90:4:181 (Apr. 1990). 22. Koch, B.; Gramith, J.T.; Dale, M.S.; & Ferguson, D.W. Control of 2- Methylisoborneol and Geosmin by Ozone and Peroxone: A Pilot Study. Water Sci. & Technol., 25:2:291 (1992). 23. Gramith, J.T. Ozone as a Treatment Option. Advances in Taste and Odor Treatment and Control. AWWA Res. Fdn., Denver (1995). 24. Van Der Kooij, D; Hijnen, W.A.M.; & Kruithof, J.C. Effects of Ozonation, Biological Filtration, and Distribution on the Concentration of Easily Assimilable Organic Carbon in Drinking Water. Ozone Sci. & Engrg., 11:297 (1989). 94 DECEMBER 2000 JOURNAL AWWA PEER-REVIEWED NERENBERG ET AL

11 SUMMARY Ozonation and biofiltration provide a particularly wellsuited combination to treat taste and odor. Ozonation alone can bring about partial destruction of MIB (and other odor compounds, such as geosmin). It also creates biological instability by transforming nonbiodegradable NOM into smaller, more oxidized compounds that are substrates for bacteria. Biofiltration following ozonation stabilizes the water by significantly reducing the concentration of biodegradable organic matter. In the case of MIB, the higher instability created by ozonating the water may enhance the biofilter s ability to degrade this musty/earthy micropollutant. 5 The literature and investigation at CLCJAWA suggest that the ozonation biofiltration tandem can effectively reduce high levels of MIB to below threshold values. Most important, this study highlights the important role that biofiltration has in reducing this micropollutant. ACKNOWLEDGMENT The authors thank the board of directors of CLCJAWA for funding the field investigation. ABOUT THE AUTHORS: Robert Nerenberg* is an AWWA member and a research assistant and doctoral student in the Department of Civil Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, IL ; <r-nerenberg@northwestern.edu>. Bruce E. Rittmann is an AWWA member and John Evans Professor of Environmental Engineering at Northwestern University. William J. Soucie is an AWWA member and a laboratory supervisor at the Central Lake County Joint Action Water Agency in Lake Bluff, Ill. *To whom correspondence should be addressed If you have a comment about this article, please contact us at <journal@awwa.org>. 25. Goel, S.; Hozalski, R.M.; & Bouwer, E.J. Biodegradation of Natural Organic Matter: Effect of NOM Source and Ozone Dose. Jour. AWWA, 87:1:90 (Jan. 1995). 26. Rittmann, B.E. & Snoeyink, V.L. Achieving Biologically Stable Drinking Water. Jour. AWWA, 76:10:106 (Oct. 1984). 27. 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Bench-scale Determination of the Removal of Odor Compounds With Biological Treatment. Water Sci. & Technol., 31:11:203 (1995). 35. Rittmann, B.E. Transformation of Organic Micropollutants by Biological Processes. The Handbook of Environmental Chemistry, Quality, and Treatment of Drinking Water (J. Hrubec, editor). 5:B:31 (1995). 36. Urfer, D.; Huck, P.M.; & Booth, D.J. Biological Filtration for BOM and Particle Removal: A Critical Review. Jour. AWWA, 89:12:83 (Dec. 1997). 37. LeChevallier, M. et al. Evaluating the Performance of Biologically Active Rapid Filters. Jour. AWWA, 84:4:136 (Apr. 1992). 38. Dewaters, J.E. & DiGiano, F.A. Influence of Ozonated Natural Organic Matter on the Biodegradation of a Micropollutant on a GAC Bed. Jour. AWWA, 82:8:69 (Aug. 1990). 39. Silvey, J.K.G. & Roach, A.W. Studies on Microbiotic Cycles in Surface Water. Jour. AWWA, 56:1:60 (1964). 40. Hoehn, R.C. Biological Methods for the Control of Tastes and Odors. 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AWWA Res. Fdn., Denver (1995). 48. Krasner, S.W.; Hwang, C.J.; McGuire, M.J. A Standard Method for Quantification of Earthy/musty Odorants in Water, Sediments, and Algal Cultures. Water Sci. & Technol., 15:127 (1983). NERENBERG ET AL PEER-REVIEWED JOURNAL AWWA DECEMBER