Long term case study of MIEX pre-treatment in drinking water; understanding NOM removal

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 3 9 e1 5 4 8

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Long term case study of MIEX pre-treatment in drinking water; understanding NOM removal Mary Drikas*, Mike Dixon, Jim Morran Australian Water Quality Centre, South Australian Water Corporation, GPO Box 1751, Adelaide SA 5001, Australia

article info

abstract

Article history:

Removal of natural organic matter (NOM) is a key requirement to improve drinking water

Received 25 June 2010

quality. This study compared the removal of NOM with, and without, the patented

Received in revised form

magnetic ion exchange process for removal of dissolved organic carbon (MIEX DOC) as

18 November 2010

a pre-treatment to microfiltration or conventional coagulation treatment over a 2 year

Accepted 18 November 2010

period. A range of techniques were used to characterise the NOM of the raw and treated

Available online 24 November 2010

waters. MIEX pre-treatment produced water with lower concentration of dissolved organic carbon (DOC) and lower specific UV absorbance (SUVA). The processes incorporating MIEX

Keywords:

also produced more consistent water quality and were less affected by changes in the

MIEX

concentration and character of the raw water DOC. The very hydrophobic acid fraction

Coagulation

(VHA) was the dominant NOM component in the raw water and was best removed by MIEX

Microfiltration

pre-treatment, regardless of the raw water VHA concentration. MIEX pre-treatment also

NOM

produced water with lower weight average apparent molecular weight (AMW) and with the

Fractionation

greatest reduction in complexity and range of NOM. A strong correlation was found

Molecular weight

between the VHA content and weight average AMW confirming that the VHA fraction was a major component of the NOM for both the raw water and treated waters. ª 2010 Elsevier Ltd. All rights reserved.

1.

Introduction

Natural organic matter (NOM) has a significant impact on drinking water quality either directly, by reacting with water treatment chemicals (to form disinfection by-products), or indirectly, by impacting water treatment processes (including fouling of membranes and reducing the effectiveness of activated carbon for contaminant removal). Therefore the water industry has focussed on improving current treatment and developing new processes to increase the removal of NOM. Conventional treatment comprising coagulation/flocculation/ sedimentation/filtration is one of the most widely used methods to remove NOM. Extensive research has been undertaken to increase the extent of NOM removal by conventional treatment, including the use of increased

coagulant doses and reduced pH, referred to as enhanced coagulation (Crozes et al., 1995; White et al., 1997; Bell-Ajy et al., 2000). A more recent technology developed specifically for the removal of NOM is the patented MIEX DOC Process (Morran et al., 1996; Drikas et al., 2002). This process utilises a strong base anion-exchange resin, incorporating magnetic iron oxide particles within its core, which is applied to raw water utilising a stirred contactor. The small resin beads facilitate rapid reaction whilst the magnetic component allows separation of the resin and recycling of the resin in a continuous process. This differs from other more traditional applications where ion exchange resin is applied as the final polishing step within a filter (Brattebo et al., 1987; Baker et al., 1995). Laboratory scale testing of the MIEX resin has proven the effectiveness of the process for rapid removal of NOM, to a greater extent than that

* Corresponding author. Tel.: þ61 8 7424 2110; fax: þ61 8 7003 2110. E-mail address: [email protected] (M. Drikas). 0043-1354/$ e see front matter ª 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2010.11.024

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possible by coagulation, or enhanced coagulation, in a range of waters (Drikas et al., 2002, 2003a; Singer and Bilyk, 2002; Morran et al., 2004; Fearing et al., 2004; Humbert et al., 2005; Boyer and Singer, 2005). Some studies have also been conducted comparing pilot plant or full scale MIEX treatment with coagulation (Drikas et al., 2003b; Allpike et al., 2005; Boyer and Singer, 2005; Warton et al., 2007; Singer et al., 2007, 2009; Jarvis et al., 2008) although all of these studies have been conducted over a short period of time. A few studies have also assessed the effectiveness of MIEX as a means of reducing fouling of microfiltration or ultra filtration membranes (Fabris et al., 2007; Humbert et al., 2007; Dixon et al., 2010). MIEX has been shown to remove both hydrophobic and hydrophilic organic acid fractions of NOM (Singer and Bilyk, 2002; Morran et al., 2004; Fearing et al., 2004; Boyer and Singer, 2005; Mergen et al., 2008, 2009) to a greater extent than possible with coagulation alone. MIEX was also found to remove a wider range of molecular weight components than coagulation with alum (Drikas et al., 2003a, b; Morran et al., 2004; Allpike et al., 2005; Humbert et al., 2005; Singer et al., 2007). The first MIEX plant was commissioned at Mt Pleasant in South Australia in August 2001 (Drikas et al., 2003b). The Mt Pleasant Water Treatment Plant (WTP) is a small (2.5 ML/d) potable water treatment plant supplying high quality treated water to the local community. However the plant is more complex, innovative and diverse in processes than necessary to enable the MIEX DOC process to be fully evaluated. The WTP has been divided into two streams of 1.25 ML/d capacity, each incorporating the MIEX DOC process but the plant also enables comparison of two separate subsequent processes for the removal of suspended matter e conventional treatment (comprising coagulation, flocculation, sedimentation, and filtration) and submerged microfiltration (MF) (Drikas et al., 2003b). This study enabled an extended evaluation of the impact of the MIEX DOC process on NOM removal by comparing the performance of the two processes operating at the Mt Pleasant WTP with separate pilot plant installations utilising the same processes (conventional treatment and submerged MF) but without MIEX pre-treatment, over a 2 year period. A quantitative assessment of the NOM removed by all the treatment processes was undertaken together with a detailed study of the character of the remaining NOM using a rapid fractionation technique and molecular weight profiles for 16 months of this period. This study has identified novel benefits of the continuous operation of the MIEX DOC process and provided a clearer understanding of the character of the NOM removed.

2.

Materials and methods

2.1.

Treatment processes

A schematic of the treatment trains used is provided in Fig. 1. A conventional pilot plant (Conv) consisting of coagulation, flocculation, sedimentation and rapid filtration was established on site at the Mt Pleasant WTP using the same raw water that supplied the WTP. The flash mixer was a vessel of 1.5 L volume which was stirred at a rate of 200 rpm with a flat paddle agitation blade. Alum was dosed directly into the top of this vessel via a piston dosing pump incorporating a flow

dampening device. The two flocculation bays held 45 L each and were separated by a plate which the water laundered over. The first vessel was stirred at 80 rpm and the second at 40 rpm by an overhead stirrer with 25 offset flat paddles. Three 50 mm pipes delivered flocculated water into the 65 L sedimentation bay. The inverted pyramid shaped sedimentation bay allowed sludge to be collected over three days and be drained off to waste. Settled water was laundered from the top of the sedimentation bay via flexible beverage tubing which ran to a peristaltic pump. The pipe was split into three via a manifold in order to reach the desired flow rate and pumped via three peristaltic heads to the top of the filter column. The filter column consisted of a 140 mm diameter acrylic column and was filled with 600 mm of gravel (of varying grades), with 300 mm of sand of size 0.5e0.6 mm and 750 mm anthracite of size 1.0e1.1 mm. The media was of the same type and depth as the filters on Stream 1 of the Mt Pleasant WTP. The conventional pilot plant operated for three days on and four days off. The pilot plant throughput was 36 L/h which gave 2.5 h flocculation and 2 h settling time. The alum dose was 40 mg/L (as Al2(SO4)3$18H2O) over the study period. This was selected by the use of a model (van Leeuwen et al., 2005) and confirmed by regular jar tests to achieve the optimum DOC removal (defined as the point of diminishing return, where an additional 10 mg/ L alum produces <0.1 mg/L DOC reduction). The pH was not optimised but was between 6.5e6.8 throughout the study. Conventional treatment (Conv) was compared with Stream 1 at the Mt Pleasant WTP which incorporates MIEX followed by conventional treatment comprising coagulation, flocculation, sedimentation, rapid filtration (MIEX Coag) (Fig. 1). During the period July 2005 to June 2007, MIEX was applied to maintain the resin dose at or above 10 mL/L for 10 min contact followed by sedimentation and removal of the resin before entering the separate particulate removal processes. The actual resin dose varied between 8 and 16 mL/L (average 12 mL/L) over this period. The resin was recirculated in a continuous process with 10% removed for regeneration using sodium chloride. Fresh regenerated resin was returned continuously to the resin contact tank to maintain a constant resin dose while regeneration was undertaken separately on a batch process as required. Virgin makeup resin was added on an infrequent basis to compensate for resin lost due to attrition. Coagulation in Stream 1 during this period varied between 6 and 10 mg/L (average 8 mg/L) (as Al2(SO4)3$18H2O) and 0.2 mg/L poly dimethyl diallyl ammonium chloride (DADMAC) as a coagulant aid to ensure filtered water turbidity was maintained below 0.2 NTU. The throughput of Stream 1 at Mt Pleasant WTP remained steady at 0.3 ML/day which gave 3.5 h flocculation and 1.5 h settling time prior to filtration. Filter run times averaged 2 days. The second stream at the Mt Pleasant WTP incorporates MIEX followed by submerged continuous microfiltration (CMFS) with polyvinylidenefluoride (PVDF) membranes which have a nominal pore size of 0.04 mm (Memcor S10 V). However the MF pilot plant was used to provide the comparison of MF with and without MIEX pre-treatment to ensure operating conditions were identical for both operating systems. The MF pilot plant consisted of a single module CMF-S membrane, the same variety as that used in the Mt Pleasant WTP. Two separate membrane modules were used in the MF pilot plant unit in a one week on, one week off rotation. The source water for

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Raw

Conventional Treatment Pilot plant

Stream1 WTP MIEX

Submerged Microfiltration Pilot Plant

WTP Conventional Treatment

Conv

Stream 1 WTP MIEX

Submerged Microfiltration Pilot Plant

MIEX Coag

Raw MF

MIEX MF

Fig. 1 e Schematic of Treatment Trains. Conv-conventional treatment pilot plant consisting of coagulation, flocculation, sedimentation and rapid filtration; MIEX Coag-Stream 1 at Mt Pleasant water treatment plant (WTP) consisting of MIEX followed by conventional treatment utilising coagulation, flocculation, sedimentation and rapid filtration; Raw MF-raw water followed by passage through submerged microfiltration membrane in pilot plant; MIEX MF-MIEX treated water sourced from Stream 1 at Mt Pleasant WTP followed by passage through submerged microfiltration membrane in pilot plant.

one module was raw water (Raw MF) and the other MIEX treated water sourced from Stream 1 (prior to coagulation) (MIEX MF) (Fig. 1).

2.2.

Analyses

Samples were taken from (a) the raw water (Raw), (b) after the filtration stage of both the conventional pilot plant (Conv) and the WTP Stream 1 (MIEX Coag) and (c) after passage through each of the MF membranes (Raw MF and MIEX MF). Monitoring of water quality included measuring the extent of NOM removal using dissolved organic carbon (DOC) and UV absorbance at 254 nm (UV254) three times a week. Characterization of NOM was determined from August 2005 to December 2006 by measuring molecular weight distribution and rapid resin fractionation. Characterization was undertaken monthly for the first 6 months and every 2 months for the remainder of the study. Samples for DOC and UV254 were filtered through 0.45 mm prerinsed membranes. UV254 was measured through a 1 cm quartz cell. DOC was measured using a Sievers 820 Total Organic Carbon Analyser (GE Analytical Instruments, USA). Specific UV absorbance (SUVA) was calculated as (UV254  100)/DOC in L/mg-m. Rapid fractionation analysis separates DOC into four fractions by adsorption onto different adsorbent resins in a sequential process based on hydrophobicity. After filtration through 0.45 mm pre-rinsed membranes a 500 mL sample was acidified and flowed through packed columns of Supelite DAX-8 and then Amberlite XAD-4 resin with intermediate samples taken for DOC analysis. Samples were then adjusted to pH 8 and flowed through a strong anion-exchange resin (Amberlite IRA-958). All resins were obtained from Supelco (Sigma Aldrich, USA). Fraction concentrations were obtained by calculation of DOC concentration measured before and after each resin. Fractions produced are defined as very hydrophobic acids (VHA), slightly hydrophobic acids (SHA), charged hydrophilics (CHA) and neutral hydrophilics (NEU).

Specifics of the technique and definitions have been described elsewhere (Chow et al., 2004). Apparent molecular weight (AMW) distribution profiles were determined using high performance size-exclusion chromatography (HPSEC) utilising UV detection after filtration through 0.22 mm pre-rinsed membranes. Weight and number average molecular weight (Mw and Mn respectively) were derived using the following equations (Chin et al., 1994). P ni M2i i (1) Mw ¼ P ni Mi i

and P

ni Mi Mn ¼ iP ni

(2)

i

where ni is the height of the HPSEC curve and Mi is the equivalent calculated molecular weight of an analyte eluted at volume i. The polydispersity (ratio of Mw to Mn) was also calculated (Chin et al., 1994). A polydispersity value of 1 indicates the presence of a single homogenous compound while greater values indicate a more disperse, complex mixture of compounds. HPSEC analysis was undertaken using a Waters Alliance 2690 separations module and 996 photodiode array detector at 260 nm (Waters Corporation, USA). Phosphate buffer (0.02 M) with 0.1 M sodium chloride was flowed through a Shodex KW802.5 packed silica column (Showa Denko, Japan) at 1.0 mL/min. AMW was derived by calibration with poly-styrene sulphonate molecular weight standards of 35, 18, 8 and 4.6 kDa.

3.

Results and discussion

3.1.

NOM removal

The raw water quality at the Mt Pleasant WTP varied over the study period; turbidity, 8.7e60 Nephelometric turbidity units

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7 Period A

Period B

Raw

Period C

Conv

6

MIEX Coag Raw MF

5

DOC (mg/L)

MIEX MF

4 3 2 1 0 Jul-05

Oct-05

Feb-06

May-06

Aug-06

Nov-06

Mar-07

Jun-07

Fig. 2 e DOC present after each treatment process (Period A, Auguste14 November 2005; Period B, 18 November 2005e15 June 2006; Period C, 19 June 2006e30 June 2007).

a

7 6

DOC (m g/ L)

5 4 3 2 1 0 Raw

b

Conv

MIEX Coag

Raw MF

MIEX MF

0.4 0.35

UVab s(/ cm )

0.3 0.25 0.2 0.15 0.1 0.05 0 Raw

c

Conv

MIEX Coag

Raw MF

MIEX MF

8 7 6

SUVA (L/m g-m )

(NTU), colour, 6e23 Hazen units, and DOC, 2.7e5.8 mg/L. The DOC variation is shown in detail in Fig. 2. Three distinct periods were observed; Period A, Auguste14 November 2005, (winter/spring), Period B, 18 November 2005e15 June 2006, (summer/autumn), and Period C, 19 June 2006e30 June 2007 (the full season cycle during drought conditions upstream). Period A consisted of variable DOC between 3.0 and 4.8 mg/L, generally decreasing, over the 4 month period. Period B began with an increase in DOC to 5.7 mg/L, due to rain and inflow from storages upstream, and remained reasonably stable within the band 4.5e5.8 mg/L over the 7 month period. Period C was less distinct but appeared to start mid June 2006 and showed a gradual steady decrease from 4.5 mg/L to 3 mg/L over the next year, under drought conditions and very little inflow upstream. The DOC concentration after each of the treatment processes over the periods A, B and C is also shown in Fig. 2. Pre-treatment with MIEX achieved the lowest DOC, with similar concentration obtained regardless of whether MIEX was followed by coagulation or MF. The DOC concentration after MIEX treatment remained nearly constant over the entire study period despite the variation in raw water DOC. Conventional treatment did not remove as much DOC as the MIEX pre-treatment whilst MF alone with no pre-treatment was found to consistently remove only a small amount of DOC. The greater DOC removal by MIEX alone compared with coagulation has been observed previously (Singer and Bilyk, 2002; Drikas et al., 2003a,b; Morran et al., 2004; Boyer and Singer, 2005; Warton et al., 2007; Jarvis et al., 2008). The extent of DOC remaining, as well as UV254 and SUVA, can also be compared over the period of the study using box and whisker plots as summarised in Fig. 3a. Outliers were excluded for each treatment train but the number of analyses conducted over this period and used in the calculations was similar for each process e Raw, 137; Conv, 132; MIEX Coag, 132; Raw MF, 139; MIEX MF, 134. As shown in Fig. 3a, the two processes incorporating MIEX pre-treatment achieved the lowest DOC concentrations with median of 1.7 mg/L after MIEX Coag and 1.6 mg/L after MIEX MF compared with 2.5 mg/L after Conv and 3.3 mg/L after Raw MF. This equates to greater average removal of DOC using MIEX pre-treatment (54% and 57%) compared with the Conv (33%) and the Raw MF (11%)

5 4 3 2 1 0 Raw

Conv

MIEX Coag

Raw MF

MIEX MF

Fig. 3 e Box and whisker plot of (a) DOC, (b) UVabs and (c) SUVA remaining after each treatment, for the total period of study. The bottom of the box is the 25th percentile and the top is the 75th. The whiskers represent the maximum and minimum for each treatment process.

processes. The 95 percent confidence intervals around the medians (not shown in this figure) indicate that the MIEX pretreatment trains are statistically different from those not including MIEX. The extent of removal of DOC by the processes incorporating MIEX pre-treatment was within the range previously observed for waters with similar SUVA values (Boyer and Singer, 2005; Singer et al., 2007). Fig. 3a also shows that the raw water over the period of study was skewed towards higher DOC and that this trend was continued after passage through the MF (Raw MF) and to a lesser extent after conventional treatment (Conv). However the data for the two treatments incorporating MIEX did not show any skew around the median results indicating that they produced more consistent water quality and were less affected by the raw water DOC with 50% of the remaining DOC results contained within a region of less than 0.5 mg/L DOC for the duration of the study.

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 3 9 e1 5 4 8

The UV254 absorbance removal followed similar trends to the DOC removal although removals obtained were significantly higher - MIEX pre-treatment achieved 81% and 78% removal for the median data compared with the Conv (55%) and the Raw MF (25%) processes. Fig. 3b indicates that the UV254 absorbance of the raw water was also skewed towards higher values but confirms that all treatments used were more effective in removal of UV absorbing components than DOC with the whiskers after all treatments significantly reduced. This is not surprising as these types of structures are generally larger and more hydrophobic which makes them easier to remove by both coagulation and physical filtration. The unexpected removal of a small amount of DOC by the Raw MF can be attributed to removal of these larger UV absorbing compounds as evidenced by the reduced range of UV254 absorbance apparent in Fig. 3b. The removal of larger and more UV absorbing organics by MF membranes has been observed by other researchers (Fan et al., 2001; Lee et al., 2005). The reduced spread in UV254 absorbance of the Conv treatment is expected based on the known preferred removal of these compounds by coagulation (Chow et al., 1999; Archer and Singer, 2006). The MIEX treated waters had a very small spread of UV254 absorbance with 50% of data within a region of less than 0.01 Abs/cm and the total spread of data less than 0.03 Abs/cm. This data confirms previous observations that the removal of UV absorbing compounds by MIEX is very effective (Drikas et al., 2002; Singer and Bilyk, 2002; Fearing et al., 2004; Boyer and Singer, 2005; Humbert et al., 2005; Singer et al., 2007). The small spread in UV254 absorbance data also confirms the consistency in treated water quality following MIEX pre-treatment regardless of raw water changes observed with DOC removal. The change in SUVA would be expected to follow similar trends for all of the treated waters as it is a parameter derived from the DOC and UV254 absorbance data. Fig. 3c confirms that Raw MF reduced the SUVA of the raw water from a median of 2.4 to 2.0 L/mg-m, while Conv achieved a median SUVA of 1.5 L/mg-m. MIEX pre-treatment consistently produced water with the lowest SUVA because more UV absorbing organics were removed by MIEX relative to the DOC. This differed from Boyer and Singer (2006) who found that the treated water SUVA was similar to the raw water SUVA but was consistent with the findings of Singer et al. (2007) who found that SUVA at each utility studied decreased with MIEX treatment. MIEX MF had a median of 1.1 L/mg-m while MIEX Coag was lower with a median of 0.9 L/mg-m. Again the differences in SUVA observed following MIEX treatment were statistically significantly based on a 95 percent confidence interval.

3.2.

Rapid fractionation

To better understand the mechanism of NOM removal, the types of organics removed by each of the treatment processes were investigated. An example of the information obtained from rapid fractionation is shown in Fig. 4 for the sample taken in November 2005. DOC of the raw water was high in the early stages of the study, especially during Period B, and contained high concentration of VHA (1.7e2.3 mg/L). The VHA fraction, consisting predominantly of higher molecular weight humic and fulvic acids, was the dominant fraction

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Fig. 4 e Rapid Fractionation for all treatments for sample taken in November 2005. Fractions are defined as very hydrophobic acids (VHA), slightly hydrophobic acids (SHA), charged hydrophilics (CHA) and neutral hydrophilics (NEU). Error bars represent limit of detection.

present and was removed best by the two processes containing MIEX pre-treatment. The SHA fraction, variously described as transphilic (Boyer and Singer, 2005) or hydrophilic acid (Fearing et al., 2004), although not present to a large extent in the raw water during this study, was also preferentially removed by the MIEX pre-treatment. The high proportion of VHA in this water and its preferential removal by the MIEX processes resulted in a significant difference in the amount of DOC removed and in the resulting character of the waters after treatment, as illustrated in Fig. 4. Similar comparative removal was obtained for all samples analysed over the period of the study. Fearing et al. (2004), Sharp et al. (2006) and Mergen et al. (2008, 2009) utilised a modified fractionation process where they further separated the VHA fraction using pH adjustment into two additional fractions, which they defined as HAF (humic acid fraction) and FAF (fulvic acid fraction), and did not separate the non-adsorbed hydrophilic fraction into CHA and NEU as in this study. Fearing et al. (2004) found that MIEX removed a larger proportion of one component of the VHA fraction (the FAF fraction) than coagulation with ferric sulphate but similar amounts of the SHA and the other component of the VHA fraction (the HAF fraction) while Chow et al. (2005) showed a strong correlation of the VHA fraction with alum dose in a conventional treatment plant. The CHA fraction was removed most effectively, particularly by those processes containing coagulation (Conv and MIEX Coag) and/or ion exchange (MIEX Coag and MIEX MF). Other studies have also shown that the CHA fraction is the most amenable to removal by coagulation (Chow et al., 2004; Bolto et al., 2002; van Leeuwen et al., 2002) and ion exchange (Bolto et al., 2002; Morran et al., 2004). Fearing et al. (2004) found that MIEX performed better at removal of the nonadsorbed hydrophilic fraction (combination of CHA and NEU) than ferric coagulation. Boyer and Singer (2005) showed that the MIEX removed all fractions to a greater extent than coagulation with removal increasing with increased resin dose while Singer et al. (2007) found that MIEX removed the VHA and SHA fraction more than the non-adsorbed hydrophilic fraction. Study of the regenerant produced by MIEX treatment of four waters by Mergen et al. (2009) showed that

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MIEX removed VHA (both HAF and FAF components), SHA and the non-adsorbed hydrophilic fraction but that the SHA fraction was the only fraction which increased in concentration in the regenerant solution for one water and after coagulation of the regenerant for the four waters studied suggesting better removal of this fraction by MIEX than by coagulation. The NEU hydrophilic fraction generally consists of lower molecular weight components such as polysaccharides and proteins and is often indicative of biologically derived material (Leenheer, 1981; Buchanan et al., 2005). Only very small amounts of the NEU were removed by any of the treatments. van Leeuwen et al. (2002) identified the NEU fraction as recalcitrant following fractionation and coagulation of two waters while Chow et al. (2004) showed that removal of all fractions, except the NEU fraction, could be improved by change in coagulation conditions. Sharp et al. (2006) also showed strong removal of the hydrophobic components by alum coagulation with poor removal of the non-adsorbed hydrophilic fraction (combination of CHA and NEU). The concentration of the dominant VHA fraction present in the raw and remaining after treatment over the period of the study is summarised in Fig. 5. Initial samples taken in Period A showed some inconsistency, particularly with the raw water sample. This was due to development and optimisation of the rapid fractionation method during this initial period. Notwithstanding the early results, it appears that the VHA content of the raw water decreased during Period A, increased at the junction of Period A and B and then steadily decreased during period B and C of the study. Fig. 5 also shows that generally the Raw MF removed little or no VHA, compared with the filtered raw water. This is despite the Raw MF achieving a measurable removal of DOC (median DOC difference of 0.4 mg/L). The lack of observation of consistent removal of VHA is attributed to the limit in sensitivity of the rapid fractionation analysis. The analysis requires measurement of the DOC of the raw sample and of each fraction after passage through the resins which results in an additive limit in detection of 0.2 mg/L for each fraction. This means that small differences in VHA concentrations are not able to be detected. It is likely that some DOC was removed by the Raw MF by adsorption onto particulates during filtration by the mechanism of cake layer formation. During the start of Period B, on the 18th November 2005, there was an increase in raw water turbidity from 30 NTU to above 50 NTU. This higher turbidity level (also associated with higher DOC) was present until late February 2006 when the

turbidity again was reduced to around 30 NTU. This higher turbidity period coincided closely with the measured removal of some VHA by the Raw MF observed at the start of Period B and provides support for this theory with enhanced cake layer formation at the higher turbidity. Conv treatment removed some VHA with the extent of removal apparently dependent on the amount of VHA in the raw water. The MIEX pre-treated waters achieved the lowest VHA concentration and this concentration remained nearly constant over the entire study period, regardless of the raw water VHA concentration. This supports the previous observation, noted with the other water quality parameters, of consistent treated water quality following MIEX pre-treatment regardless of raw water quality changes. There was some change in the NOM character of the raw water during the study. Initially the raw water consisted of 60% VHA during period A and B but this gradually decreased to 50% VHA by the end of the study period. The difference in NOM character of the waters after treatment is also apparent, particularly during period B, where the two MIEX pre-treated waters consistently had between 10 and 20% less VHA present after treatment, than either the Conv or Raw MF. This indicates that the DOC of the MIEX pre-treated waters consisted of a greater percentage of the other fractions; in particular the NEU fraction was a significant component of the remaining DOC, as this was not removed by any of the treatment processes. As stated earlier this NEU component is the most recalcitrant to all forms of treatment. The percentage of the VHA fraction removed by each of the treatment processes over the study period was also calculated (Fig. 6). This confirms that the raw MF removed little or no VHA during the study. Conv treatment removed very little VHA during Period A, a constant amount of VHA, about 35%, during Period B with decreasing removal observed during Period C. The two treatments incorporating MIEX achieved the highest percentage of VHA removal throughout the study. During Period B, when the raw water had very high VHA concentration, the two treatments incorporating MIEX pretreatment consistently removed a significantly greater proportion of the VHA fraction (w70%). During this same period, the Raw MF and the Conv treatments were not able to achieve the same removal of VHA (0% and 35% respectively), resulting in higher concentration of VHA in the treated waters (as shown in Fig. 5). The MIEX pre-treated waters continued to remove a high percentage of VHA even during Period C when the raw water VHA concentration decreased and the Conv

2.50

Period A

Period B

Period C

Raw

Period A

300

Period B

Period C

Raw MF

50

Jun-06

Aug-06

Oct-06

Dec-06

Fig. 5 e Concentration of VHA fraction present after each treatment process.

Fig. 6 e Percentage removal of VHA Fraction by each treatment process.

Dec-06

Apr-06

Aug-06

Feb-06

J un-06

Dec-05

Apr-06

Oct-05

Feb-06

0

Dec-05

Aug-05

100

-50

0.00

Raw MF

150

Nov-05

0.50

MIEX MF

200

Oc t-05

1.00

MIEX Coag Conv

Sep-05

1.50

250

Aug-05

MIEX MF

Oc t-06

MIEX Coag

% VHA Fraction Removed

VHA Concentration (mg/L)

Conv

2.00

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0.006

Raw

0.005

Raw MF

UV Abs @ 260 nm

0.004

Conv 0.003

MIEX Coag

0.002

MIEX MF

0.001

0.000 100

1000

10000

-0.001

Apparent Molecular Weight (Daltons)

Fig. 7 e Apparent Molecular Weight profiles for all samples taken in November 2005.

3.3.

Apparent molecular weight distribution profiles

AMW distribution profiles of each of the treatment processes were also determined. It should be noted that as these profiles were obtained using UV detection only UV absorbing NOM would be detected. To illustrate the type of data that was obtained, the AMW profile of the same sample selected from November 2005 that was used in Fig. 4 is shown in Fig. 7. The Raw MF removed minimal DOC, resulting in molecular weight distribution similar to the raw water. Conventional treatment (Conv) removed predominantly higher AMW material, mainly above 1000 Da. The removal of high AMW organic matter with coagulation is well documented in the literature (Collins et al., 1986; Owen et al., 1995; White et al., 1997; Chow et al., 1999). MIEX treatment removed organics across the complete AMW range, including above and below 1000 Da, as has been observed in other studies (Drikas et al., 2003a,b; Morran et al., 2004; Allpike et al., 2005; Boyer and Singer, 2005; Humbert et al., 2005; Singer et al., 2007; Mergen et al., 2009). In the example shown in Fig. 7, MIEX Coag removed more NOM than MIEX MF. This was consistent for all samples up to June 2006 (Period A and B) after which removal in the two MIEX treated trains was virtually equivalent up to the last sample analysed in March 2007. The greater removal of organics by MIEX Coag

than MIEX MF evident in the AMW profile has been attributed to the inclusion of the coagulation step, which would be expected to remove additional organics, particularly above 1000 Da. To illustrate the impact of treatment on the AMW distribution, the weight-average MW (Mw) achieved after each treatment was calculated for the period of study and is summarised in Fig. 8. The Mw of the raw water over the period of study followed a similar, albeit less pronounced, pattern to that observed with the VHA; an initial high Mw which decreased during Period A, increased slightly at the junction of Period A and B and then steadily decreased during period B and C of the study. The variation in Mw of all the treated waters also followed similar, but reduced, trends to those observed with the VHA. Fig. 8 shows that the Raw and Raw MF had virtually identical Mw. The removal of DOC by the MF was low (of the order of 0.4 mg/L) and, as apparent from the example in Fig. 7, observed differences in AMW were small, due in part to the use of tighter membranes for filtration before HPSEC analysis (0.22 mm for filtration of samples compared with 0.45 mm for the other organic analyses). Fig. 8 confirms the trends observed previously for other parameters and shows that DOC removal across the entire AMW range by the processes incorporating MIEX pre-treatment

1600

Weight Average Molecular Weight (M ) (Daltons)

treatment had a reduced VHA removal. The consistently high VHA removal compares well with the work of Mergen et al. (2008) where waters containing more than 50% hydrophobic DOC were found to have removals around 70% DOC with MIEX. However in their work MIEX applied in batch tests to water containing over 75% VHA was found to result in reduced DOC removal with repeated resin use whereas DOC removal in water containing only 20% VHA and 60% SHA remained constant. In our study, raw water VHA varied between 50 and 65% (assuming raw water VHA for Period A approximated that obtained with Raw MF) but the DOC removal with MIEX did not show a decline with time based on repeated resin use such as would occur for a proportion of the resin in a continuous process.

Period A

Period B

Period C

Raw Conv

1400

MIEX Coag Raw MF

1200

MIEX MF 1000 800 600 400 200 0 Aug-05

Oct-05

Dec-05

Feb-06

Apr-06

Jun-06

Aug-06

Oct-06

Dec-06

Fig. 8 e Weight average AMW (Mw) for all treatment processes.

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w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 3 9 e1 5 4 8

3 2.5

1.25

VHA (m g/L)

Polydispersity (M /M )

1.3

1.2

1.15

1.1

1.5 1 0.5 0

1.05 p Raw

p Conv

p MIEX Coag

p Raw MF

p MIEX MF

Fig. 9 e Median of polydispersity achieved after each treatment processes. Error bars denote interquartile range.

resulted in the lowest Mw, around 630 Da, nearly half that of the raw water and significantly less than that achieved by the Conv treatment. The remaining Mw in the MIEX pre-treated waters concurs with the observation of Humbert et al. (2005) that the components of NOM that are refractory to MIEX (and even more to coagulation) are the lowest MW components. The data from this study provides more detailed data on the range of recalcitrant organics than that observed in other studies. The polydispersity (ratio Mw/Mn) achieved after each treatment was also calculated (Chin et al., 1994). Polydispersity is a measure of the homogeneity of the NOM in a sample, with values above 1 indicating the presence of a diverse, complex mixture of compounds. The median of the polydispersity in the raw water and after each treatment for the twelve samples is shown in Fig. 9. The raw water had the greatest polydispersity, which was unchanged after passage through the MF membrane. It is likely that the higher AMW organics that may be removed by the MF membrane (Fan et al., 2001; Lee et al., 2005) would also be removed by filtration through 0.22 mm filters prior to HPSEC analysis of both the Raw and Raw MF samples and therefore differences between their molecular weight profiles and consequently polydispersity would not be measurable. Conventional treatment (Conv) reduced the range of organics somewhat whilst the two processes incorporating MIEX pre-treatments had the lowest polydispersity confirming that these treatments had the greatest reduction in complexity and range of AMW of the organics. The smaller interquartile range for MIEX Coag in Fig. 9 also confirms that MIEX Coag was more effective in reducing the range of organics than MIEX MF. This was attributed to the removal of additional organics above 1000 Da by the coagulation step.

3.4.

2

Correlations

The different characterization techniques employed whilst measuring different components are not mutually exclusive. For example, SUVA has been shown to provide a measure of the extent of conjugation and aromaticity of water (Chin et al., 1994; Weishaar et al., 2003) which is likely to be associated with both higher AMW and hydrophobic compounds such as those predominant in the VHA fraction. Therefore it would be expected that there would be a relationship between the observed SUVA and the Mw and VHA of the various waters

0

200

400

600

800

1000

1200

1400

1600

Weight Average Molecular Weight Mw (Daltons)

Fig. 10 e Correlation of VHA and weight average AMW (Mw) for raw and treated waters (r2 [ 0.83; n [ 53).

studied. These relationships were studied utilising a linear regression function for the raw water and all of the various waters following treatment. It was found that SUVA was not strongly correlated to the amount of VHA present (r2 ¼ 0.68; n ¼ 53) or the Mw (r2 ¼ 0.63; n ¼ 57). The SUVA of the raw water alone showed a similar poor relationship with both VHA (r2 ¼ 0.55; n ¼ 8) and Mw (r2 ¼ 0.47; n ¼ 10) most likely due to the large variation in the SUVA of the raw water (these 10 samples varied from 5.1 to 1.8). However the relationship between SUVA and Mw was significantly improved when the raw data was excluded from the data set (r2 ¼ 0.85; n ¼ 47) supporting the relationship between molar absorptivity and weightaveraged molecular weight observed by Chin et al. (1994). The improved correlation of SUVA with Mw following removal of the raw data was due to removal of the vast spread in the SUVA data for very similar Mw; attributed to a significantly higher concentration of UV absorbing compounds within the same molecular weight range in some raw water samples. The relationship between SUVA and VHA was not impacted when raw water data was excluded from this data set because the data range was not markedly affected by the removal of the raw data as the range in VHA concentrations in the raw water was not significantly different to the range observed after treatment. It also suggests that SUVA is not an accurate measure of the organic character for all the waters studied and while it could be correlated with the Mw of the treated waters, or those without high levels of UV absorbing organics, it could not be related to the Mw of all waters nor could it be attributed solely to the VHA component of waters. There was, however, a strong relationship between VHA and Mw when using data from both the raw water and the waters following treatment (r2 ¼ 0.83; n ¼ 53) as illustrated in Fig. 10. This relationship was also retained when the raw water data was excluded from the data set (r2 ¼ 0.85; n ¼ 44) and improved when only the raw water data was considered (r2 ¼ 0.90; n ¼ 9). This confirms that the VHA fraction was the major component of the molecular weight distribution for both the raw and treated waters supporting the premise that the hydrophobic fraction of NOM obtained using XAD-8 resin comprises a large component of the UV absorbing NOM in this natural water and concurs with Chin et al. (1994) that the relative amount of aromatic moieties in aquatic fulvic acid increases with increasing molecular weight. The close correlation between these two parameters also supports the

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 1 5 3 9 e1 5 4 8

relevance of these two techniques as valuable tools to characterise NOM. However it should also be noted that NOM arising from sources such as algal blooms may be more hydrophilic and that the observed relationship between VHA and Mw may not hold.

4.

Conclusion

Pre-treatment with MIEX prior to both conventional treatment and microfiltration over a 2 year period was found to result in consistently greater removal of DOC with resultant lower SUVA in the treated water than the same processes without MIEX over the entire period of study. The processes incorporating MIEX produced more consistent water quality and were less affected by changes in the raw water DOC. This would make treatment plants incorporating MIEX easier to operate and maintain good quality water even when subjected to large variations in the raw water DOC concentration and character. Characterization of the organics indicated that the VHA fraction was the dominant fraction present in the raw water and this was best removed by the processes incorporating MIEX pre-treatment. The high proportion of VHA in this water and its preferential removal by MIEX pre-treatment resulted in a significant difference in the amount of DOC removed and in the resulting character of the waters after treatment. The lowest VHA concentration was achieved after treatment with processes incorporating MIEX, regardless of the raw water VHA concentration. The processes incorporating MIEX pre-treatment also removed organics across the complete AMW range resulting in lower weight average AMW in the treated water than the other treatment processes. Greater removal of organics by MIEX Coag than MIEX MF was evident in some of the specific AMW profiles and this has been attributed to the inclusion of the coagulation step, which would be expected to remove additional organics, particularly above 1000 Da. The two processes incorporating MIEX pre-treatments had the lowest polydispersity confirming that these treatments had the greatest reduction in complexity and range of AMW of the organics. The greater reduction in concentration of NOM and range of organics following treatment incorporating MIEX results in more stable water which will be less reactive to disinfectants. A strong correlation was found between the VHA and weight average AMW suggesting that the VHA fraction was a major component of the AMW for both the raw water and treated waters. SUVA was found to correlate well with the weight average AMW of the treated waters, but it could not be related to the weight average AMW of all waters nor attributed to the VHA component of the waters.

Acknowledgements The authors would like to thank the following people: Nick Nedelkov for technical assistance at Mt Pleasant WTP and Edith Kozlik, Miriam Nedic and Leanne Biddiss for analytical support.

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