THM precursor rejection by UF membranes treating Scottish surface waters

Separation and Purification Technology 149 (2015) 381–388

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THM precursor rejection by UF membranes treating Scottish surface waters S. Sutherland a, S.A. Parsons a, A. Daneshkhah b, P. Jarvis b, S.J. Judd b,c,⇑ a

Scottish Water, Edinburgh, Scotland, UK Cranfield University, Bedfordshire, UK c Qatar University, Doha, Qatar b

a r t i c l e

i n f o

Article history: Received 2 April 2015 Received in revised form 8 June 2015 Accepted 9 June 2015 Available online 10 June 2015 Keywords: Trihalomethanes Natural organic matter Ultrafiltration Nanofiltration Formation propensity Yield

a b s t r a c t Trihalomethanes (THMs) are known to be generated from specific fractions of natural organic matter (NOM) present in surface waters used for potable water supply. The overall specific THM formation propensity (THMFP) can be expressed in terms of yield – the mass of THM generated per unit mass of permeate total organic carbon (TOC). A study of permeate water quality, with specific reference to TOC rejection and THM yield, has been conducted for membrane-based potable water supply plants across Scotland. The study encompassed 35 operating plants fitted with ‘‘tight’’ UF membranes, and was based on 18 months of water quality (WQ) data. Literature data used to calculate yield for comparable pilot or full scale membrane plants challenged with various real waters and operating under similar conditions indicate permeate THM yields predominantly in the range of 20–50 lg total THMs (tTHMs) per mg TOC across a range of membrane characteristics. Outcomes from the study revealed average yields within the range of 10–70 lg THMs/mg TOC, with TOC passage averaging 18%, roughly in keeping with published values for comparably selective membranes. Data for THM distribution revealed that, counter-intuitively, increased bromine substitution lead to slightly reduced total THM levels. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Membrane technology can be used for the rejection of natural organic matter (NOM) for potable water production. The key measure of efficacy for this duty is the removal of the organic matter component responsible for THM formation (the THM formation propensity, or THMFP). THMFP can be normalised against total organic carbon (TOC), or fraction thereof, to produce the yield (or, sometimes, reactivity) in terms of lg THM generated per mg TOC. Fractionation may be with reference to molecular weight range [1–6] and/or chemistry [5–13] of the TOC, since yield is generally perceived to be dependent on specific physicochemistry. A large number of chemical fractionation studies, based on ion exchange resins of specific chemistries, have been devoted to assessing the THM yield of the different chemical fractions. Experiments conducted on analogues [14] or individual NOM-laden raw water samples [15] have generally corroborated widely reported trends of decreasing yield with decreasing TOC ⇑ Corresponding author at: Cranfield University, Bedfordshire, UK. E-mail address: s.j.judd@cranfield.ac.uk (S.J. Judd). http://dx.doi.org/10.1016/j.seppur.2015.06.009 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.

hydrophilicity and molecular weight (MW). The hydrophobic organic (HPO) fraction of the NOM tends to dominate in many surface waters used for drinking water supply [11–13], and it is this fraction that also tends to have the highest yield and is coincidentally the fraction most readily removed by conventional clarification [15,16]. In terms of size fractionation it is the sub-1 kDa fraction which dominates and also tends to generate the greatest THM concentration. Whilst the above studies have been informative, their practical application in terms of ameliorating THM formation in potable water supplies is constrained by the nature of the fractionation experiments, which are laborious and demand a degree of skill. It is also the case that the generally low residual TOC concentrations persisting in membrane permeate streams do not lend themselves to fractionation, since the required fractionation time increases with decreasing sample TOC concentration. It is perhaps for the above reason that few studies of membrane performance have encompassed chemical fractionation of the permeate for THM yield determination. Notwithstanding the above, studies based on fractionation of a range of raw water samples have revealed no consistency in

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S. Sutherland et al. / Separation and Purification Technology 149 (2015) 381–388

Fig. 1. THM yield vs. membrane selectivity (calculated from data presented in [1]).

absolute yield values on the same fraction for different waters. For example, reports on chemically-fractionated surface water samples [6,15] have revealed the absolute yield to vary from 23–42 [15] to 270–414 [6] lg tTHMs/mg TOC for the HPO (hydrophobic) fraction, and from 16–33 to 178–238 for the lowest MW fraction across 2–4 samples. An exhaustive bench-scale, stirred-cell filtration test conducted on five different membranes of varying selectivity ratings and 10 different feed waters revealed permeate

THM yield to vary from 34 to 104 on average across the five different membranes [1]. In this case the yield apparently decreased with decreasing membrane molecular weight cut-off (MWCO) rating (Fig. 1), contrary to the more widely reported trend of increasing yields with decreasing TOC MW [4,6,11,15]. However, the standard deviation across all 10 water samples analysed ranged from 36% to 80%. Neither chemical nor physical fraction thus necessarily correlates with THM yield. Given the variability in the absolute yield values reported with both physical and chemical fractions, even in the underlying trends, it is instructive to view the available heuristic data from potable water supply studies. Outcomes have been reported from the application of ultrafiltration (UF), nanofiltration (NF) and direct filtration (DF) of environmental surface waters (Table 1), where DF pertains to precoagulation followed by UF, MF or media filtration. The table includes data from (a) size fractionation using membranes [1,3,4,6,7,9,19], and (b) sampling of streams from full-scale or pilot plants [6–8,10,12,17,19–21]. According to such data, from investigations conducted at bench, pilot and full-scale, the membrane permeate THM yield across 11 of the 15 studies for which yield is reported varies largely between 22 and 50 lg/mg. For two of the remaining four studies [1,11,19,20] at least a proportion of the reported data (for example two of the five data from [1], Fig. 1) fall within this THM yield range. It is thus only for two of the studies, both conducted on the same regional Chinese surface waters [3,6] that usually high

Table 1 Summary, THM yield data, surface waters, publications within the past 10 years. Water source and scale of study

Stream/fraction

TOC, mg/L or % of total

tTHMs, lg/L

THM yield, lg/mg

Reference

Five surface water sources, Sp UF/NF permeation testa Huangpu River, Ch Lab-scale size fractionation Lab-scale size fractionation, Tu Yellow River and Danjiangkou Reservoir, Ch

Feed, ave Permeate, ave Feed <1 kDa All fractions Feed 1 <1 kDa HPO Feed 2 <1 kDa HPO Feed <1 kDa Treated, NF Feed Permeate

– – 4.95 52%

– – – 350

[1]

2.23 38.4% 29.5% 1.96 41.4% 19.0% 3.12 38%

382 203 178 279 144 154 135 70 33 82 57 – 250 74 450 50 – 437 56

97a 34–82a – 136 24–42 171 238 270 142 178 414 43 34 46 50 41 32–45, 31 ave 48 25 85 26 – 107 33 17–48b 16–33b 23–42b 123 34 47 – 120 50 80 46 65 – 22–45

Lab-scale size fractionation

Han River, Ko Pilot plant, cf. size fractionation Pilot plant (DF), Ch Erlong reservoir fractionation, Ch Chaophraya River, Th Pilot plant Tatamagouche, Ca Sampling of full-scale plant Sri-Trang Reservoir, Th Five surface waters, Ca/US Lab-scale size fractionation Kao-pin River, Ta Sampling of full-scale plant Llobregat River, Sp Lab-scale membrane test West River, Ma Pilot plant Lab-scale size fractionation Tai Lake, Ta Sampling of full-scale plant Clarified reservoir water, Fr Sampling of full-scale plant

Feed Treated, MFc Feed Permeate, UF Permeate, NF Feed Permeate, UF Feeds 1–3 <1 kDa HPO Feed Treated, DFc Feed Permeate, NF Feed Permeate, UF <1 kDa Feed Permeate, NF Feed Permeate, NF

10.05 5.17 2.91 5.3 1.9 0.32 4.08 1.71 7.1, 4.0, 7.9

0.59 0.29 2.56 – 1.5–1.7 – – 7 0.791 2.5–4 <1

KEY: Ca Canada, Ch China, Fr France, Ma Macau, Sp Sain, Ta Taiwan, Th Thailand, Tu Turkey, US United States. a See Fig. 1. b Assuming a mean mass of 128 g/mol for the tTHMs. c Pre-coagulation/clarification applied.

72 10 120 <30 – – – 325 51 – 15–40

[3] [4] [6]

[7]

[8] [9] [10] [12]

[13] [15]

[17] [18] [19]

[20] [21]

S. Sutherland et al. / Separation and Purification Technology 149 (2015) 381–388

THM yields have been reported for the <1 kDa size range and HPO chemical fractions (between 136 and 414 lg/mg). The generally consistent data for the permeate THM yield (20– 50 lg/mg, 39 on average with a standard deviation of 31%) is in contrast with the mostly higher (70 lg/mg on average from 11 data) and more highly scattered (±43%) values for the untreated feed waters. Moreover, the reduced yield values for the permeate compared with the feed water does not appear to be depend significantly either on the membrane characteristics nor the process type (direct filtration vs. permeation without pre-treatment). This suggests that removal of the suspended and colloidal fraction of the NOM is sufficient to reduce the yield to a more consistent range of values regardless of the membrane pore size. It is of key interest, to practitioners in particular, to gain a further understanding of the trends in THM yield for real waters across a range of plants operating within different regions. Specifically, the nature and reliability of this relationship is of critical importance in setting a realistic TOC water quality target. A heuristic study has been conducted across all regions of Scotland, making use of WQ data captured over a 12–18 month period from 30 individual full-scale UF installations based on two membrane products. Data pertaining to the feed and permeate TOC and permeate tTHMs concentration have been processed to obtain mean yield values across the region. This constitutes a unique study, since previous studies as listed in Table 1 have been limited to: (a) A maximum of 5 water sources [1], compared with 35 in the current study. (b) 1–9 months, compared with 12–18 months in the current study.

Table 2 Membrane characteristics. Membrane module Design Polymer Configuration Area, m2 Length, m MWCO, Da Plant operation, mean Flux, L m2 h1 Approach flow rate, m3/h Approach velocity, m/s Recovery, %

PCI CA202

PCI ES

Cellulose acetate (CA) Tubular (Tube)

Polysulphone Cellulose acetate (PS) (CA) Tubular (Tube) Spiral wound (SpW) 0.146 30.2 3.7 1 4000 8000

0.146 3.7 2000 values 15 0.24

Koch UF-325

383

(c) Either bench scale analysis [1–4,9,11,13,15,19] or a limited number (1 or 2) pilot/full-scale plants or plant configurations [6–8,10,12,17,19,20,21]. 2. Materials and methods 2.1. Materials The plants are based on two different ‘‘tight’’ ultrafiltration technologies (Table 2). The spiral wound (SpW) membranes employ various levels of pre-treatment (generally microscreening and sand filtration) with cartridge filtration fitted upstream of the membranes. For the tubular membrane there is no pre-treatment. For both technologies the conversion achieved is 75–80%, with the required crossflow maintained by retentate recycling rather than staging, with rehardening and hypochlorite dosing downstream (Fig. 2). 2.2. Methods Grab samples were taken 3–5 times weekly at each site to determine total organic carbon (TOC), colour (Hz) and THMs. Laboratory analyses were undertaken by Scottish Water Laboratories using standard methods [22]. THMs were measured both in the permeate and at the customer tap: there was found to be no statistically significant difference between the two measurements. Data refer to an 18 month period from February 2012. Statistical analysis for production of boxplots was conducted using the R Statistical Computing Package. The data analysis was conducted in two stages: 1. A scoping trial comprising a regional analysis (two Scottish islands, ‘‘islands’’ data) based on the SpW technology alone and encompassing colour data. 2. A more widespread analysis across all regions. Key performance data are expressed in terms of TOC passage rather than rejection, thus relating more directly to permeate water quality, along with the yield of tTHMs with respect to feed or permeate TOC. 3. Results and discussions

15 0.24

14 7

3.1. SW technology, ‘‘islands’’ data (15 plants)

0.6

0.6

0.25

75–80

75–80

75–80

3.1.1. Water quality data Mean and standard deviation (SD) values of the key parameters (colour, TOC and tTHMs) in the feed and permeate streams of all

Fig. 2. Plant schematic.

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Table 3 Site WQ data, SpW technology, ‘‘islands’’ data. Site code

Feed Colour Hz

TOC mg/L

Col/TOC Hz L/mg

Table 4 Linear regression analyses of mean Table 3 data. % passage

Permeate

Colour

tTHMs lg/L

TOC

SpWN1 SpWN2 SpWN3

27 24 70

4.8 4.1 9.3

5.7 5.9 7.5

11 10 9

16 16 20

SpWN4 SpWN5

64 23

8.8 3.8

7.3 6.2

7

17

SpWN6 SpWN7 SpWN8 SpWN9 SpWE1 SpWN10

38 15 27 22 40 40

6.1 2.6 3.9 3.8 5.3 5.3

6.3 5.9 7.0 5.7 7.5 7.5

29 15 22 9 9 10 22

41 27 30 19 17 24

SpWN11 SpWN12 SpWN13

19 24 55

4.1 3.7 7.0

4.6 6.7 7.9

SpWE2

72

9.2

Mean % SD of mean

37 51

5.5 40

13 12

36 29 21

7.8

27 14

36 23

6.6 14

15 48

25 33

25 25 52 46 59 42 16 29 22 22 32 38 34 50 62 37 39

Correlation Feed TOC Permeate Permeate Permeate Permeate Permeate Permeate Permeate

Col/TOC Hz L/mg 3.9 3.8 3.6 3.2 4.4 3.4 4.4 3.2 3.0 3.2 4.7 2.1 3.7 5.9

a

vs. colour tTHMs vs. tTHMs vs. tTHMs vs. tTHMs vs. tTHMs vs. tTHMs vs. tTHMs vs.

feed TOCa perm TOC perm TOC, zero intercept perm SUVA perm SUVA, zero intercept feed SUVA, zero intercept perm SUVA

Grad

Int

R2

0.11 5.3 18 26 2.6 5.6 5.6 5.2

1.2 5.4 13 0 23 0 0 16

0.97 0.76 0.58 0.45 0.43 0.37 0.17 0.11

Ignoring SpWN5.

correlation with tTHMs is not (Table 4), with R2 values of 0.17 and 0.11 for the feed and permeate TOC respectively. Whilst there is a degree of scatter in these averaged data, the strongest correlation is between the permeate tTHMs and feed water TOC where, ignoring one outlier (SpWN5):

4.8 3.8 24

Outliers underscored.

plants were determined for the 18 months of data (Table 3). Attempts to correlate these parameters on a daily basis for individual plants provided no significant linear correlations. The averaged water quality indicates passage of colour and TOC in the ranges of 7–29% and 16–41% respectively. If three outliers (SpWN5, SpWN10 and SpWN13, underscored in Table 3) are ignored the TOC passage data across all plants lies between 16% and 30%. The correlation of tTHMs with permeate SUVA yields the smallest SD, with values predominantly between 20 and 40. Averaged data for permeate tTHM concentration against permeate colour or TOC yields an improved correlation over that for the instantaneous measurements. The data are nonetheless scattered with R2 (linear correlation coefficient) values of 0.43 and 0.57 for tTHM vs. colour and TOC respectively (Fig. 3a), colour closely correlating with TOC (Table 4). Against this, an analysis based on feed TOC reveals a comparatively high correlation coefficient value of 0.76 (Fig. 3b, Table 4), provided a single outlier (SpWN5) is ignored. This is one of the two sites fitted with the most rudimentary pretreatment, the other (SpWN13) also yielding a high TOC and colour passage (Table 3). Whilst the mean colour/TOC values across all sites are reasonably consistent at 6.6 Hz L/mg ± 14% SD, its

tTHMFPpermeate  5:3TOCfeed þ 5:4; R2 ¼ 0:76

ð1Þ

In terms of the key datum of permeate THM yield, the mean data indicates a value of 30 ± 8.4 lg tTHMs per mg permeate TOC. 3.2. Multi-region analysis, two technologies The outcomes of the scoping trial indicate that there is marginal gain from monitoring both colour and TOC; correlation with TOC provides the yield directly, allowing comparison with the literature data of Table 1. Analysis of data from 30 sites (Table 5) indicates a mean yield of 30 ± 14 lg tTHMs per mg permeate TOC – an identical mean to the scoping trial. Also, as with the scoping trial water quality (Table 3), no significant correlation was obtained for product tTHM concentration against either feed or permeate recorded colour or TOC concentration for most individual plants. The overall trend in TOC passage was 18% – but with considerable data scatter (±47%, almost identical to the scatter in the yield data). The mean TOC passage at some sites (SpWN5 and SpWN10) is more than double this average value, and similarly high mean THM yield values were calculated for TN7 and, in particular, TN8 (70 lg/mg). This implies that 7% of the permeate TOC is converted to THMs, cf. 3% on average across all sites. The mean THMFPfeed with respect to the feed TOC (THMFPfeed) is 5.4 lg/mg, about the same as the value of the coefficient for the scoping trial data (Eq. (1)).

Fig. 3. Permeate tTHMs correlations against (a) permeate TOC and colour, and (b) feed water TOC.

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S. Sutherland et al. / Separation and Purification Technology 149 (2015) 381–388 Table 5 Mean water quality data, 30 sites. Site

Configuration

SpWN3 SpWN4 SpWN5 SpWN6 SpWE1 SpWN10 SpWN12 SpWN13 SpWE2 TW1 TN1 TN2 TN3 TN4 TN5 TW2 TS1 TN6 TN8 TN9 TN10 TN12 TN13 TN14 TW3 TN15 TW4 TE1 TN7 TN11 Mean Standard deviation (%)

MWCO

Feed

Permeate

SpW SpW SpW SpW SpW SpW SpW SpW SpW Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube, Tube,

12.1 10.4 4.0 5.9 4.7 5.0 4.1 7.6 9.1 3.6 5.0 7.3 6.2 6.7 6.7 4.9 4.1 9.5 8.6 7.3 6.2 12.6 3.7 6.2 5.7 5.4 2.4 2.8 5.1 6.9 6.3 40

1.7 1.6 1.5 1.7 1.3 1.7 0.9 1.4 2.0 0.7 0.7 1.0 0.7b 1.2 0.6 0.7b 1.2 0.7b 0.9 0.5 0.7 1.1 0.7b 0.7b 0.7b 0.7b 0.8 0.8 0.7b 1.1 1.0 40

2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 2 kDa 4 kDa 4 kDa 4 kDa

THM yield, lg THMs/mgTOC

TOC, mg/L a

% passage

vs. feed

vs. permeate

14 15 37 29 28 35 22 18 22 19 14 14 11 18 9 14 29 7 11 7 12 8 19 11 12 13 33 28 13 16c 18 47

5.4 5.2 15.3 7.6 4.1 6.5 9.4 7.3 6.2 4.1 2.7 5.6 2.3 5.6 2.7 1.5 7.9 0.7 9.4 2.0 6.2 3.8 2.4 2.9 1.1 4.3 11.5 5.2 7.3 6.5 5.4 60

38.3 34.5 41.3 26.7 14.9 18.8 42.6 39.8 28.2 22.0 19.2 39.3 19.9 31.7 28.4 10.4 27.7 10.6 69.7 28.0 37.0 45.3 13.1 25.4 8.8 32.1 35.1 18.9 54.3 40.8 30.1 46

Outliers underscored. a Permeate taken as the average of the permeate and tap data. b Assumed values: TOC readings reported as being at or below limit of detection. c Refers to ‘‘normal’’ operation: during a period between late March and early August 2011 there were anomalously high levels of organic carbon in the treated water, attributed to the bogbean plant. During this peak period passage rose to 81%, possibly indicating post-permeation contamination.

(2) A significantly higher (by 60% compared with the most selective membrane) TOC passage for the SpW membrane plants than for the tubular membrane plants, in keeping with the MWCO values (Table 2). (3) A slightly greater average yield for the least selective tube membrane. However, whilst these mean data are consistent with the nominal membrane ratings, there are evidently significant variations between sites with respect to the yield, for example:  SpWE1 exhibits a relatively high TOC passage of 28%, and yet has amongst the lowest yield values of 15 lg/mg – half the average values across all sites.  TN8 exhibits a relatively low TOC passage of only 11%, but provides the highest mean yield value of 70 lg/mg.

Fig. 4. TOC passage and THM yield for the two membranes.

A consideration of the sites on the basis of the membrane technologies (Fig. 4) reveals: (1) Reasonably close correspondence in average THM yield (31.7 vs. 29.2 lg tTHMs/mgTOC) for permeate samples from plants fitted with the SpW and most selective tubular membrane.

The mean TOC passage values of 15% for the 2 kDa MWCO membrane and 25% for the 8 kDa one were, as expected, somewhat lower than those reported for less selective membranes associated with the studies listed in Table 1. Reported data for TOC passage generally range between 34% and 65% for membranes rated at between 100 kDa and 0.1 lm [7,12,13], with 6–12% passage provided by UF-NF combined [12,20]. However, there is no correlation between rejection and membrane pore size across different waters. In one instance, for example, the UF membrane provided no TOC rejection at all [19], attributed to the unusually low MW distribution of the NOM in the raw water.

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Fig. 5. Boxplots for levels of (a) feed TOC, and (b) permeate THMs over three seasons.

Fig. 6. Boxplots for levels of (a) TOC, and (b) permeate tTHMs over four regions.

THM yield trends from the current study suggest that organic carbon chemistry varies substantially across all sites with reference to THM precursors and their reactivity, despite the apparent greater overall selectivity of the tubular membrane. The overall range of mean values (10–70 lg THMs per mg TOC, 30 on average) is in close agreement with the published range of values (Table 1), predominantly between 25 and 48 lg/g, 39 on average. 3.3. Seasonal and regional analysis The effects of temporal and spatial variation on THM and TOC levels were examined using boxplots (or ‘‘box-and-whisker’’ diagrams) and Analysis of Variance (ANOVA). Boxplots display variation in samples of a statistical population, allowing potential outliers (or extreme values) to be detected. The bottom and top of the box are the first (Q1) and third (Q3) quartile respectively, and the band inside the box the median; the lower band or whisker is given by [Q11.5  (Q3Q1)] and the upper band or whisker by [Q3 + 1.5  (Q3Q1)]. The points outside the lower and upper bands (i.e. ([Q11.5  (Q3Q1)], [Q3 + 1.5  (Q3Q1)])) are then considered as outliers.

Based largely on anecdotal evidence of THMFP trends, the seasons chosen were 1st July–31st October (Season 1), 1st November–28th February (Season 2), and March–30th June (Season 3). The outcomes for concentrations of feed TOC (Fig. 5a) and permeate THM (Fig. 5b) over the three seasons illustrate significant differences in behaviour between the two parameters across the different seasons. In both cases there appears to be a statistically significant increase in the concentrations in Season 1 (Fig. 5). Whilst this trend is not apparent from the boxplot given in Fig. 5a, the computed p-value (p < 2  1016) obtained from one-way ANOVA demonstrates that TOC levels significant differ over the seasons. However, there are a significant number of outliers outside the whisker band in the boxplot. The accepted levels of TOC in Seasons 1, 2 and 3, are 6.6, 4.2 and 6 (mg/L) respectively; the corresponding percentages of the extreme values are 3.4%, 3.6% and 2.4%. For the THM values the levels in Season 1 are significantly higher than levels in other seasons, supported by a p-value below 2  1016 derived from ANOVA. There are also a considerable number of outliers for all three seasons. The accepted levels of THM in Seasons 1, 2 and 3, are 105, 79.6 and 66.9 lg/L respectively, and the

387

S. Sutherland et al. / Separation and Purification Technology 149 (2015) 381–388 Table 6 TOC and tTHM accepted levels (third Quartile, Q3) and percentages of outliers for each region.

a

Region

Accepted TOC level, mg/L

% outliers

Accepted tTHM level, lg/L

% outliers

tTHM yield, lg tTHM/mg TOC

East North South West

4.2 8.4 4.5 5

0.07 1 1.8 4.9

99.2 89.3 59 88

0.9 1 1 1.6

23a 11 13 18

Based on two samples.

Fig. 7. Boxplots for levels of (a) TOC, and (b) tTHMs for Isle of Skye data.

corresponding percentages of the outliers 1.2%, 1% and 1.3%. Based on the accepted upper-boundary levels (the third Quartile, Q3), the yields of permeate THM to feed TOC concentration for the three seasons are 16, 19 and 11 lg tTHM/mg TOCfeed respectively for Seasons 1, 2 and 3. This trend is in keeping with the trend in seasonal temperature, which has been observed to promote THMFP in previous studies [23,24]. An analysis of the impact of the location of the plants in the Scottish Water region, when divided generally into North, South, East and West, revealed that there are also significant differences between mean TOC and THM levels across these regions (Fig. 6). In addition to these boxplots, the computed p-value (p < 2  1016) derived from ANOVA indicates that there are significant differences between TOC and THM levels across the different geographical regions. The Tukey test was then used to identify which two locations might have similar TOC or THM levels. This detailed analysis revealed that the TOC levels of South and West are not significantly different. Similar analysis revealed that THM levels of the West and North are not significantly different. A large proportion of outliers (4.9%) for the TOC data arise in the region (West) where there is the largest sample of installations compared to 0.07% for the East region where the sample is much smaller (Table 6). For the THM data the % outliers were all within 0.9% and 1.6%. The overall yields, based on the accepted upper-boundary levels, range from 11 lg tTHM/mg TOCfeed in the North to 23 in the East. However, since the East region is based only on two samples, this datum should be ignored. A comparison between the data from the most challenging region (the Isle of Skye) and all others (Fig. 7) reveals a clear increase in the yield for the plants on Skye (to 24 lg tTHM/mg TOCfeed). It can be surmised that both selectivity and reactivity changes seasonally and regionally, unaffected by membrane characteristics, impacting significantly on the yields based on the Q3 feed TOC data (Fig. 8). It is also evident that the Q3 yield data values of Fig. 8 are considerably higher than the mean yield data of around 5.4 lg tTHM/mg TOCfeed (Table 5 and Eq. (1)).

Fig. 8. Summary of seasonal and regional data, based on Q3 data for feed TOC and permeate tTHMs.

3.4. THM speciation A possible contributor to variation in permeate THMFP is the substitution of the chlorine atom by the heavier bromine atom, increasing the molecular weight. The Isle of Skye data set was therefore further analysed to establish the impact of bromine substitution on permeate THM concentration. The data selected for this was for a one year period for SpW plants in one specific region of Scotland, thereby reducing regional and technological effects as far as possible, and the concentration of the mono-substituted species bromodichloromethane (CHBrCl2). A review of the WQ data revealed the di- and tri-brominated species to be at very low concentrations. A correlation between the tTHM and CHBrCl2 concentrations (Fig. 9) reveals that, counter-intuitively and contrary to published studies [15,25], the apparent impact of bromine substitution is to suppress the tTHM levels rather increase them. For example, for

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Acknowledgment The authors are grateful to Scottish Water, who funded the work and provided the water quality data. References

Fig. 9. Ratio of total THMs to bromodichloromethane across five sites.

the SpWN4 plant where the CHBrCl2 species made up 27–36% of the tTHMs, the tTHM concentration was largely between 20 and 80 lg/L. For the SpWN3 plant CHBrCl2 constituted only 5–15% of the tTHMs but the tTHM concentration ranged from 35 to 90 lg/L with a mean concentration of 54 lg/L. Similar trends of decreasing tTHM levels with increasing bromine substitution were recorded at three other plants in Scotland where levels of brominated species were high.

4. Conclusions The determination of permeate THM yields from organic carbon concentrations measured across a number of membrane potable water treatment plants across Scotland has revealed:  Significant data scatter exists for water quality data pertaining to individual plants, such that no recognisable pattern is evident in yield for specific sites.  Data averaged over an extended period of time (18 months) for each site indicate a weak but noticeable relationship between tTHMs and permeate TOC, with yield ranging between 10 and 70 lg tTHMs/mg TOC. This range, and the corresponding mean value of 30 ± 8 lg/L, is in broad agreement with the majority of published literature THM yield values.  There is evidence of significant seasonal and regional variations in both TOC levels and, in particular, THM yields. A more conservative estimation of permeate tTHM levels is derived using boxplots with Analysis of Variance, which indicate a yield of 11–24 lg tTHMs/mg feed TOC based on the third quartile of the measured feed TOC and permeate tTHM concentrations.  Bromine substitution appears to have no significant deleterious impact on tTHM levels with available, with increasing concentrations of the mono-substituted species (CHBrCl2) correlating with decreased tTHM concentrations. This is both counter-intuitive and contrary to published findings. The findings suggest that the risk of THM mitigation is best addressed by limiting the permeate TOC concentration to <1.1 mg/L based on the mean concentration and <0.4 mg/L based on the third quartile. These values would equate to a maximum THMFP of 80 lg/L tTHMs. The findings indicate the risk of THM concentration exceeding threshold to become significant during the March–October season in the Western region of the country.

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