Comparison of portable nephelometric turbidimeters on natural waters and effluents

Transcription

1 New Barter Zealand & Deas Portable Journal of Marine nephelometric and Freshwater turbidimeters Research, 2003, Vol. 37: /03/ $7.00 The Royal Society of New Zealand Comparison of portable nephelometric turbidimeters on natural waters and effluents PAUL J. BARTER TONI DEAS Cawthron Institute Private Bag 2 98 Halifax Street East Nelson, New Zealand paul.barter@cawthron.org.nz Abstract Five different cuvette-style portable turbidimeters using a nephelometric (side scattering of light) principle were assessed simultaneously to determine between-meter variability. The assessment was conducted using primary formazin standards, and a variety of different effluent types and receiving waters encompassing a wide range of turbidities and optical properties. For the formazin standards, the coefficient of variation (CV) ranged from 1.5% to 6.8% providing an indication of precision. For the effluents and receiving waters, the CV ranged from 6.6% to 44.1%, which compares favourably with previous interlaboratory studies reporting CVs ranging from 23% to 144%; however, the previous studies were not direct side-by-side comparisons of meters. A power analysis showed that, with the exception of very clear headwater streams, single turbidity samples are sufficient to detect changes in clarity recommended in the New Zealand guidelines for the management of water colour and clarity. Keywords turbidity; water clarity; nephelometry M02077; Published 5 August 2003 Received 26 September 2002; accepted 23 April 2003 INTRODUCTION The recently revised Australian and New Zealand guidelines for fresh and marine water quality (ANZECC 2000) incorporates the development of procedures for deriving numerical water quality criteria for aquatic ecosystems in New Zealand and Australia. A fundamental component of the guideline approach is the development of appropriate indicators for use as criteria and for routine monitoring. To address the widespread issues caused by suspended particulate matter (e.g., reduction in visual clarity, light penetration, smothering of habitat) the ANZECC (2000) guidelines recommend turbidity as one of the primary indicators. Portable field turbidimeters (or nephelometers) are widely used in New Zealand as an index of clarity of effluents and receiving waters. Major advantages of these instruments are low cost per sample and ease of operation. Unfortunately, this relative ease in acquiring seemingly quantitative data is also one of the primary drawbacks to using these types of instruments, as results are often misinterpreted or given undue weight by inexperienced operators. The most common misconception is that turbidity (measured as nephelometric turbidity units or NTU) is an absolute measure like temperature, and that different turbidimeters will yield similar NTUs in the same way that different thermometers will reliably measure temperature. However, nephelometric turbidity merely represents a relative index of the side scattering of an incident beam of light compared against an aqueous chemical standard (usually formazin). Because of differences in optical design (e.g., light source, spectral sensitivity of detector, beam configuration) between the various turbidimeters, different instruments may yield different numerical NTU values. These design limitations and diversity of response have been reported previously in the literature (McGirr 1974; McCluney 1975; Davies- Colley & Smith 2001) and are recognised in the New Zealand guidelines for the management of water colour and clarity (MFE 1994) as well as the

2 486 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37 nephelometric method itself (APHA 1998). For example, Standard methods (APHA 1998, Method 2130) states that Nephelometers that instrument manufacturers claim meet the design specifications of this method may not give the same reading for a given suspension, even when each instrument has been calibrated using the manufacturer s manual. Similarly, the colour and clarity guidelines (MFE 1994) state that different nephelometers, even though standardised identically to formazin, will give different turbidity values on the same water. However, neither of these documents provides guidance on how much variability can be expected between instruments. Although the MFE (1994) guidelines reported a CV for turbidity ranging from 23% to 104%, these data were derived from interlaboratory comparisons. Such interlaboratory comparisons are of limited value for turbidity since samples must be shipped to the respective laboratory and are neither analysed at the same time nor under similar environmental conditions. In addition, the make and model of meters used are not reported. Similarly, McGirr (1974) demonstrated that the CV among nephelometers was between 28% and 144%, but this study is almost 30 years old, used a variety of different instrument types (i.e., nephelometers, transmissometers, spectrophotometers, and optical backscatterance meters), and was also based on interlaboratory data and not a direct side-by-side comparison. McGirr (1974) acknowledged this limitation stating that storage instability can cause a 20 30% negative bias in turbidity of shipped samples within a few days. There have been a couple of studies conducted on the comparison of meters produced by the same manufacturer (e.g., Hongve & Akesson 1998) and comparative studies on low level turbidity measurements (e.g., Sadar 1999; Letterman et al. 2002); however, we are unaware of any current guidelines or references that document variability between newer portable turbidimeters. Therefore, the purpose of this study was to compare different portable field turbidimeters on a range of formazin standards, and a variety of different natural waters and effluent types. The results of the comparisons are discussed in the context of the MFE (1994) colour and clarity guidelines; specifically, whether the range of variation between meters is low enough to detect recommended maximum changes in clarity. METHODS Five different portable field turbidimeters were evaluated in this study. All five meters were cuvettestyle nephelometers which purportedly met USEPA (1993) method or ISO (1990) Method 7027 design specifications. These design criteria generally Table 1 Manufacturer s specifications for each of the five meters tested. (NTU, nephelometric turbidity units; n/a, not available.) Hach Hf Scientific Merck Turbiquant Orbeco Hellige Orbeco Hellige 2100P DRT-15CE 1000-IR 966-T 966-IR Range (NTU) Light source Tungsten filament Tungsten filament Infra-red Tungsten filament Infra-red (temp., wavelength) * (2650 K, 570 nm) (2250 K, 860 nm) (860 nm) (2250 K, 600 nm) (870 nm) Response time (s) 6 < n/a n/a Calibration 4 point 4 point 4 point 2 point 2 point (0, 20, 100, 800) (0.02, 10, 100, 1000) (0.02, 10, 100, 1000) (0, 40) (0, 40) Resolution (NTU) (on lowest range) Sample size (cuvette) 15 ml 25 ml 15 ml 20 ml 20 ml Accuracy ±2% of reading or 0 10 NTU, ±1% (±2%) ± 2% in low 2 ranges ±2% in low 2 ranges ±1 least significant digit; NTU, ±5% (±3%) ±3% of reading NTU, ±10% ( NTU) Power supply (battery) 4 AA 6 V gel-cell rechargeable 4 AAA 4 AA 4 AA Power supply (AC) Yes (optional) Yes No Yes (optional) Yes (optional) adaptor Design criteria USEPA USEPA ISO 7027 USEPA ISO 7027 * Colour temperature (for tungsten lamps) and peak spectral response of instrument. Observed response time was c s.

3 Barter & Deas Portable nephelometric turbidimeters 487 specify the light source, water path length, detector angle, and detector spectral peak response of the meter. A list of the manufacturers specifications for each meter is presented in Table 1. The Merck Turbiquant is manufactured by WTW Incorporated and is also sold in New Zealand as a WTW Turb 350 IR Portable Turbidimeter. Although turbidimeters using other measurement principles are commercially available, these were not examined in the current study. Each meter was supplied new from the manufacturer at the start of the study to eliminate problems that might otherwise arise through continued use, such as scratched cuvettes and optical degradation. Before analysis, each meter was calibrated using a primary formazin standard according to the manufacturer s specifications. Although standard analytical methods were followed (e.g., USEPA 1993; APHA 1998), the purpose of the exercise was to re-create field conditions, hence certain laboratory-specific procedures were not employed (e.g., degassing samples through the application of partial vacuum, or sonication, or the addition of surfactants to achieve dispersion). These laboratory procedures are often used to eliminate small bubbles in the sample which can affect the reading. To ensure consistent measurements, a single experienced operator conducted all of the sample preparation and analysis. Measurements were conducted on primary formazin standards to examine meter precision. A variety of different effluent types and different receiving waters were also analysed to cover a wide range of water clarity and optical properties of suspensoids. Five replicate samples were analysed simultaneously on each of the five meters. Before analysis, samples were mixed by gentle agitation and inversion, paying careful attention to avoid entrainment of air bubbles within the sample. Individual replicates consisted of a 100 ml aliquot of sample which was poured at random into each of the five instrument cuvettes. Sample cuvettes were handled from the top to avoid fingerprints on optical surfaces and each cuvette was wiped clean with a lint-free disposable cloth (Kimwipes ) before reading turbidity. Following each replicate reading, the cuvettes were emptied and allowed to dry briefly before reading the next replicate. Between samples the cuvettes were rinsed with reverse osmosis purified water. Immediately following calibration with a primary formazin standard, a dilution series of the same standard was analysed on each meter. A variety of common effluent types were also analysed: domestic waste waters; oxidation pond effluents; dairy wash waters; fish processing effluent; urban storm water; meatworks effluent; and apple processing effluent, as well as the following receiving waters: river and stream fresh waters; estuarine water; and coastal sea water. Like the formazin standards, readings on each effluent and receiving water consisted of five replicates for each meter. For comparison against MFE (1994) water clarity criteria, a statistical power analysis was conducted to determine the number of samples required to detect a certain percentage change in turbidity. The sample size calculation, using a single sided t-test for means (Zar 1999), was performed with a 95% confidence interval (a = 0.05) and a power of 0.8 (b = 0.2). RESULTS AND DISCUSSION Formazin standards The mean results on formazin samples, along with the standard deviation (SD) are presented in Table 2. Results of the dilution series show reasonable agreement between meters up to the 400 NTU range (Table 2), but a marked disparity for the 800 NTU standard. CVs across all five meters for each of these standards ranged from 1.5% for the 400 NTU standard, up to 6.8% for the 800 NTU standard, but were generally c. 3% for the remainder of the standards (Table 2). This increase in variation at the higher end of the range is common in turbidity measurements and is the underlying rationale in Standard methods (APHA 1998) for reporting different ranges of turbidities to different levels of precision. For example, Standard methods recommends that turbidities in the range of NTU be reported to the nearest 50 NTU. The within-meter precision was very good across all standards ranging from 0.1% to 2.2% and all meters met or exceeded the manufacturers specified precision listed in Table 1. As would be expected with digital instrumentation, the linearity of each meter across all standards was also excellent, with r values all in excess of 0.99 whereas the standard error (SE) of this regression (or between-meter precision) ranged from 2.1 NTU for the Hach 2100P to 24.7 NTU for the Orbeco 966-T. That is, the difference observed between meters for the formazin standards was the result of minor differences in response slope and not because of non-linear response. This was particularly evident with the two Orbeco models which only used a two-point

4 488 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37 Table 2 Mean (±SD) (n = 5) Nephelometric turbidity unit (NTU) values for each of the five turbidimeters, and all five turbidimeters combined, for a dilution series of primary formazin standard immediately following calibration. (CV, coefficient of variation.) Hach HF Scientific Merck Orbeco Orbeco All 5 meters combined (n = 25) Standard 2100P DRT-15CE 1000IR 966-IR 966-T Mean (±SD) Min. max. % CV 10 NTU (±0.07) 9.90 (±0.05) 9.83 (±0.07) 10.6 (±0.19) (±0.24) (±0.39) NTU (±0.18) (±0.17) (±0.18) 41.1 (±0.14) (±0.09) (±1.24) NTU (±0.38) (±0.11) (±0.43) (±0.37) (±0.24) (±3.41) NTU (±0.84) (±0.84) (±0.8) (±0.84) (±0.71) (±6.35) NTU (±1.92) (±1.00) (±3.27) (±1.64) (±2.17) (±5.88) NTU (±1.22) (±2.12) (±2.34) (±1.52) (±2.12) (±51.54) calibration and very small changes of the upper standard yielded very different calibration slopes. Effluent samples and receiving waters Analysis of effluent and receiving water samples are presented in Table 3. The within-meter variability was generally very good for these samples (i.e., mean CV c. 5%), so the differences between meters accounted for the bulk of the variability shown in Table 3. For comparative purposes, each sample in Table 3 has been placed in a descriptive group ranging from very low turbidities to very high. The highest CVs are seen in the groups at either end of the range of these samples (i.e., very low and very high) whereas the lowest are from the middle of the range. Obviously some of this variation is the result of sample type and not necessarily the actual turbidity level of the sample, but the trend of highest CVs at the extreme low and high end of the range of these meters is not unexpected. That is, regardless of sample type, meters perform best at turbidities within the middle of their operating range. However, apparently sample type strongly influences crosscomparability of these meters. Certain sample types were much more homogenous and therefore more stable than others. Surprisingly, some waste waters (i.e., both raw and treated sewage), were generally very stable and turbidity did not vary appreciably during the response time of the meter, whereas samples with large suspended particulates (e.g., fish processing effluent, dairy wash water) fluctuated between readings. This is an important consideration since two of the meters (i.e., Hach 2100P and Merck Turbiquant) use signal averaging while the remainder do not. For unstable samples, there is a much higher chance of introducing operator bias since the operator is forced to decide which NTU reading to record. Nevertheless, the majority of the results showed appreciably better agreement between meters than has been reported previously. This is illustrated in Fig. 1 where bias shows as a deviation away from the 1:1 relationship. With the exception of a slight positive bias at the high end of the range for the Hach 2100P, individual meters agreed closely with little or no deviation from the 1:1 ratio. Of the 17 sample types analysed, 11 exhibited a CV of <15%, seven had a CV of <10%, whereas only five had a CV >20%. These results are far better than the % CV reported by McGirr (1974) and the % reported in the MFE (1994) guidelines, which were both based on interlaboratory comparisons.

5 Barter & Deas Portable nephelometric turbidimeters 489 Compliance monitoring From a regulatory perspective, while these results indicate that the variability between meters is not as disparate as previously reported, they should not be construed as an endorsement for use of turbidity on routine monitoring programmes over other clarity measures (e.g., black disk, Secchi disk, transmissivity). Turbidimeters do offer a quick and inexpensive means for measuring the relative clarity of effluents and receiving waters, but the limitations mentioned in the introduction and outlined in the MFE (1994) guidelines and by Davies-Colley & Smith (2001) are still applicable. If turbidity is used for regulatory compliance purposes, it is best not used as the sole means of determining changes in clarity, but rather as an ancillary measure supporting the other clarity measures mentioned above. The results also highlight the importance of reporting not only NTU values, but the make and model of meter used. A further consideration is whether the variability in turbidity measures is low enough that meaningful comparisons can be made to existing guideline levels. For example, the MFE (1994) colour and clarity guidelines recommend no greater than a 20% change in visual clarity for Class A waters (i.e., where visual clarity is an important characteristic of the water body) and between 33% and 50% for all other waters. The guidelines also point out that this latter criterion roughly corresponds to a % change in turbidity, a distinction often overlooked by water quality professionals. A power analysis was Table 3 Mean (±SD) (n = 25) Nephelometric turbidity unit (NTU) values of all five turbidimeters combined for different effluents and receiving waters. (CV, coefficient of variation.) Sample Mean NTU (±SD) Min. Max. Range % CV Group Distilled water 0.14 (±0.06) Very low (<0.5 NTU) Headwater stream A 0.17 (±0.05) Headwater stream B 0.24 (±0.05) Coastal sea water 1.6 (±0.18) Low (0.5 5 NTU) Estuarine water 2.83 (±0.25) River water 3.06 (±0.31) Oxidation pond effluent A (±0.75) Medium (5 25 NTU) Oxidation pond effluent B (±1.79) Oxidation pond effluent C (±1.60) Storm water (±1.83) Fish processing effluent (±11.01) High ( NTU) Domestic waste water A (±10.87) Domestic waste water B (±18.89) Domestic waste water C (±16.52) Very high (>200) Dairy wash water (±53.04) Apple processing effluent (±65.09) Meatworks effluent (±80.36) Table 4 Results of power analysis (a = 0.05, b = 0.2) on different receiving waters showing range of replicate samples required to detect a certain percentage change in turbidity. (NTU, nephelometric turbidity units.) No. of samples required to detect a certain percentage change in NTU * Receiving water Mean NTU (±SD) 33% 50% 100% Headwater stream A 0.17 (±0.05) Headwater stream B 0.24 (±0.05) Coastal sea water 1.60 (±0.18) Estuarine water 2.83 (±0.25) River water 3.06 (±0.31) Storm water (±1.83) * Ranges are for lowest observed single meter variation to highest.

6 490 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37 Fig. 1 Individual turbidity meter results for waters and effluents plotted against the mean turbidity of all five meters combined. The 1:1 line of perfect agreement is shown.

7 Barter & Deas Portable nephelometric turbidimeters 491 performed to determine how many replicate samples would be required to detect a 33%, 50%, or 100% change in turbidity based on the highest and lowest variation observed within meters (i.e., any single meter variation) for the different receiving waters (Table 4). For example, a range of 1 4 meant that a single replicate would be required to detect the specified percentage change from the lowest variation meter whereas four replicates would be required from the meter exhibiting the highest CV. Note that the range of CVs was generally very close between meters and no one meter was obviously better or worse than the others. For instance, the Orbeco 966-T showed the lowest variation on headwater stream A but the highest variation on estuarine water. The results in Table 4 show that a single sample is usually sufficient to detect a 33% change in turbidity with the exception of very clear headwater streams. Even so, collecting a minimum of three replicates per station would still be a prudent approach. For many monitoring programmes, measuring multiple replicate samples per station would be prohibitively expensive; however, turbidity measurements are relatively quick and cost-effective and it is envisaged that this degree of replication would not be overly restrictive. For very clear receiving waters (i.e., <0.5 NTU) the degree of replication required would be much higher and careful site-specific considerations would need to be made. These clear headwater streams are often considered Class A waters and are thereby subject to tighter limits (i.e., <20% change in clarity) which would require even more numbers of replicates. In these instances, turbidity measures would be neither cost-effective nor efficient although direct clarity readings would probably be suitable. CONCLUSIONS Portable field turbidimeters offer a cost-effective and rapid means of measuring relative clarity of effluents and receiving waters. Despite the inherent limitations in turbidity as an index of clarity, its inclusion as one of the recommended indicators in the recent revision and re-release of the ANZECC (2000) guidelines suggests that it will continue to be used for a variety of different water quality monitoring programmes. This study has shown that between-meter variability (expressed as % CV) ranged from 1.5% to 6.8% for primary formazin standards providing an indication of the precision of turbidity measurement, and from 6.6% to 44.1% for effluents and receiving waters, which amounts to an index of accuracy of turbidity measurement. These results compare favourably with previous interlaboratory studies (McGirr 1974; MFE 1994) that reported CVs ranging from 23% to 144% from interlaboratory comparisons. A power analysis of the variance suggests that single replicate samples are usually sufficient to detect changes in clarity recommended in MFE (1994) guidelines. ACKNOWLEDGMENTS Turbidity meters used in this study were kindly supplied by: Kim Alexander (John Morris Scientific); Craig Trembath (Merck Limited, New Zealand); Alison Young (Biolab Scientific Ltd); and Peter Hassan (Alphatech NZ Ltd). We also thank Barrie Forrest (Cawthron) and Dr Rob Davies-Colley (NIWA) for peer review and technical advice. REFERENCES ANZECC 2000: Australian and New Zealand guidelines for fresh and marine water quality APHA 1998: Standard methods for the examination of water and wastewater, 20th ed. Washington, D.C., American Public Health Association. Davies-Colley, R. J.; Smith, D. G. 2001: Turbidity, suspended sediment and water clarity: a review. Journal of the American Water Resources Association 37: Hongve, D.; Akesson, G. 1998: Comparison of nephelometric turbidity measurements using wavelengths and 860 nm. Water Research 32: ISO 1990: ISO 7027 Water quality determination of turbidity. Geneva, International Organisation for Standardisation. Letterman, R. D.; Johnson, C. E.; Viswanathan, S.; Dwarakanathan, J. 2002: A study of low-level turbidity measurements. Denver, CO, United States, American Research Foundation and American Water Works Association. 164 p. McGirr, D. J. 1974: Interlaboratory quality control study no. 10: turbidity and filterable and nonfilterable residue. Environment Canada Report Series No p. MFE 1994: Water quality guidelines no. 2. Guidelines for the management of water colour and clarity. New Zealand Ministry for the Environment. 77 p.

8 492 New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37 Sadar, M. 1999: Turbidimeter instrument comparison: low-level sample measurements. Hach Inc. Technical Information Series Literature No. 7063: USEPA 1993: Determination of turbidity by nephelometry (Method 180.1). Methods for the determination of inorganic substances in environmental samples. Washington, D.C., United States, United States Environmental Protection Authority. 10 p. Zar, J. H. 1999: Biostatistical analysis. Prentice-Hall Inc. 663 p.