cawrpt 2602 tank calibration of turbidity · pdf fileapproximately 400 litres. ... a total of...

REPORT NO. 2602

TANK CALIBRATION OF TURBIDITY SENSORS FOR PORT OTAGO LTD

CAWTHRON INSTITUTE | REPORT NO. 2602 OCTOBER 2014

TANK CALIBRATION OF TURBIDITY SENSORS FOR PORT OTAGO LTD

PAUL BARTER

Prepared for Port of Otago Limited

CAWTHRON INSTITUTE 98 Halifax Street East, Nelson 7010 | Private Bag 2, Nelson 7042 | New Zealand Ph. +64 3 548 2319 | Fax. +64 3 546 9464 www.cawthron.org.nz

REVIEWED BY: Robyn Dunmore

APPROVED FOR RELEASE BY: Chris Cornelisen

ISSUE DATE: 27 October 2014

RECOMMENDED CITATION: Barter P 2014. Tank calibration of turbidity sensors for Port Otago Ltd. Prepared for Port Otago Limited. Cawthron Report No. 2602. 7 p. plus appendices.

© COPYRIGHT: This publication must not be reproduced or distributed, electronically or otherwise, in whole or in part without the written permission of the Copyright Holder, which is the party that commissioned the report.


1

1. INTRODUCTION AND BACKGROUND

Port Otago Ltd (POL) has recently been granted resource consent (ORC 2010.198) by the Otago Regional Council (ORC) to undertake a large capital dredging programme within Otago Harbour. Under a 2012 environment court decision which upheld the original consent, a large-scale environmental management plan (EMP) was stipulated to determine and assess any adverse effects resulting from the dredging. The EMP was also designed to clarify and elucidate some of the conditions in the original consent document. One component of the EMP was the installation, maintenance, and calibration of in-Harbour and offshore turbidity monitoring sites. Specifically, the calibration component relates to condition 10(e) of the resource consent which states: (e) a confirmation of the relationship between turbidity and suspended solids concentrations established by a sampling protocol and programme, which is specified by the Technical Group. The NTU calculations are based on a ratio of NTU to suspended sediment concentration of 1:1 plus or minus 25%. If the NTU is calibrated outside that range then the consent holder shall apply for a review of the NTU limits specified in these conditions

Cawthron Institute (Cawthron) was contracted by POL to design and build the turbidity monitoring sites and to undertake some laboratory-based calibrations of the selected sensors to ascertain the relationship between nephelometric turbidity units (NTU) and suspended sediment concentration (SSC). This short report presents the findings of the laboratory-based calibrations.

2. METHODS

2.1. Instrument selection

The original 1:1 (NTU:SSC) relationship was established using field data collected by the National Institute of Water and Atmospheric Research (NIWA) from Otago Harbour and reported in Baddock (2008). This 2008 work used a Seapoint© STM turbidity sensor to measure NTU1 in the water column and was calibrated to SSC through laboratory testing of water samples collected in-situ. The sensors selected by Cawthron for the turbidity monitoring stations (Table 1) were made by a different manufacturer (Wetlabs© Inc) and were therefore likely to show a different NTU:SSC relationship. Inherent variability between make and model of turbidity sensor has been reported since they were originally developed (McGirr

1 While the term NTU is used in the consent, and for consistency, repeatedly used throughout this report, the

technically correct term should be Formazin Turbidity Units (FTU) since the sensors themselves measure optical back-scatterance and not nephelometry (see Appendix 1).

OCTOBER 2014 REPORT NO. 2602 | CAWTHRON INSTITUTE

2

1974). While improvements in technology have significantly reduced this variability (e.g. Barter & Deas 2003), instrument and site specific calibration is still the recommended approach (e.g. MFE 1994, Davies-Colley and Smith 2001). Given the extra effort involved, the decision to change instrument model from that used in the original assessment was made because the Wetlabs© sensors have several design advantages (e.g. copper antifouling and self-wiping optics) for long-term deployments which justified the switch.

Table 1. Turbidity sensor specifications. Make Model Range (NTU) Notes

Wetlabs© NTUSB 0–250 Offshore Sites A-C NTUS 0–400 Harbour Sites A-G Seapoint© STM 0–500 Cawthron (s/n 12076) STM 0–750 NIWA Instrument

Therefore, the approach to the calibration exercise was to test not only the SSC:NTU relationship for the Wetlabs© sensors but to cross-calibrate against the original Seapoint © sensor(s) as well.

2.2. Tank tests

In order to obtain the widest range of turbidity and ensure samples were evenly distributed within that range, a tank-based method was employed rather than trying to rely solely on samples collected in the field. The tank used was made from black polyethylene with a nominal capacity of approximately 400 litres. Individual turbidity meters were mounted through circular holes on a board that sat above the tank such that the sensors were all positioned at the same height in the water column, facing downwards (Figure 1). Seawater within the tank was continually mixed using a 31 litre per minute submersible impellor pump. Sensors were occasionally moved from hole to hole within the board (i.e. different positions in the tank) and replicate readings collected to ensure homogenous mixing was taking place. Data from each serial sensor (e.g. Wetlabs©) were simultaneously fed in real time to a computer mounted adjacent to the test tank (Figure 1), whereas voltage outputs from analogue sensors (e.g. Seapoint©) were logged using a Fluke© 289 digital multi meter.


3

Figure 1. Photo of the setup for the tank testing procedure showing instrument mounting

arrangement. The start point of the test procedure was to fill the tank (~375 l) with a measured volume of filtered seawater2 and start the pump prior to taking any turbidity readings. Following collection of the clean seawater start point, a reverse serial dilution series was conducted where sequentially larger amounts of pre-weighed Otago Harbour sediments were added to the tank as individual aliquots. These individual aliquots/steps started at 5 grams and increased in weight over the course of the test. For example, in the first test, a twenty step series totalling 1 Kg of sediment was completed using the following aliquots: 5 g (×2); 10 g (×4); 25 g (×4); 50 g (×3); 100 g (×7). Individual 1 litre water samples for SSC were collected at the start of the series and at several of the individual steps during the test. SSC samples were sent to Hill Laboratories in Hamilton and analysed gravimetrically following filtration for marine total suspended solids (APHA 2540 D 22nd ed. 2012).

2 Tasman Bay seawater was passed through a three-stage filter process with successively smaller pore diameters

of 30 µm, 5.0 µm and finally 0.35 µm. Volume was calculated using a Gardena© water smart flow meter.


4

3. RESULTS AND DISCUSSION

A total of three separate tank calibration tests have been completed using a slightly different set of instruments each time (Table 2). Results of the tests, including the weight of sediment added at each step and the results of the SSC water samples are summarised in Appendix 2.

Table 2. Summary of test conditions for the three separate tank tests

Test date Instruments Reference sediment

Purpose

28/11/2013 Wetlabs Harbour Sensors A-G s/ns 586-592

Otago Harbour

Determine NTU:SSC Ratio

04/04/2014 Offhsore A-C (s/n 499-501) Harbour G (s/n 592) Seapoint (NIWA & Cawthron STMs)

Otago Harbour

Determine NTU:SSC Ratio Cross check Wetlabs & Seapoint

07/04/2014 Offhsore C (s/n 501) Seapoint (NIWA & Cawthron STMs)

Tasman Bay

Determine NTU:SSC Ratio Cross check Wetlabs & Seapoint

Examination of the data for calibration purposes was carried out two different ways: the first comparison was establishing the NTU:SSC relationship for each different manufacturer (i.e. Wetlabs© vs Seapoint©), whilst the second comparison was the relationship between different manufacturers. For the first comparison, the NTU:SSC relationship was examined by pooling all of the Wetlabs© sensors separately from the Seapoint© sensors. For those SSC results that were less than the detection limit (i.e. 3 g/m3), a value of half the detection limit (i.e. 1.5 g/m3) was used. Results of individual points as well as the linear regressions for both sets of data are shown in Figure 2. Note that the regression lines for both are forced through the origin as it’s assumed that zero SSC equals zero NTU.


5

Figure 2. Comparison of nephelometric turbidity units (NTU) to suspended sediment concentration

(SSC) for all three tank tests by manufacturer. Both sets of instruments exhibited excellent linearity across the entire NTU range with R2 values in excess of 0.97. However, there was an obvious difference between manufacturers with the Seapoint sensors yielding approximately double the NTU to SSC ratio. Interestingly, this calibration shows that neither of the sensor types would be within the ±25% of a 1:1 NTU:SSC relationship as specified in Condition 10.e of the consent. So a correction would be required regardless of which manufacturer was used. The reason for this is that the original 1:1 relationship from Braddock (2008) was based on in-situ field samples which did not cover the entire range of the instruments and were for samples with a maximum SSC of 6.54 g/m3. In fact, only two of the samples used for the linear relationship in Braddock (2008) were in excess of 2 NTU, whereas the current tank testing procedure tested SSC from near zero to approximately 750 g/m3. Nevertheless, the original consent limits were derived using Seapoint sensors and the historic in-situ data, so a comparison of the difference between the two manufacturers is also warranted. The second comparison differed from the first in that all data points where both Wetlabs© and Seapoint© sensors were simultaneously in the tank could be used and not merely those with corresponding SSC results. The data were compared by combining each Seapoint© reading with all of the readings from the various Wetlabs©


6

sensors in the tank at the same time. The results of this cross-comparison are presented in Figure 3 and show that a conversion factor of 0.4938 would need to be applied to Wetlabs© sensor turbidity values in order to report Seapoint© equivalent turbidity units.

Figure 3. Comparison of Wetlabs versus Seapoint turbidity from all three tank tests.

In terms of condition 10(e) of the consent, the simplest solution is to record the raw Wetlabs© NTU values and to adjust those readings to the equivalent SSC concentration (in g/m3) using the following simplified formula from Figure 2:

SSC = 2.5 × Wetlabs NTU These SSC values can then be applied to the existing Environmental Turbidity Limits (ETLs). It is understood that the original ETLs were based directly on SSC and the sensitivity within the receiving environment to adverse effects from increased suspended sediments. Adopting the conversion from the current set of tank tests

We

tlabs

(NT

U) 0 50

0

50W

etla

bs(N

TU

)


7

offers the added assurance that the converted SSC values will be accurate across the entire range of the instrument and not merely the lower-end of the range. This is especially important since all of the ETLs are greater than the values used during the derivation of the original 1:1 ratio.

4. REFERENCES

APHA 2012. Standard methods for the examination of water and wastewater. American Public Health Association, Washington, D.C. 22nd Edition.

Barter PJ, Deas T 2003. Comparison of portable nephelometric turbidimeters on natural waters and effluents. New Zealand Journal of Marine and Freshwater Research 37: 485–492.

Baddock E 2008. Turbidity monitoring in Otago Harbour data report. NIWA Client Report CHC2008-171. Project POL09501.

Clement D, Barter P 2011. Review of the RCEP Water Clarity, Colour, Suspended Materials and Suspended Sediment Indicators and Standards for the Canterbury Region. Prepared for Environment Canterbury. Cawthron Report No. 1838. 61 p.

Davies-Colley RJ; SmithDG 2001. Turbidity, suspended sediment and water clarity: a review. Journal of the American Water Resources Association 37: 1–17.

McGirr DJ 1974. Interlaboratory Quality Control Study No. 10: Turbidity and filterable and nonfilterable Residue. Environment Canada Report Series No. 37. 9 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

5. APPENDICES

Appendix 1. Electronic instruments for measuring clarity, adapted from Clement & Barter (2011).

Several different types of electronic devices have been invented to measure the relative clarity (or lack of) in aqueous solutions. The three most commonly used devices in New Zealand are: (i) turbidimeter (or nephelometer); (ii) optical backscatter (OBS) sensor and (iii) transmissometer. Each of these instruments uses a primary light source (generally in the form of an LED, laser diode or tungsten lamp). However, they differ markedly in how the incident light is measured after passing through a water sample (Figure A1.1). Two of the instruments (turbidimeter and OBS) relying on a side- or back-scattering of light from particles in the sample.

Figure A1.1. Schematic diagram showing the differences in how light is measured from different clarity

instruments. For comparative purposes, Figure A1.1 also includes a schematic representation of visual clarity measurements collected using traditional black-disk or Secchi disk. This shows that transmissivity is most closely related to these visual methods. Turbidimeters and optical backscatter

Turbidimeters or nephelometers measure the relative clarity of a water sample based on the 90 degree sidescatter of a beam of light. Readings are based on the relative clarity of a sample and are expressed as nephelometric turbidity units or NTU where higher NTUs represent more turbid waters. Optical backscatter sensors are very similar to turbidimeters but rely on a back-scatter (i.e. greater than 90 degree) of the

Light Source:LED, Laser diode

or Tungsten

NTU Detector90° from light source

Xmiss Detector0° from light source

OBS Detector>90° from

light source

Path length (Generally 10 or 25 cm)

Sample cell or in-situ

Secchi or Black Disc

Visual Observation

Path length (varies depending on water quality)

can be up to180° from

light source


9

light source. Readings from these sensors are often expressed as FTU or formazin3 turbidity units to avoid confusion with NTU. Both these types of sensors are calibrated to a formazin standard and as such it is not uncommon, albeit incorrect, for OBS to be reported in NTU. Optical backscatter sensors are often used with in situ monitoring situations (e.g. on a moored instrument) because the light source and detector can be incorporated into a much smaller sensor.

3 Formazin is a compound standard used that appears cloudy white in solution and can be mixed in discrete

concentrations, allowing for the preparation of quantifiable standards. Both these types of sensors are calibrated to a formazin standard.


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Appendix 2. Results of all three tank tests.

Table A2.1. Results of tank test conducted on 28 November 2013.

Sed

imen

t ad

ded

g

wet

wei

gh

t

To

tal s

edim

ent

add

ed

g w

et w

eig

ht

Har

bo

ur

A

NT

US

s/n

586

Har

bo

ur

B

NT

US

s/n

587

Har

bo

ur

C

NT

US

s/n

588

Har

bo

ur

D

NT

US

s/n

589

Har

bo

ur

E

NT

US

s/n

590

Har

bo

ur

F

NT

US

s/n

591

Har

bo

ur

G

NT

US

s/n

592

Lab

SS

C (

g/m

3)

0.00 0.3 0.1 0.1 0.2 0.2 0.2 0.1 < 3

0.00 0.1 0.2 0.1 0.1 0.2 0.1 0.1

5.06 5.06 0.3 0.3 0.3 0.3 0.3 0.4 0.3

5.10 10.16 0.9 0.8 0.8 0.8 0.7 0.8 0.8

10.00 20.16 1.9 1.8 1.8 1.9 2.0 1.8 1.7 6

10.04 30.20 3.4 3.1 3.0 3.0 3.0 3.0 3.1

10.04 40.24 4.1 3.9 3.8 4.0 4.0 4.0 4.0

10.03 50.27 5.3 5.0 5.0 5.0 5.1 5.1 4.8 12

25.09 75.36 7.4 7.5 7.4 7.5 7.6 7.5 7.7

25.02 100.38 10.9 10.8 10.4 10.3 10.3 10.5 10.0 25

25.00 125.38 13.9 13.1 12.6 12.8 13.0 13.0 12.8

25.02 150.40 16.0 15.5 14.9 15.2 15.2 15.5 14.5

50.03 200.43 21.5 20.4 20.2 20.4 20.0 20.2 19.8

50.13 250.56 27.8 26.0 25.2 25.2 24.4 24.8 24.6 64

50.04 300.60 32.3 31.1 30.0 30.4 29.3 30.1 29.2

100.05 400.65 43.1 41.3 39.8 40.1 38.7 40.1 38.6

100.08 500.73 52.6 49.9 48.5 49.8 47.6 47.8 47.6 116

100.11 600.84 61.8 58.7 56.8 58.8 57.5 56.3 57.2

100.04 700.88 74.0 68.9 65.5 66.6 66.6 64.1 65.1

100.07 800.95 78.4 75.3 74.0 74.2 73.1 71.2 71.5

100.07 901.02 87.6 83.4 81.1 81.9 80.3 79.1 79.8 198

100.04 1001.06 93.5 90.2 87.9 90.3 88.6 87.0 87.3


11

Table A2.2.Results of tank test conducted on 3 April 2014.

Sed

imen

t ad

ded

g

wet

wei

gh

t

To

tal s

edim

ent

add

ed

g w

et w

eig

ht

Off

sho

re A

N

TU

SB

s/n

49

9

Off

sho

re B

N

TU

SB

s/n

50

0

Off

sho

re C

N

TU

SB

s/n

50

1

Har

bo

ur

G

NT

US

s/n

592

NIW

A

Sea

po

int

ST

M

Caw

thro

n

Sea

po

int

ST

M

TS

S (

g/m

3)

0.00 0.00 0.2 0.3 0.2 0.3 0.4 0.4 < 3

2.53 2.53 0.4 0.4 0.4 0.5 0.8 0.8

2.51 5.04 0.5 0.5 0.5 0.6 1.2 1.2

5.03 10.07 2.8 2.6 2.6 2.3 5.2 5.3

9.99 20.06 5.9 6.4 6.1 5.2 12.3 12.2 36

10.17 30.23 9.6 10.3 9.2 8.2 21.5 20.9

10.04 40.27 14.9 14.1 14.1 12.1 26.0 28.8

10.01 50.28 19.2 16.5 16.6 14.7 33.6 35.0 49

25.14 75.42 28.5 24.9 24.1 23.1 57.6 54.7

25.08 100.50 36.1 31.9 31.8 29.5 74.3 69.9 87

25.04 125.54 43.8 40.2 39.7 37.2 85.1 90.6

25.09 150.63 50.2 44.1 47.0 41.4 108.7 106.7

50.10 200.73 71.3 64.3 62.8 60.0 148.1 138.6

50.08 250.81 90.0 81.5 80.0 76.7 186.3 184.5 195

50.01 300.82 109.3 104.6 93.9 94.1 215.9 212.2

100.29 401.11 149.4 137.5 130.3 127.6 278.6 250.1

100.04 501.15 188.5 171.5 168.9 168.3 359.8 340.2 450

100.11 601.26 224.1 205.7 213.2 191.1 423.6 422.4

100.37 701.63 248.2 238.1 231.2 223.1 471.7 481.8

100.08 801.71 >250 249.8 246.6 249.7 496.6 492.8

100.16 901.87 >250 >250 >250 278.8 524.0 498.8 780


12

Table A2.3. Results of tank test conducted on 7 April 2014.

Sed

imen

t ad

ded

g

wet

wei

gh

t

To

tal s

edim

ent

add

ed

g w

et w

eig

ht

Off

sho

re C

N

TU

s/n

501

NIW

A

Sea

po

int

ST

M

Caw

thro

n

Sea

po

int

ST

M

TS

S (

g/m

3)

0 0 0.1359 0.381 0.226 < 3

2.58 2.58 1.0117 2.04 2.064

2.55 5.13 1.9328 3.7945 3.962

5.03 10.16 3.6391 6.793 7.032

10.04 20.2 7.3235 14.884 15.284 14

10.03 30.23 10.6153 23.165 23.414

10.06 40.29 13.9977 28.23 29.442

10.05 50.34 17.3046 40.0325 37.716 32

25.03 75.37 27.4971 57.9 59.274

25.07 100.44 37.9614 80.2825 80.862 80

25.15 125.59 46.1909 102.24 103.74

24.98 150.57 56.2626 128.03 127.408

50.1 200.67 75.9681 173.66 168.702

50.06 250.73 98.7087 229.15 205.492

50.08 300.81 153.2046 357.64 245.744

100.02 400.83 182.3174 424.35 314.548

100.22 501.05 220.4449 523.6 381.46 540

cawrpt 2602 tank calibration of turbidity · pdf fileapproximately 400 litres. ... a total of...

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