New approaches for proficiency testing of wastewater sampling

CROATIAN LABORATORY CROLAB HRVATSKI LABORATORIJI CROLAB

__________________________________________

9th INTERNATIONAL CONFERENCE 9. MEĐUNARODNA KONFERENCIJA

OPATIJA, CROATIA 6th – 9th November 2013

6.- 9. studeni 2013.

LABORATORY COMPETENCE 2013

KOMPETENTNOST LABORATORIJA 2013 &

Regional Conformity Assessment Workshop

Regionalna radionica o ocjeni sukladnosti

PROCEEDINGS

KNJIGA RADOVA

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DRUGO DESETLJEĆE CROLABA

Misija , djelatnost i vizija CROLAB-a

Udruga Hrvatski laboratoriji CROLAB je osnovana 16.06.2003. u cilju razvoja hrvatskih laboratorija kao infrastrukture razvoja proizvodnje i gospodarstva u okviru zahtjevnog otvorenog tržišta, korištenjem zajedničkih potencijala i sinergijskih učinaka udruživanja. Tijekom deset godina CROLAB je dao značajan doprinos na promicanju kvalitete kao značajnog faktora razvoja gospodarstva i poboljšavanju usluga u svakodnevnom životu

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Djelatnost definirana Statutom je povezivanje u radu temeljem zajedničkog interesa sa znanstveno-nastavnim institucijama, tijelima uprave, trgovačkim društvima, te osobama koje obavljaju privatnu laboratorijsku praksu

Aktivnosti se provode: Organiziranjem stručnih konferencija, seminara, predavanja i okruglih stolova sa svrhom međusobnog obavješćivanja, sticanja novih znanja i razmjene iskustava.

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Upravni Odbor CROLABa smatra da ovako sumirane i navedene 10 godišnje aktivnosti CROLAB-a na edukaciji, širenju i promicanju kvalitete, međunarodne reputacije zaslužuju pozornost javnosti. To je potvrdila zahvala HZN za dobru suradnju, Povelje za edukaciju HDK i Povelja HGK.

CROLAB je član HDK, Eurechema i EUROLABA.

Predsjednica CROLAB-a

Prof. dr. sc. Štefica Cerjan Stefanović

LABORATORY COMPETENCE 9th INTERNATIONAL CONFERENCE Opatija, Croatia 2013

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Stručni rad

NEW APPROACHES FOR PROFICIENCY TESTING OF WASTEWATER SAMPLING

Magda Cotman, Albin Pintar

Laboratory for Environmental Sciences and Engineering,

National Institute of Chemistry, Hajdrihova 19, P.O. Box 660, SI-1001 Ljubljana, Slovenia

E-mail: [email protected]

Abstract: Since sampling errors were recognized in the context of ISO/IEC 17025 as an important factor affecting the quality of an analytical result, the needs for consistency of data arising from the European Water Framework Directive 2000/60/EC rendered enhancement of metrological knowledge in this step of the measurement chain more significant. In the year 2012, we consequently organized a collaborative field trial on wastewater sampling that is particularly important in the environmental field, e.g. for exact analysis of wastewater. The trial was the first national attempt to improve knowledge of the effect in wastewater sampling undertaken as part of regulatory monitoring. In the year 2013, we continued with the organization of such trials. Sampling teams took part in the trial in a sampling site on municipal wastewater treatment plant near Ljubljana. The monitoring parameters selected were as field parameters (pH value and temperature) and laboratory measurements (ammonia nitrogen, biochemical oxygen demand (BOD5), chemical oxygen demand (COD) and total organic carbon (TOC)). The aim of this study was to evaluate several sampling procedures, including standardized, to determine the variability induced by sampling operations in subsequent analytical processes. Experimental determination of sampling uncertainty contribution is very cost and time intensive especially for single laboratory experiment. The most convenient and robust way to estimate sampling uncertainty is participation in proficiency testing organized in agreement with international guides.

1. INTRODUCTION

Proficiency testing (PT) is defined in ISO/IEC 17043 [1] as the evaluation of participant performance against pre-established criteria by means of inter-laboratory comparisons. Laboratory for Environmental Sciences and Engineering at the National Institute of Chemistry (NIC) is a provider of proficiency testing services. PT schemes

mailto:[email protected]


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are organized for laboratories of the environmental sector, both public and private,

at national and international levels, within the frame of a permanent collaborative programme since 2000, which is in agreement with the new international standards. Participation in PT schemes is recommended by ISO 17025 [2] as a tool for:

monitoring and improving quality of measurements,

inter-laboratory measurement comparison,

greater process control,

demonstrating competency

evaluating methods and instrumentation, and

staff development and training.

All the PT schemes within are operated in accordance with the international standard ISO/IEC 17043 [1]. The statistical analysis undertaken is in accordance with the international standard ISO 13528 [3].

There are four types of samples that could be collected during the sampling by the wastewater monitoring programme: grab, time proportional composites, flow proportional composites, and hand composites. The sampling method used depends largely on the type of analyses to be run, and the nature of the waste stream being sampled. Sampling of wastewater is conducted using the procedures as described in the standard ISO 5667-10 [4]. The standard gives guidance on the selection of the sampling site to assure representative sampling. It describes both manual as well as automatic sampling of wastewater. Primarily, two types of automatic samplers are available: time-proportional and flow-proportional samplers. The guidance is rarely used, being considered too theoretical and not descriptive enough for the sampling staff. The ISO 5667 series guidances have been never validated by interlaboratory comparison.

Literature survey and communication with the European organisers of interlaboratory comparisons show that there exists no report in the open literature on interalaboratoy comparison of wastewater sampling. Cslab has provided proficiency testing schemes in the area of wastewater sampling since 2001 (personal communication). These intercomparisons are organized with the support of management of wastewater treatment plant, which enables organization of sampling and gives technical support to the action.

The purpose of this work was therefore to carry out a collaborative field trial of wastewater sampling, to thoroughly evaluate several sampling procedures, including standardized, as well as to determine the variability induced by sampling operations in subsequent analytical processes.


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2. MATERIALS AND METHODS General description of PT for wastewater sampling.

This article outlines the issues that need to be considered when planning http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5H-4MSHXW2-4&_user=634399&_coverDate=04%2F30%2F2007&_alid=1696288352&_rdoc=5&_fmt=high&_orig=search&_origin=search&_zone=rslt_list_item&_cdi=5787&_sort=r&_st=13&_docanchor=&view=c&_ct=393777&_acct=C000033678&_version=1&_urlVersion=0&_userid=634399&md5=0a934656a8475f164a461c135de0ba1c&searchtype=a – hit2water samplinghttp://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5H-4MSHXW2-4&_user=634399&_coverDate=04%2F30%2F2007&_alid=1696288352&_rdoc=5&_fmt=high&_orig=search&_origin=search&_zone=rslt_list_item&_cdi=5787&_sort=r&_st=13&_docanchor=&view=c&_ct=393777&_acct=C000033678&_version=1&_urlVersion=0&_userid=634399&md5=0a934656a8475f164a461c135de0ba1c&searchtype=a – hit4 for chemical monitoring, and provides guidance for the work required. The overview chart is illustrated in Fig. 1. In addition, it presents some existing technologies that may be used to complement automatic wastewater sampling, and considers the limitations associated with current monitoring practices based on spot http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5H-4MSHXW2-4&_user=634399&_coverDate=04%2F30%2F2007&_alid=1696288352&_rdoc=5&_fmt=high&_orig=search&_origin=search&_zone=rslt_list_item&_cdi=5787&_sort=r&_st=13&_docanchor=&view=c&_ct=393777&_acct=C000033678&_version=1&_urlVersion=0&_userid=634399&md5=0a934656a8475f164a461c135de0ba1c&searchtype=a – hit3sampling.http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5H-4MSHXW2-4&_user=634399&_coverDate=04%2F30%2F2007&_alid=1696288352&_rdoc=5&_fmt=high&_orig=search&_origin=search&_zone=rslt_list_item&_cdi=5787&_sort=r&_st=13&_docanchor=&view=c&_ct=393777&_acct=C000033678&_version=1&_urlVersion=0&_userid=634399&md5=0a934656a8475f164a461c135de0ba1c&searchtype=a – hit5

http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V5H-4MSHXW2-4&_user=634399&_coverDate=04%2F30%2F2007&_alid=1696288352&_rdoc=5&_fmt=high&_orig=search&_origin=search&_zone=rslt_list_item&_cdi=5787&_sort=r&_st=13&_docanchor=&view=c&_ct=393777&_acct=C000033678&_version=1&_urlVersion=0&_userid=634399&md5=0a934656a8475f164a461c135de0ba1c&searchtype=a#hit2




























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Fig. 1. Field wastewater sampling collaborative trial: overview chart.

The trial was organized by the Laboratory for Environmental Sciences and Engineering at NIC in partnership with the Slovenian Environment Agency and Central Wastewater Treatment Plant (CWTP) Ljubljana. At the CWTP, two possible candidate sampling sites were allocated:

sampling site 1 (located after mechanical wastewater treatment);

sampling site 2 (located at the outflow after final treatment). Preliminary tests started in 2010 at both candidate sites to choose for a trial the most convenient one. Wastewater analyses were carried out for the whole year in order to establish the concentration level of target compounds and spatio-temporal homogeneity. After the expert assessment, site 1 located after mechanical treatment was chosen as the sampling location due to the logistics, spatio-temporal homogeneity and concentration of pollution in wastewater. From the beginning of 2013, 16 sampling teams were selected considering the following criteria:

be part of the national wastewater monitoring programme;

sampling teams perform analytical activities in laboratories as well. An information questionnaire on sampling practices was sent to the participants. The objective was to acquire information before the trial about quality-assurance procedures and sampling equipment (e.g. type of sampling equipment and other technical needs). On the basis of collected data, the sampling site was prepared in advance for all participants.

The trial started on 4 June 2013 at 7 a.m. and finished in the late afternoon, with sampling focused between 8 a.m. and 2 p.m., in order to avoid the strong effect of sunlight on field measurements. Micro locations of participants were determined by organizers. Each sampler was equidistant from any other. Throughout the trial, the organizers also observed and checked different sampling practices to detect any type


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of irregular procedure. The participants used 6 hours time-proportional sampling,

but they were allowed to follow their own sampling protocols. During the sampling period, two field parameters (pH value and temperature) were monitored. The participants used various commercially available sampling equipment with different principles of sample collection, so the details of each participant were different (e.g. time interval). Homogenisation, preservation and sub-sampling were performed by participants following their own protocols and equipment. NIC was in charge to supply bottles for sample storage and to assure transportation to its laboratory, in order to minimize global uncertainty. After finishing, the collected samples of wastewater were divided into two sub-samples: samples A and samples B. In samples A, chemical measurements were performed in participants’ own laboratories, while in samples B all measurements were done at NIC, which has long-lasting experiences and the established quality control system. The samples were immediately placed in lightproof insulated box containing ice-packs to ensure rapid cooling, and transported to NIC. The time between sample collection and beginning of the analyses did not exceed 24 h. During transportation, temperature was monitored. The analyses were performed in duplicate with standardized methods. All the above described actions were performed in order to minimize the analytical impact on global uncertainty. Measurement uncertainty

Measurement uncertainty can be estimated by identifying all possible sources of uncertainty associated with a method, quantifying uncertainty components and calculating total uncertainty by combining the individual uncertainty components following appropriate mathematical rules. For the proficiency testing scheme, “teaching” function is at least as important as the monitoring component, so the need to adapt statistical and evaluation procedures of current PT schemes to include uncertainty data is very significant. The issue of whether PT should include uncertainty when most participants do not use or understand it, was considered. In our PT scheme, the assigned value and its standard uncertainty was determined in different ways depending on the number of participants. When the number was large enough (more than twenty), the assigned value can be calculated from analytical results of the laboratories as a consensus value (as the mean or median of the results). The choice which results were used for calculating consensus value, is responsibility of an organizer.

The standard uncertainty of assigned value is calculated according to ISO 13528 3 as follows:


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px s

ux

*

25.1 (1)

where ux represents standard uncertainty of assigned value, s* is the robust standard deviation, and p stands for a number of results.

The participants paid more attention to procedures for calculating measurement uncertainty as the schemes proceeded, so every year more and more participants reported these values. Some participants calculated uncertainties according to the

Guides for Quantifying Measurement Uncertainty (GUM) 5 , while other calculated the measurement uncertainty with data from validation procedure. It was also seen that some participants reported standard uncertainty (ulab), while other expanded measurement uncertainty (Ulab, k=2). In the future, more and more attention should be paid to make this procedure uniform.

The most complete information about the precision of sampling protocols can be obtained from the sampling proficiency testing, which should involve a sufficient number of experienced samplers (n ≥ 8) and enough typical targets with an appropriate range of analyte concentrations. In our trial which is specific for wastewater sampling, the MU is evaluated according to procedures previously described [6]. The approach used to combine two uncertainty sources was chosen as a quadratic propagation, thus:

UUU analysissamplingglobal

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(2)

To establish the uncertainty, we decided to focus on the reproducibility (CVR)

standard deviation (expanded uncertainty k=2 is given as 2 × CVR). Measurement uncertainty of reference measurements was evaluated accordingly to the methodology published by Drolc et al. [7]

3. RESULTS AND DISCUSSION

The scope of the exercise was to obtain realistic picture of wastewater sampling, which was performed on real sampling site, additionally settled in order to reduce impact of sampling site on the measurement uncertainty. Before the trial, all details regarding the sampling site and sampling procedure were discussed with participants. The sampling site was prepared in advance for all participants. The participants used 6 hour time-proportional sampling, but they were allowed to follow


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their own sampling protocols. The monitoring consisted of both field (pH value, temperature) as well as laboratory measurements (ammonia nitrogen, BOD5, COD and TOC). The measurements of BOD5, COD and TOC were performed in whole homogenised samples, while the concentration of ammonia nitrogen was

determined in filtered samples using a 0.45 m filter. All acquired samples were also analyzed at NIC in order to minimize the analytical impact on global uncertainty.

This trial was the first attempt carried out in Slovenia to significantly improve knowledge of the effect of wastewater sampling undertaken as part of regulatory monitoring. All sixteen sampling teams that took part in the trial, performed at the sampling site on CWPT Ljubljana, carried out the sampling procedures with their own protocols. The participants used different sampling devices from well-known producers that were equipped with either peristaltic or vacuum pumps for sample withdrawal. The latter were performed in the present study on collected wastewater samples divided into two subgroups. Homogenization, preservation and sub-sampling were performed by participants. The NIC team performed a lot of actions to minimize the impact on global uncertainty. Results of field measurements of PT for wastewater sampling

The obtained results of field measurements are presented in Fig. 2. (a)

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Fig. 2. Results of measured field parameters: pH value (a) and temperature (b).

The pH value and temperature were continuously measured during the trial by

placing probes 20 cm below the water level at the sampling site. All participants reported average values of both parameters. It can be seen in Fig. 2 that the acquired data are in the range between 7.40 and 8.12 for pH value (s = ±0.17) and between 16.4 and 17.7 °C for temperature (s = ±0.31 °C). The measured values plotted in Fig. 2 confirm good homogeneity of the measuring site. Results of monitored parameters of PT for wastewater sampling

Sampling variation can be studied only by conducting analytical measurements. The monitoring parameters included the following analyses: ammonia nitrogen, biochemical oxygen demand (BOD5), chemical oxygen demand (COD) and total organic carbon (TOC). All results derived by participants in the trial are presented in Fig. 3. On the graph are illustrated reported values accompanied by reported analytical measurement uncertainty (MU). Additionally, results of reference measurements demonstrated as reference values (solid lines) together with evaluated analytical measurement uncertainty (dashed lines) are shown as well.


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Fig. 3. Results of reported values and measurement uncertainties of analyses performed in laboratories: (a) ammonia nitrogen; (b) BOD5, (c) COD and (d) TOC. Results of reference

measurements are demonstrated as reference values (solid lines) together with evaluated analytical measurement uncertainty (dashed lines).

During the course of the trial, fifty measurements were performed by participants

and 64 measurements were done at NIC in duplicated tests. Eventually, 121 results were analyzed in statistical analyses. The seven samplers did not perform analyses of total organic carbon in their own laboratories. The variability between reported results of participants in the trial, expressed as CVs, were found to be 5.4 % for ammonia nitrogen, 11.1 % for COD, 19.2 % for BOD5 and 23.7 % for TOC, respectively. In the trial performed in 2012, in which the analytes such as COD, TSS and SO4

2- were determined in acquired aqueous-phase samples, similar values of the variability between reported results of participants were observed [6]. This again shows that errors induced by sampling procedures are much higher in comparison to errors resulting from subsequent (instrumentally supported) analyses carried out in laboratories.

4. CONCLUSIONS AND FUTURE PERSPECTIVES

The quality control of chemical measurements is currently a major priority in many countries. Only accurate results allow valid conclusions to be drawn about the harmful properties of different types of waste water and the risk related to biota. The quest of quality is a long-term process, which requires adherence to rules, such as the use of control charts, reference materials, certified reference materials and participation in interlaboratory comparisons.

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Laboratory for Environmental Sciences and Engineering at NIC is continually striving

to improve current schemes and to introduce new schemes/test materials/test parameters, when appropriate. In the year 2013, we consequently organized the second PT on wastewater sampling. Sampling is a key step in the analysis of chemical compounds. It is particularly important in the environmental field, e.g. for exact sampling of wastewater effluents. In contrast to the improvements that have occurred in analytical measurements, developments in the field of sampling are less active, although sampling errors may easily exceed by an order of magnitude those related to analytical processes. Sixteen sampling teams took part in the trial performed in a sampling site on CWPT Ljubljana. The participants exhibited required technical skills for measuring field parameters (temperature and pH value). The monitored parameters (ammonia nitrogen, BOD5, COD and TOC) were determined with sufficiently low analytical uncertainty in order to quantify sampling uncertainty. The largest variability between reported results of participants, and consequently the largest sampling uncertainty (i.e. 46.4 %), was observed in the case of TOC that resulted in global measuring uncertainty as high as 47.1 %.

The aim of this trial, which will be followed by new ones on a regular basis, has been to thoroughly evaluate several sampling procedures, including standardized, as well as to determine the variability induced by sampling operations in subsequent analytical processes. REFERENCES 1 ISO/IEC 17043, Conformity assessment general requirements for proficiency testing.

International standard Organization-Geneve, 2010.

2 ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories. International standard Organization-Geneve, 1996.

3 ISO 13528, Statistical methods for use in proficiency testing by interlaboratory comparisons. International standard Organization-Geneve, 2005. [4] International Organization for Standardization (ISO), ISO 5667–10: 1992 Water quality – Sampling – Part 4: Guidance on sampling of wastewaters, ISO, Geneva, Switzerland, 1992

5 Guide to the Expression of Uncertainty in Measurement, BIPM, IEC, IFCC, ISO, IUPAC, IUPAP, OIML, ISO, Geneve, 1995.

6 M. Cotman, A. Pintar, Sampling uncertainty of wastewater monitoring estimated in collaborative field trial, TrAC Trends in Analytical Chemistry, 51 (2013) 71-78.

7 A. Drolc, M. Cotman, M. Roš, N. Majcen, Measurement traceability and its role in proficiency testing schemes – a case study for wastewater analysis in Slovenia. Accredit. Qual. Assur., 11 (2006) 455-461.

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New approaches for proficiency testing of wastewater sampling