Proficiency testing of wastewater sampling: What did we learn?

The proficiency testing of wastewater sampling is organized by the Laboratory for Environmental Sciences and Engineering at the National Institute of Chemistry, Slovenia (NIC). Preliminary tests started in 2010 at two candidate sites to choose for a trial the most convenient one. In 2012, we organized the first collaborative field trial on wastewater sampling and such trials continued in 2013 and 2014. Between 16 and 20 sampling teams were selected from those that (1) participate in the national wastewater monitoring program and (2) perform analytical activities in laboratories. The participants used a 6-h time proportional sampling approach. They used sampling equipment with different principles of sample collection which take a series of discrete samples at fixed time intervals. Subsequent analyses involved both field and laboratory measurements. The participants, with only one exception, exhibited required technical skills for measuring field parameters (temperature and pH value). All samples taken were also analyzed at NIC. The variability between reported values of participants in the trials, expressed as CV, was found to be 8.5 %, 11.1 % and 9.5 % for chemical oxygen demand. Of all parameters, the largest variability was found for total organic carbon concentration amounting to 23.7 % and 35.7 %. Particular attention was given to separate the measurement uncertainty contributions arising from sampling and chemical analyses.