Sampling uncertainty of wastewater monitoring estimated in a collaborative field trial

•17 sampling teams took part in a collaborative field trial on wastewater sampling.•Equipment used in time proportional sampling resulted in small random effects.•Participants, with one exception, exhibited technical skills for field measurements.•Monitored parameters were determined with low analytical uncertainty (below 10%).•The largest sampling uncertainty (i.e. 28.3%) was observed in the case of sulfate.

This case study gives practical guidance on how uncertainty in monitoring programs can be reduced in wastewater sampling. Seventeen sampling teams took part in a trial. The monitoring consisted of both field and laboratory measurements. The variabilities between reported results of participants in the trial, expressed as coefficients of variation (CVs), were found to be 8.5% for chemical oxygen demand (COD), 9.5% for total suspended solids (TSS) and 14.5% for sulfate ion, respectively.