Whether you discharge to the sewer or directly to a water body, the importance of collecting truly representative samples of your treated wastewater cannot be overstated. So what does "representative" mean? It means that:
- your wastewater treatment system is operating properly,
- you've selected a day to sample that is typical of your daily operations,
- your composite sampling equipment is set up to take flow proportioned samples properly,
- your flow meter is sized appropriately for your flow rates and is calibrated at least annually, and finally,
- the sampling equipment and collection containers are clean!!
To my last point, the last thing you want is a cross-contaminated sample. If you think about it, there are actually quite a few opportunities for contamination. And with metals limits in parts per billion, it doesn't take much to tip the lab results over the limit.
Dischargers should check with their sampling contractor regarding their procedures to prevent sample contamination. Do they change the pump tubing or just rinse it out between yours and another client's sampling? Do they acid wash and rinse the collection container? And how does he or she appear when they are onsite to setup and collect the samples - is their appearance neat and clothing clean? Even dirty hands and dust/dirt on clothing can be a potential source of contamination.
Did you know that in some areas of the country, dischargers are subject to limits in parts per trillion (Maine and the Great Lake areas' mercury limits are in ug/L) and must perform "clean sampling"? The U.S. EPA's Clean Sampling guidance includes using non-metal sampling equipment, and wearing tyvek suits and gloves, and even warns against breathing near the sample (ex. mercury in dental fillings).
You might also check with your contractor to find out if they perform periodic equipment or field blanks as part of their Quality Control program? Do they have a Quality Control Program? Non-compliance is not a pleasant situation to be in and can lead to enforcement actions including fines and penalties. Don't take a chance, make sure that your samples are being collected properly to ensure that your data is "representative" of your discharge.