I had a good couple questions on my last post, and once my response in the comments reached two paragraphs, I thought it would be better to split it out into a separate post.
So, how do I clean frac tanks, and why do I worry about cleaning them, instead of the person I rented them from? And if I clean frac tanks, do I have a death spiral of cleaning ever smaller containers? (...paraphrasing)
So, the frac tank company rents out tanks to everybody. You see them by the side of the highway and on big construction sites for clean water storage. I arrange for them to store water contaminated with all sorts of stuff, although usually not with hair-raising concentrations (those get segregated separately as hazardous waste, and usually end up in smaller containers, like drums). I've also used them to store exotic mixtures of water + stuff to treat contamination, like oxidants and nutrients for bioaugmentation (feed bugs that break down contamination). The frac tank company is not expert in oily water or treatment chemicals or random contamination. So they require that the tanks be returned clean. I admit that I have not previously worried about how clean they were upon arrival to the site, because honestly, by the time you get to 21,000 gallons, that's a heck of a lot of dilution.
I always arrange for a transportation and disposal contractor to get my investigation-derived waste (IDW) off-site and to do the tank cleaning. And I usually have them deal with the tank/roll-off rental (for a modest mark-up, of course), because then it's their responsibility to clean the containers sufficiently that they'll get be accepted back.
When we're developing wells (removing fines!) or drilling, we'll always end up with sediment in the water. And we can arrange for bag filters and secondary settlement tanks and all that, but we'll inevitably end up with at least some gunk on the bottom of the containers. So the IDW contractor, who has all the appropriate certifications and equipment to go traipsing around inside contaminated tanks, takes a power washer to it and gets all the gunk out. Usually someone is standing by to suction the water+gunk into a drum. And since the water that was in the tank has been characterized and accepted for disposal (which is why the tank is empty now), the wash water is the same stuff, only diluted, and it goes off to wherever the rest of the water went. The drums get recycled (washed out, worst of the dents banged out, cooked, maybe get a new coat of paint) and then off they go to the next drilling job.
When I first started out, it was a bit of a surprise how much time was spent on things like cleaning tanks compared to science. But that's the environmental consulting business - science is fine, but a big part of the job is logistics.