A little background info may be needed then: Plastic Disks from Westchester Plant Wash onto Glen Cove Beaches - Glen Cove, NY Patch
And: N.H. sewage water disks are washing up in Mass. - The Boston Globe
"I still don't understand why these things exist and what they do." you say?
To put it in terms that even I can understand, they grow the 'good' bacteria on these plastic pieces of pasta that will consume the e. coli and other harmful bacteria and then they combine these little wonders with the effluent.
The real problem is that sewage treatment plants can't seem to keep their fly's zippered so these little darlings ended up all over the coastline, along with whatever waste water they released at the same time.
And you were worried about a few drops of diesel fuel or black water dripping in the water. Silly you!