Here is my take on it, as I am a Wisconsin state certified water and waste water operator.
Can it be done, yes. Is it legal to do it, depends on the city/village/town ordinances where you live, but more than likely it is illegal. The village were I live and work it is illegal to use the sewer lateral clean out of anything other than maintenance to the lateral. This was actually a topic of discussion at one of our meetings/classes this past year.
The reason for it being illegal is because there has been cases people dumping such things as motor oil, vehicle antifreeze, gasoline and other harmful chemicals down the clean outs. Doing such things like this will cause major problems in the sewer mains and at the treatment plant, there are two citys within 30 miles of ours that have had to do a complete restart on their treatment plants within the last year because of gas or motoroil being dumped into the city sewer. This process ends up costing thousands or even tens of thousands of dollars in damage and man power to get everything back up and working correctly.
Granted what you are talking about won't hurt anything, but it is more than likely illegal to do.
TT-2013 Passport 3220BH
TV-2004 F150 FX4, not exactly stock...
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