A modern sewer system doesn't really have a lot of standing water in it - if it did it would quickly fill in with sediment. The mains typically have only a few inches of water in the bottom. The pipes in your house are empty (except for the traps) so no need to worry about snakeheads coming out. Cockroaches on the other hand...
Already done Sir, already done.
Already done Sir, already done.
Our sewers are full of cock roaches. Sadly, that doesn't count for much. It's the grossest thing ever. They will seriously crawl up through your drains, and even the toilets. *shudder*