Hong Kong is a small island with a big population. Because of this, there is a limited amount of room for facilities such as sewage treatment plants. Instead of using freshwater, the toilets in Hong Kong are flushed with seawater.
This may come across as revolting, but it's actually a rather ingenious way to solve the problem. After the waste has been removed from the toilets by the seawater flushing system, it is carried away by the tide. It is an environmentally friendly method for getting rid of sewage, and it does not call for a significant amount of energy or resources to be operational.
This system has been in operation for several decades, and it is one of the primary factors that contributes to Hong Kong's extremely high population density. It's a straightforward approach that gets the job done well, and it's just one of the many things that sets Hong Kong apart from other places.