Because essentially, what you're doing is you're using clean water and you're using it to flush your toilet, convey it to a wastewater treatment plant which then discharges to a river, and that river, again, is a drinking water source.
因为基本上来说，你所做的 就是用干净的水 去冲厕所， 再把它运到污水处理厂， 然后再排放到河里去， 而这些河呢，又是饮用水源。