Shenzhen city (like HK and Kowloon) was built without a sewage treatment plant. There was a river nearby, however, so engineers piped the sewage untreated in to that. The Shenzhen River discharges into Inner Deep Bay (Hau Hoi Wan), at low tide, a large shallow expanse of mudflat which is internationally recognised as an important wetland for migratory birds on route between northern and southern hemispheres.
The smell from the river was, unbelievably, even worse than the Kai Tak nullah which at least flushed out into HK harbour. Being tidal for the entire length of the river, the septic sewage just slopped backwards and forwards with a little escaping only as far as the Ramsar wetland.