It would seem to be a major environmental benefit if toilets used saltwater instead of potable (drinkable) fresh water, right?
The main answer is logistical. Building a network of pipes to convey water to houses is a major effort, and most places where that's been done found it extremely expensive. Providing a second set of pipes to bring salt water to houses would probably cost about the same. Except that cities go to considerable lengths to put their water reservoirs above the city as far as possible so they don't have to use as much energy for pumping. The few cities with seawater above them pay a lot to pump water out.
A minor secondary problem is that sewage treatment plants are fairly heavily biological. They grow things in the sewage to break it down. But most of those things won't grow in salt water. It's already a challenge keeping toxic chemicals out of the sewage. For this to work it might be necessary to add a second set of sewer pipes to carry the water away, and develop new methods of treating that salty sewage (the treated sewage couldn't be spread on farmland, for example).
It would be far simpler, cheaper and more reliable to simply buy everyone a composting toilet to replace each toilet they currently have. As a halfway measure, mandating more economical toilets would help, as would education campaigns like the ones encouraging people not to flush at all when they urinate. Even with a sensible cistern that has a 2 litre "half flush", that's two litres wasted every time someone pees. Additionally, salt is not good for pipes. A lot of pipes, even today, are clay or concrete, and salt water is bad for both of these. It's not impossible to get around that, just expensive.
Catalina Island does use salt water for toilets. It's a small community, however, and they seem to be shifting towards freshwater due to corrosion issues inherent to salt water utilization.
Saltwater used for toilet-flushing and fire suppression purposes is simply filtered as it is not intended to be potable.
Unique issues for wastewater treatment include the high salt content of the wastewater resulting from the City’s use of salt water for toilet flushing throughout the system; and very large differences between average and peak flow loadings resulting from the varying tourist population
It was discovered that the use of saltwater for wastewater purposes leads to corrosion of the laterals. A majority of the laterals were broken or cracked, causing wastewater from the homes in Avalon to enter the groundwater system
The blog post has references for further reading.
It might even be more environmentally friendly to dispense with using flush toilets altogether by switching to using negative pressure toilets. Vacuum toilets are used on planes and also help to remove the unwanted odours as well as prevent the toilet plume aerosol caused by flush toilets. Vacuum toilets are available commercially for domestic use and use a small amount of water for washing the toilet bowl.
Navy/Marine ships usually have two systems to handle water, in separate tanks:
- CHT: Collection, Holding and Transfer tanks (more detail on the standards by the SBIR here)
- MSD: Marine Sanitation Device tanks
Ships use salt water from the fire main for flushing, and to clean out the CHT (shit, which is used when sewage can't be dumped, or to process before dumping over the side) tank, so some toilets do use salt water, and other ones don't. They do this because using fresh water can be taxing on the water tanks. Also the CHT tanks have more cleaning enzymes than potable water would have available to it.
Saltwater piping is difficult due to rust, and would need a full second network of piping.
We have recently heard from the news, that a city (Mykolaiv) in Ukraine had to switch to using saltwater in its water distribution network. They fully knew that the pipes will rust away from saltwater, but they are in a difficult situation in the middle of a war and have no other choice. It's either no water now, or saltwater now plus fix the damage later.
If we had saltwater piping for toilets, we would need to have two water distribution networks, one for fresh water and another for saltwater. The saltwater network would be the very troublesome network of these two, every metal in contact with saltwater would rust away. So even if the piping was built from plastic for example to avoid corrosion, every pump that has some metal in contact with water would rust away very quickly.