Using Saltwater in Toilets

[Steve81]

7 minutes ago, npts2020 said:

Very few large ocean going vessels are not steam powered and the vast majority that aren't are military. The reason being that most of the relative advantages of gas turbines (quick start up, size, no steam etc.) are not as relevant as the fact that steam turbines are generally quite a bit more efficient.

AFAIK, all the big cargo ships and super tankers are diesels. They’re much more fuel efficient, and cheaper to operate as a result. They also have nasty emissions, but that’s another story.

https://dieselforum.org/port--marine#:~:text=Moving this trade through sea,%2C performance%2C durability and reliability.

 

Quote

Moving this trade through sea and river ports requires many trucks, trains, ships, cargo handling equipment, barges, and marine workboats. Diesel fuel and engines are the predominant technology used to power these vehicles and equipment due to their fuel efficiency, power, performance, durability and reliability.

Edited August 23, 2023 by Steve81

[KrallSpace29]

On 8/22/2023 at 11:09 AM, sethoflagos said:

Based on personal experience, the major technical challenge to pumped sea water systems is that it is considerably more nutritious than fresh water. Large channels rapidly attract colonies of mussels and oysters etc, and small channels (eg cistern fill valves, filters) get blocked by salps.

Not insurmountable problems, but expensive to solve.

Anding nitrogenous waste to the mix will escalate these biological issues even further.

Maybe if it were kept moving then it might not be so susceptible to biological growth. But even freshwater isn't biology free.

There has to be an easy way to make this happen. A few small scale experiments could easily determine the right way to go about using saltwater in toilets for waste removal. I was thinking if it were all pvc that would be a good start, as for the intakes and channels, etcetera, someone (chemist, whatever), could design components that are impervious to such growth. 

 

A series of small scale experiments can do it. Now lets just put our wallets together...

[exchemist]

On 8/23/2023 at 5:04 AM, npts2020 said:

Very few large ocean going vessels are not steam powered and the vast majority that aren't are military. The reason being that most of the relative advantages of gas turbines (quick start up, size, no steam etc.) are not as relevant to commercial ships as the fact that steam turbines are generally quite a bit more efficient.

This is not correct. Almost no ships today are steam powered, of any size. The only exceptions are nuclear submarines and LNG carriers, many of which are steam turbine driven in order to run on boil-off gas from the cargo. Virtually all other ships are diesel and have been for decades. Diesels are a lot more efficient than turbines, hence their dominance. A modern large 2-stroke low speed engine, of the types used in container ships and tankers, can get to 50%thermal efficiency:

http://marineengineering.co.za/lectures/technical-information/motor-docs/sulzer-rta-series.pdf

In powergen applications the combined cycle (gas turbine with exhaust used to raise steam for a steam turbine, often running on the same shaft) can get close to 60%. But you can’t run a gas turbine on residual fuel oil, so these are not used in ships. There are indeed some gas turbines on military ships, usually combined with diesel engines, but navies burn gasoil, not RFO, to avoid the associated hassle of dealing with it.

(I used to work in the oil industry on lubricants for the marine and power sector.) 
 

Edited August 24, 2023 by exchemist

[Steve81]

2 hours ago, exchemist said:

 

http://marineengineering.co.za/lectures/technical-information/motor-docs/sulzer-rta-series.pdf

In powergen applications the combined cycle (gas turbine with exhaust used to raise steam for a steam turbine, often running on the same shaft) can get close to 60%. But you can’t run a gas turbine on residual fuel oil, so these are not used in ships. There are indeed some gas turbines on military ships, usually combined with diesel engines, but navies burn gasoil, not RFO, to avoid the associated hassle of dealing with it.

(I used to work in the oil industry on lubricants for the marine and power sector.) 
 

Thanks for explaining the powergen application, never read up on that before! 
 

I’m just a hobbyist/modeler who read up on the history of maritime propulsion, from the triple expansion engines of yore to what we have today. 😅

 

[Sensei]

4 hours ago, exchemist said:

Diesels are a lot more efficient than turbines, hence their dominance.[...] can get to 50% thermal efficiency:

https://www.google.com/search?q=steam+turbine+efficiency+formula

"Multistage (moderate to high pressure ratio) steam turbines have thermodynamic efficiencies that vary from 65 percent for very small (under 1,000 kW) units to over 90 percent for large industrial and utility sized units. Small, single stage steam turbines can have efficiencies as low as 40 percent."

 

[exchemist]

55 minutes ago, Sensei said:

https://www.google.com/search?q=steam+turbine+efficiency+formula

"Multistage (moderate to high pressure ratio) steam turbines have thermodynamic efficiencies that vary from 65 percent for very small (under 1,000 kW) units to over 90 percent for large industrial and utility sized units. Small, single stage steam turbines can have efficiencies as low as 40 percent."

 

90% is bullshit, unless you count utilisation of waste heat in industrial processes. Shipboard steam turbines can’t compete with diesel, which is why they are no longer used on ships, since fuel became expensive after the Yom Kippur war in the 1970s. There is something wrong with those numbers. A huge supercritical turbine installation can get over 50% I think, from memory, but you can’t install that on a ship.

[sethoflagos]

20 hours ago, exchemist said:

There is something wrong with those numbers.

They're isentropic or polytropic efficiencies for the turbine cycle, not overall thermal efficiencies.

On 8/23/2023 at 7:57 PM, KrallSpace29 said:

There has to be an easy way to make this happen. A few small scale experiments could easily determine the right way to go about using saltwater in toilets for waste removal. I was thinking if it were all pvc that would be a good start, as for the intakes and channels, etcetera, someone (chemist, whatever), could design components that are impervious to such growth.

90-10 CuNi is the 'book' material for sea water piping systems, with blind-flanged tees at every elbow to facilitate periodic rodding through.

But that's by the by. As others have pointed out, sea water toilets are quite viable but only where sea water is readily available, and fresh water is not.

[John Cuthber]

I am more used to discussions that start off about the thermodynamics of steam engines and end up discussing toilets, than the other way round.
But I fear we may have strayed a bit from the topic.

We pipe water to our houses at fairly high purity, and chlorinated.
That's not going to grow much biology.
After we use it to flush toilets, it has more "biology" in it than seawater does.

So the question of maintaining free flowing water  when using sea water (if one set up a pipe system to deliver it) would only relate to the inlet- the outlet would be pretty much the same (I'm assuming the mix of bugs in the water treatment works would adjust to cope with the increased salinity).

If we chlorinated the infeed sea water the problem would go away.

But it would take more chlorine to "sterilise" sea water than drinking water.
It might not be worth the effort.
 

[KrallSpace29]

17 hours ago, sethoflagos said:

They're isentropic or polytropic efficiencies for the turbine cycle, not overall thermal efficiencies.

90-10 CuNi is the 'book' material for sea water piping systems, with blind-flanged tees at every elbow to facilitate periodic rodding through.

But that's by the by. As others have pointed out, sea water toilets are quite viable but only where sea water is readily available, and fresh water is not.

That's true though but that's still good.