Best pipe material for sewage pipeline over sea channel 1

[macki]

I intend to install a 4" sewage pipeline 120' long (approx).
pipe exits from a commode on a dockhouse with a very good slope for gravity drain along a distance of 120' to shore.
there are issues with UV light on PVC, however cast iron, carbon steel etc will be overkill, will require a lot of supports, and too expensive for such a low presure system.
I am very worried about environmental hazards with a potential spill into the channel, trying to select a pipe material smooth enough to allow the solids/suspension to gravity glide to a tank at shore, strong enough to withstand 110 degrees F and a lot of UV. any advice would be appreciated.
thank you

 

[macki]

ooops HDPE is highly flammable !!!!!

 

[TomDOT]

Wooden docks are also highly flammable.

 

[rconner]

If you go with a pumped system and pipe smaller than 4" as some others have recommended, better make sure it's a "grinder pump" that in turn can be suitably maintained (otherwise maybe a carp or other unmentionables flushed could wreak havoc)!
That being said whether or not it is "overkill", a good bit of metallic e.g. ductile iron or steel pipe is used in major docking or wharfing facilities (e.g. see

http://ndbm.wbdg.org/system/html/6/2719

and NAVFAC etc.) While I won't pretend to know all reasons for such (I think some have been provided by others), in addition to what has already been mentioned such pipes are also quite resistant to force and impact type damages that can/tend to eventually befall exposed piping, including by flotsam and jetsam or flying objects on, adjacent to or under docks etc. With regard to sunlight and UV exposure on plastics mentioned by others, despite common addition of UV inhibitors (and frankly some industry downplaying of risks), sunlight over a period of time can have a rather drastic effect on particularly the structural impact strength or toughness with regard to impacts or crack propagation at flaws/damages of some plastic pipelines, exacerbating such risks.

 

[macki]

TomDOT: the dock in question is a concrete dock. HDPE has been eliminated from the shortlist because of its flammability xtics, I may have to spend more money and design for ductile iron with internal epoxy coating. I can deal with corrossion after a couple of years of service. I am grateful for the numerous contributions.

 

[jgailla]

+1 for epoxy-lined ductile iron. It has resistance to sunlight and won't break as easy. I wouldn't use PVC or HDPE aboveground without a really good reason.

 

[macki]

thank you jgailla for your input