Green sand

[Windmill]

Why do they put green sand in mains water supplies when they make changes to the supply?

There are a lot of gadgets which use solenoid-controlled water valves etc. which may leak if grit gets in, and/or have filters which can become blocked.

So why? Not because they want to force DIY work, I'm sure.

[Mr Fuxit]

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I'm very likely to be wrong, but I believe that the green sand is actually crushed glass, which is used to remove deposits from the walls of the pipe.

[polygonum]

I had no idea what you were going on about. Assumed it wasn't from the Lower Greensand Ridge - or whatever. And found this:

Manganese Greensand Iron Removal Filter

The AMPAC USA Iron-Manganese Green Sand Filter is designed to remove high levels of Iron up to 20 ppm, sulfur/hydrogen sulfide (H2S or Rotten-egg odor) up to 15 ppm, manganese, lead, silt, turbidity, and other trace amounts of metals such as copper, arsenic, and uranium

The AMPAC USA Iron-Manganese Green Sand Filter uses a high quality manganese greensand media. The system is usually installed on the main water line feeding the whole house.

Green Sand Filter Media oxidize Raw Water Supply by causing precipitation, trapping iron, manganese, hydrogen sulfide present in your water supply resulting in a clean, filtered water flow to your household.

The Green Sand Media is periodically regenerated automatically using potassium permanganate. The system comes with the Potassium Permanganate Tank. (Removal per Cubic Foot)

is probably entirely irrelevant but interesting in its own way.

[Andrew Gabriel]

It may be backfill placed around the pipe in the ground, which can get in if the pipe had had to be repaired in place. There is no market for recycled glass, so it's ground up and used instead of sand in many situations, as a way of burying it in the ground without attracting landfill tax (until the EU cottons on to this loophole, anyway).

[Windmill]

The muck that belched and spattered out of the air-filled pipes seemed to have two constituents, something gritty (could be glass as you say) and a green dye (unless that was a colloidal suspension of very finely ground green bottles!).