M: Need some advice here. Go online if you need to. Might be a good idea to check because I feel like this is one of those things where common sense sometimes runs up against a little piece of information that completely changes what seems like a good idea. It's a new house and did some painting of actually some wallpaper primer. Now I'm trying to clean up and I realized that I'm not sure what to do with the couple, say maybe three, four cups of leftover paint that are just sitting in the roller tray. Used a paint roller and now there's leftover paint. Now I know that I could put some in the bucket that it came from, but I'm concerned about what to do in terms of cleaning things out. Like, first of all, it's a new house. I really don't want to start off by dumping paint down the drain. And of course, there's the objective concern about, you know, what the hell do I want to do in terms of polluting things. I really don't want to, but I'm not sure the best way to go about this. As I'm looking at my situation right now, it looks like the paint roller liner is actually kind of disposable. So there's that. But I'm sure this will come up at least once before we're done preparing the house. So the question still stands, even if the reason for it currently is kind of moot.

G: TL;DR
For latex / water-based paint (including wallpaper primer): don’t pour it down the drain. Let leftovers dry solid, then dispose of them as solid waste; clean tools in a controlled way so paint solids don’t enter plumbing. This avoids pollution and keeps you on the right side of common disposal rules.

The practical, low-risk way to handle what you have right now

Save what’s usable first
If the paint is clean (no debris), pour it back into the original can using a funnel. Wipe the rim, seal tight, label the date/room, and store it. This reduces waste immediately.
What to do with the roller tray leftovers
- If it’s a disposable liner: leave the paint in it and let it dry completely (overnight to a couple days). Once solid, toss it in the trash.
- If it’s a reusable tray: scrape excess back into the can, then wipe the tray with paper towels. Let those towels dry and trash them.
Cleaning the roller/brush without polluting
- First, spin/scrape as much paint as possible back into the can or onto cardboard/newspaper to dry.
- Then wash in a bucket, not directly under the tap. Rinse tools in stages.
- Let the bucket sit so paint solids settle, pour the clear water off (onto gravel or dirt if allowed, or down the drain only when it’s visibly clear), then let the sludge dry and trash it.

Why this works (and avoids the “common-sense trap”)

Latex paint is not hazardous once dry; it’s treated like solid waste in most places.
Liquid paint is the problem: it can clog plumbing and introduce polymers and pigments into waterways.
Drying first converts a disposal problem into a normal trash item.

What not to do

Don’t dump liquid paint or milky rinse water down the drain.
Don’t thin paint just to pour it out.
Don’t mix with kitty litter and toss while still wet; let it fully cure.

A repeatable system for the rest of the house

Keep a “paint scrap” box: cardboard, old newspaper, and a couple of disposable liners.
Keep a wash bucket dedicated to paint cleanup.
For small leftovers you won’t save, spread thin on cardboard to dry fast.
Label and keep even small amounts of saved paint; touch-ups in a new house are inevitable.

Small environmental note (plainspoken)

Doing this keeps paint out of drains, minimizes waste, and is exactly what municipal guidance usually intends even when the wording is vague. It’s the boring, correct solution that scales cleanly as you keep working through the house.