Why You Shouldn’t Pour It Down Drains
Acrylics are a great alternative to traditional oil paint because they are water based, and so there aren’t any fumes, and you can clean up with soap and water. However, at the end of a studio session, the jar or container that you use to rinse off your brushes between colors will have quite a bit of paint in it, and you should not pour this rinse water down drains because many professional-grade paint pigments are toxic, such as the cadmium oxides used for reds, oranges and yellows. Even if you use “non-toxic” student-grade paints, the pigments and acrylic polymers are still problematic for the waste-water treatment processes, and so these shouldn’t go down the drain either.
Disposal As Solid Waste
The solution is to dispose of the material as solid waste. The question is how do you get the water out, which isn’t as simple as just letting it dry out. You may have noticed that rinse water from acrylic paint tends to dry much more slowly than regular water. This is probably due to the acrylic polymers forming an invisible scum on top of the water which acts as a barrier that inhibits evaporation. But there are ways to help the water evaporate faster.
How To Dehydrate Rinse Water
My preferred solution to how to get rinse dehydrated is actually a set of solutions that take advantage of waste energy or ambient energy. In the winter months, the rinse water can be poured into a metal coffee can or other recycled disposable container and set on a steam radiator. You can also pour it into a disposable aluminum baking pan and sit it by a heater vent or AC vent. In the summer months, there is the floorboard of your hot car with the window cracked open slightly.
I use an old plastic tote that is wide and shallow. This allows the rinse water to spread out and maximizes surface area. I keep the top covered with 1/2″ hardware cloth (wire mesh) to keep out pets and leaves. If I am painting a lot every day and generating more than my usual amount of rinse water, I will sometimes put my dehydrator tote next to or under a fan, preferably one that was already running and not turned on just for my rinse water.
Of course, I don’t try to clean out my dehydrator tote between uses. That would be problematic for several reasons (such as the potential to create hazardous dust), so I just have thin layer after thin layer accumulate on the bottom of my dehydrator, which will eventually have to be disposed of and replaced after several years.
Golden Paint’s Recommended Solution
Golden is the leading manufacturer of acrylic paints and mediums, and their website is a tremendous resource of how-to information. They have written a page explaining how to use hydrated lime and aluminum sulfate to quickly precipitate paint solids from rinse water and then filter them out using coffee filters. Keep in mind that hydrated lime and aluminum sulfate are commonly available in the fertilizer aisle of your local hardware store or land and garden center, so we aren’t talking about exotic chemical reagents that you need to special order.
Golden Paint’s Demonstration Video
Golden also has a video on YouTube demonstrating how to use their method to precipitate and filter the solids, but I think the demonstration could be improved. Specifically, they show the reagents being poured from bags, which should be avoided in general because pouring creates so much dust. Instead of pouring, slit the top of the bag completely open and scoop from the bag using an old spoon or scoop or hand shovel. There is another useful point they could have shown, especially since these powdered reagents are likely to be used intermittently and stored for extended periods:
Use Plastic Buckets With Lids To Store Hygroscopic Powders
Powdered reagents that are soluble in water also tend to be hygroscopic (bind moisture from the air) and clump over time. Examples like sugar and table salt come readily to mind, but the problem can be more than a nuisance. For example, old bags of chemical fertilizer are often unusable because the tiny pellets of fertilizer will “sweat” moisture from the air and fuse into one big lump of material that could never be spread in quantities small enough not to kill plants.
The heavy-duty plastic bags that many fertilizers and powdered reagents are sold in weren’t really designed for long-term storage and offer limited protection over time. Often times, twisting the plastic bag closed with a bread ties isn’t enough to seal out the moisture from the air. Sometimes all it takes are a few tiny holes in the bottom of the bag, especially if the material isn’t consumed for months. That is why I save old plastic pails and buckets with lids to store things like grout and thinset, and I would probably recommend them for anyone using lime and aluminum sulfate for precipitating the solids from their paint rinse water. Plastic buckets with snap-on lids are relatively cheap, and you can also get them for free from restaurants, bakeries and house painters.
IMPORTANT: CUT THE TOP OF THE BAG OPEN NEATLY INSTEAD OF STRETCHING THE PLASTIC UNTIL IT TEARS. LOWER THE WHOLE BAG INTO THE BUCKET INSTEAD OF POURING THE BAG OUT INTO THE BUCKET.
Scoop from the bag-in-bucket. Cut or fold the bag down as needed.