Oil paints and the solvents used for thinning and cleaning shouldn't
be disposed of down the sink. It's possible to paint without using solvents; oil paint can be used directly from the tube with a painting knife. Materials and brushes can be cleaned with a rag and a little linseed oil to help remove the more stubborn pigment.
 To save paint, don't squeeze large amounts out onto your palette.
As oil paint remains usable for many days, if not weeks, you aren't battling with a quick drying medium, and by using a palette which can be sealed or covered, you can store your paints
before the next session. Any pre-mixed quantities of paint that need to be saved can be decanted into airtight jars or even empty paint tubes. These are available with an open end, into which you can put the paint you want to save and twist and crimp the end to create a seal.

If you need to use solvent, it's fairly easy to reuse it multiple times without needing to dispose of it. Don't allow solvents to evaporate in an open container but use a sealable jar or metal container – some have an inner pot that lifts out and acts as a sieve. Put the lid on and leave it for a few days. The pigment suspended in the solvent will settle at the bottom, leaving clean solvent that can be decanted into a new container to be reused. Occasionally you'll need to remove the build-up of sludge.
 Only dry paint should be disposed of in general waste. Oil paint takes time to dry, so  you could collect the sludge into a separate lidded container and use an absorbent material like cat litter, sawdust or a dedicated paint hardener to dry it out. Some recycling centres have specialist paint disposal points, and sealed containers can be taken there. Alternatively, use the 'sludge' as a ground for underpainting.
Rags or paper that have been in contact with oil paint and solvents run the risk of spontaneous combustion. However, this is more of a concern with rags soaked in these materials – not usually the case in everyday painting, but safe storage and disposal are essential. Avoid scrunching or piling up rags used with oils/solvents, and store them flat until thoroughly dry, which may take a couple of days. They can also be wetted with water and allowed to dry, which allows the solvent to disperse, the oil to cure and avoids overheating. A metal bin with a lid can be used to store the used rags safely; keeping them away from direct sunlight. Avoid disposing of rags in plastic household waste bins.

Water-mixable oils are oil paints, but they have been modifi ed to be able to be thinned and cleaned with water. Like traditional oil paint, there should be very litt le paint waste as you can store and reuse it over several days. If you do have any paint left over, why not create small abstracts or studies to use it up? As you're using water instead of solvents, when washing brushes you can allow the sediment to sett le in a similar way to the solvent by leaving it in a jar or container. Then the cleanish water can be put in an open bucket and evaporated. Any sludge can either be left to dry and disposed of or placed in an airtight container and used as a wash.

Water-based mediums use water as a vehicle. When washing brushes, this can be left to evaporate, or once the pigments have settled at the bottom, the remaining water can be disposed of down the drain.
You can make a filter system using a container and a fine mesh; pour the dirty water through cut-up tights or coffee filters. Cat litter or sawdust in the bottom of the container will also capture the paint remnants, once dried, and can be disposed of in general waste.
If you're working in a commercial or educational environment where more waste can be created, consider fi tt ing a drain filter or commercial filter. 

There should be very little waste from watercolour or gouache as these can be rewetted and used. If you're using tubes, you can squeeze them onto a lidded palette and just shut the lid when you have finished, to use another time. 
Acrylic paint can be kept wetter for longer use. A lidded, 'keep wet' or 'stay wet' palette uses a moist sheet that allows moisture into the paint by osmosis, keeping it workable for a few days or even weeks, reducing the need to dispose of the quickly drying paint. Once finished, you can easily dispose of the paint sheets in your general waste. If the paint has dried on the palette, you can often peel off the 'skin' and either dispose of it in the general waste or save it and use it in other creative endeavours. The used sheets can then be disposed of into the general waste and not down the sink. 

Wipe down your brushes, palette knives, and other tools with a paper towel before rinsing them;
it helps keep paint out of the water supply. Watercolour brushes, and especially acrylic brushes, do need to be cleaned at the end of a session. Remove as much pigment as possible so the brushes take little cleaning with an artists' soap and a little warm water.
With good storage and cleaning practices, an artist's brush and tools will last for many years, and even when they become a little tired; they can be repurposed for different marks or mediums.

These can be disposed of in general waste – keep the lid off to allow
any residual paint to dry within
the tube before disposal. Squeeze as much paint out of the tube as possible, and even consider carefully cutting off the bottom of the tube to extract every bit of pigment you can. Consider keeping the lids to replace lost or damaged ones.
Some empty paint pans can be reused or recycled. It will depend on the local authority rules and facilities in your area. 

• Work in a well-ventilated room – especially when using oil paints and solvents or volatile mediums like alcohol inks or pens
• Avoid eating or drinking in the studio
• Ensure solvents are kept in a lidded container, which should be sealed when not being used. Consider the amount of solvent used and decanted, as this can cause dizziness or nausea.  
• Clean up spills immediately
• Avoid skin contact as much as possible, as irritation to the skin can occur. Wash hands immediately after painting
• Consider what you wear, as some mediums can ruin clothing
• Wear a mask when dealing with dry pigments to avoid inhalation
• Wear protective gloves or barrier cream if you have sensitive skin
• Take regular breaks
• Store your materials safely