The best way to dispose of paint is to deliver it to your local specialist waste recycling centre. If you do not have access to one of these, let the paint dry fully, adding sawdust or cat litter as needed, before putting it in the dustbin. Don't throw it down the drain, because it is toxic and it interferes with the processes of water treatment plants.
It is largely biodegradable, less polluting to aquatic habitats and just as effective a cleaning agent as less environmentally friendly options. Look for eco-friendly dishwashing detergents, such as those labelled with the European flower eco-label (see www.eco-label.com to find one in your country).
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