Here's something I'm sure you'll find interesting from the No More Coffee Down the Toilet Dept:

According to Onboard Updates: you can pour any liquids down galley drains on all fleet types.  Some points they mention to consider are:

• To prevent clogged galley drains or more serious plumbing problems, never permanently remove the strainer. However, you will need to periodically clear the strainer of residue.
• After pouring milk down the drain, flush the drain with hot water to avoid curdling in the line.
• Always rinse drains with hot water after pouring liquids containing pulp, grounds or residue. If a coffeepot contains a large amount of grounds, pour it down the lavatory toilet or in the trash.
• Galley and lavatory sinks empty outside the aircraft so use them only when the aircraft is in motion to prevent spills on ramp employees. During extended gate delays, resort to alternatives, such as lavatory toilets.
• Use back-up options – lavatory toilets and sinks – if a galley sink clogs or has a placard directing you not to use it.
• Write up any slow-draining or clogged galley sinks.