Is it okay to throw the food, collected in the mesh in the sink, down the drain, if it doesn't contain plastic?
It's not safe to assume that food won't cause a blockage if there are other processes at work.

I've known multiple people who have had tree roots manage to break into their sewer pipes.  (odds are, there was a leak of water, the roots went towards it, and then opened the pipe up further).  If the drain is already restricted, larger particles can make the situation problematic.

In my case, when the state highway was installing sidewalks and curbs on my street, their contractors ended up crushing sewer lines along our side of the street.  I know that at least four of us were affected, as we started having issues with blockages and basements getting flooded by sewage ... but it took a couple of weeks for other matter to start filling in the crevices until we reached a complete blockage.

Even in the case where you don't already have an obstruction in your pipes, garbage disposals are quite inefficient as a way of disposal; you're wasting clean water in the process, and you're putting extra work on the sewage treatment plants.

Sewage treatment plant are designed to handle a specific volume of water and waste, and in cases of periods above their capacity, they may have to divert untreated water and hope that dilution will be sufficient.  This means increased nutrient loads in your waterways, which can cause algae blooms and kill fish populations.  (although the unprocessed food isn't as bad as the other stuff that's typically in wastewater).

Most carbon digestion is an aerobic process, with the result of the breakdown of food in the wastewater using up the oxygen in the water.  Discharging this low-ox water can also lead to  deadzones.

You also have to look at what a treatment facilities does with their extracted solids.  In some newer facilities, there is biogas extraction, and the decomposition of the materials is used to power the facility.  (sometimes, with excess capacity sold off).  In others, they have to truck it away ... sometimes it's spread on fields as ferilizer.  Other times it goes through a 'de-watering' process ... either pumped to the bottom of a lagoon to digest or spread out into troughs to air-dry, then hauled to a landfill for disposal.

Garbage disposals are convenient, and they're useful for the occassional scraps ... but if you can, compost your food scraps.  As for normal garbage vs. garbage disposal, there too many local factors -- if you're in a water restricted area that doesn't have a biogas system or sufficient capacity at the water treatment facility, normal trash is likely your best yet.  If you're in an area with lots of water, sufficient treatment capactiy and a biogas system, garbage disposal may be better.

For more reading on some of the hidden costs of garbage disposals, see:


http://www2.buildinggreen.com/blogs/garbage-disposals-and-worms-face-over-environmental-food-waste-disposal
http://www.slate.com/articles/health_and_science/the_green_lantern/2008/09/should_we_dispose_of_disposals.html
http://www.treehugger.com/corporate-responsibility/trash-talking-the-garbage-disposal-examination-of-a-not-so-green-us-export.html