I have an ongoing discussion with my friends on how
environmentally-friendly plastic bottles (e.g. water bottles from the
supermarket) are. My friends maintain that they don't do any
environmental harm if they are dropped into recycling bins. In their
opinion, recycling is as good as reuse. My own understanding is,
however, that plastic products cannot really be recycled, but only
downcycled. E.g. the plastic recovered from a recycled water bottle
can only be used for, say, park benches, but it cannot be turned into
a new water bottle, since it is of inferior quality.

So, the core of my question is: Can the environmentally conscious
citizen buy water bottles at the supermarket, or should he/she avoid
plastic products in general?

Hello,

The question you have asked is a complex one.

Recycling used products into similar ones is called "closed-loop
recycling." As you thought, when you recycle a plastic bottle, it
generally does not become another plastic bottle. Instead, it turns
into material for plastic bags, so-called plastic lumber and fiber for
polyester products. So the products are not going directly into a
landfill, but the production of new plastic bottles does not decrease.

The International Plastics Task Force, an environmental group,
recommends that consumers reduce and reuse plastics as much as
possible.

According to the American Plastics Council:

"Today, some recycled plastics are used in food and beverage
containers. Technical and economic barriers currently limit widespread
use of recycled plastic packaging in direct contact with food."

By comparison, most of the material in aluminum beverage cans is
recycled content. So the environmentally friendly choice should be
aluminum over plastic, assuming of course you recycle the can.

I hope this answer meets your needs. If not, please request clarification

Sources:

IPTF, "Recycled Content," 2002,
http://www.ecologycenter.org/iptf/recycling/recycledcontent.html.

Dave Aftandilian, "Coke's Broken Promise," Feb. 2000,
http://www.consciouschoice.com/note/note1302.html

APC, "Plastic Recycling and Recovery,"
http://www.plastics.org/s_plastics/doc.asp?TRACKID=&CID=41&DID=412

City of Clearwater, "Remarkable Recycling Facts," 26 Mar. 2004,
http://www.clearwater-fl.com/gov/depts/solid_waste/docs_pub/facts.asp.

Search strategy:

Plastics recycling
://www.google.fr/search?q=plastics+recycling&hl=en&safe=off&rls=GGLD%2CGGLD%3A2003-34%2CGGLD%3Aen

plastics recycling closed-loop
://www.google.fr/search?q=plastics+recycling+closed-loop&hl=en&safe=off&rls=GGLD%2CGGLD%3A2003-34%2CGGLD%3Aen

plastics
://www.google.fr/search?sourceid=navclient&ie=UTF-8&rls=GGLD,GGLD:2003-34,GGLD:en&q=plastics

I have no idea what precentage of plastic is recycled insted of
'downcycled' as you suggested. But I would not wory about it because
you won't need to make the park bench out of new plastic. So as long
as the old plastic is used anywhere in place new plastic, it was
enviromentaly correct.