Now Mark that is the question, I agree it takes more water to flush out a DE filter than others and yes I know DE is bad for lungs which is why I would never use it (other methods are just as good) but water harvesting for rain water is becoming the norm in Europe so a little education on saving water isn't a bad thing long term and I am sure a 100 gallon tank would do just as well.
Now the other chemicals, a little bit of copper perhaps 0.8ppm is about twice what is allowed in bottled drinking water so not really toxic although there are plans in Europe to outlaw it's use in pools as they have done with borates. A small amount of aluminium from floc, again the amount is tiny. Cyanuric acid is made from urea so it will be broken down by bacteria in the soil. Of course what's killing your neighbors grass may be salt but from experience you would be backwashing very frequently and on the same spot to really have an effect.
I had customers who backwashed their pool at shock chlorine level on to a field and it slowed the crop development for 2 weeks in an area around the pipe but then the crops (wheat) recovered and developed as normal. As I mentioned before the water could also be used for flushing toilets rather than wasted, people are begining to use rainwater harvesting for that in Europe as mains water is getting more expensive.
I think Rick summed it up very well, right up to the last sentence about sand being hard to beat but that is the part we will debate for the future.
Different areas require different techniques. This includes everything from balancing chems used to filter selections. I know many pools that benefit from the very small amount of dilution from a backwash. Helps to keep TDS, CYA, Hardness and others from climbing too high. The problem I see far too often is 30gpm-40gpm sand filters married to 75gpm and larger pumps. In other words, too many sand filters are undersized. Filter slower and your clarity will improve and the backwash frequency and volume will decrease. Properly sized its difficult to beat sand in many areas.
We recommend cartridge filters to all our customers unless they specifically ask for something else (and then we'll give them our opinion again).
Not a big fan of cartridges, but our business is in the SouthWest. Sand filters are easier to deal with here. We don't have many regulations with waste water. From a maintenance aspect Cartridge Filters are harder to deal with. We have a mixture of Sand and Cartridge filters on our routes with Sand being the preferred. If its and indoor setting Cartridge would be the preferred choice for me. On an outdoor system I would go with sand. Cartridges in our area have to be cleaned out once every 3-6 months on an outdoor system. Sand Filters last forever, if maintained properly. Sand needs changed about every 5 years and it would be a good idea to replace the lateral assembly while you are in there. Spider gaskets and multi-port valves need to be maintained as well, 90% of the time, multi-port valve is the point of failure with the Sand filter system