Most cities and towns have made provision for disposal of TX and RX batteries. Your hobby shop or Radio Shack should also be aware of a method of disposal. It's time to dump old batteries when they will no longer hold a charge for a given period of time or will not reach their rated voltage when charged. Often a cycle and recharge will restore a battery that you have given up for lost.
If you are talking of recycle/charging. They should be recycled by the use of a battery dumper that will discharge them to a safe point before recharging. If you only use the batter for a short period and then recharge a nicad will develope a memory. Try to run it down to safe level before recharging. Just don't do it in the air.