You can also look at discharging the backwash effluent into a gravity, geotextile container allowing the supernatant to exfiltrate out the bottom. Try Flo-trend WWW.FLOTREND.COM
and you can put GAC in the bottom of the unit to give you dechlorination.
You can also try contacting the AA suppliers, they can give you a list of assemblers in your area. I would start with a top notch AA guy:
Jon Mogan
Alcan Specialty Aluminas
(jon.mogan@alcan.com)