I also recommend using bleach, but I use a 50/50 mix of Clorox Outdoor Bleach and Simple Green, then dilute at 10:1 by running it through the injection system. The combination of outdoor bleach and Simple Green gives a thick/sudsy concoction that will cling to the surfaces really well and both are safe to use around plants, grass, etc. I use this mix on siding, concrete, wood, brick with great results.
I let it sit for 3-5 mins. and use high volume/low pressure to rinse well. Contrary to popular belief, using bleach with downstream injection will cause no harm if a good rinse is done after each use. I also use compressed air to blow all water out of hoses, use Pump Saver and lubricate all o-rings/couplings after each use. It's keeping up with Preventive Maintenance that makes alot of difference down the road.