SUSTAIN SUMMER SHIELDSustain Summer Shield provides pool water with an extra level of protection against algae. It does this by creating a backup of chlorine that is released on demand when routine sanitizing levels drop (below 1ppm), such as after a pool party or thunderstorm. Sustain Summer Shield chlorine extender also contains a clarifier to assist your filter keeping your pool crystal clear. Sustain Summer Shield is designed for use with the Sustain 3 Part System.