HRP follow the AFP in AIWP systems. Aerobic bacteria break down dissolved organic matter and algae take up nutrients and further BOD. HRPs are designed to promote the symbiosis between the microalgae and aerobic bacteria, each utilizing the major metabolic products of the other. Microalgae grow profusely releasing oxygen from water by photosynthesis. Since at the rapid growth of algae pH can raise to above 9 (because at peak algal activity carbonate and bicarbonate ions react to provide more carbon dioxide for the algae leaving an excess of hydroxyl ions), most pathogens are also killed.