A floway system for a building roof includes a substrate for supporting a periphyton culture that removes nutrient pollution from surface water and isolates the building envelope from solar-induced heat. The substrate is seedable to form a floway. A basin is provided for holding water to be treated, and a pump for pumping water therefrom to the floway inlet. At the floway outlet is positioned an effluent-collection gutter, which channels effluent to a collection vessel. Algal biomass can be harvested. A flexible divider is positioned in the gutter along the long axis, a top edge of securable adjacent the floway outlet so that, during harvesting, algal biomass is scraped into a distal section of the gutter, and thence to a biomass collection basin. The harvester can include a scraper and mechanism for traveling down the floway. The harvester can also move between adjacent floways and to the building roof.