High rate algal pond systems for low‐energy wastewater treatment, nutrient recovery and energy production

High rate algal pond (HRAP) systems provide opportunities for low‐energy wastewater treatment and energy recovery from wastewater solids, as well as biofuel production from the harvested algal biomass. The wastewater is pretreated using covered anaerobic ponds or gravity settlers and covered digester ponds which remove and digest the wastewater solids. The effluent is then treated in shallow gently mixed HRAP which efficiently breakdown the dissolved organic matter. The algae assimilate wastewater nutrients to provide both secondary and partial tertiary‐level treatment. HRAP also provide more efficient natural disinfection. HRAP performance can be further enhanced by bubbling CO 2 into the pond during the day to promote algal growth when it is often carbon‐limited. This paper discusses the design and operation and performance of HRAP systems and their application for economical, low‐energy upgrade of conventional wastewater treatment ponds combined with energy recovery and biofuel production.