Nitrogen and Phosphorous Removal from Municipal Wastewater Using High Rate Algae Ponds

Eutrophication due to uncontrolled discharge of sewage water rich in nitrogen and phosphorous is one of the most significant water quality problems world-wide. Nitrogen and phosphorous can be removed by activated sludge process, one of the widely applied technologies among other conventional methods for municipal wastewater treatment. However, such technology requires high capital and operational costs, making it unaffordable for many developing nations, including Ethiopia. This study aimed to investigate the performance of two high rate algal ponds (HRAPs) in nitrogen and phosphorous removal from primary settled municipal wastewater under high land tropical climate conditions in Addis Ababa. The experiment was run under semi-continuous feed for 2 months at hydraulic retention times (HRT) ranging from 2 to 8 days and organic loading rates ranging from 44.3 to 9.08 g COD/m2/day using two HRAPs 250 and 300 mm deep, respectively. In this experiment, Chlorella sp., Chlamydomonas sp., and Scenedesmus sp. in the class of Chlorophyceae were identified as the dominant species. The maximum TN and TP removal of 91.70 and 82.81% was achieved in the 300 mm deep HRAP during 8 and 6 day HRT operations, respectively. Increased HRT and pond depth increased nutrient removal but high chlorophyll-a biomass was observed in the 250 mm deep HRAP. Therefore, the 300 mm deep HRAP is promising for scaling up nutrient removal from municipal wastewater at a daily average organic loading rate in the range of 14.3–15.33 g COD/m2/day or 10.34–11.46 g BOD5/m2/day and a 6 day HRT. We conclude that HRAP is a dependable approach to remediate nitrogen and phosphorous from primary settled municipal wastewater in Addis Ababa climate with appropriate control of pond depth, organic loading rates and HRT.