Removal of Organic Pollutants from Municipal Wastewater by Applying High-Rate Algal Pond in Addis Ababa, Ethiopia

Abstract

The discharge of inadequately treated municipal wastewater has aggravated the pollution load in developing countries including Ethiopia. Conventional wastewater treatment methods that require high capital and operational costs are not affordable for many developing nations, including Ethiopia. This study aimed to investigate the performance of two high-rate algal ponds (HRAPs) in organic pollutant removal from primary settled municipal wastewater under highland tropical climate conditions in Addis Ababa. The experiment was done for 2 months at hydraulic retention times (HRTs) ranging from 2 to 8 days using an organic loading rates ranging 333–65 kg $$ {\text{BOD}}_{5} $$ /ha/day using two HRAPs, 250 and 300 mm deep, respectively. In this experiment, Chlorella sp., Chlamydomonas sp., and Scenedesmus sp., the class of Chlorophyceae, were identified as the dominant species. Chlorophyll-a production was higher in the shallower ponds (250 mm) throughout the course of the study, whereas the deeper HRAP (300 mm) showed better dissolved oxygen production. The maximum COD and $$ {\text{BOD}}_{5} $$ removal of 78.03 and 81.8% was achieved at a 6-day HRT operation in the 250-mm-deep HRAP. Therefore, the 300-mm-deep HRAP is promising for scaling up organic pollutant removal from municipal wastewater at a daily average organic loading rate of 109.3 kg $$ {\text{BOD}}_{5} $$ /ha/day and a 6-day HRT. We conclude that the removal of organic pollutants in HRAP can be controlled by pond depth, organic loading rate, and HRT.