Role of organic matter, nitrogen and phosphorous on granulation and settling velocity in wastewater treatment

Abstract

In this study, the ability of the aerobic biogranulation process to remove organic matter, total nitrogen, and total phosphorous from wastewater was investigated. The effect of organic matter load on the quality of the produced biogranules and settling velocity of the generated biomass was also investigated. Four different aerobic scenarios were studied in sequential batch bioreactors (SBR) to identify the best operating conditions. The experimental setup consisted of four bioreactors containing 300 mL samples (100 mL of sludge and 200 mL of sweet whey at four different concentrations) under constant stirring (150 rpm), room temperature, and controlled pH (e.g., 7.98 ± 0.25). The different organic matter loads were calculated as the chemical oxygen demand (COD) concentration at 1157 (A), 719 (B), 357 (C), and 202 mg/L (D), respectively. The process consisted of cycles of initial feast followed by 72 h of famine. During the experiments, COD, total nitrogen (TN), and total phosphorus (TP) were measured at the beginning (t0) and the end of the process. The results confirmed that the best conditions were found in the bioreactor with the highest COD load (1157 mg/L), which achieved 98.7 % reduction in organic matter, 45.6 % reduction in TP, and 99.4 % reduction in TN after 9 days with final average sludge settling velocity of 0.7 m/h. The biogranulation process achieved improved clarification of the supernatant, showing increased settling velocity in all of the bioreactors. Microscopic observation confirmed uniform morphology (e.g., spherical or cylindrical) with the biggest biogranules being observed in bioreactors A and B. Organic load was found to play a significant role in biogranules formation and the achievement of the highest organic matter removal, sludge volume reduction, and water clarification. The overall trend in final sludge settling velocity measured in the different bioreactors was C > B > D > A, and in all of them settling was faster than the untreated sample. The results of this study confirm the high potential of biogranulation process for the treatment of wastewater with high organic matter content.