Employability of BAC for Tertiary Treatment of Waste Water

Abstract

In developing countries, Sewage Treatment Plants (STPs) effectively remove BOD (Biochemical Oxygen Demand), but they struggle to eliminate total nitrogen and phosphorous. In order to improve nutrient removal, we propose the use of Biologically Activated Carbon (BAC) technology, which involves developing a biofilm on exhausted Granular Activated Carbon. This approach combines the adsorption capabilities of activated carbon with biological wastewater treatment. By accumulating or artificially immobilizing microbes on the surface of activated carbon under specific nutrient and temperature conditions, we can create a carrier for BAC technology. The objective of this study was to develop a cost-effective tertiary treatment system that can remove nutrients and residual organics, including trace organics such as emerging contaminants, from poorly operated STPs. To achieve this, we seeded exhausted GAC with ethanol and sludge for seven days, maintaining a high Empty Bed Contact Time (EBCT). Once a steady state was reached, the EBCT was reduced to 25 minutes. At the steady state, the removal rates for Chemical Oxygen Demand (COD), phosphate, Dissolved Organic Carbon (DOC), and nitrate were 20%, 29%, 59%, and 30% respectively. In the next phase, we operated two BAC columns in series. The first column was subjected to inlet Dissolved Oxygen (DO) levels at saturation, while the second column was operated under anoxic conditions. The influent wastewater used was secondary treated wastewater from a well-operated full-scale sequential batch reactor. Carbon and ammonia sources were added to simulate the conditions found in poorly functioning STPs in other parts of India. The total EBCT for this setup was 45 minutes (20+25). The removal efficiencies observed for COD, ammonia, and phosphate were significantly higher in comparison to the single column with a similar EBCT, reaching 69%, 87%, and 32% respectively. Furthermore, when the EBCT was further reduced to 35 minutes (15+20), even higher removal efficiencies were achieved, with COD, phosphate, and ammonia reaching 75%, 19%, and 78% respectively. Notably, a substantial 50% removal of DOC was also observed at an EBCT of 25 minutes (10+15). Scanning Electron Microscopy (SEM) analysis revealed the presence of biofilm on the surface of BAC, along with various unknown bacteria and other microorganisms. Some elements were also detected on the surface of both new GAC and BAC. In conclusion, this study demonstrates that exhausted GAC can be transformed into BAC, enabling efficient tertiary treatment of wastewater even at lower and practical EBCTs.