A study by the Indian Institute of Technology Bombay (IIT Bombay) on biofilters has revealed that partially treated wastewater from refineries already carry bacteria that can remove the organic contaminants and harmful pollutants when flowing through sand, offering a new, cost-effective method to treat industrial wastewater.
Refineries, which transform crude oil into products like gasoline and diesel, generate large amounts of wastewater that often contains harmful organic and inorganic pollutants, including nitrogen-containing compounds. This wastewater then undergoes multiple levels of treatment before it can be safely discharged into the environment. However, IIT Bombay researchers have found that pollutant-eating bacteria can form naturally in wastewater as it flows through sand, further purifying the water by removing the organic contaminants from the wastewater.
Quartz sand as biofilter
Led by Professor Suparna Mukherji from the Department of Environmental Science and Engineering, the study utilised a biofilter made from pure quartz sand, which allowed bacteria to cling to the sand grains and form biofilms. “Sand was chosen (as a biofilter) since it is commonly used in deep bed filters used for water and wastewater treatment,” said Mukherji.
“As the water flows through the sand bed, bacteria present in the water/wastewater get adsorbed onto the sand. Subsequently, the attached bacteria replicate and secrete extracellular polymeric substances to form a biofilm on the surface of the sand grains. Bacteria grow using dissolved oxygen, organic carbon, and other nutrients from the water flowing through the sand bed,” Mukherji explained. This biofilm in turn eats away at the organic contaminants in the water, the study added.
Findings of the study
The study further revealed that tests on refinery wastewater showed that after just two recirculations through the sand biofilter, the bacteria removed significant amounts of harmful organic compounds like Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC). And after 12 recirculations, more than half of COD and TOC were removed.
“Recirculating the wastewater up to 12 times resulted in maximum reduction in COD and TOC of 62% and 55% respectively,” said Dr. Prashant Sinha, an author of the study.
The team also used a technique called Gas chromatography Time of Flight Mass spectrometry (GcxGC-TOF-MS) to detect and quantify specific organic compounds in the water. “GcxGC-TOF-MS revealed that several of the identified target (harmful) compounds could not be detected in the wastewater after 12 recirculations, suggesting 100% removal,” Sinha added.
Nitrates build-up in treated water
Despite the promising results, the study also noted a build-up of nitrates in the treated water, a byproduct of the bacterial breakdown of nitrogen-containing compounds.
“The build-up of nitrates observed is not desirable," Mukherjee said. "However, refineries commonly employ RO as the final treatment step. This process can reduce the level of nitrates in the final effluent,” she added.
According to the study, predominant bacteria carried by wastewater belonged to a group called Proteobacteria. The group is known for their ability to break down complex organic compounds like polynuclear aromatic hydrocarbons (PAHs), which are harmful to living organisms.
Sand biofiltration stands out for its cost-effectiveness as it uses simple and easily available materials like quartz sand, and its simplicity allows it to be adopted by industrial plants globally, according to the study.
