Leaching of Trace-pollutants from Wastewater Sludge through the Ultrasonic Treatment

Abstract

The generation amount of sludge has increased along economic growth. In accordance with the London Convention and the Marine Environment Management Act, dumping sewage sludge and live stock manure in the ocean was banned since 2012. Ultrasonic treatment has been mainly used among various reduction techniques because of low expenses and high efficiency. Sludge is decomposed and a lot of pollutants in sludge were leached by using ultrasonic treatment. When trace-pollutants were leached from the sludge sediment, they can cause contamination on supernatant. Also, trace-pollutants such as heavy metals and PAHs (polycyclic aromatic hydrocarbons) can have a significant impact. However, the fate of trace-pollutants in supernatant and sediment of sludge after being treated using ultrasonic treatment has been rarely reported. In this study, wastewater sludge was collected from chemical plants in Ulsan, Korea, and the effect of ultrasonic treatment on sludge reduction and leaching of trace-pollutants was investigated. Sludge samples were irradiated at 600 W for 5-30 min by ultrasonicaters and centrifuged at 3,000 rpm for 30 min. To confirm the effect on the reduction of sludge, we investigated microorganism flocs in the sludge due to ultrasonic irradiation times using microscope. Volume change of sediment in the sludge and TSS (Total Suspended Solid) were also analyzed. To confirm concentration of heavy metals in supernatant and sediment, the samples were analyzed using Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-OES, 720-ES, Varian, USA) after the stage of acid treatment. We also analyzed the concentration of PAHs in supernatant and sediment using Gas chromatograph-mass spectrometer (GC/MS, Agilent 7890A, Agilent, USA) and calculated the fate of PAHs. Changes of microorganism in sludge were observed using a microscope. The cell walls and flocs of microorganism were disintegrated by increasing irradiation time. According to these results, it was expected that the ultrasonic treatment have a great effect on the leaching of trace-pollutants and the disintegration of microorganism. The TSS concentration was increased by increasing irradiation times because suspended materials can be increased by decreasing size of flocs. While the concentration of heavy metals and PAHs decreased in the sediment, the concentration of heavy metals and PAHs increased in the supernatant according to the result. The reason for the leaching of heavy metals and PAHs was the disintegration of cell walls of microorganism. Therefore, it was expected that the treatment has great effect on leaching of trace-pollutants and reducing sludge by increasing irradiation times and irradiation intensity. According to these results, leached trace-pollutants in supernatants were possible to be the source of sludge treatment in WWTP (wastewater treatment plant). Therefore, treatment of supernatant after ultrasonic treatment should be considered appropriately. On the other hand, the sediment in sludge was able to be used more safely by leaching trace-pollutants in it using ultrasonic treatment. Therefore, ultrasonic treatment which has effect on both reducing sludge and recycling it safely should be developed by carrying out continuous monitoring of the effect on the sludge.