A study on the dewatering of industrial waste sludge by fry-drying technology

In sludge treatment, drying sludge using typical technology with high water content to a water content of approximately 10% is always difficult because of adhesive characteristics of sludge. Many methods have been applied, including direct and indirect heat drying, but these approaches of reducing water content to below 40% after drying is very inefficient in energy utilization of drying sludge. In this study, fry-drying technology with a high heat transfer coefficient of approximately 500 W/m 2 °C was used to dry industrial wastewater sludge. Also waste oil was used in the fry-drying process, and because the oil's boiling point is between 240 and 340 °C and the specific heat is approximately 60% of that of water. In the fry-drying system, the sludge is input by molding it into a designated form after heating the waste oil at temperatures between 120 and 170 °C. At these temperatures, the heated oil rapidly evaporates the water contained in the sludge, leaving the oil itself. After approximately 10 min, the water content of the sludge was less than 10%, and its heating value surpassed 5300 kcal/kg. Indeed, this makes the organic sludge appropriate for use as a solid fuel. The wastewater sludge used in this study was the designated waste discharged from chemical, leather and plating plants. These samples varied in characteristics, especially with regard to heavy metal concentration. After drying the three kinds of wastewater sludge at oil temperatures 160 °C for 10 min, it was found that the water content in the sludge from the chemical, leather, and plating plants reduced from 80.0 to 5.5%, 81.6 to 1.0%, and 65.4 to 0.8%, respectively. Furthermore, the heat values of the sludge from the chemical, leather, and plating plants prior to fry-drying were 217, 264, and 428 kcal/kg, respectively. After drying, these values of sludge increased to 5317, 5983 and 6031 kcal/kg, respectively. The heavy metals detected in the sludge after drying were aluminum, lead, zinc, mercury, and cadmium. Most importantly, if the dried sludge is used as a solid fuel, these heavy metals can be collected from the dust collector after combustion.