Tae-In Ohm

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) degrees C was used to dry industrial wastewater sludge. Also waste oil was used in the fry-drying process, and because the oils boiling point is between 240 and 340 degrees 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 degrees 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 degrees 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.

The evaporative drying of sludge by immersion in hot oil: Effects of oil type and temperature.

We investigated the evaporative drying by immersion in hot oil (EDIHO) method for drying sludge. This involved heating oil to a temperature higher than that needed for moisture to be evaporated from the sludge by turbulent heat and mass transfer. We fry-dried sewage and leather plant sludge for 10 min in each of four different oils (waste engine, waste cooking, refined waste, and B-C heavy) and three different temperatures (140 degrees C, 150 degrees C, and 160 degrees C). Drying efficiency was found to be greater for higher temperatures. However, giving consideration to energy efficiency we suggest that the optimal temperature for fry-drying sludge is 150 degrees C. At 150 degrees C, the water content of sewage sludge reduced from 78.9% to between 1.5% (with waste cooking oil) and 3.8% (with waste engine oil). The reduction in water content for leather plant sludge fry-dried at 150 degrees C was from 81.6% to between 1% (with waste cooking oil) and 6.5% (with refined waste oil). The duration of the constant rate-drying period was also influenced by the type of oil used: refined waste oil>waste engine oil>B-C heavy oil>waste cooking oil. The duration at 150 degrees C with waste cooking oil was 3 min for sewage sludge and 2 min for leather plant sludge. It is likely that the drying characteristics of oil are influenced by its thermal properties, including its specific heat, and molecular weight.

Experimental study of fry-drying and melting system for industrial wastewater sludge.

In South Korea, ocean dumping of organic sludges has been prohibited by the London Convention and by Korean regulations. Therefore, the Government of South Korea has sought an alternative process for the disposal of organic sludges. Recently, the combined fry-drying and melting system has been recognized as an efficient way to utilize the energy content of organic industrial sludge. Three kinds of fry-dried industrial sludges (obtained from industrial sites DG, DJ and GM), which had average heating value of 20,470kJ/kg and less than 5% water content, were tested. Unlike sewage sludge, industrial sludge contains high concentrations of heavy metals and thus cannot be directly utilized as refuse-derived fuel. The dried sludges were melted in a furnace and then rapidly cooled to form vitrified slags; the vitrification of SiO2 securely encapsulates hazardous heavy metals within the crystalline structure of the slag. To evaluate the hazard of vitrified slag, the heavy metal elution concentration was analyzed. Following vitrification, Hg, Cd, Cr(+6), HCN and Pb concentrations were not detectable, whereas Cu concentration decreased from 26.78mg/L to 0.42mg/L in DJ sludge, from 27.10mg/L to 0.13mg/L in DG and from 49.47mg/L to 0.047mg/L in GM sludge.

Characteristics of Fry-Drying and Solid Refuse Fuels for Organic Wastes with High Water Content

With the dramatic increase in the quantity of organic wastes, economic and environmentally friendly technologies are urgently required for reducing the volume of sludge and remediating its harmful impacts. In this study, drying experiments were performed on sludge through fry-drying technology and the characteristics of the resulting fuels were investigated to identify proper methods for treating sewage sludge, wastewater sludge, swine excreta, and food waste and converting them into fuels. The four types of organic wastes were fry-dried, and the best drying conditions were found to be 140 °C for 8 min for sewage sludge and 150 °C for 10 min for wastewater sludge, swine excreta, and food waste. The water contents of sewage sludge, wastewater sludge, swine excreta, and food waste were 2.40 wt%, 2.70 wt%, 2.90 wt% and 5.82 wt%, respectively. Based on the results of fuel ratio, C/H ratio, thermogravimetric analysis (TGA), and derivative thermogravimetric (DTG) analysis on the four types of fry-dried solid refuse fuels, mixing the waste fuels with a certain proportion of coal for incineration in order to control the early ignition and rapid incineration rate was found to be effective.