Elsevier

Journal of Hazardous Materials

Volume 313, 5 August 2016, Pages 78-84
Journal of Hazardous Materials

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

https://doi.org/10.1016/j.jhazmat.2016.03.084Get rights and content

Highlights

  • After fry-drying, the moisture content of each sludges was less than 5%.

  • In three sludge samples, Pb, As, HCN and Cu were detected.

  • In molten slags of dried sludges, the concentrations of Cu and As were minuscule.

  • In molten slags of dried sludges, no other heavy metals were detected.

  • The molten slag is suitable for recycling as aggregate.

Abstract

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 20,470 kJ/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 26.78 mg/L to 0.42 mg/L in DJ sludge, from 27.10 mg/L to 0.13 mg/L in DG and from 49.47 mg/L to 0.047 mg/L in GM sludge.

Introduction

The South Korean economy has grown rapidly since 1970, and thus living standards have improved. However, as the population is concentrated in cities and the number of industrial complexes has increased, the generation of domestic sewage and industrial wastewater sludge has sharply increased. Many environmental problems have been solved by providing environmental infrastructures, centered on regions of high population. However, sludge generated from sewage and industrial wastewater disposal plants is increasing as a new source of contamination.

Ocean dumping of organic sludge has been gradually reduced, from 77% in 2005 to 47% in 2009 and 14% in 2013 [1], [2]. As direct landfilling of organic sludge and marine dumping of sewage sludge had been prohibited from 2012 by the London Convention and by Korean regulations, organic sewage sludge has been recycled or incinerated at public disposal facilities and contracted-out treatment equipment [3]. However, treatment capacity is limited by the mass-generation of organic sludges, and the cost of sludge treatment is also continuously increasing with rising energy prices [3], [4], [5]. Thus, to reduce operating costs, new technological advances are required, both to treat the sludge and reduce the generation of sludge at source. The treatment options for organic sludge include landfill, fertilizing, incineration, melting, and pyrolysis. A sludge-drying technology has been proposed as an alternative method for using solid refuse as fuel [4], [5], [6].

Since organic sewage and industrial wastewater sludge have water contents of 80% or more, it is very important to identify appropriate techniques. Among them, a fry-drying technique can rapidly evaporate a large quantity of water from organic sludge [7], thus increasing the heating value, thereby fuelizing the sludge [8], [9], [10], [11], [12], [13], [14], [15]. However, unlike sewage sludge, industrial wastewater sludge contains high concentrations of heavy metals [16], [17], [18]. Furthermore, since these various heavy metals originate from industrial processes, the dried sludge cannot be directly recycled as a fuel, and thus another method is required for recycling wastewater sludge [19], [20], [21], [22], [23], [24].

In this study, the fry-drying technique was used to treat and recycle industrial wastewater sludge discharged from various disposal facilities. Subsequently, proximate analysis, ultimate analysis, heavy metal analysis, and measurement of heating value were performed before and after fry-drying. Changes in the sludge water content according to drying conditions (such as residence time and oil temperature) were measured to establish a stable and efficient method of treating the waste water sludge using the proposed fry-drying technology [14], [15]. Additionally, a melting experiment was performed to minimize the elution volume when the wastewater sludge discharged from various industrial facilities contains high concentrations of heavy metals. The intention was to reuse or recycle the molten product, based on its characteristics, and thereby establish a new, eco-friendly, and economical technique for treating industrial wastewater sludge.

Section snippets

Experimental system

Fig. 1 illustrates a batch type fry-drying system used in this study. The objective of the batch fry-drying experiment is to determine optimal temperature and time for fry-drying of industrial wastewater sludge, through experiments under various conditions (including the kind of oil, oil temperature, and drying time); and to apply these to a continuous-type experimental system. An electronic scale was provided at the lower end of the fry-drying system to automatically measure the change in

Results of fry-drying experiment

The three types of industrial wastewater sludge were fry-dried for approximately 10 min at oil temperatures of 150 °C. Since the sludge processing method and dewatering method differ in each industrial complex, the fry-drying experiment should be performed under the same conditions.

Fig. 4 illustrates the moisture contents of the wastewater sludges after fry-drying, according to variations in drying time. After drying at 150 °C for 6 min, two of the sludges (except DG) had moisture contents of 5%,

Conclusion

This study reports on the use of fry-drying and melting technologies as an economical and environmentally friendly method for processing organic industrial wastewater sludge. These technologies were evaluated for their drying and thermal characteristics, and an elution experiment was performed to evaluate the environmental performance of the resulting vitrified slag.

  • 1.

    Temperature of 150 °C and duration of 10 min were found to be the optimal conditions for fry-drying the three kinds of industrial

Acknowledgements

This study was supported by the “R&D Center for Reduction of Non-CO2 Greenhouse Gases (201300 1690006)" funded by the Korea Ministry of Environment (MOE) as a ‘Global Top Environment R&D Program’; the Korea Energy Technology Evaluation Institute (20133030100860); and a Grant from the Human Resources Development Program (No. 20134030200230) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Korea Ministry of Trade, Industry and Energy.

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