Enhancement of microwave effect with addition of chemical agents in solubilization of waste activated sludge

doi:10.1016/j.jiec.2014.12.019

Journal of Industrial and Engineering Chemistry

Volume 24, 25 April 2015, Pages 359-364

https://doi.org/10.1016/j.jiec.2014.12.019 Get rights and content

Abstract

Microwave (MW) is a promising method for solubilization of wasted activated sludge (WAS) due to the accelerated reaction rates, environmental friendliness and low overall cost compared to conventional heating. In this study, to enhance the MW effect on solubilization of WAS, several chemical agents were applied including H₂SO₄ as the acidic material and NaCl and CaCl₂ as ionic materials. In the experiment of salt-assisted MW pretreatment, CaCl₂ turned out to be ineffective on WAS solubilization, whereas NaCl had a positive effect on WAS solubilization compared to that of MW only assisted pretreatment. Thus, NaCl was considered to be appropriate salt to assist MW pretreatment for WAS solubilization. In the chemically assisted MW pretreatments, H₂SO₄ assisted MW pretreatment was the most effective for WAS solubilization. However, the application of H₂SO₄ to MW pretreatment has the drawbacks of reuse, which may increase the operating costs. Thus, food wastewater was used for enhancement of MW efficiency on WAS solubilization due to its high NaCl and acidity. As a result, the application of food wastewater to WAS afforded the solubilization efficiency higher than that of MW-H₂SO₄ pretreatment.

Introduction

The management of waste activated sludge (WAS) from wastewater treatment plants has attracted increasing concern because of the continuous increase in sludge generation [1]. In Korea, conventional WAS treatment method has been ocean dumping, but other WAS disposal routes should be adopted because ocean disposal was recently banned and the costs of treatment for WAS cause over 50–60% of total operating cost [2]. In fact, WAS, which is mainly composed of microbial cells, consists of 59–88% (w/v) organic materials and can therefore become a source of renewable energy like methane or hydrogen [3].

Anaerobic digestion of WAS has been a widely promising method for WAS treatment because it produces safe and clean energy and also contributes to reducing this abundant waste [4]. However, as the organic compounds of WAS are generally within the cell wall and extracellular polymeric substances (EPS) and as semi rigid structure of the cell envelope inhibits the osmotic lysis of the cells [5], producing biogas from anaerobic digestion requires a long hydraulic retention time. In this aspect, increasing the hydrolysis of WAS, which is the rate-limiting step, is the main concern for anaerobic digestion process. Therefore, many studies to increase biogas recovery have focused on accelerating the hydrolysis of WAS.

Several different pretreatment methods for sludge destruction are being investigated, including acid/alkali, ultrasound and heating [6], [7], [8]. Microwave (MW) has emerged recently because it is superior to conventional thermal heating due to its ability to heat rapidly, accelerate reaction rates [9] and provide environmental friendly energy [3]. The main effects of MW irradiation on WAS solubilization reactions are distinguished as thermal effects and non-thermal effects. Thermal effects are based on direct coupling of electromagnetic energy with the molecules of the solvents and reagents, whereby the magnitude of the heating depends on the dielectric properties of the molecules. Non-thermal effects are electrostatic polar effects, leading to dipole-dipole type interactions between the dipolar molecules and charges in the electric field [10], [11].

In MW irradiation process, high temperature and pressure treatment conditions increase WAS solubilization, but energy consumption of the process needs to be decreased. In this scenario, chemically assisted MW pretreatment can give a way to save energy and to achieve high sludge solubilization efficiency. Some studies showed that combined MW-alkali treatment could produce a synergy effect and become a cost effective alternative for sludge solubilization [12], [13]. On the other hand, some studies reported that the presence of salts, which increase the effects of dielectric loss and MW coupling, could produce superheating condition since the addition of salts to solutions could enhance conductivity and have a considerable effect on the rate of heating [14]. Thus, superheating effects can be amplified by the supplement of NaCl. In this sense, the mixture of food wastewater and WAS for the acceleration of solubilization is promising due to the high NaCl concentration of food wastewater.

The objective of the study is to enhance the MW efficiency for the cell destruction of WAS. Many materials were evaluated as the accelerating agents for enhancing the MW efficiency on WAS solubilization. H₂SO₄ was used as the acidic material and CaCl₂ and NaCl were used as the ionic materials. We also evaluated the effect of food wastewater for the enhancement of MW effect on WAS solubilization by varying the mixture ratio of WAS and food wastewater.

Section snippets

WAS characteristics

WAS samples were sourced from the return line of the secondary settling tank and thickener of Suyoung wastewater treatment plant in Busan, Korea. The sludge used in the study was from conventional aerobic activated process operated at an average hydraulic retention time of 6 h and a sludge retention time of 13 d. Table 1 presents the characteristics of return sludge (WAS 1) in conventional process and thickener sludge (WAS 2). The collected WAS samples were immediately sieved with mesh no. 24

Comparison of MW irradiation and CH

The main organic compounds in sludge are carbohydrates, proteins and lipids. Under MW irradiation, floc structure of activated sludge would be disrupted and extracellular and intracellular biopolymers are released [16]. Lipids are transferred into palmitic acid, stearic acid and oleic acid. Proteins are degraded into saturated and unsaturated acids, ammonia and carbon dioxide. Carbohydrates are hydrolyzed into lower molecular weight polysaccharides and even into monosaccharides, thereby

Conclusion

To enhance the MW efficiency for the cell destruction of WAS, several kinds of materials were evaluated. In the experiment of MW-CaCl₂ pretreatment, CaCl₂ was ineffective for WAS solubilization because Ca²⁺ was considered to be reacted with PO₄³⁻ and re-flocculation of dissolved organic polymers might have occurred in the presence of Ca²⁺. On the other hand, NaCl has a positive effect on WAS solubilization compared to that of MW only assisted pretreatment Thus, NaCl is thought to be appropriate

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Education, Science and Technology (2012R1A1A1043827) and the Brain Korea 21 Plus Project in the Division of Creative Low Impact Development and Management for Ocean Port City Infrastructures.

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Citation Excerpt :
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Enhancement of microwave effect with addition of chemical agents in solubilization of waste activated sludge

Abstract

Introduction

Section snippets

WAS characteristics

Comparison of MW irradiation and CH

Conclusion

Acknowledgements

J. Hazard. Mater.

Bioresour. Technol.

Int. J. Hydrogen Energy

Water Sci. Technol.

Bioresour. Technol.

J. Hazard. Mater.

J. Mol. Catal. A: Chem.

Chem. Eng. J.

Water Res.

Carbohydr. Res.

Water Res.

Renew. Sustain. Energy Rev.

Bioresour. Technol.