Dewatering of bio-sludge from industrial wastewater plant using an electrokinetic-assisted process: Effects of electrical gradient Chih-Huang Weng a , Yao-Tung Lin b,⇑ , Ching Yuan c , Yu-Hao Lin b a Department of Civil and Ecological Engineering, I-Shou University, Kaohsiung 84008, Taiwan b Department of Soil and Environmental Sciences, National Chung Hsing University, TaiChung 40227, Taiwan c Department of Civil and Environmental Eng., National University of Kaohsiung, Kaohsiung 811, Taiwan article info Article history: Available online 21 June 2013 Keywords: Bio-sludge Dewatering Electrokinetic-assisted Moisture content abstract An electrokinetic-assisted sludge dewatering (EASD) process was used to evaluate the dewaterability of the bio-sludge cake with moisture content of 68.9%. The cake sample was collected immediately after a belt filter press of an industrial wastewater treatment plant. Results showed that an average of electro- osmosis permeability of 2.2  10 5 cm 2 /V-s was observed. After treatment, the sludge pH values near the anode and cathode were in the range of 6.5–7.5 and 9.5–10.8, respectively. The effectiveness of the EASD process was directly related to the magnitude of the electric current in the system. Increasing the poten- tial gradient would enhance the extent of dewatering efficiency. As a constant electrical gradient of 1.25 V/cm was applied, the moisture content of sludge decreased to 63.6% after 4-h treatment. The mois- ture content of sludge decreased further to 48.8% as the process operated at an applied potential gradient of 5.0 V/cm. Energy requirement per unit ton of sludge treated was 17.7–86.5 kW h/ton (dry weight). Pre- liminary cost analysis of the EASD dewatering process showed that approximately a saving of 23% sludge disposal cost could be realized. Results indicated that the EASD process is an economic, viable and effec- tive technology in assisting sludge dewatering. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Water contained in sludge can be categorized in four types of water [1]: (1) free water, (2) interstitial water, (3) vicinal water, and (4) water of hydration. Free water and most of the interstitial water are removable by mechanical dewatering without using polymers. The vicinal water is partially removable by mechanical dewatering with addition of polymers. The so-called bound water is usually refers to the waters are physically bound inside flake of sludge, biologically or chemically bound in intracellular form, and physically bound in colloid. The bound water in intracellular form is arduous to be removed unless an external force applied by breaking the cellular wall using thermal drying or freezing [2]. Dewatering the sludge generated in industrial and water/ wastewater treatment plants can be achieved with conventional processes, including vacuum filtration, centrifugation, mechanical press, and sun-drying. Owing to the low energy requirement, sludge dewatering via mechanical press is selected as comparing to thermal process. However, the water content of the sludge after mechanical press dewatering process is normally exceeding 70 wt% [3]. The use of sun-drying method would be economically fea- sible if the space, sufficient time, and weather condition allow. A water content of 50–70% can be achieved via sun-drying. Although freeze–thaw is capable of decreasing the water content of sludge to less than 50% with the addition of polymer electrolytes [4], it is ex- pected that the consumption of energy and polymer electrolytes may become limiting factors under economical consideration. To achieve a sufficiently low water content, an attempt of improving sludge dewatering has been made by Lu et al. [5], they reported that Fenton’s reagent could improve the filtration and dewatering efficiencies of industrial wastewater sludge. However, applying Fenton’s reagent in sludge matrix would acidify the sludge and sig- nificantly change the sludge characteristics. The superimposition of ultrasounds [6] and magnetic fields [7] are other alternatives for the enhancement of sludge cake filtration [2,8]. Electrokinetic (EK) process involves the application of an elec- trical field across a porous medium to induce the movement of electrolyte solution and soluble contaminants toward the elec- trodes. Owing to its unique applicability to low-permeability soils, this process has received increasing attention worldwide in soil remediation [9], and activated carbon regeneration [10]. Recently, interests in electrokinetic-assisted sludge dewatering (EASD) have increased because electro-osmosis (EO) enables to enhance sludge dewatering [11–18]. Weng et al. [9] have shown that the use of electro-osmosis to enhance sludge dewatering was applicable in wastewater treatment process. The electro-dewatering process can be operated under either constant voltage or constant current 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.06.013 ⇑ Corresponding author. Tel.: +886 4 22852137; fax: +886 4 22862043. E-mail address: yaotung@nchu.edu.tw (Y.-T. Lin). Separation and Purification Technology 117 (2013) 35–40 Contents lists available at SciVerse ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur