Chemical Engineering Journal 164 (2010) 85–91 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Effect of freeze/thaw conditions, polyelectrolyte addition, and sludge loading on sludge electro-dewatering process Pham-Anh Tuan a,∗ , M. Sillanpää a,b a University of Eastern Finland, Laboratory of Applied Environmental Chemistry, Patteristonkatu 1, FI-50100 Mikkeli, Finland b LUT Technology, Lappeenranta University of Technology, Patteristonkatu 1, FI-50100 Mikkeli, Finland article info Article history: Received 14 December 2009 Received in revised form 11 August 2010 Accepted 12 August 2010 Keywords: Electro-dewatering Electro-osmosis Freeze/thaw conditions Polyelectrolyte addition Sludge abstract Laboratory scale pressure-driven electro-dewatering reactor was set up to study the effect of polyelec- trolyte addition and freeze/thaw conditions on solid content in the final sludge cake at different sludge loading rates. An increase in freezing temperature and extended natural freezing periods resulted in a significant increase in sludge dewatering ability. However, dry solid (DS) content in the final sludge cake after electro-dewatering was similar (39.3–41.5%) regardless the experimental strategies. The reduction in sludge loading rate from 20 to 3 kg DS/m 2 resulted in the increase in DS content of the final sludge cake (35.8–48.7%) using both, the polyelectrolyte addition and freezing condition sludge samples. During electro-dewatering using sludge amended with polymers, the DS content in the final sludge cake by the anode and the cathode was similar or a bit higher at the cathode either at high sludge loading or using high amount of polymers. It was found that polymer addition had a negative effect on the electro-osmotic flow. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The most important disadvantage of aerobic municipal wastew- ater treatment is the generation of large amounts of waste sludge. Original biological sludge from wastewater treatment plant such as activated sludge and anaerobically digested sludge are well known to have a poor dewaterability. It is known that freeze/thaw conditioning is a highly effective sludge dewatering technique. The main principle of this technique is that during freezing, ice crystals grow incorporating water molecules [1,2]. Because the structure of ice crystals is highly organized and symmetrical, it cannot accommodate any additional atoms or molecules. Each ice crystal continues to grow as long as there are water molecules available. All other impurities and solid particles are forced to the boundaries of the ice crystal where they become compressed or dehydrated. This technique changes sludge floc into a compacted form, reduces sludge bound water content and makes sludge more apt for settling and filtration [3]. In general, sludge freezing at slow freezing rates shows better dewaterability than fast freezing. Nonetheless, it has been reported that even fast freezing signifi- cantly improves sludge dewaterability [4]. Ormeci and Vesilind [2] who studied freeze/thaw conditioning effect on alum and activated sludge, concluded that freeze/thaw conditioning effectively dewa- ∗ Corresponding author. Tel.: +358 40 355 3404; fax: +358 15 333 6013. E-mail addresses: tuan.pham@uef.fi (P.-A. Tuan), mika.sillanpaa@lut.fi (M. Sillanpää). tered alum and activated sludge, however, alum sludge was likely to freeze/thaw better than activated sludge due to its low dissolved ion and organic matter content. High concentrations of dissolved ions and organic material present in activated sludge promote par- ticle entrapment during freezing and decrease the effectiveness of freeze/thaw conditioning. Alum sludge ice crystals predominantly grow in columns, whereas activated sludge ice crystals grow in dendrite [5]. The addition of dissolved solids (NaCl) to alum sludge changes ice crystals growth from columnar to dendrite. Because the dendrite ice crystals are formed at the ice/water interface, sludge particles are trapped in the ice front, resulting in a decrease in sludge dewaterability. The freeze/thaw conditioning does not only increase the sludge dewaterability but also reduces pathogenic microorganisms in sludge [6]. It is also considered as a low-cost sludge treatment technique at moderate to cold climates [7]. Electrically assisted treatments have gained popularity in envi- ronmental engineering in recent years [8–13]. Electro-dewatering method, when low level electric field is applied to the sludge cake to induce the migration of water, has been reported to significantly reduce water content in the final sludge cake [14,15]. However, the effectiveness of electro-dewatering process strongly depends on electric field strength and the contact time [16–18]. Moreover, an increase in voltage application subsequently results in reduced water content in the final sludge cake as reported by numerous researchers [14,19–21]. During electro-dewatering process, sludge type and alkalinity plays an important role in water removal rate and the final dry solid (DS) content of the sludge cake [22]. The effect of different types of polyelectrolyte and doses on electro- 1385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.08.028