Performance of electro-osmotic dewatering on different types of sewage sludge

Abstract

A B S T R A C TThe feasibility of pressure-driven electro-dewatering (EDW) on sludge samples taken after different biological processes, stabilisation methods or mechanical dewatering techniques was assessed. First, the influence of potential values on EDW of anaerobically and aerobically stabilised, mechanically dewatered, sludge samples was investigated. Preliminary tests carried out by applying a constant potential (10, 15 and 20 V) in a lab-scale device confirmed the possibility to reach a dry solid (DS) content of up to 42.9%, which corresponds to an increase of 15% of the dry content in dewatered sludge without the application of the electrical field. Dewatering increased with the applied potential but at the expense of a higher energy consumption. A potential equal to 15 V was chosen as the best compromise for EDW performance, in terms of DS content and energy consumption. Then, the influence of the mechanical dewatering was studied on aerobically stabilised sludge samples with a lower initial DS content: the higher initial water content led to a lower final DS content but with a considerable reduction of energy consumption. Finally, the biological process, studied by comparing sludge samples from conventional activated sludge and membrane bioreactor processes, didn't evidence any influence on EDW. Experimental results shown that DS obtained after mechanical dewatering, volatile solids and conductivity are the main factors influencing EDW. Anaerobically digested sludge reached the highest DS content, thanks to lower organic fraction.

Assessment of pressure-driven electro-dewatering as a single-stage treatment for stabilized sewage sludge

“…The EDW treatment is composed of two successive phases [16,18]: (i) filtration under mechanical pressure (duration of 10 min), (ii) application of a constant electric potential in the presence of mechanical pressure (duration of 25 min). As already observed by Visigalli et al [27,34], a polarisation phase of 15 min is enough to reach a DS content higher than that achieved by mechanical dewatering. In this work, the authors tested a 10 min-longer duration.…”

“…The low value of the zeta potential of sludge from WWTP 3 can be an indicator of a higher electrophoretic rate during EDW. CST is a good indicator of mechanical dewatering but does not give any information on the suitability of EDW [34,37]. On the contrary, high conductivity values can improve the dewatering rate at the beginning of the polarisation phase due to increased measured current densities (Section 3.3).…”

“…Indeed, as already discussed, the lower fraction of volatile solids of the anaerobically digested sludge makes the mechanical dewatering to be more effective (see Table 2) [38]: a high fraction of volatile solids, typical of aerobically stabilised sludge, means that the sludge is rich in organic compounds, which are usually rich in bound water, and requires a great energy expenditure to be drained away by conventional treatments [27].…”

“…The present work is a follow-on of our last publication [27]. The sludge samples have been supplied from four different WWTPs around the metropolitan area of Milan (Italy), including both aerobically and anaerobically stabilised samples.…”

“…The EDW process has also been used as post-treatment to further dewater the sludge samples [8,11,14,[24][25][26]. Visigalli et al [27] tested sludges from different treatment processes and found that the EDW can achieve higher final DS than the conventional mechanical dewatering processes.…”