A comparative study of electro-dewatering process performance for activated and digested wastewater sludge.

Electro-dewatering (EDW) is an alternative emerging and energy-efficient technology that provides improved liquid/solids separations in the dewatering of wastewater sludge. The EDW technology is not only an innovative dewatering method for significantly reducing the volume of wastewater sludge before re-utilization or disposal, but is also a promising emerging method which may potentially be used for decontamination purposes. In this study, the influence of the sludge properties (e.g. electrical conductivity, zeta potential, specific cake resistance, among others) on their mechanical and electrical behaviour in terms of dewaterability and electro-dewaterability, the applied current (current density from 20 to 80 A/m2), and filter cloth position relative to the electrode was investigated. A two-sided filter press at lab-scale with moving anode was used, and the treatment performance of the EDW process on two different types of wastewater sludge (activated and digested) was thoroughly assessed from both an electrochemical viewpoint and in terms of the dewatering rate. The results showed that the conditioned digested sludge was more easily dewatered by mechanical dewatering (MDW) with 34-35% (w%) of dry solids content compared to 19-20% (w%) for the activated sludge, thanks to the lower content of both the microbial extracellular polymeric substances (EPS) and the volatile suspended solids fraction. For the EDW results, the electrical conductivity of the sludge was pivotal to the dryness of the final solids and therefore also to the dewatering kinetics. The results demonstrated that the activated sludge arrived at an equilibrium much faster (after approximately 3600 s) compared with digested sludge, thanks to its lower electrical conductivity (0.8 mS/cm) providing a greater voltage drop across the cathode and therefore more repulsion of the solids from the cathode leading to continuously high filtrate flowrate. Also the EDW performance was analysed by comparing the ratio of the filtrate volume collected at the anode to the volume collected at the cathode side. For digested sludge at 5 bar, 40 A/m2 different positions of the filter cloth were tested but these configurations barely impacted the EDW performance, despite having a significant impact on the energy requirements. At industrial scale, it would be useful to position the filter cloths at some distance from the electrodes, but this study shows that this benefit may be quickly outweighed by the loss in EDW energy efficiency.

Influence of process operating parameters on dryness level and energy saving during wastewater sludge electro-dewatering.

Electrically assisted mechanical dewatering, known as electro-dewatering (EDW), is an alternative emerging technology for energy-efficient liquid/solids separation in the dewatering of wastewater sludge. In this study, the performance of the electro-dewatering (EDW) process for activated wastewater sludge was investigated. The influence of the operating modes; being the timing of voltage (U-EDW) or current (I-EDW) application to either or both the filtration and compression stages, and the influence of the applied pressure (in successive 30 min pressure steps) were studied. The results showed that by delaying the application of the electric field to the filter cake compression stage, there was a potential saving in power consumption of around 10-12% in the case of U-EDW and about 30-46% in the case of I-EDW. The increase of the applied pressure from 0.5 to 12 bar during the filter cake compression stage leads to an increase in electro-dewatering kinetics. The results also reveal that at a low electric field level the increase of the processing pressure has a relatively pronounced effect on the dewatering process. At high levels of the electric field, a minimum processing pressure (4-6 bar) is required to improve the electrical contact between the electrode and the sludge and thus lower the energy consumption.

An evaluation of a hybrid ion exchange electrodialysis process in the recovery of heavy metals from simulated dilute industrial wastewater.

Hybrid ion exchange electrodialysis, also called electrodeionization (IXED), is a technology in which a conventional ion exchange (IX) is combined with electrodialysis (ED) to intensify mass transfer and to increase the limiting current density and therefore to carry out the treatment process more effectively. It allows the purification of metal-containing waters, as well as the production of concentrated metal salt solutions, which could be recycled. The objective of this paper was to investigate the ability of the IXED technique for the treatment of acidified copper sulphate solutions simulating rinsing water of copper plating lines. A single-stage IXED process at lab-scale with a small bed of ion exchanger resin with a uniform composition was evaluated, and the treatment performance of the process was thoroughly investigated. The IXED stack was assembled as a bed layered with the ion exchanger resin (strong acid cation-exchange Dowex™) and inert materials. The stack configuration was designed to prevent a non-uniform distribution of the current in the bed and to allow faster establishment of steady-state in the cell for IXED operation. The influence of operating conditions (e.g. ion exchanger resin with a cross-linking degree from 2 to 8% DVB, and current density) on IXED performance was examined. A response surface methodology (RSM) was used to evaluate the effects of the processing parameters of IXED on (i) the abatement yield of the metal cation, which is a fundamental purification parameter and an excellent indicator of the extent of IXED, (ii) the current yield or the efficiency of copper transport induced by the electrical field and (iii) the energy consumption. The experimental results showed that the performance at steady-state of the IXED operation with a layered bed remained modest, because of the small dimension of the bed and notably the current efficiency varied from 25 to 47% depending on the conditions applied. The feasibility of using the IXED in operations for removal of heavy metals from moderately dilute rinsing waters was successfully demonstrated.

Electro-dewatering of wastewater sludge: influence of the operating conditions and their interactions effects.

Electric field-assisted dewatering, also called electro-dewatering (EDW), is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. The application of these additional fields can be applied to either or both dewatering stages (filtration and/or compression), or as a pre-or post-treatment of the dewatering process. In this study, the performance of the EDW on wastewater sludge was investigated. Experiments were carried out on a laboratory filtration/compression cell, provided with electrodes, in order to apply an electrical field. The chosen operating conditions pressure (200-1200 kPa) and voltage (10-50 V) are sufficient to remove a significant proportion of the water that cannot be removed using mechanical dewatering technologies alone. A response surface methodology (RSM) was used to evaluate the effects of the processing parameters of EDW on (i) the final dry solids content, which is a fundamental dewatering parameter and an excellent indicator of the extent of EDW and (ii) the energy consumption calculated for each additional mass of water removed. A two-factor central composite design was used to establish the optimum conditions for the EDW of wastewater sludge. Experiments showed that the use of an electric field combined with mechanical compression requires less than 10 and 25% of the theoretical thermal drying energy for the low and moderate voltages cases, respectively.

Electrical field: a historical review of its application and contributions in wastewater sludge dewatering.

Electric field-assisted dewatering, also called electro-dewatering, is a technology in which a conventional dewatering mechanism such a pressure dewatering is combined with electrokinetic effects to realize an improved liquid/solids separation, to increase the final dry solids content and to accelerate the dewatering process with low energy consumption compared to thermal drying. Electro-dewatering is not a new idea, but the practical industrial applications have been limited to niche areas in soil mechanics, civil engineering, and the ceramics industry. Recently, it has received great attention, specially, in the fields of fine-particle sludge, gelatinous sludge, sewage sludge, pharmaceutical industries, food waste and bull kelp, which could not be successfully dewatered with conventional mechanical methods. This review focuses on the scientific and practical aspects of the application of an electrical field in laboratory/industrial dewatering, and discusses this in relation to conventional dewatering techniques. A comprehensive bibliography of research in the electro-dewatering of wastewater sludges is included. As the fine-particle suspensions possess a surface charge, usually negative, they are surrounded by a layer with a higher density of positive charges, the electric double layer. When an electric field is applied, the usually negative charged particles move towards the electrode of the opposite charge. The water, commonly with cations, is driven towards the negative electrode. Electro-dewatering thus involves the well-known phenomena of electrophoresis, electro-osmosis, and electromigration. Following a detailed outline of the role of the electric double layer and electrokinetic phenomena, an analysis of the components of applied voltage and their significance is presented from an electrochemical viewpoint. The aim of this elementary analysis is to provide a fundamental understanding of the different process variables and configurations in order to identify potential improvements. Also discussed herein is the investigation of the electrical behaviour of a porous medium, with particular emphasis on porous medium conductivity determination.