Electrokinetic Dewatering of Sewage Sludge with Fixed and Moving Electrodes: Attenuation Mechanism and Improvement Approach

Researchers from the MOE Key Laboratory of Soft Soils and Geo-environmental Engineering in Zhejiang University (China) presented a laboratory study on Electrokinetic Dewatering (EKD) of sewage sludge with fixed and moving electrode. The paper appeared in Journal of Environmental Engineering.

Dewatering of sewage sludge due to increasing sludge volume has been a critical step in curbing detrimental impact it offers to the society. Mechanical means such as centrifugation and filter pressing has been widely used in dewatering but it offers technical difficulty and relative cost as it only dewaters sludge to water content of 80%. This led to Electrokinetic Dewatering (EKD) of sludge based on electroosmosis mechanism to drive water movement from anode to cathode by applying direct current electric field porous medium.

The use of Electrokinetic Dewatering with non-fixed anode has been reported. The use of rotational anode driven by a motor could enhance the Electrokinetic Dewatering process significantly due to refreshment of dry sludge cake near the anode. However in the application of this technique location of anode and distance between anode and cathode did not change during Electrokinetic Dewatering.

Zhan et al. (2016), hereby proposed a moving anode technique to resolve attenuation in Electrokinetic Dewatering. Further comparative study on moving-anode Electrokinetic Dewatering and conventional fixed-dewatering Electrokinetic dewatering was made to provide insight on their dewatering effect and energy consumption.

The sludge sample used for the experiment had water content of 82.1% and remaining organic content of 28.72% with bulk density of 1.05g/cm³. Electrokinetic Dewatering experiments set up for fixed-spacing electrodes were conducted in two sizes of horizontal perplex cylinder cells; the small size and middle size. The position of cells in middle-size was made adjustable in order for changes in electrode distance.

Set up for Electrokinetic Dewatering experiments with moving anode consisted of a rectangular perplex container with a fixed cathode made of stainless steel plate and movable anode array made of 25 stainless steel rods. Movement of anode arrays toward the cathode is driven manually or by a motor.

Sludge specimens were treated under different experimental conditions with respect to voltage gradient, electrode distance and treating time. For the small-sized cell, voltage gradient from 2 to 8V/cm was applied with decreasing treating time (min) at different test from T1 to T6. Middle-sized cell made use of two different electrode distance of 16cm and 1cm from T7 to T12 where constant current instead of constant voltage was applied at some stages (T13 to T14).

Six test was carried out for moving anode Electrokinetic Dewatering experiment. First three test (M1 to M3) allowed movable anode array move slowly towards the cathode while the other three test had movable array fixed (F1 to F3) throughout with both having a treatment time of 50, 100 and 150min.

Attenuation results for experiment goes thus; From T1 to T4, the higher the voltage applied, the more significant the observed attenuation. For T7 to t10, attenuation in electric current and flow rate was more significant when electrode spacing reduced. This was attributed to decrease in length of electrokinetic flow path from anode to cathode. Electroosmotic permeability K_e of the sludge at the beginning of the test decreased from beginning at 7.29×10^-5cm²/s to 1.02-4.5×10^-5cm²/s after electrokinetic dewatering (the higher the applied voltage gradient, the lower the K_e). This results show a linear relationship between electric current and electroosmotic flow rate.

Dewatering effect for fixed electrode test T1 to T6 was shown to be non-uniform and gradually decreased from anode to cathode in distribution profile to residual water content. Dewatering boundary gradually moved from anode to cathode with an increase in treating time. From test T11, T12 and T7 confirmed attenuation of dewatering effect. As dewatering boundary advanced, dewatering effect gradually became less significant.

Results from dewatering processes and Electrokinetic attenuation associated with fixed-electrode Electrokinetic Dewatering showed that attenuation in voltage gradient for underwatered section result in a proportional attenuation in  both electrical current and flow rates as well as attenuation in dewatering effect along dewatering direction. The linear relationship formed between residual water content and corresponding voltage for previous test on T1-T4, T7 and T8 demonstrated that dewatering effect near the dewatering boundary was determined by effective voltage gradient left for underwatered section.

Comparison results between fixed-electrode and moving-anode tests showed that only a slight attenuation was observed for electroosmotic flow and electric current was recorded for M3 while significant attenuation was observed for measurement in test F3 (fixed anode). For M1 to M3, relative uniform distribution of residual water content of about 75.5% (dewatering limit) was obtained with M3 and treatment duration of 150min and M1-M2 at lower treatment time. For F1-F3 significant attenuation was confirmed as only the residual water content near the fixed-anode reached dewatering limit of 75.5% at 150min.

Estimated energy consumption using (Yuan and Weng, Advances in Environmental Research,, 7, 727–732, 2003) equation for moving-anode Electrokinetic Dewatering increases significantly with an increase in velocity gradient. However, it offers lower energy consumption and better dewatering effect than fixed-electrode experiments. At 8V/cm voltage gradient, sludge water content reduced from 82.1% to 62.2% for moving-electrode while fixed electrode decreased to 73.6% and its estimated energy consumption can be as low as 18KWh/m³.