Electro-dewatering of sewage sludge: Influence of combined action of constant current and constant voltage on performance and energy consumption

Electro-dewatering performance of sewage sludge under interrupted pulsating voltage: A comparison between square shape and half-sine shape waveform

Electro-dewatering of sewage sludge with pulsating voltage was conducted under the two different wave shapes (square wave (SQW) and half-sine wave (HSW)) to investigate the influence of wave shape and duty cycle on sludge dewatering performance. The results indicated that, under the same average voltage, the moisture content of dewatered sludge with HSW was 10.3%-35.4% lower than that with SQW, suggesting the better dewatering performance of HSW. The optimal dewatering performance was achieved at duty cycle of 80% for SQW and 60% for SHW. The chemical oxygen demand of filtrate from HSW could be 13% higher than that from SQW, indicating the higher capacity of HSW in breaking sludge cells/floc structure. The applied voltage during electrochemical treatment promoted the hydrolysis of protein in filtrate, and the main components in the electro-dewatered filtrate were fulvic acid- and humic acid-like substances. The specific energy consumption for sludge electro-dewatering were 0.015-0.269 kWh/(kg removed water), and it was almost in linear relationship with duty cycle. By overall considering the energy consumption and electro-dewatering performance, the condition of 60% duty cycle with HSW was obviously better than other conditions, which provides a meaningful guidance for future application of sludge electro-dewatering technology with pulsating voltage.

Research progress in improving sludge dewaterability: sludge characteristics, chemical conditioning and influencing factors

Sludge from wastewater treatment processes with high water content and large volume has become an inevitable issue in environmental management. Due to the challenging dewatering properties of sludge, current mechanical dewatering methods are no longer sufficient to meet the escalating water content standards of sludge. This paper summarizes the characteristics of various sludge and raises reasons for the their dewaterability differences. Affected by extracellular polymeric substances, biological sludge is hydrophilic and negatively charged, which limits the dewatering degree. The rheological properties, flocs, ionic composition, and solid phase concentration of the sludge also influence the dewatering to some extent. For these factors, the chemical conditioning measures with simple operation and excellent effect improve its dewaterability, which mainly include flocculation/coagulation, acid/alkali treatment, advanced oxidation, surfactant treatment and combined treatment. There is a growing necessity to explore the development of new chemical conditioning agents, even though traditional agents continue to remain widely used. However, the development of these new agents should prioritize finding a balance between various factors such as efficiency, effectiveness, ease of operation, environmental safety, and cost-effectiveness. Electrochemical dewatering enhances solid-liquid separation, and its coupling with chemical conditioning is also an excellent means to further reduce water content. In addition, the improvement of press filter is an effective way, which is influenced by pressure, processing time, sludge cake thickness and pore structure, filter media etc. In general, it is essential to develop new conditioning agents and enhance mechanical filtration press technology based on a thorough understanding of various sludge properties. Concurrently, an in-depth study of the principles of mechanical pressure filtration will contribute to establishing a theoretical foundation for effective deep sludge dewatering and propel further advancements in this field.

Influence of alternating electric field on deep dewatering of municipal sludge and changes of extracellular polymeric substance during dewatering

A self-prepared experimental device made of plexiglass with alternating power supply system was used to study the deep dewatering of municipal dewatered sludge. Considering the reduction rate of sludge water content (W_r) as the index, factors affecting enhanced electric settlement of sludge such as exchange electrode method, voltage gradient, sludge thickness, and mechanical pressure were studied, and the dewatering mechanism was elucidated. The single-factor experiment combined with the surface response method based on the Box-Behnken central experimental design was performed. With W_r as the response value, the voltage gradient conditions, time ratio, and sludge thickness were optimized. Pearson correlation analysis showed that the reduction of proteins/polysaccharides was beneficial to improving the sludge dewatering effect. Tightly bound extracellular polymeric substances (TB-EPSs) showed a significant influence on the sludge dewatering effect. Under the action of the external electric field, particles with negative charge moved toward the anode sludge, water with partial positive charge flowed to the cathode, and the sludge cellular structure was damaged. This resulted in the dissolution of a large number of EPSs and the release of bound water. The anode sludge cake got thickened due to the accumulation of the sludge particles, leading to the increase in resistance. The TB-EPS was deconstructed by the ohmic heating to improve the sludge dewatering effect and achieve deep dewatering. Scanning electron microscopy results showed that the drying problem of anode sludge was alleviated during the dewatering process.

Experimental investigation on a combination of soil electrokinetic consolidation and remediation of drained water using composite nanofiber-based electrodes

A novel electrokinetic geosynthetic (EKG) can be efficient in achieving multiple objectives. In this study, a new EKG as an electrode and a drainage channel in the electro-osmotic consolidation was fabricated by electrospun nanofibers containing graphene nanoparticles (GNs) attached to a carbon fiber substrate. To investigate the effectiveness of the fabricated electrodes in electro-osmotic consolidation and remediation of water drained from the system, an experimental apparatus was constructed while considering loading capability in expanded ranges and applying the electric field, and was filled with copper (Cu)-contaminated kaolinite. Experiments were divided into control (CT) and EKG groups, and three categories, C-EK, ES₁-EK, and ES₂-EK (using carbon fiber, electrospun nanofibers containing 1 wt% GNs, and electrospun nanofibers consisting of 2 wt% GNs, respectively). All the experiments were conducted with the same conditions, loading, drainage condition, and duration. However, EKG experiments were performed by employing the electric field under the vertical pressure in the range of 7-113 kPa, while the CT was conducted without the electric field. According to experimental results, 18 wt% polymethyl methacrylate in the dimethylformamide solvent containing 1 and 2 wt% GNs was selected for making a nanofibrous layer on the carbon fiber. The average diameters of the fibers were 404 ± 36 and 690 ± 62 nm and yielded at 1 and 2 wt% GNs, respectively. The results revealed that using the EKG accelerated kaolinite consolidation. The average degree of consolidation was 68 and 85% in the CT and EKG experiments, respectively. Furthermore, the fabricated electrodes were highly effective as a drainage channel for remediating water drained from the system. Moreover, the highest Cu removal efficiency was obtained in ES₂-EK (97%) and ES₁-EK (92%), respectively. Conversely, the lowest Cu removal efficiency was observed in the C-EK group (85%).

Cationic polyacrylamide (CPAM) enhanced pressurized vertical electro-osmotic dewatering of activated sludge

Pressurized vertical electro-osmotic dewatering (PVEOD) has been regarded as a feasible method to achieve sludge deep-dewatering, but the dewatering efficiency is still challenged by high electric resistance. This study employed cationic polyacrylamide (CPAM) as a skeleton builder to enhance electro-osmotic flow in PVEOD. The sludge dewatering efficiency and synergistic effect of CPAM and PVEOD were elucidated. The sludge morphology, surface property, extracellular polymeric substances (EPS) destruction and migration, spatial distributions of proteins and polysaccharides, and current changes were investigated. After the addition of optimal CPAM dose, the sludge formed a uniform and porous structure that provided water channels and enhanced electric transport, thus promoting EPS destruction. The sludge moisture content (MC) analysis indicated the more liberation of bound water due to EPS destruction. Besides, the re-flocculation of disintegrated sludge flocs improved the sludge filtration and thus dewaterability. Instantaneous energy consumption (E_t,0.5) was optimized and two-step synergistic mechanism was thus proposed. These findings indicated that the combination of CPAM and PVEOD is a promising strategy to broaden the scope of industrial application of sludge deep-dewatering.

Impact of pH and removed filtrate on E. coli regrowth and microbial community during storage of electro-dewatered biosolids

Residual biosolids can be land applied if they meet microbiological requirements at the time of application. Electro-dewatering technology is shown to reduce biosolids bacterial counts to detection limits with little potential for bacterial regrowth during incubations. Here, we investigated the impacts on Escherichia coli regrowth and microbial communities of biosolids pH, removed nutrients via the filtrate, and inhibitory compounds produced in electro-dewatered biosolids. Findings suggest pH as the primary mechanism impacting E. coli regrowth in electro-dewatered biosolids. Propidium monoazide treatments were effective at removing DNA from dead cells, based on the removal of obligate anaerobes observed after anaerobic incubation. Analyses of high throughput sequenced data showed lower alpha-diversities associated with electro-dewatering treatment and incubation time. Moreover, biosolids pH and incubation period were the main factors contributing to the variations in microbial community compositions after incubation. Results highlight the role of electro-dewatered biosolids' low pH on inhibiting the regrowth of culturable bacteria as well as reducing the microbial community variance.

Enhancement of sludge electro-dewatering by anthracite powder modification

Electro-dewatering of sludge has received considerable attention due to its low energy consumption for sludge deep-dewatering. However, prior studies have shown the resistance of dried sludge near anode significantly hinders electro-dewatering. The dewatering performance may be improved by reducing the resistance with the addition of conductive material into sludge. We conditioned municipal sludge by anthracite powder, an inexpensive product, to increase solid conductivity, followed by electro-dewatering. After running for 20 min under a constant voltage of 30 V, when the anthracite powder mass was 10%-22% of raw sludge dry solids mass (DS), the final dry solids content of the mud cake after dehydration was 6.2%-12.9% higher than that from dehydration of unconditioned sludge. The average filtrate flow rate ranged from 0.0243 to 0.0285 g s^-1. The lowest unit energy consumption, 0.19 kW h·kg_water^-1, which was 14% lower than that of control, was reached when 18% DS of anthracite was added. Our theoretical analysis indicates that properly increasing solid conductivity of sludge can reduce the adverse effect caused by the high electrical resistance of sludge near anode. The experimental results, along with the theoretical analysis, show that using anthracite powder for sludge modification is an economical approach to improve sludge dewatering rate and reduce energy consumption.

Electro-dewatering of sewage sludge: Effect of near-anode sludge modification with different dosages of calcium oxide

The efficiency of common sludge electro-dewatering (EDW) is restrained by the following issues: 1, the near-anode sludge dries out quickly, causing a rapid increase in electrical resistance; 2, the pH at anode decreases by the accumulation of H⁺ from the electrolysis of moisture, resulting in a decrease in Zeta potential (ζ). Alleviating the negative impact of these problems is the key to improving the dewatering efficiency of EDW. Therefore, in this study, calcium oxide (CaO) was used for near-anode sludge modification to increase its pH and electrical conductivity. With increasing CaO dosage, pH rose from 6.0 to 12.2, electrical conductivity increased from 368 ± 16 μS/cm to 6285 ± 21 μS/cm and the ζ declined from -15.3 ± 0.6 mV to -8.8 ± 0.4 mV. The EDW tests were conducted at 30 V and 25.5 kPa. The results indicate near-anode sludge modification with CaO weighing 3%-5% mass of raw sludge (m_u(RS)) improved the EDW effect, while the energy consumption increased slightly. When 3%-5% m_u(RS) of CaO was added, the final moisture content of sludge was 54.5%-44.3%, below that of the blank group (no CaO added), which was 57.9%; the time to obtain target moisture content (60%) was 910 s-590 s, lower than the blank group's 1060 s; and the energy consumption to obtain target moisture content was 0.233 kW h/kg H₂O-0.271 kW h/kg H₂O, higher than the blank group's 0.157 kW h/kg H₂O. A quantitative criterion (Ksi_EDW) was adopted to assess the feasibility of EDW. Economically and energetically, the experiment with 4% m_u(RS) of CaO added for near-anode modification was the optimal condition in this research, due to its second smallest Ksi_EDW, the best sludge reduction effect (67.2%), lower final moisture content (46.2%) and less time (640 s) to obtain target moisture content. The results show some mechanisms of EDW and provide experience for practical application.

Dewaterability and energy consumption model construction by comparison of electro-dewatering for industry sludges and river sediments

Electro-dewatering (EDW) is an emerging technology for improved sludge/sediment dewatering enabling subsequent cost effective treatment for toxicity and pathogenic reduction if required and/or disposal, but the effects of sediment/sludge properties on the efficacy of EDW remain unclear. Here we investigate EDW in the absence of chemical conditioning which can result in secondary pollution. The influence of sediment/sludge volatile solids content (VS), electrical conductivity (EC), pH and zeta potential (ζ), on mechanical and electrical behaviors determining dewaterability and energy consumption (P_E) was investigated. Optimization of EDW parameters increased the final solids content (DS_f) from 40 wt% to more than 55 wt% for river sediment, while the solids content in municipal sludge was only increased from 10 wt% to 15-20 wt%. Multiple linear regression and statistical analysis showed that electro-dewatering performance is primarily affected by VS and P_E is mainly affected by EC. A theoretical basis for engineering design and selection of operational parameters for sludge/sediment electro-dewatering is provided by this study.

Electric Field Induced Dewetting of Hydrophobic Nanocavities at Ambient Temperature

The understanding of water dewetting in nanoporous materials is of great importance in various fields of science and technology. Herein, we report molecular dynamics simulation results of dewetting of water droplet in hydrophobic nanocavities between graphene walls under the influence of electric field. At ambient temperature, the rate of dewetting induced by electric field is significantly large. Whereas, it is a very low rate of dewetting induced by high temperature (423 K) due to the strong interaction of the hydrogen-bonding networks of water droplets in nanocavities. In addition, the electric filed induced formation of a water column has been found in a vacuum chamber. When the electric field is turned off, the water column will transform into a water droplet. Importantly, the results demonstrate that the rate of electric field-induced dewetting increases with growth of the electric field. Overall, our results suggest that electric field may have a great potential application for nanomaterial dewetting.

Effects of frequency and duty cycle of pulsating direct current on the electro-dewatering performance of sewage sludge

Electro-dewatering of sewage sludge with pulsating direct current (PDC-dewatering) was conducted to investigate the effects of pulsating frequency (0.01-60 Hz) and duty cycle (DTC) (20-100%) on sludge dewatering. The results indicated that both the frequency and DTC showed marked influences on electro-dewatering. Compared with the condition under the stable direct current (SDC-dewatering) of 30 V, the filtrate discharged from PDC-dewatering (at DTC of 40-60% and frequency of 30 Hz) was about 8% higher than that from SDC-dewatering. At DTC of 40%, the sludge electro-dewatering performance was promoted when the frequency increased from 0.01 Hz to 30 Hz. Compared with SDC-dewatering, PDC-dewatering can effectively mitigate ohmic heating. Layered tests were also conducted to investigate the differences of SDC- and PDC-dewatering in the distributions of water, pH, organic matters, zeta potential and conductivity in the upper, middle and lower layer of sludge cake. The results indicated that the variation tendencies of these parameters were similar between SDC- and PDC-dewatering, but the water, organic matters and charged ions in sludge cake were more homogeneously distributed during PDC-dewatering than SDC-dewatering. In addition, the anodic pH of PDC-dewatering was higher than that of SDC-dewatering, suggesting the potential of mitigating anodic corrosion during PDC-dewatering. Finally, energy consumptions of PDC- and SDC-dewatering were calculated and compared. The effects of frequency and DTC on energy consumption were investigated. PDC-dewatering was found to be more energy efficient than that of SDC-dewatering, making PDC-dewatering a promising electro-dewatering technology in future.

Enhancement of sludge electro-dewaterability during biological conditioning

Electro-dewatering (EDW) is considered as one of the most promising dewatering technologies due to saving power consumption. In this study, the potential effects of anaerobic biological conditioning (BC) on sludge EDW treatments was investigated. The results showed that without any additives BC pretreatment of sludge enhanced EDW dewaterability and energy efficiency. At 35 °C BC for 3 days, the dry solids (DS) of sludge dewaterability limit could increase up to 49%, which corresponded to an increase of 13% of DS in dewatered sludge cake without BC pretreatment, and the dewatering time was shortened by 22%. There was an economic advantage saving in energy consumption of around 49.5% in the case of BC-EDW when the DS of sludge was up to 38%. Then, the mechanism of BC to improve EDW performance was studied. The principal component regression (PCR) analysis showed that the DS content of dewaterability limit mainly depended on the degradation of organic matter and the change of conductivity in sludge. Fourier transform infrared spectroscopy (FTIR), zeta potential and bound water in sludge were also determined in an attempt to explain the observed changes in sludge BC-EDW. It was indicated that the increase of negatively charged hydroxyl groups on the surface of sludge particles resulted in an increase of the absolute value of the zeta potential and significantly promoted EDW. The tightly bound EPS (TB-EPS) decreased and it loosened the bond between water or metal cations and sludge particles, and the bound water was also found to be released into free water in sludge during BC.

An effective deep dewatering process of coupling biological conditioning and electro-dewatering was proposed and analyzed.

What Advanced Treatments Can Be Used to Minimize the Production of Sewage Sludge in WWTPs?

Similar to other types of waste, sewage sludge (SS) must be minimized, not only to respect the European Directive 2018/851 on waste, but also because the cost of sludge management is approximately 50% of the total running costs of a wastewater treatment plant (WWTP). Usually, minimization technologies can involve sewage sludge production with three different strategies: (i) adopting a process in the water line that reduces the production of sludge; (ii) reducing the water content (dewatering processes) or (iii) reducing the fraction of volatile solids (stabilization). This review, based on more than 130 papers, aims to provide essential information on the process, such as the advantages, the drawbacks and the results of their application. Moreover, significant information on the technologies still under development is provided. Finally, this review reports a discussion on the impact of the application of the proposed processes in the sludge line on a WWTP with a capacity exceeding 100,000 population equivalent (PE).