The sewage sludge dewatering produced by Wastewater Treatment Plants is a multifaceted process due to the presence of colloid fractions. Electro-osmosis could be a suitable technique to reduce the water content of the final sludge. Electric fields of 10 V/cm, 15 V/cm and 20 V/cm have been studied for electro-osmosis tests under the pressure of a static or rotating piston, obtaining a dry solids content up to 40-45%, with respect to 25-30% obtained by mechanical methods. In order to optimise the process, the corrosion behaviour and the wear of the anodic material appear the main critical aspects, due to the high circulating current density and the use of a rotating electrode. We compared the efficiency and the corrosion resistance of dimensionally stable anodes (DSA) with respect to bare stainless steel (AISI 304) and stainless steel coated by PVD technique with TiN, AlTiN and DLC. Characterization of the anode surfaces by SEM and potentiodynamic tests, show that DSA is the most suitable material for our application. However, efficiencies of the electro-osmosis processes have been found comparable, in terms of developed current densities and total energy consumptions, for short-test duration.Keywords: electro-osmosis; dewatering; sludge; DSA; anode; corrosion. IntroductionThe so-called "activated sludge process" is adopted by almost the totality of urban and industrial wastewater treatment plants to achieve the concentration limits of biodegradable pollutants stated by the law to allow the discharge of treated effluents to natural water bodies (rivers, lakes and the sea).About half of the organic pollution load treated by the activated sludge process is oxidised and converted into water and carbon dioxide, while the remaining is converted into biomass, called "excess biological sludge" or "waste sludge". After reducing both the content of biodegradable matter and the water content through mechanical dewatering or thermal drying, the sludge becomes a product suitable for its final disposal.When compared with thermal (evaporative) processes for water reduction, mechanical dewatering is often selected due to its low energy requirement. The processes of mechanical dewatering are largely developed on the industrial scale and can produce sludge with 20-25% of dry solid (DS) content and, in some cases, up to 30%. However, the high DS values demanded for thermal valorisation of sludge cannot be achieved by mechanical dewatering techniques. , and Tuan et al. [10] reported that the pH drop relates to the reduction of the absolute value of the zeta potential, thus the decrease in pH (specifically at the anode) reduced the electro-osmotic flow during direct current application [1, 5,8,11].The oxidation of the anode material, due to oxygen evolution, ohmic heating and pH decrease, reduces the process efficiency [12] and can cause in some applications the contamination of the filter cake or filtrate, increasing the operating cost. Anodes such as stainless steel will be subject to

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.

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The objective of this study is to evaluate the feasibility of implementing electro-dewatering (EDW) as an add-on unit to the existing conventional dewatering units with the aim of increasing the final dry solids content and reducing the subsequent handling and energy costs of sewage sludge management. The assessment was carried out by focusing on a case study, a small wastewater treatment plant (WWTP) in the Milan metropolitan area. Various indicators were used to evaluate the environmental impact and economic performance. Primary data, such as operating data from the case study WWTP and economic data from an EDW equipment manufacturer, were extracted and used in the modelling. Four scenarios were set up and compared, which address the current and future sludge management schemes in Italy. The results suggest that it is environmentally and economically feasible to implement the EDW upgrade if the sludge disposal follows the incineration route. More specifically, when small WWTPs deliver their EDW-dewatered sludge to a centralised incineration facility, this will enable to reduce the global warming impact of the system up to 135 kg CO2-eq. per dry tonne of sludge. In addition, good profitability (incremental return on investment > 15.1%) can be obtained when the market disposal cost is above 30.5-39.6 € per wet tonne of sludge. Based on our recent market survey, the sludge disposal price is well above the break-even values.

Digital textile printing (DTP) is a game-changer technology that is rapidly expanding worldwide. On the other hand, process wastewater is rich in ammoniacal and organic nitrogen, resulting in relevant issues for discharge into sewer system and treatment in centralized plants. The present research is focused on the assessment of the partial nitritation/anammox process in a single-stage granular sequencing batch reactor for on-site decentralized treatment. The technical feasibility of the process was assessed by treating wastewater from five DTP industries in a laboratory-scale reactor, in one case investigating long-term process stabilization. While experimental results indicated nitrogen removal efficiencies up to about 70%, complying with regulations on discharge in sewer system, these data were used as input for process modelling, whose successful parameter calibration was carried out. The model was applied to the simulation of two scenarios: (i) the current situation of a DTP company, in which wastewater is discharged into the sewer system and treated in a centralized plant, (ii) the modified situation in which on-site decentralized treatment for DTP wastewater is implemented. The second scenario resulted in significant improvements, including reduced energy consumption (− 15%), reduced greenhouse gases emission, elimination of external carbon source for completing denitrification at centralized WWTP and reduced sludge production (− 25%).