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eco-friendly and energy efficient sewage SLUDGE dewaTeRing through novEl nanomAterials and elecTro-osmotic process

Final Report Summary - SLUDGETREAT (eco-friendly and energy efficient sewage SLUDGE dewaTeRing through novEl nanomAterials and elecTro-osmotic process)

Project objectives
The main aim of the SLUDGETREAT project is to develop an innovative dewatering process capable to reduce the water content in sludge and increase the dry solid (DS) content through electro-dewatering (EDW): from 18-25% DS (typical of most sludges after mechanical dewatering) to 35-40% DS by the application of an electric field. This will reduce the overall energy balance as it will reduce transportation and disposal costs and produce sludge with a higher calorific value suitable for direct incineration without preliminary thermal drying.

Description of the work performed since the beginning of the project
During the entire duration of the project, the team focused on the following aspects: design and building of a lab-scale device for electro-dewatering tests; study of operating conditions (time, electric field and pressure) that could give the highest DS content in sludge, with the lowest energy consumption; investigate sludge characteristics affecting pressure-driven electro-dewatering; research of wear and corrosion resistant materials to be used as anode; assess the life cycle of the electro-dewatering process; assess the costs of the electro-dewatering process (compared to conventional dewatering methods) and its influence on sludge disposal costs; design and build a full scale electro-dewatering prototype; test the electro-dewatering prototype on field.

Preliminary tests were carried out by a lab scale device on sludge samples taken from five different wastewater treatment plants.

Generally, the DS content of sewage sludge samples increased up to 45-50% at the highest electric potentials applied and the lowest cake thicknesses. Moreover, when the electric potential was lower than 25 V, EDW allowed to consume less specific energy than thermal drying (in terms of primary energy).
The characteristics and properties of sludge that affect electro-dewatering have been studied and reported in three papers presented at three conferences (ASSM 2016, Cracow, PL; FILTECH 2016, Cologne, DE; SLUDGETECH 2017, London, UK) and in a paper published in Environmental Research (doi: 10.1016/j.envres.2017.05.015).
The data achieved from the EDW tests have been used for a life-cycle assessment and for an economical analysis of the process. These results have been reported in two papers published in Journal of Environmental Chemical Engineering (doi: 10.1016/j.jece.2017.11.034) and Journal of Cleaner Production (doi: 10.1016/j.jclepro.2018.11.044) and shown at a conference in Rome (SMICE 2018).
In order to optimise the process, corrosion resistance of the anodic material is the main critical aspect, due to the high circulating current density, the increase of acidity and the oxygen evolution. We compared the efficiency and the corrosion resistance of dimensionally stable anodes (DSA, the most used in this application) with respect to bare stainless steel (AISI 304) and stainless steel coated by PVD and HIPIMS techniques with TiN, AlTiN, DLC, AlTiSiN and TaN. Aiming at the same goal, multi-walled carbon nanotubes (MWCNTs) composite coatings on carbon steel have been studied. Characterization of the anodes samples by SEM and potentiodynamic tests have shown that corrosion resistance of the DSA is the most suitable for our application. Localized corrosion was visible on both bare and PVD coated stainless steel, while generalized corrosion was observed on MWCNT coatings. However, efficiencies of the electro-osmosis process for all the materials used have been found comparable, in terms of developed current densities and total energy consumptions, for short-test durations. Preliminary results on the materials to be used as anodes in the EDW process have been shown at the Conference SMT30 (Milan, 2015) and in a paper published in Materials and Manufacturing Processes (doi: 10.1080/10426914.2017.1279313).
From the data collected by means of the EDW tests with the lab scale device, the full scale EDW prototype have been designed, built and tested at Carpi's wastewater treatment plant. The results achieved with the first release of the EDW machine evidenced an increase in DS content of the sewage sludge when the electric field was switched on. A second release of the EDW prototype was built starting from January 2019 and will be tested in April 2019.
In order to study the potential market of the EDW machine, lab scale tests have been carried out on olive paté for animal feed and on two types of industrial sludge.

Expected final results and their potential impact and use
During the initial stages of the SLUDGETREAT project, the EDW technology was thought to be applied in addition to existing dewatering machines, to reach a dry matter of 40% or more. However, the lab-scale tests and the preliminary tests with the EDW prototype evidenced two different possible uses of the machine:
- as a standalone process to be applied on liquid thickened sludge (DS = 2-4%), in order to increase the DS content up to 30-35% with limited energy consumptions. In this case, the EDW technology is thought to share the market with highly efficient centrifuges
- as an additional stage to be applied to mechanically dewatered sludge (DS = 15-20%), in order to get a final DS content of 45-50%, leading to an increase of the calorific value of sludge and allowing self-sustained incineration without preliminary thermal drying treatments.

In particular, with the possible industrial application of the novel technology as a standalone system, the team may enter deeply the niche markets. In addition to improved solid content from conventional dewatering machines, the innovative solution shall be further improved to cover the interesting market of small municipal wastewater treatment plants (0-10.000 PE) that usually have no equipment and are currently used to transport sludge to a WWTP or to pay subcontractor with a mobile unit (with very expensive management costs).
In addition to sewage sludge dewatering, there are other possible exploitable markets such as dewatering of residues from the food industry. The demand for niche and novel solutions for dewatering of slurries, sludges and other semi-solid-wastes will grow further due to rising demand from emerging markets and will increase its market share.
This will open sound opportunities for electro-dewatering as a leading solution in the field of dewatering. As it can affordably reduce the water content of these wastes in a sustainable manner, it has the potential to enhance and implement conventional dewatering processes, producing a drier material that has a higher potential to be exploited as biomass in recycling schemes (composting, thermal treatments, etc.) within the concept of circular economy. This will widen also the choice for more effective dewatering technologies, increase market opportunities and create highly qualified job opportunities. The patent regarding the final version of the machine is pending (italian patent application n° 102017000122179, deposited on the 26th October 2017, titled “Dispositivo per il trattamento di fanghi")

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