Scientific Research

Sewage sludge originates from the process of waste water treatment. Assuming to take away as many “pollutants” as possible from wastewater, how to efficiently and effective treat sewage sludge for further added-value re-uses? The progressive implementation of the Urban Waste Water Treatment Directive 91/271/EEC in all Member States is increasing the quantities of sewage sludge requiring disposal: in the EU-27, the current practices for sewage sludge disposal are landfill (18%), thermal treatment/incineration (23%), composting (7%), agricultural use (45%) and others re-uses (7%).
Beneficiaries intend to move away from sewage sludge disposal as land filling, as this gradually has to be phased out by EU (as stated, i.e., in the Waste Framework Directive 2008/98/EC, and Directive 99/31 on landfill of waste), and agricultural use as fertilizers, as the latter presents strong limitations coming from organic and inorganic pollutants that sewage sludge may contain. On the other hand, sludge thermal drying for further re-use instead of disposal, still needs considerable amounts of fossil fuel to provide the heat necessary to the process and thus remains an unexplored (but added-value potential) research and market area.

Current dewatering technologies are one of the key bottlenecks limiting the share of sewage sludge which can be re-used, since water content remain too high (up to 75-80%). Today’s dewatering standard dryness of the sludge produced by industrial wastewater treatment, treatment of organic liquid wastes and anaerobic digestion of agricultural wastes (animal slurries, manure, agricultural residues), usually lies in the usual range 20 to 30% dry solids.
The minimum legislative requirements for thermo-valorisation requires that the sewage sludge is dried up to about 30-45% dry solids in order to self-sustain (with a sufficient calorific value) combustion at 850°C . At lower dry-solid content, pre-heating the combustion air by adding additional fossil fuel (such as methane) is necessary to remove the water content from the sludge.
Alternatively, sewage sludge can be dried up to 90% dry solid content in drying ovens, which are usually heated by fossil fuels and then used in cement kilns or brick furnaces as both additional fuel (for its combustible volatile fraction) and inert filler (for its inert fraction).
Current commercial centrifuges can reach up to around 30% dry-solid content when treating anaerobically digested municipal sewage sludge, but this figure generally reduces to 23-24% for aerobically stabilised sludge, especially for sludge deriving from the combined treatment of industrial wastewater with domestic sewage. Filter-presses achieve up to 40% dry-solid content in digested municipal wastewater sludge and 30% in aerobically stabilised sludge, at the cost of adding 15 to 25% (w/w dry solids) chemical additives, usually a combination of lime and ferric chloride.
Any action addressed to recover useful materials or energy from sludge could be positively influenced by application of actions aimed at reducing the amount of water in the sludge.

Therefore it is important to develop an innovative dewatering process capable to increase water removal from sludge (expected up to 50% of dry content), well beyond the range of conventional technologies, thus optimising the overall energy balance (lower mass to be transported to final disposal/recycling sites and increasing of the calorific power in the dewatered sludge).

Electro-osmosis, forcing water molecules to move out from sewage sludge and pass through a porous filter when dragged by an applied electrical field, appears an energy efficient alternative solution to current technologies to bring sewage sludge from 20-25% up to 45-50% dry-solid content. While sludge electro-dewatering has been successfully applied for specific industrial application in food and pharmaceutical sectors, electro-dewatering in wastewater treatment is still in an R&D phase mainly because of the complexity that comes from controlling the electrochemical process in the wastewater sludge, variable in both its chemical composition and physical properties.
The basic principle of electro-osmosis application to sewage sludge dewatering has been formerly taken into account in literature, using different machines based on the electro-osmotic process. But there is still much to do to improve sewage sludge quality and dry content for further re-use, from the ST point of view.
The SLUDGEtreat beneficiaries intend to carry out a sound Transfer of Knowledge (ToK) in the sewage sludge treatment domain, with these two main objectives:

1. Boosting the researchers of the future, with a broad view of the environmental, engineering, electro-chemical and regulative aspects of the sewage sludge treatment.
• Creating a long lasting research network involving both the commercial and non-commercial sectors to foster co-operation based on joint research projects;
Promoting innovation and knowledge transfer through secondment of researchers;
• Providing different career possibilities & research experience for researchers, knowledge sharing/exchange;
• Enabling researchers to orient their innovative results to the actual needs of SMEs in the field, that are typically different from the needs of large organizations in the software market, thus supporting SME-academia partnerships.

2. Investigating the application i) of the electro-osmosis process to sewage sludge dewatering resolving issues related to reliability, efficiency (right dimensioning, process parameters, energy consumption) and ii) of nanomaterials against corrosion as a basis for the development of a novel dewatering system as well as advanced coatings deposition technologies, leading to the following benefits:
• Water extracted from sewage sludge through the electro-osmotic process requires significantly lower energy (about 1/3) than the one required by thermal drying.
• The increase of dry-solid content up to 50% drastically reduces sludge volumes to be transported to final disposal, limiting CO2 emissions and costs of transport.
• The increased calorific power of the dried sludge can be efficiently used in a self-sustaining thermo-valorization process, apt to energy recovery, as both electric power and heat.

The proposed project addresses specific scientific and technological objectives:

a. To investigate novel eco-friendly nanomaterials and coating techniques able to minimize the corrosion due to acid environment and friction, to be applied in the anode. New materials for anode should be studied to improve the performances of the machine, by reducing anodic corrosion. The main research questions to be addressed are: the conjugation of sufficient conductivity with anticorrosion properties as well as mechanical resistance to abrasion and shear forces at the anode.

b. To maintain the highest possible energy-efficiency of the electrochemical process when sludge dryness increases during the process. The project will quantify the actual energy balance of an entire sludge treatment train at a conceptual level, from the sludge thickening with the proposed solution to its final re-use. In the case of thermal valorization of sludge, the study will show the extent to which the electro-dewatering process can help in achieving a highly positive energy output by producing a high dry-content sludge, compatible with self-sustaining combustion in an incinerator or a gasifier. The overall energy result will be expressed as the net energy produced per unit mass of Dry Solids (kWh/kgDS) or per unit mass of additional water removed (kWh/kgH2O). The overall environmental impact will also be evaluated through a Life Cycle Assessment procedure. A deep comparison with conventional sludge treatments will be performed on the technical aspects and by means of lab-scale tests.

c. To design and develop a proof-of-concept dewatering system for sewage sludge based on electro-osmosis.  The beneficiaries will jointly prepare the specifications to lead the researchers design the new system. The prototypal machine will consist of:

• An electro-osmotic chamber, inside a casing, in rotating status during filtration.
• An electrode apparatus located inside the chamber in order to apply a difference of potential.

The main results of the project will be to 1) provide researchers with new research skills and broad horizons in electro-osmotic process, oriented to the eco-friendly dewatering of sewage sludge and to 2) develop a proof-of-concept energy efficient dewatering system.