PROJECT DESCRIPTION
BACKGROUND
Overall waste generation is stable in the EU. However, generation of sewage sludge is still increasing, partly as a result of the progressive implementation of the Urban Waste Water Treatment Directive (91/271/EEC). Between 1992 and 2005, production of dry sewage sludge in the EU increased from around 5.5 million tonnes to nearly 9 million tonnes. Current trends suggest that EU27 production will reach 13 million tonnes by 2020. By 2020, all waste should be managed as a resource and landfilling of organic material should be virtually eliminated. However, to achieve this for sewage sludge and avoid landfilling or incineration new technologies are necessary. These technologies need to take account of the fact that sludge tends to concentrate heavy metals and poorly biodegradable trace organic compounds, as well as potentially pathogenic organisms. Current environmental issues related to the recycling of sewage sludge on land include the risk of nutrient leaching, impacts on soil biodiversity and greenhouse gas emissions. As well as the presence of heavy metals and other inorganic elements, there is also concern over the presence of endocrine-disrupting chemicals, including natural and synthetic hormones, and less potent industrial agents such as phthalates. Wastewater treatments need a multidisciplinary approach to addressing water quality, sewage sludge management, pollution - organics, nitrates and phosphorous - derived from sludge application, and the overall water-energy nexus.
OBJECTIVES
The Lo2x project aimed to show the environmental and socio-economic benefits of using wastewater treatment plants as a means of resource recovery, not only from sludge but also from other waste streams with similar or worse effects on water quality. It specifically set out to demonstrate the synergies resulting from co-treatment of sewage sludge and other wastes - including raw/digested manure, high-load food processing wastes, pesticides and leachates based on supercritical water co-oxidation technology, with energy and phosphorous recovery. The project's goal was to design and construct a prototype for the co-oxidation in supercritical water of mixtures of sludge and other wastes. This would be tested at a wastewater treatment plant in Valencia, a region of high water stress and high levels of nitrates and pesticides in sources of drinking water. The mix was expected to provide enhanced performance through synergies, with the sludge providing embedded energy from the organic content, animal manure and slurries providing for phosphorous recovery and ammonia elimination, and hazardous and toxic streams such as landfill leachates and drencher wastewater providing a gate fee for their treatment. The process was expected to result in the complete destruction of organic toxic substances - pesticides and leachates. The beneficiary planned to define optimum operating conditions and mix ratios for best process yield including optimum recovery of phosphorous - and energy balance. Another objective was to reduce the final amount and environmental impact of waste generated by treatment plants. It also expected to show economic benefits from: reductions in the cost of sludge management treatment and disposal; exploitation of recovered phosphorous; and improved energy balance. The success of the project can contribute to the achievement of European objectives related to reducing landfill of organic wastes, energy efficiency, water quality, and the European Resource Efficiency Roadmap.
RESULTS
The Lo2x project built a demonstrative supercritical water co-oxidation plant at the Paterna wastewater treatment facility in Valencia. This is capable of treating near half a tonne of dry matter per day, this means a quarter of the sludge processed during wastewater treatment. The pilot plant was tested in 7 scenarios, operating in continuous mode, to study the versatility of the process. The beneficiary conducted successful trials with sludges and different co-substrates, including olive mill wastewater, pig slurry, cow manure, drencher wastewater and leachates. The Lo2x technology was found to mineralise nearly 100% of nitrogen in ammonia form and close to 100% in the liquid phase, facilitating the application of different commercial nitrogen recovery technologies (e.g., stripping). The nutrients recovered could be commercialised as an alternative to mineral fertilisers. Phosphorous content of the sediment was around 23% on average. It can be recycled for use as a secondary raw material, in line with the goals of circular economy policy and the Waste Framework Directive's provisions on the recovery of waste. The supercritical water co-oxidation treatment performed in the prototype reduced chemical oxygen demand by above 99% and removed more than 90% of total solids, that means a reduction of amount of sewage leaving the wastewater treatment plant (i.e. lorries) about 98%. It also removed 100% of pathogens from sludge containing pesticides and recaptured above 85% of heavy metals for safe handling. The process produces no harmful gases and reduces sludge treatment costs by 10% on average thanks to co-substrate gate fee. Trials carried out during the project showed how the Lo2x technology can be a fully thermal energy self-sufficient process if organic matter is higher than 8% or 70,000 COD mg/l. The project calculated that the total cost of water sanitation could be close to zero euros per cubic metre of wastewater. To summarise, the technology proposed in LIFE Lo2x could change the current paradigm in wastewater treatment plants, because it could imply the elimination of the sludge line. Its development with the participation of Valencian companies located in the area should help in promoting environmental services and technology with a broad market, creating highly-qualified jobs in the near future. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).
