PROJECT DESCRIPTION
BACKGROUND
Pollution from agriculture is a major pressure on Europe’s freshwater resources. Nutrients (nitrogen and phosphorous) from fertilisers, pesticides, organic material, pathogenic micro-organisms in sediment and micropollutants are washed into waterways. This is especially the case in areas with intense rainfall and poorly drained soils. In some river basins, the high nitrate concentration levels can even promote eutrophication in receiving coastal waters.
The EU Nitrates Directive (91/676/EEC) was adopted to protect waters against pollution caused/induced by nitrates from agricultural sources. However, nitrate pollution of water bodies remains a problem. According to Eurostat data (2009), although nationally averaged nitrate concentrations were well below the Nitrates Directive limit (50 mg/L), the value was exceeded at about 13% of the measuring points. Exceedance of limits was most significant in Belgium (30%), Denmark (26%), Spain (22%) and Cyprus (19%). Other countries including Austria, Bulgaria, the Czech Republic, Germany and Montenegro also had high exceedance levels.
OBJECTIVES
LIFE STO3RE would implement an innovative and cost-efficient technology to protect aquatic environments against pollution caused by diffusion of nitrates and micropollutants. The technology would allow conversion of manure and sludge from waste water treatment plants (WWTPs) into a high environmental quality biofertiliser.
The technology is called CavO3+DAG-TPAD (dual acid-gas temperature phased anaerobic digestion configuration coupled with ozone oxidation and hydrothermal cavitation). It was developed in a previous R&D project (Sludge4Energy), carried out by FACSA and co-funded by Spain's Competitiveness and Economy Ministry. STO3RE would test this technology in a newly developed demonstration plant that will centralise the treatment of secondary sludge from small WWTPs and cattle manure from surrounding farms. The facility would be located in Totana (Murcia, Spain) and be able to deal with sewage sludge from five surrounding WWTPs (Aledo, Librilla, Alhama, Mazarrón and Puerto Lumbreras) and several farms, covering an area of about 2 700 square kilometres. The total amount of sewage sludge treated in the demonstration plant was expected to reach 5-10 cubic metres of mixed substrate per day.
RESULTS
The LIFE STO3RE project team implemented innovative and cost-effective STO3RE technology at the Totana wastewater treatment plant (WWTP) in Murcia (Spain), to convert manure and sludge into a high-quality biofertiliser. This technology comprises a dual acid-gas temperature phased anaerobic digestion configuration coupled to ozone oxidation and hydrothermal cavitation (CavO3+DAG-TPAD). The project team constructed and operated a prototype, processing a wastewater volume of 1-2 m3/day, to demonstrate the technology. It also treated sewage sludge received from another 5 WWTPs (Aledo, Alhama of Murcia, Librilla, Mazarrón, and Puerto Lumbreras) and pig slurry from 5 farmers located in Totana and Librilla.
STO3RE technology produces a sanitised biofertiliser with less micropollutant content, and an increase in biogas production by 40% in comparison with conventional anaerobic digestion. The percentage of methane in the biogas of the control digester is 64%, while in the mesophilic digester the average value is 68%. Volatile organic matter is reduced by 33% in the STO3RE process, compared by a 25% reduction achievable in the conventional process.
The beneficiaries demonstrated that the STO3RE process with post-ozonation achieves the best sanitation conditions. The pathogens E. coli and Salmonella were totally removed, whereas in the control reactor sometimes they were not completely removed. On the other hand, Clostridium perfringens is abated 65% using post-ozonation. The results highlight that the ozonation process significantly improves the hygienic quality of the digestate.
The project team also showed that the STO3RE process with post-ozonation reduces the amount of micropollutants applied on the soil. In fact, Diclofenac, Imazalil, Thiabenzole and Oxytetracycline are totally depleted (i.e. concentrations below quantification limit), while Sulfadiazine and technical Nonylphenol are reduced by 90% and 50%, respectively.
Regarding nutrients, all nitrogen, phosphorus and potassium present in sewage sludge and pig slurry were found in the digestate, after being transformed by microbial processes (i.e. mineralised). Therefore, the fertiliser has a good NPK value and provides good stabilised products; though the beneficiaries noted that it should be considered more an organic soil amendment than a true fertiliser.
Carbon footprint is significantly reduced thanks to the process, as it reduces the greenhouse gas (GHG) emissions caused by pig slurry storage and indirect emissions of GHG. Thus, GHG emissions were reduced from 2 172 kg CO2eq/ton of dry matter (baseline) to 100 kg CO2eq/ton of dry matter (DPAD+O3). The project team also highlighted that the STO3RE model based on DPAD+O3 also avoids odour nuisance, due to the process of ozonation resulting in biofertiliser that smells like wetted straw.
LIFE STO3RE’s technology can contribute to upcoming EU requirements on the use of sewage sludge in agriculture. This aspect is very important, as small- and medium-sized WWTPs are not yet ready to comply with this upcoming legislation because current technologies do not reach the necessary level of sanitation. By improving the removal of pathogens and micropollutants, the STO3RE process can help WWTPs meet policy objectives; though the pathogen Clostridium perfringens could be a barrier to the application of digestate as an organic amendment.
Further information on the project can be found in the project's layman report (see "Read more" section).
