The Sewage Sludge Directive 1 (SSD) aims at encouraging the use of sewage sludge in agriculture while preventing negative health and environmental impacts. It sets quality requirements for the sludge and the soil on which it is to be used, by setting upper limits on their content in seven heavy metals (cadmium, copper, nickel, lead, zinc, mercury, chromium). It also requires sludge treatment before application, and consideration of the nutrient needs of the plants.
The Directive has never been substantially amended since its adoption almost 40 years ago. Meanwhile, in parallel to progress on the knowledge of sludge properties, treatment and use, the wider environmental legislative and policy framework has considerably changed. Also, wide differences in implementation are observed, linked to the fact that sludge management strongly depends on local conditions, or policy choices by Member States (e.g. some of them ban the use of sludge in agriculture). Broadly speaking, looking across all heavy metals thresholds and parameters set by the Directive, over time seventeen of them have adopted stricter requirements by setting stricter limits or limits for additional contaminants.
This evaluation was carried out in view of that, and in line with the European Commission’s Better Regulation principles and methodology.
About 40% of the 2 to 3 million tons of sludge yearly produced in the EU (17 kg/ha) are applied on farmland. The other share is incinerated (27%), composted (about 10%), or landfilled (currently estimated at 11%, and phasing out). Sludge use in agriculture has remained the main route for sludge management in the EU, allowing to curb sewage sludge disposal through landfilling, while acting as a fertiliser which also shows to improve further soil properties.
The level of heavy metals in sludge used in agriculture significantly decreased over time, to level below the limits set by the SSD, very often 10 times lower. While it can be partly attributed to the Directive, it is challenging to distinguish its effect from that of national action or of legislation controlling the emission of these pollutants at source.
The evaluation found very limited information linking soil quality and use of sewage sludge, in particular on long-term effects, and research is going on in this regard. The current reporting system also showed limitations, not only suffering from low compliance by Member States, but also not generating information allowing to assess the impact of sludge use on land, including verifying that plant needs are taken into account (thereby preventing groundwater pollution by excess of nitrates). The evaluation also identified room for data improvement in streamlining reporting under the SSD and the Urban Waste-Water Treatment Directive 2 (UWWTD) and data flow management at EU level (currently shared between the European Environmental Agency and Eurostat).
Quality standards voluntarily set up between sludge producers and farmers have contributed to achieving the objectives of the SSD. Conversely, negative public perception, the lack of EU-wide end-of-waste criteria for sludge and the use of alternative organic fertilisers e.g. manure, or, increasingly, biowaste, are identified as hindering factors.
The SSD also has had unintended effects, positive (sludge use in agriculture presents an overall negative carbon footprint) or negative (sludge use in agriculture leading to antibiotic resistance genes and microplastics presence in soil, associated emissions of methane and of other contaminants not regulated by the Directive).
Using sludge in agriculture entails costs, for treatment to make it suitable for use (for safety reasons e.g. hygienisation, or to ease transport), and for transport. These amount to several hundreds of euros per ton of dry sludge (tDS). However, it is overall significantly less costly than other sludge treatment options, especially (mono-)incineration, the main alternative to the agriculture route. A hypothetical shift of the sludge currently used in agriculture towards (mono-)incineration could increase costs by €41–488 million/yr, and, for mono-incineration only, by €391-488 million/yr. Also, in a theoretical case where sludge would fully substitute a mineral fertiliser, savings for farmers could be some €96 and €44/tDS, for nitrogen for phosphorus respectively.
The SSD is in line with the waste hierarchy set by the Waste Framework Directive 3 , prioritising recovery of nutrients over energy recovery and disposal, notably through landfilling.
In principle, the objectives of the Directive are also aligned with other environmental and health legislation and the linked policies outlined in the Zero Pollution Action Plan and the EU Soil Strategy for 2030. However, in practice this coherence would be fully ensured if the risks linked to contaminants present in sludge were reassessed, notably reviewing the limit values and the set of pollutants which it regulates.
The revised UWWTD, as proposed by the Commission 4 in 2022, could affect the composition of sludge, by potentially increasing its content in microplastics, while decreasing the level of contaminants which the UWWTD would reduce at the source. It is also bound to influence sludge treatment practices in view of increasing the energy efficiency of the treatment plants.
More broadly, the Directive serves the European Green Deal, and EU policies on climate, health, circular economy, security of food supply, and independence in fertilisers, critical raw materials and energy. These policies influence sludge management policies differently, depending on local conditions e.g. the agronomic needs of soils, energy mix and available infrastructure. It can also favour specific treatments, with varying impacts on carbon footprint. Anaerobic digestion allows to produce biogas while treating the sludge, but is not always sufficient to remove certain pollutants. Treatment may not always be feasible, where infrastructure lacks, or technically, depending on the contaminants to be abated in the sludge. In such cases, (mono)incineration could be a last recourse, possibly with recovery of heat from a renewable resource.
EU added value
The SSD maintains its added value, as the sole legal instrument providing an EU wide framework for soil protection from sludge use in agriculture, setting a minimum level of harmonisation for control of pollution and health risks. It also promotes a rather inexpensive sludge management route.