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3.3 Extracting resources from sewage and sewage sludge
3.3.1 Sewage sludge
Sewage sludge has been used for centuries as a fertiliser (Mulder, 2019). Addition of sewage sludge to land can provide nutrients such as nitrogen and phosphorus, and humus which can help the soil structure. Long term experiments show fertility enhancement after sewage sludge application, resulting from lower soil bulk density and higher soil carbon concentration (Börjesson and Kätterer, 2018). Where lime has been used to treat the sludge, the sludge can also help reduce the acidity of agricultural soils. The Sewage Sludge Directive (EEC, 1986) sets minimum treatment and standards to protect against health and pollution risks from sludge application to land. This old Directive is currently under evaluation (autumn 2021) and is planned for revision under the Green Deal. However, concerns remain that the treated sludge has effectively collected many of the persistent pollutants present in waste water, which then may be dispersed on to the land and become a source of diffuse pollution. A study in Norway considered microplastics in sludge, concluding that they could be a major source to the environment though with no assessment of the risk that this might present (Lusher et al, 2017). Knowledge about the sources and presence of microplastics in water is still limited (EEA, in prep).
A study investigating concentrations of metals and several pharmaceuticals in the sludge itself, from 11 UWWTPs over a year, calculated that concentrations would be below soil predicted no effect concentrations (UKWIR, 2018).
Across Europe, there are polarising opinions as to the fate of sewage sludge. In Germany there is a national strategy to end the application of sewage sludge to soil, with deadlines by 2032 for UWWTPs over 50 000 p.e., though sludge from smaller plants may still be used (Ricardo, 2021). In parallel, the ProgGress strategy requires the recovery of phosphorus from the sludge through mono-incineration (BMUB, 2016). Meanwhile, in Sweden, policy has shifted towards treated sludge going to land as part of a more circular approach (text box).
Text box: REVAQ-CERTIFIED WASTE WATER TREATMENT PLANTS IN SWEDEN
In Sweden, concerns about contaminants in sludge led to recommendations not to apply sludge to land during the early 2000s (Ricardo, 2021). However, in 2008 a collaboration between farmers, regulators and the water and food industries led to a certification scheme “REVAQ”, which assures the safety of sludge application to agricultural land, both in relation to the quality of soil and food and to water quality. This has led to an increase in the amount applied to land rising from 22 % in 2011 to 45 % in 2018 (Ricardo, 2021). Studies showed that “there is clear evidence that sludge fertiliser application supplies plant nutrients and humus that agriculture demands.” (Ministry of the Environment of Sweden, 2020).
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"the APPLICATION OF LIMED (or SUCH A) sludge can also help reduce the acidity of agricultural soils"-> because it is thanks to the lime, not the sludge.
Indeed, however where soil lack of organic matter and they are acidic, you combined the positive effects.
"calculated that concentrations would be below soil" -> what was the application period? after 35 years? would be nice to have that info here.
"end the application of sewage sludge to soil" all soils or only agricultural ones?
"In Sweden", here the same, do we speak about agricultural land/soil? I think that in Sweden, Sewage Sludge mixed with Pulp & Paper Industrial waste is applied on soil but for extractive waste facilities closure or site rehabililtation.
Please consider adding the underlined text:
"Sewage sludge has been used for centuries as a fertiliser (Mulder, 2019). Addition of sewage sludge to land can provide nutrients such as nitrogen, phosphorus, potassium and micronutrients (sulphur, zinc, copper, selenium) and also humus which can help the soil structure."
"However, concerns remain that the treated sludge has effectively collected many of the persistent pollutants and metals present in waste water, which then may be dispersed on to the land and become a source of diffuse pollution."
"...(Lusher et al, 2017). However two more recent Swedish studies show that only 40-60% of the microplastics in the incoming wastewater were found in the anaerobically digested sludge. The rest of the microplastics have been removed or disintegrated/degraded during the treatment in an ordinary WWTP. The concentration of microplastics in sludge from Rya WWTP, Käppala WWTP and Sjölunda WWTP, corresponds to 5, 8 and 8 g/(PE year) respectively. Sludge (normal dose) used as fertilizer for 35 years seems not to cause an elevated microplastic concentration in the soil. Data indicate that the microplastics might be disintegrated/degraded in the soil."
"In parallel, the ProgGress strategy requires the recovery of phosphorus from the sludge through mono-incineration (BMUB, 2016) from WWTP larger than 50 000 pe. "
"However, in 2008 a collaboration between farmers, regulators and the water and food industries led to a certification scheme “REVAQ”, which assures a certain level of control at source and upstream measures and the safety of sludge application to agricultural land, both in relation to the quality of soil and food and to water quality."
"This has led to an increased popularity and confidence among farmers and food industry to use sludge, in the amount applied to land rising from 22 % in 2011 to 4539 % in 2018 (Ricardo, 2021)."
Regarding the UKWIR study refering to metals and pharmaceuutical, perhaps there are studies available on other CECs that should be considered too.
Regarding the microplastics, several references can be used as reference:
Project 1: Malmö, Sjölunda WWTP http://vav.griffel.net/filer/svu-rapport-2018-13.pdf (Summary in English)
Project 2: Göteborg (Rya WWTP) and Stockholm WWTP https://www.svensktvatten.se/contentassets/22657293353d44ecaca7721d0b1c907c/svu-rt228.pdf (Summary in English)
EurEau briefing note on wastewater and microplastics (http://www.eureau.org/resources/briefing-notes/3940-briefing-note-on-microplastics-and-the-water-sector/file)
For the REVAQ system, the following reference can be used: https://www.ieabioenergy.com/wp-content/uploads/2018/01/REVAQ_CAse_study_A4_1.pdf
Revaq has so far focused on metals and nutrients and missed organic pollutants. This is changing but it is still unclear what it will mean in practice. To ensure safety, more substances should be analysed, e.g. the SIN list and the 100 most common pharmaceutical residues.