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3.3.3        Resource recovery from sewage sludge

Owing to its high nutrient and organic matter content, and the energy content of dried sludge being comparable to that of woody biomass, sewage sludge is a potential secondary resource which can contribute to Europe’s transition to a circular economy (Ricardo, 2021). A potential obstacle for promoting sludge reuse and recovery stems from two policy objectives which may be in conflict:

• Protection of the environment and human health, which require that  sludge for reuse  complies with specific quality standards
• Resource efficiency promoting the use of sludge in agriculture, ensuring the recycling and recovery of valuable and finite nutrients.

 Prevention of contamination of sewage by persistent, hazardous pollutants would allow recycling of sewage sludge on land without concerns that this might lead to diffuse pollution of soils, plants and water.

The estimated annual amount of nutrients that could be potentially recovered from sewage sludge produced in UWWTPs in the EU 27 ranges between 6 900 and 63 000 t of phosphorus and between 12 400 and 87 500 t of nitrogen (EEA-32: between 8 100 and 68 100 t of phosphorus and between 14 600 and 94 700 t of nitrogen). These amounts correspond respectively to 0.6-6% of total P fertilizers and 0.1-1% of total N fertilizers used in the EU in 2018 (Anderson et al, 2021).

Sludge could also contribute to renewable energy policy and targets as feedstock for energy generation, for example, through anaerobic digestion, incineration, pyrolysis and gasification. Estimations of additional energy potential (i.e. on top of that already recovered) has been subject to several studies assessing different sewage sludge options. Anderson et al (2021) estimated that the EU-27 could potentially recover between 1 800 GWh and 3 200 GWh of energy (net heat and electricity) through anaerobic digestion of the total generated sewage sludge currently intended for landfilling and composting, and 250 GWh (net electricity) through incineration of the total generated sewage sludge currently sent to landfill.  This respectively represents 7 % , 13 % and 1 % of the total waste water sector energy needs in the EU-27 in 2018.

Using an alternative approach to calculation, JRC (ref) estimated that up to an additional 3285 GWh could potentially be recovered through anaerobic digestion of the total sewage sludge applied directly on land (30%) and 850 GWh resulting from incineration of sludge currently sent to landfill. 

Selection of the optimum sewage sludge option for energy recovery must consider additional energy demands and GHG losses that arise from sludge processing. Energy savings and burdens need to be summed to derive a net value for assessed energy recovery options. The JRC study concluded that across different sewage sludge options, anaerobic digestion followed by use on land and co-incineration were the options that have the lowest GHG emissions.