Effluent sludge

Effluent sludge is produced during the treatment of waste water in waste water treatment plants. Effluent sludge contains plant nutrients such as phosphorus and nitrogen, but also some heavy metals such as lead, cadmium, copper and zinc.

Effluent sludge contains plant nutrients such as phosphorus and nitrogen, but also some heavy metals such as lead, cadmium, copper and zinc. Moreover, organic compounds that are not easily degradable can accumulate in the effluent sludge, from cleaning agents, body care products or medicines. Potential pathogens such as bacteria, viruses and parasites can also occur in effluent sludge. The use of effluent sludge as an agricultural fertilizer has therefore been prohibited in Switzerland since 1 October 2006.

Effluent sludge counts as waste and must be disposed of or recycled in accordance with the Technical Ordinance on Waste (TVA). In Switzerland effluent sludge is thermally disposed of in municipal household waste incinerators (KVA), sludge incineration plants (SVA) and cement works. Before incineration, organic compounds and water are removed from the raw sludge in several treatment stages. This is done to improve its stability, transportability and recoverability. The first step is to subject the raw sludge to fermentation or digestion processes. This produces biogas, which can be used as a renewable energy source. The mechanical dehydration process that follows this takes superfluous water from the sludge which can then be thermally disposed of. When recovery takes place in cement works, the water content must be further reduced in an additional drying stage. In 2012 around 195,000 tonnes of effluent sludge were produced. Of this 43 percent was disposed of in SVAs and 27 percent each in KVAs and cement works. The other 3 percent were exported. The main treatment for effluent sludge today is therefore thermal. The fertilizer ban of 2006 had a big effect on this development.

Recovery of phosphorus from effluent sludge

Against the background of sustainable waste management, the Federal Office for Environment has been looking for some time at the problem of recovering phosphorus from effluent sludge. Dehydrated effluent sludge contains approximately 1% phosphorus and effluent sludge ash over 6%. If phosphorus has not first been recovered, thermal disposal of the effluent sludge will cause the irreversible loss of the nutrient phosphorus.

According to a FOEN study around 90 percent of the phosphorus could be recovered from the effluent sludge and its ash. This would correspond to around 6000 tonnes of phosphorus per year. The aim is therefore to establish phosphorus recycling on a long-term basis so that this high-quality nutrient can be used for fertilizer production. The legal basis for the recycling of phosphorus will be created when the TVA is completely revised.

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Last modification 29.06.2018

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