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Sustainable and Safe Re-use of Municipal Sewage Sludge for Nutrient Recovery

Final Report Summary - SUSAN (Sustainable and Safe Re-use of Municipal Sewage Sludge for Nutrient Recovery)

Municipal sewage sludge (MSS) is a carrier of valuable nutrients - most important phosphorus (P) - but is often contaminated by pathogens, hazardous organic pollutants such as pharmaceuticals, pesticides, polyaromatic hydrocarbons (PAH), chlorobenzenes, phenols, polychlorinated biphenyls and phthalates and inorganic pollutants such as arsenic, nickel, cadmium, lead, mercury, chromium, copper and zinc. Therefore, municipal sewage sludges must be disposed of or the pollutants must be removed before agricultural application to protect farmland and human health.

In the last couple of years, the agricultural application of sewage sludge has decreased, while the interest in alternative sludge disposal routes to protect farmland and human health has increased. However, following the disposal route, nutrients are irreversibly lost and the need for mineral fertiliser products will increase. Concerning the scarce resource phosphorus, around 300,000 tons of P will be lost per year in the European Union if the entire amount of sewage sludge would be disposed of. This potential phosphorus resource could be recovered if a sound recycling strategy is developed and applied which was the target of the 'Sustainable and safe re-use of municipal sewage sludge for nutrient recovery' (SUSAN) project.

In the SUSAN project an alternative (sustainable and safe) management option for municipal sewage sludges based on thermal treatment was developed and assessed. Mono-incineration of sewage sludges completely destructs pathogens and the organic pollutants in a first step. For adjacent application of phosphorus-recovery techniques it is required to incinerate the sludges in mono-incineration facilities where municipal sewage sludges are incinerated only.

Co-incineration of sewage sludges with other wastes containing less P would dilute the ashes resulting in low P-concentrations, less attractive for recovery. The incineration residues from mono-incineration are sewage sludge ashes (SSA) with a high phosphorus content in the range 15-25 % P2O5. However, these ashes often contain heavy metal compounds above the limits for agricultural use according to EU- and national fertiliser ordinances. Furthermore, phosphorus in the ashes exhibits low bioavailability - a disadvantage in farming. Therefore, a second step is required after mono-incineration to:

i) reduce the heavy metal concentrations below the relevant limits and
ii) to transfer phosphorus into mineral phases with high P-bioavailability.

In the SUSAN project an environmentally friendly technology for sewage sludge treatment was developed and optimised aiming at nutrient recovery. The SUSAN option is a two step thermal treatment. In a first step sewage sludge is mono-incinerated under energy recovery and destruction of organic pollutants.

In a second thermochemical step heavy metals are removed and the valuable nutrient phosphorus is transferred into a form that is available for plants. This treatment step works at 850-1000 degrees Celsius. The thermochemically treated ashes contain approx. 20 % P2O5 in a bio-available form and are thus suitable raw materials for manufacturing marketable fertilisers such as P-, PK- or NPK-fertilisers. The nutrients phosphorus, calcium, magnesium and potassium that were present in sewage sludge are transferred into the ash based fertilisers and thus can be recovered with this technology. Recovery of phosphorus is most important in this context as phosphorus is a non-renewable resource that can become scarce in the future.

The P-resources of the European Union are negligible. Consequently, the EU depends on a world market that is dominated by a small number of exporting nations. Main phosphorus reserves are located in China, Morocco, South Africa and the United States of America. Dramatic increases of the prices for rock phosphate were observed in the years 2007 and 2008 with significant effects on the agronomic sector. However, those developments on the fertiliser market positively affect the evolution of recycling strategies such as the SUSAN management option for nutrient recovery.

An economic operation of large-scale facilities treating sewage sludge ashes according to the SUSAN technology is expected in the near future. A pilot plant for fertiliser production from sewage sludge ashes is currently in operation. The SUSAN participant ASH DEC Umwelt AG will build up the first demonstration facility with an annual capacity of approx. 15,000 tons of ashes in the years 2009-2010.