Heavy Metal Depletion of Sewage Sludge Ashes – Less is More

Compliance with limit values for fertilizers – current and perspective

Heavy metals, such as copper, zinc, nickel and lead, enter wastewater through various pathways. During wastewater treatment, a large proportion of the heavy metals enter the sewage sludge through the mechanical and biological treatment stages. When sewage sludge is thermally recycled, the majority of the heavy metals remain in the sewage sludge ash.

The input quantities of heavy metals vary depending on the municipal industrial input pathways. Therefore, sewage sludge ash is also contaminated with heavy metals to varying degrees. Phosphorus recyclates from this – such as fertilizers – must comply with corresponding limit values for the content of heavy metals.

Of course, this also applies to fertilizer recyclates produced using PHOS4green. Here, the fertilizer ordinances – German Düngemittelverordnung (DüMV) and Regulation EG No. 2019/1009 – set clear limits.

For sewage sludge ashes that comply with the limits of the fertilizer ordinances, the PHOS4green technology is used without heavy metal depletion.

Comparison of the pollutant content in fertilizer produced with PHOS4green with the permitted limit values according to Table 1.4 of the Fertilizer Ordinance (DüMV)

Content of copper and zinc in fertilizer produced with PHOS4green in comparison with the permitted limits

More disposal safety for sewage sludge ashes with excessive heavy metal content

Currently, a significant portion of sewage sludge ash does not meet the quality requirements of the Fertilizer Ordinance (DüMV) and Regulation EC No. 2019/1009, making heavy metal depletion necessary.

Economical heavy metal depletion increases the raw material potential (amount of usable sewage sludge ash) available for PHOS4green. The PHOS4green process, which has been modified with regard to heavy metal depletion, thus provides greater planning security with regard to possible future tighter limits or strongly fluctuating ash loads.

The ability to remove heavy metals from incoming sludge means there is no need for lot to lot selection – all available raw material can be used. This will also simplify regional and inter-municipal solutions for sewage sludge treatment, making the process viable over the long term. It is equal which ashes with which quality are used for fertilizer production from phosphorus recovery. This guarantees future disposal security and thus contributes to sustainable sewage sludge utilization.

Currently, Glatt offers various approaches to heavy metal depletion as part of a modified PHOS4green process.