Variation of the element composition of municipal sewage sludges in the context of new regulations on phosphorus recovery in Germany

Abstract

Phosphorus (P) recovery is obligatory for all sewage sludges with more than 20 g P/kg dry matter (DM) from 2029 in Germany. Nine wastewater treatment plants (WWTPs) were chosen to investigate variations of phosphorus contents and other parameters in sewage sludge over the year. Monthly sewage sludge samples from each WWTP were analyzed for phosphorus and other matrix elements (C, N, H, Ca, Fe, Al, etc.), for several trace elements (As, Cr, Mo, Ni, Pb, Sn) and loss of ignition. Among the nine WWTPs, there are four which have phosphorus contents both above and below the recovery limit of 20 g/kg DM along the year. Considering the average phosphorus content over the year, only one of them is below the limit. Compared to other matrix elements and parameters, phosphorus fluctuations are low with an average of 7% over all nine WWTPs. In total, only hydrogen and carbon are more constant in the sludge. In several WWTPs with chemical phosphorus elimination, phosphorus fluctuations showed similar courses like iron and/or aluminum. WWTPs with chamber filter presses rather showed dilution effects of calcium dosage. As result of this study, monthly phosphorus measurement is highly recommended to determine whether a WWTP is below the 20 g/kg DM limit.

Supplementary information: The online version contains supplementary material available at 10.1186/s12302-022-00658-4.

Keywords: Elemental variations; Phosphorus elimination; Phosphorus recovery; Sewage sludge; Wastewater.

Fig. 1

Average sludge composition (out of all monthly samples) of 9 wastewater treatment plants (WWTPs)

Fig. 2

Phosphorus content ranges for sludges from 9 wastewater treatment plants (WWTPs). Relative standard variations below

Fig. 3

Examples for congruent trend of phosphorus and precipitation elements and opposite trend of phosphorus and calcium