Agronomic efficiency and heavy metal contamination of phosphorus (P) recycling products from old sewage sludge ash landfills

A modified Neubauer pot trial with rye was carried out to investigate the agronomic efficiency of different fertilizer products based on a sewage sludge monoincineration ash (SSA) which had been stored below ground for about ten years. Test products included the stored raw ash from chemically precipitated sewage sludge (SSA 1), an acid leaching treatment without (SSA 2) and with subsequent Ca-P precipitation (SSA 3), and a product subjected to an ion exchange process after acid leaching (SSA 4). For comparison, a thermo-chemically treated ash (SSA-TC) representing the currently available technology was also included. A commercial water soluble P fertilizer (SSP) was used as a reference. NAC-solubility of SSA 2–4 was increased by the acid leaching procedure up to a level around 87–92% of total P, compared to 36.5% in the raw ash (SSA 1) and 63% in SSA-TC. SSA 4 was the only test product with high water solubility (77% of total P). Rel- ative agronomic efficiency related to dry matter yield (RAE-DMY) did not show large variation between prod- ucts, ranging from 84% (SSA 1) to up to 91% (SSA 2) in comparison to the control (SSP) at the optimum P level (20 mg). A larger variation was displayed when relative agronomic efficiency regarding P uptake (RAE-PU) was calculated. At P level 20, RAE-PU ranged between 53.5% for SSA 1 up to 99% for SSA 4. Thus, the water soluble product clearly distinguished itself from the non-water soluble variants. However, at the highest P level (40 mg), there was a significant decrease in both DMY and PU for SSA 4, which could be explained by Na toxicity related to the high Na content of the product originating from the treatment procedure. The acid leaching treatment successfully reduced heavy metal contents (As, Cd, Cr, Cu, Ni, Pb, Tl, Zn) of the raw ash, so that the limit values of the German Fer- tilizer Ordinance were safely met. U concentration was already low (2.9 mg/kg DM) in the raw material. However, even the Ca-precipitation (SSA 3) was not able to reduce the initially high Fe and Al co ncentrations to a sufficiently low level to prevent the (re-)formation of insoluble Fe/Al- phosphates during the experiment. While the exchange of the cations Al and Fe by Na (SSA 4) was able to enhance the plant availability and uptake of P, it induced other dif- ficulties related to the increase in Na content. It was concluded that, to make the ash-based recycling fertilizers more attractive, fu rther research is needed to find a procedure which will make the ash-P plant avail- able and at the same time result in a product which is low in growth-reducing contamin ants such as sodium or heavy metals.