Phosphordünger aus Klärschlamm: Evaluierung von Pyrolysekohle und Alkalipyrolyse

Summary

Phosphorus (P) in sewage sludge holds the main proportion of P currently lost in waste streams, and options to recycle it for agriculture are needed. Pyrolysis is a promising local alternative to centralized sewage sludge incineration. Pathogens and organic pollutants are destroyed and a carbon-rich substrate known as biochar is formed. We evaluated pyrolysis of sewage sludge as well as sintering of pyrolysis char with alkali salts. The products were applied in a pot experiment with calcareous and acidic soils to determine P uptake by ryegrass (Lolium multiflorum) and changes in soil
pH, available P and cation exchange capacity. Two products were also tested in a field experiment with maize (Zea mays) on a neutral soil. Cadmium and mercury were partly lost during pyrolysis. The pot experiment revealed interactions between soil pH and P use efficiency of the various products, including reference fertilizers permitted in organic farming. Pyrolysis changed plant P availability only slightly compared to the parent sewage sludge, but sintering with alkali salts increased plant P availability to
levels comparable to triple superphosphate. Potential implications for organic agriculture are discussed.