Phosphorus bioavailability in ash from straw and sewage sludge processed by low-temperature biomass gasification

Reuse of phosphorus (P) from waste streams used for bioenergy conversion is desirable to reduce dependence on nonrenewable P resources. Two different ash materials from low-temperature biomass gasification of wheat straw and sewage sludge, respectively, were investigated with regard to their P bioavailability. A set of pot experiments with spring barley was carried out to compare the ash P fertiliser value with mineral P fertiliser and the sewage sludge feedstock. An indirect radioactive labelling approach with P-33 was used to determine the amount of P taken up from the fertiliser materials. Depending on the application rate, straw gasification ash produced a fertiliser response comparable to mineral P. However, P uptake from the ash was generally less than uptake from equivalent amounts of mineral P, and the calculated relative effectiveness was 44% after 6weeks of plant growth. In contrast, the P fertiliser value of Fe-rich sewage sludge after low-temperature gasification was practically zero. These results suggest that ash from low-temperature gasification could be developed into alternative P fertilisers; however, as the P bioavailability depends greatly on the feedstock used, a greater emphasis on feedstock composition is required.