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

Re-use of phosphorus (P) from waste streams used for bioenergy conversion is desirable in order to reduce dependence on non-renewable 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 33P was used to determine the amount of P taken up from the fertiliser materials.
Depending on the dosage applied, straw gasification ash had a fertiliser effect comparable to mineral P. However, P uptake from the ash was generally lower than uptake from equivalent amounts of mineral P, and the calculated relative effectiveness was 44 % after six weeks of plant growth. In contrast, low- temperature gasification of Fe-rich sewage sludge reduced its P fertiliser value to practically zero. The results suggest that ashes from low-temperature gasification could be developed into alternative P fertilisers, however since their P bioavailability varies strongly depending on the feedstock used, a greater emphasis on feedstock composition is required.