Phosphorus bioavailability in alum biosolids used in agriculture

Abstract

Effluent discharged from wastewater treatment plants to inland water bodies has the potential to cause eutrophication unless the phosphorus (P) concentration is reduced by methods such as dosing with alum (Al2(SO4)3), to precipitate P and capture it in the solid sludge. As a consequence, the aluminium (Al) and P concentration of the alum sludge is greater than sludge (or biosolids) produced without chemical treatment. Alum biosolids are typically landfilled and not used for crop production and consequently there is little research that has examined the potential to make better use of the P value in the sludge. The use of alum residuals from drinking water treatment, however, has indicated that the Al may contribute to a reduction in P uptake by plants. A field experiment was established to investigate the ability of alum biosolids to provide P for cereal production. Alum biosolids (7.5% Al, 3.7% P) were applied at six rates (0, 3.4, 6.7, 10.1, 13.5 and 20.1 Mg DS ha-1) on a P deficient sand.In addition, one rate of inorganic fertiliser at district practice containing 20 kg P ha-1 was applied. The inorganic fertiliser treatment yielded 44% higher than the nil fertiliser treatment at final grain harvest, demonstrating the requirement for P on this site for crop production. The addition of alum biosolids at rates of up to 20.1 Mg DS ha-1 produced comparable grain yields (>0.05) to that of the inorganic fertiliser treatment. The concentration of P in the shoots of wheat sown in alum biosolids treatments at tillering (61 days after sowing) were P deficient, whereas P was adequate in the inorganic fertiliser treatment. There was no evidence of any other nutrient deficiency in plant shoot samples other than P. Therefore, it is suggested that on this P deficient soil, the ability of alum biosolids to provide P for cereal production was poor. The concentration of bicarbonate extractable soil P measured over the growing season increased in alum biosolids treatments, though this was not related to improved plant growth and therefore this relationship is being investigated further in the second cropping season.