Recycling of phosphorus in sewage sludge

In order to examine the fertilizing effect of P in different sewage sludge a pot experiment with rye grass growing in a moraine and a clay soil with application of different kinds of sewage sludge was carded out. Data on pH, water extractable P and ammonium acetate lactate extractable P in soil, and yield P content showed that Sewage sludge application significantly influenced the soil pH and may thereby indirectly influence the plants general access to essential nutrients in the soil. Beside of the purification history of the sewage sludge, both the soil content of available P and the P adsorption capability of the soil must be considered when advising sludge application to crop production. Biological purification without chemical additives and Ca treatment of the sludge gave the highest amount of plant available P and also the best utilization of the P applied as sludge. Although low concentrations of water extractable P after addition of sewage sludge the considerable accumulation of total P in the surface soil (50-95% increase) must be considered a potential environmental risk due to possible surface P runoff by erosion.