Influence of chemically and biologically stabilized sewage sludge on plant-available phosphorous in soil

Abstract

The fertilizing effect of P in different sewage sludges was investigated in a pot experiment. Five different sludge types were applied to rye grass growing in either a moraine or a clay soil. The flocculating agents used to precipitate P in the sludges were aluminium chemicals, iron chloride or biological P-reduction without use of chemicals. The total P content in the sludges varied approximately from 7500 to 30,950 mg kg −1, and the pH varied between 5.45 and 12.4. Biological purification without chemical additives and lime treatment produced sludges with a P fertilizing effect comparable to inorganic fertilizers. P precipitated by use of Fe and Al chemicals and without liming treatment produced sludges with very low P fertilization values. Sewage sludge P applied in combination with water-soluble inorganic P increased the availability of the sewage sludge P. Sewage sludge application significantly influenced soil pH, and may therefore indirectly influence the plants’ general access to essential soil nutrients. Repercussion values of P in the sewage sludge were calculated indicating the amount of plant available P left in the soil at the end of the first growing season. This unused surplus of plant-available P may be utilized by the plants in the subsequent growing season. The repercussion was highest for the biological and lime treated sludges and low in the sludges treated with Fe and Al flocculating agents. Although low concentrations of water-extractable P were measured after addition of sewage sludge, considerable accumulation of P took place in the soil (50–95% increase). This accumulation must be considered as a potential environmental risk due to the possibility of erosion and subsequent surface runoff.