Sewage sludge as a phosphate fertilizer.

In the Netherlands the average production of sewage is appr. 50 m' per PE (population equivalent) per year, containing appr. 1 kg of phosphorus (P = 0.44 P205; P205 = 2.3 P). After mechanical-biological treatment 25% of the phosphorus is still present in the sewage sludge. Assuming a sludge production of 20 kg DM (dry matter) per PE per year, the P concentration in the dry matter would be appr. 1.25%. By additional chemical treatment, 90% or more of the phosphorus can be removed. In this case the P concentration in the sludge amounts to appr. 5% (of DM). In 570 sludge samples, analysed in the period 1972/76, the P concentration varied from 0.5 to 4.1%, with a mean value of 2.0%. Over 1967/70 the mean value was 1.6%. Apparently there is a tendency to increase in the course of time. A further increase in the future depends on the extent to which chemical treatment to remove additional P from sewage is introduced. On the other hand, legal measures are now in preparation in the Netherlands to gradually halt the use of detergents containing P, which may lead to a lower P concentration in the sludge. The amount of phosphorus in sludge currently amounts to about 0.4 kg per PE per year in the Netherlands. With 7 people per hectare of agricultural land this would mean an average supply of 2.8 kg P/ha, if all sewage was treated and all sludge was applied to agricultural land. Only about 80% of the sewage, however, is treated as yet and only about 40% of the sludge is applied to agricultural land. Consequently, on average only 1 kg P from sludge is available per ha of agricultural land per year. Corresponding values for P from fertilizers and animal waste are 20 and 30 kg, respectively. The contribution of sludge in the P-supply of agricultural land therefore currently amounts to less than 2%.