According to our analyses, the de-watered sludge produced at the plant contained 27% air-dry material. The following values were measured in the air-dry material: organic matter: 45%, organic C: 26%, total N: 4.26%, C/N ratio: 6, “total” salt: 1.65%, inorganic salt: 0.27%, mineral N: 7% of total N, NH4-N: 3104 mg kg-1, NO3-N: 38 mg kg-1, NH4-lactate acetic acid (AL) soluble P2O5: 8218 mg kg-1, AL-K2O: 1385: mg kg-1. The Zn content of sludge was 0.11% and the Cu content was 0.08% which shows industrial contamination. These do not reduce the sludge value, as the majority of Hungarian soils are especially low on Zn and Cu. The prescribed loading limit is 10 kg ha-1 year-1 Cu which is reached with 47 t ha-1 year-1, while the 30 kg ha-1 year-1 Zn loading limit could be reached with 97 t ha-1 year-1 in the case of field use. High N content could be a restricting factor. If the total N stock of the sludge mineralised in the first year and transformed to NO3-N, only 15 t ha-1 year-1 could be used in the N-sensitive area. In other cases, this dose could be 30 t ha-1 year-1. If the breakdown of sludge is considered to be the usual 3-4 years similarly to to other fertilisers, even 100 t ha-1 year-1 is acceptable. The Govt. decree no. 50/2001. (IV.3.) prohibits the use of sludge in the field for no reason, thereby increasing the risk of illegal application. The supervision of this decree is justified because it neglects the typical Hungarian conditions and professional arguments. Based on our previous experiments, the N stock of composts and sewage sludges could be utilised in 3-10 years, depending on soil and farming conditions. It is an objective to increase the agricultural use of sludges poor in harmful substances and rich in essential elements from the currently estimated 10% to the 50-80% typical in EU countries. This could increase the organic matter and nutrient stock, as well as the productivity of our soils, while their water management and environmental protection features would also improve.

Keywords: sewage plant, sewage sludge, composition, element turnover