Undersøgelses- og moniteringsprogram for omsætningen af miljøfremmede
stoffer i slammineraliseringsanlæg og slamlager.

English Summary

Limit values

In 1997 the Danish Environmental Protection Agency introduced limit values for organic pollutants in sewage sludge bound for agricultural purposes. In the years 2000 and 2002 the limit values will be lowered (bekendtgørelse nr. 49 af 20. januar 2000 – "slambekendtgørelsen"). Due to both economical and environmental reasons it will be necessary to find methods that can reduce the concentration of organic pollutants in sewage sludge thus enabling a continuos use of sewage sludge as an organic fertiliser on agricultural land.

Purpose

The purpose of this project was to examine the degradation of the organic pollutants: LAS, NPE, DEHP og å PAH, under long term storage, under long term storage with mechanical turning and during treatment in a reed bed treatment system (in the following called constructed wetland).

Experiments

Full-scale experiments were carried out from February 1999 until November 1999. Experiments were made with both biological (activated) sludge and anaerobically digested sludge.

The storage experiments were carried out by storing approx. 15 tons of biological sludge and approx. 15 tons of anaerobically digested sludge (with a dry weight of 20% and 26% respectively) in containers in nine months. Parallel to these experiments a stack of anaerobically digested sludge was placed on a concrete floor and turning carried out every 14 to 20 days during the nine months.

For the constructed wetland experiments one of eight basins at Kallerup Sewage Treatment Plant was used (Kallerup is situated west of Copenhagen). The constructed wetland at Kallerup Sewage Treatment Plant was established in 1996. For experimental use approx. 240 m3 of anaerobically digested sludge were pumped into the basin (dry weight approx. 2-3%).

Degradation of the organic pollutants mentioned were observed by taking samples in the respective experiments during the nine months in 1999. Analyses of the organic pollutants were supplemented with analyses of volatile solids, pH, BOD, COD and N, P, C. Furthermore the redoxpotential and temperature were sampled. Five times during the experiments ecotoxicological tests were made of the stored sludge.

Results and conclusions - Storage experiments

Storage experiments with anaerobically digested sludge showed reductions of approx. 41% for LAS, 0-27% for å PAH, 0-14% for DEHP and 0% for NPE, after nine months of storage in containers. Reductions were not found for the organic pollutants in the experiments with the activated sludge. During the experiments with both activated sludge and anaerobically digested sludge a column (covering top to bottom) was taken. When compared with the stored sludge in the depth 20 to 120 cm reductions in the concentration of organic pollutants were found in the upper 20 cm only. The reduction found in the storage experiments may therefore only be ascribed to the degradation found in the upper 20 cm.

Turning (oxidation) of the sludge had a positive effect on the reduction of the organic pollutants. NPE, DEHP, LAS and å PAH were reduced with 43%, 47%, 90% and 32% respectively.

Results and conclusions - Constructed wetland experiments

Constructed wetland was found to be an efficient method in regard to degradation of the organic pollutants. Reductions of approx. 93%, 60%, 98% and 60% were obtained for NPE, DEHP, LAS and PAH respectively. The organic pollutants were mineralised in the whole depth and not only in the upper layer of sludge. The size of the reduction obtained is comparable with newly published results from experiments with composting and after-oxidation of sludge.

General conclusion

It was found that oxygen is the limiting factor in the degradation of organic pollutants while temperature has influence on rate of degradation. Degradation under anaerobic conditions were negligible, which is comparable with the known knowledge regarding organic pollutants from literature.