Report No FR0330

DETOXIFICATION OF SLUDGE AND SLUDGE DRYING

An Assessment of the Loss of Organic Contaminants during Composting

FR0330

Oct 1992

SUMMARY

I BENEFITS

A theoretical predictive approach has been developed to assess the fate of organic contaminants present in sewage sludge during composting.

The predictive approach has been applied to assess the fate of 'Red List' organic compounds.

The predictive approach could easily be applied as a method for assessing the fate of other organic contaminants present in sludge during composting.

II OBJECTIVES

To review the potential extent of degradation and loss of organic contaminants from sludge during composting.

III REASONS

There is increasing concern over the presence of organic pollutants in sewage sludge and their fate during disposal. Composting of sewage sludge produces a potentially useful and marketable product and provides an environmentally acceptable disposal route. However, the extent of use of the compost produced may well depend on the extent to which organic pollutants are attenuated during composting. The intense aerobic biological activity and heat generated during composting may result in attenuation of organic pollutants by biological and chemical degradation and by volatilization, but there is very little information available.

IV CONCLUSIONS

The theoretical predictive approach indicated that more than half of the selected `U.K. Red List' compounds would be significantly attenuated during composting. The predictive approach recommended can be applied to any organic chemical suspected to be present in sewage sludge in order to provide an assessment of its persistence during composting. There is insufficient experimental data for sewage sludge composting available to be able to comment on the accuracy or validity of the theoretical predictions. However, there were two chemicals for which there was experimental data that could be compared to the theoretical predictions, and the data showed good agreement.

V RECOMMENDATIONS

Experimental data on the fate of selected organics during composting are required in order to test the validity of the theoretical predictions and assess how widely this approach can be applied. Experimental information is also required on the fate and behaviour of potentially toxic organics in sewage sludge during other non-destructive processes such as pelletization (Swiss Combi process) in order to assess which chemicals persist in the final product. There is a need for production of a definitive database of physico-chemical data of organic compounds in which the data has been fully validated according to a set of standard guidelines.

VI RESUME OF CONTENTS

Experimental data for the fate of organic contaminants present in sewage sludge during composting is presented. Due to the paucity of information available in the literature, a theoretical predictive approach for assessing the loss of organic contaminants present in sewage sludge during composting is presented. The problems and assumptions associated with this approach are discussed and comparisons made between experimental and theoretical data.

Copies of the report are available from FWR, price 15.00, less 20% to FWR Members.