Volatile organic compounds released from thermal drying of sewage sludge

Abstract

Thermal drying of sewage sludge is a treatment to obtain a material that can be used as co-fuel for some chemical industries, but some volatile organic compounds (VOCs) can be emitted and consequently must be considered. In this work the emissions of these VOCs were studied. The sewage sludge produced by a domestic water treatment plant was dried in a simple laboratory scale distillation/evaporation apparatus at different temperatures (80, 90, 100 and 120 °C). The sludge was heated and a flow of air released water and volatile compounds. The water was condensed and separated, and the VOCs were analysed in the resultant gas fraction. More than forty compounds were identified and the total yield ranged from 545 to 591 mg kg-1 dry weight, without significant differences for the temperatures used. The compounds with the highest yields were dimethyl disulfide, toluene, dimethyl trisulfide and 1,2,4- trimethylbenzene, with great differences in yields from the rest of the compounds found. Between the compounds identified, several types were found: aliphatic hydrocarbons, aromatic hydrocarbons, halogenated or not, and compounds with sulphur. Since most of the identified VOCs are carcinogenic, toxic or harmful by inhalation, there is a need to control the emitted gases from thermal drying.