Thermochemical conversion characteristics of biosolid samples from a wastewater treatment plant in Brisbane, Australia

Abstract

Environmental context Biosolids are nutrient-rich organic materials. They can be used as fertiliser and solid amendments in agriculture if treated according to regulatory requirements. If farming applications of biosolids decline due to potential pollution from their heavy metal content, an alternative to traditional methods of biosolid disposal is required. In this context, thermal processing of biosolids is an economically and environmentally suitable option to convert large quantities of biosolids into useful energy. Rationale Due to more stringent environmental regulations and frequently required long-distance transportation, the traditional disposal of biosolids from wastewater treatment plants in landfills and farms is becoming unsustainable. A potentially economical and environmental option is the thermochemical conversion of biosolids into energy and value-added products. This paper describes the chemical composition and energy content of a representative biosolid sample collected from a major wastewater-treatment plant in Queensland, Australia. Methodology The thermochemical behaviour and compositional changes in biosolids were investigated under a wide range of pyrolysis and gasification conditions using a horizontal tube furnace (HTF), a fixed-bed reactor and a thermogravimetric analyser (TGA). In terms of practical application of by-products, we describe mineral matter transformations in char and ashes during pyrolysis and volatilisation as well as under different gasification conditions. Results HTF experiments revealed that at pyrolysis below 800°C, mainly organic species were released, while losses of inorganic elements (phosphorus, magnesium and zinc) occurred at higher temperatures. In-situ gasification behaviour of biosolid chars in the TGA reactor showed that the gasification reaction of biosolid chars occurred rapidly at temperatures above 720°C, regardless of the pyrolysis temperatures at which those chars were produced. Mineral matter transformations began at temperatures above 600°C, and mainly involved the transformation of amorphous phases into crystalline oxide and phosphide forms. Under gasification conditions, all crystalline phases appeared as different phosphates and alumino-silicates. Discussion The methods described here provide different options for the disposal of biosolids from wastewater by adjusting and optimising thermochemical conversion processes.