Year

1996

Degree Name

Doctor of Philosophy

Department

Department of Civil and Mining Engineering

Abstract

Waste disposal has became one of the major problems of the industrialised world. With increasing world population the amount of solid waste generated has increased dramatically. Sewage sludge and fly ash, are two waste products which are produced in large quantities and pose several environmental hazards. With better wastewater treatment processes, the quantity of sewage sludge generated worldwide has risen significantly. In Sydney area alone, 125 dry tonnes of sewage sludge is produced daily, and safe and reliable disposal method need to be found. Fly ash is a waste product generated from coal fired power plants and most of these waste are stockpiled in storage areas. An estimated 6 million tonnes of fly ash is produced in N S W in one year. In an effort to introduce a satisfactory means of disposal, these two wastes were combined to produce a structural material called "Biofly brick". It is an innovative approach to convert largely unacceptable wastes to a beneficial and useful material. This thesis contains two volumes. Volume I describes the detailed work undertaken for the Biofly brick process. Volume JJ gives the summary of all test results. A comprehensive investigation into the production of full size Biofly bricks in a plant scale has been undertaken. Biofly bricks were made from various proportions of fly ash, four types of sewage sludge and clay/shale mixture. The structural and environmental suitability of the Biofly brick was evaluated. Full size Biofly and 100% clay bricks were manufactured at the University of Wollongong, Civil Engineering laboratories, and 2200 bricks were made. These Biofly and clay bricks have been tested for various engineering and environmental properties. The results indicate that the average compressive strength ranged between 21.4 to 49.7 M P a for Biofly brick and 39.1 M P a for ordinary clay brick, average transverse strength ranged between 4.5 to 10.5 M P a for Biofly brick and 4.2 M P a for conventional clay brick, absorptivity averaged between 0.83 to 1.09 for Biofly brick and 0.97 for clay brick, characteristic expansion ranged between 0.15 to 1.86 m m / m for Biofly brick and 1.70 m m /m for ordinary clay brick, and initial rate of absorption ranged between 0.17 to 1.52 kg/m^/min for Biofly brick and 0.50 kg/m^/min for conventional clay brick. Test results for efflorescence, examination for pitting due to lime particles and resistance to salt attack are also reported. Other properties, such as linear shrinkage was 8.7% for Biofly brick and 13.9% for clay brick, bulk density ranged between 1520 to 1750 kg/m3 for Biofly brick and 2170 kg/m3 for ordinary clay brick, and weight loss was 28.5% for Biofly brick and 2 0 % for clay brick. Assessment of these structural properties were evaluated according to the Australian standard 1226 which indicates that the Biofly bricks exhibit superior structural properties in comparison to the ordinary clay brick. This is primarily attributed to the combination of fly ash and sewage sludge used as replacement materials. A comprehensive leachate study was undertaken for three different size fractions on all the bricks made. All leachate samples were analysed for trace metals: copper, iron, manganese, nickel, lead, zinc, cadmium, chromium and aluminium, utilising atomic absorption spectrophotometry. The concentration of heavy metals from the Biofly brick leachate was similar or lower than that of the conventional clay brick except in the case of cadmium and iron, but it is well within the standard limits proposed by Victoria E P A and U S CFR. It was expected that the concentration of metals in the leachate should be increased with the increased exposed surface area of the smaller particles. However, this was not the case, and the metals remained "locked up" even in the small particles. This may be due to the greater ion exchange capacity of the smaller particles which prevents significant leaching of the metals. The gas consumption and gas emission study indicated that the Biofly brick process uses less energy and emits smaller amount of air pollutants compared to a standard clay making process. The Biofly process has the potential to control the emission of air pollutants during the firing of bricks. Heavy metals, fluoride, chloride and acid gases emissions, generally associated with the conventional clay brick manufacturing process, are also reduced during the Biofly brick process. The concentration of metals emission from Biofly brick were found to be lesser than that of the conventional clay bricks and lower than the standard limits proposed by Victoria EPA. The contained metals were believed to be physicochemically locked inside the vitrified brick, possibly due to a silicate-based entrapment mechanism. The large scale laboratory experimental results indicated that, Biofly bricks have been successfully made using fly ash, sewage sludge and clay/shale. The characteristics of the full size Biofly bricks have been found to depend on the sludge type and waste mix however by suitably selecting the sludge type and mix composition, the Biofly bricks can be produced 20 - 2 5 % lighter, 10 - 3 0 % stronger and with significant energy savings of upto 4 4 % in comparison to conventional clay bricks. The Biofly brick also was fired in approximately less than one half of the time required for firing clay brick and hence the kiln throughput will be increased by almost 100%.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.