Use of biosolids in enhanced fired clay bricks

Abstract

The ever growing quantities of waste materials and their management costs, strict regulations declared for landfilling of waste, shortage of landfill spaces, and scarcity of natural earth materials highlight the urgent need to investigate sustainable routes for the recycling and reusing of waste materials. Biosolids are solid and treated organic residual by products from the wastewater treatment process and are available for beneficial use in accordance with relevant regulations. The production of biosolids is increasing annually as a result of increasing water demand and wastewater generation in metropolitan areas throughout the world, which, consequently, presents an urgent need for landfill spaces to dispose of biosolids and other by products. The State of Victoria, Australia, alone produces approximately 95,600 dry tonnes of biosolids annually on top of an approximated 2 million dry tonnes of existing biosolids, which involves a biosolids management cost of approximately A$90 million every year. Therefore, incorporating biosolids into building materials, for instance, fired clay bricks, could be a winwin strategy for the comprehensive recycling of biosolids. Bricks are one of the longest lasting and strongest manufactured building materials, and have been used over a long period of time. Due to their strength, reliability, weather resistance, flexibility in composition, simplicity, and durability, bricks are used extensively throughout the world. Due to a shortage of natural brick soil, brick production has been limited in some countries, such as China, in order to protect the virgin clay quarries. Therefore, innovative approaches to producing fired clay bricks with fewer virgin resources are highly encouraged from the perspective of environmental protection and sustainable development. Attempts have been made to manufacture bricks incorporating various types of waste materials, such as sludge, fly ash, wood sawdust, silica fume, cigarette butts, and polystyrene. The results of these studies showed positive effects from the recycling of waste materials into a ceramic body, such as lightweight bricks with higher thermal insulation properties. This thesis presents and discusses the possible reuse of biosolids in fired clay bricks and the effect of incorporating biosolids on the compressive strength, density, and other physical and mechanical properties of bricks. In this study, biosolids from the stockpiles of the Eastern Treatment Plant (ETP) and Western Treatment Plant (WTP) in Melbourne were used to assess their suitability as a partial replacement material for clay in the formulation of fired clay bricks. The major research outcome in this research is the establishment of a process for manufacturing fired clay bricks incorporating biosolids. Furthermore, the possible levels of environmental impacts, such as the leaching of heavy metals, emissions, and life cycle assessment (LCA) related to the biosolids amended bricks were investigated. In order to achieve the aforementioned objectives, this study was conducted in three stages. In the first stage of the study, extensive laboratory experiments were conducted to investigate the geotechnical properties of different biosolids samples, as the existing knowledge is still limited in this context. The geotechnical properties, such as Atterberg limits, particle size distribution, linear shrinkage, specific gravity, and organic content, were determined. A series of standard Proctor compaction tests were carried out to investigate the variation in the maximum dry density and optimum moisture content of biosolids soil mixtures with different percentages of biosolids. The chemical composition, mineral composition, and thermal analysis of all biosolids samples were also determined. Thereafter, four sets of brick samples were manufactured incorporating 25% of three different ETP biosolids and one WTP biosolids. The physical and mechanical properties of biosolids amended bricks were tested to assess their suitability as a partial replacement material for the brick soil in fabricating clay bricks. Furthermore, all the properties of biosolids amended bricks were compared with conventional bricks. In the second stage of the study, bricks were manufactured incorporating five different percentages (5, 15, 25, 35, and 50%) of selected ETP biosolids, and the physical and mechanical properties of bricks were evaluated. In addition, the effects of the addition of ETP biosolids on the microstructure of the bricks were investigated by means of scanning electron micrographs. A novel method was implemented at the laboratory scale for measuring the energy consumption of bricks during the firing stage. Moreover, the leachate analysis was undertaken for the biosolids amended bricks according to the Australian and US EPA standards. The leaching concentrations of toxic heavy metals were then compared with the respective regulatory limits. In addition, the effect on the physical and mechanical properties, energy consumption, and microstructure of biosolids amended bricks of the organic content present in the brick mixture was investigated. As a final part of stage two, the multivariate statistical analysis was carried out to investigate the effect of organic content on the properties tested, and determine the variations and interdependency of the results. In addition, the effect on the properties of biosolids amended bricks from a change in the heating rates for the firing was investigated. Higher heating rates can significantly reduce the firing time, which, consequently, reduces the energy demand for the firing process. Therefore, investigating the physical and mechanical properties of bricks at different heating rates would provide a better understanding of the use of a precise heating rate without compromising the properties of bricks. Therefore, the effect of incorporating different percentages (5, 10, 15, 20, and 25%) of WTP biosolids and the effect of heating rates (0.7, 1.0, 1.5, and 2.0 °C per min) during the sintering process on the physical and mechanical properties of biosolids amended bricks were investigated. In the third stage of this study, a new technique was developed and used for the measuring of gaseous emissions during the firing of bricks. The emission factors were then developed for CO, CO2, NO, HCN, and SO2 for ETP and WTP biosolids amended bricks. Furthermore, in this stage, a comparative Life Cycle Assessment (LCA) was carried to compare the environmental footprint between biosolids amended bricks and conventional bricks. Potential environmental impacts, such as climate change, ozone depletion, acidification, human toxicity, terrestrial and marine ecotoxicity, urban land occupation, and water depletion, were analysed and compared through the ReCipe midpoint life cycle impact assessment (LCIA) method using SimaPro 8.0.5.13 software. Overall, the experimental results and LCA results found in this study were promising and encouraging for the recycling of biosolids into fired clay bricks. The findings of this study showed that the incorporation of biosolids produced bricks with higher porosity, lower density, better thermal insulation properties, and acceptable compressive strength. Also, it was found that the percentage of organic content present in the brick mixture has a significant effect on the physical and mechanical properties, energy consumption, as well as the microstructure of biosolids amended bricks. The quality of ETP biosolids bricks was found to be relatively better than that of WTP biosolids bricks, which was mainly due to the lower organic content present in the ETP brick mixture than that in the WTP brick mixture. The leachate analysis results showed that the addition of biosolids in a ceramic body immobilised some of the toxic heavy metals present in the biosolids and the organic content present in the brick mixture has an insignificant effect on the leaching concentrations of heavy metals. Interestingly, the leachate test results revealed that the concentrations of toxic heavy metals were well below the Australian and International regulatory limits for industrial solid wastes. The comparative LCA between the biosolids amended bricks and the control bricks showed that the addition of biosolids is beneficial because the biosolids amended bricks had a positive environmental impacts compared to conventional bricks