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Title: Calcite-precipitating indigenous bacteria in landfills and their application towards ground improvement
Author: Rajasekar, Adharsh
ISNI:       0000 0004 7658 4778
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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This project offers a new outlook on landfills as potential locations from which calcite precipitating bacteria could be isolated and/or stimulated for ground improvement purposes. The research presented in this thesis confirms the presence of indigenous calcite-precipitating bacteria in landfills and investigates their potential application for ground improvement. For instance, this technique can be employed to reduce the permeability of the soil at landfill sites to prevent or minimise possible contamination of ground water from leachate. The novel research presented here, describes the isolation and identification of seven unique indigenous calcite-precipitating bacteria from landfill leachate and groundwater. In addition, their proof-of-principle application for ground improvement within geomaterials has been successfully demonstrated. A review of existing literature has highlighted the need for identifying indigenous calcite precipitating bacteria in harsh environments such as landfills. Most studies focussed on a number of known calcite precipitating bacteria that are readily available from cell culture collection laboratories and only a few researchers have tried to investigate the potential calcite precipitation of indigenous bacteria. A culture independent technique was implemented to investigate a selected landfill environment in detail, sampling its leachate, soil and groundwater. An urban sampling site located at a significant distance away from the landfill was selected for contrasting purposes. Using NGS, it was found that the bacterial consortia consisted of up to 16 phyla in which Proteobacteria, Actinobacteria and Bacteroidetes were found to be dominant in both sampling sites. Only Chloroflexi was detected at urban site and Pseudomonas was the dominant (67-93%) genus in landfill leachate. Arsenic concentrations were 1.11x103 μg/L and 1.78x103 μg/L for the landfill raw leachate (RL) and fresh leachate (FL2), respectively. Similarly, the mercury concentrations measured were 10.9 μg/L and 7.37 μg/L, for RL and FL2, respectively. These values were higher than the recommended values of those heavy metals in the Chinese State Environmental Protection Administration (SEPA) standards for leachate in landfills. Shannon diversity index and Chao 1 richness estimate showed RL and FL2 lacked bacterial richness and diversity when compared with other samples. A total of two indigenous bacteria from the landfill groundwater and five from the landfill leachate were selected and isolated using a nutrient broth media. The isolated bacteria were studied for their ability to mediate calcite precipitation. The media consisted of calcium chloride, urea and nutrient broth. The biomineralisation experiment was conducted at a starting pH of 7.5 at 30°C for 168 hours. The results from the isolates proved that indigenous calcite precipitating bacteria can be found in environments ranging from landfill leachate to groundwater. As a qualitative assessment, SEM images showed the difference in crystal morphology between the bacterial and abiotic solutions having spherical shaped crystals and trigonal shaped crystal formations, respectively. For quantitative analysis, carbonate titration experiments were performed following the biomineralisation experiment to determine the amount of carbonate precipitated by each bacterial strain during the biomineralisation process. The most prolific bacteria (bacteria that precipitated the most calcium carbonate) from landfill leachate and landfill groundwater were determined based on the amount of carbonate precipitated by the bacteria. The carbonate titration experiment revealed that the seven selected bacteria precipitated between 4.66 to 6.1 g/L of carbonate which was three times more than that observed for the abiotic solution. One prolific bacterium isolated from landfill groundwater and two prolific bacteria from landfill leachate were further investigated in column-based porous media studies. Porous media studies were conducted using specially designed polyvinyl chloride columns and sand was used as the porous media to determine the effects of biocementation exerted by the bacteria through biomineralisation. The experimentally generated compressive strength of the bacterial columns ranged from 150-260 kPa. Improved permeability ranging from 10-6 to 10-7 m/s was observed in the bacterial columns. Superior cementation between sand particles was observed under SEM in the columns where the bacteria were added. In summary, calcite-precipitating bacteria have been shown to survive even in contaminated leachate conditions. The extent of their calcite-precipitating abilities is shown through laboratory and porous media experiments. This means that the biohazard from the landfill is not necessarily a barrier towards remediation and applying microbially induced calcite precipitation (MICP) methodology along with monitored natural attenuation would prove to be beneficial in treating the leachate along with prevention of its interaction with groundwater.
Supervisor: Wilkinson, Stephen ; Bridge, Jonathan ; Raju, Sekar ; Medina-Roldan, Eduardo ; Loo Chin Moy, Charles Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral