Portfolio of Anaerobic Digestion Projects

    Ph.D Thesis

    My Ph.D Thesis, entitled "Support Matrix and Feed Flow Effects in Anaerobic Fixed-Bed Reactors", was completed under the supervision of Professor Emer Colleran (RIP) at the University College Galway, Ireland in 1987. This involved examined the relationship between the potential for methane production in anaerobic digesters operated using agricultural waste in the following ways:

    • Upflow - Fixed Film Packed Reactor
    • Upflow Hybrid/Sludge-Bed Fixed Film - Effect of varying the matrix quantity
    • Downflow Fixed Film

    Anaerobic reactors were studied for optimisation in:

    • Start-up phase
    • Long term operation
    • Methanogenic activity in attached and non-attached sludges
    • Liquid flow dispersion

    Together with Professor Colleran we developed and improved upon specific methanogenic activity tests that existed during the 1980s. These included the Cofactor F420 assay and the measurement of specific methanogenic potential of sludges.

    Anaerobic Activity Tests

    Development of a Rapid Methanogenic Activity Test Procedure

    The specific methanogenic activity of anaerobic sludges is usually assayed by gas chromatographic analysis for methane in the headspace of sealed test vials. Gas is routinely sampled with a pressure lock syringe, which allows quantification independent of the pressure prevailing in the vials. An alternative method was developed using pressure transducer to monitor the increase in headspace gas pressure as CH4/CO2. Application of a simple formula, related the increase at each sample point to millilitres of CH4 produced, gave values for the specific methanogenic activity of a variety of anaerobic sludges, which were in good agreement with the values obtained by the more labor-intensive gas chromatographic method. This method has been subsequently developed by other researchers to measure hydrogenotrophic activity of sludges and for the methanogenic activity of pure and mixed cultures of methanogenic bacteria.

    Coenzyme F420 Content of Methanogenic Sludges

    The levels of this coenzyme have been used in anaerobic digestion studies as a measure of the total methanogen content of digester or seed sludges and as a predictive value of their potential methanogenic activity. Determination of the coenzyme F420 content of anaerobic sludges from operational digesters by the various methods reported in the literature was shown to give conflicting results. The main methods in general use were compared and individual steps in the extraction and quantitation protocols were evaluated, modified and optimised. A standardised procedure, which gave accurate and reproducible results for all sludge samples tested, was developed.

    Anaerobic Sulphate-Based Trials

    Postdoctoral Study: Treatment of Landfill Leachate by Methanogenic and Sulphate-Based Anaerobic Digestion

    High concentrations of sulphate in wastewater streams subjected to anaerobic digestion have been considered to be undesirable because of the production of sulphide during digestion, which can inhibit methanogenesis. Appreciable sulphate may be present normally in many industrial effluents, and anaerobic digestion of such effluents results in low biogas formation and high concentrations of sulphide in the effluent.

    Sulphate-reducing bacteria (SRB) compete with methanogenic bacteria (MB) for substrates such as acetate and hydrogen, outcompeting MB when sulphate is present in the environment. While the methanogenic bacteria metabolise only a restricted range of substrates (principally acetate and hydrogen), the SRB metabolise a far wider range of substrates, including long chain fatty acids, aromatic compounds etc. The SRB might be expected, therefore, to be more resilient to challenge by novel organic molecules than the MB, and digestion based upon sulphate-reduction therefore more resilient than methanogenic digestion. Furthermore, the production of sulphide during digestion may precipitate many metals as their insoluble sulphides, a process which has been used to reduce high metal content in waste waters. In contrast, methanogenic digestion has a reputation for being sensitive to toxicity by organic inhibitors such as toluene or chloroform and by metals. Inhibition of methanogenesis can also be caused by organic overload when the rates of acid production exceeds methane production, leading to decreased pH and souring of the digester.

    This study was designed to investigate the effect of high sulphate concentrations, which may be present in landfill leachates. The study was commissioned by the UK Department of Environment (now Defra) who needed to understand the impact of elevated sulphate levels in landfill leachate, which would be expected to occur if coal containing high sulphate levels was imported from outside of the UK, and the disposal option for the processed coal waste (to remove sulphate) was co-disposal with household and/or industrial waste in landfill. This work investigated the treatability of landfill leachate with elevated sulphate and heavy metals by anaerobic digestion, comparing methanogenic with sulphate-based digestion, in upflow anaerobic digesters where biomass was retained as biofilms on support material.

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