PhD Thesis Defence Presentations - Konstantinos Papadopoulos

Developing low-cost technologies that offer the potential of energy and resource recovery is key to achieving efficiency and sustainability in the wastewater treatment field. Integration of photosynthetic microorganisms in aerobic microbial communities could be a step towards this direction: photosynthetic oxygen production replaces mechanical aeration, which is the main energy sink of aerobic wastewater treatment, and photosynthetic microorganisms fix inorganic carbon and convert it into harvestable biomass. This study aims to develop an algal-bacterial process for treatment of organic-rich wastewater that can serve as a sustainable alternative to conventional technologies, while investigating underlying phenomena with the use of mathematical modelling.

Using brewery wastewater as a case study, the performance of two different algal bacterial communities is evaluated: (i) an activated-sludge derived community and (ii) a community dominated by Arthrospira platensis. It is shown that, although both communities were viable, higher growth rates, higher pollutant removal rates, and lower microbial population fluctuations were observed in the case of the activated-sludge-derived one. The diversity of the community, the metabolic flexibility of Leptolyngbya sp., which dominated the culture, and symbiosis with heterotrophic bacteria allowed the microbial community to remove both organic and inorganic pollutants, and form large, settable aggregates, which allows efficient biomass harvesting.

Following on from this work, a semi-continuous algal-bacterial process that utilizes the unique characteristics of the microbial community isolated from activated sludge is described. The elementary operating principles of conventional bacterial activated sludge (i.e., sedimentation and biomass recirculation) are maintained, while aeration is solely provided by photosynthetic populations. The results revealed that performance was stable over consecutive operating cycles, and replicable in a pilot scale high-rate algal pond setup, while a biomass recirculation strategy not only enhanced the formation of larger, mature aggregates that settle more effectively, but also pollutant removal. Bioethanol production is also explored as a potential biomass utilization strategy. It is shown that bioethanol production coupled with high-performance wastewater treatment using algal-bacterial aggregates is feasible, albeit less productive than systems exclusively designed for third and fourth-generation biofuel production and conventional technologies like anaerobic digestion.

Experimental results are then fitted and analyzed with the aid of a mechanistic integral model, which utilizes IWA’s ASM model package as a foundation. It is revealed that light intensity and light attenuation are the main rate-controlling factors of the process. The role of aggregation is also further highlighted; it does not only serve as a means of biomass harvesting, but also appears to enhance light dilution and is estimated to be an effective strategy against predation. The model is also used to simulate operation of a large-scale wastewater treatment facility and estimate operating costs using literature data. It is shown that algal-bacterial wastewater treatment coupled with bioenergy production with anaerobic digestion can eliminate operating costs, although increased land requirements for high-rate algal pond installation ought to be taken under consideration.