Microbial (Microalgal-Bacterial) Biomass Grown on Municipal Wastewater for Sustainable Biofuel Production
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
High biomass productivity and efficient harvesting are currently recognised challenges in microbial biofuel applications that were addressed by this research using ecological engineering principles and an integrated systems approach. Microbial (microalgal-bacterial) biomass was grown in laboratory reactors using municipal wastewaters from the Christchurch Wastewater Treatment Plant (CWTP) in New Zealand. Reactors were inoculated with native microbes, fed with primary and secondary treated wastewaters, and subjected to various hydraulic and solids retention times (i.e., 1.4- to 9-d HRT and 4- to 80-d SRT, respectively) under cold, warm, and ambient climate conditions. Biomass settleability and productivity (i.e., settleable productivity) were sequentially improved over the course of experiments to optimise settleable productivity at 21 g/m2/d on average using primary treated wastewater, 2-d HRT, 12-d SRT, and warm climatic conditions. Secondary treated wastewater was a poor substrate most likely because of low C, elevated pH, and supersaturated oxygen levels limiting growth. Biomass recycling generally improved settleable productivity of primary treated wastewater cultures since productivity increased at short HRT and settleability increased at longer SRT. No overriding trends were found relating productivity or settleability to biomass ecology or biochemistry.
Growth rate modelling of warm climate cultures indicated that heterotrophy was mostly C limited at long (≥ 4-d) HRT and DO limited at short (≤ 2-d) HRT of primary treated wastewater while photoautotrophy was probably always light limited. Nevertheless, almost 50% greater C fixation was achieved using these systems compared to conventional activated sludge systems. Cold climate cultures, with up to 66% less biomass than warm climate cultures, were limited by lower light and/or temperature (i.e., 13 °C mean water temperature with 410 μmol/m2/s photosynthetically active radiation [PAR] for 9.6 h/d vs. 21 °C mean water temperature with 925 μmol/m2/s PAR for 14.7 h/d).
Biomass settleability was facilitated by microbial aggregation into stable, compact flocs over time and also by bioflocculation during 1-h sedimentation periods. These mechanisms were largely influenced by wastewater loading and microbial growth rate, but also to a lesser extent by monitoring methods (i.e., light, duration, and sedimentation container). Settleability of primary treated wastewater cultures was mainly greater than 70% and more consistent when operated at longer SRT and shorter HRT compared to only 22% on average for secondary treated wastewater cultures. Symbiotic growth of native microalgae and bacteria promoted efficient O2/CO2 exchange to improve productivity and enhanced natural floc formation to improve settleability while requiring low energy inputs and providing some wastewater treatment. These capabilities greatly increased the biomass’ sustainability for biofuel production compared to other feedstocks. This research demonstrated the value of biomass recycling to concurrently achieve greatest productivity and settleability to maximise harvestable yield since the overall growth rate of more total biomass was reduced at longer SRT which thereby facilitated excellent floc formation and sedimentation at shorter HRT. The resulting biomass was best suited for biofuel conversion pathways such as anaerobic digestion or thermochemical liquefaction. Potential other uses included animal feed and fertiliser since biomass was harvested without additional chemicals.