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Microalgal cultivation to treat anaerobically digested domestic effluent (ADDE)

Gibson, Leia (2020) Microalgal cultivation to treat anaerobically digested domestic effluent (ADDE). Other thesis, Murdoch University.

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In this study, the cultivation of microalgae in anaerobically digested domestic effluent (ADDE) was evaluated. Based on the knowledge from other literature, this is the first study of its kind. The ADDE was sourced from a local wastewater treatment plant (WWTP) which had a total ammonia content of approximately 1000 - 1500 mg L-1 NH3+-N. The overarching aim of this study was to identify the optimal nutrient concentration to maximise the growth of the mixed microalgal consortium (Chlorella and Scenedesmus species) in the ADDE supernatant. Apart from this, the growth comparison of microalgae cultivated in sand-filtered and unfiltered, ADDE supernatant was also assessed.

In this research, two experimental studies were conducted. The first study evaluated the growth of the mixed microalgal consortium in different dilutions (25%, 50%, 75% and 100%) of the unfiltered, ADDE supernatant. Based on the first study, the mixed consortium produced the highest mean specific growth rate (μ) in higher dilutions (25% and 50%) of the unfiltered, ADDE. The observations from the first study led to the creation of the second experiment, which observed the same consortium in the sand-filtered and unfiltered, ADDE at 25% and 50% dilution. No statistical difference, however, was observed between the average biomass productivities (Pr) and μ of different treatments in the second study. The algal growth, however, in the 50% diluted, unfiltered ADDE treatment (50% UA treatment) possessed a higher μ when compared to the other treatments.

The 50% UA treatment was also the most efficient treatment at removing nitrogen sources in comparison to the other treatments. It removed 63% and 77% more total nitrogen (ammonia, nitrate and nitrite) when compared to the average removal rates of the same nitrogen sources observed in other treatments. No statistical difference was, however, detected between the mean nitrogen (ammonia, nitrate and nitrite) removal rates of all treatments.

The mixed consortium in all treatments was, however, not very efficient at removing phosphorus and COD. The mean phosphate removal rates observed in this second study was also much lower when compared to the phosphate or total phosphorus removal rate reported in other literature. The cause for this might be due to the non-ideal N: P ratio of the wastewater. The unfiltered, ADDE supernatant contained higher levels of phosphorus instead of ammonia. More nitrogen is typically required for microalgal growth instead of excess phosphorus which makes only 1% of the microalgal dry weight. In terms of COD removal inefficiency, microalgal cells can release substances that increase COD levels, especially in conventional microalgae-based wastewater treatment systems (WWTS). Most of the COD in wastewater are also non-soluble and take a long time to biodegrade.

As for the aerobic bacteria population observed in the second study, no statistical difference was detected between the aerobic cell counts of different treatments on day 0, 1 and 3. There was an increase (by about 40% - 50%) in aerobic bacterial cells observed in all treatments between day 1 and 3. An average of 1× 107 CFU mL-1 aerobic bacteria cells was observed in all treatments towards the end of the experiment of the second study. There were no E.coli or coliform bacteria observed in all the treatment replicates through the entire experimental period of the second study.

In terms of heavy metal absorption, the same consortium that was grown in the 50% UA treatment, removed about 21% and 25% of Cd, Cr and As. The microalgal biomass cultivated in this treatment was more efficient at assimilating Cr and Cd over As. The concentration of As and Cd present in the mixed consortium’s biomass was respectively, about 83% - 99% higher than the maximum levels of these heavy metals (HMs) recommended by the Codex Alimentarius set by the FAO. It is unknown if the Cr levels in the mixed consortium’s biomass exceed the maximum levels since it was not stated in the Codex Alimentarius.

While the 50% UA treatment was the most efficient at removing nitrogen sources (ammonia, nitrate and nitrite) in comparison to the other treatments. It was still relatively low in comparison to nitrogen (nitrate, nitrite and ammonia) removal rates reported in other published studies. The phosphorus or phosphate removal rates published in other studies were also much higher in comparison to this research. Based on the heavy metal analysis; the microalgal biomass produced in the same treatment, cannot be used to produce food and feed. It does not meet the international safety standards outlined in the Codex Alimentarius set by the FAO. The high level of aerobic bacteria detected in all treatments was due to the dissolved oxygen produced by the photosynthetic activity of microalgal cells. More work is still required before this indoor lab study can be conducted outdoors. Further analysis is needed to fully understand how to optimise the growth of this consortium in the ADDE supernatant.

Item Type: Thesis (Other)
Murdoch Affiliation(s): Environmental and Conservation Sciences
Notes: This is a Research Masters with Training thesis
Supervisor(s): Moheimani, Navid and Vadiveloo, Ashiwin
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