Development and modelling of immobilised algal-bacterial consortia for nitrogen removal from wastewater

Microalgal-bacterial consortia can remove nitrogen with less energy consumption than conventional N removal, taking advantage of microalgae’s photosynthesis to provide bacteria required oxygen. Immobilization of algae and bacteria can potentially improve the settling velocity and performance of the consortia in the wastewater treatment systems. This thesis discusses the immobilisation of microalgal-bacterial consortia and its potential for ammonium removal from synthetic wastewater. Polyvinyl alcohol (PVA) and sodium alginate (SA) were screened for the optimal immobilising material and their concentration. Nitrification activity test was conducted with the immobilized consortia compared with free consortia. The results showed that 2% SA was more favourable by algae since it was less opaque. Thus, SA was used as immobilizing material. Immobilised bacteria, immobilised algae and immobilised microalgal-bacterial consortia were made. Fabricated double-layer beads were made to immobilise algae and bacteria separately, but the core-shell structure was unstable. In the batch mode activity test, immobilised consortia had the ammonium removal rate of 1 .13 mg N L-1 h-1, while the free consortia had a higher removal rate of 1.85 mg N L-1 h-1, but lower than the reported value from literature. Compared with bacteria and algae, the consortia had higher removal efficiency both in free and immobilised form. Moreover, the oxygen profile in the beads showed that immobilised consortia had lower oxygen production than the immobilised algae solely. Algae in the beads could produce O2 through photosynthesis, but immobilized nitrifiers did not show vigorous activity. Besides, AQUASIM software was used to model the immobilized microalgal-bacterial consortia in a batch mode. The model could predict some characteristics of the consortia, but needs to be further calibrated with more experimental data. The research demonstrates the immobilisation technique can hinder the mass-transfer and consequently, inhibit the microbial activity or even reduced the concentration of bacteria inside the beads. The interaction and mass-transfer of immobilised microalgal-bacterial consortia still need to be studied