Nutrient and organic matter removal from wastewaters with microalgae
|Tila||Julkaistu - 22 toukokuuta 2019|
|Nimi||Tampere Univeristy Dissertations|
Use of microalgae in wastewater treatment has been increasingly studied to integrate with or replace the present treatment systems for removal of nutrients and other pollutants. The potential advantages of this integration (wastewater treatment and microalgal cultivation) could be simultaneous recovery of nitrogen and phosphorus and the use of produced microalgal biomass as feedstock for e.g. biofuel, fertilizer and/or energy. However, the use of microalgae in wastewater treatment is mainly in research stage due to e.g. low nutrient removal and microalgal biomass growth. The aim of this thesis was to
enable efficient nutrient and organic matter removal from wastewaters by microalgae while promoting microalgal biomass production.
Chlorella vulgaris and Scenedesmus acuminatus were successfully grown in batch photobioreactors with liquid digestates from anaerobic digestion (AD) of biosludge from a municipal wastewater treatment plant (ADMW) and a pulp and paper mill wastewater treatment plant (ADPP). The final ammonium removal efficiencies were above 97% when cultivating both microalgae separately in ADPP, however, only 24% and 44% of ammonium were removed from ADMW by C. vulgaris and S. acuminatus, respectively. Both microalgae efficiently removed phosphate (>96%), while color (74–80%) and soluble COD (27–39%) were partially removed from ADMW and ADPP. The obtained highest S. acuminatus biomass concentration (7.8–10.8 g L-1 VSS) in ADPP is among the highest yields reported for microalgae in real wastewaters. Higher S. acuminatus biomass yields were obtained in thermophilic ADPP (without and with pretreatment prior to AD: 10.2±2.2 and 10.8±1.2 g L-1, respectively) than in pretreated mesophilic ADPP (7.8±0.3 g L-1). In addition, the highest microalgal biomass concentration and methane yields were obtained in the same integrated AD and microalgal culti-
vation system (thermophilic AD with pretreatment). The iron (0.1, 1.0, and 1.9 mg L-1) and sulfate-sulfur (3.7, 20, and 35.8 mg L-1) concentrations were found to affect nitrogen removal efficiency and microalgal biomass concentration more in the media with nitrate than with ammonium, probably due to different microalgal assimilation mechanisms for nitrate and ammonium. In this study, synthetic medium with nitrate as nitrogen source with 1.0 mg L-1 iron and 35.8 mg L-1 sulfate-sulfur enabled the highest microalgal biomass concentration. The effect of iron concentration on nitrate removal efficiency and microalgal growth was more significant than that of sulfate concentration, while the interaction effect between sulfate and iron was not observed.
The average ammonium removal efficiency (14 to 30%) and microalgal biomass concentration (0.50 to 1.17 g particulate organic carbon per L) in continuous-flow membrane photobioreactor were promoted by adding a low concentration of zeolite (0.5 g L-1). The zeolite likely provided a habitat for attached growth of microalgae and high availability of ammonium for growth on the surface of the zeolite due to ammonium adsorption to zeolite. Further increase in zeolite concentration (from 0.5 to 1 and 5 g L-1) did not improve ammonium removal efficiency or biomass concentration. This was likely due to the increased solution turbidity caused by breaking apart of added zeolite particles into finer particles, which reduced light availability.
In summary, this work showed the possibility of utilizing microalgae in wastewater treatment to efficiently remove nutrients and organic matter, and simultaneously promote microalgal growth. Selecting suitable microalgal species for the specific wastewater to remove nutrients and organic matter is essential to promote algae-based wastewater treatment applications.