Process Regulation Of Wastewater Advanced Treatment By Oleaginous Microalgae

As water scarcity and energy crisis are two of the biggest challenges nowadays, microalgae have attracted much attention due to their ability to remove nitrogen and phosphorus from waste water and their potential in producing biodiesel. Thus, the coupling of advanced wastewater treatment and biodiesel production based on microalgae is a promising technology. Furthermore, the microalgae use CO2 to create biomass, which can reduce CO2 emission and slow down the greenhouse effect. Two problematic areas of the coupling system are the low biomass density and the high harvesting cost. In this study, the microalgae with high growth rate and high lipid content was chosen to treat wastewater, and the cultivation conditions were optimized to promote the growth of microalgae, wastewater treatment efficiency and lipid productivity. The CO2 fixation efficiency and the effect of carbon source on microalgal growth and lipid production was further studied. Based on the study of microalgal settleability, the forward osmosis membrane technology was employed for microalgae seperation and the continuous operation of the coupling system. The main research of the thesis included the following five aspects:(1) Four species of microalgae were chosed to investigate the algal growth and lipid production potential within wastewater. Chlorella vulgaris and Scenedesmus obliquus have higher growth rate and nutrients removal efficiency. Both of them could grow well within real municipal secondary wastewater, but came into the decline phase soon after due to the contamination of bacteria, which had become a matter of urgent doubt. Scenedesmus obliquus had higher growth rate and could remove the organic pollutants within wastewater, while the Chlorella vulgaris could not utilize organic carbon. However, the lipid content of Chlorella vulgaris was higher than that of Scenedesmus obliquus. Chlorella vulgaris could remove nitrogen and phosphorus and obtain high lipid productivity at the same time.(2) Chlorella vulgaris could remove nitrogen and phosphorus and obtain high lipid productivity. The removal efficiencies of TN and TP were found to be 92.2-96.6% and over 99%, respectively, after a batch cultivation of 20 days, when the initial TN concentration was 10-40 mg L-1. The removal efficiencies of TN and TP were 96.1% and over 99%, respectively, when the initial TP concentration was 0.5-5 mg L-1. To illustrate the response of lipid accumulation to nutrients removal, C. vulgaris was further cultivated in the recycling experiment within the photobioreactor. The lipid accumulation was triggered upon the almost depletion of nitrate (<5 mg L-1). The optimum CO2 concentration was 1% when cultivated within municipal secondary wastewater, and the maximum CO2 fixtion rate reached 109.5 mgL-1h-1.(3) To intensify wastewater treatment and microalgal biofuel production, the effect of organic and inorganic carbon on algal growth and nutrient removal of Scenedesmus obliquus were examined by varying TOC (Total Organic Carbon) concentrations and feeding CO2 concentrations. The results showed that the algal growth rate, lipid productivity and nutrients removal efficiency increased with the initial TOC concentrations and could be utilized for simultaneous organic pollutants reduction, N, P removal and lipid accumulation. The optimum CO2 concentration was 5% when cultivated within municipal secondary wastewater, and the maximum CO2 fixtion rate and average CO2 fixtion efficiency were 101.3 mg L-1h-1 and 4.8%, respectively.(4) The effective microalgal harvest is the key to algal recycling, and the good settleability of microalgae would prompt the harvest efficiency. The settleability of stable phase was best during entire cultivation days, and increased with the increasing of pH.S. obliquus had better settleability than C. vulgaris and the 12h removal efficiency reached over 98% during the stable phase. The experiments also investigate extracellular polymeric substances (EPS) on the auto flocculation efficiency. The results showed that the BEPS of C. vulgaris and S. obliquus might hinder auto flocculation, and SEPS could promotes auto flocculation (mainly through adsorption bridging role of polysaccharides), and the promotion effect of SEPS was greater than the hinder effect of BEPS.(5) Based on the optimized operation parameters mentioned above, S. obliquus was disposed by photobioreactor for research of nutrients removal and lipid accumulation. Microalgae could be recycled and utilized for continuous treatment due to its good settleability and FO membrane technology. According to sequence treatment, the suitable initial algal density should be 1.5-2.0 g L-1 and the HRT of continuous treatment was 48h. The harvest efficiency would raise owing to the combination of auto flocculation and membrane technology.