Development of an effective swine manure-based algal cultivation system for biofuel & animal feed production and wastewater treatment

Abstract

Microalgae have great potential to replace current crop feedstocks for biofuel and animal feed production. However, the algal industry is still far from being economically applicable. The dissertation was inspired by the idea of integrating algal biomass production and wastewater treatment. The overall objective of the study was to develop an effective algal cultivation system in which inorganic nutrients and organic substances in swine manure could be utilized by robust microalgae strains for the production of algal biomass with high lipids or protein contents. The first step in the study was bioprospecting for mixotrophic microalgae strains that adapted well to diluted swine manure. Through the multi-step screening strategy, two locally isolated microalgae strains, UMN271 and UMN266, were found to be facultative heterotrophic, manure-tolerant, and obviously robust in the algae storage including 98 UTEX strains and 50 indigenous strains. The two strains were identified as Chlorella sp. and Hindakia sp., respectively, through morphological observation and genetic identification, and were utilized for further studies in the dissertation. Since algae growth on highly diluted swine manure was still relatively low, a second step in the study was to assess the limiting factors of using anaerobically decomposed swine manure as nutrient supplement for algae cultivation. In the study, Chlorella sp. (UMN271) and Hindakia sp. (UMN266) were used to investigate the effects of two potential factors, which were trace elements and carbon compounds in swine manure, on algal growth and waste nutrient recovery. The results indicated that the algal growth and nutrient removal rates were independent of trace metal content in diluted swine manure, but were significantly improved when the initial liquid COD content in swine manure was high. Moreover, it was demonstrated that Chlorella sp. (UMN271) was able to utilize acetic, propionic and butyric acids, the main water-soluble organic carbon compounds in digested swine manure, for algal growth, lipid production, and waste nutrient removal. Therefore, it was considered that algal growth in highly diluted swine manure was limited by the deficiency of volatile fatty acids (VFAs). The third step was to modify anaerobic digestion process for liquid swine manure (LSM) rich in VFAs, so that the liquid effluent could be more suitable than the conventionally decomposed LSM effluent as nutrient for the cultivation of Chlorella sp. (UMN271). The results showed that the modified acidogenic digestion successfully promoted VFA concentrations in swine manure. The obtained acidogenically digested LSM supported the growth of Chlorella sp.(UMN271) in a 5-day batch experiment with higher algal growth rates and fatty acid contents in comparison with those on the conventionally decomposed LSM. High removal efficiencies on water-soluble nutrients, including COD, PO4-P, TN and NH3-N, were also observed for the raw, acidogenically digested manure sample (58.02%, 44.74%, 31.58%, and 44.73%, respectively). Finally, the fatty acid profile analysis showed that harvested algal biomass could be used as feedstock to produce high-quality biodiesel. In the fourth step, central composite design (CCD) was used to investigate the influences of two key parameters, namely wastewater dilution rate (DR) and hydraulic retention time (HRT), on algal biomass productivity and waste nutrient removal rates. According to the response surface analyses of the CCD results, statistically valid quadric models for the response variables, including algal biomass productivity and the liquid nutrient removal rates including COD, PO4-P, TN and NH3-N, were obtained. The regression analyses illustrated that both DR and HRT had significant influences on the five response variables. The optimal conditions estimated from the significant second-order quadratic models (p<0.05) were 8-fold DR and 2.26-d HRT. The final step was the development of an effective algal cultivation system for the semicontinuous production of algal biomass and waste nutrient removal. In the study, the predicted optimal conditions were applied in a bench-scale multi-layer photobioreactor (PBR) for algae growth on acidogenically digested swine manure. The 17-day semicontinuous cultivating experiment achieved high and stable algal productivity and nutrient removal rates, which fitted the predictive models well. Moreover, relatively high and stable protein and lipid contents (58.78% and 26.09% of the dry weight, respectively) were observed for the harvested algal sample, indicating the suitability of the algal biomass as ideal feedstock for both biofuel and animal feed production.