Browsing by Subject "Operational parameter optimization"

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    Cultivation of algae on highly concentrated municipal wastewater as an energy crop for biodiesel production.
    (2012-06) Li, Yecong
    There has been renewed interest in bio-fuel production over the past decade due to the sharp rise in fossil fuel prices and increasing concerns about the global climate change. This dissertation was inspired by the idea of coupling algae based biofuel generation and municipal wastewaster treatment. The objectives of this study are to employ potential algae strains for efficient simultaneous biodiesel production and wastewater nutrients removal, and to develop an operational strategy optimal for biomass and biodiesel production as well as nutrient removal from a highly concentrated municipal wastewater stream, centrate, which is generated during the sludge thickening process. In the first stage, the feasibility of growing Chlorella sp. in the centrate for simultaneous wastewater treatment and energy production was tested. The characteristics of algal growth, biodiesel production, wastewater nutrient removal and the viability of scale-up and the stability of continuous operation were examined. Two culture media, namely autoclaved centrate (AC) and raw centrate (RC) were used for comparison. The results showed that by the end of a 14-day batch culture, algae could remove ammonia, total nitrogen, total phosphorus, and chemical oxygen demand (COD) by 93.9%, 89.1%, 80.9%, and 90.8%, respectively from raw centrate, and the fatty acid methyl ester (FAME) content was 11.04% of dry biomass providing a biodiesel yield of 0.12g-biodiesel/L-algae culture solution. The system could be successfully scaled up, and continuously operated at 50% daily harvesting rate, providing a net biomass productivity of 0.92 g-algae/(L·day). The second stage was targeted at screening one or several algae species/strains that can survive and grow well in centrate with high biomass and lipid productivity as well as superior nutrient removal efficiency, and investigating the significance of environmental factors including ligh intensity, light/dark cyle, and exogenous CO2 on biomass accumulation and biodiesel production using Plackett-Burman experiment design. In this stage the study was carried out in two sections, namely using aglae strains collected from local lake areas and those purchased from algae commercial bank, respectivity. In the first section, sixty algae-like microorganisms collected from different sampling sites in Minnesota were examined using multi-step screening and acclimation procedures to select high-lipid producing facultative heterotrophic microalgae strains capable of growing on centrate for simultaneous energy crop production and wastewater treatment. Twenty-seven facultative heterotrophic microalgae strains were found, among which seventeen strains were proved to be tolerant to centrate. These seventeen top-performing strains were identified through morphological observation and DNA sequencing as Chlorella sp., Heynigia sp., Hindakia sp., Micractinium sp., and Scenedesmus sp.. Five strains were chosen for other studies because of their ability to adapt to centrate, high growth rates (0.455 - 0.498 d-1) and higher lipid productivities (74.5 – 77.8 mg L-1 d-1). These strains are considered highly promising compared with other strains reported in the literature. In the second section, 143 different algae strains from the family of Chlamydomonas, Chlorella, Scenedesmus, Ankistrodesmus, Euglena, Chloroccum, and Botryococcus, etc. purchased from a number of U.S. institutions were screened use the same multi-step screening and acclimation procedures. The results showed that 14 algae strains from the genus of Chlorella, Haematococcus, Scenedesmus, Chlamydomonas, and Chloroccum were capable of growing on centrate. Since the highest net biomass accumulation (2.0143 g/L) was observed with Chlorella kessleri (UTEX398) followed by Chlorella protothecoides (UTEX25, 1.3089g/L), the two strains were used in further studies. It is found that both algae strains, UTEX398 and UTEX25 were capable of mixotrophic growth when cultivated on centrate. Environmental factors had significant effect on algal biomass accumulation, wastewater nutrients removal and biodiesel production for both strains. Higher light intensity and exogenous CO2 concentration with longer lighting period promote biomass accumulation, fatty acid methyl ester (FAME) formation, removal of chemical oxygen demand (COD) and total nitrogen (TN), while, lower exogenous CO2 concentration promotes phosphorus removal due to higher pH increase. In the third stage, the single effect of light intensity on biomass accumulation, wastewater nutrient removal through algae cultivation, and biodiesel productivity was investigated with algae species Chlorella kessleri and Chlorella protothecoide. The light intensities studied were 0, 15, 30, 60, 120, 200 μmolm-2s-1. The results showed that light intensity had profound impact on tested responses for both strains, and the dependence of these responses on light intensity varied with different algae strains. For Chlorella kessleri, the optimum light intensity was 120 μmol·m-2·S-1 for all responses except for COD removal. For Chlorella protothecoide, the optimum light intensity was 30 μmol·m-2·S-1. The major components of the biodiesel produced from algae biomass were 16-C and 18-C FAME, and the highest biodiesel contents were 24.19% and 19.48% of dried biomass for Chlorella kessleri and Chlorella protothecoide, respectively. Both species were capable of wastewater nutrients removal under all lighting conditions with high removal efficiencies. Since Chlorella kessleri showed superior capability of biomass accumulation and biodiesel production, it was used in the following sections.The fourth stage was aimed at investigating the single effect of light/dark cycle at low light frequency on algae based biodiesel production using centrate wastewater stream with mixotrophic strain Chlorella kessleri. The data suggested that the length of lighting period during a day greatly affects the algal biomass accumulation, biodiesel production and wastewater nutrients removal. The biomass concentration, biodiesel content and the removal of ammonia, total nitrogen and total phosphorus all increased with increasing lighting period. However, the removal of COD showed different trend, with higher removal rate observed under shorter lighting period. The results showed that for maximum biomass accumulation, biodiesel production and wastewater nutrients removal in batch culture system, the optimum lighting period is 16 hours and the cultivation time should be controlled at three days.In the fifth stage, the effect of supplying different concentrations of exogenous CO2 uder various light intensities at simulated light/dark cycle on biomass accumulation and wastewater nutrients removal was tested with aglae strain Chlorella kessleri. The results confirmed that there was an optimum CO2 level for algae growth, which is 2.5% for this investigation. When the light intensity reached a certain lower limit, the effect of CO2 supplementation became minimum because the photoautotrophic efficiency was significantly reduced. It is also found that a future study to investigate the effects of environmental factors on biomass and biodiesel production under continuous or fed-batch cultivation mode, where wastewater is continuously replenished and algae was harvested, would be valuable due to the fact that the nutrients was depleted and the algae biomass concentration decreased on Day 8. The objective of the last stage was to optimize the operational parameters, including light intensity, light-dark cycle, exogenous carbon dioxide concentration, and hydraulic retention time (HRT), for high algae biomass accumulation, biodiesel production and wastewater nutrients removal in designed fed-batch culture system. The results showed that response surface methodology with central composite design is effective in system optimization for algal-based biodiesel production using centrate wastewater in fed-batch culture. The regression analysis illustrates that, at p-value< 0.05, harvesting rate, light intensity and lighting period had significant effect on biomass concentration, biomass and biodiesel yield, as well as ammonia removal rate. Hydraulic retention time and exogenous CO2 concentration played important role in FAME content, while the removal rates of toal nitrogen (TN), total phosphorous (TP) and chemical oxygen demand (COD) could not be explained by quadratic models. The optimum conditions for biodiesel production are hydraulic retention time of 2.86 days, which means harvesting 35% of the total volume daily, light intensity of 190µmolm-2s-1 and lighting period of 10h.