by CHIH-TING KUO
Although combining algae cultivation with wastewater treatment has been highlighted as a promising pathway for sustainable biofuels, there are still several challenges that limit the ability to use algae for biological wastewater treatment. First, the treatment performance of an algal wastewater system is less stable than current systems using aerobic heterotrophic bacteria. For instance, Garcia et al. (2005) showed significantly increased effluent ammonia concentrations during the nighttime. Secondly, the turbidity of wastewater limits the penetration of light into the system, which reduces photosynthesic efficiency. Third, previously reported algae cultivation systems require up to 10 times more land area because of longer hydraulic retention times (HRTs) and shallow tankage designed to maximize solar energy capture.
To address the challenges listed above, this study reports on the development of a novel adsorbent-based algae cultivation system that improves the efficiency and reliability of integrated systems that provide both wastewater treatment and sustainable algal biomass production at a reasonable cost. Chapter 3 investigated the benefits of integrating adsorbents into an algal wastewater treatment system and found that adding granular activated carbon (GAC) and/or zeolite was able to improve both the effluent water quality and biomass productivity.
In Chapter 4, a commercially available rotating algal biofilm system (Algaewheel®) was used to evaluate the technical barriers of using algae to treat swine wastewater and subsequently convert the wet mixed biomass to biocrude oil via hydrothermal liquefaction (HTL). Three different hydraulic retention times (HRT) were used to study the effects of nutrient loading on the removal of nutrients and biocrude oil yield. The results were used to develop an advantageous operational strategy aimed at maximizing algal biofuel yield combined with relatively high nutrient removal efficiency.
In Chapter 5, the long-term benefits of integrating adsorbents into algal wastewater treatment systems were investigated. The adsorbent amended system was able to recover faster from shock loading events and provided more stable effluent quality. Moreover, this research demonstrated for the first time that algae wastewater treatment systems can be successfully operated with continuous recycling of HTL aqueous product (PHWW). The system without adsorbents had a significant reduction of biomass productivity when the PHWW concentration in the influent was above 1.5%. In contrast, the system with adsorbents had increased biomass productivity when PHWW added. The effects of service time on the adsorbents were also investigated. After 10 months of usage, activated carbon capacity was reduced by 40%, but the adsorption rate was not significantly different than virgin activated carbon. In contrast, after 10 months usage, zeolite had only a slight reduction in adsorption capacity, but the adsorption rate was reduced by an order of magnitude. These results indicated activated carbon might need to be regenerated after longer term usage (years) and zeolite may need some occasional washing operations to control the surface biofilm thickness and restore adsorption kinetics.
Chapter 6 conducted a techno-economic analysis for three different scenarios of algal wastewater treatment system coupled with biofuel production and nutrient recycling methods. The results showed that an adsorbent integrated Algaewheel® system coupled with HTL and recycling PHWW had the lowest biofuel production cost ($10.7/gal). In comparison,other alternative scenarios with Algaewheel/HTL/Catalytic Hydrothermal Gasification (CHG) and High rate pond/Extraction/Anaerobic digestion had biofuel production costs of $11.7/gal and $13.2/gal, respectively. Wastewater treatment credits and electricity credits were then estimated and included to calculate the minimum fuel selling price. The results showed wastewater treatment credits could potentially cover all the costs for biofuel production. Sensitivity analysis suggested that the HRT of the system and the biocrude oil yield had the most impact on costs.
Chapter 7 provides a summary and describes future work to facilitate the commercialization of algal wastewater treatment and biofuel production system. Recommended future work includes: 1) Investigate the effects of lower HRT and long-term effects of continuous PHWW recycle; 2) Study the tailoring of the selected adsorbents and mixing ratios to address different influent wastewater qualities. 3) Develop biomass pretreatment to reduce ash content of algal biomass for improved HTL biocrude oil yield. All in all, this study proposed a novel idea of integrating different types of adsorbents into the algae cultivation system to facilitate integration with wastewater treatment and improve biofuel production. This novel adsorbent-based algal cultivation system could overcome many of the current challenges for algae systems used for wastewater treatment and sustainable biofuel production.