Improving integrated systems for algal biofuels and wastewater treatment

The primary goal of this PhD work is to promote synergistic integration of wastewater treatment and algal biofuel production. In particular, we proposed and then analyzed the impacts of a novel integrated system that treats wastewater and maximizes the beneficial reuse of wastewater nutrients and organics to produce bio-crude oil. Centered around this novel system, we investigated several important aspects that can improve performance of the integrated system and resolve several of the key limitations of current algal biofuel production schemes. The novel system proposed in this study is referred to as “Environment-Enhancing Energy” (E2-Energy) because it can simultaneously improve conventional wastewater treatment by providing enhanced nutrient removal, while simultaneously producing a large amount of low-cost feedstock for biofuel production. The main components of the system are a mixed algal-bacterial cultivation process and a hydrothermal liquefaction (HTL) process with internal recycling of key resources between these processes. A series of algal cultivation and HTL experiments were conducted to confirm the feasibility of four key steps of the integrated system, which together facilitate multi-cycle nutrient reuse and biomass amplification to dramatically increase the biofuel potential of wastewater treatment systems. A mathematical model was developed using STELLA to simulate the dynamic mass balances of E2-Energy operations that showed the potential to amplify biofuel production from wastewater by up to 10 times. Thus, the current amount of biosolids in manure, food processing wastes and municipal wastewater (153 million dry tons), could be amplified using E2-Energy to provide enough biofuel feedstocks to replace all U.S. petroleum imports (0.5 billion tons) using only wastewater inputs and CO2. The mixed algal-bacterial cultivation process in the E2-Energy system was further investigated using a hybrid photobioreactor (PBR) that incorporated hollow-fiber membranes and granular activated carbon (GAC). The PBR was used to monitor the long-term performance when treating a combination of municipal wastewater and post-HTL wastewater (PHWW), which is an important step in the E2-Energy system. During 800 days of operation covering a range of operating conditions (15-20 day SRT, 1% to 4% PHWW loading rate, with and without GAC), the PBR supported stable and efficient removal of organic carbon (79%-93% COD) and nitrogen (50%-99% NH4+, and 27%-30% TN). Increasing PHWW loading rates generally reduced the removal of COD and NH4+. Solids retention time (SRT) below 15 days lead to reactor failure without GAC present. The addition of GAC facilitated stable PBR performance with shorter SRTs and higher PHWW loading rates. GAC also improved COD removal from 70% to over 90%, improved NH4+ removal from 26% to 100%, and improved cytotoxicity removal of the combined wastewater from 40% to 60%. The feasibility of treating PHWW via anaerobic digestion (AD) was also investigated, as a pretreatment step before the mixed algal cultivation. Successful AD occurred at concentrations of PHWW below 6.7%, producing a biogas yield of 0.5 ml/mg CODremoved, with a methane content of ~70%, and organic removal efficiency of ~50%. Higher concentrations of PHWW (≥ 13.3%) had an inhibitory effect on the AD process, as indicated by delayed, slower, or no biogas production. Activated carbon effectively mitigated this inhibitory effect by enhancing biogas production and allowing digestion to proceed at PHWW concentrations up to 33.3%. Thus, AD is a feasible approach to remove and recover carbon from PHWW, which can improve the overall energy efficiency of E2-Energy system. An algal-growth inhibition assay and response surface methodology were developed and used to study how HTL operating conditions affect the inhibition of algae by PHWW. The IC50 values of 15 PHWW samples generated under various HTL operating conditions (260-300⁰C, 30-90min, 15-35% solids ratio) ranged from 0.34%-1.9%. As solids ratio increased from 15% to 25%, PHWW inhibition of algae increased significantly, and then decreased slightly as solids ratio increased from 25% to 35%. PHWW inhibition also generally increased as temperature and retention time increased. GC-MS analysis revealed chemical compound differences between the most toxic and least toxic PHWW regarding the content of N, O heterocyclic compounds and straight chain oxygenates etc. COD, NH4+, and TN were all found to have moderate correlation with the PHWW inhibition. 280⁰C, 60min, and 15% solids ratio is recommended as HTL operating conditions that yields relatively high energy recovery (66.1%), and relatively low inhibition effect on algal growth (IC50 of 0.92%). Finally, in order to provide useful insights about potential ways to improving photosynthetic efficiency in algal biofuel production systems, biochemical characteristics of a potentially advantageous “mutant” strain (IM) of Chlamydomonas reinhardtii were quantified and compared with its progenitor (KO), and to its wild-type (WT). IM grown under low light (10 and 20 µmol photons m-2 s-1) had up to 27% higher dry weight than WT and KO cells. IM and KO cells, grown under 60 µmol photons m-2 s-1, also showed higher rates of net oxygen evolution and respiration than WT cells. The slow SM phase of chlorophyll a fluorescence transient was much reduced in IM cells, which is ascribed to a regulatory event called, “state 2 to state 1 change”. Additionally, when the IM strain is grown under low light, non-photochemical quenching of excited chlorophyll rose faster and recovered faster than in the other strains. Compared to WT cells, IM cells had more metabolites related to carbon metabolism and protection against oxidative stress. These results suggest that the IM strain has unique features that could improve algae productivity in optically dense cultures that are expected when integrating with wastewater treatment