The primary objective of this research was to improve our understanding of the water quality effects of thermochemical bioenergy production processes that can be applied to wet organic-laden wastes, such as animal manures, municipal wastewater, and food processing wastes. In particular, we analyzed the impacts of a novel integrated process combining algal wastewater treatment with hydrothermal liquefaction (HTL) on the fate of emerging bioactive contaminants (i.e., pharmaceuticals, estrogenic compounds, antibiotic resistant genetic material, etc.) and the potential for wastewater reuse. We hypothesized and then confirmed that the elevated temperature and pressure of an HTL process can effectively convert the bioactive organic compounds into bioenergy products or otherwise break them down to inactive forms. High performance liquid chromatography (HPLC) analysis of samples before and after HTL treatment showed removal of specific emerging bioactive compounds (florfenicol, ceftiofur and estrone) to below detection limits when HTL was operated at temperatures 250°C for 60 min or at 300°C for ≥30 min. Complete destruction and/or inactivation of antibiotic resistant genes in wastewaters by the HTL process was also obtained at all tested HTL conditions (250–300°C, 15–60 min reaction time). The presence of HTL feedstock such as swine manure or Spirulina algae reduced the removal of bioactive compounds (11–15%) and plasmid DNA (2–3%) when HTL was operated at lower temperatures (250°C) and short retention time (15 min). However, this effect was negligible when HTL was operated at 250°C for 60 min or at 300°C for ≥ 30 min. Analysis of the organic compounds in the HTL wastewater using liquid-liquid extraction in conjunction with nitrogen-phosphorus derivatization and gas chromatography-mass spectrometry (GC-MS) showed the occurrence of hundreds of nitrogenous organic compounds (NOCs). Purified chemical reference standards for nine of the most significant NOC chromatography peaks were obtained and then used to positively identify and quantify the concentrations of these predominant NOCs. The chronic cytotoxicity effects of these NOCs were also evaluated using a Chinese hamster ovary (CHO) cell assay as an indicator of mammalian cell cytotoxicity. This analysis found that the rank order for chronic cytotoxicity of these nine NOCs was 3-dimethylamino phenol> 2,2,6,6-tetramethyl-4-piperidinone >2,6-dimethyl-3-pyridinol > 2-picoline>pyridine > 1-methyl-2-pyrrolidinone > σ-valerolactam > 2-pyrrolidinone > ε-caprolactam. However, none of the individual NOC compounds exhibited cytotoxicity at the concentrations found in HTL wastewater (HTL-WW). In contrast, the complete mixture of organics extracted from HTL-WW showed significant cytotoxicity, with our results indicating that only 7.5% of HTL-WW would induce 50% reduction in CHO cell density. The effect of identified NOCs in HTL-WW on algal growth was also investigated to provide insight on combining algal wastewater treatment with HTL biofuel production. Experimental results showed three out of eight tested NOCs from HTL-WW could cause at least 50% inhibition of algal growth at their detected concentration in HTL-WW. In addition, we found that treatment of HTL-WW with a batch fed algal bioreactor could effectively remove more than 99% of these eight specific NOCs with 7 days or less of treatment. HTL-WW was also fractionated into hydrophobic and hydrophilic fractions using XAD 8 resin. Algal bioassays with fractionated HTL-WW demonstrated that dissolved organic nitrogen (DON) in the hydrophilic fraction was effectively utilized for algal growth, whereas hydrophobic DON remained nearly constant during the 3 week incubation period. Removal of total nitrogen and dissolved organic nitrogen in the hydrophilic HTL-WW fraction by algal bioreactor treatment was 99% and 82%, respectively. Meanwhile, only 32% removal of total nitrogen was obtained for hydrophobic HTL-WW fraction during algal bioreactor treatment, and no removal of dissolved organic nitrogen in the hydrophobic HTL-WW fraction was observed. The effects of three key HTL operating parameters (reaction temperature, reaction time and feedstock solids concentration) on the chemical characteristics and cytotoxicity of HTL-WW was also investigated for fifteen different combinations of operating conditions in the range considered to be practical for bio-oil production.We found that HTL-WW contained a substantial quantity of suspended solids, nutrients and organics. Comparing the three tested HTL operating parameters, the feedstock solids concentration was the most dominant factor in determining the concentration of nutrients and organics in the HTL-WW. The higher the feedstock solids concentration, the more nutrients and organics were found in HTL-WW. Looking at the effect of different operating conditions on cytotoxicity, we found that prolonged reaction times (≥60 min) generally decreased the HTL-WW toxicity. Meanwhile, cytotoxicity generally increased with increases in either reaction temperature or feedstock solids content.Unfortunately, increases in reaction temperature or solids content also generally enhanced bio-oil yield, such that there will be trade-offs between oil yield and toxicity. There was also a moderate positive correlation (r=0.58 and p=0.024) between increasing cytotoxicity of HTL-WW and increasing concentrations of chemical oxygen demand (COD) in HTL-WW. We also noticed that operating conditions providing a high bio-oil yield generally produced a more cytotoxic HTL-WW.The most cytotoxic HTL-WW was generated when 35% solids content Chlorella pyrenoidosa (C. pyrenoidosa) was liquefied at 300°C for 30 min reaction time. Experimental data also showed that HTL of 35% solids content C. pyrenoidosa feedstock could be carried out at 280°C and 60 min reaction time for an advantageous balance of fairly high oil yield and lower cytotoxicity in the HTL-WW.Treatment of 10% HTL-WW diluted in municipal wastewater with a semi-batch algal bioreactor provided 50% removal of COD and 30% removal of the HTL-WW cytotoxicity. Subsequent post-treatment of algal treated HTL-WW with granular activated carbon provided an additional of 40% removal of COD and 62% removal of cytotoxicity. Thus, a combination of algal and GAC treatment provided of 90% removal of COD and 92% removal of HTL-WW cytotoxicity. These post-treatments of HTL-WW synergistically integrate with HTL bioenergy production because biomass from algal bioreactor processes and the GAC used to treat HTL-WW can both be fed back into HTL to generate additional bio-crude oil. Full strength HTL-WW was also treated via catalytic hydrothermal gasification (CHG),which provided removal of 96.7% of COD and 37.5% of cytotoxicity. All in all, integration of adsorption and algal bioreactor with HTL bioenergy production offers significant potential for an advanced wastewater treatment system that can simultaneously provide significant biofuels, decrease the cytotoxicity and nutrient levels of effluent wastewater and support water reuse applications.
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Characterizing the effects of hydrothermal processes on bioactive compounds in wastewater bioenergy systems
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