|Abstract:||Animal manure and domestic wastewater contain chemicals of emerging concern (CECs) that can threaten ecosystems and human health. For instance, estrone (E1) and 17β-estradiol (E2) can disrupt the reproductive biology of vertebrates at very low concentrations. This study investigates the effects of novel manure management systems that can simultaneously produce bioenergy and reduce the discharge of CECs to improve energy security and water quality.
Natural occurrence of estrogenic hormones of swine manure from three different swine barns were investigated using a serial extraction method. Daily production of total hormones per pig were as follows: farrowing (1,333 ± 19 µg/day-hd) > gestation (789 ± 15 µg/day-hd) > finishing (518 ± 5 µg/day-hd). Sedimentation, screening, and filtration processes were used to remove the solids content (up to 98%) and produce a liquid portion of animal manure (LPAM) which was used as an influent for biological processes that cultivated bacteria and algae to provide improved LPAM water quality and a bioenergy feedstock. The mixed algal-bacterial bioreactor (MABB) was operated with and without the addition of granular activated carbon (GAC) to enhance the removal of CECs and other organics from the LPAM. The resulting biomass was harvested for biofuel conversions via hydrothermal liquefaction (HTL) and catalytic hydrothermal gasification (CHG), which were performed under 16 different conditions to study the effects of hydrothermal processes on the removal of bioactive CECs. Solid Phase Extraction (SPE) of estrogenic hormones and hydrolysis of florfenicol (FF) followed by GC/MS analysis was used to measure the concentration of CECs before and after algal bioreactor treatment and hydrothermal conversion processes.
The research demonstrated that the algal manure treatment and thermochemical-waste-to-energy processes could simultaneously remove the CECs from animal manure by 96.5% to 99.9% in total estrogenic hormones and 93.0% to 99.9% in antibiotics, while converting biomass into biocrude oil with up to a 40.0% yield (dry basis) and a bio-char solid residue with a 12.0% yield (dry basis) using the HTL. In addition, the CHG process converted the same biomass to syn-gas with an average yield of up to 54.4% (dry basis). The most favorable process conditions for both bioenergy production and hormones removal were 300°C/60minutes for HTL and 500°C/60minutes with Ruthenium catalyst (Ru) for CHG. The Xenoscreen YES assay was used to investigate the effects hydrothermal processes on eliminating the estrogenic activity of LPAM. Increasing reaction temperature in hydrothermal processes generally reduced the estrogenic potency and the concentration of residual hormone compounds. Antibiotic resistant bacteria (ARB) assay tests were conducted with HTL-WW and CHG-WW to determine if HTL and CHG can reduce the capacity of bacterial cells to generate antibiotic resistance. Despite the complex matrix of wastewaters from hydrothermal processes, the ARB assay was sensitive in measuring the antibiotic resistance, and the HTL and CHG processing could eliminate the capacity of samples to generate bacteria with resistance to the target antibiotic, FF.
Water quality parameters such as soluble COD, TN, TP, NH3-N in the effluents from various bioreactor treatments were investigated to demonstrate the ability to extract bioactive CECs and other dissolved organics from LPAM, while also creating a feedstock for bioenergy production. The percent removal of soluble COD, TN, TP, NH3-N in MABB with GAC were 74.6, 30.1, 39.5, and 97.0%, respectively. Mammalian cell cytotoxicity assays using Chinese hamster ovary (CHO) cells were conducted using LPAM, bioreactor effluents from a MABB and a conventional activated sludge reactor (CAS), as well as the effluent wastewaters after HTL (HTL-WW) and CHG conversions (CHG-WW). In this study, adding GAC was synergistic with MABB and CAS in reducing the cytotoxicity of LPAM by adsorption of toxic compounds and/or stabilized performance of each bioreactor. More importantly, HTL-WW and CHG-WW exhibited the lowest cytotoxic characteristics under the optimal conditions for bioenergy production, 300°C/60minutes for HTL and 500°C/60minutes with Ruthenium catalyst (Ru) for CHG. To quantify the acute toxicity of wastewater samples, a Microtox® assay was also conducted. MABB with GAC showed the lowest acute toxicity due to slightly better organics removal via adsorption and higher temperature. The acute toxicities of the HTL-WW and CHG-WW demonstrated a similar trend as the corresponding CHO cell cytotoxic data, and exhibited the lowest acute toxicity under the optimal conditions for HTL and CHG processes.
In the MABB with GAC, the average percent distribution of heavy metals to biomass and effluents were 6.1 and 80.4%, respectively. After HTL and CHG treatment of biomass, the highest concentration of heavy metals was found mostly in the solids residue (84.2 and 73.1%, respectively), followed by the aqueous product (10.4 and 20.8%, respectively), with only a miniscule fraction in the biocrude oil (0.1 and 2.2%, respectively). The concentrations of arsenic (As), lead (Pb), copper (Cu), zinc (Zn), and cadmium (Cd) ranged from 0.02 to 37.3 mg/L in the aqueous phase after HTL and CHG tests, which are higher than the typical limits allowed for livestock drinking water and irrigation. (Ayers et al., 1985; U.S.EPA. 1974). However, if these hydrothermal process aqueous products were blended back with the separated water fraction of raw manure, the combined concentration would be below recommended heavy metal limits.
In conclusion, these findings will contribute to the development of cost-effective systems to increase bioenergy production, reduce water pollution, and enhance opportunities for the treatment and reuse of the aqueous fraction of livestock manure.