Carbon dioxide-assisted thermochemical conversion of magnetically harvested harmful algae into syngas and metal biochar.

With rising of harmful algae blooming and toxin exposure, practical utilization of harmful algae has been developed. This work aimed to magnetically harvest Microcystis aeruginosa (MA) using iron oxides and investigate the feasibility of algae/iron oxides mixture as feedstock in pyrolytic platform to produce syngas and metal biochar. Carbon dioxide (CO2) was used as a feeding gas to enhance the production efficiency of syngas and also functioned pH controller for better MA harvesting and toxin removal. CO2 support brought multiple benefits: magnetite (Fe3O4) and maghemite (γ-Fe2O3) recovered MA in a relatively short period of time (∼1 min), the recovered biomass generated 34-fold increased carbon monoxide, and metal biochar adsorbed higher amount of toxin from MA (2.8-fold). Pyrolytic utilization of harmful algae supported by CO2 and iron oxides could be one of promising techniques for evolution of metal biochar to remove toxin, while efficiently recover biomass and enhance syngas production.

Continuous cultivation of mixed-culture microalgae using anaerobic digestion effluent in photobioreactors with different strategies for adjusting nitrogen loading rate.

This study examined continuous mixed-culture microalgae cultivation for nutrient removal from anaerobic digestion (AD) effluents in photobioreactors, while altering the NH4+-N loading rate (NLR) by adjusting either the hydraulic retention time (HRT) (reactor set RH) or the influent NH4+-N concentration (reactor set RS). Both RH and RS demonstrated efficient nutrient removal and microalgae cultivation at NLRs of 4-10 mg NH4+-N/L∙d, reaching peak performance at 10 mg NH4+-N/L∙d. Within this range, RH obtained greater biomass yield and productivity, while RS maintained higher microalgal concentrations. The cultivated biomasses obtained from RH and RS had good settleability and suitable fatty acid compositions as a biodiesel feedstock, although their organic composition varied considerably with NLR and HRT. Parachlorella overwhelmingly dominated the reactors' microalgal communities throughout the experiment, co-existing with various microalgae-associated bacteria. Changes in NLR significantly influenced the bacterial community structures, underscoring its critical role in determining reactor performance and microalgal-bacterial community behavior.

Direct interspecies electron transfer enables anaerobic oxidation of sulfide to elemental sulfur coupled with CO2-reducing methanogenesis.

Electric syntrophy between fatty acid oxidizers and methanogens through direct interspecies electron transfer (DIET) is essential for balancing acidogenesis and methanogenesis in anaerobic digestion. Promoting DIET using electrically conductive additives proved effective in enhancing methanogenesis; however, its possibility to affect other microbial redox reactions in methanogenic systems has been little studied. This study provides the first confirmation of the electro-syntrophic coupling of sulfide oxidation to S0 with CO2-reducing methanogenesis in sulfur-rich methanogenic cultures supplemented with conductive magnetite (100-700-nm particle size). The H2S content in biogas, initially exceeding 5000 ppmv, decreased to below 1 ppmv along with an accumulation of extracellular S0 (60-70 mg/L; initially <1 mg/L) at a magnetite dose of 20 mM Fe, while there were no significant changes in methane yield. A comprehensive polyphasic approach demonstrated that the S0 formation occurs through electro-syntrophic oxidation of sulfide coupled with CO2-reducing methanogenesis, involving Methanothrix as the dominant methanogen.

Potential treatment of aged cow manure using spare capacity in anaerobic digesters treating a mixture of food waste and pig manure.

With the increasing production of cow manure (CM) and the continuing decrease in the demand for manure compost, CM management has become an urgent and challenging task in Korea. In most cattle farms in Korea, CM mixed with bedding materials is left in pens exposed to the open air for several months before treatment, which makes CM an unsuitable feedstock for anaerobic digestion. This study examined the co-digestion of aged CM with a mixture of food waste and pig manure as the base substrate to assess the possibility of treating and valorizing CM using spare capacity in existing anaerobic digesters dealing with other wastes. The duplicate digesters initially fed with the base substrate were subjected to the addition of increasing amounts of CM (3-10% in the feed, w/v) over nine months. Co-feeding CM up to 5% in the feed (w/v) did not compromise the methanogenic degradation of the substrates, but adding more CM led to a significant performance deterioration likely related to the buildup of inhibitory free ammonia and H2S. Adding CM substantially influenced the digester microbial communities, especially methanogenic communities, and induced a dominance shift from aceticlastic Methanothrix to hydrogenotrophic methanogens as the CM fraction increased. The overall results suggest that the CM fraction should not exceed 5% in the feed (w/v) for its stable treatment with the base substrate in the experimental digesters. Although further studies are needed, anaerobic treatment using spare capacity in existing digesters can be a useful strategy for the management of aged CM.

Effects of Different pH Control Strategies on Microalgae Cultivation and Nutrient Removal from Anaerobic Digestion Effluent.

This study investigated nutrient removal from anaerobic digestion effluent by cultivating mixed-culture microalgae enriched from anaerobic sludge under different pH conditions: RUC (uncontrolled), R7-8 (maintained at 7-8), and R<8 (maintained below 8). Significant amounts of NH4+-N were lost by volatilization in RUC cultures due to increased pH values (≤8.6) during the early period of cultivation. The pH control strategies significantly affected the biological NH4+-N removal (highest in R7-8), microalgal growth (highest in R7-8), biomass settleability (highest in R<8), and microalgal growth relative to bacteria (highest in R<8) in the cultures. Parachlorella completely dominated the microalgal communities in the inoculum and all of the cultures, and grew well at highly acidic pH (<3) induced by culture acidification with microalgal growth. Microalgae-associated bacterial community structure developed very differently among the cultures. The findings call for more attention to the influence and control of pH changes during cultivation in microalgal treatment of anaerobic digestion effluent.

Human urine as a forward osmosis draw solution for the application of microalgae dewatering.

Human urine is a unique solution that has the right composition to constitute both a severe environmental threat and a rich source of nitrogen and phosphorous. In fact, between 4-9% of urine mass consists of ions, such as K+, Cl-, Na+ or NH4+. Because of its high ionic strength, urine osmotic pressure can reach values of up to 2000 kPa. With this in mind, this work aimed to study the effectiveness of real urine as a novel draw solution for forward osmosis. Water flux, reverse nitrogen flux and membrane fouling were investigated using fresh or hydrolysed urine. Water flux as high as 16.7 ± 1.1 L m-2 h-1 was recorded using real hydrolysed urine. Additionally, no support layer membrane fouling was noticed in over 20 h of experimentation. Urine was also employed to dewater a Chlorella vulgaris culture. A fourfold increase in algal concentration was achieved while having an average flux of 14.1 L m-2 h-1. During the algae dewatering, a flux decrease of about 19% was noticed; this was mainly due to a thin layer of algal deposition on the active side of the membrane. Overall, human urine was found to be an effective draw solution for forward osmosis.

Nutrient removal and microalgal biomass production from different anaerobic digestion effluents with Chlorella species.

Potential of microalgal cultivation as an alternative approach to the treatment of anaerobic digestion (AD) effluents was examined using two representative Chlorella species, Chlorella vulgaris (CV) and Chlorella protothecoides (CP). Both species effectively removed NH4+-N from the AD effluents from four digesters treating different wastes under different operating conditions. In all experimental cultures on the AD effluents, NH4+-N (initial concentration, 40 mg/L) was completely removed within 10 days without residual NO3--N or NO2--N in batch mode. Compared to CP, CV showed greater biomass and lipid yields (advantageous for biodiesel production), regardless of the media used. Prolonged nitrogen starvation significantly increased the lipid accumulation in all cultures on the AD effluents, and the effect was more pronounced in the CV than in the CP cultures. On the other hand, compared to CV, CP showed significantly faster settling (advantageous for biomass harvesting) in all media. Our results suggest that the Chlorella cultivation on AD effluents under non-sterile, mixed-culture conditions may provide a viable way to manage and valorize the problematic effluents. Diverse bacteria derived from the AD effluents co-existed and presumably interacted with the Chlorella species in the cultures.

Potential of mixed-culture microalgae enriched from aerobic and anaerobic sludges for nutrient removal and biomass production from anaerobic effluents.

This study examines the potential of the mixed-culture microalgal consortia enriched from aerobic sludge (AeS) and anaerobic sludge (AnS) with regard to nutrient removal and biomass production from four different anaerobic digestion (AD) effluents. Both the inocula achieved the complete removal of the NH4+-N (initial concentration of 40 mg/L) within 14 days from all the effluents. The AeS cultures showed faster and greater microalgal growth, although the NH4+-N removal rate was comparable or higher in the case of the AnS cultures. Further, the AeS and AnS cultures showed significantly different lipid production characteristics in terms of the fatty acid composition and the response to nitrogen deficiency. Nitrogen starvation caused changes in the microbial community structures in all the experimental cultures, which may have influenced the lipid metabolism and the microalgal growth. The overall results suggest that both the inocula exhibit good potential with regard to the treatment of AD effluents.