Effect of combining adsorption-stripping treatment with acidification on the growth of Chlorella vulgaris and nutrient removal from swine wastewater

After swine wastewater (SW) was treated with adsorption-stripping stage, the concentration of NH₄⁺-N and Total phosphorus (TP) in SW significantly decreased from 598.04, 42.95 to 338.02, 8.36 mg L^-1, respectively. The concentration of heavy metals, especially Zn²⁺ (96.78%), decreased by the ion exchange of artificial zeolite (AZ). The acidification of SW could significantly improve the nutrient utilization efficiency and promote the growth rate of C. vulgaris due to the hydrolysis of macromolecular substances into smaller molecules usable for algae. By combining adsorption (Part I), stripping (Part II) and cultivation (Part III), the highest removal rates of NH₄⁺-N, TP, chemical oxygen demand (COD) and total organic carbon (TOC) from SW were 80.50, 96.90, 72.91, and 84.17%, respectively, and the OD₆₈₀ value was 1.129 (1.48 times of control) at pH 6.0. The combined system (Part I-III) can significantly enhance the removal efficiency of nutrient and biomass production by acidification.

Ex-situ catalytic upgrading of vapors from fast microwave-assisted co-pyrolysis of Chromolaena odorata and soybean soapstock

Fast microwave-assisted catalytic co-pyrolysis of Chromolaena odorata (C. odorata) and soybean soapstock with HZSM-5 as an ex-situ catalyst was investigated. Effects of catalytic temperature, feedstock: catalyst ratio and C. odorata: soybean soapstock ratio on the yield and composition of the bio-oil were discussed. Results showed that catalytic temperature greatly influenced the bio-oil yield. Co-pyrolysis of C. odorata and soybean soapstock improved the bio-oil yield, and the maximum bio-oil yield of 55.14% was obtained at 250 °C. However, the addition of HZSM-5 decreased bio-oil yield but improved the quality of bio-oil. Moreover, the proportion of oxygen-containing compounds decreased dramatically with the addition of soybean soapstock. The C. odorata: soybean soapstock ratio of 1:2 and feedstock: catalyst ratio of 2:1 were the optimal condition to upgrade the bio-oil. In addition, the resulted biochar contained various essential elements and could be used as soil repair agent.

Improving hydrocarbon yield from catalytic fast co-pyrolysis of hemicellulose and plastic in the dual-catalyst bed of CaO and HZSM-5

The high concentration of oxygenated compounds in pyrolytic products prohibits the conversion of hemicellulose to important biofuels and chemicals via fast pyrolysis. Herein a dual-catalyst bed of CaO and HZSM-5 was developed to convert acids in the pyrolytic products of xylan to valuable hydrocarbons. Meanwhile, LLDPE was co-pyrolyzed with xylan to supplement hydrogen during the catalysis of HZSM-5. The results showed that CaO could effectively transform acids into ketones. A minimum yield of acids (2.74%) and a maximum yield of ketones (42.93%) were obtained at a catalyst to feedstock ratio of 2:1. The dual-catalyst bed dramatically increased the yield of aromatics. Moreover, hydrogen-rich fragments derived from LLDPE promoted the Diels-Alder reactions of furans and participated in the hydrocarbon pool reactions of non-furanic compounds. As a result, a higher yield of hydrocarbons was achieved. This study provides a fundamental for recovering energy and chemicals from pyrolysis of hemicellulose.

A comparative study between fungal pellet- and spore-assisted microalgae harvesting methods for algae bioflocculation

Fungi assisted microalgae bioflocculation is an emerging, efficient and cost-effective microalgal harvesting method, but no study has systematically evaluated and compared fungal spore-assisted (FSA) and fungal pellet-assisted (FPA) microalgal harvesting methods. In this study, harvesting Chlorella sp. cells by co-culture with Penicillium sp. spores or pellets was compared. Temperature, glucose concentration, pH and fungi:algae ratio were the critical parameters for harvesting efficiency. The highest flocculation efficiency (99%) of FSA method was achieved in 28 h at 40 °C, 160 rpm, 5 g glucose/L and 1.1 × 10⁴ cells/mL (spore). FPA method can harvest 98.26% algae cells in 2.5 h at 34 °C, 160 rpm, pH 4.0 with the fungi:algae ratio of 1:2. The carbon input for FPA is only half of that for FSA. FPA takes less time and needs less glucose input compared with FSA and may be more promising to be further developed as an effective microalgae bioflocculation method.

Microwave-assisted co-pyrolysis of brown coal and corn stover for oil production

The controversial synergistic effect between brown coal and biomass during co-pyrolysis deserves further investigation. This study detailed the oil production from microwave-assisted co-pyrolysis of brown coal (BC) and corn stover (CS) at different CS/BC ratios (0, 0.33, 0.50, 0.67, and 1) and pyrolysis temperatures (500, 550, and 600 °C). The results showed that a higher CS/BC ratio resulted in higher oil yield, and a higher pyrolysis temperature increased oil yield for brown coal and coal/corn mixtures. Corn stover and brown coal showed different pyrolysis characteristics, and positive synergistic effect on oil yield was observed only at CS/BC ratio of 0.33 and pyrolysis temperature of 600 °C. Oils from brown coal mainly included hydrocarbons and phenols whereas oils from corn stover and coal/corn mixtures were dominated by ketones, phenols, and aldehydes. Positive synergistic effects were observed for ketones, aldehydes, acids, and esters whereas negative synergistic effects for hydrocarbons, phenols and alcohols.

New Insight into the Mechanism of the Hydrogen Evolution Reaction on MoP(001) from First Principles

Molybdenum phosphide-based catalysts have recently exhibited excellent catalytic activities for the hydrogen evolution reaction (HER) in wide pH range conditions; the intrinsic reaction mechanism, on the other hand, has not been well established. Herein, by employing the MoP as the prototypical molybdenum phosphide-based catalyst, HER activities in both acid and neutral conditions were studied by conducting periodic density functional theory calculations. Thermodynamic analysis of hydrogen atoms absorbed on both P- and Mo-terminated surfaces were compared, as well as all the reaction energy and activation energy barriers for reactions involved in the HER process. Calculation results revealed that, in an acid condition, the Volmer-Heyrovsky and Volmer-Tafel reaction mechanisms were dominated on the P-terminated and Mo-terminated catalyst surfaces, where Heyrovsky and Volmer reactions were the rate-determining step, respectively. Additionally, water splitting was introduced to the current reaction mechanism and a small reaction activation energy barrier was revealed on the P-terminated surface. Besides, a relevant small activation energy was obtained in the Tafel reaction on the defect of the P-terminated surface in a neutral solution. Theoretical results proved that HER could take place readily on both P- and Mo-terminated catalyst surfaces via different reaction mechanisms in the acid condition from the view of atom scale. More important, computational results uncovered that HER could also occur on the P-terminated surface with the assistance of surface defect in the neutral condition, which sheds new light on the HER mechanism on transition metal phosphite-based catalysts. The doping effect on HER activity was further investigated in theory and calculation results, indicating that catalytic performance could be improved by substitutional doping of the Mo atom with metals such as Mn and W.

Development and application of a continuous fast microwave pyrolysis system for sewage sludge utilization

A continuous fast microwave-assisted pyrolysis system was designed, fabricated, and tested with sewage sludge. The system is equipped with continuous biomass feeding, mixing of biomass and microwave absorbent, and separated catalyst upgrading. The effect of the sludge pyrolysis temperature (450, 500, 550, and 600 °C) on the products yield, distribution and potentially energy recovery were investigated. The physical, chemical, and energetic properties of the raw sewage sludge and bio-oil, char and gas products obtained were analyzed using elemental analyzer, GC-MS, Micro-GC, SEM and ICP-OES. While the maximum bio-oil yield of 41.39 wt% was obtained at pyrolysis temperature of 550 °C, the optimal pyrolysis temperature for maximum overall energy recovery was 500 °C. The absence of carrier gas in the process may be responsible for the high HHV of gas products. This work could provide technical support for microwave-assisted system scale-up and sewage sludge utilization.

Using Chlorella vulgaris to treat toxic excess sludge extract, and identification of its response mechanism by proteomics approach

Chlorella vulgaris was cultivated in varying proportions of toxic sludge extracts obtained from a sequencing batch reactor for treating synthetic wastewater containing chlorophenols. C. vulgaris could reduce the ecotoxicity from sludge extracts, and a positive correlation was noted between ecotoxicity removal and total organic carbon removal. In terms of cell density, the optimal proportion of sludge extracts required for the cultivation of C. vulgaris was lower than 50%. The correlation between protein content in per 10⁶ algae and inhibition extent of ecotoxicity of the 5 groups on the day of inoculation (0.9182, p < .05) indicated a positive relationship between algal protein secretion and ecotoxicity. According to the protein expression and differential protein expression analysis, we concluded that C. vulgaris produced proteins that involved in the stress response/redox system and energy metabolism/biosynthesis to respond to the toxic environment and some other proteins related to mixotrophic metabolism.

In situ plasma-assisted atmospheric nitrogen fixation using water and spray-type jet plasma

In this study, a sustainable nitrogen fixation process was presented under atmospheric conditions and without introducing hydrogen or any catalyst. The novel in situ synthesis in this study used an advanced spray-type jet plasma, which significantly improved the fixation rate of nitrite, nitrate, and ammonium. Furthermore, the mechanism focusing on the co-synthesis of the abovementioned three nitrogen compounds was proposed based on the synergistic interactions between the gas-phase plasma and liquid surface dissociation.

Bamboo (Phyllostachys pubescens) as a Natural Support for Neutral Protease Immobilization

Lignin polymers in bamboo (Phyllostachys pubescens) were decomposed into polyphenols at high temperatures and oxidized for the introduction of quinone groups from peroxidase extracted from bamboo shoots and catalysis of UV. According to the results of FT-IR spectra analysis, neutral proteases (NPs) can be immobilized on the oxidized lignin by covalent bonding formed by amine group and quinone group. The optimum condition for the immobilization of NPs on the bamboo bar was obtained at pH 7.0, 40 °C, and duration of 4 h; the amount of immobilized enzyme was up to 5 mg g^-1 bamboo bar. The optimal pH for both free NP (FNP) and INP was approximately 7.0, and the maximum activity of INP was determined at 60 °C, whereas FNP presented maximum activity at 50 °C. The K_m values of INP and FNP were determined as 0.773 and 0.843 mg ml^-1, respectively; INP showed a lower K_m value and V_max, than FNP, which demonstrated that INP presented higher affinity to substrate. Compared to FNP, INP showed broader thermal and storage stability under the same trial condition. With respect to cost, INP presented considerable recycling efficiency for up to six consecutive cycles.

Microwave-assisted acid pretreatment of alkali lignin: Effect on characteristics and pyrolysis behavior

This study performed microwave-assisted acid pretreatment on pure lignin. The effects of microwave temperature, microwave time, and hydrochloric acid concentration on characteristics and pyrolysis behavior of lignin were examined. Results of ultimate analysis revealed better properties of all pretreated samples than those of raw lignin. Fourier transform infrared spectroscopy analysis showed breakage of βO4 bond and aliphatic side chain, decrease in OH groups, and formation of CO groups in pretreatment. Microwave temperature exerted more significant influence on lignin structure. Thermal stability of treated lignin was improved and insensitive to short microwave time and acid concentration under mild conditions. Resulting from improved alkyl-phenols and decreased alkoxy-phenols, microwave-assisted acid pretreatment of lignin yielded bio-oil with excellent quality. Total yield of phenols in pyrolysis vapors (200 °C) improved to 14.15%, whereas that of guaiacols decreased to 22.36%. This study shows that microwave-assisted acid pretreatment is a promising technology for lignin conversion.

Adsorption of AgNO3 onto bamboo hemicelluloses in aqueous medium

An insight into the adsorption behavior between hemicellulose biomass and metal ions is important to utilize hemicelluloses as adsorbent. In this study, bamboo hemicelluloses were used as adsorbent for AgNO₃ in aqueous medium. The adsorption amount of AgNO₃ onto hemicelluloses was determined through an inductively coupled plasma-atomic emission spectrometer (ICP-AES). The morphology and the elemental composition of the recovered hemicelluloses were characterized using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) techniques, respectively. The filtrate after removing undissolved hemicelluloses was analyzed by ultraviolet-visible spectroscopy (UV-vis). Results demonstrated that the obtained highest adsorption amount of AgNO₃ onto hemicelluloses was 42.82 mg/g under the conditions discussed. Some Ag⁺ was in situ reduced by hemicelluloses to form silver nanoparticles. Ag⁺ and the formed silver nanoparticles possibly destroyed the hydrogen-bonding network of hemicelluloses, resulting in the stretch of molecules and formed rod-like agglomerates with irregular length. Even an agglomerate with the length of 420 nm was found. The side chains and the newly formed carboxyl groups through oxidation of hemicelluloses by silver ions removed away from the hemicelluloses during adsorption. A part of Ag⁺ and silver nanoparticles were adsorbed on the unresolved hemicelluloses, and the other part was dispersed in the aqueous solution.

Balancing carbon/nitrogen ratio to improve nutrients removal and algal biomass production in piggery and brewery wastewaters

To improve nutrients removal from wastewaters and enhance algal biomass production, piggery wastewater was mixed with brewery wastewaters. The results showed that it was a promising way to cultivate microalga in piggery and brewery wastewaters by balancing the carbon/nitrogen ratio. The optimal treatment condition for the mixed piggery-brewery wastewater using microalga was piggery wastewater mixed with brewery packaging wastewater by 1:5 at pH 7.0, resulting in carbon/nitrogen ratio of 7.9, with the biomass concentration of 2.85 g L^-1, and the removal of 100% ammonia, 96% of total nitrogen, 90% of total phosphorus, and 93% of chemical oxygen demand. The application of the established strategies can enhance nutrient removal efficiency of the wastewaters while reducing microalgal biomass production costs.

Development and operation of innovative scum to biodiesel pilot-system for the treatment of floatable wastewater scum

A novel process was developed for the biorefining of floatable wastewater scum and other waste oils from water treatment facilities into biodiesel and other value-added bio-products. To test the scalability and commercial potential of the technology, a 7000 l/year pilot-scale system was designed and built. Scum from a wastewater treatment facility, located in St. Paul, Mn, was collected and converted into methyl esters (biodiesel) according to the process chemistry. All of the incoming and outgoing process streams were sampled, tested, weighed and recorded to calculate both the process efficiency and product quality. Data from the pilot-scale system operation was compared to laboratory results and the theoretically expected values for each individual unit operation. The biodiesel was tested using a third party laboratory and confirmed it met all of the US EPA's test requirements for commercial-grade biodiesel.

A techno-economic evaluation of anaerobic biogas producing systems in developing countries

Biogas production has been the focus of many individuals in the developing world; there have been several investigations that focus on improving the production process and product quality. In the developing world the lack of advanced technology and capital has hindered the development of energy production. Renewable energy has the potential to improve the standard of living for most of the 196 countries which are classified as developing economies. One of the easiest renewable energy compounds that can be produced is biogas (bio-methane). Biogas can be produced from almost any source of biomass through the anaerobic respiration of micro-organisms. Low budget energy systems are reviewed in this article along with various feedstock sources. Adapted gas purification and storage systems are also reviewed, along with the possible economic, social, health and environmental benefits of its implementation.

Carbon-dependent alleviation of ammonia toxicity for algae cultivation and associated mechanisms exploration

Ammonia toxicity in wastewater is one of the factors that limit the application of algae technology in wastewater treatment. This work explored the correlation between carbon sources and ammonia assimilation and applied a glucose-assisted nitrogen starvation method to alleviate ammonia toxicity. In this study, ammonia toxicity to Chlorella sp. was observed when NH₃-N concentration reached 28.03mM in artificial wastewater. Addition of alpha-ketoglutarate in wastewater promoted ammonia assimilation, but low utilization efficiency and high cost of alpha-ketoglutarate limits its application in wastewater treatment. Comparison of three common carbon sources, glucose, citric acid, and sodium bicarbonate, indicates that in terms of ammonia assimilation, glucose is the best carbon source. Experimental results suggest that organic carbon with good ability of generating energy and hydride donor may be critical to ammonia assimilation. Nitrogen starvation treatment assisted by glucose increased ammonia removal efficiencies and algal viabilities.

Fast microwave-assisted ex-catalytic co-pyrolysis of bamboo and polypropylene for bio-oil production

The ex-catalytic co-pyrolysis of bamboo and polypropylene (PP) with HZSM-5 was investigated with microwave assistance. The influences of catalytic temperature, feedstock/catalyst ratio, and bamboo/PP ratio on the product yields and chemical components of bio-oil from the co-pyrolysis were studied. When the catalytic temperature, feedstock/catalyst ratio, and bamboo/PP ratio were 250 °C, 1:2, and 2:1, respectively, the bio-oil yield reached its maximum value at 61.62 wt%. The oxygenate proportion compounds decreased with increasing catalyst content. The PP addition improved the proportions of aromatics and naphthenic hydrocarbons. The bio-oil was upgraded significantly from the jet fuel perspective. A synergistic effect also existed between bamboo and PP.

Waste-to-biofuel integrated system and its comprehensive techno-economic assessment in wastewater treatment plants

Combining wastewater treatment and biofuel production is considered the cost-effective way for better waste remediation and lowering the environmental impact for biofuel production. In this study, an innovative integrated system incorporating sludge, scum and centrate treatment and biofuel production was developed. A comprehensive techno-economic analysis was conducted to evaluate the technology and economic feasibility of the integrated system with the consideration of biofuel production, wastewater treatment improvement, tax credits, carbon credit, and coproducts utilization. Benefited from the integrated system that the intermediate byproducts can be used in between the sub-systems, such as the glycerol generated from the scum-to-biodiesel production can be used as an organic carbon for the centrate-to-algae production, the estimated breakeven selling price of the bio-oil ($1.85/gallon) is very close to the 5-year averaged crude oil price. The assessment result showed the payback period and the IRRs of the integrated system are superior in comparison with others.

In-situ and ex-situ catalytic upgrading of vapors from microwave-assisted pyrolysis of lignin

In-situ and ex-situ catalytic upgrading with HZSM-5 of vapors from microwave-assisted pyrolysis of lignin were studied. The in-situ process produced higher bio-oil and less char than ex-situ process. The gas yield was similar for both processes. The ex-situ process had higher selectivity to aromatics and produced more syngas and less CO₂ than the in-situ process. Additional experiments on ex-situ process found that the bio-oil yield and coke deposition decreased while the gas yield increased at higher catalyst-to-lignin ratios and catalytic upgrading temperatures. The increased catalyst-to-lignin ratio from 0 to 0.3 reduced the selectivity of methoxy phenols from 73.7% to 22.6% while increased that of aromatics from 1.1% to 41.4%. The highest selectivity of alkyl phenols (31.9%) was obtained at 0.2 of catalyst-to-lignin ratio. Higher catalytic temperatures favored greater conversion of methoxy phenols to alkyl phenols and aromatics. Appropriate catalyst-to-lignin ratio (0.3) together with higher catalytic temperatures favored syngas formation.

Hydrothermal pretreatment of bamboo sawdust using microwave irradiation

In the present study, the effect of temperature and residence time during microwave hydrothermal pretreatment (MHT) on hydrochar properties and pyrolysis behaviors was investigated. Experimental results indicated that higher heating value (HHV) and fixed carbon content gradually increased with increased pretreatment severity. Obvious reduction of oxygen content was found under MHT at 230°C-15min and 210°C-35min. Although lower mass yield was observed under severe conditions, corresponding energy yield was relatively higher. Crystallinity indexes of hydrochar demonstrated an upward trend with increased residence time. Unlike hydroxyl group, dissociation of acetyls was more favorable under prolonged residence time rather than increased temperature. Peaks in thermogravimetric and derivative thermogravimetric curves shifted to higher temperature region under severe conditions, indicating better thermal stability. Py-GC/MS analysis suggested that acids content was decreased but sugars increased with increased MHT severity. Moreover, compared to temperature, residence time was mainly responsible for acetic acid formation.

Cultivation of Chlorella vulgaris on anaerobically digested swine manure with daily recycling of the post-harvest culture broth

In this work, a cultivation system with daily recycling of the post-harvest culture broth was set up and performed in order to reuse the water and nutrients in pretreated anaerobically digested swine manure, which was used as media to cultivate Chlorella vulgaris (UTEX 2714) at different recycling ratios. Results showed that the alga grew well in the system with an accumulative algal biomass and productivity of 1.68-3.47g/L and 234.1-532.2mg/L/d, respectively, at the end of the cultivation. Additionally, chemical compositions in this alga varied with the change of recycling ratios, and the highest productivities of carbohydrate, protein and lipids (76.4, 257.2 and 183.7mg/L/d, respectively) were obtained in the system with a recycling ratio of 1/4 or 1/6. Fatty acid profiles indicated that this alga could be used as a good-quality biodiesel feedstock with a biodiesel productivity of 9.65-40.1mg/L/d.

Co-cultivation of microalgae in aquaponic systems

Aquaponics is a sustainable system for the future farming. In aquaponic systems, the nutrient-rich wastewater generated by the fish provides nutrients needed for vegetable growth. In the present study, the role of microalgae of Chlorella sp. in the floating-raft aquaponic system was evaluated for ammonia control. The yields of algal biomass, vegetable, and removal of the key nutrients from the systems were monitored during the operation of the aquaponic systems. When the systems were in full operation, the algae production was about 4.15±0.19g/m²·day (dry basis) which is considered low because the growth conditions are primarily tailored to fish and vegetable production. However, it was found that algae had a positive effect on balancing pH drop caused by nitrifying bacteria, and the ammonia could be controlled by algae since algae prefer for ammonia nitrogen over nitrate nitrogen. The algae are more efficient for overall nitrogen removal than vegetables.

Cultivation of Chlorella vulgaris in a pilot-scale photobioreactor using real centrate wastewater with waste glycerol for improving microalgae biomass production and wastewater nutrients removal

To improve nutrients removal from real centrate wastewater and enhance the microalgae biomass production, cultivation of Chlorella vulgaris in lab and a pilot-scale photobioreactor with waste glycerol was studied. The results showed the optimal concentration of the crude glycerol was 1.0gL^-1 with the maximum biomass productivity of 460mgL^-1d^-1 TVS, the maximum lipid content of 27%, the nutrient removal efficiency of all above 86%, due to more balanced C/N ratio. The synergistic relationship between the wastewater-borne bacteria and the microalgae had significant good influence on nutrient removal. In pilot-scale wastewater-based algae cultivation, with 1gL^-1 waste glycerol addition, the average biomass production of 16.7gm^-2d^-1, lipid content of 23.6%, and the removal of 2.4gm^-2d^-1 NH₄⁺-N, 2.7gm^-2d^-1 total nitrogen, 3.0gm^-2d^-1 total phosphorous, and 103.0gm^-2d^-1 of COD were attained for 34days semi-continuous mode.

An innovative intermittent-vacuum assisted thermophilic anaerobic digestion process for effective animal manure utilization and treatment

Intermittent-vacuum stripping (IVS) was developed as a pretreatment for thermophilic anaerobic digestion (TAD) to improve methanogenesis and hydrolysis activity through preventing free ammonia and hydrogen sulfide (H₂S) inhibition from liquid swine manure (LSM). Over 98% of ammonia and 38% organic nitrogen were removed in 60min from 55°C to 85°C with vacuum pressure (from 100.63±3.79mmHg to 360.91±7.39mmHg) at initial pH 10.0 by IVS. Thermophilic methanogenesis and hydrolysis activity of pretreated LSM increased 52.25% (from 11.56±1.75% to 17.60±0.49%) in 25days and 40% (from 10days to 6days) in bio-methane potential assay. Over 80% H₂S and total nitrogen were removed by IVS assistance, while around 70% nitrogen was recycled as ammonium sulfate. Therefore, IVS-TAD combination could be an effective strategy to improve TAD efficiency, whose elution is more easily utilized in algae cultivation and/or hydroponic system.