Laboratory of Environment-Enhancing Energy (E2E), Key Laboratory of Agricultural Engineering in Structure and Environment of Ministry of Agriculture and Rural Affairs, College of Water Resources and Civil Engineering, China Agricultural University, Beijing 100083, China
The conversion of biomass into biocrude oil by hydrothermal liquefaction (HTL) is a potential way to produce renewable liquid fuels. However, the aqueous phase (HTL-AP) by-products have high yields, complex compositions and high environmental risk which limit the green development of hydrothermal liquefaction technology. Based on the relevant research of our research group in recent 10 years (2012-2021), this paper summarizes the formation mechanism, characteristics and resource recovery path of HTL-AP. The factors affecting the formation of HTL-AP were introduced, and the ways of compound transformation and element migration in different hydrothermal reaction variables were summarized. The approaches and research progress of aqueous biotransformation, including aerobic microbial degradation, microalgae culture, anaerobic treatment and microbial electrochemistry, were reviewed. The physical methods such as membrane separation and adsorption were introduced to separate aqueous substances and obtain high-value components. The potential of using HTL-AP to prepare agricultural fungicides was discussed. Finally, the treatment principle and research direction are prospected in order to provide reference for the treatment and resource recovery of HTL-AP.
Contents
1 Introduction
2 Formation mechanism of HTL-AP
2.1 Hydrothermal reaction conditions
2.2 Raw material conversion path
2.3 Element migration path
3 Physical and chemical methods
3.1 Separation of organic elements
3.2 Separation of inorganic elements
3.3 Mixed component utilization
4 Biological treatment method
4.1 Cytotoxicity
4.2 Aerobic biological treatment
4.3 Microalgae treatment
4.4 Anaerobic fermentation
4.5 Microbial electrochemistry
5 Challenges and prospects of HTL-AP resource recovery
Yongdong Xu, Zhidan Liu.
Formation Mechanism and Resource Recovery Perspectives of Aqueous Phase from Hydrothermal Liquefaction of Biomass.
Progress in Chemistry, 2021, 33(11): 2150-2162 DOI:10.7536/PC210420
The environmental problems and total reserves of fossil energy have become an important factor restricting the sustainable development of the global society[1,2]. Therefore, it is very important to find alternative resources for fossil energy, especially oil, to alleviate the global energy crisis. Renewable energy has attracted more and more attention because of its wide source, clean and pollution-free, and renewable advantages[3].
Hydrothermal liquefaction (HTL) can convert wet biomass such as microalgae, livestock manure and straw into bio-crude oil under high pressure (4 ~ 22 MPa) and high temperature (250 ~ 400 ℃), which is a renewable energy conversion technology with great application prospects[4⇓~6]. In 2018, Chen et al. Further upgraded and refined the bio-crude oil produced by hydrothermal liquefaction for internal combustion engine. The experimental results show that the viscosity, acidity and energy content of the crude oil meet the current requirements of internal combustion engine, and it is a good liquid fuel[7]. Many research teams at home and abroad have carried out related research on hydrothermal liquefaction[8,9]. Since 2011, our research group has carried out research on the characteristics of hydrothermal liquefaction raw materials, the mechanism of oil production process, the reaction equipment of hydrothermal liquefaction and the comprehensive treatment and utilization of products, and has made some progress[7,10,11].
Hydrothermal liquefaction produces by-products such as aqueous phase, gas and residue while converting various biomass resources to produce bio-crude oil, among which the main by-product is HTL aqueous phase (HTL-AP) (Fig. 1). 26% ~ 55% of the substances in the substrate were transferred to the aqueous phase and converted into organic acids, amides, esters, ketones, alcohols and phenols, resulting in strong biological toxicity and pungent odor in the aqueous phase[12,13]. Untreated HTL-AP has great potential hazards to soil, water and air, and the treatment of aqueous phase has become an important obstacle to the green development of hydrothermal liquefaction and the recycling of waste resources.
In recent years, research teams at home and abroad have mainly explored the formation mechanism of substances in the aqueous phase and the sources of toxic substances, and explored the potential of recycling substances in the aqueous phase through biological treatment methods such as aerobic microbial degradation, microalgae treatment, anaerobic treatment, electrochemical treatment and physicochemical technologies such as adsorption precipitation and membrane separation. Based on the research results of our group, this paper summarizes the research results of HTL-AP by-products at home and abroad in the past 10 years (2012-2021). Firstly, the characteristics of hydrothermal aqueous phase are reviewed, and the formation path and influencing factors of aqueous phase in hydrothermal conversion reaction are introduced. Then the direct utilization of aqueous products and the separation of specific products were introduced. Then four biological methods, including aerobic biodegradation, microalgae treatment, anaerobic biodegradation and microbial electrochemistry, were reviewed for the recovery of aqueous resources. Finally, the bottleneck of the recycling of hydrothermal liquefaction resources is summarized, and the future research direction is prospected.
2 Formation mechanism of aqueous phase in hydrothermal liquefaction
The water molecules in HTL-AP mainly come from the free water in the reaction system and the bound water in the raw materials[14]. In the process of hydrothermal liquefaction, water molecules are in the subcritical state, the hydrogen bonds in the molecules are destroyed, the dielectric constant, viscosity and surface tension decrease, and the properties of water gradually change from strong polarity to non-polarity, which promotes the solubility of organic reaction substances to increase, the diffusion coefficient to increase, and the effective collision to increase in the reaction. In hydrothermal reactions, water molecules, on the one hand, act as solvent and reaction medium to dissolve raw materials, provide space for the reaction of substances and improve the reaction rate, on the other hand, act as reactants to directly participate in the hydrolysis of substances or as products to participate in dehydration reactions[14]. Under lower treatment intensity, macromolecular substances in the raw materials are hydrolyzed into micromolecular substances by utilizing water molecules in a reaction system; With the increase of treatment intensity, water molecules are produced by further reaction dehydration[15,16]. The water molecules added in the reaction system, the water molecules formed by dehydration and the water-soluble substances formed by raw material conversion constitute the HTL-AP. Operating conditions and raw materials are the main factors affecting the physical and chemical properties of HTL-AP.
2.1 Hydrothermal operating condition
The operating conditions such as reaction temperature, residence time, solid content, heating rate, initial pressure, gas atmosphere, catalyst and product separation method have effects on the composition and yield of HTL-AP, among which reaction temperature and residence time have the most significant effects[17,18]. At present, considering the two factors of temperature and residence time, our research group introduced the concept of hydrothermal treatment intensity (formula 1) to quantify the severity of operating conditions[19].
Wherein T is the reaction temperature; The t1 and t2 are that time required for the reactor to rise to and fall from 100. degree. C., respectively, dure hydrothermal liquefaction.
The intensity of hydrothermal treatment has an important effect on the yield of aqueous phase, the element composition and the content of substances in the aqueous phase. At low treatment intensity, the yield of aqueous products increases with the increase of hydrothermal treatment intensity, and then decreases when the treatment intensity reaches a certain value. It is worth noting that this is consistent with the change of the yield of biocrude oil, which may be related to the fact that the substances produced by the conversion of organic substances enter the water phase and the oil phase at the same time. As the hydrothermal intensity continues to increase, the small molecules in the water phase are further decomposed into the gas phase, thus reducing the yield of the water phase[19,20].
The intensity of the reaction affects the elemental composition in the hot aqueous phase of the water. Our research group found that with the increase of reaction intensity, the relative content of C and H in the hot water phase of straw water increased significantly, while the relative content of N decreased. This is because the C, H and O elements in the straw are degraded into water-soluble compounds with the increase of reaction intensity, resulting in the increase of the relative content of C and H and the decrease of the relative content of N[19].
The intensity of the reaction affects the material composition in the hot water phase of the water (fig. 2). Five typical components (oil, protein, lignin, cellulose and hemicellulose) of hydrothermal liquefaction raw materials were used to explore the composition of HTL-AP under different hydrothermal treatment intensities.The results showed that with the increase of reaction intensity, the reaction of substances in the aqueous phase became more complex, resulting in the increase of the species of substances in the aqueous phase and the decrease of the relative content of each substance, and further analysis was still in progress[21].
2.2 Feedstock conversion path
The organic component in the reaction feed is the determining factor for the aqueous product characteristics. The raw materials of hydrothermal liquefaction include agricultural and forestry wastes such as straw, manure and kitchen waste, and the organic components of raw materials include oil, protein, carbohydrate and lignin.
In the hydrothermal liquefaction reaction, oils and fats are first decomposed into fatty acids and glycerol, which further react to produce hydrocarbons, formaldehyde, acetaldehyde and alcohols[22]. The higher lipid content in kitchen waste, some algae and oil-producing crop residues resulted in more fatty acids and glycerol in HTL-AP, and the pH of the aqueous phase was generally acidic (pH 3. 6-5. 8).
Protein mainly exists in kitchen waste, pig manure, chicken manure and some algae raw materials. Proteins were hydrolyzed to amino acids at low reaction intensity. With the increase of reaction intensity, ammonia nitrogen, amides, indoles, alcohols, phenols, pyrrolidones and pyrazines were produced and entered HTL-AP[22]. Compared with low-protein feedstock, high-protein feedstock HTL-AP had higher ammonia nitrogen content, which accounted for more than 50% of the total nitrogen in the aqueous phase, and the high concentration of ammonia nitrogen made HTL-AP generally alkaline (pH 7. 0-9. 0)[23].
Lignocellulosic biomass such as rice straw, shrubs and corn stalks are widely distributed and are important raw materials for hydrothermal liquefaction. Under hydrothermal conditions, cellulose is first decomposed into oligomers, and then converted into monosaccharides such as glucose. With the increase of reaction intensity, cyclic ketones, furan derivatives, short-chain organic acids and aldehydes are produced through dehydration, isomerization and cyclization reactions[17,24]. Hemicellulose was converted to oligomers first, and then to xylose, galactose, glucose and fructose. With the increase of reaction intensity, hemicellulose was converted to lactic acid, acetic acid, aldehydes (5-hydroxymethyl-2-furfural and 2-furfural) and furans[25]. Hydrolysis and cleavage of C — O — C and C — C bonds, demethoxylation, alkylation and condensation are the main reactions of lignin in the hydrothermal system, which compete with each other to produce phenols, among which phenol, methoxyphenol and catechol are the main products in the aqueous phase[26][17]. The lignocellulosic HTL-AP contained more acids, phenols, ketones and aldehydes, which resulted in the acidity of the aqueous phase (pH 3. 8-5. 6)[22].
Biomass raw materials for hydrothermal liquefaction have complex components, and various components and substances generated by conversion react with each other to inhibit or promote the conversion of HTL organic matter, which further increases the complexity of HTL-AP material components[27]. Our group found that protein, lipid, carbohydrate (Maillard reaction), lignin and soybean oil inhibited each other in HTL, which reduced the yield of oil phase and water phase[28,29].
On the whole, 26% ~ 55% of the components in the raw material were transferred to HTL-AP, the pH value was 3.0 ~ 9.0, and the COD was concentrated in the range of 20 ~ 110 G/L (Table 1). From the perspective of TOC, the contribution of each component in biomass to the TOC concentration in the aqueous phase was protein, carbohydrate and lipid[30].
2.3 Element Migration Path
The material transformation caused by the migration of elements such as C and N in the raw material is the key to the formation of HTL-AP. Yu et al. Found that 65% -70% of N and 35% -40% of C in microalgae were transferred into HTL-AP[31].
Based on six kinds of livestock and poultry manure (pig manure, laying hen manure, sheep manure, dairy cow manure, broiler manure and beef cow manure), our research group explored the migration law of C and N elements in the process of hydrothermal liquefaction (Fig. 3)[32]. The results showed that more than 30% of C in pig manure, layer manure and sheep manure was transferred to HTL-AP, and 48.2% and 46.1% of C in dairy manure and broiler manure were transferred to HTL-AP. Except 45.5% of N in beef cattle manure was transferred into solid residue, 45% ~ 65% of N in other five kinds of livestock and poultry manure was transferred into water phase with the increase of hydrothermal treatment intensity. The analysis of nitrogen species in the aqueous phase showed that nitrogen in the aqueous phase mainly existed in the form of N2O2 and N2O3[32].
Reaction raw materials such As sludge and feces contain a variety of metal elements, such as Zn, Cu, As, Pb, Cd and so on. During the reaction, more than 70% of Zn, Cu, Pb and Cd were transferred to the residue and less than 1% to the aqueous phase[35]. 20.0% ~ 75.0% of As was transferred into the solid residue, and the rest of As was transferred into the aqueous phase in the form of sodium arsenate, potassium arsenate or nitrophenylarsonic acid. In addition, Shi et al. Found that more than 80% of the P in the sludge was transferred to the aqueous phase[31].
3 Physicochemical method
HTL-AP contains a variety of organic substances (small molecular acids, esters, alcohols, etc.) and inorganic salts (N, P, etc.), which has a high potential for resource recovery (Table 1)[22,33,34].
3.1 Organic separation
HTL-AP contains a variety of high-value compounds, such as ethanol, glycolic acid, phenol, amide, pyridine, pyrazine and so on, which is an important chemical intermediate[36]. The separation and utilization of HTL-AP compounds may be an important path for water phase resource utilization, but the composition of HTL-AP is complex and the relative content of single component is low, which makes the separation and purification difficult. Rao et al. Used membrane distillation technology to concentrate the aqueous phase, and after concentration, the concentration of organic matter in the membrane permeate increased by 4 times[37]. Chen et al. Used modified resin to separate phenols from rice straw aqueous phase. After adsorption and desorption, an ethanol solution containing 78% phenols was obtained[38]. Some researchers also used the three-stage nanofiltration technology to separate the substances in the aqueous phase, such as Lyu, which concentrated the substances in the aqueous phase into three parts: sugar concentrate, phenol and cyclopentenone concentrate, and acetic acid concentrate[39]. Zhang et al. recovered the carboxylic acids in the aqueous phase by adjusting the pH value of the nanofiltration solution[40].
3.2 Separation of inorganic elements
HTL-AP contains N(5~20 g·L-1), P(1~10 g·L-1) and K(1~8 g·L-1) elements, and has the potential of fertilizer utilization (Table 1). After the kitchen waste and feces HTL-AP were concentrated by membrane distillation, the P of 100% kitchen waste and 99.5% feces HTL-AP was separated into the retentate, and the P concentrations were 18 000±2000 mg·L-1 and 15 800±50 mg·L-1, respectively[37]. Shanmugam and Li et al. Have proposed a way to recover N and P in the aqueous phase to prepare MgNH4PO4·6H2O[41][42]. By adding HCl, NaOH and MgCl2 to the aqueous phase, Shanmugam et al. converted 99% of P and 40% to 100% of ammonium nitrogen into MgNH4PO4·6H2O precipitate[41]. Li et al. Used electrochemical stripping method to dissolve aqueous ammonium nitrogen and H2SO4The MgNH4PO4·6H2O was generated by adding MgCl2, and the 78 kg MgNH4PO4·6H2O and 158 kg(NH4)2SO4 could be produced by the thermal liquefaction of 1 000 kg of sewage algae water[42].
3.3 Mixed component utilization
3.3.1 The aqueous phase is recycled to serve as a reaction solvent
HTL-AP contains a variety of organic substances, which can be used as a hydrothermal liquefaction reaction raw material and reaction medium. Several studies have shown that hydrothermal aqueous phase recycling can improve the yield of bio-oil from algal hydrothermal liquefaction[43]. Klemmer et al. Found that after circulation, the content of various substances in the water phase was increased, which had the effect of concentrating the water phase, and the nitrogen content in the bio-oil was also increased[44]. However, some studies have shown that aqueous circulation has an inhibitory effect on the conversion of lignin and oil production, and organic aqueous solvents will intensify the polymerization of phenolic compounds in the process of lignin conversion, thereby inhibiting the liquefaction of lignin and oil production, resulting in a decline in oil production[45]. In different studies, the variation of organic acid content in the aqueous phase after circulating liquefaction is not consistent, which may be related to the raw materials and reaction conditions of the circulating aqueous phase[45,46].
3.3.2 Aqueous phase gasification
Biomass gasification is an effective way to convert organic matter into combustible gas, which is widely used in the energy conversion of fruit shells, sewage sludge and municipal solid waste[47]. HTL-AP has a high content of organic matter, so it is feasible to gasify HTL-AP into combustible gas.
Our research group used supercritical water gasification technology to treat human fecal HTL-AP, and the results showed that hydrogen and methane accounted for 28% -57% and 4% -20% of the gas produced by gasification at 400-600 ℃ for 30 min, respectively, and the conversion rate of C was 24% -58%[48]. Under the condition of holding at 400 ℃ for 30 min, the proportion of H2 in the generated gas is the largest, reaching 56. 3%. The increase of gasification temperature can improve the conversion rate of C. When Zhang et al treated sludge and waste newspaper HTL-AP at 700 ℃, hydrogen and methane accounted for 44. 7% -54. 9% and 12. 1% -15. 8% of the gas, respectively, and the conversion rate of C was 90. 1% -96. 6%[49].
3.3.3 Utilization of insecticides and fungicides
HTL-AP has high biological toxicity. Phenols, aldehydes, pyrimidines, pyridines and ketones in aqueous phase are important raw materials for bactericidal and insecticidal pesticides. Our research group recently explored the inhibitory effect of HTL-AP on plant pathogenic fungi Fusarium oxysporum, Fusarium graminearum, Botrytis cinerea, Rhizoctonia solani, Ralstonia solanacearum and soft rot of Chinese cabbage. The results showed that aqueous phase could significantly inhibit the growth of plant pathogens, change the morphology of hyphae, and damage the cell membrane and energy metabolism pathways of fungi and bacteria.
4 Biological treatment method
HTL-AP has high chemical oxygen demand (COD) and high bioavailability potential (Table 1)[22,33,34]. The biological treatment method has the advantages of being eco-friendly, environment-friendly, energy-saving and the like, and is widely applied to the aspects of resource recovery, sewage treatment and the like[50,51]. At present, the main biological methods for HTL-AP treatment are aerobic biodegradation, microalgae cultivation, anaerobic digestion and microbial electrochemical degradation.
4.1 Cytotoxicity
The composition of aqueous phase is complex, including phenols, aldehydes, pyrimidines, pyridines, ketones, ammonia nitrogen and heavy metals, which can damage cells and inhibit biological growth[13,33]. Pham et al. evaluated the cytotoxicity of the aqueous phase using mammalian (mouse) cells[6]. The results showed that the HTL-AP organic mixture had strong cytotoxicity. When 7.5% of the aqueous phase was added to the culture medium, 50% of the cells died. The nitroxides in the aqueous phase were the main source of cytotoxicity, and the cytotoxicities were in the order of 3-dimethylaminophenol > 2,2,6,6-tetramethyl-4-piperidone > 2,6-dimethyl-3-pyridinol > 2-pyridine > pyridine > 1-methyl-2-pyrrolidone > σ-valerate > 2-pyrrolidone > ε-caprolactam[6]. Zheng et al. Found that the activity of 50% of anaerobic fermentation microorganisms was inhibited when 6% HTL-AP was added to the fermentation system[52]. It is inferred that the half-inhibitory concentration (IC50) of HTL-AP is below 10%. Our research group found that there were great differences in the toxicity of different hydrothermal raw materials and hydrothermal conditions, and the bactericidal effect of the aqueous product produced by microalgae at 320 ℃ on Escherichia coli and Staphylococcus aureus was similar to that of streptomycin produced by 0.1 mg·mL-1, and the diameter of inhibition zone of Oxford cup reached 11 mm[21]. In addition to nitrogen and oxygen, aldehydes, ketones, ammonia nitrogen and heavy metals are the main reasons for the biological toxicity of water phase[12].
4.2 Aerobic microbial treatment
Aerobic microorganisms play an important role in the treatment of domestic wastewater and livestock wastewater[53,54]. It is an important way for further biological treatment of aqueous phase to use aerobic microorganisms to treat HTL-AP and recover resources, thereby reducing the toxicity of aqueous phase.
It was reported that Pseudomonas putida, Rhodococcus sp. And Saccharomyces cerevisiae could tolerate and degrade HTL-AP. When HTL-AP was used as the sole source of carbon, nitrogen and phosphorus, and the concentration of water phase was 10% ~ 40%, Pseudomonas putida grew well, and when the concentration of water phase was 20%, Pseudomonas putida grew fastest[55]. He et al. Used Rhodococcus to treat HTL-AP from algae and pine, and the results showed that Rhodococcus transformed more than 92.7% of COD in the culture environment within 11 days, and removed more than 1/3 of the total chemical substances in the water phase[56]. The different components of the culture environment resulted in different metabolic pathways of Rhodococcus, and the accumulation of Rhodococcus lipids in pine-HTL-AP was 0.10 g·L-1, while that in microalgae-HTL-AP was 0.46 g·L-1[56]. Yeasts did not grow well in the aqueous phase, but the growth of yeasts could be improved by adding glucose as a C source to the medium[57].
Four strains, including two bacteria (one Bacillus and one Monomonas) and two fungi (one Aspergillus and one Mucor), which can degrade HTL-AP, were isolated from soil by our research group. The screened Bacillus and Monas bacteria had high tolerance and transformation ability to HTL-AP produced by microalgae and straw raw materials at 320 ℃. In the nutrient broth containing 30% aqueous phase, the removal rates of Bacillus and Stenotrophomonas were 55. 0% and 44. 09%, respectively, and the inhibitory effect of the treated water on Escherichia coli and Staphylococcus aureus was reduced. However, the conversion ability of the two strains to the aqueous phase was low when the aqueous phase product was used as a single carbon and nitrogen source.
Although the selected strains have certain tolerance and degradation ability to the aqueous phase, the higher cytotoxicity makes the microbial degradation efficiency lower. At present, genetic engineering methods have been widely used to improve the stress resistance and the yield of target products of the screened strains[58]. Jayakody et al. Used genetic engineering technology to modify Pseudomonas putida and induced the overexpression of groEL, groES and clpB genes, thus improving the tolerance of the strain to phenolic substances in HTL-AP[55].
4.3 Microalgae treatment
Microalgae are photoautotrophic in water, with high photosynthetic efficiency and fast growth rate, which are widely used in the resource transformation of domestic sewage, biogas slurry and livestock and poultry breeding sewage, and are also important raw materials for hydrothermal liquefaction[59,60]. The treatment of HTL-AP by microalgae is the focus of the current research on the utilization of water resources (Table 2)[33]. Based on microalgae and hydrothermal liquefaction, Zhang yuanhui's research team (China Agricultural University and University of Illinois at Urbana-Champaign) proposed the concept of "environmental value-added energy", which uses wet biomass such as microalgae as raw materials.With hydrothermal liquefaction technology as the core process, chemicals and bio-crude oil are obtained, and by-products such as water phase and gas are used for microalgae cultivation, thus realizing material resource recycling and zero pollution emission[61].
HTL-AP was used to culture Chlorella regularis, Chlorella pyrenoidosa, Scenedesmus quadricauda, Microcystis aeruginosa and Chlorella vulgaris. The results showed that the microalgae utilized HTL-AP for heterotrophic metabolism of organic carbon and organic nitrogen under continuous dark conditions when the concentration of HTL-AP in the aqueous phase was low, and the photosynthesis of microalgae increased with the increase of light intensity. TOC (total organic carbon), TN (total nitrogen) and TP (total phosphorus) were absorbed and utilized by microalgae in the culture environment[62,63]. With the increase of HTL-AP concentration in the culture system, the growth of microalgae was gradually inhibited, and it was reported that microalgae could tolerate 0. 2% ~ 6.7% hydrothermal aqueous phase in the culture system[64]. Du et al showed that algae could remove 45.5% ~ 59.9% of total nitrogen, 85.8% ~ 94.6% of TP and 50.0% ~ 60.9% of COD in the system at dilution of 50 ~ 200[65]. In addition, the heavy metals in HTL-AP are precipitated and volatilized by microalgae through their own absorption and transformation or the change of the environmental characteristics of the aqueous phase, so as to achieve the effect of heavy metal removal[66].
HTL-AP phenols, ammonia nitrogen and nitrogen and oxygen organic compounds were the main substances inhibiting the growth of microalgae. The raw material HTL-AP with high protein content contains high concentration of ammonia nitrogen, and NH4+ and NH3 enter the microalgae through the cell membrane and combine on the thylakoid to inhibit the photosynthesis of the microalgae. In addition, the initial ratio of N and P in HTL-AP and the nutrient imbalance caused by the consumption of nutrients (such as P) during the culture process would inhibit the growth of microalgae[67]. Different separation methods of HTL-AP affect the effect of microalgae culture. Our research group compared the effects of HTL-AP separated by ether and vacuum filtration on the growth of microalgae with high protein and low lipid, high lipid and low protein[63]. C. vulgaris 1067 increased from 1.9% to 6.7% in the aqueous phase of high protein and low oil raw materials by ether separation. The daily productivity reached the C. vulgaris, which was 1.9 times that of the vacuum filtration method. The removal rates of TOC, TN, NH3-N and TP were significantly improved. The HTL-AP obtained by vacuum filtration was beneficial to the growth of C. vulgaris 1067, and the daily biomass production rate and the final bioaccumulation reached C. vulgaris and 1.4 g·L-1, respectively, which were twice as much as those obtained by organic solvent extraction[63].
The adsorption of HTL-AP by activated carbon and zeolite can significantly improve the conversion rate of COD and promote the biomass accumulation of microalgae. Lim et al. treated the aqueous phase diluted by 500 times, 1000 times and 2000 times with activated carbon, the contents of organic nitrogen, organic carbon, phenols and nickel in the system were significantly reduced, and the accumulation of microalgal biomass reached 0.41±0.09 g·L-1(1000 times diluted condition), which was slightly lower than that of traditional medium (0.49±0.10 g·L-1)[68].
It is an important way to improve the utilization efficiency of microalgae for water resources by screening algae species and constructing multiple algae species or bacteria-microalgae symbiotic system. In the medium containing HTL-AP, the treatment effect of mixed culture of multiple algae species is better than that of single algae species[69,70][71]. Our research group carried out the research on the treatment of HTL-AP by bacteria-microalgae symbiotic system and bacteria-microalgae segmented system. Compared with the bacteria-microalgae symbiotic system, the bacteria-microalgae segmented system has higher degradation efficiency of organic matters in the treatment of hydrothermal liquefaction water, the removal rate of COD and TOC reaches 71.2% and 55.8% within 6 days, and the total output of biomass is 0.96 g·L-1. Further domestication of algae species improves the tolerance of microalgae to hydrothermal water phase and the utilization of nutrient elements. However, after acclimation, the photoautotrophic ability of microalgae was weakened, and the activity of algal cells decreased, which led to the decrease of biomass accumulation and daily productivity[71]. In addition, the growth performance of microalgae in HTL-AP can be improved by increasing the ratio of NH3 to NH4+ in the culture medium and adding inorganic salt ions (Mg2+), which is the key for microalgae growth, to promote the synthesis of specific growth components of microalgae[13,22,62,63].
4.4 Anaerobic fermentation
Anaerobic fermentation is an important way to treat organic waste, which has the advantages of simple operation, large treatment capacity and relatively low construction cost, and is widely used in the treatment of livestock and poultry manure, kitchen waste and sewage[72,73].
HTL-AP has a high COD content (10 ~ 170 G/L), which has a great potential for resource recovery from anaerobic digestion[22]. Currently, there are some reports on anaerobic fermentation of HTL-AP. Generally speaking, the efficiency of methane production is in the 14~314 mL·g-1COD, focusing on the 200 mL·g-1COD, where the energy recovery efficiency reaches 57%, and the methane production is significantly lower than the theoretical methane production[22].
The material, reaction intensity and separation method of hydrothermal reaction affect the composition of the aqueous phase, and then affect the treatment efficiency of anaerobic fermentation and the degradation effect of HTL-AP. It was found that when microalgae were used as raw materials, the reaction time was 1 H, and the reaction temperatures were 260, 280, 300 and 320 ℃, the COD value of the aqueous phase of hydrothermal liquefaction at 300 ℃ was the lowest, the time lag of anaerobic fermentation was the largest, and the methane production rate was the lowest[52,74]. In addition, ethyl acetate, petroleum ether and n-hexane were the most favorable solvents for HTL-AP extraction and separation, and the methane production of HTL-AP extracted by dichloromethane was not obvious. The methane production of ethyl acetate HTL-AP was about 325 mL·g-1COD, which was close to the theoretical production of 350 mL[75]. In low concentration HTL-AP system, 44% ~ 61% of COD was removed by anaerobic fermentation. In high concentration HTL-AP system, ammonia nitrogen, nitrogen-oxygen heterocyclic compounds, furan and phenolic compounds inhibited the activity of anaerobic microorganisms[76]. The higher ammonia concentration (1.9~12.7 g·L-1) in the aqueous phase of hydrothermal liquefaction results in the change of intracellular acid-base environment of methanogenic microorganisms, and inhibits the activities of key enzymes in the process of methanogenesis, thus inhibiting the efficiency of anaerobic fermentation and reducing methane yield[77]. Nitrogen-containing compounds such as pyrimidine, pyridine, pyrazine and the like have synergistic cytotoxicity effects with each other and play a role in inhibiting microorganisms in the anaerobic fermentation reactor[6].
The concentration of inhibitors decreased after HTL-AP adsorption and dilution, which was also an important way to improve the anaerobic treatment effect. Our research group explored the effect of zeolite adsorption pretreatment on HTL-AP anaerobic fermentation (Fig. 4)[76]. After zeolite adsorption, 5.73% of C and 75.4% of N were removed, and butylated hydroxytoluene and N- [2-hydroxyethyl] succinimide were completely removed. Most of the sulfate is removed, and the toxic effect of sulfides such as H2S on microorganisms is reduced. Before HTL-AP was adsorbed, the lag phase reached 8.8 days at the optimum substrate concentration 4 g·L-1. After adsorption, the lag period was shortened to 1. 03 days, and the cumulative methane production was increased to 14.79 mmol·g-1·COD, which was 35. 7% higher than that before adsorption. 60. 3% of C and 13% of N were converted to use, the energy recovery rate increased by 6. 24%, the carbon recovery rate increased by 4. 04%, and the fermentation time was shortened by 5 days[76].
Struvite, activated carbon, polyurethane and ozonation have also been used to remove anaerobic inhibitors in the aqueous phase. After the treatment of struvite, activated carbon and polyurethane, the lag phase of anaerobic fermentation was significantly shortened, and the total methane yield was increased by 10%, 38% and 37%, respectively[52]. After Yang et al. Used activated carbon to treat the aqueous phase of hydrothermal liquefaction, the yield of methane increased by 67.7%, and the removal rate of COD reached 97.29%, which was higher than that of the untreated group (42.66%) and the ozonated group (71.13%)[52]. Shanmugam et al. increased the methane yield by 3.5 times (182±39 mL·g-1COD) by adding HCl, NaOH, and MgCl2 to remove ammonia nitrogen and P from the solution[41]. Mixed fermentation is an important means for anaerobic fermentation to improve COD degradation effect and increase gas production[78]. Fernandez et al. Added 20-30% of algae HTL-AP to the feces fermentation system for mixed fermentation[79]. The biogas production of HTL-AP from Tetraselmis and Chlorella under semi-continuous anaerobic fermentation mode was 327.2 mL·g-1 and 263.4 mL·g-1COD, respectively. With the increase of the concentration of wastewater, the gas production decreased gradually, and the author believed that the increase of salt concentration inhibited the action of anaerobic microorganisms[79].
Two sets of continuous Anaerobic fermentation systems for HTL-AP treatment were established by using high efficient anaerobic reactors. UASB (Up-flow Anaerobic Sludge Blanket) Reactor was used to establish a continuous methane production system, and UASB and PBR (Packed Bed Reactor) reactors were used to establish a two-stage hydrogen alkane production system. In the continuous methane-producing system, when 30% HTL-AP was added to the reaction system, there was no significant change in the fermentation effect compared with the non-aqueous phase, but when the proportion of aqueous phase increased to 50%, the anaerobic process was completely inhibited. After zeolite adsorption, the removal capacity of the system for aqueous phase was significantly improved. When the proportion of aqueous phase was 60%, the methane content, methane yield and COD removal rate were still 58. 9%, 1.25 L·L-1·d-1 and 66. 3%. When the proportion reached 80%, the degradation was obviously inhibited (COD removal rate was 25. 3%).
In the UASB and PBR hydrogen alkane production systems, more clean biological hydrogen alkane is produced. The fermentative hydrogen-producing reactor had a good degradation effect on the inhibitors of methanogenic microorganisms, thus improving the efficiency of anaerobic fermentation.Compared with the methanogenic system alone, the two-stage fermentation system had a higher COD removal efficiency, and the yields of H2 and CH4 reached 29 mL·g-1COD and 254 mL·g-1COD, respectively[80].
HTL-AP, as a fermentation substrate, significantly changed the microbial system of anaerobic fermentation (Fig. 5)[80]. Metagenomic analysis found that with the addition of hydrothermal liquefaction wastewater, Desulfovibrionaceae, Bacillaceae, Geobacteraceae,The relative abundance of microorganisms such as Pseudomonadaceae and Anaerolineaceae, which can degrade phenols and aldehydes, increased. In the two-stage fermentation process, the abundance of microorganisms related to the degradation of inhibitors such as phenols and aldehydes was higher in the hydrogen-producing reactor, and the abundance of microorganisms related to the oxidation of acetic acid was higher in the methanogenic reactor[80]. However, Chen et al. believed that the microbial composition and function in HTL-AP anaerobic fermentation could not be well analyzed by using 16s rRNA gene technology and molecular microbial species with 97% and 98.65% confidence intervals, and no flora with corresponding functions was found for a variety of degraded substances[75].
4.5 Microbial electrochemistry
Microbial Electrochemical Technology (MET) is widely used in the research of wastewater treatment and energy recovery, which is a new approach of wastewater treatment with Electrochemical coupling of Microbial redox. According to whether external power supply is provided, microbial electrochemical system can be divided into microbial fuel cell (MFC) and microbial electrolysis cell (MEC)[81]. Our research group investigated the effect of two microbial electrochemical technologies applied to HTL-AP treatment, and is the only team at home and abroad to carry out electrochemical treatment of HTL-AP.
In our research group, carbon nanotube supported MFC was used to treat the [82] of HTL-AP. When the organic load is 8.16 g·L-1·d-1, the MFC obtains the maximum voltage of 480 mV and the average process voltage of 381 mV. When the organic loading was 3.01 g·L-1·d-1, the maximum TOC removal rate (84.67%) was obtained, and 5-HMF was removed by 100%. The lignocellulosic HTL-AP contains high refractory phenols. It was found that when the phenol concentration was 250 mg·L-1, the maximum power density of MFC for phenol degradation rate reached 15.6 mg·L-1·h-1,MFC reached 34 W·m-3.
Our research group carried out the treatment of HTL-AP by MEC, and found that MEC had a good removal effect on nitrogen-containing organic compounds and refractory macromolecular organic compounds (hexadecane, phenol and diphenylamine)[83,84]. When the pig manure HTL-AP with high ammonia nitrogen was used as the substrate, the auxiliary voltage was positively correlated with the hydrogen production rate and hydrogen yield, and the maximum hydrogen production rate and hydrogen yield were 168.01 mL·L-1·d-1 and 5.14 mmol·kg-1COD, respectively. When the initial substrate concentration was low, the removal rate of CODcr was up to 97. 87%, but the removal rate of NH3-N by MEC was poor (36. 66% ~ 76.53%). This may be related to the fact that NH3-N is the main product of nitrogen in the degradation and transformation of MEC: part of the organic nitrogen in the solution is transformed into NH3-N by microbial metabolism, and part of the inorganic nitrogen is gradually reduced to NH4+ by the electrons generated by the degradation of organic matter at the anode.
HTL-AP significantly changed the microbial composition of the anodic biofilm in MEC and reduced the microbial diversity. The microorganisms in the anode were mainly composed of bacilli and archaea. At the phylum level, Chlorobi, Proteobacteria and Actinobacteria accounted for more than 90% of the total number of bacteria in the anode biofilm of MEC[83]. Chlorobiaceae, Mycobacteria ceae and Methylocystaceae were the dominant species at the family level, while Chlorobium and Mycobacterium were the dominant species at the genus level[83].
5 Challenge and prospect of aqueous resource recovery
5.1 Aqueous resource recovery challenge
Aqueous products of hydrothermal liquefaction are characterized by large production, complex composition and high environmental safety risk, which has become a resistance to the further development of hydrothermal liquefaction technology at this stage. Therefore, it is necessary to treat and utilize the aqueous phase. At present, the treatment of HTL-AP mainly uses physical, chemical and biological methods to recycle the valuable components in the water phase, degrade and remove toxic substances, and then obtain clean water to meet the discharge standards[37]. It is in line with the original intention of the green development of hydrothermal liquefaction to recycle the components in the aqueous phase, but the complexity of the chemical composition of raw materials, reaction conditions and reaction process leads to the complexity of the components in HTL-AP, and the low content of single substance further increases the difficulty and cost of recycling[38]. Biochemical treatment is widely used in waste resource utilization.Although the higher COD concentration of HTL-AP provides conditions for biochemical treatment, the high toxicity limits the activity of microorganisms, and the pretreatment process further increases the treatment cost, treatment cycle and process complexity[13]. Chemical methods such as advanced oxidation further cause waste of resources and pollution.
5.2 Prospect of Aqueous Phase Resource Recovery
To sum up, the treatment of HTL-AP should follow the principle of first reducing, then using, and ensuring the discharge up to the standard[43].
Reduce the amount at the source to reduce the generation of HTL-AP. The utilization of HTL-AP should enhance the understanding of the material conversion process of hydrothermal liquefaction technology. The reaction of hydrothermal liquefaction is complex, and the mechanism is still unclear. Natural complex biomass raw materials further lead to the complex nature of the aqueous phase[22,85]. Limited by the accuracy of current detection methods, the composition and content of substances in aqueous phase can not be accurately determined. A systematic understanding of the nature of the hydrothermal aqueous phase and the transformation process should be further established[21,32,33]. Regulating the yield and component composition of the aqueous phase product by controlling the process parameters of the hydrothermal conversion; Through the further recycling of the water phase, the consumption of water in the raw material configuration and the discharge of the water phase product are reduced.
Strengthen utilization and recycle HTL-AP resources. In view of the chemical components in HTL-AP, the potential of direct resource utilization should be demonstrated first. Valuable components in the water phase are separated by a membrane separation technology or an adsorption and desorption technology; Additional substance are added to that aqueous phase for further conversion to valuable component[38]. Secondly, it is necessary to explore the way to utilize the water phase as a whole, including aerobic biological treatment, microalgae treatment, anaerobic digestion, microbial electrochemistry and other biological methods to recover the material components in the water phase (Table 3). The removal of toxicity in aqueous phase is the main problem of biological treatment technology. In order to improve the effect of biological treatment, it is necessary to further optimize the conditions of aqueous phase treatment and screen efficient biological species for aqueous phase treatment[57]. Higher N and P contents in aqueous phase make fertilizer utilization have certain potential, but the current research lacks reports on fertilizer efficiency and safety of aqueous phase fertilizer application[41].
Discharge up to standard to ensure HTL-AP pollution-free discharge. In the case of failing to meet the discharge requirements after resource recovery and treatment, conventional sewage treatment processes and paths such as advanced oxidation are used as the guarantee for HTL-AP to meet the discharge standards, so as to avoid environmental pollution. It is worth noting that a single treatment method generally can not meet the treatment requirements of the aqueous phase, and a combination of multiple treatment technologies should be constructed to achieve the resource utilization and discharge of the aqueous phase.
6 Conclusion.
Aqueous by-product of hydrothermal liquefaction (HTL-AP) has high organic content and toxicity, which limits the energy conversion efficiency of hydrothermal liquefaction and the development of green cycle. In this paper, the influencing factors, material transformation and element migration pathways in the formation process of HTL-AP were summarized, and the research progress of HTL-AP treatment and resource utilization was discussed. Physicochemical methods such as membrane separation and adsorption are used to separate inorganic salts and organic substances in aqueous phase. The complex and unstable characteristics of aqueous phase components lead to low separation efficiency of aqueous phase. Microalgae cultivation, anaerobic treatment and microbial electrochemistry have been used for the biotransformation of aqueous resources, but the high toxicity of aqueous phase inhibits the activity of microorganisms. The research on hydrothermal liquefaction of aqueous phase is still in its early stage, and most of the current studies are still focused on the reduction of pollutants in aqueous phase. Recently, based on the research on bacteriostasis and resource utilization of aqueous chemicals,The efficacy of using aqueous phase to prepare agricultural fungicides has been found and reported, which shows some potential applications in plant protection, food preservation and other fields, but its mechanism and evaluation need to be further studied.
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