Activation of sawdust biochar with water and wastewater treatment residuals for sorption of perfluorooctanesulfonic acid in water.

Recent research has found biochar to be a cost-effective adsorbent for removal of perfluoroalkyl substances in water. To promote cleaner production and sustainable waste management, this study explored the potential to produce activated biochars by co-pyrolyzing sawdust with iron-rich biosolids and polyaluminum sludge. The maximum capacity to adsorb perfluorooctanesulfonic acid (PFOS) reached 27.2 mg g-1 with biosolids-activated biochar and 19.2 mg g-1 with aluminum sludge-activated biochar, compared to 6.2 mg g-1 with sawdust biochar. The increased adsorption capacities were attributed to electrostatic interactions between the anionic PFOS and metal functionalities on the biochar surface. In contrast, hydrophobic interaction was the dominant adsorption mechanism of sawdust biochar. The presence of dissolved organic matter at 5-50 mg L-1 was found to inhibit adsorption of PFOS in water, while pH as low as 3.0 and sodium chloride concentrations up to 100 mM enhanced removal of PFOS by all the three adsorbents. In batch adsorption tests at environmentally relevant PFOS dosages and adsorbent dosage of 0.25 g L-1, the biosolids-sawdust biochar and Al sludge-sawdust biochar removed 71.4% and 66.9% of PFOS from drinking water and 77.9% and 87.9% of PFOS from filtrate of sludge digestate, respectively. The biosolids-sawdust biochar additionally removed Fe, although the Al sludge-sawdust biochar released Al into the alkaline drinking water and filtrate. Overall, this study proved co-pyrolyzing sawdust and Fe-rich biosolids to be an effective approach to activate sawdust biochar for enhanced removal of PFOS while recycling wastewater treatment residuals and sawdust.

Characteristics and practical treatment technologies of winery wastewater: A review for wastewater management at small wineries.

The wine-making industry drives tourism and rural revitalization in several countries. Meanwhile, winemaking generates wastewater at all production stages, mainly from cleaning of equipment, floors, vessels, and bottles. This review presents a comprehensive analysis with statistical characteristics on the overall quality and generation rate of winery wastewater since 2007, identifies the technologies used by wineries in pilot- and full-scale wastewater treatment systems, and offers insights on practical wastewater treatment at small wineries. The median wastewater generation rate has been reduced to 1.58 L/L-wine, with a weekly peaking factor of 1.6-3.4 and monthly peaking factor of 2.1-2.7. Winery wastewater is acidic and of high organic strength. The organic substances are largely biodegradable and constituent concentrations do not exceed 50% inhibitory levels for biological treatment. However, the small ratios of nitrogen and phosphorus to biochemical oxygen demand indicate substantial needs to supplement nutrients for aerobic biological treatment. The frequency of processes used to pretreat winery wastewater was in the order of sedimentation > coarse screening > equalization > neutralization. The most frequently reported treatment methods were constructed wetland, activated sludge process, membrane bioreactor, and anaerobic digestion. Advanced oxidation processes have been pilot tested for polishing. The best wastewater management practice at small wineries is physical pretreatment, followed by land-based treatment systems. Covered anaerobic lagoons and underground digesters are practicable anaerobic digestion designs to reduce organic loading to land-based treatment systems. Research is needed to develop sufficient design criteria for the best practicable treatment processes and compare land-based treatment systems at pilot and full scales.

A spatial distribution - Principal component analysis (SD-PCA) model to assess pollution of heavy metals in soil.

With the rapid development of urbanization, heavy metal pollution of soil has received great attention. Over-enrichment of heavy metals in soil may endanger human health. Assessing soil pollution and identifying potential sources of heavy metals are crucial for prevention and control of soil heavy metal pollution. This study introduced a spatial distribution - principal component analysis (SD-PCA) model that couples the spatial attributes of soil pollution with linear data transformation by the eigenvector-based principal component analysis. By evaluating soil pollution in the spatial dimension it identifies the potential sources of heavy metals more easily. In this study, soil contamination by eight heavy metals was investigated in the Lintong District, a typical multi-source urban area in Northwest China. In general, the soils in the study area were lightly contaminated by Cr and Pb. Pearson correlation analysis showed that Cr was negatively correlated with other heavy metals, whereas the spatial autocorrelation analysis revealed that there was strong association in the spatial distribution of eight heavy metals. The aggregation forms were more varied and the correlation between Cr contamination and other heavy metals was lower. The aggregation forms of Mn and Cu, Zn and Pb, on the other hand, were remarkably comparable. Agriculture was the largest pollution source, contributing 65.5 % to soil pollution, which was caused by the superposition of multiple heavy metals. Additionally, traffic and natural pollution sources contributed 17.9 % and 11.1 %, respectively. The ability of this model to track pollution of heavy metals has important practical significance for the assessment and control of multi-source soil pollution.

Capacitive Removal of Pb ions via Electrosorption on Novel Willow Biochar - Manganese Dioxide Composites.

AbstractBiochar derived from lignocellulosic biomass has been used as a low-cost adsorbent in wastewater treatment applications. Due to its rich porous structure and good electrical conductivity, biochar can be used as a cost-effective electrode material for capacitive deionization of water. In this work, willow biochar was prepared through carbonization of shrub willow chips, activated with potassium hydroxide, and loaded with manganese dioxide (WBC-K-MnO2 nanocomposite). The prepared materials were used to electrochemically adsorb Pb2+ from aqueous solutions. Under the applied potential of 1.0 V, the WBC-K-MnO2 electrode exhibited a high Pb2+ specific electrosorption capacity (23.3 mg/g) as compared to raw willow biochar (4.0 mg/g) and activated willow biochar (9.2 mg/g). KOH activation followed by MnO2 loading on the surface of raw biochar enhanced its BET surface area (178.7 m2/g) and mesoporous volume ratio (42.1%). Moreover, the WBC-K-MnO2 nanocomposite exhibited the highest specific capacitance value of 234.3 F/g at a scan rate of 5 mV/s. The electrosorption isotherms and kinetic data were well explained by the Freundlich and pseudo-second order models, respectively. The WBC-K-MnO2 electrode demonstrated excellent reusability with a Pb2+ electrosorption efficiency of 76.3% after 15 cycles. Thus, the WBC-K-MnO2 nanocomposite can serve as a promising candidate for capacitive deionization of heavy metal contaminated water.

Occurrence and dietary exposure risks of phthalate esters in food in the typical valley city Xi'an, Northwest China.

Diet is an important exposure pathway of phthalate esters (PAEs) for humans. A total of 174 food samples covering 11 food groups were collected from Xi'an, a typical valley city in Northwest China, and analyzed to assess the occurrence and exposure risks for PAEs in the food. Twenty-two PAEs were detected. The sum of the 22 PAEs (∑22PAEs) varied between 0.0340 and 56.8 µg/g, with a mean of 3.94 µg/g. The major PAEs were di-iso-butyl phthalate (DiBP), di-n-butyl phthalate (DnBP), bis(2-ethylhexyl) phthalate (DEHP), di-iso-nonyl phthalate (DiNP), and di-iso-decyl phthalate (DiDP), which were associated mainly with the usage of plasticizers. Bio-availability of the PAEs in the combined gastro-intestinal fluid simulant of digestion was higher than that in the single gastric or intestinal fluid simulant. Bis(2-methoxyethyl) phthalate exhibited the highest bio-availability in each of the three simulants. Bio-availability of the PAEs was negatively correlated with the molecular weight and octanol-water partition coefficient of the PAEs and positively correlated with the solubility and vapor pressure of the PAEs. The estimated daily intake (EDI) of PAEs based on national and municipal food consumption data was lower than the reference dose (RfD) of the United States Environmental Protection Agency and the tolerable dairy intake (TDI) of European Food Safety Authority (EFSA), except for the EDI of DnBP and DiBP being higher than the TDI of EFSA. Grains and vegetables were the major sources of human dietary exposure to PAEs. The hazardous quotient for human dietary exposure to PAEs was less than the critical value of 1 and the cancer risk of butyl benzyl phthalate and DEHP was in the range of 10-11-10-6, suggesting relatively low health risks. The results indicated that human exposure to DnBP, DiBP, DEHP, DiNP, and DiDP in food is considerable and a health concern.

Stable, high-rate anaerobic digestion through vacuum stripping of digestate.

This study coupled anaerobic digestion with vacuum stripping to achieve stable digestion at higher organic loading rates. Besides mitigation of ammonia inhibition, vacuum stripping of digestate improves solids solubilization and dewaterability due to vacuum-enhanced low-temperature thermal and mild-alkaline treatment under the vacuum stripping conditions (65 °C, 25-27 kPa, and pH 9). Batch vacuum stripping for 8 h removed 97.4-99.4% of ammonia, increased the dissolved fraction of volatile solids (VS) by 72.5%, and improved dewaterability with 30% decreases in time-to-filter and viscosity. The digesters having 2.9% of digestate replaced daily by vacuum stripped digestate were stable up to organic loading rate of 4.3 g-VS/Lreactor/d with biogas production at 3.15 L/Lreactor/d, while the digesters without stripping attained biogas production of 1.90 L/Lreactor/d at its highest stable organic loading rate of 2.5 g-VS/Lreactor/d. Acetoclastic Methanosaeta were the dominant methanogens, which became more resistant to ammonia stress in the digesters with vacuum stripping.

Elucidating the kinetics of ammonia inhibition to anaerobic digestion through extended batch experiments and stimulation-inhibition modeling.

Ammonia can be accumulated to a level inhibitory to methanogenesis. There are large discrepancies in the reported inhibition thresholds. Through extended batch digestion experiments (up to 110 d) at 6 ammonia concentrations (0.70-13 g N/L), this study discovered sequential occurrence of adaptable and unadaptable inhibition that reveals the discrepancies. Lag phase length representing adaptable inhibition increased exponentially with ammonia concentration. The kinetics of specific biogas yield that reveals unadaptable inhibition was best simulated with the Han & Levenspiel model. The 50% unadaptable inhibition thresholds were 10.7 g N/L with active inoculum and 6.8 g/L with stressed inoculum. The digesters with stressed inoculum had faster adaptation to adaptable inhibition though less resistance to unadaptable inhibition. The inhibition sequence was evidenced by microbial population shifts and confirmed by earlier studies employing short (2-65 d) and long (80-198 d) batch experiments. Distinguishing adaptable from unadaptable inhibition provides precise guidance for mitigating ammonia inhibition.

Enhanced degradation of organic contaminants using catalytic activity of carbonaceous structures: A strategy for the reuse of exhausted sorbents.

Generation of hydroxyl radicals (⋅OH) is the basis of advanced oxidation process (AOP). This study investigates the catalytic activity of microporous carbonaceous structure for in-situ generation of ⋅OH radicals. Biochar (BC) was selected as a representative of carbon materials with a graphitic structure. The work aims at assessing the impact of BC structure on the activation of H2O2, the reinforcement of the persistent free radicals (PFRs) in BC using heavy metal complexes, and the subsequent AOP. Accordingly, three different biochars (raw, chemically- and physiochemically-activated BCs) were used for adsorption of two metal ions (nickel and lead) and the degradation of phenol (100 mg/L) through AOP. The results demonstrated four outcomes: (1) The structure of carbon material, the identity and the quantity of the metal complexes in the structure play the key roles in the AOP process. (2) the quantity of PFRs on BC significantly increased (by 200%) with structural activation and metal loading. (3) Though the Pb-loaded BC contained a larger quantity of PFRs, Ni-loaded BC exhibited a higher catalytic activity. (4) The degradation efficiency values for phenol by modified biochar in the presence of H2O2 was 80.3%, while the removal efficiency was found to be 17% and 22% in the two control tests, with H2O2 (no BC) and with BC (no H2O2), respectively. Overall, the work proposes a new approach for dual applications of carbonaceous structures; adsorption of metal ions and treatment of organic contaminants through in-situ chemical oxidation (ISCO).

Reversibility of propionic acid inhibition to anaerobic digestion: Inhibition kinetics and microbial mechanism.

Anaerobic digestion is a technology that simultaneously treats waste and generates energy in the form of biogas. Unfortunately, when a high organic loading rate is applied, anaerobic digestion can suffer from volatile fatty acid accumulation that results in pH drop and decreased biogas production. In particular, propionic acid has shown to inhibit biogas production even at a very low concentration. Therefore, the kinetics of biogas production in relation to propionic acid concentration needs to be investigated. In batch experiments on anaerobic co-digestion of food waste and dairy manure in the present study, cumulative biogas production showed little inhibition by propionic acid in the concentration range of 6.5-14.6 mM, but a lag phase of 9.4, 16.3 and 60.8 d was detected in the digesters with initial propionic acid concentrations of 22.7, 36.2, and 56.4 mM, respectively. After the lag phase, these digesters accelerated to specific biogas yields of 0.59-0.70 L g-VS-1. The similar specific biogas yields across all of the digesters at initial propionic acid concentrations of 6.5-56.4 mM indicated reversibility of the inhibition. The reversibility was made possible by microbial acclimation and the shift to hydrogenotrophic methanogenesis in syntrophy with acetogenic bacteria. Evidently, an increase of hydrogenotrophic Methanobacterium and Methanoculleus abundances was found at 36.2 and 56.4 mM. Batch digestion experiments must be extended beyond the lag phase in order to fully reveal the inhibition kinetics. This paper highlights the need for a standard protocol that experimentally evaluates inhibition in anaerobic digestion.

Microbial community similarity and dissimilarity inside and across full-scale activated sludge processes for simultaneous nitrification and denitrification.

Simultaneous nitrification and denitrification under low dissolved oxygen conditions is an energy-saving modification of the activated sludge process to achieve efficient nitrogen removal. Geographically distinct full-scale treatment plants are excellent platforms to address the links of microbial community with operating parameters. Mixed liquor samples were collected from a sequencing batch reactor plant, oxidation ditch plant, and step-feed activated sludge plant. Next-Generation Sequencing of the samples showed that the microbial communities were similar at the phylum level among the plants, being dominated by Proteobacteria. Microbial composition of functional groups was similar between the react fill and react phases of the sequencing batch reactors, among four sequencing batch reactors, and among four oxidation ditches. Nitrospira was the only identified genus of autotropic nitrifying bacteria with a relative abundance of 2.2-2.5% in the oxidation ditches and 0.4-0.7% at the other plants. Heterotrophic nitrifying-aerobic denitrifying bacteria were dominated by Dechloromonas with a relative abundance of 0.4-1.0%. Microbial community composition and nitrogen removal mechanisms were related to overall level and local zonation of dissolved oxygen, mixed liquor suspended solids concentration, nitrogen and organic loadings, and solids retention time. Low dissolved oxygen and low organic and nitrogen loadings favored growth of Nitrospira.

Phthalate esters in atmospheric PM2.5 and PM10 in the semi-arid city of Xi'an, Northwest China: Pollution characteristics, sources, health risks, and relationships with meteorological factors.

PM2.5 and PM10 samples were collected in the semi-arid city of Xi'an in Northwest China from November 2016 to November 2017 and analyzed to assess pollution characteristics, sources, health risks, and influencing factors of 6 priority phthalate esters (PAEs). The results showed that the sum of the 6 PAEs (Σ6PAEs) was 85.5 ng m-3 in PM2.5 and 94.5 ng m-3 in PM10, being higher at the suburban site than the urban site and winter > spring > summer > autumn. The most abundant PAE was bis(2-ethylhexyl phthalate) (DEHP). PM2.5- and PM10-bound PAEs were associated mainly with the use of plasticizers plus the uses of cosmetics and personal care products, construction materials, and home furnishings. Temperature, relative humidity, and visibility had stronger influences on the concentrations of PM and PM-bound PAEs than pressure and wind speed. Pressure and relative humidity were positively correlated with the concentrations of PM and most of the PM-bound PAEs, while temperature, visibility and wind speed had negative correlations with the concentrations of PM and PM-bound PAEs. The non-carcinogenic risks of human inhalation exposure to PM-bound PAEs were in the range of 10-7 to 10-3, suggesting low non-cancer risks, which were higher at the suburban site than the urban site and higher to children than adults. The cancer risks of human inhalation exposure to PM-bound DEHP and butyl benzyl phthalate (BBP) were in the range of 10-12 to 10-10, suggesting low carcinogenic risks, being in the order of the suburban site > the urban site and DEHP > BBP. Special attention should be paid to long-term low dose exposure to PAEs in the suburb, especially in winter and spring.

Three-stage treatment for nitrogen and phosphorus recovery from human urine: Hydrolysis, precipitation and vacuum stripping.

Source separation of human urine has not been widely adopted because of scaling on urine collecting fixtures and lack of verified technologies for on-site utilization of waterless urine. This study investigated the effects of flushing liquid, temperature and urease amendment on hydrolysis of urea to ammonia, explored ammonia recovery via vacuum stripping in connection with phosphorus recovery via struvite precipitation in different sequences, and performed economic analysis of a proposed nutrient recovery strategy. It was found that acetic acid could be dosed at 0.05-0.07 N to flush urine-diverting toilets and urinals for hygiene and prevention of scaling. However, a high dosage of 0.56 N completely inhibited urea hydrolysis. Source-separated urine could be stored at 25 °C with ample urease for complete urea hydrolysis within approximately 20 h. Fully hydrolyzed waterless urine contained 9.0-11.6 g/L ammonia-N, 0.53-0.95 g/L phosphate-P and only 2.3-9.1 mg/L magnesium. When magnesium was supplemented to attain the optimum Mg2+: PO43- molar concentration ratio of 1.0 in hydrolyzed urine, batch operation of a pilot-scale air-lift crystallizer removed 93-95% of phosphate and produced 3.65-4.93 g/L struvite in 1-5 h. Batch operation of a pilot-scale vacuum stripping - acid absorption system for 12 h stripped 72-77% of ammonia and produced 37.6-39.7 g/L (NH4)2SO4. Compared with the ammonia → phosphorus recovery sequence, the struvite precipitation → vacuum stripping sequence produced more struvite and ammonium sulfate. The strategy of urea hydrolysis → struvite precipitation → vacuum stripping of ammonia is a sustainable alternative to the conventional phosphorus fertilizer production and ammonia synthesis processes.

Microbial removal and plant uptake of nitrogen in constructed wetlands: mesocosm tests on influencing factors.

Macrophytes and bacteria are key drivers of nitrogen removal in constructed wetlands. Through mesocosm experiments with vegetated submerged beds and free water surface wetlands in various operational modes, wetland configurations, and system layouts, this study developed empirical models for non-destructive estimation of plant biomass growth and associated nitrogen assimilation and explored the combined effects of multiple factors that influence microbial nitrogen removal. The above-ground biomass of individual plants was a power function of plant height for both Cyperus alternifolius and Typha angustifolia. Below- to above-ground biomass ratio was 0.38 for C. alternifolius and 2.73 for T. angustifolia. Because of greater tolerance to ammonia stress, C. alternifolius and C. papyrus grew faster than T. angustifolia. There were no significant effects of wetland type, vegetation, and plant species on microbial nitrogen removal. Microbial nitrogen removal was inhibited by free ammonia at 13.3-16.2 mg N/L. Denitrification and anammox were suppressed at dissolved oxygen greater than 1.9 mg/L. Microbial removal of ammonia in vegetated submerged beds was sensitive mainly to dissolved oxygen, pH, and influent ammonia concentration, while in free water surface wetlands, it was sensitive to influent ammonia concentration, pH, and temperature.

Recovery of ammonia in anaerobic digestate using vacuum thermal stripping - acid absorption process: scale-up considerations.

A vacuum thermal stripping process coupled with acid absorption has been developed at laboratory scale to recover ammonia in anaerobic digestate. To make this ammonia recovery process scalable, this study investigated the effects of feed depth on vacuum thermal stripping in a pilot system, developed sodium hydroxide dosages required to raise feed pH for stripping, and simulated the dynamics of ammonia reduction in batch stripping tests. As feed depth was increased from 8.5 to 34.0 cm, the ammonia mass transfer coefficient and ammonia stripping efficiency decreased while the mass of stripped ammonia increased. Digested municipal sludge had a greater ammonia mass transfer coefficient than digested dairy manure at each feed depth, which could be attributed to the difference in suspended and dissolved solids concentrations. The optimum feed depth was 18 cm of the digested sludge and 14 cm of the digested manure. Sodium hydroxide dosage for the digested manure was higher than that for the digested sludge and co-digested foodwaste. The dosages were correlated to concentrations of total dissolved solids and ammonia. Total ammonia concentration decreased exponentially in batch stripping of the digested sludge at 25.5 cm deep, with a first-order stripping rate coefficient of 0.087-0.144 h-1.

Anaerobic co-digestion of foodwaste with liquid dairy manure or manure digestate: Co-substrate limitation and inhibition.

Process instability has been a challenge to anaerobic digestion of foodwaste at higher organic loading rates. Co-digestion is one of the measures to improve stability. This study conducted batch experiments to compare liquid dairy manure and dairy manure digestate as a co-substrate for anaerobic digestion of foodwaste. The batch co-digestion experiments showed a two-stage biogas production process, which could be simulated with a modification of the Gompertz model. The specific biogas yields derived with the two-stage biogas production model was further simulated against the co-substrate ratios with substrate limitation - inhibition models for identifying the optimal co-substrate ratio. The Haldane model was the best to simulate co-substrate limitation - inhibition kinetics in anaerobic co-digestion of foodwaste. A higher ratio of dairy manure could result in co-substrate inhibition to biogas production due to recalcitrance of cellulose and toxicity of lignin and lignin derivatives. Kinetic modeling shows that the optimal volatile solids (VS) ratio of liquid dairy manure is 16.6%, at which the maximum specific methane yield is 0.54 L/g VS. Semi-continuous co-digestion of 88% foodwaste and 12% liquid dairy manure at a hydraulic retention time of 14 d attained 94% of the simulated maximum methane yield. Although co-digestion of foodwaste and manure digestate resulted in lower biogas yields than co-digestion with liquid dairy manure, manure digestate is still an attractive co-substrate that has several operational advantages compared with liquid dairy manure.

Pollution characteristics and health risk assessment of phthalate esters in urban soil in the typical semi-arid city of Xi'an, Northwest China.

A total of 62 urban soil samples were collected in the city of Xi'an in Northwest China, and analyzed for six U.S. Environmental Protection Agency priority phthalate esters (PAEs). Unlike earlier studies on PAEs in agricultural soil as well as urban soil in humid climates, this paper for the first time comprehensively assessed pollution characteristics and health risks of human exposure to PAEs in urban soil in a typical semi-arid climate. The total concentrations of the six PAEs (Σ6PAEs) in the urban soil varied between 193.0 and 19146.4 µg kg-1 with a mean of 1369.3 µg kg-1. The PAEs were dominated by di-n-butyl phthalate and di(2-ethylhexyl) phthalate. Magnetic susceptibility and soil texture were controlling factors influencing the concentrations of PAEs in the urban soil. The concentrations of benzyl butyl phthalate, di(2-ethylhexyl) phthalate, and Σ6PAEs increased from the first to third ring roads, while the concentrations of di-n-octyl phthalate decreased. Relatively higher levels of PAEs were observed in industrial, traffic, and residential areas. The PAEs in the urban soil originated mainly from the application of plasticizers or additives, use of cosmetics and personal care products, emissions of construction materials and home furnishings, industrial processes, and atmospheric deposition. The concentrations of some PAEs in the urban soil exceeded soil allowable concentrations and environmental risk levels. The non-cancer and carcinogenic risks of human exposure to the PAEs were relatively low.

Investigating into composition, distribution, sources and health risk of phthalic acid esters in street dust of Xi'an City, Northwest China.

Phthalic acid esters (PAEs) are widely used as plasticizers and in consumer products, which may enter the environment and present risks to human health. U.S. EPA classifies six PAEs as priority pollutants, which could be accumulated in street dust at the interface of atmosphere, biosphere and geosphere. This study collected a total of 58 street dust samples from Xi'an City in Northwest China and analyzed for concentrations of the priority PAEs. Composition, distribution, sources and health risk of the PAEs were further examined. All the priority PAEs were detected in the street dust. The concentrations of individual PAEs varied between not detected and 183.19 mg/kg. The sum of the 6 priority PAEs (∑6PAEs) ranged from 0.87 to 250.30 mg/kg with a mean of 40.48 mg/kg. The most abundant PAEs in the street dust were di-n-butyl phthalate and di (2-ethylhexyl) phthalate (DEHP). Higher concentrations of ∑6PAEs in the street dust were found in the south and west parts of Xi'an City as well as its urban center, which were possibly attributed to the prevailing northerly Asian winter monsoon. The PAEs in the street dust originated mainly from wide application of plasticizers as well as cosmetics and personal care products. The main pathways of human exposure to PAEs in the street dust were ingestion and dermal adsorption of dust particles. The non-cancer risk of human exposure to PAEs in the street dust was relatively low, while the risk to children was higher than that to adults. The cancer risk of human exposure to DEHP in the street dust was lower than the standard limit value of 10-6.

Developing a vacuum thermal stripping - acid absorption process for ammonia recovery from anaerobic digester effluent.

To prevent acetoclastic methanogens from ammonia inhibition in anaerobic digestion of protein-rich substrates, ammonia needs to be removed or recovered from digestate. This paper presents an innovative ammonia recovery process that couples vacuum thermal stripping with acid absorption. Ammonia is stripped out of digestate boiling at a temperature below the normal boiling point due to vacuum. Stripped ammonia is absorbed to a sulfuric acid solution, forming ammonium sulfate crystals as a marketable product. Three common types of digestate were found to have boiling point temperature-vacuum curves similar to water. Seven combinations of boiling temperature and vacuum (50 °C 16.6 kPa, 58 °C 20.0 kPa, 65 °C 25.1 kPa, 70 °C 33.6 kPa, 80 °C 54.0 kPa, 90 °C 74.2 kPa, and 100 °C 101.3 kPa) were tested for batch stripping of ammonia in dairy manure digestate. 93.3-99.9% of ammonia was stripped in 3 h. The Lewis-Whitman model fitted ammonia stripping process well. Ammonia mass transfer coefficient was significantly higher at boiling temperature 65-100 °C and vacuum pressure 25.1-101.3 kPa than 50-58 °C and 16.6-20.0 kPa. The low ammonia saturation concentrations (0-24 mg N/L) suggested a large driving force to strip ammonia. The optimum boiling point temperature - vacuum pressure for ammonia recovery in a recirculation line of a mesophilic digester was 65 °C and 25.1 kPa, at which the ammonia mass transfer coefficient was as high as 37.3 mm/h. Installation of a demister and liquid trap could avoid negative effects of higher stripping temperature and stronger vacuum on formation of ammonium sulfate crystals. Pilot tests demonstrated that high-purity ammonium sulfate crystals could be produced by controlling sulfuric acid content and maintaining acid solution saturated with ammonium sulfate. Although volatile organic compounds such as cyclohexene were found in the final acid solutions, no volatile organic compounds were found in the recovered crystals.

Characteristics, sources, and health risk of polycyclic aromatic hydrocarbons in urban surface dust: a case study of the city of Xi'an in Northwest China.

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants. Urban surface dust is an important carrier of PAHs. To investigate the characteristics, sources, and health risk of PAHs in urban surface dust, this study collected urban surface dust samples from Xi'an, the largest city in Northwest China and one of the cities with severe smog occurrences in China. The total concentration of 16 US EPA priority PAHs (∑16PAHs) ranged from 5.0 to 48 mg/kg, with an average of 14 mg/kg. The seven carcinogenic PAHs accounted for 21 to 65 % of the ∑16PAHs. Higher levels of PAHs were found in its industrial, traffic, and mixed commercial and traffic districts. The PAHs were dominated by four-ring PAHs, and the predominant components were Fla, Phe, Chy, and Pyr. Multivariate statistical analyses showed that the PAHs originated mainly from the combustion of fossil fuel as well as coal and wood, and petroleum emission. The toxic equivalency quantities (TEQs) of urban surface dustborne PAHs ranged from 0.25 to 8.3 mg/kg, with a mean of 1.8 mg/kg. The 95 % upper confidence limit of incremental lifetime cancer risk (ILCR) due to human exposure to urban surface dustborne PAHs was 8.2 × 10(-5) for children and 7.3 × 10(-5) for adults.

Struvite recovery from anaerobically digested dairy manure: A review of application potential and hindrances.

Anaerobically digested dairy manure is rich in ammonium, orthophosphates, and magnesium, indicating a high potential for struvite recovery. Continuous generation of large amounts of dairy manure plus increasing global interest in anaerobic digestion of dairy manure suggest a huge market for struvite production with anaerobically digested dairy manure. However, the complex chemical composition of digested dairy manure presents hindrances to struvite recovery. This review paper assesses the significance and potential of struvite recovery from anaerobically digested dairy manure, identifies the factors hindering struvite recovery, and discusses the methods to overcome hindrances and the measures to improve phosphorus speciation of dairy manure for struvite formation. This paper proposes using "struvite recovery potential" or Pstruvite based on the least molar activity of struvite component ions in addition to "supersaturation ratio" to identify the potential for struvite recovery. The probable hindrances mainly include high Ca(2+) concentration and molar activity ratios of Ca(2+): Mg(2+) and Ca(2+): PO4(3-), high ionic strength, and high alkalinity. Struvite formation and purity is likely a function of all the interfering variables, rather than just a single factor with digested dairy manure. Potential enhancement measures need to be tested for technical and economic feasibility and applicability to various sources of digested dairy manure. This review paper provides guidance to overcoming the hindrances of digested dairy manure to struvite formation.