Enhanced Methane Production from Food Waste Using Cysteine To Increase Biotransformation of l‑Monosaccharide, Volatile Fatty Acids, and Biohydrogen

The enhancement of two-stage anaerobic digestion of polysaccharide-enriched food waste by the addition of cysteinean oxygen scavenger, electron mediator, and nitrogen sourceto the acidification stage was reported. It was found that in the acidification stage the accumulation of volatile fatty acids (VFA), which mainly consisted of acetate, butyrate, and propionate, was increased by 49.3% at a cysteine dosage of 50 mg/L. Although some cysteine was biodegraded in the acidification stage, the VFA derived from cysteine was negligible. In the methanogenesis stage, the biotransformations of both VFA and biohydrogen to methane were enhanced, and the methane yield was improved by 43.9%. The mechanisms study showed that both d-glucose and l-glucose (the model monosaccharides) were detectable in the hydrolysis product, and the addition of cysteine remarkably increased the acidification of l-glucose, especially acetic acid and hydrogen generation, due to key enzymes involved in l-glucose metabolism being enhanced. Cysteine also improved the activity of homoacetogens by 34.8% and hydrogenotrophic methanogens by 54%, which might be due to the electron transfer process being accelerated. This study provided an alternative method to improve anaerobic digestion performance and energy recovery from food waste

Acute and Chronic Responses of Activated Sludge Viability and Performance to Silica Nanoparticles

Recently, the potential health and environmental risks of silica nanoparticles (SiO₂ NPs) are attracting great interest. However, little is known about their possible impacts on wastewater biological nitrogen and phosphorus removal. In this study, the acute and chronic effects of SiO₂ NPs on activated sludge viability and biological nutrient removal performance were investigated. It was found that the presence of environmentally relevant concentration (1 mg/L) of SiO₂ NPs caused no adverse acute and chronic effects on sludge viability and wastewater nitrogen and phosphorus removal. However, chronic exposure to 50 mg/L SiO₂ NPs induced the increase of effluent nitrate concentration, and thus depressed the total nitrogen (TN) removal efficiency from 79.6% to 51.6% after 70 days of exposure, which was due to the declined activities of denitrifying enzymes, nitrate reductase and nitrite reductase. Wastewater phosphorus removal was insensitive to 1 and 50 mg/L SiO₂ NPs after either the acute or chronic exposure, because the critical factors closely related to biological phosphorus removal were not significantly changed, such as the activities of exopolyphosphatase and polyphosphate kinase and the intracellular transformations of polyhydroxyalkanoates and glycogen. Denaturing gradient gel electrophoresis (DGGE) analysis revealed that the bacterial community structure was changed after long-term exposure to 50 mg/L SiO₂ NPs, and the quantitative PCR assays indicated that the abundance of denitrifying bacteria was decreased, which was consistent with the declined wastewater nitrogen removal

Polycyclic Aromatic Hydrocarbon Affects Acetic Acid Production during Anaerobic Fermentation of Waste Activated Sludge by Altering Activity and Viability of Acetogen

Till now, almost all the studies on anaerobic fermentation of waste activated sludge (WAS) for bioproducts generation focused on the influences of operating conditions, pretreatment methods and sludge characteristics, and few considered those of widespread persistent organic pollutants (POPs) in sludge, for example, polycyclic aromatic hydrocarbons (PAHs). Herein, phenanthrene, which was a typical PAH and widespread in WAS, was selected as a model compound to investigate its effect on WAS anaerobic fermentation for short-chain fatty acids (SCFAs) accumulation. Experimental results showed that the concentration of SCFAs derived from WAS was increased in the presence of phenanthrene during anaerobic fermentation. The yield of acetic acid which was the predominant SCFA in the fermentation reactor with the concentration of 100 mg/kg dry sludge was 1.8 fold of that in the control. Mechanism exploration revealed that the present phenanthrene mainly affected the acidification process of anaerobic fermentation and caused the shift of the microbial community to benefit the accumulation of acetic acid. Further investigation showed that both the activities of key enzymes (phosphotransacetylase and acetate kinase) involved in acetic acid production and the quantities of their corresponding encoding genes were enhanced in the presence of phenanthrene. Viability tests by determining the adenosine 5′-triphosphate content and membrane potential confirmed that the acetogens were more viable in anaerobic fermentation systems with phenanthrene, which resulted in the increased production of acetic acid

Response of Anaerobic Granular Sludge to a Shock Load of Zinc Oxide Nanoparticles during Biological Wastewater Treatment

The increasing use of zinc oxide nanoparticles (ZnO NPs) in consumer and industrial products highlights a need to understand their potential environmental impacts. In this study, the response of anaerobic granular sludge (AGS) to a shock load of ZnO NPs during anaerobic biological wastewater treatment was reported. It was observed that the extracellular polymeric substances (EPS) of AGS and the methane production were not significantly influenced at ZnO NPs of 10 and 50 mg per gram of total suspended solids (mg/g-TSS), but they were decreased when the dosage of ZnO NPs was greater than 100 mg/g-TSS. The visualization of EPS structure with multiple fluorescence labeling and confocal laser scanning microscope revealed that ZnO NPs mainly caused the decrease of proteins by 69.6%. The Fourier transform infrared spectroscopy analysis further indicated that the C–O–C group of polysaccharides and carboxyl group of proteins in EPS were also changed in the presence of ZnO NPs. The decline of EPS induced by ZnO NPs resulted in their deteriorating protective role on the inner microorganisms of AGS, which was in correspondence with the observed lower general physiological activity of AGS and the death of microorganisms. Further investigation showed that the negative influence of ZnO NPs on methane production was due to their severe inhibition on the methanization step

Short-Chain Fatty Acid Production from Different Biological Phosphorus Removal Sludges: The Influences of PHA and Gram-Staining Bacteria

Recently, the reuse of waste activated sludge to produce short-chain fatty acids (SCFA) has attracted much attention. However, the influences of sludge characteristics, especially polyhydroxyalkanoates (PHA) and Gram-staining bacteria, on SCFA production have seldom been investigated. It was found in this study that during sludge anaerobic fermentation not only the fermentation time but also the SCFA production were different between two sludges, which had different PHA contents and Gram-negative bacteria to Gram-positive bacteria (GNB/GPB) ratios and were generated respectively from the anaerobic/oxic (AO) and aerobic/extended-idle (AEI) biological phosphorus removal processes. The optimal fermentation time for the AEI and AO sludges was respectively 4 and 8 d, and the corresponding SCFA production was 304.6 and 231.0 mg COD/g VSS (volatile suspended solids) in the batch test and 143.4 and 103.9 mg COD/g VSS in the semicontinuous experiment. The mechanism investigation showed that the AEI sludge had greater PHA content and GNB/GPB ratio, and the increased PHA content accelerated cell lysis and soluble substrate hydrolysis while the increased GNB/GPB ratio benefited cell lysis. Denaturing gradient gel electrophoresis profiles revealed that the microbial community in the AEI sludge fermentation reactor was dominated by <i>Clostridium sp.</i>, which was reported to be SCFA-producing microbes. Further enzyme analyses indicated that the activities of key hydrolytic and acids-forming enzymes in the AEI sludge fermentation reactor were higher than those in the AO one. Thus, less fermentation time was required, but higher SCFA was produced in the AEI sludge fermentation system

Effect of CO₂ on Microbial Denitrification via Inhibiting Electron Transport and Consumption

Increasing anthropogenic CO₂ emissions have been reported to influence global biogeochemical processes; however, in the literature the effects of CO₂ on denitrification have mainly been attributed to the changes it causes in environmental factors, while the direct effects of CO₂ on denitrification remain unknown. In this study, increasing CO₂ from 0 to 30 000 ppm under constant environmental conditions decreased total nitrogen removal efficiency from 97% to 54%, but increased N₂O generation by 240 fold. A subsequent mechanistic study revealed that CO₂ damaged the bacterial membrane and directly inhibited the transport and consumption of intracellular electrons by causing intracellular reactive nitrogen species (RNS) accumulation, suppressing the expression of key electron transfer proteins (flavoprotein, succinate dehydrogenase, and cytochrome c) and the synthesis and activity of key denitrifying enzymes. Further study indicated that the inhibitory effects of CO₂ on the transport and consumption of electrons were caused by the decrease of intracellular iron due to key iron transporters (AfuA, FhuC, and FhuD) being down-regulated. Overall, this study suggests that the direct effect of CO₂ on denitrifying microbes via inhibition of intracellular electron transport and consumption is an important reason for its negative influence on denitrification

Long-Term Effects of Copper Nanoparticles on Wastewater Biological Nutrient Removal and N₂O Generation in the Activated Sludge Process

The increasing use of copper nanoparticles (Cu NPs) raises concerns about their potential toxic effects on the environment. However, their influences on wastewater biological nutrient removal (BNR) and nitrous oxide (N₂O) generation in the activated sludge process have never been documented. In this study the long-term effects of Cu NPs (0.1–10 mg/L) on BNR and N₂O generation were investigated. The total nitrogen (TN) removal was enhanced and N₂O generation was reduced at any Cu NPs levels investigated, but both ammonia and phosphorus removals were not affected. The mechanism studies showed although most of the Cu NPs were absorbed to activated sludge, the activated sludge surface was not damaged, and the released copper ion from Cu NPs dissolution was the main reason for TN removal improvement and N₂O reduction. It was also found that the transformation of polyhydroxyalkanoates and the activities of ammonia monooxygenase, nitrite oxidoreductase, exopolyphosphatase, and polyphosphate kinase were not affected by Cu NPs, whereas the decreased metabolism of glycogen and the increased activities of denitrification enzymes were observed. Further investigation revealed that Cu NPs increased the number of denitrifiers (especially N₂O reducing denitrifiers) but decreased nitrite accumulation. All these observations were in correspondence with the enhancement of TN removal and reduction of N₂O generation

Effects of CuNPs on phosphorus anaerobic release and aerobic uptake during one cycle of EBPR process.

<p>Error bars represent the standard deviations of triplication tests.</p

The effect of CuNPs on the flocculating ability of activated sludge.

<p>Error bars represent standard deviations of triplicate measurements.</p

Influence of Copper Nanoparticles on the Physical-Chemical Properties of Activated Sludge

<div><p>The physical-chemical properties of activated sludge, such as flocculating ability, hydrophobicity, surface charge, settleability, dewaterability and bacteria extracellular polymer substances (EPS), play vital roles in the normal operation of wastewater treatment plants (WWTPs). The nanoparticles released from commercial products will enter WWTPs and can induce potential adverse effects on activated sludge. This paper focused on the effects of copper nanoparticles (CuNPs) on these specific physical-chemical properties of activated sludge. It was found that most of these properties were unaffected by the exposure to lower CuNPs concentration (5 ppm), but different observation were made at higher CuNPs concentrations (30 and 50 ppm). At the higher CuNPs concentrations, the sludge surface charge increased and the hydrophobicity decreased, which were attributed to more Cu²⁺ ions released from the CuNPs. The carbohydrate content of EPS was enhanced to defense the toxicity of CuNPs. The flocculating ability was found to be deteriorated due to the increased cell surface charge, the decreased hydrophobicity, and the damaged cell membrane. The worsened flocculating ability made the sludge flocs more dispersed, which further increased the toxicity of the CuNPs by increasing the availability of the CuNPs to the bacteria present in the sludge. Further investigation indicated that the phosphorus removal efficiency decreased at higher CuNPs concentrations, which was consistent with the deteriorated physical-chemical properties of activated sludge. It seems that the physical-chemical properties can be used as an indicator for determining CuNPs toxicity to the bacteria in activated sludge. This work is important because bacteria toxicity effects to the activated sludge caused by nanoparticles may lead to the deteriorated treatment efficiency of wastewater treatment, and it is therefore necessary to find an easy way to indicate this toxicity.</p></div