Diurnal cycling of dissolved gas concentrations in a willow vegetation filter treating landfill leachate

Membrane inlet mass spectrometry (MIMS) was used to monitor continuously and simultaneously the concentrations of dissolved gases (O₂, CO₂, CH₄), in the willow root zone at the laboratory scale, and within the treatment bed of a willow vegetation filter treating leachate at a landfill site in mid Wales. These results demonstrate that willows are able to release oxygen into the root zone which accumulates during daylight. Diurnal cycles of oxygen, carbon dioxide, and methane were observed, whereby CO₂ and CH₄ varied reciprocally in relation to O₂. The intensity of these cycles appeared to be related to light intensity and temperature. Oxygen was shown to fluctuate between completely anaerobic and fully aerated (300 μM), between day and night in sunny conditions.     

An integrated anaerobic–aerobic bioreactor (IAAB) for the treatment of palm oil mill effluent (POME): Start-up and steady state performance

Palm oil mill effluent (POME) with average chemical oxygen demand (COD) and biochemical oxygen demand (BOD) of 70,000 and 30,000 mg/L, respectively, can cause serious environmental hazards if discharged untreated. There are conventional palm oil mill effluent (POME) treatment systems that require large footprint, long HRT and fail to meet the Malaysian Department of Environment (DOE) discharge limit. Hence, the current research is aimed to design a novel integrated anaerobic–aerobic bioreactor (IAAB) for POME treatment in order to overcome these shortcomings of the conventional system. IAAB is a new bioreactor configuration which integrates anaerobic and aerobic digestion in one reactor. The overall removal efficiencies in steady state condition in terms of chemical oxygen demand (COD), biochemical oxygen demand (BOD) and total suspended solids (TSS) were more than 99% at the organic loading rate (OLR) of 10.5 g COD/L day with methane yield of 0.24 L CH₄/g COD removed. The effluent quality remained stable (BOD < 70 mg/L) and complied with the discharge limit (BOD < 100 mg/L). Overall, the IAAB system exhibited good stability and pH adjustment was unnecessary. The results show that the IAAB achieves higher performance in terms of organic removal efficiency and methane yield at higher OLR and shorter HRT as compared to the conventional system. Further evaluations of its long-term performance are proposed for the subsequent study.     

Anaerobic submerged membrane bioreactor (AnSMBR) treating low-strength wastewater under psychrophilic temperature conditions

An anaerobic submerged membrane bioreactor (AnSMBR) treating low-strength wastewater was operated for 90 days under psychrophilic temperature conditions (20 °C). Besides biogas sparging, additional shear was created by circulating sludge to control membrane fouling. The critical flux concept was used to evaluate the effectiveness of this configuration. Biogas sparging with a gas velocity (U_G) of 62 m/h together with sludge circulation (94 m/h) led to a critical flux of 7 L/(m² h). Nevertheless, a further increase in the U_G only minimally enhanced the critical flux. A low fouling rate was observed under critical flux conditions. The cake layer represented the main fouling resistance after 85 days of operation. Distinctly different volatile fatty acid (VFA) concentrations in the reactor and in the permeate were always observed. This fact suggests that a biologically active part of the cake layer contributes to degrade a part of the daily organic load. Hence, chemical oxygen demand (COD) removal efficiencies of up to 94% were observed. Nevertheless, the biogas balance indicates that even considering the dissolved methane, the methane yield were always lower than the theoretical value, which indicates that the organic compounds were not completely degraded but physically retained by the membrane in the reactor.     

Decomposition processes in soil of a healthy and a declining Phragmites australis stand

Decomposition processes were investigated in the soil of a declining, more eutrophic and a healthy, less eutrophic freshwater reed (Phragmites australis (Cav.) Trin. ex Steudel) stand in the littoral zone of Rožmberk fishpond, Czech Republic. Soil and pore water were sampled five times from April to October 1998. Chemical properties, CO₂ production in oxic and anoxic conditions, CH₄ production, denitrifying enzyme activity (DEA) and bacterial biomass were measured under laboratory conditions in suspensions prepared from homogenised soil samples. The more eutrophic West stand was more anaerobic than the East stand, with lower redox potential, lower pH and with a higher amount of organic acids, mainly acetic and lactic acid. Mean seasonal concentrations of total nitrogen in pore water, nitrogen of amino acids and proteins, and reducing sugars were all higher in the soil at the more eutrophic stand. Higher nutrient status and more reduced conditions at the more eutrophic stand were accompanied by (i) a limitation of aerobic microbial activities (CO₂ production in oxic conditions: 0.35 versus 0.54 μmol CO₂ cm⁻³ h⁻¹); lower DEA (4.0 versus 20.2 nmol N₂O cm⁻³ h⁻¹) and a lower proportion of bacteria that were active in aerobic conditions; (ii) by a prevalence of anaerobic over aerobic microbial processes; (iii) by a higher rate of methanogenesis (15.0 versus 11.5 nmol CH₄ cm⁻³ h⁻¹) and (iv) by an overall lower rate of microbial processes as compared to less eutrophied stand. The shift from aerobic to anaerobic microbial metabolism, and a coinciding restriction of metabolic activities at the more eutrophic stand are indicative of an elevated oxygen stress in the soil, associated with accumulation of metabolites toxic to both the micro-organisms and the reed. Possible links between eutrophication, decomposition processes in the soil and reed decline are discussed.     

Research on soybean protein wastewater treatment by the integrated two-phase anaerobic reactor

The start-up tests of treating soybean protein wastewater by the integrated two-phase anaerobic reactor were studied. The results showed that the soybean protein wastewater could be successfully processed around 30 days when running under the situation of dosing seed sludge with the influent of approximately 2000 mg/L and an HRT of 40 h. When the start-up was finished, the removal rate of COD by the reactor was about 80%. In the zone I, biogas mainly revealed carbon dioxide (CO₂) and hydrogen (H₂). Methane was the main component in the zone 2 which ranged from 53% to 59% with an average of 55%. The methane content in biogas increased from the zone I to II. It indicated that the methane-producing capacity of the anaerobic sludge increased. It was found that the uniquely designed two-phase integrated anaerobic reactor played a key role in treating soybean protein wastewater. The acidogenic fermentation bacteria dominated in the zone I, while methanogen became dominant in the zone II. It realized the relatively effective separation of hydrolysis acidification and methanogenesis process in the reactor, which was benefit to promote a more reasonable space distribution of the microbial communities in the reactor. There were some differences between the activities of the sludge in the two reaction zones of the integrated two-phase anaerobic reactor. The activity of protease was higher in the reaction zone I. And the coenzyme F₄₂₀ in the reaction zone II was twice than that in the reaction zone I, which indicated that the activity of the methanogens was stronger in the reaction zone II.     

Effect of chlorate,molybdate, and shikimic acid on Salmonella enterica serovar Typhimurium in aerobic and anaerobic cultures

Experiments were conducted to determine factors that affect sensitivity of Salmonella enterica serovar Typhimurium to sodium chlorate (5 mM). In our first experiment, cultures grown without chlorate grew more rapidly than those with chlorate. An extended lag before logarithmic growth was observed in anaerobic but not aerobic cultures containing chlorate. Chlorate inhibition of growth during aerobic culture began later than that observed in anaerobic cultures but persisted once inhibition was apparent. Conversely, anaerobic cultures appeared to adapt to chlorate after approximately 10 h of incubation, exhibiting rapid compensatory growth. In anaerobic chlorate-containing cultures, 20% of total viable counts were resistant to chlorate by 6 h and had propagated to 100% resistance (>10⁹ CFU mL^?1) by 24 h. In the aerobic chlorate-containing cultures, 12.9% of colonies had detectable resistance to chlorate by 6 h, but only 1% retained detectable resistance at 24 h, likely because these cultures had opportunity to respire on oxygen and were thus not enriched via the selective pressure of chlorate. In another study, treatment with shikimic acid (0.34 mM), molybdate (1 mM) or their combination had little effect on aerobic or anaerobic growth of Salmonella in the absence of added chlorate. As observed in our earlier study, chlorate resistance was not detected in any cultures without added chlorate. Chlorate resistant Salmonella were recovered at equivalent numbers regardless of treatment after 8 h of aerobic or anaerobic culture with added chlorate; however, by 24 h incubation chlorate sensitivity was completely restored to aerobic but not anaerobic cultures treated with shikimic acid or molybdate but not their combination. Results indicate that anaerobic adaptation of S. Typhimurium to sodium chlorate during pure culture is likely due to the selective propagation of low numbers of cells exhibiting spontaneous resistance to chlorate and this resistance is not reversible by molybdenum supplementation.     

Methane Hydrate: Killer cause of Earth's greatest mass extinction

The cause for the end Permian mass extinction, the greatest challenge life on Earth faced in its geologic history, is still hotly debated by scientists. The most significant marker of this event is the negative δ¹³C shift and rebound recorded in marine carbonates with a duration ranging from 2000 to 19 000 years depending on localities and sedimentation rates. Leading causes for the event are Siberian trap volcanism and the emission of greenhouse gases with consequent global warming. Measurements of gases vaulted in calcite of end Permian brachiopods and whole rock document significant differences in normal atmospheric equilibrium concentration in gases between modern and end Permian seawaters. The gas composition of the end Permian brachiopod-inclusions reflects dramatically higher seawater carbon dioxide and methane contents leading up to the biotic event. Initial global warming of 8–11 °C sourced by isotopically light carbon dioxide from volcanic emissions triggered the release of isotopically lighter methane from permafrost and shelf sediment methane hydrates. Consequently, the huge quantities of methane emitted into the atmosphere and the oceans accelerated global warming and marked the negative δ¹³C spike observed in marine carbonates, documenting the onset of the mass extinction period. The rapidity of the methane hydrate emission lasting from several years to thousands of years was tempered by the equally rapid oxidation of the atmospheric and oceanic methane that gradually reduced its warming potential but not before global warming had reached levels lethal to most life on land and in the oceans. Based on measurements of gases trapped in biogenic and abiogenic calcite, the release of methane (of ∼3–14% of total C stored) from permafrost and shelf sediment methane hydrate is deemed the ultimate source and cause for the dramatic life-changing global warming (GMAT > 34 °C) and oceanic negative-carbon isotope excursion observed at the end Permian. Global warming triggered by the massive release of carbon dioxide may be catastrophic, but the release of methane from hydrate may be apocalyptic. The end Permian holds an important lesson for humanity regarding the issue it faces today with greenhouse gas emissions, global warming, and climate change.     

Effects of nitrobenzene concentration and hydraulic retention time on the treatment of nitrobenzene in sequential anaerobic baffled reactor (ABR)/continuously stirred tank reactor (CSTR) system

The effects of increasing nitrobenzene (NB) concentrations and hydraulic retention times (HRT) on the treatment of NB were investigated in a sequential anaerobic baffled reactor (ABR)/aerobic completely stirred tank reactor (CSTR) system. In the first step of the study, the maximum COD removal efficiencies were found as 88% and 92% at NB concentrations varying between 30 mg L^?1 and 210 mg L^?1 in ABR. The minimum COD removal efficiency was 79% at a NB concentration of 700 mg L^?1. The removal efficiency of NB was nearly 100% for all NB concentrations in the ABR reactor. The methane gas production and the methane gas percentage remained stable (1500 mL day^?1 and 48–50%, respectively) as the NB concentration was increased from 30 to 210 mg L^?1. In the second step of the study it was found that as the HRT decreased from 10.38 days to 2.5 days the COD removal efficiencies decreased slightly from 94% to 92% in the ABR. For maximum COD and NB removal efficiencies the optimum HRT was found as 2.5 days in the ABR. The total COD removal efficiency was 95% in sequential anaerobic (ABR)/aerobic (CSTR) reactor system at a minimum HRT of 1 day. When the HRT was decreased from 10.38 days to 1 day, the methane percentage decreased from 42% to 29% in an ABR reactor treating 100 mg L^?1 NB. Nitrobenzene was reduced to aniline under anaerobic conditions while aniline was mineralized to catechol with meta cleavage under aerobic conditions.     

Performance of an UASB reactor treating synthetic wastewater at low-temperature using cold-adapted seed slurry

The present study is an attempt to investigate if a long-term acclimation of digester contents to low-temperatures would improve wastewater treatment at low-temperatures similar to mesophilic ranges. The feasibility of low-temperature (15 °C) anaerobic treatment of synthetic wastewater in an upflow anaerobic sludge blanket reactor was studied using inoculum from a cattle manure digester adapted to 15 °C. The effect of varying hydraulic retention time was studied by decreasing the retention time from 7 days to 1 day. Under a constant temperature of 15 °C with a hydraulic retention time of 1 day and a corresponding loading rate of 7.2 g-chemical oxygen demand (COD)/l/day, 90–95% removal efficiency was achieved. The methane production of 250 l/kg-COD removed at standard temperature pressure (STP) is a major highlight of the study complementing the high treatment efficiency achieved. Loading rates >5 g-COD/l/day was accompanied by increase in effluent volatile fatty acids (VFA) concentrations. Due to the presence of a high concentration of active granular sludge in the lower compartment of the reactor, 80% reduction of COD occurred within the granular bed of the reactor. Treatment of low strength wastewater for a short period showed 70–75% removal efficiencies with methane yield of 300 l/kg-COD removed. Specific methanogenic activity profiles of the anaerobic biomass revealed low-temperature (15 °C) optima, indicating selection of cold-active microorganisms during the acclimation process. The SMA assays also indicate the development of a putatively psychrophilic acetoclastic methanogenic community and biogas analysis showed 75% efficiency in energy recovery as methane.     

The interactive effects of elevated carbon dioxide and water table draw-down on carbon cycling in a Welsh ombrotrophic bog

The effects of elevated atmospheric CO₂ (eCO₂) and water table draw-down on soil carbon sequestration in an ombrotrophic bog ecosystem were examined. Peat monoliths (11 cm diameter, 25 cm deep) with intact bog vegetation were exposed to ambient or elevated (ambient + 200 mg l^?1) atmospheric CO₂, combined with a natural water table (level with the peat surface) or a water table draw-down (?5 cm). Eight observations per treatment were included in the study, which was conducted over a 12 week period. Concentration of dissolved organic carbon (DOC), phenolic compounds and the fluxes of CO₂ and CH₄ were measured. The eCO₂ treatment caused an increase in the CH₄ and CO₂ fluxes and a small decrease in both the DOC and phenolic concentrations. The water table draw-down invoked decreases in phenolic and DOC concentrations, a decrease in CH₄ flux and a small increase in CO₂ flux. The combined (eCO₂ + water table draw-down) treatment caused a larger than expected CH₄ flux decrease and CO₂ flux increase and an increase in DOC concentration. Our results suggest very different effects on the system dependent on the treatment applied. The draw-down treatment principally increased oxidation of the rhizosphere resulting in increased decomposition and as such a removal of material from the dissolved carbon pool. The data also suggest labile carbon availability may be limiting the rate of decomposition and so slowing inorganic nutrient and carbon pool turn-over. The elevated CO₂ addressed the labile-carbon limitation. Under the environment of the combined treatment, these limitations were effectively removed, culminating in a destabilisation of the carbon-sequestering environment to a weaker sink (or even a source) of atmospheric carbon.     

Aerobiosis–anaerobiosis transition has a significant impact on organic acid production by Corynebacterium glutamicum

Corynebacterium glutamicum is known to produce organic acids under anaerobic culture conditions, in particular, lactic, succinic, and acetic acids. Our study is focused on acetic and succinic acid production using a lactate dehydrogenase-deficient strain of C. glutamicum. Usually, with this bacterium, the organic acid production process is based on an initial aerobic growth phase, followed by a rapid deoxygenation and an anaerobic production phase. In our study, we demonstrated that this strategy was unfavorable for the production of organic acids. Conversely, we showed that applying the best transition strategy based on progressive deoxygenation significantly increased the concentration of organic acids up to 640%. This was observed either by applying controlled dissolved oxygen concentrations or by decreasing the steps of gas flow rates. Our results also showed that applying constant oxygen transfer flux throughout the culture, and thus in the absence of the anaerobic phase, promoted constant production yields (approximately 0.5 mol of succinate or acetate per mole of glucose). In this case, acetate production (120 mM) was favored over succinate production (132 mM), resulting in a decrease in the molar ratio of products (succinate/acetate) from 4.8 to 1.1 between progressive deoxygenation and constant OTR cultures.     

Anaerobic digestion of harvested aquatic weeds: water hyacinth (Eichhornia crassipes), cabomba (Cabomba Caroliniana) and salvinia (Salvinia molesta)

The aim of this study was to explore the potential of three aquatic weeds, water hyacinth, cabomba, and salvinia, as substrates for anaerobic digestion. A set of four pilot-scale, batch digestions were undertaken to assess the yield and quality (% methane) of biogas from each plant species, and the rate of degradation. A set of 56 small-scale (100 mL) biological methane potential (BMP) tests were designed to test the repeatability of the digestions, and the impact of drying and nutrient addition.The results of the pilot-scale digestions show that both water hyacinth and cabomba are readily degradable, yielding 267 L biogas kg^?1 VS and 221 L biogas kg^?1 VS, respectively, with methane content of approximately 50%. There is evidence that the cabomba fed reactor leaked midway through the digestion therefore the biogas yield is potentially higher than measured in this case. Salvinia proved to be less readily degradable with a yield of 155 L biogas kg^?1 VS at a quality of 50% methane.The BMPs showed that the variability was low for water hyacinth and cabomba but high for salvinia. They also showed that the addition of nutrient solution and manure did not significantly increase the biogas yields and that drying was detrimental to the anaerobic degradability of all three substrates.Based on these results treatment of both water hyacinth and cabomba by anaerobic digestion can be recommended. Anaerobic digestion of Salvinia cannot be recommended due to the low biogas yields and high variability for this substrate.     

Denitrification activity,wood loss,and N2O emissions over 9 years from a wood chip bioreactor

Loss of nitrate in subsurface drainage water from agricultural fields is an important problem in the Midwestern United States and elsewhere. One possible strategy for reducing nitrate export is the use of denitrification bioreactors. A variety of experimental bioreactor designs have been shown to reduce nitrate losses in drainage water for periods up to several years. This research reports on the denitrification activity of a wood chip-based bioreactor operating in the field for over 9 years. Potential denitrification activity was sustained over the 9-year period, which was consistent with nitrate removal from drainage water in the field. Denitrification potentials ranged from 8.2 to 34 mg N kg^?1 wood during the last 5 years of bioreactor operation. Populations of denitrifying bacteria were greater in the wood chips than in adjacent subsoil. Loss of wood through decomposition reached 75% at the 90–100 cm depth with a wood half-life of 4.6 years. However, wood loss was less than 20% at 155–170 cm depth and the half-life of this wood was 36.6 years. The differential wood loss at these two depths appears to result from sustained anaerobic conditions below the tile drainage line at 120 cm depth. Pore space concentrations of oxygen and methane support this conjecture. Nitrous oxide exported in tile water from the wood chip bioreactor plots was not significantly higher than N₂O exports in tile water from the untreated control plots, and loss of N₂O from tile water exiting the bioreactor accounted for 0.0062 kg N₂O-N kg^?1 NO₃-N.     

Biological treatment of low-salinity shrimp aquaculture wastewater using sequencing batch reactor

In order to improve the water quality in shrimp aquaculture operated under low-salinity conditions, a sequencing batch reactor (SBR) was tested for treatment of the wastewater. This water from the backwash of a single-bead filter from the Waddell Mariculture Center, South Carolina, contained high concentrations of carbon and nitrogen and was successfully treated using the SBR. By operating the reactor sequentially in aerobic, anoxic and aerobic modes, nitrification and denitrification were achieved, as well as removal of carbon. Specifically, the initial chemical oxygen demand (COD) concentration of 1201 mg l⁻¹ was reduced to 32 mg l⁻¹ within 8 days of reactor operation. Ammonia in the sludge was nitrified within 3 days. The denitrification of nitrate was achieved by the anoxic process and total removal of nitrate was observed.     

Real evidence about zeolite as microorganisms immobilizer in anaerobic fluidized bed reactors

Using the scanning electronic microscopy, it was observed that natural zeolite possesses excellent physical characteristics as a support medium in anaerobic fluidized bed reactors (AFBR). Samples for biomass analysis were taken from two identical laboratory-scale AFBR (R-1 and R-2), which were operated with 25% of fluidization. These reactors treated distillery wastewaters (vinasses) at mesophilic temperature (30 ± 2 °C). The experiments were carried out with 0.25–0.50 and 0.50–0.80 mm zeolite particle diameter in reactors R-1 and R-2, respectively. The biomass concentration attached to zeolite in both reactors was found to be in the range of 40–45 g volatile solids/l. COD removal efficiencies as high as 90% were achieved at organic loading rate (OLRs) of up to 20 g COD/l day. The volatile fatty acid (VFA) levels were always lower that the suggested limits for digester failure. The yield coefficient of methane production was 0.29 l CH₄(at STP)/g COD consumed and was virtually independent of the OLR applied. A hybridization technique (fluorescence in situ hybridization, FISH) helped determine the predominant anaerobic microorganisms that colonized the natural zeolite, which were found to be Methanosaeta and Methanosarcinaceae, observing a reduced number of sulphate reducing bacteria. The results obtained for reactors R-1 and R-2 were very similar, showing that the particle size did not significantly influence the microbial community immobilized on zeolite.     

Fungal post-treatment of pulp mill effluents for the removal of recalcitrant pollutants

The objective of this work was to evaluate the post-treatment of an anaerobic recalcitrant effluent (anaerobically-treated weak black liquor, AnE) in an aerobic, upflow reactor packed with “biocubes” of Trametes versicolor immobilized onto small cubes of holm oak wood. The treated effluent (named anaerobic effluent; AnE) from an anaerobic fluidized bed reactor was fed to an up-flow aerobic fungal packed bed reactor (PBR). Two HRT were tested in this unit, namely 5 and 2.5 days; the PBR operated 60 days at 5-day HRT and 35 days at 2.5-day HRT. The aerobic packed bench scale reactor was a glass column 1.5 L total geometric volume containing 0.75 L biocubes of T. versicolor immobilized onto holm oak wood small cubes of 5 mm side. The reactor was operated at 25 °C. The pH of the AnE was adjusted to 4.5 before feeding; no carbohydrates or other soluble carbon source was supplemented. The fungal packed bed bioreactor averaged organic matter removals of 30% and 32% COD basis, during an experimental run of 60 days at 5-day HRT and 35 days at 2.5-day HRT, respectively. Colour and ligninoids contents were removed at higher percentages (69% and 54% respectively, average of both HRT). There was no significant difference between reactor performance at 5- and 2.5-day HRT, so, operation at 2.5-day HRT is recommended since reactor throughput is double. Activity of manganese peroxidase and laccase was found during the entire operation of the fungal PBR whereas lignin peroxidase activity practically disappeared in the second operation period. In general, enzyme activities were higher in the first period of operation (5-day HRT) than at 2.5-day HRT. To the best of our knowledge, this is one of the few works that demonstrated extended performance (3 months) of a fungal bioreactor for the treatment of a recalcitrant wastewater with no supplementation of glucose or other expensive, soluble carbohydrate.     

Improvement of sludge anaerobic degradability by combined electro-flotation and electro-oxidation treatment

Recently, bioenergy recovery from sludge biomass has attracted increasing attention due to the high demand for renewable energy resources. In order to enhance methane production from sludge biomass, electrochemical treatment can be used as a novel and efficient pretreatment for the hydrolysis of sludge biomass. In this study, a combined electro-flotation and electro-oxidation pretreatment was employed to improve the anaerobic degradability of sludge biomass. Electro-flotation was efficient in separating flocs in the mixed liquor and led to a sludge volume reduction greater than 60% after 10 min of operation at a current density of 4.72 mA cm⁻². Electro-oxidation using IrO₂/Ti anode was performed to improve the anaerobic degradability of sludge and resulted in a 30% increase in COD solubilization after 30 min of operation at current density of 9.45 mA cm⁻². The factors affecting electro-oxidation, i.e. the gap width between anode and cathode, current density and applied voltage, were investigated to optimize the operating conditions. A biochemical methane potential assay demonstrated that the anaerobic biodegradability of sludge was enhanced by combined electro-flotation and electro-oxidation pretreatment.     

Operation of partial nitrification to nitrite of landfill leachate and its performance with respect to different oxygen conditions

The coupled system of partial nitrification and anaerobic ammonium oxidation (Anammox) is efficient in nitrogen removal from wastewater. In this study, the effect of different oxygen concentrations on partial nitrification performance with a sequencing batch reactor (SBR) was investigated. Results indicate that, partial nitrification of landfill leachate could be successfully achieved under the 1.0–2.0 mg L⁻¹ dissolved oxygen (DO) condition after 118 d long-term operation, and that the effluent is suitable for an Anammox reactor. Further decreasing or increasing the DO concentration, however, would lead to a decay of nitrification performance. Additionally, the MLSS concentration in the reactor increased with increasing DO concentration. Respirometric assays suggest that low DO conditions (<2 mg L⁻¹) favor the ammonia-oxidizing bacteria (AOB) and significantly inhibit nitrite oxidizing bacteria (NOB) and aerobic heterotrophic bacteria (AHB); whereas high DO conditions (>3 mg L⁻¹) allow AHB to dominate and significantly inhibit AOB. Therefore, the optimal condition for partial nitrification of landfill leachate is 1.0–2.0 mg L⁻¹ DO concentration.     

Comparative kinetic study between moving bed biofilm reactor-membrane bioreactor and membrane bioreactor systems and their influence on organic matter and nutrients removal

New technologies regarding wastewater treatment have been developed. Among these technologies, the moving bed biofilm reactor combined with membrane bioreactor (MBBR-MBR) is a recent solution alternative to conventional processes. This paper presents the results obtained from three wastewater treatment plants working in parallel. The first wastewater treatment plant consisted of a membrane bioreactor (MBR), the second one was a MBBR-MBR system containing carriers both in anoxic and aerobic zones, and the last one consisted of a MBBR-MBR system which contained carriers only in the aerobic zone. The reactors operated with a hydraulic retention time of 26.47 h. During the study, the difference between the experimental plants was not statistically significant concerning organic matter and nutrients removal. However, different tendencies regarding nutrients removal are shown by the three wastewater treatment plants. In this sense, the performances in terms of nitrogen and phosphorus removal of the MBBR-MBR system which contained carriers only in the aerobic zone (67.34 ± 11.22% and 50.65 ± 11.13%, respectively) were slightly better than those obtained from another experimental plants. As a whole, the pilot plant which consisted of a MBR showed better performance from the point of view of the kinetics of the heterotrophic and autotrophic biomass with values of μ_m,H = 0.00858 h⁻¹, μ_m,A = 0.07646 h⁻¹, K_M = 2.37 mg O₂ L⁻¹ and K_NH = 1.31 mg N L⁻¹.     

Oxygen enhances the recovery of Potamogeton maackianus from prolonged exposure to very low irradiance

Recovery ability in relation to carbohydrate content of Potamogeton maackianus growing in two dissolved oxygen concentrations (8 and 2 mg L⁻¹) was investigated during 28 days exposure to very low irradiance (about 0.06 μmol m⁻² s⁻¹). Plant weight remained relatively constant (0.19 g dry wt plant⁻¹) within the initial 21 days in the high oxygen treatment, but decreased to 0.14 g dry wt plant⁻¹ at the end of the experiment. In low oxygen environments, plant weight was similar within the initial 14 days, but decreased to 0.08 g dry wt plant⁻¹ at 21 day. During the experimental period, both soluble sugar and starch contents in shoots decreased with time. Compared to high oxygen treatment, plants in the low oxygen treatment depleted starch more quickly (25 versus 18 mg g⁻¹ at 28 day) but remained a relatively high soluble sugar content (0.9 versus 1.8 mg g⁻¹ at 28 day). After recovery in high light and high dissolved oxygen conditions for 1 week, plant growth rate, new branch number, stem elongation rate and leaf recruitment number were significantly higher in high oxygen than in the low oxygen treatments. These data suggest that the ability of the plant to recover from prolonged exposure to very low irradiance is related to the depletion level of carbohydrate stored in plant tissues, which is regulated by oxygen availability in the water.