Treating landfill leachate using passive aeration trickling filters; effects of leachate characteristics and temperature on rates and process dynamics

Biological ammoniacal-nitrogen (NH₄⁺-N) and organic carbon (TOC) treatment was investigated in replicated mesoscale attached microbial film trickling filters, treating strong and weak strength landfill leachates in batch mode at temperatures of 3, 10, 15 and 30 °C. Comparing leachates, rates of NH₄⁺-N reduction (0.126-0.159 g m^− 2 d^− 1) were predominantly unaffected by leachate characteristics; there were significant differences in TOC rates (0.072-0.194 g m^− 2 d^− 1) but no trend relating to leachate strength. Rates of total oxidised nitrogen (TON) accumulation (0.012-0.144 g m^− 2 d^− 1) were slower for strong leachates. Comparing temperatures, treatment rates varied between 0.029-0.319 g NH₄⁺-N m^− 2 d^− 1 and 0.033-0.251 g C m^− 2 d^− 1 generally increasing with rising temperatures; rates at 3 °C were 9 and 13% of those at 30 °C for NH₄⁺-N and TOC respectively. For the weak leachates (NH₄⁺-N < 140 mg l^− 1) complete oxidation of NH₄⁺-N was achieved. For the strong leachates (NH₄⁺-N 883-1150 mg l^− 1) a biphasic treatment response resulted in NH₄⁺-N removal efficiencies of between 68 and 88% and for one leachate no direct transformation of NH₄⁺-N to TON in bulk leachate. The temporal decoupling of NH₄⁺-N oxidation and TON accumulation in this leachate could not be fully explained by denitrification, volatilisation or anammox, suggesting temporary storage of N within the treatment system. This study demonstrates that passive aeration trickling filters can treat well-buffered high NH₄⁺-N strength landfill leachates under a range of temperatures and that leachate strength has no effect on initial NH₄⁺-N treatment rates. Whether this approach is a practicable option depends on a range of site specific factors.     

Ammonium adsorption in aerobic granular sludge, activated sludge and anammox granules

The ammonium adsorption properties of aerobic granular sludge, activated sludge and anammox granules have been investigated. During operation of a pilot-scale aerobic granular sludge reactor, a positive relation between the influent ammonium concentration and the ammonium adsorbed was observed. Aerobic granular sludge exhibited much higher adsorption capacity compared to activated sludge and anammox granules. At an equilibrium ammonium concentration of 30 mg N/L, adsorption obtained with activated sludge and anammox granules was around 0.2 mg NH₄-N/g VSS, while aerobic granular sludge from lab- and pilot-scale exhibited an adsorption of 1.7 and 0.9 mg NH₄-N/g VSS, respectively. No difference in the ammonium adsorption was observed in lab-scale reactors operated at different temperatures (20 and 30 °C). In a lab-scale reactor fed with saline wastewater, we observed that the amount of ammonium adsorbed considerably decreased when the salt concentration increased. The results indicate that adsorption or better ion exchange of ammonium should be incorporated into models for nitrification/denitrification, certainly when aerobic granular sludge is used.     

Alkaline solubilization and microwave irradiation as a combined sludge disintegration and minimization method

Commonly used pretreatment method of alkaline solubilization (using NaOH) and a relatively new technology of microwave (MW) irradiation (160 °C) were combined as a pretreatment method of waste activated sludge (WAS) in this study. First alkaline and MW pretreatment methods were examined separately, then their combination for different conditions was investigated in terms of their effect on COD solubilization, turbidity and capillary suction time (CST). For combined pretreatments, soluble COD to total COD ratio (SCOD/TCOD) of WAS increased from 0.005 (control) to 0.18, 0.27, 0.34 and 0.37 for combined methods of MW and pH-10, 11, 12 and 12.5, respectively. Deteriorated dewaterability due to alkaline pretreatment was also improved due to the incorporation of MW irradiation. Further, with small scale batch anaerobic reactors, pH-10, pH-12, MW (alone), MW + pH-10 and MW + pH-12 pretreated WAS samples were anaerobically digested. Highest total gas and methane productions were achieved with MW + pH-12 pretreatment with 16.3% and 18.9% improvements over control reactor, respectively. Finally the performance of MW + pH-12 pretreatment was examined with 2 L anaerobic semi-continuous reactors for 92 days and compared to that of the control reactors. These reactors were operated at an SRT of 15 days. After steady state, 43.5% and 55% improvements were obtained in respective daily total gas and methane productions. TS, VS and TCOD reductions were improved by 24.9%, 35.4% and 30.3%, respectively based on a relative calculation with respect to control reactors. This way combined alkaline-microwave treatment proved to be an effective sludge minimization method. Pretreated digested sludge had 22% improved dewaterability than unpretreated digested sludge. Higher SCOD and NH₃-N concentrations were measured in the pretreated digested sludge supernatant; however, PO₄-P concentration did not increase much.     

Long-term (1243 days), low-temperature (4-15 °C), anaerobic biotreatment of acidified wastewaters: Bioprocess performance and physiological characteristics

The feasibility of long-term (>3 years), low-temperature (4-15 °C) and anaerobic bioreactor operation, for the treatment of acidified wastewater, was investigated. A hybrid, expanded granular sludge bed-anaerobic filter bioreactor was seeded with a mesophilic inoculum and employed for the mineralization of moderate-strength (3.75-10 kg chemical oxygen demand (COD) m⁻³) volatile fatty acid-based wastewaters at 4-15 °C. Bioprocess performance was assessed in terms of COD removal efficiency (CODRE), methane biogas concentration, and yield, and biomass retention. Batch specific methanogenic activity assays were performed to physiologically characterise reactor biomass.Despite transient disimprovements, CODRE and methane biogas concentrations exceeded 80% and 65%, respectively, at an applied organic loading rate (OLR) of 10 kg COD m⁻³ d⁻¹ between 9.5 and 15 °C (sludge loading rate (SLR), 0.6 kg COD kg[VSS]⁻¹ d⁻¹). Over 50% of the granular sludge bed was lost to disintegration during operation at 9.5 °C, warranting a reduction in the applied OLR to 3.75-5 kg COD m⁻³ d⁻¹ (SLR, c. 0.4-0.5 kg COD kg[VSS]⁻¹ d⁻¹). From that point forward, remarkably stable and efficient performance was observed during operation at 4-10 °C, with respect to CODRE (≥82%), methane biogas concentration (>70%) and methane yields (>4 l_Methane d⁻¹), suggesting the adaptation of our mesophilic inoculum to psychrophilic operating conditions.Physiological activity assays indicated the development of psychroactive syntrophic and methanogenic populations, including the emergence of putatively psychrophilic propionate-oxidising and hydrogenotrophic methanogenic activity. The data suggest that mesophilic inocula can physiologically adapt to sub-optimal operational temperatures: treatment efficiencies and sludge loading rates at 4 °C (day, 1243) were comparable to those achieved at 15 °C (day 0). Furthermore, long-term, low-temperature bioreactor operation may act as a selective enrichment for psychrophilic methanogenic activity from mesophilic inocula. The observed efficient and stable bioprocess performance highlights the potential for long-term, low-temperature bioreactor operation.     

Pre-treatment mechanisms during thermophilic-mesophilic temperature phased anaerobic digestion of primary sludge

Pre-treatment is used extensively to improve degradability and hydrolysis rate of material being fed into digesters. One emerging process is temperature phased anaerobic digestion (TPAD), which applies a short (2 day) 50-70 °C pre-treatment step prior to 35 °C digestion in the main stage (10-20 days). In this study, we evaluated a thermophilic-mesophilic TPAD against a mesophilic-mesophilic TPAD treating primary sludge. Thermophilic-mesophilic TPAD achieved 54% VS destruction compared to 44% in mesophilic-mesophilic TPAD, with a 25% parallel increase in methane production. Measurements of soluble COD and NH₄⁺-N showed increased hydrolysis extent during thermophilic pre-treatment. Model based analysis indicated the improved performance was due to an increased hydrolysis coefficient rather than an increased inherent degradability, suggesting while TPAD is suitable as an intensification process, a larger main digester could achieve similar impact.     

Removal of refractory compounds from stabilized landfill leachate using an integrated H2O2 oxidation and granular activated carbon (GAC) adsorption treatment

This study investigated the treatment performances of H₂O₂ oxidation alone and its combination with granular activated carbon (GAC) adsorption for raw leachate from the NENT landfill (Hong Kong) with a very low biodegradability ratio (BOD₅/COD) of 0.08. The COD removal of refractory compounds (as indicated by COD values) by the integrated H₂O₂ and GAC treatment was evaluated, optimized and compared to that by H₂O₂ treatment alone with respect to dose, contact time, pH, and biodegradability ratio. At an initial COD concentration of 8000 mg/L and NH₃-N of 2595 mg/L, the integrated treatment has substantially achieved a higher removal (COD: 82%; NH₃-N: 59%) than the H₂O₂ oxidation alone (COD: 33%; NH₃-N: 4.9%) and GAC adsorption alone (COD: 58%) at optimized experimental conditions (p ≤ 0.05; t-test). The addition of an Fe(II) dose at 1.8 g/L further improved the removal of refractory compounds by the integrated treatment from 82% to 89%. Although the integrated H₂O₂ oxidation and GAC adsorption could treat leachate of varying strengths, treated effluents were unable to meet the local COD limit of less than 200 mg/L and the NH₃-N of lower than 5 mg/L. However, the integrated treatment significantly improved the biodegradability ratio of the treated leachate by 350% from 0.08 to 0.36, enabling the application of subsequent biological treatments for complementing the degradation of target compounds in the leachate prior to their discharge.     

The effect of pH on N2O production under aerobic conditions in a partial nitritation system

Ammonia-oxidising bacteria (AOB) are a major contributor to nitrous oxide (N₂O) emissions during nitrogen transformation. N₂O production was observed under both anoxic and aerobic conditions in a lab-scale partial nitritation system operated as a sequencing batch reactor (SBR). The system achieved 55 ± 5% conversion of the 1 g NH₄⁺-N/L contained in a synthetic anaerobic digester liquor to nitrite. The N₂O emission factor was 1.0 ± 0.1% of the ammonium converted. pH was shown to have a major impact on the N₂O production rate of the AOB enriched culture. In the investigated pH range of 6.0-8.5, the specific N₂O production was the lowest between pH 6.0 and 7.0 at a rate of 0.15 ± 0.01 mg N₂O-N/h/g VSS, but increased with pH to a maximum of 0.53 ± 0.04 mg N₂O-N/h/g VSS at pH 8.0. The same trend was also observed for the specific ammonium oxidation rate (AOR) with the maximum AOR reached at pH 8.0. A linear relationship between the N₂O production rate and AOR was observed suggesting that increased ammonium oxidation activity may have promoted N₂O production. The N₂O production rate was constant across free ammonia (FA) and free nitrous acid (FNA) concentrations of 5-78 mg NH₃-N/L and 0.15-4.6 mg HNO₂-N/L, respectively, indicating that the observed pH effect was not due to changes in FA or FNA concentrations.     

Nitrous oxide emissions for early warning of biological nitrification failure in activated sludge

Experiments were carried out to establish whether nitrous oxide (N₂O) could be used as a non-invasive early warning indicator for nitrification failure. Eight experiments were undertaken; duplicate shocks DO depletion, influent ammonia increases, allylthiourea (ATU) shocks and sodium azide (NaN₃) shocks were conducted on a pilot-scale activated sludge plant which consisted of a 315 L completely mixed aeration tank and 100 L clarifier. The process performed well during pre-shock stable operation; ammonia removals were up to 97.8% and N₂O emissions were of low variability (<0.5 ppm). However, toxic shock loads produced an N₂O response of a rise in off-gas concentrations ranging from 16.5 to 186.3 ppm, followed by a lag-time ranging from 3 to 5 h ((0.43-0.71) × HRT) of increased NH₃-N and/or NO₂⁻ in the effluent ranging from 3.4 to 41.2 mg L⁻¹. It is this lag-time that provides the early warning for process failure, thus mitigating action can be taken to avoid nitrogen contamination of receiving waters.     

Evaluating the influence of process parameters on soluble microbial products formation using response surface methodology coupled with grey relational analysis

Soluble microbial products (SMPs) present a major part of residual chemical oxygen demand (COD) in the effluents from biological wastewater treatment systems, and the SMP formation is greatly influenced by a variety of process parameters. In this study, response surface methodology (RSM) coupled with grey relational analysis (GRA) method was used to evaluate the effects of substrate concentration, temperature, NH₄⁺-N concentration and aeration rate on the SMP production in batch activated sludge reactors. Carbohydrates were found to be the major component of SMP, and the influential priorities of these factors were: temperature > substrate concentration > aeration rate > NH₄⁺-N concentration. On the basis of the RSM results, the interactive effects of these factors on the SMP formation were evaluated, and the optimal operating conditions for a minimum SMP production in such a batch activated sludge system also were identified. These results provide useful information about how to control the SMP formation of activated sludge and ensure the bioreactor high-quality effluent.     

Biohydrogen production from xylose at extreme thermophilic temperatures (70 °C) by mixed culture fermentation

Biohydrogen production from xylose at extreme thermophilic temperatures (70 °C) was investigated in batch and continuous-mode operation. Biohydrogen was successfully produced from xylose by repeated batch cultivations with mixed culture received from a biohydrogen reactor treating household solid wastes at 70 °C. The highest hydrogen yield of 1.62 ± 0.02 mol-H₂/mol-xylose_consumed was obtained at initial xylose concentration of 0.5 g/L with synthetic medium amended with 1 g/L of yeast extract. Lower hydrogen yield was achieved at initial xylose concentration higher than 2 g/L. Addition of yeast extract in the cultivation medium resulted in significant improvement of hydrogen yield. The main metabolic products during xylose fermentation were acetate, ethanol, and lactate. The specific growth rates were able to fit the experimental points relatively well with Haldane equation assuming substrate inhibition, and the following kinetic parameters were obtained: the maximum specific growth rate (μ_max) was 0.17 h⁻¹, the half-saturation constant (K_s) was 0.75 g/L, and inhibition constant (K_i) was 3.72 g/L of xylose. Intermittent N₂ sparging could enhance hydrogen production when high hydrogen partial pressure (>0.14 atm) was present in the headspace of the batch reactors. Biohydrogen could be successfully produced in continuously stirred reactor (CSTR) operated at 72-h hydraulic retention time (HRT) with 1 g/L of xylose as substrate at 70 °C. The hydrogen production yield achieved in the CSTR was 1.36 ± 0.03 mol-H₂/mol-xylose_sonsumed, and the production rate was 62 ± 2 ml/d·L_reactor. The hydrogen content in the methane-free mixed gas was approximately 31 ± 1%, and the rest was carbon dioxide. The main intermediate by-products from the effluent were acetate, formate, and ethanol at 4.25 ± 0.10, 3.01 ± 0.11, and 2.59 ± 0.16 mM, respectively.     

Quantitative effects of composting state variables on C/N ratio through GA-aided multivariate analysis

It is widely known that variation of the C/N ratio is dependent on many state variables during composting processes. This study attempted to develop a genetic algorithm aided stepwise cluster analysis (GASCA) method to describe the nonlinear relationships between the selected state variables and the C/N ratio in food waste composting. The experimental data from six bench-scale composting reactors were used to demonstrate the applicability of GASCA. Within the GASCA framework, GA searched optimal sets of both specified state variables and SCA's internal parameters; SCA established statistical nonlinear relationships between state variables and the C/N ratio; to avoid unnecessary and time-consuming calculation, a proxy table was introduced to save around 70% computational efforts. The obtained GASCA cluster trees had smaller sizes and higher prediction accuracy than the conventional SCA trees. Based on the optimal GASCA tree, the effects of the GA-selected state variables on the C/N ratio were ranged in a descending order as: NH₄⁺-N concentration > Moisture content > Ash Content > Mean Temperature > Mesophilic bacteria biomass. Such a rank implied that the variation of ammonium nitrogen concentration, the associated temperature and the moisture conditions, the total loss of both organic matters and available mineral constituents, and the mesophilic bacteria activity, were critical factors affecting the C/N ratio during the investigated food waste composting. This first application of GASCA to composting modelling indicated that more direct search algorithms could be coupled with SCA or other multivariate analysis methods to analyze complicated relationships during composting and many other environmental processes.     

An effective pathway for the removal of microcystin LR via anoxic biodegradation in lake sediments

Aerobic biodegradation has been considered to be the main attenuation mechanism for microcystins, but the role of anoxic biodegradation remains unclear. We investigated the potential for anoxic biodegradation of microcystin and the effects of environmental factors on the process through a series of well-controlled microcosm experiments using lake sediments as inocula. Microcystin LR could be degraded anoxically from 5 mg L⁻¹ to below the detection limit at 25 °C within 2 days after a lag phase of 2 days. The rate was highly dependent on temperature, with a favorable temperature range of 20-30 °C. The addition of glucose or low levels of NH₄-N had no effect on the anoxic biodegradation of microcystin, whereas the addition of NO₃-N significantly inhibited the biodegradation at all experimental concentrations, and the inhibition increased with increasing amount of NO₃-N-amended. Adda (3-amino-9-methoxy-2,6,8-trimethyl-10-phenyl-deca-4,6-dienoic acid), a previously reported nontoxic product of aerobic degradation of microcystin, was identified as the anoxic biodegradation product. This is the first report of Adda as a degradation product of microcystin under anoxic conditions. No other product containing Adda residue was detected during the anoxic degradation of microcystin. These results strongly indicated that anoxic biodegradation is an effective removal pathway of microcystin in lake sediments, and represents a significant bioremediation potential.     

Systematic evaluation of nitrate and perchlorate bioreduction kinetics in groundwater using a hydrogen-based membrane biofilm reactor

To evaluate the simultaneous reduction kinetics of the oxidized compounds, we treated nitrate-contaminated groundwater (∼9.4 mg-N/L) containing low concentrations of perchlorate (∼12.5 μg/L) and saturated with dissolved oxygen (∼8 mg/L) in a hydrogen-based membrane biofilm reactor (MBfR). We systematically increased the hydrogen availability and simultaneously varied the surface loading of the oxidized compounds on the biofilm in order to provide a comprehensive, quantitative data set with which to evaluate the relationship between electron donor (H₂) availability, surface loading of the electron acceptors (oxidized compounds), and simultaneous bioreduction of the electron acceptors. Increasing the H₂ pressure delivered more H₂ gas, and the total H₂ flux increased linearly from ∼0.04 mg/cm²-d for 0.5 psig (0.034 atm) to 0.13 mg/cm²-d for 9.5 psig (0.65 atm). This increased rate of H₂ delivery allowed for continued reduction of the acceptors as their surface loading increased. The electron acceptors had a clear hydrogen-utilization order when the availability of hydrogen was limited: oxygen, nitrate, nitrite, and then perchlorate. Spiking the influent with perchlorate or nitrate allowed us to identify the maximum surface loadings that still achieved more than 99.5% reduction of both oxidized contaminants: 0.21 mg NO₃-N/cm²-d and 3.4 μg ClO₄/cm²-d. Both maximum values appear to be controlled by factors other than hydrogen availability.     

Potential nanosilver impact on anaerobic digestion at moderate silver concentrations

Silver nanoparticles (AgNPs, nanosilver) entering the sewers and wastewater treatment plants (WWTPs) are mostly accumulated in the sludge. In this study, we determined the impact of AgNPs on anaerobic glucose degradation, sludge digestion and methanogenic assemblages. At ambient (22 °C) and mesophilic temperatures (37 °C), there was no significant difference in biogas and methane production between the sludge treated with AgNPs at the concentrations up to 40 mg Ag/L (13.2 g silver/Kg biomass COD) and the control. In these anaerobic digestion samples, acetate and propionic acid were the only detectable volatile fatty acids (VFAs) and they were depleted in 3 days. On the other hand, more than 90% of AgNPs was removed from the liquid phase and associated with the sludge while almost no silver ions were released from AgNPs under anaerobic conditions. Quantitative PCR results indicated that Methanosaeta and Methanomicrobiales were the dominant methanogens, and the methanogenic diversity and population remained largely unchanged after nanosilver exposure and anaerobic digestion. The results suggest that AgNPs at moderate concentrations (e.g., ≤40 mg/L) have negligible impact on anaerobic digestion and methanogenic assemblages because of little to no silver ion release.     

High-rate anaerobic treatment of Fischer-Tropsch wastewater in a packed-bed biofilm reactor

This study investigates the anaerobic treatment of an industrial wastewater from a Fischer-Tropsch (FT) process in a continuous-flow packed-bed biofilm reactor operated under mesophilic conditions (35 °C). The considered synthetic wastewater has an overall chemical oxygen demand (COD) concentration of around 28 g/L, mainly due to alcohols. A gradual increase of the organic load rate (OLR), from 3.4 gCOD/L/d up to 20 gCOD/L/d, was adopted in order to overcome potential inhibitory effects due to long-chain alcohols (>C6). At the highest applied OLR (i.e., 20 gCOD/L/d) and a hydraulic retention time of 1.4 d, the COD removal was 96% with nearly complete conversion of the removed COD into methane. By considering a potential of 200 tCOD/d to be treated, this would correspond to a net production of electric energy of about 8 × 10⁷ kWh/year.During stable reactor operation, a COD balance and batch tests showed that about 80% of the converted COD was directly metabolized through H₂⁻ and acetate-releasing reactions, which proceeded in close syntrophic cooperation with hydrogenotrophic and acetoclastic methanogenesis (contributing to about 33% and 54% of overall methane production, respectively). Finally, energetic considerations indicated that propionic acid oxidation was the metabolic conversion step most dependent on the syntrophic partnership of hydrogenotrophic methanogens and accordingly the most susceptible to variations of the applied OLR or toxicity effects.     

Synergetic pretreatment of sewage sludge by microwave irradiation in presence of H2O2 for enhanced anaerobic digestion

A microwave-enhanced advanced hydrogen peroxide oxidation process (MW/H₂O₂-AOP) was studied in order to investigate the synergetic effects of MW irradiation on H₂O₂ treated waste activated sludges (WAS) in terms of mineralization (permanent stabilization), sludge disintegration/solubilization, and subsequent anaerobic biodegradation as well as dewaterability after digestion. Thickened WAS sample pretreated with 1 g H₂O₂/g total solids (TS) lost 11-34% of its TS, total chemical oxygen demand (COD) and total biopolymers (humic acids, proteins and sugars) via advanced oxidation. In a temperature range of 60-120 °C, elevated MW temperatures (>80 °C) further increased the decomposition of H₂O₂ into OH^• radicals and enhanced both oxidation of COD and solubilization of particulate COD (>0.45 micron) of WAS indicating that a synergetic effect was observed when both H₂O₂ and MW treatments were combined. However, at all temperatures tested, MW/H₂O₂ treated samples had lower first-order mesophilic (33 ± 2 °C) biodegradation rate constants and ultimate (after 32 days of digestion) methane yields (mL per gram sample) compared to control and MW irradiated WAS samples, indicating that synergistically (MW/H₂O₂-AOP) generated soluble organics were slower to biodegrade or more refractory than those generated during MW irradiation.     

Application of high rate, high temperature anaerobic digestion to fungal thermozyme hydrolysates from carbohydrate wastes

The objective of this study was to examine the feasibility of using a two-step, fully biological and sustainable strategy for the treatment of carbohydrate rich wastes. The primary step in this strategy involves the application of thermostable enzymes produced by the thermophilic, aerobic fungus, Talaromyces emersonii, to carbohydrate wastes producing a liquid hydrolysate discharged at elevated temperatures. To assess the potential of thermophilic treatment of this hydrolysate, a comparative study of thermophilic and mesophilic digestion of four sugar rich thermozyme hydrolysate waste streams was conducted by operating two high rate upflow anaerobic hybrid reactors (UAHR) at 37 °C (R1) and 55 °C (R2). The operational performance of both reactors was monitored from start-up by assessing COD removal efficiencies, volatile fatty acid (VFA) discharge and % methane of the biogas produced. Rapid start-up of both R1 and R2 was achieved on an influent composed of the typical sugar components of the organic fraction of municipal solid waste (OFMSW). Both reactors were subsequently challenged in terms of volumetric loading rate (VLR) and it was found that a VLR of 9 gCOD l⁻¹ d⁻¹ at a hydraulic retention time (HRT) of 1 day severely affected the thermophilic reactor with instability characterised by a build up of volatile fatty acid (VFA) intermediates in the effluent. The influent to both reactors was changed to a simple glucose and sucrose-based influent supplied at a VLR of 4.5 gCOD l⁻¹ d⁻¹ and HRT of 2 days prior to the introduction of thermozyme hydrolysates. Four unique thermozyme hydrolysates were subsequently supplied to the reactors, each for a period of 10 HRTs. The applied hydrolysates were derived from apple pulp, bread, carob powder and cardboard, all of which were successfully and comparably converted by both reactors. The % total carbohydrate removal by both reactors was monitored during the application of the sugar rich thermozyme hydrolysates. This approach offers a sustainable technology for the treatment of carbohydrate rich wastes and highlights the potential of these wastes as substrates for the generation of second-generation biofuels.     

Nitrogen pollution and source identification in the Haicheng River basin in Northeast China

A survey was conducted in the Haicheng River near Liaodong Bay to analyze the characteristics and sources of the in-stream nitrogen pollution throughout the year 2010. The results indicated that the total nitrogen (TN) concentrations in the river water all exceeded the GB3838-2002 standard for Class V guideline of 2.0 mg/l at all sampling sites during the sampling seasons. Ammoniacal nitrogen (NH₃-N) dominated TN during the spring season, while nitrate nitrogen (NO₃-N) dominated during the summer and autumn seasons. Different forms of nitrogen had significant seasonal variations (p < 0.01 or p < 0.05). Only NH₃-N and NO₃-N displayed distinct spatial differences at p < 0.05 and p < 0.001, respectively. Most forms of nitrogen were interrelated with physicochemical parameters during different seasons, displaying nitrification and denitrification processes that occurred in the river rather than seasonal biological demand. Based on the nitrogen parameters, the 30 sampling sites were divided into three clusters, by which the pollution sources from chemical nitrogen fertilizers, animal wastes, domestic sewage, and industrial wastewater were identified. Generally, the Haicheng River basin provided about 700 tons of the annual TN flux, contributing to the eutrophication of the Liaodong Bay and Bohai Sea.     

Moisture effects on greenhouse gases generation in nitrifying gas-phase compost biofilters

Gas-phase compost biofilters are extensively used in concentrated animal feeding operations to remove odors and, in some cases, ammonia from air sources. The expected biochemical pathway for these predominantly aerobic systems is nitrification. However, non-uniform media with low oxygen levels can shift biofilter microbial pathways to denitrification, a source of greenhouse gases. Several factors contribute to the formation of anoxic/anaerobic zones: media aging, media and particle structure, air velocity distribution, compaction, biofilm thickness, and moisture content (MC) distribution. The present work studies the effects of media moisture conditions on ammonia (NH₃) removal and greenhouse gas generation (nitrous oxide, N₂O and methane, CH₄) for gas-phase compost biofilters subject to a 100-day controlled drying process. Continuous recordings were made for the three gases and water vapor (2.21-h sampling cycle, each cycle consisted of three gas species, and water vapor, for a total of 10,050 data points). Media moisture conditions were classified into three corresponding media drying rate (DR) stages: Constant DR (wetter media), falling DR, and stable-dry system. The first-half of the constant DR period (0-750 h; MC = 65-52%, w.b.) facilitated high NH₃ removal rates, but higher N₂O generation and no CH₄ generation. At the drier stages of the constant DR (750-950 h; MC = 52-48%, w.b.) NH₃ removal remained high but N₂O net generation decreased to near zero. In the falling DR stage (1200-1480 h; MC = 44-13%) N₂O generation decreased, CH₄ increased, and NH₃ was no longer removed. No ammonia removal or greenhouse gas generation was observed in the stable-dry system (1500-2500 h; MC = 13%). These results indicate that media should remain toward the drier region of the constant DR (in close proximity to the falling DR stage; MC = 50%, approx.), to maintain high levels of NH₃ removal, reduced levels of N₂O generation, and nullify levels of CH₄ generation.     

Total and stable washout of nitrite oxidizing bacteria from a nitrifying continuous activated sludge system using automatic control based on Oxygen Uptake Rate measurements

Partial nitrification (ammonium oxidation to nitrite) has gained a lot of interest among researchers in the last years because of its advantages with respect to complete nitrification (ammonium oxidation to nitrate): decrease of oxygen requirements for nitrification, reduction of COD demand and CO₂ emissions during denitrification and higher denitrification rate and lower biomass production during anoxic growth.In this study, an extremely high-strength ammonium wastewater (3000-4000 mg N L⁻¹) was treated in a continuous pilot plant with a configuration of three reactors in series plus a settler. The system was operated under the maximum possible volumetric nitrogen loading rate, at mild temperature (around 25 °C), with high sludge retention time (around 30 d) and significant nitrifying biomass concentration (average of 1800 ± 600 mg VSS L⁻¹). The implemented control loops transformed the system, which was operating with complete nitrification, into a continuous partial nitrification system. Nitrite oxidizing bacteria (NOB) washout was accomplished with local control loops for pH and dissolved oxygen (DO) with proper setpoints for NOB inhibition (pH = 8.3 and DO = 1.2-1.9 mg O₂ L⁻¹) and with an inflow control loop based on Oxygen Uptake Rate (OUR) measurements, which allowed working at the maximum ammonium oxidation capacity of the pilot plant in each moment. This operational strategy maximized the difference between ammonia oxidizing bacteria (AOB) and NOB growth rates, which is the key point to achieve a fast and stable NOB washout. The results showed a stable operation of the partial nitrification system during more than 100 days and NOB washout was corroborated with fluorescence in-situ hybridization (FISH) analysis.