Treatment of a sanitary landfill leachate using combined solar photo-Fenton and biological immobilized biomass reactor at a pilot scale

A solar photo-Fenton process combined with a biological nitrification and denitrification system is proposed for the decontamination of a landfill leachate in a pilot plant using photocatalytic (4.16 m² of Compound Parabolic Collectors - CPCs) and biological systems (immobilized biomass reactor). The optimum iron concentration for the photo-Fenton reaction of the leachate is 60 mg Fe²⁺ L⁻¹. The organic carbon degradation follows a first-order reaction kinetics (k = 0.020 L kJ_UV⁻¹, r₀ = 12.5 mg kJ_UV⁻¹) with a H₂O₂ consumption rate of 3.0 mmol H₂O₂ kJ_UV⁻¹. Complete removal of ammonium, nitrates and nitrites of the photo-pre-treated leachate was achieved by biological denitrification and nitrification, after previous neutralization/sedimentation of iron sludge (40 mL of iron sludge per liter of photo-treated leachate after 3 h of sedimentation). The optimum C/N ratio obtained for the denitrification reaction was 2.8 mg CH₃OH per mg N-NO₃⁻, consuming 7.9 g/8.2 mL of commercial methanol per liter of leachate. The maximum nitrification rate obtained was 68 mg N-NH₄⁺ per day, consuming 33 mmol (1.3 g) of NaOH per liter during nitrification and 27.5 mmol of H₂SO₄ per liter during denitrification. The optimal phototreatment energy estimated to reach a biodegradable effluent, considering Zahn-Wellens, respirometry and biological oxidation tests, at pilot plant scale, is 29.2 kJ_UV L⁻¹ (3.3 h of photo-Fenton at a constant solar UV power of 30 W m⁻²), consuming 90 mM of H₂O₂ when used in excess, which means almost 57% mineralization of the leachate, 57% reduction of polyphenols concentration and 86% reduction of aromatic content.     

Nitrification of high strength ammonia wastewaters: comparative study of immobilisation media

Due to legislative pressures, sludge production and processing in the UK will increase substantially in the future resulting in a supernatant liquid high in ammonia (500-1000 mg l-1) and "hard" COD (approximately 500 mg l-1). A small footprint reactor is required to effectively nitrify this effluent, and the aim of this work was to compare a number of immobilisation media under a variety of conditions in order to determine which media held the most promise for future development. Laboratory-scale continuously stirred tank reactors containing freely suspended and immobilised biomass were operated with a high-strength synthetic ammonia wastewater (500 mg N l-1) to determine the nitrification rates at various temperatures, and ammonia and COD loadings. COD:NH3 ratios in sludge liquors vary widely depending on the treatment processes employed, and therefore ratios of 1:1 and 2:1 were tested as being fairly typical. The freely suspended nitrifiers were washed out of the reactors at a 1 d hydraulic retention time (HRT), whereas the reactors containing adsorption particles (Linpor and Kaldnes) and PVA-encapsulated nitrifiers continued partially nitrifying down to 12 h, and oxygen addition enhanced nitrification. A decrease in temperature from 25 to 16 degrees C only caused a small (10%) decrease in nitrification in the immobilised cell reactors, demonstrating that nitrification was mass transfer rather than kinetically controlled. A reduction in nitrification occurred when glucose (500 mg l-1) was added to the feed due to the growth of a heterotrophic population. The adsorbed biomass reactors lost 35% of nitrification compared to only 7% with PVA, and it appears that the colonisation of PVA by heterotrophs is more difficult than for Linpor and Kaldnes. Respiration rates for all particles increased with time in the reactors, and nitrifiers immobilised in PVA retained approximately 40% of their viability after immobilisation. Volumetric nitrification rates were generally higher for the PVA reactor than for Linpor and Kaldnes, and were: suspended biomass reactor: 0.36; Linpor: 0.57; Kaldnes: 0.53 and PVA: 0.70 kg N m-3-reactor d-1 for a 25% reactor fill. These equate to 2.28, 4.24 and 3.97 g N m-2-media d-1 for Linpor, Kaldnes and PVA respectively, hence other reactor fill rates for Kaldnes warrant further investigation. However, the PVA particles with the highest nitrification rates under all conditions showed promise as an immobilisation medium, and are amenable to further optimisation for the nitrification of high-strength ammonia wastewaters.     

Nitrification kinetics in activated sludge at various temperatures and dissolved oxygen concentrations

Activated sludge from a domestic sewage works was enriched with nitrifying bacteria by running a laboratory fermenter on ammonia-supplemented sewage. This enriched culture was used to determine respirometrically the kinetics of microbial nitrification. It was demonstrated that the reaction fits the Michaelis-Menten model for temperatures from 10 to 35°C, having a temperature optimum at 15°C (K₃ 0.72 mg 1⁻¹ NH₃). Nitrification is unaffected by high dissolved oxygen concentration 38 mg 1⁻¹ O₂ at 30°C) after acclimatisation. Nitrite concentrations > 20 mg 1⁻¹ are inhibitory to the reaction.     

Alumina nanoparticles-induced effects on wastewater nitrogen and phosphorus removal after short-term and long-term exposure

Alumina nanoparticles (Al₂O₃ NPs) have been widely used in many fields, which causes a growing concern about their potential health and environmental risks. However, their possible impacts on wastewater nitrogen and phosphorus removal have not yet been reported. In this study, both short-term and long-term effects of Al₂O₃ NPs on wastewater nutrient removal were investigated. Scanning electron microscope (SEM) analysis showed that most of Al₂O₃ NPs were adsorbed onto activated sludge, but these NPs had no adverse effects on the surface integrity and viability of activated sludge. It was found that short-term exposure to 1 and 50 mg/L Al₂O₃ NPs induced marginal influences on wastewater nitrification, denitrification and phosphorus removal. Nevertheless, the prolonged exposure to 50 mg/L Al₂O₃ NPs was observed to decrease the total nitrogen (TN) removal efficiency from 80.4% to 62.5% due to the suppressed denitrification process, although biological phosphorus removal and the transformations of intracellular polyhydroxyalkanoates and glycogen were not affected. Quantitative PCR assays indicated that compared with the control, 50 mg/L Al₂O₃ NPs decreased the abundance of denitrifying bacteria in activated sludge. Further enzyme activity tests showed that the activities of key denitrifying enzymes (nitrate reductase and nitrite reductase) were inhibited, which might be responsible for the negative effects of 50 mg/L Al₂O₃ NPs on wastewater nitrogen removal after long-term exposure.     

Effect of free ammonia concentration on monochloramine penetration within a nitrifying biofilm and its effect on activity, viability, and recovery

Chloramine has replaced free chorine for secondary disinfection at many water utilities because of disinfection by-product (DBP) regulations. Because chloramination provides a source of ammonia, there is a potential for nitrification when using chloramines. Nitrification in drinking water distribution systems is undesirable and may result in degradation of water quality and subsequent non-compliance with existing regulations. Thus, nitrification control is a major issue and likely to become increasingly important as chloramine use increases. In this study, monochloramine penetration and its effect on nitrifying biofilm activity, viability, and recovery was investigated and evaluated using microelectrodes and confocal laser scanning microscopy (CLSM). Monochloramine was applied to nitrifying biofilm for 24 h at two different chlorine to nitrogen (Cl₂:N) mass ratios (4:1 [4.4 mg Cl₂/L] or 1:1 Cl₂:N [5.3 mg Cl₂/L]), resulting in either a low (0.23 mg N/L) or high (4.2 mg N/L) free ammonia concentration. Subsequently, these biofilm samples were allowed to recover without monochloramine and receiving 4.2 mg N/L free ammonia. Under both monochloramine application conditions, monochloramine fully penetrated into the nitrifying biofilm within 24 h. Despite this complete monochloramine penetration, complete viability loss did not occur, and both biofilm samples subsequently recovered aerobic activity when fed only free ammonia. When monochloramine was applied with a low free ammonia concentration, dissolved oxygen (DO) fully penetrated, but with a high free ammonia concentration, complete cessation of aerobic activity (i.e., oxygen utilization) did not occur and subsequent analysis indicated that oxygen consumption still remained near the substratum. During the ammonia only recovery phase, different spatial recoveries were seen in each of the samples, based on oxygen utilization. It appears that the presence of higher free ammonia concentration allowed a larger biomass to remain active during monochloramine application, particularly the organisms deeper within the biofilm, leading to faster recovery in oxygen utilization when monochloramine was removed. These results suggest that limiting the free ammonia concentration during monochloramine application will slow the onset of nitrification episodes by maintaining the biofilm biomass at a state of lower activity.     

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.     

Simultaneous nitrification and denitrification process in a new Tubificidae-reactor for minimizing nutrient release during sludge reduction

Nutrient release is reported as one of the main disadvantage of sludge reduction induced by aquatic worm. In this study, a Static Sequencing Batch Worm Reactor (SSBWR) was proposed with novel structure of perforated panels, combined aeration system and cycle operation. Effective simultaneous nitrification and denitrification were obtained owing to the stratified sludge layer containing aerobic and anoxic microzone formed on each carrier during most of the operation time in the SSBWR, which created suitable conditions for remarkable sludge reduction and nutrient removal. The results showed that the total nitrogen (TN) concentration, NO₃^?–N + NO₂^?–N concentration and NH₄⁺–N release could be reduced by 67.5%, 98.5% and 63.0%, respectively. And the soluble chemical oxygen demand (sCOD) released by sludge predation was also proved to provide a carbon source for denitrification leading to carbon release control and substantial cost savings. A schematic diagram of the stratified sludge layer and the mass balance of the nitrification–denitrification cycle were given, providing further insight into the nutrient (sCOD and nitrogen compounds) transformation during the worm predation in the SSBWR. For the mixed sludge liquid of 3000 mg TSS/L, 30 mg/L sCOD and 40 mg/L NO₃^?–N, the NO₃^?–N and NO₂^?–N came close to zero, and the sludge concentration, NH₄⁺–N release and sCOD release was reduced by 33.6%, 63.0% and 72.5%, respectively, during 48 h’ predation.     

Monitoring off-gas O2/CO2 to predict nitrification performance in activated sludge processes

Nitrification/denitrification (NDN) processes are the most widely used technique to remove nitrogenous pollutants from municipal wastewater. The performance of nitrogen removal in the NDN process depends on the metabolism of nitrifying bacteria, and is dependent on adequate oxygen supply. Off-gas testing is a convenient and popular method for measuring oxygen transfer efficiency (OTE) under process conditions and can be performed in real-time. Since carbon dioxide is produced by carbonaceous oxidizing organism and not by nitrifiers, it should be possible to use the off-gas carbon dioxide mole fraction to estimate nitrification performance independently of the oxygen uptake rate (OUR) or OTE. This paper used off-gas data with a dynamic model to estimate nitrifying efficiency for various activated sludge process conditions. The relationship among nitrification, oxygen transfer, carbon dioxide production, and pH change was investigated. Experimental results of an online off-gas monitoring for a full-scale treatment plant were used to validate the model. The results showed measurable differences in OUR and carbon dioxide transfer rate (CTR) and the simulations successfully predicted the effluent ammonia by using the measured CO₂ and O₂ contents in off-gas as input signal. Carbon dioxide in the off-gas could be a useful technique to control aeration and to monitor nitrification rate.     

Insights into function of hydrocyclone separator and modified volcanic rock coagulant on formation of aerobic granular sludge

Hydrocyclone separator and novel coagulants were adopted to separately enhance the granulating process of aerobic granular sludge. For hydrocyclone separator, although there was no obvious large particle, it was feasible to improve the sludge sedimentation capacity, indicated by the sludge volume index. Moreover, the nitrogen removal performance was promoted by the separator with a nitrogen removal rate of 8.2 mg/(L·h), nearly two times of system without a separator. Meanwhile, nitrogen-related microbial communities were stimulated, for example, Thauera and Nitrosomonas. Another method of coagulation by the modified volcanic rock promoted the sludge settling performance. The optimum coagulating conditions were the coagulant size of 100 μm with dosage of 6 g/L. The addition of polyacrylamide played a weak role in granulation. Overall, the role of hydrocyclone separator and modified volcanic rock coagulant belonged to the improvement of the settleability of sludge and the aggregation of sludge, respectively.     

Toxic effects of disyston and furadan on the bimodal pattern of oxygen consumption in the climbing perch, Anabas testudineus (Bloch)

A significant change was noted in the aquatic (vO₂), aerial (VO₂) and total [(v + V)O₂] consumption of Anabas testudineus (Bloch) exposed to 4.0 and 10.5 mg l⁻¹ disyston and 0.56 and 1.56 mg l⁻¹ furadan. After 1 h of exposure to all the concentrations significant decreases were noticed in the (v + V)O₂. Though significant increases were noted in all the measures of respiration after 3 h of exposure in both the pesticides, their effects vary from one exposure period to another. The fish held in 4.0 mg l⁻¹ disyston and 0.56 mg l⁻¹ furadan concentrations resumed normal pattern of bimodal respiration after 120 and 96 h respectively. Further the pattern of changes in (v + V)O₂ closely followed the changes observed in VO₂ rather than vO₂ indicating that the fish held in pesticide medium predominantly relied on aerial respiration. A direct correlation was noticed between the VO₂ consumption and the surfacing frequency of the fish exposed to disyston and furadan. Relatively furadan is more toxic than disyston to Anabas testudineus if their effects on O₂ consumption are taken into consideration.     

The influence of reactor mixing characteristics on the rate of nitrification in the activated sludge process

The effect of longitudinal mixing on nitrification was evaluated in two bench scale activated sludge reactors of equal volume, one approximating complete mixing (∂ = 0.62) and one approximating plug-flow mixing (∂ = 0.07). The onset of nitrification was more rapid under plug-flow conditions and a higher rate constant for nitrification was observed. Both the numbers and species of nitrifying bacteria were the same in both reactors and thus this did not contribute to the observed differences. Lower reaction rates in the complete mix reactor were shown to result from a high concentration of free ammonia in the mixed liquor, which gave rise to inhibition of nitrifying bacteria. Over an extended operating period, the plug flow reactor produced a sludge which demonstrated superior settling properties to that of the complete mix reactor. In addition incidences of sludge bulking were absent, whereas they were a regular feature of the complete mix system.     

Field analysis of PSA O2–ozone production process for water recycling

The use of an oxygen generator based on pressure swing adsorption technology (PSA) has been investigated as an alternative method for supplying oxygen for onsite ozone production. During the investigation period of 1040 h, the oxygen purity from the PSA process fluctuated within a range of 90.5–93 O₂% (v/v). Using the working ratio of 2.8 mol of O₂ to 1 m³ of raw water, the PSA process in series with a corona discharge ozone generator yielded an oxidation–reduction potential of ca. 200 mV and a concentration of 22.8 mg/L dissolved oxygen. The average efficiency of the raw water treatment was as follows: 47% chemical oxygen demand removal, 78% biological oxygen demand removal, 36% sulphide removal and 34% colour removal. The operating cost of the PSA O₂ generation was reduced 3 times compared with the cost of high‐pressure oxygen cylinders.     

Effect of loading rate and oxygen supply on nitrification in a non-porous membrane biofilm reactor

A nitrifying membrane biofilm reactor (MBfR) was operated over 170 days, to assess the effect of ammonia loading rate under O₂-excess conditions, and the effect of dissolved oxygen under O₂-limiting conditions on nitrification efficiency. The MBfR was fed pure oxygen by diffusion through a non-porous membrane. Five different loading rates, ranging from 1.92 to 5.53 g N/m² d, were tested, yielding specific nitrification rates (SNR) ranging from 1.54 to 2.60 g N/m² d. SNR increased linearly with specific loading rate, up to the load of 3.5 g N/m² d, which indicated that mass transfer was linearly related to the bulk ammonia concentration. Beyond that load, substrate diffusion limitation inhibited further increase of SNR. When operating the system under limited oxygen supply conditions, 100% oxygen utilization was achievable. Maintenance of higher oxygen supply allowed a slightly higher SNR due to the growth of nitrifiers at the outer side of the biofilm (away from the membrane surface). Nitrification batch tests confirmed that the fraction of nitrifiers in the solids detached from the surface of the biofilm (and washed out with the effluent), was twice as high during oxygen-excess conditions when compared to oxygen-limiting conditions.     

A quantitative measure of nitrifying bacterial growth

Nitrifying bacteria convert ammonia (NH3) to nitrate (NO3-) in a nitrification reaction. Methods to quantitatively separate the growth rate of these important bacterial populations from that of the dominant heterotrophic bacteria are important to our understanding of the nitrification process. The changing concentration of ammonia is often used as an indirect measure of nitrification but ammonification processes generate ammonia and confound this approach while heterotrophs remove nitrate via denitrification. Molecular probe methods can tell us what proportion of the microbial community is nitrifying bacteria but not their growth rate. The technique proposed here was able to quantify the growth rate of the nitrifying bacterial populations amidst complex ecological processes. The method incubates [methyl-3H] thymidine with water samples in the presence and absence of an inhibitor of nitrification-thiourea. The radioactively labeled DNA in the growing bacteria was extracted. The rate of incorporation of the label into the dividing bacterial DNA was used to determine bacterial growth rate. Total bacterial community growth rates in full-scale and pilot-scale fixed-film nitrifying reactors and an activated sludge reactor were 2.1 x 10(8), 4.1 x 10(8) and 0.4 x 10(8)cell ml(-1)d(-1), respectively; the growth rate of autotrophic-nitrifying bacteria was 0.7 x 10(8), 2.6 x 10(8) and 0.01 x 10(8)cell ml(-1)d(-1), respectively. Autotrophic-nitrifying bacteria contributed 30% and 60% of the total bacterial community growth rate in the nitrifying reactors whereas only 2% was observed in the activated sludge reactor that was not designed to nitrify. The rates of ammonia loss from the nitrifying reactors corresponded to the rate of growth of the nitrifying bacteria. This method has the potential to more often identify factors that enhance or limit nitrifying processes in both engineered and natural aquatic environments.     

Microbial degradation of quinoline by immobilized cells of Burkholderia pickettii

A quinoline-biodegrading microorganism was isolated from activated sludge of coke-oven wastewater treatment plant using quinoline as sole carbon and nitrogen source. It is a gram negative, rod-shaped and aerobic strain, which was identified as Burkholderia pickettii. The biodegradation of quinoline was carried out with this isolated strain. Analysis by high performance liquid chromatography and gas chromatography/mass spectrum (GC/MS) revealed that 2-hydroxyquinoline (2-OH-Q) was the first intermediate in the course of quinoline biodegradation. A novel immobilization carrier, that is, polyvinyl alcohol (PVA)-gauze hybrid carrier, was developed. The isolated strain was immobilized by two different immobilizing techniques and used for the quinolinerdegradation. It was found that biodegradation rate of quinoline by the microorganisms immobilized on PVA-gauze hybrid carrier was faster than that by the microorganisms immobilized in PVA gel beads. Kinetics of quinoline biodegradation by cells of Burkholderia pickettii immobilized on PVA-gauze hybrid carrier was investigated. The results demonstrate that quinoline degradation could be described by zero-order reaction rate equation when the initial quinoline concentration was in the range of 50-500 mg l(-1).     

Biodegradation potential of bulking agents used in sludge bio-drying and their contribution to bio-generated heat

Straw and sawdust are commonly used bulking agents in sludge composting or bio-drying. It is important to determine if they contribute to the biodegradable volatile solids pool. A sludge bio-drying process was performed in this study using straw, sawdust and their combination as the bulking agents. The results revealed that straw has substantial biodegradation potential in the aerobic process and sawdust has poor capacity to be degraded. The temperature profile and bio-drying efficiency were highest in the trial that straw was added, as indicated by a moisture removal ratio and VS loss ratio of 62.3 and 31.0%, respectively. In separate aerobic incubation tests, straw obtained the highest oxygen uptake rate (OUR) of 2.14 and 4.75 mg O₂ g⁻¹VS h⁻¹ at 35 °C and 50 °C, respectively, while the highest OUR values of sludge were 12.1 and 5.68 mg O₂ g⁻¹VS h⁻¹ at 35 °C and 50 °C and those of sawdust were 0.286 and 0.332 mg O₂ g⁻¹VS h⁻¹, respectively. The distribution of biochemical fractions revealed that soluble fractions in hot water and hot neutral detergent were the main substrates directly attacked by microorganisms, which accounted for the initial OUR peak. The cellulose-like fraction in straw was transformed to soluble fractions, resulting in an increased duration of aerobic respiration. Based on the potential VS degradation rate, no bio-generated heat was contributed by sawdust, while that contribution by straw was about 41.7% and the ratio of sludge/straw was 5:1 (w/w, wet basis).     

Floc-based sequential partial nitritation and anammox at full scale with contrasting N2O emissions

New Activated Sludge (NAS^®) is a hybrid, floc-based nitrogen removal process without carbon addition, based on the control of sludge retention times (SRT) and dissolved oxygen (DO) levels. The aim of this study was to examine the performance of a retrofitted four-stage NAS^® plant, including on-line measurements of greenhouse gas emissions (N₂O and CH₄). The plant treated anaerobically digested industrial wastewater, containing 264 mg N L⁻¹, 1154 mg chemical oxygen demand (COD) L⁻¹ and an inorganic carbon alkalinity of 34 meq L⁻¹. The batch-fed partial nitritation step received an overall nitrogen loading rate of 0.18-0.22 kg N m⁻³ d⁻¹, thereby oxidized nitrogen to nitrite (45-47%) and some nitrate (13-15%), but also to N₂O (5.1-6.6%). This was achieved at a SRT of 1.7 d and DO around 1.0 mg O₂ L⁻¹. Subsequently, anammox, denitrification and nitrification compartments were followed by a final settler, at an overall SRT of 46 d. None of the latter three reactors emitted N₂O. In the anammox step, 0.26 kg N m⁻³ d⁻¹ was removed, with an estimated contribution of 71% by the genus Kuenenia, which constituted 3.1% of the biomass. Overall, a nitrogen removal efficiency of 95% was obtained, yielding a dischargeable effluent. Retrofitting floc-based nitrification/denitrification with carbon addition to NAS^® allowed to save 40% of the operational wastewater treatment costs. Yet, a decrease of the N₂O emissions by about 50% is necessary in order to obtain a CO₂ neutral footprint. The impact of emitted CH₄ was 20 times lower.     

Kinetic and metabolic study of benzene, toluene and m-xylene in nitrifying batch cultures

The effect of benzene, toluene, and m-xylene (BTX) compounds on the nitrifying activity of a sludge produced in steady-state nitrification was evaluated in batch cultures. Benzene and m-xylene at 10 mg C/L decreased ammonium consumption efficiency by 57% and 26%, respectively, whereas toluene did not affect the ammonium oxidation process. The consumed NH4+-N was totally oxidized to NO3- -N. There was no significant effect at 5 mg C/L of each aromatic compound. BTX (5-20mg C/L) induced a significant decrease in the values for specific rates of NH4+ -N consumption (76-99%) and NO3- -N production (45-98%). At 10 mg C/L of BTX compounds, the inhibition order on nitrate production was: benzene > m-xylene > toluene while at 20 mg C/L, the sequence changed to m-xylene > toluene > benzene for both nitrification inhibition and BTX compounds persistence. At 5 mg C/L of BTX compounds, there was no toxic effect on the sludge whereas from 10 to 50 mgC/L, bacteria did not totally recover their nitrifying activity. At a concentration of 5 mg C/L, toluene was first oxidized to benzyl alcohol, which was later oxidized to butyrate while m-xylene was oxidized to acetate and butyrate.     

Impact of water boundary layer diffusion on the nitrification rate of submerged biofilter elements from a recirculating aquaculture system

Total ammonia nitrogen (TAN) removal by microbial nitrification is an essential process in recirculating aquaculture systems (RAS). In order to protect the aquatic environment and fish health, it is important to be able to predict the nitrification rates in RAS's. The aim of this study was to determine the impact of hydraulic film diffusion on the nitrification rate in a submerged biofilter. Using an experimental batch reactor setup with recirculation, active nitrifying biofilter units from a RAS were exposed to a range of hydraulic flow velocities. Corresponding nitrification rates were measured following ammonium chloride, NH₄Cl, spikes and the impact of hydraulic film diffusion was quantified.The nitrification performance of the tested biofilter could be significantly increased by increasing the hydraulic flow velocity in the filter. Area based first order nitrification rate constants ranged from 0.065 m d⁻¹ to 0.192 m d⁻¹ for flow velocities between 2.5 m h⁻¹ and 40 m h⁻¹ (18 °C). This study documents that hydraulic film diffusion may have a significant impact on the nitrification rate in fixed film biofilters with geometry and hydraulic flows corresponding to our experimental RAS biofilters. The results may thus have practical implications in relation to the design, operational strategy of RAS biofilters and how to optimize TAN removal in fixed film biofilter systems.     

Low temperature removal of nitrate by bacterial denitrification

The efficiency of two denitrifying sludges enriched at 5 and 20°C were compared using methanol as an electron donor. Both sludges were exposed to the same hydraulic and chemical conditions using an influent containing methanol and mineral salts. The low temperature sludge seemed to have several advantages over the sludge selected for at the higher temperature. In the range 0–17°C, the specific denitrification rate was 1.5–4 times the rate for the high temperature sludge, temperatures below 8°C being the most favourable. At 2°C, under nitrate limiting conditions, 98% nitrate reduction was obtained at a hydraulic residence time of 3.5 h, with an effluent concentration of 0.8 mg NO₃---Nl⁻¹. Sedimentation characteristics were always better for the low temperature sludge, and the utilization of methanol equally good as the high temperature sludge. The low temperature sludge appeared to be biochemically and microbiologically stable to temperature changes within the range 0–17°C, the latter temperature being close to the limit for maintaining the psychrophilic characteristics of the sludge. Studies on pure culture isolates of the denitrifying bacteria showed >90% dominance of one bacterial strain in both sludges. Studies of the isolates also showed that the low-temperature sludge consisted predominantly of psychrotrophs/psychrophiles, and not well-adapted mesophiles, which were only present in low concentrations. The dominant strain in both sludges was unable to grow on methanol in pure culture without access to nutrient growth factors. Only a few minor strains were obligate methylotrophs.Low temperature sludges were tested in a 3-stage biological process receiving domestic sewage. Each stage; carbon oxidation, nitrification and denitrification had separate sludge recycle, and methanol was added to the denitrification stage. These sludges were grown and selected for at temperatures 5°C. At 5°C the laboratory scale process gave 90% removal of total nitrogen at hydraulic residence times of 1.5, 9 and 4 h for the two aeration stages and the anaerobic stage respectively. Overall nitrification/denitrification was 95%, while denitrification separately was 98%. The effluent contained 0.4 mg NO₃---Nl⁻¹. The critical step in the process was unquestionably nitrification. Oxidation of ammonium was satisfactory at low temperature, but the reaction was somewhat vulnerable to changes in external conditions. The low temperature denitrifying sludge was originally enriched on synthetic waste but did not appear to change its microbial composition or characteristics by exposure to municipal wastewater.