Effect of substrate concentration on biodegradation of 2,4-dichlorophenol using modified rotating biological contactors

2,4-Dichlorophenol used in the manufacture of pesticides, germicides, resins, seed disinfectants and antiseptics, if disposed untreated causes greater havoc for land and aquatic environment. In all the earlier works, 2,4-dichlorophenol has been fed along with easily biodegradable substrate, glucose as one of the constituents. A modified 4-stage RBC was used for the biodegradation studies of 2,4-dichlorophenol. The micro organisms attached to the disks were specially acclimatised to the extent that the 2,4-dichlorophenol alone serves as the sole carbon source supporting their metabolic activities. The RBC was operated at 12?rpm. The toxic substrate removal studies were carried out in the hydraulic loading rates ranging from 0.005?m³/m²/d to 0.035?m³/m²/d and organic loading rates from 0.35?g/m²/d to 6.15?g/m²/d. A correlation plot between 2,4-dichlorophenol removal and organic loading rate is presented. A mathematical model is proposed using regression analysis.     

Influence of COD/NH3–N ratio on organic removal and nitrification using a modified RBC

Synthetic wastewaters were prepared with different influent concentrations of ammonia nitrogen (NH₃–N) and COD and the treatment studies were conducted using a rotating biological contactor (RBC). If organic removal and nitrification can be simultaneously effected in one process, it will be an ideal solution to water pollution control. The RBC used in the present study was a four stage laboratory model and the discs were modified by attaching porous netlon sheets to enhance biofilm area. The COD loads (S ₀) used were about 1000 and 1500?mg/l whereas NH₃–N concentrations used were in the range of 20 to 185?mg/l. Hydraulic load (q) of 0.03?m³?.?m^-2?.?d^-1 and ammonia nitrogen loadings in the range of 0.66 to 5.5?g NH₃–N?.?m^-2?.?d^-1 were used. The RBC was operated at two different rotating speeds of 6 and 12?rpm. The results showed that the nitrification and percentage of COD removal were not affected up to the value of the COD/NH₃–N in the range from 47 to 23 at w=6?rpm and for an average influent COD of 1003?mg/l. Beyond that range only the nitrification rate decreased much whereas the percentage of COD removal was not affected. Similarly, at an influent COD load of 1557?mg/l, the nitrification and percentage COD removal were not affected for the value of the COD/NH₃–N in the range from 44 to 23 but beyond that range only the nitrification rate decreased while the percentage of COD removal was approximately constant and still high. A correlation plot between the NH₃–N removed and NH₃–N applied was presented at a rotating speed of 6?rpm and it was found that the nitrification rate of 3.93?g NH₃–N?.?m^-2?.?d^-1 was achieved at ammonia loading of 5.55?g NH₃–N?.?m^-2?.?d^-1. Also the results at w=12?rpm showed improvement of nitrification rate over those at 6?rpm.     

Organic reduction of tapioca wastewaters using modified RBC

A modified Rotating Biological Contactor (RBC) was used for the treatability studies of synthetic tapioca wastewaters. The RBC used was a four stage laboratory model and the discs were modified by attaching porous nechlon sheets to enhance biofilm area. Synthetic tapioca wastewaters were prepared with influent concentrations from 927 to 3600 mg/l of COD. Three hydraulic loads were used in the range of 0.03 to 0.09 m³·m^–2·d^–1 and the organic loads used were in the range of 28 to 306 g COD· m^–2·d^–1. The percentage COD removal were in the range from 97.4 to 68. RBC was operated at a rotating speed of 18 rpm which was found to be the optimal rotating speed. Biokinetic coefficients based on Kornegay and Hudson models were obtained using linear analysis. Also, a mathematical model was proposed using regression analysis.List of Symbols A m² total surface area of discs - d m active depth of microbial film onany rotating disc - K _s mg ·l^–1 saturation constant - P mg·m^–2·^–1 area capacity - Q l·d^–1 hydraulic flow rate - q m³·m^–2·d^–1 hydraulic loading rate - S ₀ mg·l^–1 influent substrate concentration - S _e mg·l^–1 effluent substrate concentration - w rpm rotational speed - V m³ volume of the reactor - X _f mg·l^–1 active biomass per unit volume ofattached growth - X _s mg·l^–1 active biomass per unit volume ofsuspended growth - X mg·l^–1 active biomass per unit volume - Y _s yield coefficient for attachedgrowth - Y _A yield coefficient for suspendedgrowth - Y yield coefficient, mass of biomass/mass of substrate removed Greek Symbols hr mean hydraulic detention time - (_max)_A d^–1 maximum specific growth rate forattached growth - (_max)_s d^–1 maximum specific growth rate forsuspended growth - _max d^–1 maximum specific growth rate - d^–1 specific growth rate - _v mg·l^–1·hr^–1 maximum volumetric substrateutilization rate coefficient     

Domestic wastewater treatment using multi-electrode continuous flow MFCs with a separator electrode assembly design

Treatment of domestic wastewater using microbial fuel cells (MFCs) will require reactors with multiple electrodes, but this presents unique challenges under continuous flow conditions due to large changes in the chemical oxygen demand (COD) concentration within the reactor. Domestic wastewater treatment was examined using a single-chamber MFC (130 mL) with multiple graphite fiber brush anodes wired together and a single air cathode (cathode specific area of 27 m²/m³). In fed-batch operation, where the COD concentration was spatially uniform in the reactor but changed over time, the maximum current density was 148?±?8 mA/m² (1,000 Ω), the maximum power density was 120 mW/m², and the overall COD removal was >90 %. However, in continuous flow operation (8 h hydraulic retention time, HRT), there was a 57 % change in the COD concentration across the reactor (influent versus effluent) and the current density was only 20?±?13 mA/m². Two approaches were used to increase performance under continuous flow conditions. First, the anodes were separately wired to the cathode, which increased the current density to 55?±?15 mA/m². Second, two MFCs were hydraulically connected in series (each with half the original HRT) to avoid large changes in COD among the anodes in the same reactor. The second approach improved current density to 73?±?13 mA/m². These results show that current generation from wastewaters in MFCs with multiple anodes, under continuous flow conditions, can be improved using multiple reactors in series, as this minimizes changes in COD in each reactor.     

Suitability of nutrients removal from brewery wastewater using a hydroponic technology with <Emphasis Type="Italic">Typha latifolia</Emphasis>

Background

This study aims to assess suitability of hydroponic technology for treatment of brewery wastewater in a hydroponic bioreactor using Typha latifolia. Triplicated hydroponic bioreactor treatment units were designed, constructed and operated at a hydraulic retention time of 5?days with different surface loadings and mean hydraulic loading rate 0.023?m³ m^?2d^??1. Young T. latifolia shoots were collected in the vicinity of study site. Wastewater characteristics, plant growth and nutrient accumulation during experiment were analyzed as per APHA standard methods and nutrient removal efficiency was evaluated based on inlet and outlet values.

Results

T. latifolia established and grew well in the hydroponics under fluctuations of wastewater loads and showed a good phytoremedial capacity to remove nutrients. Significant removal efficiencies (p?<?0.05) varied between 54 and 80% for Total Kjeldahl Nitrogen, 42 and 65% for NH₄⁺ -N, 47 and 58% for NO₃^? -N, and 51 and 70% for PO₄^3?-P. The system improved the removal up to 29% compared to control and produced biomass of 0.61–0.86?kg dry weight (DW) m^??2. Nutrients retained were up to 21.17?g?N?kg^??1 DW and 2.87?g P kg^??1 DW.

Conclusion

The significant nutrients reduction obtained and production of biomass led us to conclude that hydroponics technology using T. latifolia has suitability potential for treatment of brewery wastewater and similar agro-industrial wastewaters. Thus it could be considered as a promising eco-friendly option for wastewater treatment to mitigate water pollution. Integration of treatment and production of biomass needs further improvement.

     

Effect of influent pH and alkalinity on the removal of chlorophenols in sequential anaerobic–aerobic reactors

This study was carried out to determine the effect of influent pH and alkalinity on the performance of sequential UASB and RBC reactors for the removal of 2-CP and 2,4-DCP from two different simulated wastewaters. The performance of methanogens at low (<6.0) to high (>8.0) pH values and at sufficiently high alkalinity (1500–3500 mg/l as CaCO₃) is described in this paper. Sequential reactors were capable of handling wastewaters with influent pH, 5.5–8.5. However, with influent pH 7.0 ± 0.1 UASB reactor showed best performance for 2-CP (99%) and 2,4-DCP (88%) removals. Increase in alkalinity/COD ratio in the influent (>1.1) caused gradual decrease in the chlorophenol removal in UASB reactors. The UASB reactors could not tolerate wastewater with higher alkalinity/COD ratio (2.6) and showed significant deterioration of its performance in terms of chlorophenols removal achieving only 74.7% 2-CP and 60% 2,4-DCP removals, respectively.     

The influence of anaerobic pretreatment on the nitrogen removal from biosynthetic pharmaceutical wastewaters

The C:N ratio of the pharmaceutical wastewaters is usually suitable for a combination of the anaerobic pretreatment with the high COD removal and aerobic posttreatment with the efficient biological N removal. This kind of anaerobic-aerobic process was tested in semipilot scale by using a UASB reactor and an activated sludge system with a predenitrification (total volume 100 1). It was found that at a total HRT of 2.3 days an average of 97.5% of COD and 73.5% of total N was removed. The UASB reactor was operated at 30°C with a volumetric loading rate of 8.7 kg.m^-3.d^-1, the efficiency of COD removal was 92.2%. The processes, which take part in the biological removal of nitrogen, especially the nitrification, were running with lower rates than usually observed in aerobic treatment systems.Abbreviations AAO anaerobic anoxic oxic configuration - AOO anaerobic oxic oxic configuration - B _V volumetric organic loading rate (kg COD.m^-3. d^-1) - dB _x specific COD removal rate (mg COD. g^-1 VSS. d^-1) - DNR denitrification rate (mg N–NO₃. g^-1 VSS. h^-1) - E_COD efficiency of COD removal (%) - HRT hydraulic retention time (d) - NR nitrification rate (mg N–NO₃. g^-1 VSS. h^-1) - R recirculation ratio (%) - SBP specific biogas production (m³.kg^-1 removed COD) - SRT solids retention time; sludge age (d) - SS suspended solids (g.1^-1) - UASB upflow anaerobic sludge blanket reactor - VSS volatile suspended solids (g.1^-1)     

Chemical oxygen demand,nitrogen and phosphorus removal by subsurface wetlands with Phragmites vegetation in different models

To improve the removal efficiency of subsurface wetlands vegetated mainly by Phragmites, pilot‐scale gravel‐based wetlands were used to treat sewage characterized by chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) pollution. For Phragmites vegetation, COD, TP and TN removal loads of wetland vegetation with Phragmites australis–Typha angustata–Scirpus validus as main species reached 0.517 g m^?2 d^?1, 0.277 g P m^?2 d^?1 and 0.023 g N m^?2 d^?1. The COD removal loads in pilot‐scale and medium‐scale (260 m² in area) wetlands with Phragmites‐monoculture vegetation were 0.62–0.64 g m^?2 d^?1, while that of P. australis–T. angustata–S. validus wetland reached 0.974 g m^?2 d^?1. Thus, the preferable poly‐culture model for Phragmites wetland vegetation was P. australis, T. angustata, S. validus and Zizania latifolia with stem density ratio of 8:1:5:1. After harvest, nitrogen and phosphorus standing stocks of wetland vegetations ranged only 2.2–9.93 g N m^?2 and 5.39–13.5 g P m^?2, respectively, as both the above ground biomass and the nitrogen and phosphorus contents of the wetland vegetation harvested in late autumn were low.     

Anaerobic degradation of 2,4-dichlorophenoxyacetic acid by <Emphasis Type="Italic">Thauera</Emphasis> sp. DKT

Thauera sp. strain DKT isolated from sediment utilized 2,4-dichlorophenoxyacetic acid (2,4D) and its relative compounds as sole carbon and energy sources under anaerobic conditions and used nitrate as an electron acceptor. The determination of 2,4D utilization at different concentrations showed that the utilization curve fitted well with the Edward model with the maximum degradation rate as 0.017?±?0.002 mM/day. The supplementation of cosubstrates (glucose, acetate, sucrose, humate and succinate) increased the degradation rates of all tested chemical substrates in both liquid and sediment slurry media. Thauera sp. strain DKT transformed 2,4D to 2,4-dichlorophenol (2,4DCP) through reductive side-chain removal then dechlorinated 2,4DCP to 2-chlorophenol (2CP), 4-chlorophenol (4CP) and phenol before complete degradation. The relative degradation rates by the isolate in liquid media were: phenol?>?2,4DCP?>?2CP?>?4CP?>?2,4D?≈?3CP. DKT augmentation in sediment slurry enhanced the degradation rates of 2,4D and chlorophenols. The anaerobic degradation rates in the slurry were significantly slower compared to the rates in liquid media.     

Wastewater treatment efficiency of a multi-media biological aerated filter (MBAF) containing clinoptilolite and bioceramsite in a brick-wall embedded design

A multi-media biological aerated filter (MBAF) with clinoptilolite media was used to treat synthetic wastewater. Coal ash bioceramsite with supplemental metallic iron was added to the clinoptilolite media of MBAFs in a brick-wall embedded design. Performance parameters, such as hydraulic, organic, N and P loading capacity and microbial community composition were studied for different quantity of supplemental metallic iron contained in three MBAFs. The MBAFs with more metallic iron were found to have superior hydraulic and organic loading, and higher N and P capacities. COD, NH₃-N and TP removal dropped by 7-10%, 6-7% and 4-5%, respectively, with when hydraulic loading was raised from 2.8 to 7.5 m³ m⁻² d⁻¹. NH₃-N removal also decreased 8-9% when ammonia loading was elevated from 0.078 to 0.156 kg NH₃-N m⁻³ d⁻¹. Real-time PCR revealed a relatively stable bacterial community composed primarily of eubacteria that formed after an initial 120 d operational period. Doubling the amount of metallic iron in the bioceramsite media resulted in a twofold increase of eubacteria in the MBAF, but a decrease in the ratio of anaerobic ammonia-oxidizing bacteria to total bacteria.     

Cloning and characterisation of a novel 2,4-dichlorophenol hydroxylase from a metagenomic library derived from polychlorinated biphenyl-contaminated soil

A novel 2,4-dichlorophenol hydroxylase (TfdB, EC 1.14.13.20) gene, designated as tfdB-JLU, was identified from a metagenome constructed from polychlorinated biphenyl-contaminated soil by functional screening and heterologously expressed in Escherichia coli. The deduced amino acid sequence of tfdB-JLU exhibited less than 48% homology with other known TfdBs. The enzyme exhibited a wider substrate spectrum than the previously reported TfdBs and higher relative activity towards ortho-substituted dichlorophenols, 2-chlorophenol, and 3-chlorophenol than towards 2,4-dichlorophenol, the preferred substrate of other known TfdBs. The enzyme had a K _m of 5 μM for 2,4-dichlorophenol and 6 μM for NADPH. The optimal temperature and pH of the enzyme were 25°C and 7.5, respectively. Activity of the purified TfdB-JLU was slightly enhanced by Ca²⁺, Mn²⁺, Co²⁺, and Fe²⁺, and completely inhibited by Cu²⁺, Hg²⁺, and Zn²⁺. This study is the first report to identify a novel TfdB from a metagenome.     

Comparison of RNA- and DNA-based bacterial communities in a lab-scale methane-degrading biocover

Methanotrophs must become established and active in a landfill biocover for successful methane oxidation. A lab-scale biocover with a soil mixture was operated for removal of methane and nonmethane volatile organic compounds, such as dimethyl sulfide (DMS), benzene (B), and toluene (T). The methane elimination capacity was 211?±?40 g?m^?2 d^?1 at inlet loads of 330–516 g?m^?2 d^?1. DMS, B, and T were completely removed at the bottom layer (40–50 cm) with inlet loads of 221.6?±?92.2, 99.6?±?19.5, and 23.4?±?4.9 mg m^?2 d^?1, respectively. The bacterial community was examined based on DNA and RNA using ribosomal tag pyrosequencing. Interestingly, methanotrophs comprised 80 % of the active community (RNA) while 29 % of the counterpart (DNA). Types I and II methanotrophs equally contributed to methane oxidation, and Methylobacter, Methylocaldum, and Methylocystis were dominant in both communities. The DNA vs. RNA comparison suggests that DNA-based analysis alone can lead to a significant underestimation of active members.     

Repetitive somatic embryogenesis in peanut cotyledon cultures by continual exposure to 2,4-d

Somatic embryos from immature cotyledons in peanut (Arachis hypogaea) were initiated on media supplemented with 2,4-dichlorophenoxyacetic acid (2,4-d). Over 90% primary embryogenesis and 41–46% repetitive embryogenesis were obtained 12 weeks after initiation by maintaining embryogenic cultures on medium containing 20 mg 1^-1 2,4-d. Maintenance of cultures on medium with 30 or 40 mg I^-1 2,4-d resulted in lower primary and secondary embryogenesis, and proliferation of nonembryogenic callus. Transfer of embryogenic cultures to a secondary medium with 10 or 20 mg I^-1 2,4-d significantly enhanced secondary embryogenesis compared to basal medium without the growth regulator. The use of Murashige & Skoog versus Finer's media had no significant effect on embryogenesis (85–95%), repetitive embryogenesis (11–37%) or mean embryo number. Secondary embryogenesis was also maintained for over one year by repeated subculture of isolated somatic embryos on medium with 20 mg I^-1 2,4-d.Abbreviations B5 Gamborg et al. medium (Gamborg et al. 1968) - 2,4-d 2,4-dichlorophenoxyacetic acid - FN Finer & Nagasawa medium (Finer & Nagasawa 1968) - MS Murashige & Skoog medium (Murashige & Skoog 1962)     

Improving hydrogen production in microbial electrolysis cells through hydraulic connection with thermoelectric generators

Using alternative power sources to drive hydrogen production in microbial electrolysis cells (MECs) is important to implementation of MEC technology. Herein, thermoelectric generators (TEG) were to power MECs using simulated waste heat. With the MEC anolyte as a cold source for TEG, current generation of the MEC increased to 2.46 ± 0.06 mA and hydrogen production reached 0.14 m³ m⁻³ d^-1, higher than those of the TEG-MEC system without hydraulic connection (1.16 ± 0.07 mA and 0.07 ± 0.01 m³ m⁻³ d^-1). A high recirculation rate of 30 mL min^-1 doubled both current generation and hydrogen production with 10 mL min^-1, benefited from a stronger cooling effect that increased the TEG voltage output. However, the optimal recirculation rate was determined as 20 mL min^-1 because of comparable performance but potentially less energy requirement. Reducing anolyte hydraulic retention time to 4 h has increased hydrogen production to 0.25 ± 0.05 m³ m⁻³ d^-1 but decreased organic removal efficiency to 69 ± 2%. Adding three more TEG units that captured more heat energy further enhanced hydrogen production to 0.36 m³ m⁻³ d^-1. Those results have demonstrated a successful integration of TEG with MEC through both electrical and hydraulic connections for simultaneous wastewater treatment and energy recovery.     

Nitrification of synthetic wastewater in a cross flow medium trickling filter

The nitrification performance of a synthetic wastewater was investigated in a cross flow medium trickling filter. The attachment of biomass on packing medium was studied. The reactor was operated continuously for three different hydraulic loading rates 5, 9 and 13?m³/m²?·?d under various influent ammonium concentrations. The maximum specific surface nitrification rate was about 1.21?g/m²?·?d. There was a good agreement between actual and predicted alkalinity consumption. The ammonium conversion along the filter depth was also investigated. The external surface of the nitrifying biofilm was captured by scanning electron microscope. The results are found to be satisfactory.     

Modeling of rotating biological contactor systems

An investigation of the rotating biological contractor (RBC) process variables to determine the efficiency of biological oxygen demand (BOD) removal is presented. Operating parameters including influent BOD content (<355 mg/liter), flow rate, disk surface area, hydraulic loading, disk rotational speed, liquid retention time, stage number, and wastewater temperature were evaluated. The BOD predictive model was developed using literature data with multiple regression analysis. This study shows that influent BOD concentration, hydraulic loading, stage number, and wastewater temperature are the most significant variables in predicting the RBC system performance. The model presently developed was verified by field data concerned with the treatment of both domestic and low-strength industrial wastewaters. Also, the results calculated by this model were compared to those obtained from Weng's model.     

Transport and metabolism of glucose and arabinose in Bifidobacterium breve

Glucose was required for the transport of arabinose into Bifidobacterium breve. The non-metabolisable glucose analogue 2-deoxy-d-glucose (2-DG) did not facilitate assimilation of arabinose. Studies using d-[U-¹⁴C]-labelled arabinose showed that it was fermented to pyruvate, formate, lactate and acetate, whereas the principal metabolic products of d-[U-¹⁴C]-labelled glucose were acetate and formate. In contrast to glucose, arabinose was not incorporated into cellular macromolecules. A variety of metabolic inhibitors and inhibitors of sugar transport (proton ionophores, metal ionophores, compounds associated with electron transport) were used to investigate the mechanisms of sugar uptake. Only NaF, an inhibitor of substrate level phosphorylation, and 2-DG inhibited glucose assimilation. 2-DG had no effect on arabinose uptake, but NaF was stimulatory. High levels of phosphorylation of glucose and 2-DG by PEP and to a lesser degree, ATP were seen in phosphoenolpyruvate: phosphotransferase (PEP:PTS) assays. These data together with strong inhibition of glucose uptake by NaF suggest a role for phosphorylation in the transport process. Arabinose uptake in B. breve was not directly dependent on phosphorylation or any other energy-linked form of transport but may be assimilated by glucose-dependent facilitated diffusion.Abbreviations (2,4-DNP) 2,4-dinitrophenol - (2,4-DNP) carbonylcyanide m-chlorophenylhydrazone - (CCCP) (phosphoenolpyruvate phosphotransferase system) - PEP: PTS trichloroacetic acid - (TCA) 2-deoxy-d-glucose - (2-DG) 2-deoxy-d-glucose     

Anaerobic pre-treatment of slaughterhouse wastewater using fixed-film reactors

The aim of this work was to study the performance of anaerobic fixed-film reactors with non-random support, for poultry slaughterhouse wastewater pre-treatment, including the influence of operating conditions. The work was carried out with two lab-scale reactors, one upflow and the other downflow, both equipped with vertical corrugated PVC tubes as support and a recirculation circuit. Both reactors were operated at 35°C.COD removal efficiencies ranging from 85% to 95% were achieved for organic loading rates of 8 kg COD m⁻³ d⁻¹, while the highest organic loading rates (35 kg COD m⁻³ d⁻¹) led to efficiencies of 55–75%. The reactors did not show destabilization after 12 h shock loads of 50 kg COD m⁻³ d⁻¹ .Reactor stability was easily achieved under intermittent operation, with weekend breaks, after which the reactors rapidly returned to their optimal performance. The influences of the hydraulic retention time, temperature, the recirculation ratio and flow direction were also studied.     

Dairy wastewater polluting load and treatment performances of an industrial three-cascade-reactor plant

An industrial three-cascade-reactor plant treating 45 m³ d^?1 of dairy wastewater (DW) was monitored for approx. one year to investigate the effect of variable daily influent loads. It removed more than 85% COD, NH₄-N and non-ionic and anionic surfactants from DW within the loads 7–24, 0.4–2.3, 0.4–0.7 and 0.1–0.5 kg d^?1, respectively; NH₄-N removal, in particular, was almost quantitative. Although the degradation of the above parameters below the lower load thresholds declined to 78.7, 87.5, 50.2 and 64.7%, respectively, their residual concentrations met effluent discharge standards. The biomass settling properties, assessed as sludge volume index (SVI), were satisfactory (generally lower than 150 ml g^?1) regardless of the organic load of the influent. The depletion of the pollutant load took mainly place in the first reactor albeit a significant contribution to the removal of the slowly degradable organic matter fraction was given by the two subsequent reactors.     

Factors controlling hydrochemical and trophic state variables in 86 shallow lakes in Europe

The total nitrogen mass balance was studied over a two-year period in two reservoirs located in south-eastern Poland. Theobserved nitrogen retention (N_ret) inthe reservoirs ranged from 0.18 to 2.12g m^-2d^-1 (mean 0.83g m^-2d^-1) in case of theRzeszów Reservoir and from -0.02 to0.41 g m^-2d^-1 (mean 0.13g m^-2d^-1) in case of the SolinaReservoir, while the percentage retention(N_ret) ranged from 7 to 42% (mean22%) and -15 to 73% (mean 35%),respectively. A good relationship betweenN_ret and nitrogen loads was observedfor the Solina Reservoir. The data obtainedby the author and collected from theresearches of 6 other reservoirs located insouthern Poland were used to develop anempirical model suitable to predict thenitrogen seasonal dynamics in a reservoir.The model allows for the determination ofboth the annual mean N_ret value andits seasonal variation in a reservoir.