Chemical Oxygen Demand Decay Research Articles

Nitrogen removal from poultry slaughterhouse wastewater in anaerobic-anoxic-aerobic combined reactor: Integrated effect of recirculation rate and hydraulic retention time

The aim of this work was to assess the nitrogen removal from slaughterhouse wastewater in an anaerobic-anoxic-aerobic combined reactor, evaluating the integrated effect of recirculation rate and hydraulic retention time. The recirculation of the liquid phase from the aerobic zone to the anoxic zone was applied to promote the denitrification through the use of endogenous electron donors. Three recirculation rates (R: 0.5, 1 and 2) and three hydraulic retention times (14, 11 and 8 h) were applied. The operation of the reactor was divided into 3 steps (I, II, and III) according to the factors evaluated (recirculation rate and HRT), to achieve operational conditions that would allow satisfactory performance in the different compartments of the reactor. During the experiment the reactor was fed with average total nitrogen (TN) and chemical oxygen demand (COD) of 65 mg L−1 and 580 mg L−1, respectively. The denitrification efficiency (theoretical) and kinetics parameters for COD decay were calculated. The highest performance was verified in the Step III (R = 2) and HRT of 11 h with NH4+ and TN removals of 84% and 65%, respectively. The TN removal efficiency (65%) was considered satisfactory, since the theoretical denitrification efficiency expected for this condition (R = 2) is 67%, without addition of an external carbon source. The lowest nitrification efficiency values were obtained in HRT of 8 h in the Step I and II (R = 0.5 and 1, respectively), indicating that the nitrification time (3 h - aerobic phase) may be the limiting factor in this HRT. The COD removal efficiency was high in all assays (>95%). The values of the kinetic degradation constants of organic matter were close for all recirculation rates, and the highest values were recorded for the HRT of 8 h and R = 1 and R = 2 (−0.48 and −0.43, respectively).

Electrochemical advanced oxidation discoloration and removal of three brown diazo dyes used in the tannery industry

Three different advanced oxidation processes, electrochemical oxidation (EO), electro-Fenton (EF) and photoelectro-Fenton (PEF) were tested for the treatment of individual and mixed solutions of Bismark Brown G, Bismark Brown R and Brown DGI dyes. The experiments were performed in a 500 mL stirred undivided cell with a 25 cm2 BDD anode, using 0.050 M Na2SO4 as electrolyte and pH 3.0. The cathodes used for the different processes were a stainless-steel plate in EO and another BDD electrode in EF and PEF. In the two latter processes, pumped air was bubbled for H2O2 generation and 0.50 mM Fe2+ was added as the Fenton's reagent. For PEF, the solution was irradiated using an UVA lamp. The oxidation power of the processes increased in the order: EO < EF < PEF. This trend is explained by the prevalent formation of hydroxyl radicals at the anode surface from water oxidation and in the bulk from Fenton's reaction. The best performance of PEF is considered to be related with the photolytic action of UVA. In all cases, the discoloration presents pseudo-first order kinetics. The performance of PEF treatment of individual dye solutions was improved by raising diazo dye concentration and current density. When PEF was tested with a mixture with 60 mg L−1 of each dye, 80%, 92% and 97% discoloration, and 58%, 76% and 95% chemical oxygen demand decay were found after 60 min using 20 mA cm−2. Small concentrations of succinic, malic and oxalic acids were detected at the end of the PEF process.

Study on Anodic Oxidation parameters for removal of pesticide Imidacloprid on a modified tantalum surface by lead dioxide film

&lt;p&gt;The commercial imidacloprid (IMD) insecticide [1-(6-chloro-3-pyridinyl) methyl-4,5-dihydro-N-nitro-1H-imidazole-2-amine] is widely used for the enhancement of crop production, but the intensive use of this insecticide has caused serious environmental problems. This work presents an electrochemical process for the removal of this insecticide using galvanostatic electrolysis at modified tantalum surface by lead dioxide film anode (Ta(PbO2)) anode. The electrolytic process was monitored by chemical oxygen demand (COD). The influence of operating parameters, such as current density, initial concentration of IMD, temperature and initial pH value was investigated. The COD decay follows a pseudo first-order kinetic and the process was under mass transport control. COD removal reach 97% when using an apparent current density of 100 mA cm−2, initial COD of 953 mg L−1 and at 25 °C after 4.5 h electrolysis time.&lt;/p&gt;&#x0D;

Organics deterioration from aqueous solutions by electro-Fenton process

A MnO2-graphite electrode was used in the treatment of aqueous solution from phenolic compounds by an electro-Fenton process. The effect of operating conditions, i.e. current density and externally added Fe2+, on the efficiency of the electro-Fenton Process performance was investigated. The performance of the electro-Fenton process was estimated by the chemical oxygen demand (COD) removal percentage from the treated solution after 6 h elector-oxidation at 30 °C. Significantly, the removal increased with increasing the applied current density and Fe2+ added, while the maximum removal percentage was achieved when 1mM Fe2+ added and 8mA/cm2 applied. Kinetic study of COD deterioration from the treated aqueous solution at the applied values of the operating parameters was carried out and the results were indicated that the COD decay over the MnO2-graphite anode by electro-Fenton process followed zero order kinetics. Current efficiency and specific power consumption of the process were also predicted.

Electrochemical degradation of Mordant Blue 13 azo dye using boron-doped diamond and dimensionally stable anodes: influence of experimental parameters and water matrix.

In this work, the electrooxidation as environmentally clean technology has been studied to the degradation of Mordant Blue 13 azo dye (MB13) using boron-doped diamond (p-Si/BDD) and oxide ruthenium titanium (Ti/Ru0.3Ti0.7O2 (DSA)) anodes in various water matrices: distilled water (DW), hot tap water (HTW), and simulated wastewaters with (SWS) and without surfactant (SW). The influence of experimental parameters, such as current density, initial dye concentration, electrolysis time/specific charge, and pH on the MB13 degradation rate, current efficiency, and energy consumption, has been determined. The enhanced rate of both color and chemical oxygen demand (COD) removal in sulfate aqueous solutions with BDD was observed, which indicates that sulfate (SO4-•) radicals along with •OH ones might be responsible for the degradation process. The MB13 decolorization process obeyed a pseudo-first-order reaction kinetics with the apparent rate constant from 7.36 × 10-2 min-1 to 4.39 × 10-1 min-1 for BDD and from 9.2 × 10-3 min-1 to 2.11 × 10-2 min-1 for DSA depending on the electrolysis conditions. The effect of water matrix on the decolorization and COD removal efficiency has been evaluated. Inorganic ions, mordant salt, and surfactant contained in simulated effluents decelerated the COD decay compared to DW and HTW for the both anodes; meanwhile, they differently affected the discoloration process. A comparison of the specific energy consumption for each electrocatalytic material under different experiment conditions has been made. The BDD electrode was more efficient than the DSA to oxidize the MB13 dye in all kinds of water.

Electrochemical advanced oxidation processes (EAOPs) as alternative treatment techniques for carwash wastewater reclamation

Electrochemical advanced oxidation processes such as electrooxidation (EO), electrooxidation with hydrogen peroxide generation (EO-H2O2) and electro-Fenton process (EF) have been investigated as alternative treatment techniques for complete removal of anionic surfactants and organic matters from real carwash wastewater. The electrochemical processes were performed with acidified real carwash wastewater using boron doped anode and carbon felt cathode. In all cases, the chemical oxygen demand (COD) removal efficiency was always increased with rise in applied current and complete organic matter decay was achieved at applied current of 500 mA or above after 6 h of electrolysis. Faster and higher COD decay was observed with EF compared to either EO or EO-H2O2 treatment, at all currents and electrolysis time. Besides, complete degradation of anionic surfactants – the major organic content of the wastewater could be achieved at all applied currents studied irrespective of the process used, indicating the efficacy of processes for total remediation of real carwash wastewater. The short-chain carboxylic acids formed as the final organic byproducts were identified and quantified by ion-exclusion chromatography. More so, lower energy consumption and higher current efficiency were achieved with EF compared to EO-H2O2. Electrochemical treatment was found to be a powerful technology for the complete abatement of organic matter in carwash wastewater for possible reuse.

Electrochemical technology for the treatment of real washing machine effluent at pre-pilot plant scale by using active and non-active anodes

Real effluent of washing machine is one of the major sources of microplastics and fabrics in municipal and surface water. The effluents constitute large volume of wastewater with different surfactants since every household and industries nowadays use washing machine rather than traditional hand washing.In this work, a real effluent of washing machine was treated by electrooxidation (EO) in a pre-pilot plant scale electrochemical flow reactor using active (Ti/Pt) or non-active (boron doped diamond (BDD)) anodes and Ti cathode. The effect of anode material (Ti/Pt or BDD) and the applied current density (16.6, 33.3 and 66.6 mA cm−2) on the decay of the organic matter, in terms of chemical oxygen demand (COD) during the electrochemical treatment was examined using the as-received effluent or by adding Na2SO4 as supporting electrolyte (as auxiliary in the conductivity). A decreased in organic matter (COD) was always observed when an increase on the applied current density was achieved at both anodes; however, BDD anode showed a well-defined trend as well as higher removal efficiency at all current densities studied when compared to Ti/Pt. Besides, faster and higher COD decay was attained by adding Na2SO4 to the real effluent compared to as-received effluent regardless of anode material used. Active chlorine species were also electrochemically produced at both anodes from the Cl− ions in the real effluent, contributing to the elimination of the pollutants. The mineralization of organic matter in the effluent was also determined, obtaining >90% of total organic carbon (TOC) reduction after 360 min with BDD anode at 33.3 and 66.6 mA cm−2. Based on the achieved results, EO is an efficient and effective treatment approach for the remediation of organic micropollutants in washing machine effluent especially with BDD anode.

Veratric acid removal from water by electrochemical oxidation on BDD anode

The efficiency of boron doped diamond (BDD) in the electrochemical treatment of synthetically contaminated water with veratric acid (VA), one kind of polyphenolic type compounds, is investigated in this work. A BDD electrode was practically fabricated using hot filament chemical vapor deposition (HFCVD). Later on, the BDD electrode was implemented as an anode in a batch electrolytic reactor. The effect of operating factors such as the initial concentration of VA, NaCl addition, and supporting electrolyte type (H2SO4, H3PO4 and Na2SO4) was studied. The chemical oxygen demand (COD) measurements were conducted to study the VA electrolysis kinetics. The experimental data suggested that sodium sulfate was the best supporting electrolyte as the COD removal reached a percentage of 100% using 1 mmol/dm3 as VA concentration. The kinetics of the COD decay of the VA electrolysis were found to obey the pseudo-first order model. Remarkably, the electrolysis process is significantly speeded up once chloride is added to the reaction. The complete COD removal was achieved in 60 minutes of treatment.

Anodic oxidation of slaughterhouse wastewater on boron-doped diamond: process variables effect.

A non-sacrificial boron-doped diamond electrode was prepared in the laboratory and used as a novel anode for electrochemical oxidation of poultry slaughterhouse wastewater. This wastewater poses environmental threats as it is characterized by a high content of recalcitrant organics. The influence of several process variables, applied current density, initial pH, supporting electrolyte nature, and concentration of electrocoagulant, on chemical oxygen demand (COD) removal, color removal, and turbidity removal was investigated. Results showed that raising the applied current density to 3.83 mA/cm2 has a positive effect on COD removal, color removal, and turbidity removal. These parameters increased to 100%, 90%, and 80% respectively. A low pH of 5 favored oxidants generation and consequently increased the COD removal percentage to reach 100%. Complete removal of COD had occurred in the presence of NaCl (1%) as supporting electrolyte. Na2SO4 demonstrated lower efficiency than NaCl in terms of COD removal. The COD decay kinetics follows the pseudo-first-order reaction. The simultaneous use of Na2SO4 and FeCl3 decreased the turbidity in wastewater by 98% due to electrocoagulation.

Electrochemical treatment of shrimp farming effluent: role of electrocatalytic material.

This study investigated the electrochemical oxidation of organic matter present in shrimp farming effluent using three types of electrocatalytic materials: Ti/Ru0.34Ti0.66O2, Ti/Pt, and boron-doped diamond (BDD). An electrochemical cell with 300mL under stirring agitation was used by applying 20, 40, and 60mAcm-2. A Ti/Ru0.34Ti0.66O2 anode showed a reduction of chemical oxygen demand (COD) about 84% after 1h of electrolysis, while at the same time, 71% of COD decay was achieved at Ti/Pt. Conversely, only 71% of COD was removed after 2h with a BDD anode. Regarding the temperature effect, BDD showed better performances than those achieved for Ti/Ru0.34Ti0.66O2 and Ti/Pt anodes during an electrochemical treatment of a shrimp farming effluent, obtaining 72% of COD removal by applying 20mAcm-2 at 40°C after 15min. Energy consumption and cost were estimated in order to established the engineering applicability of this alternative process.

Green electrochemical process for metronidazole degradation at BDD anode in aqueous solutions via direct and indirect oxidation

Abstract Anodic oxidation, an environmentally friendly process has been deeply investigated to remove the antibiotic metronidazole (MTZ), dissolved in synthetic wastewaters, via direct and indirect oxidation. Experiments were conducted by boron-doped diamond (BDD) anode in the absence and presence of sodium chloride (NaCl) in the solution. However, no studies have focused on the removal of MTZ using BDD anode or under chlorination process. The chemical oxygen demand (COD) measurement during the processing allowed the evaluation of the kinetics of organic matter decay and the instantaneous current efficiency (ICE). The results have shown that the degradation of MTZ is dependent on its initial concentration, pH, applied current density and NaCl concentration. COD decay follows a pseudo first-order kinetics, and the process was under mass transport control within the range studied, regardless of the experimental conditions. The electrochemical degradation is significantly accelerated by the presence of NaCl in the solution (94% COD removal in just 120 min).

Electrochemical behavior of Auramine O dye in aqueous solutions at lead dioxide electrode

Electrochemical behavior of Auramine O (AO) was studied by galvanostatic electrolysis at lead dioxide (Ta/PbO 2 ) anodes. The influence of operating parameters, such as current density, initial concentration of AO, temperature and initial pH value was investigated. The oxidation kinetics of the AO was followed by HPLC, UV / visible spectrophotometry and the measurement of chemical oxygen demand (COD). The experimental results showed that the electrochemical process was suitable for almost completely removing COD, due to the production of the hydroxyl radicals at the anode surface. The oxidation of AO is dependent on its initial concentration, current density, temperature and pH. COD decay follows a pseudo first-order kinetic and the process was under mass transport control. The HPLC analysis showed that the main AO oxidation products formed during electrolysis are the hydroquinone (HQ) representing the reduced form of benzoquinone (BQ) and carboxylic acids (CA). The proposed AO anodic oxidation into (BQ) is mainly based on an alternance of indirect monoelectronic discharge by the mediation of hydroxyl radicals and hydrolysis steps. The electrochemical oxidation of AO can be used as an economically adequate way for a pre-treatment of waters containing this dye followed by biological treatment.

Kinetic modeling of electrochemical oxidation of phenol on boron-doped diamond anode in the presence of some inorganic species

Phenolic wastewater from many industries contains inorganic species such as cyanide, sulfide, ammonium, thiocyanate, etc. Presence of these inorganic pollutants in phenolic wastewater could hinder the proper understanding of phenol oxidation kinetics when chemical oxygen demand (COD) decay data are used in the kinetic modeling, because these inorganic species exert oxygen demand as phenol. Unlike COD decay data, total organic carbon (TOC) decay data does not have any correlation with such inorganic species, and also available TOC measurements techniques are currently better in terms of automation, accuracy, and reliability compared with COD measurement techniques. In the present study, the kinetics of electrochemical oxidation of phenol on boron-doped diamond anodes in synthetic wastewater containing sulfide, cyanide, and ammonia species in single, binary, tertiary, and quaternary mix matrixes were modeled via employing current limiting modeling (CLM) using TOC in addition to traditional COD decay data. For all the different mix matrixes investigated, comparable trends describing the kinetics of electrochemical oxidation process were obtained for both the COD and the TOC decay models. Statistical diagnostics of the developed COD and TOC decay models parameters revealed that the operating regimes of the oxidation processes were accurately predicted to be limited by mass transfer. These results establish the suitability of using TOC decay for the CLM of phenol oxidation kinetics both in the presence and absence of the inorganic species. Based on these results in relation to the disparity between COD and TOC as parameters for evaluating phenol oxidation kinetics, it was concluded that TOC decay data are more appropriate compared with COD decay data for the CLM of kinetics of electrochemical oxidation of phenol in wastewater containing inorganic species that exert oxygen demand.

Performance and behaviour of planted and unplanted units of a horizontal subsurface flow constructed wetland system treating municipal effluent from a UASB reactor

A system composed of two horizontal subsurface flow constructed wetlands operating in parallel was evaluated for the post-treatment of UASB (upflow anaerobic sludge blanket) reactor effluent, for a population equivalent of 50 inhabitants per unit. One unit was planted with cattail (Typha latifolia) and the other was unplanted. The study was undertaken over a period of 4 years, comprising monitoring of influent and effluent constituents together with a full characterization of the behaviour of the units (tracer studies, mathematical modelling of chemical oxygen demand (COD) decay, characterization of solids in the filter medium). The mean value of the surface hydraulic load was 0.11 m(3)m(-2)d(-1), and the theoretical hydraulic retention time was 1.1 d in each unit. Using tracer tests with (82)Br, dispersion number (d) values of 0.084 and 0.079 for the planted and unplanted units were obtained, indicating low to moderate dispersion. The final effluent had excellent quality in terms of organic matter and suspended solids, but the system showed low capacity for nitrogen removal. Four-year mean effluent concentration values from the planted and unplanted units were, respectively: biochemical oxygen demand (BOD(5)): 25 and 23 mg L(-1); COD: 50 and 55 mg L(-1); total suspended solids (TSS): 9 and 9 mg L(-1); N-ammonia: 27 and 28 mg L(-1). The COD decay coefficient K for the traditional plug-flow model was 0.81 and 0.84 d(-1) for the planted and unplanted units. Around 80% of the total solids present in the filter medium were inorganic, and most of them were present in the interstices rather than attached to the support medium. As an overall conclusion, horizontal subsurface flow wetlands can be a very suitable post-treatment method for municipal effluents from anaerobic reactors.

First-order COD decay coefficients associated with different hydraulic models applied to planted and unplanted horizontal subsurface-flow constructed wetlands

A system comprising two parallel horizontal subsurface-flow constructed wetlands following a UASB (Upflow Anaerobic Sludge Blanket) reactor, treating municipal wastewater from the city of Belo Horizonte, Brazil, served as the basis for the evaluation of simple first-order kinetic performance models. One unit was planted (Typha latifolia) and the other was unplanted. Tracer (Br−) studies were undertaken and samples of filtered COD (chemical oxygen demand) were collected from the inlet, outlet and three intermediate points along the longitudinal length of the units. The following kinetic models were used and the related reaction coefficients were calculated: (i) plug-flow model; (ii) dispersed-flow model; and (iii) tanks-in-series model. For the three models, the following variants were analysed: without and with residual COD (C*), and without and with taking into account water losses due to evapotranspiration. In general, the dispersed-flow model and the tanks-in-series model, both adjusted for residual COD and incorporating water losses, were able to give the best predictions and led to the same reaction coefficients, which were also likely to best represent the actual first-order kinetic coefficients.

Electrochemical degradation of chlorpyrifos pesticide in aqueous solutions by anodic oxidation at boron-doped diamond electrodes

Laboratory experiments were carried out on the kinetic of the electrochemical degradation of aqueous solutions containing chlorpyrifos as model compounds of organophosphorus pesticides. The process was studied under galvanostatic polarization mode in acidic medium using boron-doped diamond (BDD) anodes and graphite carbon bar as cathode. The chemical oxygen demand (COD) measurement during the processing permitted the evaluation of the kinetic of organic matter decay and the instantaneous current efficiency. The results showed that the degradation of this pesticide is dependent on its initial concentration, current density and temperature. COD decay follows a pseudo first-order kinetic and the process was under mass transport control within the range studied, regardless of the experimental conditions. The removal rate of COD increases with applied current density until 20 mA cm −2 and decreases for higher values. The degradation rate increased significantly with increasing temperature and initial concentration of chlorpyrifos. The best obtained conditions for COD removal on the BDD anode to degrade chlorpyrifos solutions (COD(0) = 456 mg L −1) include operating at 20 mA cm −2 and 70 °C. This arrangement allows to completely degrade chlorpyrifos in just 6 h. Compared with PbO 2, BDD anode has shown more performance and rapidity in the COD removal in the same electrolysis device.

Degradation of paraquat herbicide by electrochemical advanced oxidation methods

Various electrochemical advanced oxidation processes (EAOPs) including anodic oxidation (AO), electro-Fenton (EF) and photoelectro-Fenton (PEF) methods were applied to study the degradation of paraquat (1,1′-dimethyl-4,4′-bipyridinium dichloride), a highly toxic and widely used herbicide, in aqueous acidic solution at pH 3.0. After initial optimization studies, EAOPs were compared to classic Fenton treatment (CFT) for their effectiveness at mineralizing the herbicide solution. Chemical oxygen demand (COD), parent compound and degradation intermediates were analyzed over the course of the treatment time. Results indicate that AO and CFT were the least efficient methods, yielding less than 64% and 51% of COD abatement, respectively. CFT exhibited the fastest initial removal rate, due to the rapid generation of OH during the first few minutes of treatment. On the other hand, PEF and EF displayed the most efficient treatments (yielding 97% and 94% COD decay, respectively), with PEF being slightly more efficient due to additional effect of UV-irradiation. Paraquat decay kinetics during EAOPs treatments were found to obey the pseudo-first order model very well and the absolute second-order rate constant with hydroxyl radicals was determined as ( k PQ = 2.55 × 10 9 M −1 s −1). Three aromatic degradation intermediates were formed and subsequently degraded, although not always completely, leading to short-chain carboxylic acids (oxalic, formic, acetic and succinic) and nitrate ions. It was finally concluded that only PEF and EF are sufficiently powerful and effective detoxification processes towards paraquat herbicide.

Degradation and Mineralization of Organic Pollutants Contained in Actual Pulp and Paper Mill Wastewaters by a UV/H2O2Process

In this work, UV/H2O2 treatment of actual pulp and paper mill wastewater (PPMW) was investigated. To determine optimal conditions, the effects of some experimental parameters (initial pH, initial H2O2 concentration, temperature, and organics load) on the UV/H2O2 process efficiency in terms of absorbance at 330 and 281 nm and chemical oxygen demand (COD) and total organic carbon (TOC) removals were studied. The results show that the UV/H2O2 system leads successfully to the almost-complete elimination of absorbance at 330 and 281 nm, COD, and TOC from an actual PPMW containing 270 mg of C L−1 under natural pH (pH 11.25) using 2.1 g of H2O2 L−1 at 28 °C. Under these optimal conditions, pseudo-first-order kinetics was determined for absorbance at 330 and 281 nm and COD decay with rate constants of 0.029, 0.02, and 0.008 min−1, respectively. Furthermore, the evolution with time of the H2O2 concentration exhibits two parts: a linear decrease to 300 min and then exponential change until the end of the treatment....

Phenol and substituted phenols AOPs remediation

The oxidation of phenol and two substituted species (4-nitrophenol and 4-chlorophenol) has been carried out by means of the O 3, UV–vis, O 3 + UV–vis, TiO 2 + UV–vis, O 3 + UV–vis + TiO 2 and O 3 + TiO 2 systems. From UV–vis experiments, the quantum yield of these organics has been calculated (0.018, 0.005 and 0.017 mol per Einstein for phenol, 4-nitrophenol and 4-chlorophenol, respectively). Broadly speaking, the addition of titania powder results in a slight inhibition of the parent compound degradation rate, although a positive effect is experienced when measuring the chemical oxygen demand (COD) and total organic carbon (TOC) removals. Amongst the technologies investigated, those combining ozone and radiation show the best efficiency in terms of phenols elimination and also COD and TOC decay rates. A simple economy analysis of the processes illustrates how the combinations O 3 + UV–vis and O 3 + UV–vis + TiO 2 are the most attractive technologies, although some additional considerations have to be taken into account.

Identification and modelling of dry weather processes in gully pots

This paper presents the results of a monitoring and modelling programme, carried out to study the processes occurring in gully pots during dry weather. The monitoring programme involved estimation of the change in gully pot liquor quality, under field and laboratory conditions. The contents (i.e. liquor and sludge) of pots draining five different types of roads were monitored over dry periods in the winter and summer. A bench scale study was carried out to study the influence of temperature variations and sludge digestion by-products on gully liquor quality. The change in quality was measured in terms of chemical oxygen demand (COD), dissolved oxygen (DO) and ammonium concentration. The trends for change in COD and DO, were found to be broadly similar for all road types. However, ammonium transformation was found to follow different patterns at different locations. Several dry weather processes, such as COD decay, ammonium transformation, oxygen depletion and uptake by sludge, oxygen transfer from the atmosphere and benthic release of COD, have been identified. Other processes, found to take place during dry weather, include development of a scum layer over the surface of the gully liquor and sludge bulking. A model is proposed which can predict the change in gully liquor quality in terms of COD, DO and ammonium concentrations. Data collected during the study has been used to successfully calibrate and verify the model.