Minimum Hydraulic Retention Time Research Articles

Optimizing nitrogen removal in advanced wastewater treatment using biological aerated filters.

Reducing total nitrogen (TN) presents a significant challenge for numerous wastewater treatment facilities. In order to address this issue, the current study employed a biological aerated filter for the treatment of wastewater containing low nitrogen concentrations. Both lab-scale and pilot-scale biofilters were constructed to investigate the denitrification performance and maximum denitrification load. The findings indicated that the anaerobic denitrification process of established biofilm adhered to pseudo-first-order kinetics. The results of batch testing and continuous-flow experiments confirmed that the minimum hydraulic retention time (HRT) required for mature biofilm was determined to be 0.5h. The optimal operating parameters were found to be as follows: influent NO3 --N concentration of 25mg/L, HRT of 0.5h, resulting in effluent TN levels below 1mg/L. Under these conditions, the denitrifying load for the lab-scale I-BAF system was calculated to be 1.26kg (TN)/(m3·d). Furthermore, it was observed that the maximum denitrifying load could reach 2.2kg (TN)/(m3·d) when the influent NO3 --N concentration was increased to 50mg/L while maintaining an HRT of 0.5h. For the mature biofilter, the appropriate HRT ranged from 2h to 0.5h. Microbial diversity analysis revealed that the genus Enterobacter was dominant in all denitrification systems, followed by Comamonas and Rhodococcus. The operational parameters described in the paper could be recommended for a full-scale wastewater treatment facility.

Efficacy of a Fine Bubble Diffuser in Enhancing Attached Biofilm Hydrogenotrophic Denitrification Reactor Performance

The efficacy of a fine bubble (FB) diffuser in enhancing the performance of an attached biofilm hydrogenotrophic denitrification (HD) reactor was evaluated. HD reactors equipped with an FB diffuser (FB reactor) and an air stone (AS) diffuser that produced ordinary bubbles (AS reactor) were operated in parallel at different hydraulic retention times (HRTs) in a synthetic groundwater treatment experiment. A reduction in H2 consumption of approximately 77% was achieved using the FB diffuser to reach a gas-liquid mass transfer coefficient similar to that of the AS diffuser. The high gas dissolution efficiency of the FB diffuser resulted in an effective nitrogen removal rate (NRR) enhancement, requiring less H2 supply. The highest value of NRR at 53.0±9.8 g-N/m3/d was obtained in the FB reactor at a minimum HRT of 3 h, which was two-fold higher than the corresponding value from the AS reactor. The FB reactor also had the lowest requirement of H2 for denitrification reaching 0.1 m3-H2/g-N in this condition, which was ten-fold lower than that in the AS reactor. Furthermore, the suspended sludge concentration in the FB reactor was lower than that in the AS reactor, indicating that the application of the FB diffuser can minimize excess suspended sludge accumulation inside the HD reactor. Microbial community analysis showed the predominance of Thauera spp. reaching a relative abundance of 15.7–27.3% in the FB reactor, suggesting a contribution to the HD. This finding can provide insight into the application of the FB diffuser for optimizing nitrate-contaminated groundwater treatment technology by HD.

Expanding mechanistic models to represent purple phototrophic bacteria enriched cultures growing outdoors

The economic feasibility of purple phototrophic bacteria (PPB) for resource recovery relies on using enriched-mixed cultures and sunlight. This work presents an extended Photo-Anaerobic Model (ePAnM), considering: (i) the diverse metabolic capabilities of PPB, (ii) microbial clades interacting with PPB, and (iii) varying environmental conditions. Key kinetic and stoichiometric parameters were either determined experimentally (with dedicated tests), calculated, or gathered from literature. The model was calibrated and validated using different datasets from an outdoors demonstration-scale reactor, as well as results from aerobic and anaerobic batch tests. The ePAnM was able to predict the concentrations of key compounds/components (e.g., COD, volatile fatty acids, and nutrients), as well as microbial communities (with anaerobic systems dominated by fermenters and PPB). The results underlined the importance of considering other microbial clades and varying environmental conditions. The model predicted a minimum hydraulic retention time of 0.5 d−1. A maximum width of 10 cm in flat plate reactors should not be exceeded. Simulations showed the potential of a combined day-anaerobic/night-aerobic operational strategy to allow efficient continuous operation.

Anaerobic domestic wastewater treatment in a sequencing granular UASB bioreactor: Feasibility study of the temperature effect on the process performance

A laboratory-scale sequencing granular up-flow anaerobic sludge blanket (UASB) reactor was applied for the treatment of synthetic domestic wastewater at 15, 25 and 35 °C. The experiments were performed at different hydraulic retention times (gradually decreasing from a maximum of 22 h to a minimum of 9 h). Results at 25 and 35 °C showed similar COD removal efficiency and specific biogas production in the range of 84–94% and 0.14–0.27 m3/kgCODremoved, respectively. At 15 °C lower COD removal kinetics were observed, and a minimum hydraulic retention time of 14 h was necessary to obtain an effluent in accordance with current Italian legislation. At all temperatures, high quality effluent was achieved in terms of total suspended solids (TSS) concentration, while nitrogen and phosphorus exceeded the Italian law limits, as they are not removed during the anaerobic digestion process. Moreover, dissolved methane in the liquid phase was analysed: methane lost with the effluents was on average ≈ 37% at 25 °C and ≈ 24% at 35 °C of the produced amount, thus revealing higher losses at lower temperatures for the increase of gas solubility. These results show the need of research on technologies aimed at removing or recovering such energetic greenhouse gas. However, effective biogas production and potentialities for nutrients’ recovery demonstrated in this study confirm the feasibility of the anaerobic process as sustainable treatment of low-strength wastewaters.

Treatment of agro-food industrial waste streams using osmotic microbial fuel cells: Performance and potential improvement measures

Osmotic microbial fuel cell (OsMFC) is an emerging wastewater treatment technology which incorporates multiple functions such as bioelectricity generation, organic substrate removal, and wastewater reclamation. This study firstly reported the performance and potential improvement measures of OsMFCs in treating selected agro-food industrial waste streams, i.e., brewery wastewater (BW), sweetener process wastewater (ST), and swine wastewater (SW). BW and SW wastewaters with good biodegradability, showed high power density (11.1 and 9.3 W/m 3 ) and soluble chemical oxidation demand (sCOD) removal (89.2 ± 4.1 and 70.9 ± 3.8%) in OsMFCs. In contrast, OsMFC showed low performance with ST wastewater despite its high sCOD load. Possibilities in improving the treatment performance through modifying the operational and process settings were further explored. Shortening anode hydraulic retention time (HRT) was able to improve the bioelectricity generation and water production, whereas slightly compensated the sCOD and nutrient removal efficiencies. However, this approach showed limited effect in enhancing the ST-OsMFC. A pre-fermentation process with a minimum HRT of approximately 20 h was recommended to enhance the treatability of ST wastewater by previously converting the complex organic substrates in the raw water into volatile fatty acid (VFA). • First report in OsMFC treating industrial wastewater. • Treatment of different agro-food industrial waste streams by OsMFC was demonstrated. • An anode HRT of 9 h is recommended for OsMFC treating selected agro-food waste streams. • Pre-fermentation is an option to improve the treatability of sweetener wastewater in OsMFC.

Performance of AnMBRs treating low strength wastewater with different carbon sources at decreasing HRTs and its linkage to Methanosaeta with high specific affinity

Minimum hydraulic retention time needs to be considered to avoid methanogenesis inhibition for complex substrates in anaerobic membrane bioreactor for low-strength wastewater treatment.

Thermophilic anaerobic digestion of olive mill wastewater in an upflow packed bed reactor: Evaluation of 16S rRNA amplicon sequencing for microbial analysis

Olive mill wastewater, a by-product of olive oil production after the operation of three-phase decanters, was used in a thermophilic anaerobic digester targeting efficient bioconversion of its organic load into biogas. An active anaerobic inoculum originating from a mesophilic reactor, was acclimatized under thermophilic conditions and was filled into a high-rate upflow packed bed reactor. Its performance was tested towards the treatment efficacy of olive mill wastewater under thermophilic conditions reaching the minimum hydraulic retention time of 4.2 d with promising results. As analysis of the microbial communities is considered to be the key for the development of anaerobic digestion optimization techniques, the present work focused on characterizing the microbial community and its variation during the reactor's runs, via 16S rRNA amplicon sequencing. Identification of new microbial species and taxonomic groups determination is of paramount importance as these representatives determine the bioprocess outcome. The current study results may contribute to further olive mill wastewater exploitation as a potential source for efficient biogas production.

Effect of Hydraulic Retention Time (HRT) and Organic Loading Rate (OLR) to the Nata de coco Anaerobic Treatment Eficiency and its Wastewater Characteristics

In this study, experiments were conducted to investigate the effect of hydraulic retention time (HRT) and organic loading rate (OLR) on process stability of nata de coco wastewater anaerobic treatment using semi-continuous digester. The standard-rate anaerobic digester with working volume of 8.5 L was used to investigate the effect of three different hydraulic retention times (15, 20, and 25 days), and a standard-rate anaerobic digester with working volume of 9.1 L was operated at different organic loading rates of 0.5, 1, and 1.5 g/L/day. The findings revealed that minimum HRT for nata de coco wastewater anaerobic treatment using semi-continuous digester was achieved at HRT 20 days. Based on data from this study, the reduction of organic content in nata de coco wastewater increased when OLR increased until 1 g/L/day. But then those parameters value decreased when OLR being increased further to 1.5 g/L/day. It showed that at 1.5 g/L/day the amount of substrate fed to the system was exceeding the total degradation capacity of methanogenic microorganisms, hence the organic overload happened and decreased the efficiency of organic content reduction in anaerobic treatment of nata de coco wastewater.

Treatment of synthetic refinery wastewater in anoxic–aerobic sequential moving bed reactors and sulphur recovery

ABSTRACTObjective of the present study was to simultaneously biodegrade synthetic petroleum refinery wastewater containing phenol (750 mg/L), sulphide (750 mg/L), hydrocarbon (as emulsified diesel of 300 mg/L), ammonia-nitrogen (350 mg/L) at pH >9 in anoxic–aerobic sequential moving bed reactors. The optimum mixing speed of anoxic reactor was observed at 20 rpm and beyond that, removal rate remained constant. In anoxic reactor the minimum hydraulic retention time was observed to be 2 days for complete removal of sulphide, 40–50% removal of phenol and total hydrocarbons and 52% of sulphur recovery. The optimum HRT of aerobic moving bed reactor was observed as 16 h (total HRT of 64 h for anoxic and aerobic reactors) for complete removals of phenol, total hydrocarbons, COD (chemical oxygen demand) and ammonia-nitrogen with nitrification.

Biomethanization of the Mixture of Cattle Manure, Pig Manure and Poultry Manure in Co-Digestion with Waste Peels of Pineapple Fruit and Content of Chicken-Gizzard - Part II: Optimization of Process Variables

Background: The indiscriminate discharge of industrial waste, agricultural-biomass waste, waste of municipal, domestic and kitchen waste has negatively impacted on the environment and human health. It is very pertinent to reduce these impacts to the barest minimum through conversion of the waste to useful products. The conversion of these wastes to generate alternative energy to fossil fuel through the technology of anaerobic fermentation is one of the viable and more fascinating options for the management of waste. Objective: To investigate the interactive effect and optimization of process parameters of temperature, total solid content and feed/inoculum ratio on the biomethanization of the mixture of cattle manure, pig manure and poultry manure in co-digestion with waste peels of pineapple fruit and content of chicken-gizzard. Method: Full-factorial central composite design of experiment (RCCD) of the response surface method (RSM) was adopted to assess the possible interactive effects of the process variables and the optimal parameters (i.e. optimization) for biogas/biomethane production in an anaerobic digester. Result: The process variables had a significant (P < 0.05) positive and negative interactive effect on the biomethanization process. A second-order quadratic polynomial regression model which is statistically significant (p < 0.0001) was respectively obtained for cumulative biogas yield (CBY), biomethane content (BC) and hydraulic retention time (HRT). Temperature of 55.2°C; total solid content of 6.25%; and feed/inoculums ratio of 1:2 were found to be the optimum values required to attain a predicted optimum values of 6.261 dm3/g CBY, 71.54% BC within a minimum 8 days of HRT. At this optimum process conditions, the experimental observed maximum CBY, and BC with a minimum HRT were found to be 6.217 dm3/g of slurry, 71.10% and 7 days, respectively. Conclusion: Biogas/biomethane generation through co-substrate anaerobic fermentation of animal waste with waste of fruits constitute a reasonable and applicable renewable energy alternative and this can be optimized through response surface methodology.

Comparative column testing of three reactive mixtures for the bio-chemical treatment of iron-rich acid mine drainage

Optimization of the hydraulic properties of reactive mixtures in passive systems can improve treatment efficiency of iron-rich acid mine drainage (Fe-rich AMD). The use of highly permeable and porous substrates could limit clogging and flow-related issues. The efficiency and evolution of hydraulic conductivity (ksat) of three reactive mixtures – two types of dispersed alkaline substrate (DAS), composed of wood ash (50% v/v – WA50) or calcite (20% v/v – C20), and one mixture consisting mainly of organic matter (70% w/w), typically used in passive biochemical reactors (PBRs) – were tested in 1.5L columns.The reactors were operated with hydraulic retention times (HRTs) of 1 to 5 d over a period of 16–63days. In the WA-DAS reactors, results showed that a minimum HRT of 2 d was required to remove 33–62% of Fe in AMD with 2500mg/L Fe. The calcite-DAS showed limited Fe removal (<10%) in AMD at >1500mg/L Fe, but was around 40% at <1500mg/L Fe, over a 7-day period, at 2 d of HRT. Slight increase of the PBRs efficiency was found (77% and 91%), at initial Fe concentration of 500mg/L, when the HRT was doubled (from 2.5 d to 5 d). All reactors removed other metals (37–99.9% for Al, Zn, and Pb; 20–98% for Ni, except in C20) and SO42− (5–37%). Evolution of the hydraulic parameters of all reactors showed insignificant variation of the initial porosity of 0.68–0.74 and ksat around 10−2cm/s, indicating no evidence of clogging throughout the testing period, even at 5 d of HRT. Nonetheless, Fe removal was HRT-dependent. Therefore, water quality, especially Fe concentration, should be among the design criteria for a long-term satisfactory treatment of Fe-rich AMD.

Application of the activated sludge model to aerated lagoons

The different kinds of aerated lagoons, which exclude anaerobic pre-treatment ponds, are described and the design approach for aerated lagoons is explained. This hinges around ensuring that the 1st lagoon is suspension mixed and the second and any additional are facultative. Selection of the retention time for the 1st lagoon is important to ensure complete utilization of the influent biodegradable organics. Minimum retention times to achieve this at 14 degreesC and 22 degreesC were determined with the general activated sludge kinetic simulation model for (i) readily biodegradable soluble organics (BSO) only, (ii) slowly biodegradable particulate organics (BPO) only, (iii) real municipal wastewater (20% BSO and 80% BPO) and (iv) real municipal wastewater with 5% OHO active VSS mass seed. The minimum hydraulic retention times for these four cases are: at 14 degreesC 1.3, 3.0, 2.0 and 1.5 d, respectively, and at 22 degreesC 0.3, 2.0, 1.2 and 1.0 d, respectively. From a comparison of the simulation results with the steady-state model calculations, washout of OHOs takes place at about 75% of these retention times. Approximate equations to estimate the power requirements for aeration by mechanical surface aerators and mixing are given. These equations are combined with those of the steady-state activated sludge lagoon model for calculating the oxygen requirements and the aeration power density (W/m3) in each lagoon. With these equations, it is shown that influent COD concentration needs to be between an upper and lower limit band to ensure that the 1st lagoon is suspension mixed and the second lagoon is facultative. This COD concentration band decreases as the influent flow increases. The important conclusion arising from this is that if the aerated lagoon system is applied for small rural communities, where land for these large systems is likely to be available, then additional mixing energy above that for aeration will need to be provided to ensure that the 1st lagoon is suspension mixed - this additional aeration cost makes it unlikely that aerated lagoons will be applied for municipal wastewater treatment. Matching mixing and aeration power requirements for industrial organic wastewaters is easier because these usually are significantly stronger than municipal wastewaters.

Feasibility of phosphate precipitation from digested anaerobic sludge in a continuous aerated reactor

AbstractThis study investigates the effectiveness of a complete stirred and aerated reactor for deliberate phosphate precipitation as struvite from anaerobic sludge in a domestic wastewater treatment plant and, to achieve the optimum operational conditions in order to reduce the potential formation of struvite downstream of the digester and decrease the phosphate (P) and nitrogen (N) load on the treatment plant. The reactor was fed with anaerobic sludge and operated at different hydraulic retention times (HRT) and aeration rates. Theoretical calculations indicated that a minimum pH of 7.5 would be necessary to attain struvite precipitation to its potential. To reach this pH value, a minimum HRT of 1.0 h with aeration flow rate of 46.7 m3 air/h/m3 reactor was necessary. Struvite precipitation kinetics were observed to be much faster than other minerals that can precipitate and a decrease in HRT promotes a precipitate richer in struvite. An increase in the HRT to 2.0 h, for the same aeration rate led to a h...

Biomethanization of sugar beet byproduct by semi-continuous single digestion and co-digestion with cow manure

Dried pellet of exhausted sugar beet cossettes were digested alone and combined with cow manure as co-substrate in a mesophilic semi-continuous anaerobic system. In single digestion assay, the stable biogas production and stable reactor operation was observed at the hydraulic retention time (HRT) of 20days (OLR: 3.26gVS/Lreactord) which was the minimum HRT tolerated by the system. However, co-digestion with cow manure allowed to decrease the HRT until 15days (OLR: 4.97gVS/Lreactord) with 32% higher biogas generation and efficient reactor operation. Propionic acid was the predominant VFA observed during single digestion assay failure, while acetic acid accumulation was observed in the co-digestion assay. In both single and co-digestion assays, the recovery of digesters was possible by ceasing the feeding and re-inoculation with a well-adapted inoculum.

Full scale evaluation of combined sewer overflows disinfection using performic acid in a sea-outfall pipe

Pollution of surface waters with pathogens from combined sewer overflows limits recreational use of surface waters. Large retention basins are a satisfactory solution but they are rarely sufficient for economic or space reasons. Fast disinfection during the overflow is an alternative, but few methods are known and each has problems. This work evaluated for the first time the full-scale disinfection using performic acid by the removal of the two currently regulated indicator bacteria for bathing water quality, Escherichia coli and Enterococcus. Experiments were performed at a sewage bypass through a sea-outfall pipe with a minimum hydraulic retention time of 24min.The disinfection efficiency in the field was measured by analyzing samples taken before and after the treatment. Samples were also treated with performic acid in the laboratory to measure the disinfection effectiveness and kinetic of degradation of performic acid. Doses of 1–8ppm of performic acid achieved 1.0–3.5 log removal of E. coli and 1.0–2.44 log removal of Enterococcus in the field, but were somewhat higher in laboratory conditions at 1.69–4.38 and 1.0–4.27 log units, respectively. Studies of the degradation of performic acid in collected real samples showed more than 50% was degraded in 20min, and mostly degraded by 120min. Comparison of field and laboratory dosed samples detected that performic acid synthesis did not start in one event and clogging of the sampler in another event.Overall the tests showed that the treatment was successful but it is indicated that online control could benefit treatments efficiency.

Application of QUAL2K model to assess ecological purification technology for a polluted river.

Industrialization and urbanization have caused water pollution and ecosystem degradation, especially in urban canals and rivers in China; accordingly, effective water quality improvement programs are needed. In this study, the Tianlai River in Jiangsu, China was taken as a research site, and a combination of ecological purification technologies consisting of biological rope, phytoremediation, and activated carbon were applied in a laboratory-scale study to examine degradation coefficients under dynamic water conditions. Coefficients were then input into the QUAL2K model to simulate various hypothetical scenarios and determine the minimum density of ecological purification combination and hydraulic retention time (HRT) to meet Grade V or IV of the China standard for surface water. The minimum densities for Grade V and IV were 1.6 times and 2 times the experimental density, while the minimum HRTs for Grade V and IV were 2.4 day and 3 day. The results of this study should provide a practical and efficient design method for ecological purification programs.

Enhanced Electrolytic Removal of Ammonia from the Aqueous Phase with a Zeolite-Packed Electrolysis Reactor under a Continuous Mode

This research investigated the mechanism and effect of influencing factors for aqueous ammonia removal by electrolysis in continuous mode with a zeolite-packed electrolysis reactor. Under the conditions of 30-min hydraulic retention time (HRT), 1.8-A current, and 300-mg/L chloride ion concentration, the ammonia concentration decreased from 25 to 0.3 mg N/L within 1.5 h electrolysis time. A total of 4.4 mg N/L nitrate, 0.03 mg N/L nitrite, and 0.3–0.8 mg N/L chloramines were detected in effluent, which accounted for 76% removal of total nitrogen from the aqueous phase. Moreover, the ammonia-presaturated zeolite was almost completely regenerated during the same time period. HRT, current, and chloride ion concentration were found to have a significant effect on ammonia removal as well as zeolite regeneration. Whereas other conditions were fixed, a minimum of 10-min HRT, 1.0-A current, and 210-mg/L chloride ion concentration were required to keep the effluent ammonia concentration below 1 mg N/L. For the treatment of municipal wastewater, minimum HRTs of 7.5, 11, and 21 min were suggested for initial ammonia concentrations of 13.5, 30.2, and 53.5 mg N/L under the conditions of 300-mg/L chloride and 1.8-A current.

Effects of the reduction of the hydraulic retention time to 1.5 days at constant organic loading in CSTR, ASBR, and fixed-bed reactors – Performance and methanogenic community composition

The hydraulic retention time (HRT) is one of the key parameters in biogas processes and often it is postulated that a minimum HRT of 10–25 days is obligatory in continuous stirred tank reactors (CSTR) to prevent a washout of slow growing methanogens. In this study the effects of the reduction of the HRT from 6 to 1.5 days on performance and methanogenic community composition in different systems with and without immobilization operated with simulated thin stillage (STS) at mesophilic conditions and constant organic loading rates (OLR) of 10 g L−1d−1 of volatile solids were investigated. With the reduction of the HRT process instability was first observed in the anaerobic sequencing batch reactor (ASBR) (at HRT of 3 days) followed by the CSTR (at HRT of 2 days). The fixed bed reactor (FBR) was stable until the end of the experiment, but the reduction of the HRT to 1.5 days caused a decrease of the specific biogas production to about 450 L kg−1 of VS compared to about 600 L kg−1 of VS at HRTs of 4–5 days. Methanoculleus and Methanosarcina were the dominant genera under stable process conditions in the CSTR and the ASBR and members of Methanosaeta and Methanospirillum were only present at HRT of 4 days and lower. In the effluent of the FBR Methanosarcina spp. were not detected and Methanosaeta spp. were more abundant then in the other reactors.

Investigating the life-cycle and growth rate of Pediastrum boryanum and the implications for wastewater treatment high rate algal ponds

The colonial alga Pediastrum boryanum has beneficial characteristics for wastewater treatment High Rate Algal Ponds (HRAP) including high biomass productivity and settleability. Our previous work has shown that these characteristics are enhanced when a portion of gravity harvested algae is recycled back to the pond. To help understand the mechanisms behind the improved performance of P. boryanum dominated HRAP with algal recycling, this study investigated the life-cycle of P. boryanum. Experiments determined the exact timing and growth rate of P. boryanum life-cycle stages (‘juvenile’, ‘growth’ and ‘reproductive’) under four combinations of light and temperature (250 or 120 μMol/m2/s; 20 or 10 °C). Single juvenile 16-celled colonies were grown in microcosms on an inverted microscope and photographed every 15 min until reproduction ceased. Two asexual life-cycles and a rarely occurring sexual life-cycle were observed. The time required to achieve asexual reproductive maturity increased from 52 h (high light and temperature) to 307 h (low light and temperature), indicating that the minimum hydraulic retention time or mean cell residence time (MCRT) must be higher than these values to sustain a P. boryanum HRAP culture under ambient conditions. The net growth rate of a P. boryanum colony varied between life-cycle stages (growth > juvenile > reproductive). This suggests that the higher biomass productivity measured in HRAP with algal recycling could be due to both the increased MCRT and an increase in the net growth rate of the HRAP culture by ‘seeding’ with faster growing colonies.

Biological oxygen demand as a tool to predict membrane bioreactor best operating conditions for a photo-Fenton pretreated toxic wastewater

BACKGROUND Coupled systems of photocatalytic and biological oxidation processes for the treatment of non-biodegradable wastewater have been widely studied. Additionally, the use of membrane bioreactors (MBR) can improve the coupled system since the photocatalytic pretreatment can be shortened. Optimization of the combined process involves adjusting the intensity of the photocatalytic pretreatment to minimize the hydraulic retention time (HRT) in MBRs thus ensuring the shortest total time possible. The present study presents a new fast and easy assay based on biological oxygen demand (BOD) measurement to predict the optimum HRT in the MBR for a photo-Fenton pretreated effluent. RESULTS A non-biodegradable wastewater was mineralized by photo-Fenton to 40% and 57%. Both pretreated effluents were further treated in a MBR and the minimum HRTs were experimentally determined, being 7.7 h for both 40% and 57% mineralization. The BOD based tool comprises the parameters α' and β' related to the biodegradation rate and the effluent quality, respectively. The estimated HRTs were 7.5 h for both 40% and 57% mineralization, confirming the results obtained in the coupled system. CONCLUSION The use of the BOD based tool allowed the correct HRT values to be obtained in order to achieve the maximum treatment capacity of the system maintaining the best effluent quality. © 2013 Society of Chemical Industry