High Hydraulic Retention Time Research Articles

Hydraulic retention time affects bacterial community structure in an As-rich acid mine drainage (AMD) biotreatment process.

Arsenic removal consecutive to biological iron oxidation and precipitation is an effective process for treating As-rich acid mine drainage (AMD). We studied the effect of hydraulic retention time (HRT)-from 74 to 456min-in a bench-scale bioreactor exploiting such process. The treatment efficiency was monitored during 19days, and the final mineralogy and bacterial communities of the biogenic precipitates were characterized by X-ray absorption spectroscopy and high-throughput 16S rRNA gene sequencing. The percentage of Fe(II) oxidation (10-47%) and As removal (19-37%) increased with increasing HRT. Arsenic was trapped in the biogenic precipitates as As(III)-bearing schwertmannite and amorphous ferric arsenate, with a decrease of As/Fe ratio with increasing HRT. The bacterial community in the biogenic precipitate was dominated by Fe-oxidizing bacteria whatever the HRT. The proportion of Gallionella and Ferrovum genera shifted from respectively 65 and 12% at low HRT to 23 and 51% at high HRT, in relation with physicochemical changes in the treated water. aioA genes and Thiomonas genus were detected at all HRT although As(III) oxidation was not evidenced. To our knowledge, this is the first evidence of the role of HRT as a driver of bacterial community structure in bioreactors exploiting microbial Fe(II) oxidation for AMD treatment.

Lessons learnt from a pilot study on residual dye removal by an aerated treatment wetland

Treatment wetlands (TWs) have shown good capacity in dye removal from textile wastewater. However, the high hydraulic retention times (HRTs) required by these solutions and the connected high area requirements, remain a big drawback towards the application of TWs for dye treatment at full scale. Aerated TWs are interesting intensified solutions that attempt to reduce the TW required area. Therefore, an aerated CW pilot plant, composed of a 20 m2 horizontal subsurface flow TW (HF) and a 21 m2 Free Water System (FWS), equipped with aeration pipelines, was built and monitored to investigate the potential reduction of required area for dye removal from the effluent wastewater of a centralized wastewater treatment plant (WWTP). During a 8 months long study, experimenting with different hydraulic retention times (HRTs – 1.2, 2.6 and 3.5 days) and aeration modes (intermittent and continuous), the pilot plant has shown a normal biological degradation for organic matter and nutrients, while the residual dye removal has been very low, as demonstrated by the absorbance measure at three wavelengths: at 426 nm (blue) the removal varies from −55% at influent absorbance of 0.010 to 41% at 0.060; at 558 nm (yellow) the removal is negative at 0.005 (−58%) and high at higher influent concentrations (72% at 0.035 of absorbance for the inlet); at 660 nm (red) −82% of removal efficiency was obtained at influent absorbance of 0.002 and 74% at 0.010. These results are a consequence of the biological oxidation processes taking place in the WWTP, so that the residual dye seems to be resistant to further aerobic degradation. Therefore, TWs enhanced by aeration can provide only a buffer effect on peak dye concentrations.

Bioaugmentation of membrane bioreactor with Achromobacter denitrificans strain PR1 for enhanced sulfamethoxazole removal in wastewater

Achromobacter denitrificans strain PR1, previously found to harbour specific degradation pathways with high sulfamethoxazole (SMX) degradation rates, was bioaugmented into laboratory-scale membrane bioreactors (MBRs) operated under aerobic conditions to treat SMX-containing real domestic wastewater. Different hydraulic retention times (HRTs), which is related to reaction time and loading rates, were considered and found to affect the SMX removal efficiency. The availability of primary substrates was important in both bioaugmented and non-bioaugmented activated sludge (AS) for cometabolism of SMX. High HRT (24 h) resulted in low food to microorganism ratio (F/M) and low SMX removal, due to substrate limitation. Decrease in HRT from 24 h to 12 h, 6 h and finally 4 h led to gradual increases in primary substrates availability, e.g. organic compounds and ammonia, resulted in increased SMX removal efficiency and degradation rate, and is more favorable for high-rate wastewater treatment processes. After inoculation into the MBRs, the bioaugmentation strain was sustained in the reactor for a maximum of 31 days even though a significant decrease in abundance was observed. The bioaugmented MBRs showed enhanced SMX removal, especially under SMX shock loads compared to the control MBRs. The results of this study indicate that re-inoculation is required regularly after a period of time to maintain the removal efficiency of the target compound.

Feasibility of using a novel algal-bacterial biofilm reactor for efficient domestic wastewater treatment

ABSTRACTCurrent algal-bacterial consortia require high hydraulic retention times (HRTs, 2–10 days) to efficiently remove pollutants from domestic wastewaters. A novel algal-bacterial biofilm reactor was developed for a much lower HRT. The results showed that an HRT of 12 h ensured 90% removal of organic matter and ammonium, and phosphate removal was approximately 30%. Decreasing the HRT to 8 h significantly deteriorated the reactor's pollutant removal efficiencies and increasing the HRT to 24 h did not improve these efficiencies. Illumination, which was light source for algae, was provided by a LED light. Activity tests showed that organic matter and ammonium removal rates resulting from illumination were 70% and 50%, respectively, of the rates when dissolved oxygen concentration was maintained at 2 mg/L. Chemical oxygen demand (COD) removal rates resulted from illumination and aeration were 18.63 and 25.38 mg COD/L.h, respectively. The phosphate removal rate was 0.26 and 0.43 mg/L.h when illumination and aeration were applied, respectively. The ammonium removal rates were approximately 10,390 and 5000 mg when the reactor was aerated or illuminated, respectively. These two rates were significantly higher than reported nitrification rates. Moreover, the percentage of Oscillatoria sp. increased from below 10% to over 90% under the applied organic load and temperature, while the percentage of fast growing algae, Chlorella, chroococcus sp and Scenedesmus sp., decreased from over 90% to below 10%. These results showed that an algal-bacterial biofilm reactor with a low reactor footprint was developed.

Reduction of adsorbed dyes content in the discharged sludge coming from an industrial textile wastewater treatment plant using aerobic activated sludge process

Dye mass balance study at full-scale industrial textile wastewater (ITW) treatment plant showed that 1.5 ton of excess waste sludge, containing 304.5 Kg of dyes, are daily produced and discharged in landfills. Therefore, this by-product of activated sludge process (ASP) presents a serious environmental problem. In this work, a laboratory and pilot scale investigations were carried out to optimize aerobic biodegradation efficiency to reduce the amount of residual adsorbed dye that will be found in the waste sludge. The resistance of acclimated biomass to the toxicity of ITW was studied in 2.5 L batch reactors using different dye to biomass (D/B) ratios of 0.102, 0.25 and 0.72 g CODS/g VSS. Results of respirometric analyses showed that acclimated activated sludge (AS) biomass is able to treat ITW at high D/B ratio of 0.72 g CODS/g VSS. Moreover, biodegradation kinetic study using Monod law showed that COD and color removal were better for the highest D/B ratio. The half saturation coefficient of heterotrophs for indigo dye (KSind) of 20.01 g/m3 showed high affinity between biomass and dye molecules. Optimization of the process at pilot-scale with different hydraulic retention time (HRT) of 2–5 days, and different sludge recycling rates (SRR) of 220–680 m3/d, showed that high HRT of 5 days and a SRR of 0.22 allowed the best dye biodegradation efficiency (95%). Application of the best conditions at full-scale reduced significantly (89%) the amount of the discharged dyes from 304.5 Kg/d to 33 Kg/d. Results were numerically validated using a mathematical model based on the activated sludge model 1 (ASM1).

Fate of sulfadiazine and its corresponding resistance genes in up-flow microbial fuel cell coupled constructed wetlands: Effects of circuit operation mode and hydraulic retention time

Few studies have evaluated sulfadiazine (SDZ) removal and the potential risks of antibiotic resistance gene (ARG) accumulation in up-flow microbial fuel cell coupled constructed wetlands (UCW-MFCs). This study aims to investigate the removal of SDZ, accumulation of ARGs (sulI, sulII, intI), and bacterial community changes in four UCW-MFC reactors with two hydraulic retention times (HRTs) and two circuit operation modes. The results indicated that effluent from a closed circuit UCW-MFC had a lower SDZ concentration due to a higher electrode absorption capacity for SDZ and the dehydrogenase activity of the bacteria. Additionally, the ARG abundance in closed circuit system was higher than that in open circuit system. A lower HRT was correlated with a higher SDZ concentration accumulation on the electrode, which led to a higher ARG abundance. Also, an obvious increase in the ARG abundance was observed during treatment periods. The relative abundance of target ARGs in the anode was higher than in the cathode and the bottom. A cluster analysis of bacterial communities at the phyla level showed they were more significantly affected by HRT. Furthermore, a closed circuit system and a high HRT contributed to high microbial diversity. In addition, five phyla of bacteria might be potential hosts for ARGs. Enrichment of ARG potential hosts induced by SDZ may be responsible for ARG abundance increase. Electricity production characteristics of the UCW-MFCs were also affected by HRT.

Obtaining green energy from dry-thermophilic anaerobic co-digestion of municipal solid waste and biodiesel waste

This study has been conducted to optimise the dry-thermophilic anaerobic co-digestion of two different types of real waste (municipal solid waste and biodiesel waste) in a semi-continuous feeding regime, under high organic loading rates (OLRs) or low hydraulic retention times (HRTs). For this purpose, different high OLRs (from 10 to 34 g [VS] l−1 d−1) or low HRTs (from 10 to 4.4 d) were investigated in a continuously stirred tank reactor. Optimal conditions (257 ml [CH4] g−1 VS; 6.03 ± 0.43 l [CH4] l−1 d−1; 16.3 ± 2.3 × 10−12 l [CH4] cell−1 d−1) were obtained at 4.4 d HRT (OLR = 23 g [VS] l−1 d−1), in which the average values of the ratios of Eubacteria:Archaea and hydrolytic-acidogenic bacteria:acetogens were 95:5 and 87:8, respectively.

Assessment of low concentration wastewater treatment operations with dewatered alum sludge-based sequencing batch constructed wetland system

Competition of volatile fatty acids between anoxic denitrification and anaerobic phosphorus release is prominent. Therefore, low concentration wastewater has restricted effects on nitrogen and phosphorus removal. The purpose of this study is to treat dormitory sewage with a biochemical oxygen demand (BOD) ranging from 50 to 150 mg/L using dewatered alum sludge-based sequencing batch constructed wetland system. Vegetation in the wetland system was chosen to be Phragmites australis. Three parallel cases were carried out to assess impacts due to different hydraulic retention time (HRT) and artificial aeration. The results showed that this system is effective in removing total nitrogen (TN), ammonia nitrogen (NH3-N) and total phosphorus (TP) under different HRT. However, nitrous oxide (N2O) emission poses to be the greatest challenge in the high HRT cases. Artificial aeration could reduce N2O emission but is associated with high operational cost. Results indicate that dewatered alum sludge-based sequencing batch constructed wetland system is a promising bio-measure in the treatment of low concentration wastewater.

Evaluation of fly ash pellets for phosphorus removal in a laboratory scale denitrifying bioreactor

Nitrate and orthophosphate from agricultural activities contribute significantly to nutrient loading in surface water bodies around the world. This study evaluated the efficacy of woodchips and fly ash pellets in tandem to remove nitrate and orthophosphate from simulated agricultural runoff in flow-through tests. The fly ash pellets had previously been developed specifically for orthophosphate removal for this type of application, and the sorption bench testing showed a good promise for flow-through testing. The lab-scale horizontal-flow bioreactor used in this study consisted of an upstream column filled with woodchips followed by a downstream column filled with fly ash pellets (3 and 1 m lengths, respectively; both 0.15 m diameter). Using influent concentrations of 12 mg/L nitrate and 5 mg/L orthophosphate, the woodchip bioreactor section was able to remove 49–85% of the nitrate concentration at three hydraulic retention times ranging from 0.67 to 4.0 h. The nitrate removal rate for woodchips ranged from 40 to 49 g N/m3/d. Higher hydraulic retention times (i.e., smaller flow rates) corresponded with greater nitrate load reduction. The fly ash pellets showed relatively stable removal efficiency of 68–75% across all retention times. Total orthophosphate adsorption by the pellets was 0.059–0.114 mg P/g which was far less than the saturated capacity (1.69 mg/g; based on previous work). The fly ash pellets also removed some nitrate and the woodchips also removed some orthophosphate, but these reductions were not significant. Overall, woodchip denitrification followed by fly ash pellet P-sorption can be an effective treatment technology for nitrate and phosphate removal in subsurface drainage.

Continuous hydrogen and methane production from Agave tequilana bagasse hydrolysate by sequential process to maximize energy recovery efficiency

Continuous H2 and CH4 production in a two-stage process to increase energy recovery from agave bagasse enzymatic-hydrolysate was studied. In the first stage, the effect of organic loading rate (OLR) and stirring speed on volumetric hydrogen production rate (VHPR) was evaluated in a continuous stirred tank reactor (CSTR); by controlling the homoacetogenesis with the agitation speed and maintaining an OLR of 44 g COD/L-d, it was possible to reach a VHPR of 6 L H2/L-d, equivalent to 1.34 kJ/g bagasse. In the second stage, the effluent from CSTR was used as substrate to feed a UASB reactor for CH4 production. Volumetric methane production rate (VMPR) of 6.4 L CH4/L-d was achieved with a high OLR (20 g COD/L-d) and short hydraulic retention time (HRT, 14 h), producing 225 mL CH4/g-bagasse equivalent to 7.88 kJ/g bagasse. The two-stage continuous process significantly increased energy conversion efficiency (56%) compared to one-stage hydrogen production (8.2%).

Behaviour of biopolymeric substances in the activated sludge of an MBR system working with high hydraulic retention time

ABSTRACTThis study was undertaken to analyse the activated sludge of a membrane bioreactor (MBR), the behaviour of extracellular polymeric substances (EPS) and soluble microbial products (SMP) as well as their biopolymers composition, in the activated sludge of a membrane bioreactor (MBR) and their influence on membrane fouling were analysed. For the experiment an experimental fullscale MBR working with real urban wastewater at high hydraulic retention time with a variable sludge-retention time (SRT) was used. The MBR system worked in denitrification/nitrification conformation at a constant flow rate (Q = 0.45 m3/h) with a recirculation flow rate of 4Q. The concentrations of SMP in the activated sludge were lower than the concentrations of EPS over the entire study, with humic substances being the main components of the two biopolymers. SMP and, more specifically, SMP carbohydrates, were the most influential biopolymers in membrane fouling, while for EPS and their components, no relation was found with fouling. The SRT and temperature were the operational variables that most influenced the SMP and EPS concentration, causing the increase of SRT and temperature a lower concentration in both biopolymers, although the effect was not the same for all the components, particularly for the EPS carbohydrates, which increased with longer SRTs. Both operational variables were also the ones most influential on the concentration of organic matter of the effluent, due to their effect on the SMP. The volatile suspended solid/total suspended solid (VSS/TSS) ratio in the activated sludge can be applied as a good indicator of the risk of membrane fouling by biopolymers in MBR systems.

New operational mode of an electrochemical reactor and its application to the degradation of levofloxacin

The flow-by electrochemical reactor is generally used in batch mode with extended process time in order to achieve high efficiency in the removal of organic contaminant from aqueous solution. Moreover, the batch mode system is inappropriate for the treatment of wastewater in high volume or as a continuous stream. This paper presents a new operational mode for the electrochemical reactor in which the electrolyte passes over the electrodes just once at a reduced flow rate and with high hydraulic retention time. The concentration of hydrogen peroxide generated in the reactor running in the single-pass flow mode attained 10.2mgL−1cm−2 in acid medium, a value similar to that obtained previously with the same type of reactor operating in batch mode. In the single-pass mode, the electrochemical reactor showed high rates of removal of the antibiotic levofloxacin (initial 50mgL−1) and organic load (initial 130mgL−1) up to 99 and 86%, respectively, when operated under electro- or photo-Fenton degradation conditions (acid medium with Fe2+ ions; absence or presence of UV irradiation). Degradation of levofloxacin generated high levels of nitrate (up to 4.5mgL−1) and various by-products that could be identified by liquid chromatography-mass spectrometry.

Simultaneously energy production and dairy wastewater treatment using bioelectrochemical cells: In different environmental and hydrodynamic modes

Abstract A successful design, previously adapted for treatment of complex wastewaters in a microbial fuel cell (MFC), was used to fabricate two MFCs, with a few changes for cost reduction and ease of construction. Performance and electrochemical characteristics of MFCs were evaluated in different environmental conditions (in complete darkness and presence of light), and different flow patterns of batch and continuous in four hydraulic retention times from 8 to 30 h. Changes in chemical oxygen demand, and nitrate and phosphate concentrations were evaluated. In contrast to the microbial fuel cell operated in darkness (D-MFC) with a stable open circuit voltage of 700 mV, presence of light led to growth of other species, and consecutively low and unsteady open circuit voltage. Although the performance of the MFC subjected to light (L-MFC) was quite low and unsteady in dynamic state (internal resistance = 100 Ω, power density = 5.15 W·m − 3 ), it reached power density of 9.2 W·m − 3 which was close to performance of D-MFC (internal resistance = 50 Ω, power density = 10.3 W·m − 3 ). Evaluated only for D-MFC, the coulombic efficiency observed in batch mode (30%) was quite higher than the maximum acquired in continuous mode (9.6%) even at the highest hydraulic retention time. In this study, changes in phosphate and different types of nitrogen existing in dairy wastewater were investigated for the first time. At hydraulic retention time of 8 h, the orthophosphate concentration in effluent was 84% higher compared to influent. Total nitrogen and total Kjeldahl nitrogen were reduced 70% and 99% respectively at hydraulic retention time of 30 h, while nitrate and nitrite concentrations increased. The microbial electrolysis cell (MEC), revamped from D-MFC, showed the maximum gas production of 0.2 m 3 H 2 ·m − 3 ·d − 1 at 700 mV applied voltage.

Effect of hydraulic retention time on deterioration/restarting of sludge anaerobic digestion: Extracellular polymeric substances and microbial response

In this study, the transformation of the sludge-related extracellular polymeric substances (EPS) during mesophilic anaerobic digestion was characterized to assess the effect of hydraulic retention time (HRT) on reactor deterioration/restarting. Experimental HRT variations from 20 to 15 and 10d was implemented for deterioration, and from 10 to 20d for restarting. Long-term digestion at the lowest HRT (10d) resulted in significant accumulation of hydrolyzed hydrophobic materials and volatile fatty acids in the supernatants. Moreover, less efficient hydrolysis of sludge EPS, especially of proteins related substances which contributed to the deterioration of digester. Aceticlastic species of Methanosaetaceae decreased from 36.3% to 27.6% with decreasing HRT (20–10d), while hydrogenotrophic methanogens (Methanomicrobiales and Methanobacteriales) increased from 30.4% to 38.3%. Proteins and soluble microbial byproducts related fluorophores in feed sludge for the anaerobic digester changed insignificantly at high HRT, whereas the fluorescent intensity of fulvic acid-like components declined sharply once the digestion deteriorated.

Continuous nitrogen removal by a single-stage reactor packed with ring-laced string medium

The efficiency of nitrogen removal by a partial-nitritation/anammox (PNA) reaction was investigated using a packed-bed reactor in which ring-laced strings were used as the supporting medium. A stable population of PNA microorganisms was established from typical activated sludge, after less than two months of acclimation in the packed-bed reactor, by applying a high nitrogen-loading rate (NLR: 0.53kg/m3/d) and short hydraulic retention time (HRT: 1.8h). The stability of reactor performance was confirmed in industrial wastewater (IW), demonstrating a nitrogen removal efficiency (NRE) of greater than 77% during 260 days of continuous operation, between 0.19 and 0.53kg/m3/d of NLR. Partial nitrification was adequately controlled by low-level oxygen supply to the reactor. Pyro-tag sequencing analysis of the biofilm revealed a clear abundance of anammox bacteria in the inner part of the biofilm and ammonium-oxidizing bacteria in the outer part. In the synthetic inorganic medium (SIM), the microbial community structure did not change drastically between the early and late phases of the experiment's continuous operation, which lasted over 200 days. In IW, however, the existence ratio of anammox bacteria decreased to 4% on day 249 of continuous operation. The number of detected operational taxonomic units (OTUs) increased in the IW, implying that the community structure was widely diversified. However, anammox bacteria could propagate sufficiently to catalyze nitrogen removal under this condition because the NRE was stable at approximately 88%.

Non-equilibrium model for solute transport in differently designed biofilters targeting agricultural drainage water.

Biogeochemical processes in subsurface flow constructed wetlands are influenced by flow direction, degree of saturation and influent loading position. This study presents a simulation tool, which aims to predict the performance of the unit and improve the design. The model was developed using the HYDRUS program, calibrated and verified on previously measured bromide (Br-) pulse tracer tests. Three different hydraulic designs (Horizontal (HF), Vertical upward (VF-up), Vertical downward (VF-down) and two different flow rates: Low (L), and High (H)) were investigated. The model simulated well the Br- transport behaviour and the results underline the importance of the hydraulic design. Calibrated model parameters (longitudinal dispersivity, immobile liquid phase, mass transfer coefficient) showed a common trend for all the designs, for increasing flow rates within the investigated range. The VF-down performed best, i.e. had the highest hydraulic retention time.

Treatment of wastewater from syngas wet scrubbing: Model‐based comparison of phenol biodegradation basin configurations

AbstractA treatment process for the biological removal of phenol from wastewater generated from the wet scrubbing of syngas is discussed with the objective of reducing Hydraulic Retention Time (HRT) for the process. In phenol biodegradation, a major challenge is posed by the inhibitory effect of high inlet phenol concentrations on the functioning of degrading microorganisms, leading to excessively high HRT values. This study provides a theoretical insight on the effect of reactor configuration and operation mode on the residence time for high strength phenolic wastewaters. Two pre‐treated wastewater streams mainly contaminated with phenol, with different flow rates and phenol concentrations, have been considered for evaluation in this work. Various configurations to remove phenol have been studied and compared. The kinetics in the reactors have been estimated using the Haldane‐Andrews equation for a variety of microorganisms. The configurations which require minimum residence time for a given extent of phenol biodegradation are those employing two reactors in series with a certain reflux stream. Gulosibacter sp. YZ4 is found to be the most effective microorganism in removing phenol within the minimum retention period.

Behaviour of the main nonsteroidal anti-inflammatory drugs in a membrane bioreactor treating urban wastewater at high hydraulic- and sludge-retention time

The behaviour and removal efficiency of ibuprofen (IBU), diclofenac (DCF), ketoprofen (KPF), and naproxen (NPX) during the real urban wastewater treatment by an experimental full-scale MBR working at high sludge and hydraulic retention time (SRT, HRT) were determined. The MBR worked in denitrification/nitrification conformation at 35.4h of HRT (Q=0.45m3/h), 37 d of SRT and a recirculation flow rate of 4Q. The experiments were made under steady-state conditions, reaching a biodegradable organic matter removal higher than 99.5%. The MBR system showed similar removal capacity for IBU, NPX, and KTP (>95%), whose main transformation occurred in the aerobic reactor with a low contribution from the anoxic reactor. The system worked with complete nitrification, also achieving an effective retention of the unbiodegradable organic matter due to recirculation. DCF removal was low with negative removal yields for several samplings. Both removal and increase transformation of DCF also occurred in the aerobic reactor, this not being observed in the anoxic one. DCF tends to accumulate in the system and to be recirculated. Thus, during the sampling in which DCF influent concentration decreases, removal yields turn negative. The increase of DCF concentration in the aerobic bioreactor also contributes to the negative removal yields.

Long-term performance of an integrated constructed wetland for advanced treatment of mixed wastewater

The aim of this study was to investigate long-term performance of a full-scale integrated constructed wetland (CW) system for advanced treatment of sanitary and industrial mixed wastewater. The system consisted of a vertical subsurface flow (VSF) CW, a free water surface (FWS) CW, and a horizontal subsurface flow (HSF) CW in series. Results from six-year operation showed that the system resulted in average removal efficiencies of 70%, 70%, 34%, 52%, 45%, 74%, 21%, 43%, and 98% for COD, NH4-N, NO3-N, TN, TP, TSS, F−, Ni, and E. coli, respectively. Especially, NH4-N was effectively removed by the VSF CW. With relatively high hydraulic loading rates (18.0–43.6cmd−1) and short hydraulic retention times (1.04–2.51 d), the removal efficiencies of the system were quite stable for most of the water quality parameters, while the removal efficiency of TN increased from 49% to 60% during the six years. The removal efficiencies of the system were generally stable with the change of seasons. This study demonstrates that the VSF+FWS+HSF CW system can be a valuable approach for advanced wastewater treatment to meet the more stringent discharge requirement.

Influence of hydraulic retention time on the psychrophilic hydrolysis/acidogenesis of proteins.

The influence of the hydraulic retention time (HRT) on the anaerobic hydrolysis of complex substrates has been studied under psychrophilic conditions. For this purpose, a continuous stirred tank reactor was operated at 15 °C and neutral pH and gelatin was considered as a model protein. Three HRTs have been tested: 12, 21 and 36 h. Gelatin hydrolysis was greatly dependent on HRT, increasing from 40% at 12 h-HRT to a maximum of 65% at 36 h-HRT. Molecular size distribution analyses of the effluent showed that hydrolysation of compounds larger than 10 kDa was poor at 12 h-HRT, whereas the fraction of 1-10 kDa was completely transformed into compounds smaller than 1 kDa. Higher HRT (36 h) improved the degradation of the recalcitrant fraction (>10 kDa), obtaining an effluent with around 95% of soluble molecules (<1 kDa). In that way, the use of membrane bioreactors for the treatment of this type of macromolecules could improve the degradation efficiencies by enabling to increase the residence time of the non-hydrolyzed molecules, with what would be possible to achieve higher organic loading rate operation.