Effect of hydraulic retention time on anaerobic hydrogenesis in CSTR

The objective of this work was to evaluate the production of hydrogen in a continuous system as a function of hydraulic retention time (HRT). The intermediates accumulated and other parameters of pH, oxidation-reduction potential were quantified. The heat treatment (103 degrees C for 24 h) of the compost from a cattle dung composting facility was able to select H2-producing spores; this product was used as a seed for continuous systems. The brewery waste was used as substrate. For the eight runs with combinations of five HRTs and four pHs, the results indicate that at pH=5.5, a maximum H2 production of 47% H2 concentration, 43 ml H2/g COD(added), and 3.1 l H2/l reactor d was achieved at HRT=18 h. Nevertheless, at HRT=18 h, pH 5.5 was also the optimum pH for the maximum H2 production among four pHs evaluated from 5 to 6.5. There was a significant accumulation of volatile acid and alcohols during the entire study.     

Treatment of screened dairy manure by upflow anaerobic fixed bed reactors packed with waste tyre rubber and a combination of waste tyre rubber and zeolite: effect of the hydraulic retention time

Two laboratory-scale anaerobic fixed bed reactors were evaluated while treating dairy manure at upflow mode and semicontinuous feeding. One reactor was packed with a combination of waste tyre rubber and zeolite (R1) while the other had only waste tyre rubber as a microorganism immobilization support (R2). Effluent quality improved when the hydraulic retention time (HRT) increased from 1.0 to 5.5 days. Higher COD, BOD5, total and volatile solids removal efficiencies were always achieved in the reactor R1. No clogging was observed during the operation period. Methane yield was also a function of the HRT and of the type of support used, and was 12.5% and 40% higher in reactor R1 than in R2 for HRTs of 5.5 and 1.0 days, respectively. The results obtained demonstrated that this type of reactor is capable of operating with dairy manure at a HRT 5 times lower than that used in a conventional reactor.     

Immobilized <Emphasis Type="Italic">Kluyveromyces marxianus</Emphasis> cells in carboxymethyl cellulose for production of ethanol from cheese whey: experimental and kinetic studies

Cheese whey fermentation to ethanol using immobilized Kluyveromyces marxianus cells was investigated in batch and continuous operation. In batch fermentation, the yeast cells were immobilized in carboxymethyl cellulose (CMC) polymer and also synthesized graft copolymer of CMC with N-vinyl-2-pyrrolidone, denoted as CMC-g-PVP, and the efficiency of the two developed cell entrapped beads for lactose fermentation to ethanol was examined. The yeast cells immobilized in CMC-g-PVP performed slightly better than CMC with ethanol production yields of 0.52 and 0.49 g ethanol/g lactose, respectively. The effect of supplementation of cheese whey with lactose (42, 70, 100 and 150 g/l) on fermentative performance of K. marxianus immobilized in CMC beads was considered and the results were used for kinetic studies. The first order reaction model was suitable to describe the kinetics of substrate utilization and modified Gompertz model was quite successful to predict the ethanol production. For continuous ethanol fermentation, a packed-bed immobilized cell reactor (ICR) was operated at several hydraulic retention times; HRTs of 11, 15 and 30 h. At the HRT of 30 h, the ethanol production yield using CMC beads was 0.49 g/g which implies that 91.07 % of the theoretical yield was achieved.     

Effects of pH and hydraulic retention time on hydrogen production versus methanogenesis during anaerobic fermentation of organic household solid waste under extreme-thermophilic temperature (70 degrees C)

Two continuously stirred tank reactors were operated with household solid waste at 70 degrees C, for hydrogen and methane production. The individual effect of hydraulic retention time (HRT as 1, 2, 3, 4, and 6 days) at pH 7 or pH (5, 5.5, 6, 6.5, 7) at 3-day HRT was investigated on the hydrogen production versus methanogenesis. It was found that at pH 7, the maximum hydrogen yield was 107 mL-H(2)/g VS(added) (volatile solid added) but no stable hydrogen production was obtained as after some time methanogenesis was initiated at all tested HRTs. This demonstrated that sludge retention time alone was not enough for washing out the methanogens at pH 7 under extreme-thermophilic conditions. Oppositely, we showed that keeping the pH level at 5.5 was enough to inhibit methane and produce hydrogen stably at 3-day HRT. However, the maximum stable hydrogen yield was low at 21 mL-H(2)/g VS(added).     

Continuous biohydrogen production from fruit wastewater at low pH conditions

Biohydrogen production from a simulated fruit wastewater (soluble COD = 3.17 ± 0.10 g L^?1) was carried out in a continuous stirred tank reactor (CSTR) of 2 L operational volume without biomass inoculation, heat pre-treatment or pH adjustment, resulting in a low operational pH (3.75 ± 0.09). The hydraulic retention time (HRT) varied from 15 to 5 h. A strong negative correlation (p < 0.01) between the biogas production rate and the HRT was observed. Biogas production rates were higher at 30 °C than at 25 °C (p < 0.01), when the CSTR was operated under the same HRT. The biogas hydrogen content was estimated as high as 55.8 ± 2.3 % and 55.4 ± 2.5 % at 25 and 30 °C, respectively. The main fermentation end products were acetic and butyric acids, followed by ethanol. Significant differences (p < 0.01) during the operation of the CSTR at 25 or 30 °C were identified for butyric acid at almost all HRTs examined. Simulation of the acidogenesis process in the CSTR (based on COD and carbon balances) indicated the possible metabolic compounds produced at 25 and 30 °C reactions and provided an adequate fit of the experimental data.     

Influence of temperature on the performance of anaerobic fixed-film reactors utilizing screened dairy manure

Thermophilic (55°C) and mesophilic (35°C and 22°C) anaerobic digestions in laboratory scale (4 litre) fixed-film reactors fed with screened dairy manure were successfully operated over a range of hydraulic retention times, from 1 to 20 days. Maximum methane production rates of 1·82, 1·68 and 1·28 litres CH₄ litre⁻¹ day⁻¹ occurred at 1, 1·5 and 1 days HRT for the respective 55°C, 35°C and 22°C reactors. Both thermophilic and mesophilic digestions achieved maximum biodegradation efficiency at 10 days HRT. The thermophilic fixed-film reactor performed better than completely-mixed reactors in terms of methane production at HRTs shorter than 2 days. From the results, mesophilic fixed-film reactor operated at 35°C provided optimum methane production and net energy output between 1 and 5 days HRT.     

The effect of temperature and retention time on methane production and microbial community composition in staged anaerobic digesters fed with food waste

Background

Food waste is a large bio-resource that may be converted to biogas that can be used for heat and power production, or as transport fuel. We studied the anaerobic digestion of food waste in a staged digestion system consisting of separate acidogenic and methanogenic reactor vessels. Two anaerobic digestion parameters were investigated. First, we tested the effect of 55 vs. 65 °C acidogenic reactor temperature, and second, we examined the effect of reducing the hydraulic retention time (HRT) from 17 to 10 days in the methanogenic reactor. Process parameters including biogas production were monitored, and the microbial community composition was characterized by 16S amplicon sequencing.

Results

Neither organic matter removal nor methane production were significantly different for the 55 and 65 °C systems, despite the higher acetate and butyrate concentrations observed in the 65 °C acidogenic reactor. Ammonium levels in the methanogenic reactors were about 950 mg/L NH₄ ⁺ when HRT was 17 days but were reduced to 550 mg/L NH₄ ⁺ at 10 days HRT. Methane production increased from ~ 3600 mL/day to ~ 7800 when the HRT was decreased. Each reactor had unique environmental parameters and a correspondingly unique microbial community. In fact, the distinct values in each reactor for just two parameters, pH and ammonium concentration, recapitulate the separation seen in microbial community composition. The thermophilic and mesophilic digesters were particularly distinct from one another. The 55 °C acidogenic reactor was mainly dominated by Thermoanaerobacterium and Ruminococcus, whereas the 65 °C acidogenic reactor was initially dominated by Thermoanaerobacterium but later was overtaken by Coprothermobacter. The acidogenic reactors were lower in diversity (34–101 observed OTU_0.97, 1.3–2.5 Shannon) compared to the methanogenic reactors (472–513 observed OTU_0.97, 5.1–5.6 Shannon). The microbial communities in the acidogenic reactors were > 90% Firmicutes, and the Euryarchaeota were higher in relative abundance in the methanogenic reactors.

Conclusions

The digestion systems had similar biogas production and COD removal rates, and hence differences in temperature, NH₄ ⁺ concentration, and pH in the reactors resulted in distinct but similarly functioning microbial communities over this range of operating parameters. Consequently, one could reduce operational costs by lowering both the hydrolysis temperature from 65 to 55 °C and the HRT from 17 to 10 days.

     

Degradation of quinolizidine alkaloids of lupin by Rhizopus oligosporus

Rhizopus oligosporus has proven beneficial in the detoxification of lupin seeds. The fermentation process is mainly affected by the initial pH in the medium. In the range of growth of mold, there are maximum enzymatic activities in pH of 3.5 and 5.5. Metabolism change occurs at these pH levels; therefore, we studied the growth, pH changes, dry matter intake, and alkaloid degradation within 48 h of fermentation. Cultures of lupin agar (LA) with pH of 3.5 and 5.5 were made in Petri dishes with lupin flour. Results showed pH directly affects the degradation of alkaloids and fungal growth. Detoxification levels achieved were 16.58 and 63.23 % in treatments LA 3.5 and LA 5.5, respectively. Fungal growth was 0.919 mg/cm² in LA 3.5 and 1.081 mg/cm² in LA 5.5. Maximum degradation rate in LA 5.5 was given between 16 and 20 h, which coincided with maximum fungal growth. Despite having similar dry matter intake in both treatments, a pH of 3.5 did not show the same degree of detoxification. The analysis with exponential, yield of growth, yield of dry matter intake and luedeking and piret equations, confirm the relation between intake and growth with detoxification. Dry matter intake equation predicts with R ² of 0.94 the detoxification in LA 5.5. A pH of 5.5 is directly related with detoxification and fungal development.     

Improvement of mannitol production by Lactobacillus brevis mutant 3-A5 based on dual-stage pH control and fed-batch fermentations

Lactobacillus brevis 3-A5 was isolated and expected to produce mannitol efficiently by regulating pH in batch and fed-batch fermentations. In 48 h batch fermentations with free and constant pH, the optimal pH for cell growth and mannitol production in the first 24 h of incubation was 5.5, whereas that for mannitol production in the second 24 h of incubation was 4.5. To achieve high cell density and mannitol yield simultaneously, a dual-stage pH control strategy was proposed based on the kinetic analysis of mannitol production. The pH value was controlled at 5.5 for the first 12 h of fermentation and subsequently shifted to 4.5 until the fermentation was completed. Under dual-stage pH control fermentation, a 103 g/L yield of mannitol with a volumetric production rate of 3.7 g/L/h was achieved after 28 h. The dual-stage pH control fed-batch fermentation strategy was further developed to improve mannitol yield, wherein the yield increased by 109 % to 215 g/L after 98 h of fermentation. This value is the highest yield of mannitol ever reported using L. brevis.     

Rapid formation of hydrogen-producing granules in an anaerobic continuous stirred tank reactor induced by acid incubation

A novel approach to rapidly initiate granulation of hydrogen-producing sludge was developed in an anaerobic continuous stirred tank reactor at 37 degrees C. To induce microbial granulation, the acclimated culture was subject to an acid incubation for 24 h by shifting the culture pH from 5.5 to 2.0. The culture was resumed to pH 5.5 after the incubation and the reactor was operated at hydraulic retention times (HRTs) of 12, 6, 2, 1, and 0.5 h in sequence. Microbial aggregation took place immediately with the initiation of acid incubation and granules were developed at 114 h. No granule was observed in the absence of acid incubation in the control test. Changing the culture pH resulted in improvement in surface physicochemical properties of the culture favoring microbial granulation. The zeta potential increased from -11.6 to -3.5 mV, hydrophobicity in terms of contact angle improved from 31 degrees to 43 degrees and extracellular proteins/polysaccharides ratio increased from 0.2 to 0.5-0.8. Formation of granular sludge facilitated biomass retention of up to 32.2 g-VSS/L and enhanced hydrogen production. The hydrogen production rate and hydrogen yield increased with the reduction in HRT at an influent glucose concentration of 10 g/L once steady granular sludge layer was formed, achieving the respective peaks of 3.20 L/L x h and 1.81 mol-H(2)/mol-glucose at 0.5 h HRT. The experimental results suggested that acid incubation was able to initiate the rapid formation of hydrogen-producing granules by regulating the surface characteristics of microbial aggregates in a well-mixed reactor, which enhanced the hydrogen production.     

Continuous methane fermentation and the production of vitamin B12 in a fixed-bed reactor packed with loofah

A fixed-bed reactor with acclimated methanogens immobilized on a loofah support was studied on a laboratory scale to evaluate the system producing methane from the mixture of CO(2) and H(2) gas, with the production of vitamin B(12) as a by-product. Fermentation using CO(2)/H(2) acclimated methanogens was conducted in a jar fermentor with hydraulic retention times (HRTs) of three and six days. The performance of the reactor was mainly dependent on the HRT. With an HRT of three days, the methane production rate and the vitamin B(12) concentration in the culture broth were 6.18 l/l-reactor/h and 2.88 mg/l-culture liquid; these values were 11.96 l/l-reactor/h and 37.54 mg/l-culture liquid for an HRT of six days. A higher total cell mass of methanogens retained 42.5 g dry cell/l-culture liquid was achieved in the HRT of six days. The loofah carrier immobilized almost 95% of the methanogens, which led to a more effective bio-reaction. It was also observed that the fermentation system had a better ability to buffer pH, especially for an HRT of six days.     

Improving EGSB reactor performance for simultaneous bioenergy and organic acid production from cheese whey via continuous biological H<Subscript>2</Subscript> production

Objectives

To evaluate the influence of hydraulic retention time (HRT) and cheese whey (CW) substrate concentration (15 and 25 g lactose l^?1) on the performance of EGSB reactors (R15 and R25, respectively) for H₂ production.

Results

A decrease in the HRT from 8 to 4 h favored the H₂ yield and H₂ production rate (HPR) in R15, with maximum values of 0.86 ± 0.11 mmol H₂ g COD^?1 and 0.23 ± 0.024 l H₂ h^?1 l^?1, respectively. H₂ production in R25 was also favored at a HRT of 4 h, with maximum yield and HPR values of 0.64 ± 0.023 mmol H₂ g COD^?1 and 0.31 ± 0.032 l H₂ h^?1 l^?1, respectively. The main metabolites produced were butyric, acetic and lactic acids.

Conclusions

The EGSB reactor was evaluated as a viable acidogenic step in the two-stage anaerobic treatment of CW for the increase of COD removal efficiency and biomethane production.

     

Lactic Acid Production in a Mixed-Culture Biofilm Reactor

Novel solid supports, consisting of polypropylene blended with various agricultural materials (pp composite), were evaluated as supports for pure- and mixed-culture continuous lactic acid fermentations in biofilm reactors. Streptomyces viridosporus T7A (ATCC 39115) was used to form a biofilm, and Lactobacillus casei subsp. rhamnosus (ATCC 11443) was used for lactic acid production. For mixed-culture fermentations, a 15-day continuous fermentation of S. viridosporus was performed initially to establish the biofilm. The culture medium was then inoculated with L. casei subsp. rhamnosus. For pure-culture fermentation, L. casei subsp. rhamnosus was inoculated directly into the reactors containing sterile pp composite chips. The biofilm reactors containing various pp composite chips were compared with a biofilm reactor containing pure polypropylene chips and with a reactor containing a suspension culture. Continuous fermentation was started, and each flow rate (0.06 to 1.92 ml/min) was held constant for 24 h; steady state was achieved after 10 h. Lactic acid production was determined throughout the 24-h period by high-performance liquid chromatography. Production rates that were two to five times faster than those of the suspension culture (control) were observed for the pure- and mixed-culture bioreactors. Both lactic acid production rates and lactic acid concentrations in the culture medium were consistently higher in mixed-culture than in pure-culture fermentations. Biofilm formation on the chips was detected at harvest by chip clumping and Gram staining.     

Reducing the risk of foaming and decreasing viscosity by two-stage anaerobic digestion of sugar beet pressed pulp

Anaerobic digestion (AD) of sugar beet pressed pulp (SBPP) is a promising treatment concept. It produces biogas as a renewable energy source making sugar production more energy efficient and it turns SBPP from a residue into a valuable resource. In this study one- and two-stage mono fermentation at mesophilic conditions in a continuous stirred tank reactor were compared. Also the optimal incubation temperature for the pre-acidification stage was studied. The fastest pre-acidification, with a hydraulic retention time (HRT) of 4 days, occurred at a temperature of 55 °C. In the methanogenic reactor of the two-stage system stable fermentation at loading rate of 7 kg VS/m³ d was demonstrated. No artificial pH adjustment was necessary to maintain optimum levels in both the pre-acidification and the methanogenic reactor. The total HRT of the two-stage AD was 36 days which is considerably lower compared to the one-stage AD (50 days). The frequently observed problem of foaming at high loading rates was less severe in the two-stage reactor. Moreover the viscosity of digestate in the methanogenic stage of the two-stage fermentation was in average tenfold lower than in the one-stage fermentation. This decreases the energy input for the reactor stirring about 80 %. The observed advantages make the two-stage process economically attractive, despite higher investments for a two reactor system.     

Continuous fill intermittent decant type sequencing batch reactor application to upgrade the UASB treated sewage

The performance of continuous flow intermittent decant type sequencing batch (CFID) reactor treating the effluent of an UASB reactor treating domestic wastewater and operated at 8 h hydraulic retention time (HRT) was investigated. The CFID was operated at three different HRTs (22, 8 and 6 h) and three different dissolved oxygen (DO) patterns (<0.5, 2.5–3.5 and 3.5–4.5 mg/L). The highest effluent quality was observed at the 8 h HRT and 2.5–3.5 mg/L DO concentration. At this operational condition, the average BOD, TSS, ammonia nitrogen and fecal coliform removal efficiencies were 83, 90, 74 and 99 %, respectively. The CFID is a promising post-treatment option for existing UASB systems, with a final effluent quality that comply with receiving water and effluent reuse criteria.     

Biotransformation of nitrophenols in upflow anaerobic sludge blanket reactors

Four identical bench-scale upflow anaerobic sludge blanket (UASB) reactors, R1, R2, R3 and R4, were used to assess nitrophenols degradation at four different hydraulic retention times (HRT). Reactor R1 was used as control, whereas R2, R3, and R4 were fed with 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), and 2,4-dinitrophenol (2,4-DNP), respectively. The concentration of each nitrophenol was gradually varied from 2 to 30 mg/l during acclimation. After acclimation reactors were operated under steady-state conditions at four different HRTs – 30, 24, 18, and 12 h, to study its effect on the removal of nitrophenols. Overall removal of 2-NP and 4-NP was always more than 99% but 2,4-DNP removal decreased from 96% to 89.7% as HRT was lowered from 30 to 12 h. 2-Aminophenol (2-AP), 4-aminophenol (4-AP) and 2-amino,4-nitrophenol (2-A,4-NP) were found to be the major intermediates during the degradation of 2-NP, 4-NP and 2,4-DNP, respectively. Out of the total input of nitrophenolic concentration (30 mg/l), on molar basis, about 41.2–48.4% of 2-NP, 59.4–68% of 4-NP, 30–26.6% of 2,4-DNP was recovered in the form of their respective amino derivatives at 30–12 h HRT. COD removal was 98–89%, 97–56%, 97–52%, and 94–46% at 30–12 h HRT for R1, R2, R3 and R4, respectively. Average cell growth was observed to be 0.15 g volatile suspended solid (VSS) per g COD consumed. Methanogenic inhibition was observed at lower HRTs (18 and 12 h), however denitrification was always more than 99% with non-detectable level of nitrite. The granules developed inside the reactors were black in color and their average size varied between 1.9 and 2.1 mm.     

Population dynamics of methanogens and methane formation associated with different loading rates of organic acids along with ammonia: redundancy analysis

In anaerobic processes, the population dynamics of methanogens in the methanogenic stage were monitored along with hydraulic retention times (HRTs) shift. Decreasing HRTs increased the loading rates of volatile fatty acids (VFAs) and ammonia. Methanomicrobiales (MMB) began to be dominant at longer than 12.5 days HRT, Methanosarcinales (MSL) were dominant at 8, 10, and 12.5 days HRT, and Methanobacteriales (MBT) were dominant at shorter than 6 days HRT. Increased loading rates of VFAs and ammonia increased MBT, decreased MMB, and had no significant effect on MSL. Maximal daily methane production was observed at 1.57 L/L when MSL copy numbers also reached 3.60 × 10⁷ copy/mL as a peak, which were expressed as positive correlation between DMA and MSL. No sooner had methane yield (MY) increased from 1.15 to 1.32 L/g VS_removed along with HRT reduction from 25 to 22.5 days, then MY gradually decreased from 1.32 to 0.04 L/g VS_removed.     

A pilot-scale study of total volatile fatty acids production by anaerobic fermentation of sewage in fixed-bed and suspended biomass reactors

A comparative study of a fermentation process for total volatile fatty acids (TVFA) production using pilot-scale fixed-bed (FAS) and suspended biomass (FER) reactors in which similar operational conditions was carried out. The influence of the changes of ambient temperatures at fixed operational conditions was also studied. Oxidation–reduction potential (ORP) increased and effluent pH decreased as the hydraulic retention time (HRT) decreased, which was favourable for TVFA production. Equations describing the ORP and pH variations with the HRT were obtained. ORP variation with HRT for FAS and FER reactors followed a logarithmic function with a regression coefficient, R², equal to 0.98. The variations of pH with HRT followed polynomial functions with regression coefficients of 0.96 and 0.98 for FAS and FER reactors, respectively. Hydrolysis process increased with the experiment duration. At the beginning of the experiment, effluent soluble COD (SCOD) decreased with respect to the influent but further effluent SCOD increased showing higher values compared to the influent. Cold temperatures were more favourable than summer temperatures for the accumulation of TVFA at the liquid effluent. The FAS reactor was more effective in the production of TVFA than the FER reactor. The maximum yields of TVFA were obtained at an organic volumetric loading rate (B_V) of 1.9 g COD/l per day, corresponding to an HRT of 3.4 h, for both reactors. A maximum increase of ammonia and phosphorus was observed at the maximum value of HRT coinciding with an increase of pH and a decrease of ORP, as could be previously observed. The average P/SCOD ratio for the influent and effluent were 0.06 and 0.05, respectively, for FAS and FER reactors. The average Ammonia/SCOD ratio for the influent and effluent were 0.15 and 0.14, respectively. These results demonstrate that effluent quality was improved by the treatment employed in case a further process of nutrient removal is carried out.     

固定载体卧式厌氧反应器处理糖蜜废水的快速启动

为高效处理高浓度有机废水而设计了固定载体卧式厌氧反应器R1和R2, 它是厌氧折流板反应器(ABR)的改进, 以活性炭纤维作为生物膜载体固定并充当反应器的折流板, 在实验室规模上对R1和R2处理糖蜜废水进行快速启动运行。HRT和ORL是影响R1和R2稳定高效运行及启动的2个重要工艺参数。实验证明: HRT为2 d时, 反应器运行最佳。在第30天时, R1的COD去除率达到84.88%, R2达到81.72%。随着进水ORL由1.25 kg/(m3·d)提升到10 kg/(m3·d), 沼气容积产气率由0.35 L/(L·d)逐渐增加到4.98 L/(L·d)。进水pH值为3.9?4.5之间, 整个启动运行过程中, 未调节pH值, R1和R2的出水pH值均在6.7?7.6之间, 2个反应器均有较强的抗酸能力, R1的pH波动更为平缓。在整个实验过程中, 污泥流失量小, 没有发生堵塞现象, 在处理酸性高浓度有机废水时, 2个反应器均表现出较强的抗负荷冲击能力。     

The effect of hydraulic retention time on the stability of aerobically grown microbial granules

AIMS: The aim of this study is to evaluate the effect of hydraulic retention time (HRT) on the development of aerobically grown microbial granules. METHODS AND RESULTS: Five column-shaped sequential aerobic sludge blanket reactors (SASBRs) were seeded with aerobically grown microbial granules and operated in a cyclic mode at different HRTs. At the shortest HRT of 1 h, the strong hydraulic pressure triggered biomass washout and led to reactor failure. At the longest HRT of 24 h, which represented the weakest hydraulic selection in this study, aerobic granules were gradually substituted by bioflocs because of the lower frequency of volumetric exchange. Within the optimum range of HRTs from 2 to 12 h, however, aerobic granules became stabilized in the presence of adequate hydraulic selection in the reactors, with good mixed liquor volatile suspended solids (MLVSS) retention, high volumetric chemical oxygen demand (COD) removal, low sludge volume index (SVI) values, good effluent quality, low sludge production rate, stronger and more compact structures, high cell hydrophobicity and high ratios of extracellular polysaccharides (PS) to extracellular proteins (PN). CONCLUSIONS: HRTs between 2 and 12 h provided the hydraulic selection pressures favourable for the formation and maintenance of stable aerobic granules with good settleability and activity. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first systematic study on the effect of HRT on heterotrophic aerobic granules. The results of the investigation are useful in understanding how aerobic granules can be applied for wastewater treatment.