High Sludge Retention Time Research Articles

Economic and environmental sustainability of an AnMBR treating urban wastewater and organic fraction of municipal solid waste

The objective of this study was to evaluate the economic and environmental sustainability of a submerged anaerobic membrane bioreactor (AnMBR) treating urban wastewater (UWW) and organic fraction of municipal solid waste (OFMSW) at ambient temperature in mild/hot climates. To this aim, power requirements, energy recovery from methane (biogas methane and methane dissolved in the effluent), consumption of reagents for membrane cleaning, and sludge handling (polyelectrolyte and energy consumption) and disposal (farmland, landfilling and incineration) were evaluated within different operating scenarios. Results showed that, for the operating conditions considered in this study, AnMBR technology is likely to be a net energy producer, resulting in considerable cost savings (up to €0.023 per m3 of treated water) when treating low-sulphate influent. Life cycle analysis (LCA) results revealed that operating at high sludge retention times (70 days) and treating UWW jointly with OFMSW enhances the overall environmental performance of AnMBR technology.

Characteristics of free cells and aggregated flocs for the flocculation and sedimentation of activated sludge

Laboratory experiments were conducted to investigate the correlation of the free cells/aggregated flocs characteristics and the sludge flocculation and separation behavior during the activated sludge process. Activated sludge was cultured in three laboratory-scale batch reactors with same carbon sources of glucose but different sludge retention times (SRTs) of 5, 10 and 20 days. The variation in the operation condition produced sludge with different flocculation and separation characteristics. The reactor performance in terms of bioflocculation as measured by the amount of suspended solids in the effluent, sludge sedimentation and compression as measured by the sludge value index improved considerably as the SRT lengthened. The higher SRT was related to less negatively charged surface of free cells and more hydrophobic of aggregated flocs. The negative zeta potential of aggregated flocs (11–13 mV) was smaller than that of free cells (15–23 mV), and free cells carrying lesser negative surface charges resulted in effluent clarification. Hydrophobicity of aggregated flocs (10–17 %) in each reactor was higher than that of free cells (5–8 %). The increase in hydrophobicity of aggregated flocs had positive effect on settleability. Floc strength measured by breakage/erosion index was closely related to hydrophobicity of aggregated flocs, but no correlation could be established with zeta potential of aggregated flocs. In addition, different bacterial species existed in free cells and aggregated flocs. Microbial similarity between free cells and aggregated flocs increased with shorter SRT and was closely related to effluent clarification.

Physiological adaptation of growth kinetics in activated sludge

Physiological adaptation as it occurs in bacterial cells at variable environmental conditions influences characteristic properties of growth kinetics significantly. However, physiological adaptation to growth related parameters in activated sludge modelling is not yet recognised. Consequently these parameters are regarded to be constant. To investigate physiological adaptation in activated sludge the endogenous respiration in an aerobic degradation batch experiment and simultaneous to that the maximum possible respiration in an aerobic growth batch experiment was measured. The activated sludge samples were taken from full scale wastewater treatment plants with different sludge retention times (SRTs). It could be shown that the low SRT sludge adapts by growth optimisation (high maximum growth rate and high decay rate) to its particular environment where a high SRT sludge adapts by survival optimization (low maximum growth rate and low decay rate). Thereby, both the maximum specific growth rate and the decay rate vary in the same pattern and are strongly correlated to each other. To describe the physiological state of mixed cultures like activated sludge quantitatively a physiological state factor (PSF) is proposed as the ratio of the maximum specific growth rate and the decay rate. The PSF can be expressed as an exponential function with respect to the SRT.

Study of the influence of different variables on membrane fouling in submerged membrane bioreactors

Membrane fouling is still the greatest handicap to overcome in membrane bioreactor (MBR) technology. In this study, a multi-factorial experimental design was carried out in order to optimize the MBR process, studying the effect of major variables involved in the fouling phenomenon as well as the interaction between them. To that end, a submerged membrane bioreactor was operated at three different levels of: sludge retention time (SRT) [25 and 60 d], mixed liquor suspended solids (MLSS) [6 and 14 g/L], and ratio between soluble and particulate chemical oxygen demand (sCOD/pCOD) [1 and 4] in the wastewater. The concentration of extracellular polymeric substances (EPS), as one of the most relevant factor responsible for membrane fouling, and the permeate flux were measured. After an exhaustive statistical analysis, it was confirmed that there is a relationship between the concentration of EPS and membrane fouling. It also appears that the ratio sCOD/pCOD in the composition of the wastewater plays a very important role in the fouling which becomes less pronounced as the soluble fraction increases. Moreover, high fouling potential was observed at high SRT values. Finally, it was proved that the membrane fouling was slightly less pronounced at higher MLSS concentration.

Diagnosis of the anaerobic reject water effects on WWTP operational characteristics as a precursor of bulking and foaming.

This study investigates the effects observed on operational parameters in a large and full-scale wastewater treatment plant subjected to anaerobic reject water (ARW) diversion off the main line for a 3-month period and further monitoring for a 2-year period. The plant's secondary unit consists of a two-stage plug-flow-modified Bardenpho process receiving wastewater from both municipal and industrial origins. As a result, ARW was found to have a direct effect on bulking in secondary clarifiers and foaming in anaerobic digesters (AD) despite its relatively small flow rate. During the cut-off period a highly stable sludge volume index at 80 mL g(-1) level was obtained in the secondary clarifiers, effluent suspended solids concentration was reduced and continuous feeding to AD was recovered. Sludge density increased in the thickeners during hot season. Secondary clarifiers showed good and stable settleability despite low dissolved oxygen, food/microorganism ratio and high sludge retention time and ammonium levels in the biological unit. The bulking and foaming effect was presented on the plant's internal flow balance. ARW needs serious consideration for elimination by appropriate technologies because of its high potential as a multi-dimensional pollutant source, not only as a carrier of nutrients but also as a possible carrier of filamentous bacteria, which might promote chronic seeding and retention in the system.

Toward energy-neutral wastewater treatment: A high-rate contact stabilization process to maximally recover sewage organics

The conventional activated sludge process is widely used for wastewater treatment, but to progress toward energy self-sufficiency, the wastewater treatment scheme needs to radically improve energy balances. We developed a high-rate contact stabilization (HiCS) reactor system at high sludge-specific loading rates (>2kgbCODkg−1TSSd−1) and low sludge retention times (<1.2d) and demonstrate that it is able to recover more chemical energy from wastewater organics than high-rate conventional activated sludge (HiCAS) and the low-rate variants of HiCS and HiCAS. The best HiCS system recovered 36% of the influent chemical energy as methane, due to the combined effects of low production of CO2, high sludge yield, and high methane yield of the produced sludge. The HiCS system imposed a feast-famine cycle and a putative selection pressure on the sludge micro-organisms toward substrate adsorption and storage. Given further optimization, it is a promising process for energy recovery from wastewater.

Acute and chronic responses of denitrifying culture to diclofenac

Acute and chronic effect as well as biodegradation potential at different sludge retention times (SRTs) of a priority pollutant, diclofenac on denitrification process was assessed. The continuous amendment of the culture for 6months with 1μg/L diclofenac resulted in 30% decrease in gas production. The average diclofenac removal observed in the diclofenac-acclimated culture was less than 15%. Batch tests showed that nitrate was removed in diclofenac free-control reactor at a higher rate compared to diclofenac amended reactor. Although, SRT did not have any progressive effect on diclofenac degradation, the system operated at low SRT was more sensitive to diclofenac and resulted in an increase in N2O emission. Wastewater treatment plants (WWTPs) operated at higher SRTs may tolerate and recover from the adverse effects of such micropollutants. The study can lead to other researchers to understand the fate and effect of other emerging pollutants in the anoxic unit of WWTPs.

Evaluation of a MBR pilot treating industrial wastewater with a high COD/N ratio

BACKGROUND In this study the performance of a membrane bioreactor (MBR) pilot plant treating industrial wastewater at the Heraklion Industrial Park, at three different sludge retention times (SRTs), was investigated. Membrane fouling in the MBR was monitored through the transmembrane pressure build up and by measuring accumulation of extracellular polymeric substances (EPS) and soluble microbial products (SMP). The effect of hydraulic residence time (HRT) on effluent quality and membrane fouling was examined as well. RESULTS The highest removal efficiencies (COD, TN, P) of the order of 99.2%, 94.3% and 89.3%, respectively, as well as moderate formation of EPS and SMPs were observed at a SRT of 20 days. Changes in HRT had a small effect on carbon and ammonium removal, but a strong impact on denitrification and phosphate removal. Total EPS and SMP were higher at higher SRTs in contrast to reported common observations. However, the shorter SRT of 15 days led to a more rapid increase of TMP. CONCLUSION The system's performance regarding COD, TN and P removal was very good despite the high COD/N input ratio and the huge variations in influent quality. The observed membrane fouling behavior reflects the complexity of the biological process. © 2014 Society of Chemical Industry

Effects of sludge retention times on nutrient removal and nitrous oxide emission in biological nutrient removal processes.

Sludge retention time (SRT) is an important factor affecting not only the performance of the nutrient removal and sludge characteristics, but also the production of secondary pollutants such as nitrous oxide (N2O) in biological nutrient removal (BNR) processes. Four laboratory-scale sequencing batch reactors (SBRs), namely, SBR5, SBR10, SBR20 and SBR40 with the SRT of 5 d, 10 d, 20 d and 40 d, respectively, were operated to examine effects of SRT on nutrient removal, activated sludge characteristics and N2O emissions. The removal of chemical oxygen demand or total phosphorus was similar under SRTs of 5–40 d, SRT mainly affected the nitrogen removal and the optimal SRT for BNR was 20 d. The molecular weight distribution of the effluent organic matters was in the range of 500–3,000 Da under SRTs of 5–40 d. The lowest concentration of the effluent soluble microbial products concentration was obtained at the SRT of 5 d. Nitrifier growth was limited at a short SRT and nitrite existed in the effluent of SBR5. With increasing SRTs, mixed liquor suspended solids concentration increased while the excess sludge production was reduced due to the high endogenous decay rate at high SRTs. Endogenous decay coefficients were 0.020 d−1, 0.036 d−1, 0.037 d−1 and 0.039 d−1 under SRTs of 5–40 d, respectively. In BNR, the N2O emission occurred mainly during the aerobic phase and its emission ratio decreased with increasing SRTs. The ratio between the N2O-N emission and the removed ammonium nitrogen in the aerobic phase was 5%, 3%, 1.8% and 0.8% at the SRT of 5 d, 10 d, 20 d and 40 d, respectively. With low concentrations of dissolved oxygen and high concentrations of oxidized nitrogen, the N2O emission was significantly accelerated due to heterotrophic denitrification activities.

Effect of temperature on membrane bioreactor performance working with high hydraulic and sludge retention time

This study analysed the effect of temperature on MBR performance. For this purpose, a full-scale pre-denitrification MBR system was used. This system operated at a constant flow of urban wastewater (0.42m3/h) with continuous monitoring of mixed liquor temperature (9–31°C). Its sludge retention time (SRT) was 35 days and its hydraulic retention time (HRT) was 35h. Variations in temperature did not affect the effluent COD concentration. Permeability was reduced both by the increase in membrane flux resistance (Rt) at temperatures <15°C and by viscosity. A lower temperature produced a decrease both in the endogenous respiration rate (bH) and the observable biomass yield coefficient for heterotrophs. This effect was particularly evident in the bH and led to an upward trend in the concentration of the TSS and the VSS/TSS ratio. The system was found to have a high nitrification and denitrification potential regardless of temperature. However, residual dissolved oxygen due to membrane aeration or recirculation reduced nitrate removal at low temperatures.

Alkaline Pretreatment of Waste Activated Sludge in Anaerobic Digestion

Slow degradation of waste activated sludge (WAS) is a disadvantage of anaerobic digestion leading to high sludge retention time in conventional digesters. So, this study was proposed to increase biodegradability of WAS by alkaline pretreatment, and effect of pretreatment on the performance of anaerobic digestion and treatment efficiency was evaluated under both ambient and thermophilic condition. The soluble chemical oxygen demand (SCOD) and biochemical oxygen demand (BOD) fractions increased when pH values were increased. The solubilization had the highest increase in the pH range of 11 to 12 and gradually increased in the pH range of 8-11. However, the BOD 20 /COD ratios decreased for adjusted pH 10 and pH 11, and biodegradation was limited at pH 12. And pH 8 was the optimal value this alkaline pretreatment on anaerobic digestion. In alkaline pretreatment thermophilic anaerobic condition (PTAN), the result remove were 42.16%, 43.15% and 50.64% for TS, VS and COD removal, respectively, which are higher efficiency and gas production when compared to other conditions.

Successful application of nitritation/anammox to wastewater with elevated organic carbon to ammonia ratios

The nitritation/anammox process has been mainly applied to high-strength nitrogenous wastewaters with very low biodegradable organic carbon content (<0.5 g COD∙g N−1). However, several wastewaters have biodegradable organic carbon to nitrogen (COD/N) ratios between 0.5 and 1.7 g COD∙g N−1 and thus, contain elevated amounts of organic carbon but not enough for heterotrophic denitrification. In this study, the influence of elevated COD/N ratios was studied on a nitritation/anammox process with suspended sludge. In a step-wise manner, the influent COD/N ratio was increased to 1.4 g COD∙g N−1 by supplementing digester supernatant with acetate. The increasing availability of COD led to an increase of the nitrogen removal efficiency from around 85% with pure digester supernatant to >95% with added acetate while the nitrogen elimination rate stayed constant (275 ± 40 mg N∙L−1∙d−1). Anammox activity and abundance of anammox bacteria (AMX) were strongly correlated, and with increasing influent COD/N ratio both decreased steadily. At the same time, heterotrophic denitrification with nitrite and the activity of ammonia oxidising bacteria (AOB) gradually increased. Simultaneously, the sludge retention time (SRT) decreased significantly with increasing COD loading to about 15 d and reached critical values for the slowly growing AMX. When the SRT was increased by reducing biomass loss with the effluent, AMX activity and abundance started to rise again, while the AOB activity remained unaltered. Fluorescent in-situ hybridisation (FISH) showed that the initial AMX community shifted within only 40 d from a mixed AMX community to “Candidatus Brocadia fulgida” as the dominant AMX type with an influent COD/N ratio of 0.8 g COD∙g N−1 and higher. “Ca. Brocadia fulgida” is known to oxidise acetate, and its ability to outcompete other types of AMX indicates that AMX participated in acetate oxidation. In a later phase, glucose was added to the influent instead of acetate. The new substrate composition did not significantly influence the nitrogen removal nor the AMX activity, and “Ca. Brocadia fulgida” remained the dominant type of AMX. Overall, this study showed that AMX can coexist with heterotrophic bacteria at elevated influent COD/N ratios if a sufficiently high SRT is maintained.

Influence of the Presence of Long Chain Fatty Acids (Lcfas) in the Sewage on the Growth of M. Parvicella in Activated Sludge Wastewater Treatment Plants

Foaming and bulking problems in activated sludge treatment plants are associated to the presence of a variety of filamentous bacteria. However, it has been observed that M. parvicella is the most frequent filamentous microorganism causing sludge bulking and foaming, especially in treatment plants involving nutrients removal. High sludge retention time, low DO, low temperature, presence of anoxic, anaerobic, and intermittently aerated zones, are the most commonly cited conditions associated with its growth. Substrate composition is another significant factor, as it has been found that slowly degradable organic material may favour the growth of M. parvicella. If has also been suggested that M. parvicella may preferably store long chain fatty acids (LCFAs) under anaerobic conditions and subsequently use it for growth. The paper presents the results of an investigation conducted at the wastewater treatment plant of Ioannina, aiming to establish a cause-effect relationship between the presence of LCFAs and the abundance of M. parvicella. This investigation is a part of a wider study sponsored by the Greek Secretariat for Research, under the PENED programme. The duration of the investigation covers 8 months, from January 2004 to August 2004. During this period samples were taken from the sewage collection network and wastewater treatment plant of Ioannina and analysed for, among other parameters, LCFAs and total fatty acids while samples of the mixed liquor and the foam in the biological reactors were microscopically analysed in order to determine the presence and amount of various types of filamentous bacteria. Two main conclusions were drawn. The first is related to the effect of temperature on the growth of M. parvicella, indicating that the growth of this specific filamentous bacterium is favoured by low temperatures (generally below 20 oC), while higher temperatures cause the practical elimination of M. parvicella, irrespectively of other factors. This conclusion verifies previous studies in pilot units and full scale plants. The second conclusion is that during winter periods there seems to be a positive correlation, between the presence of fatty acids and more specifically LCFAs and the amount of M. parvicella.

The performance of immobilized membrane bioreactor with different membrane operation modes

The immobilized membrane bioreactor (IMBR) was investigated for the removal of organic matter and its membrane fouling condition in treating food processing wastewater. The IMBR contains 5,000 mg/L mixed liquor suspended solids with hydraulic retention time of 24 h. The advantages of IMBR include high sludge retention time, improved removal of COD, ammonia nitrogen, and reduced membrane fouling frequency with much less production of soluble microbial products (SMPs). The results showed that the IMBR was an effective organic matter removal system because it achieved 96–97% removal of COD consistently. The concentration of total SMP in the IMBR was measured at 46.4 mg/L which included 24.8 mg/L of protein and 21.6 mg/L of carbohydrate. Steadily, approximately 33% of carbohydrate and 11% of protein were rejected by the microfiltration (MF) membrane. For this reason, it was concluded that carbohydrate poses a more significant impact on membrane fouling through formation of cake/gel layer than protein. Further, various operating conditions during membrane filtration were experimented which included continuous and intermittent filtration, aeration and non-aeration, and with fiber filter (FF) as pretreatment. It was discovered that while adding an additional FF filtration before MF might improve suspended solid retention, SMP was instead discovered to be the major cause of membrane fouling. In addition, aeration in the membrane tank could significantly improve membrane performance by scouring lightly attached particles from the membrane surface.

Fate and removal of typical pharmaceuticals and personal care products by three different treatment processes

The presence and distribution of typical of pharmaceuticals and personal care products (PPCPs), which comprise two types of polycyclic musks (PCMs) including Galaxolide (HHCB) and Tonalide (AHTN) as well as six types of estrogens containing estrone (E1), 17β-estradiol (E2), estriol (E3), 17α-ethynylestradiol (EE2), diethylstilbestrol (DES), and bisphenol A (BPA), were investigated at two wastewater treatment plants (WWTPs) in Jiangsu, China. Only raw wastewater was treated in WWTP A while WWTP B was serving an urban-industrialized area. In the influent, the concentrations of EE2 (2193–4437ngL−1), E2 (1126–1170ngL−1), and DES (268–421ngL−1) were generally higher than the previously reported values, whereas the concentrations of HHCB (306–316ngL−1), E1 (29–129ngL−1), E3 (53ngL−1), and BPA (26–176ngL−1) were much lower than those reported in other previous studies. In addition, AHTN was not detected in either WWTP and E3 was not found in WWTP B. The detected processes including anaerobic/oxic process (A/O), combined orbal oxidation ditch process (C-orbal OD) and anaerobic/anoxic/anoxic/oxic membrane biological reactor (A/A/A/O-MBR) showed higher removal efficiencies for HHCB (67–71%) and EE2 (87%) than those in other previous studies. Besides, the total hydraulic retention time (HRT) ranged between 6.7 and 20.0h, sludge retention time (SRT) ranged between 8 and 23d, and water temperature ranged from 24.8 to 28.2°C. The removal efficiencies for estrogens in biological processes were related to the following factors: the level of hydrophobic estrogens, the type of removal process (C-orbal OD was consistently less efficient in removing estrogens than A/O and A/A/A/O-MBR), and a high SRT or HRT (A/A/A/O-MBR with higher SRT and HRT showed higher and more stable removal of hydrophobic estrogens).

Nanofiltration membrane bioreactor for removing pharmaceutical compounds

The application of a nanofiltration membrane bioreactor (NF-MBR) has been investigated for the treatment of two pharmaceutical compounds (cyclophosphamide and ciprofloxacine). A new type of ceramic nanofiltration (NF) membrane has been employed for the filtration of acclimated activated sludge artificially doped by these antibiotics. The performance of the NF-MBR was monitored during approximately 4 months, in order to evaluate the long term operational stability of the system and the potential effect of the high sludge retention time on antibiotics degradation. The hypothesis of this study lies in the development of NF membrane and its cut-off support in order to (i) retain micropollutants and (ii) increase micropollutant concentration in the bioreactor. The two antibiotics are weakly degraded even with acclimated sludge, and only a small fraction of these compounds was retained when the fouling of the membrane increased. COD, total suspended solids (TSS) and colour were effectively removed. No unclogging operation was necessary (4 months of operation without chemical cleaning). The biodegradability, retention and adsorption mechanisms have been also studied.

Immobilized bioprocess for organic carbon and nitrogen removal

Abstract The immobilized bioprocess (IBP) was investigated for the removal of organic carbon and ammonia nitrogen from wastewater. Two wastewaters (a food industry wastewater and a composite industrial wastewater) were investigated, one containing high concentration (820–1300 mg/l) and the other medium concentration (250–450 mg/l) of chemical oxygen demand (COD). Three mixed liquor suspended solids (MLSS) (3000, 5000, and 9000 mg/l) and two hydraulic retention times (HRT) (12 h and 24 h) were employed representing different surface loading and volume loading factors. IBP provides high sludge retention time (SRT), enabling improved removal of COD and ammonia nitrogen. The results showed 78–93% and 83–96% removals of soluble COD (SCOD) and ammonia (NH4 +–N), respectively, from the food industry wastewater, when the surficial removal rate and volumetric removal rate in the IBP were 0.011–0.056 kg COD/m2-d and 2.3–12 kg COD/m3-d, respectively. For the composite industrial wastewater, removal of SCOD and NH4 +...

Respirometric Operational Issues Related to mbr Systems Working at High Sludge Retention Times

Respirometric Operational Issues Related to mbr Systems Working at High Sludge Retention Times

Improved elimination of organic micropollutants by a process combination of membrane bioreactor (MBR) and powdered activated carbon (PAC)

Abstract Membrane bioreactors (MBR) are increasingly considered for de-centralized wastewater treatment. As MBR can be operated with higher sludge retention time and total suspended solids than conventional wastewater treatment plants (WWTP) the elimination of micropollutants is slightly better for MBR than for WWTP. However to be able to use the MBR filtrate for artificial ground water recharge it is necessary to further improve elimination of micropollutants. In this study, the MBR process has therefore been combined with the adsorption on powdered activated carbon (PAC). PAC was dosed to the MBR filtrate of a pilot plant and removed again after a contact time of 30 min. PAC was recycled to the MBR process tank to use the residual adsorption capacity. The process was operated continuously for a period of 110 d. With 5–10 mg l−1 PAC dosage a 50–80% improved elimination of carbamazepine and diclofenac could be achieved compared to the process without PAC.

Influence of organic loading rate on the performance of ultrafiltration and microfiltration membrane bioreactors at high sludge retention time

This paper presents the results of 220 days of two bench scale submerged membrane bioreactor (MBR) experiments using micro (MF-MBR) and ultrafiltration (UF-MBR) flat sheet membranes. Both systems operated with a volumetric loading rate of between 0.4 and 1.3kgCODm−3d−1. Results showed that the growth rate of MLSS increased when the loading rate increased and that results related to removal efficiency rendered values higher than 93% chemical oxygen demand (COD) removal in both MBRs, indicating their suitability for operating at different loading rates. As regards membrane fouling, it was observed that the total number of chemical cleanings performed in the two MBRs was similar. The cleaning frequency decreased when the concentration of mixed liquor suspended solids (MLSS) and organic loading increased in both MBRs. MF membranes worked better at extended periods of operation and high concentrations of MLSS, whereas the UF-MBR maintained lower permeability at the beginning of the experiments at MLSS concentrations lower than 2–3g/L. Dead-end filtration experiments showed that the highest filtration resistance was found at the highest soluble extracellular polymeric substance (EPS) concentrations, that is, at low sludge ages or low MLSS concentrations, indicating that membrane fouling is attributed to a large extent to the existence of this kind of EPS.