High Solids Retention Time Research Articles

Study on enhanced denitrification using particulate organic matter in membrane bioreactor by mechanism modeling

Particulate organic matter (POM) in wastewater is a potential denitrification carbon source, while the optimal operational mode using denitrification mechanism with POM is still unclear in wastewater treatment plants. In this work, we investigated the denitrification rates (DNRs) in a full-scale membrane bioreactor (MBR) coupled with two-stage pre-anoxic (pre-AN), and then evaluated the POM denitrification efficiency using mechanism modeling. The results indicate that POM related fraction accounted for the majority of the obtained specific DNR of 1.39±0.46mgNg−1 MLVSS h−1 in the second pre-AN without available soluble carbon source. The modeling approaches with calibration and validation procedures estimated a high residual POM concentration of 0.17g COD g−1 MLVSS in the activated sludge, which provided specific DNR of 1.14mgNg−1 MLVSS h−1. High POM retention time in the reactor was the result of high solid retention time used in the MBR. In particular, post-AN of high biomass concentration could provide the highest POM denitrification efficiency in MBR. The MBR process combined with additional sludge reduction technology could further enhance denitrification by POM.

Role of Solids Retention Time on Complete Nitrification: Mechanistic Understanding and Modeling

Effects of solids retention time (SRT) on both steps of the nitrification process (e.g., ammonia oxidation and nitrite oxidation) and the nitrifier community in a complete-mix activated sludge process were studied. At a SRT less than or equal to 20 days, the effluent ammonia concentration was lower than the effluent nitrite concentration. At the 40-day SRT, however, the effluent nitrite concentration became equal to or less than the effluent ammonia concentration. Quantitative Polymerase Chain Reaction (QPCR) assays indicated that increasing SRT significantly increased nitrite-oxidizing bacteria/ammonia-oxidizing bacteria (NOB/AOB) ratio. Further investigation indicated that Nitrosomonas europaea/eutropha were the dominant AOB for ammonia oxidation under all SRTs. However, for nitrite oxidation, Nitrobacter-like NOB played the key role at low SRTs, while Nitrospira-like NOB played the key role at high SRTs. Modeling results indicated that, for AOB and NOB, the values of maximum specific growth rate (μ) were 0.24 and 0.18/day, respectively, while the values of decay coefficient (Kd) were 0.066 and 0.045/day, respectively. The half-velocity constants (KS) for both AOB and NOB were approximately 0.02 mg/L, and the maximum specific substrate utilization rate (k) for AOB and NOB were 1.3 and 3.0 g-N/g-VSS-day, respectively. These values suggest that AOB had an advantage over NOB at a lower SRT, mainly because AOB grew faster than NOB did. On the contrary, the increased NOB/AOB ratio at a longer SRT was likely due to NOB having a smaller kd. The very low KS values for both AOB and NOB imply that a plug-flow reactor may not have much kinetic advantage over a complete-mixed reactor for a complete nitrification process. The mass concentrations of AOB and NOB in the activated sludge can also be calculated based on the measured nitrification capacity and their k value.

Performance of anaerobic membrane bioreactor for sewage sludge treatment: Mesophilic and thermophilic processes

The viability of the anaerobic membrane bioreactor (AnMBR) technology for the treatment of sewage sludge has been studied under mesophilic and thermophilic conditions. Coupling membrane filtration with the digestion process allows the digestion to be operated with longer solids retention time (SRT), and the volumetric load can be increased considerably compared to the conventional completely stirred reactor process. Maximum SRT is conditioned by the total solids concentration and the viscosity of the digested sludge. Values of 50 and 30 days for 55 and 35°C were obtained in the experimental platforms. Organic loads of 6.4 and 4.6gCOD/(L·d) were successfully treated at 55 and 35°C, respectively, representing a significant digester volume reduction compared to conventional systems. The performance of thermophilic and mesophilic AnMBRs were compared, and the same sludge biodegradability was measured. The difference was the distribution of the inert COD among the soluble and particulate fractions. Better filtration performance was observed in the thermophilic system, where a larger flow could be attained and the lower viscosity enabled operation with higher SRT. However, increased irreversible fouling at higher temperature was shown to require optimised chemical cleaning strategies to extend membrane lifetime. In conclusion, AnMBR offers an extensive opportunity for the treatment of waste streams with high-solids content waste streams, where membrane filtration is one of the most promising technologies for decoupling solids and hydraulic retention times.

Physicochemical and Morphological Characteristics of Granular Sludge in Upflow Anaerobic Sludge Blanket Reactors

Granular sludge formed in upflow anaerobic sludge blanket reactors play an important role in the field of anaerobic treatment due to their engineering advantages over conventional flocculent forms, such as rich microbial diversity, high solid retention time due to its superior settling characteristics, maximizing the microorganisms-to-space ratio, and greater ability to withstand shock loadings or temperature changes. A better understanding of granule characteristics is highly useful and vital for efficient operation of bioreactors. Recently, developments in the characterization of microbial granules have been made with innovative approaches and sophisticated technologies. This article provides an up-to-date review from the past two decades in the understanding of physicochemical and morphological characteristics of granular sludge with regard to granule size, settling velocity, specific gravity, sludge volume index, volatile suspended solids–to–suspended solids ratio, ash content, inorganic chemical content, crystalline structure, molecular functional groups, microbial structure and composition, microbial communities, and methanogenic activity. Addition of external additives such as synthetic and natural polymers and vitamins may enhance the characteristics of granular sludge. Bioaugmentation might be a useful tool for improving the stability and significantly reducing the length of start-up periods. Application of easily degradable cosubstrates such as glucose, sucrose, cellulose, molasses solution, and volatile fatty acids is highly beneficial in treating toxic wastewaters and improving sludge characteristics. Emerging knowledge on such characteristics might exhibit the optimization of microbial granulation as one of the most reliable techniques in anaerobic treatment. Future research directions are also highlighted.

Micropollutants removal in an anaerobic membrane bioreactor and in an aerobic conventional treatment plant

The paper expresses an attempt to tackle the problem due to the presence of micropollutants in wastewater which may be able to disrupt the endocrine system of some organisms. These kinds of compounds are ubiquitously present in municipal wastewater treatment plant (WWTP) effluents. The aim of this paper is to compare the fate of the alkylphenols-APs (4-(tert-octyl)) phenol, t-nonylphenol and 4-p-nonylphenol and the hormones (estrone, 17β-estradiol and 17α-ethinylestradiol) in a submerged anaerobic membrane bioreactor (SAMBR) pilot plant and in a conventional activated sludge wastewater treatment plant (CTP). The obtained results are also compared with the results obtained in a previous study carried out in an aerobic MBR pilot plant. The results showed that the APs soluble concentrations in the SAMBR effluent were always significantly higher than the CTP ones. Moreover, the analyses of the suspended fraction revealed that the AP concentrations in the SAMBR reactor were usually higher than in the CTP reactor, indicating that under anaerobic conditions the APs were accumulated in the digested sludge. The aerobic conditions maintained both in the CTP system and in the aerobic MBR favoured the APs and hormones degradation, and gave rise to lower concentrations in the effluent and in the reactor of these systems. Furthermore, the results also indicated that the degradation of APs under aerobic conditions was enhanced working at high solid retention time (SRT) and hydraulic retention time (HRT) values.

The influence of solids retention time on activated sludge bioflocculation and membrane fouling in a membrane bioreactor (MBR)

Membrane fouling remains a significant drawback for membrane bioreactors (MBRs). The solids retention time (SRT) has been widely acknowledged to be an important factor influencing membrane fouling. In general, lower membrane fouling rates are observed at elevated SRTs, however, the direct mechanisms through which a high SRT alleviates fouling are unclear. Since it has also been reported that activated sludge bioflocculation is an important factor in membrane fouling, this paper studies the impact of SRT on bioflocculation with respect to membrane fouling. A pilot-scale MBR was operated for more than two years at three different SRTs during which bioflocculation was closely monitored by means of an automated image analysis procedure while the fouling rate was recorded on-line for different fluxes and different filtration/relaxation cycles. In addition, the Delft filtration characterization method (DFCm) was employed to assess the activated sludge fouling propensity. Based on these data, it is shown that stable operation of a membrane bioreactor requires a good activated sludge condition and that bioflocculation is a crucial factor within that context. In the tested SRT range (10–30–50 days), a higher SRT contributes to better activated sludge bioflocculation and as a consequence, to lower fouling rates.

Toward Long Solids Retention Time of Activated Sludge Processes: Benefits in Energy Saving, Effluent Quality, and Stability

The activated sludge process is the most common method of secondary municipal wastewater treatment; solids retention time (SRT) is the key control parameter for this process. Typically, operating at long SRT is considered only for nitrification, but there are additional benefits of high SRT operation. This paper presents experimental and literature evidence to demonstrate three major additional benefits of long SRT operation: increased oxygen transfer efficiency; improved biomass particle size distribution, which results in more efficient clarification with fewer effluent particles and suspended solids; and enhanced removal of many emerging contaminants such as pharmaceuticals, personal care products, and endocrine disrupting compounds. This paper presents experimental results from several treatment plants that showed increasing oxygen transfer efficiency and particle size with increasing SRT, and evidence documenting improved removal of emerging contaminants and biodegradable organic carbon. A long-term survey of three treatment plants concludes that operating at higher SRT is not as energy intensive as typically assumed.

ChemInform Abstract: Recalcitrant Industrial Wastewater Treatment by Membrane Bioreactor (MBR)

AbstractReview: 125 refs.

Investigation of long-term operation and biomass activity in a membrane bioreactor system

The aim of this work was to evaluate the long-term performance of a Membrane Bioreactor (MBR) that operated continuously for 2.5 years and to assess membrane fouling and biomass activity under various operating conditions. Furthermore, a method for the characterisation of influent wastewater was developed based on its separation into various fractions. The MBR system operated at the solids retention times (SRT) of 10, 15, 20 and 33 days. The increase of SRT resulted in a decrease of the fouling rate associated with the reduction of extracellular polymeric substances. Moreover, the SRT increase resulted in a significant reduction of the Oxygen Uptake Rate (OUR) due to the lower availability of substrate and in a notable decrease of the maximum OUR since high SRT allowed the development of slower growing microorganisms. Biomass consisted of small flocs due to extensive deflocculation caused by intense aeration. Finally, the method developed for wastewater characterisation is straightforward and less time consuming than the usual method that is employed.

An investigation of the long-term filtration performance of a membrane bioreactor (MBR): The role of specific organic fractions

The scope of this paper is to systematically examine the long-term filtration performance of a laboratory scale MBR pilot, under nine different operating states, for elucidating the role of specific organic fractions of the mixed liquor. It is shown that the Solids Retention Time ( SRT) is a major parameter influencing the concentration of both Dissolved Organic Matter ( DOM) – mainly comprising Soluble Microbial Products ( SMP) – and bound Extracellular Polymeric Substances ( bEPS). Increasing SRT results in a noticeable reduction of both total and biodegradable fraction of DOM; at high SRT, the total bEPS concentration slightly increases whereas the bEPS biodegradable fraction decreases. The results underpin a close interrelation between DOM and bEPS, which are apparently under a dynamic equilibrium; biosorption phenomena are suggested as a mechanism significantly influencing this equilibrium. Membrane filtration performance has been examined over a wide range of SRT and HRT values. Temporal profiles of reversible and irreversible filtration resistance have been evaluated in connection with DOM and bEPS concentration data. It appears that low DOM concentration and bEPS content inhibit the self-accelerating phenomena leading to a Trans-membrane Pressure ( TMP) jump, thus permitting a longer sustainable filtration operation; possible mechanisms are also suggested. This study provides useful data and ample evidence on the critical effect of DOM and bEPS on membrane fouling, helpful for further investigations and eventually for MBR operation.

Removal of steroid estrogens in carbonaceous and nitrifying activated sludge processes

A carbonaceous (heterotrophic) activated sludge process (ASP), nitrifying ASP and a nitrifying/denitrifying ASP have been studied to examine the role of process type in steroid estrogen removal. Biodegradation efficiencies for total steroid estrogens (Σ EST ) of 80 and 91% were recorded for the nitrifying/denitrifying ASP and nitrifying ASP respectively. Total estrogen biodegradation (Σ EST ) was only 51% at the carbonaceous ASP, however, the extent of biodegradation in the absence of nitrification clearly indicates the important role of heterotrophs in steroid estrogen removal. The low removal efficiency did not correlate with biomass activity for which the ASP carbonaceous recorded 80 μg kg −1 biomass d −1 compared to 61 and 15 μg kg −1 biomass d −1 at the ASP nitrifying and ASP nitrifying/denitrifying respectively. This finding was explained by a moderate correlation ( r 2 = 0.55) between total estrogen loading (Σ EST mg m −3 d −1) and biomass activity (μg Σ EST degraded kg −1 d −1) and has established the impact of loading on steroid estrogen removal at full-scale. At higher solids retention time (SRT), steroid estrogen biodegradation of >80% was observed, as has previously been reported. It is postulated that hydraulic retention time (HRT) is as important as SRT as this governs both reaction time and loading. This observation is based on the high specific estrogen activity determined at the ASP carbonaceous plant, the significance of estrogen loading and the positive linear correlation between SRT and HRT.

Kinetics Study of Sewage Sludge Treatment by an Aerobic Digestion

This study was carried out to evaluate the applicability of three known kinetics models (Monod, Contois and Chen and Hashimoto), overall microbial kinetics, as well as to experimentally assess the influence of the organic loading rates and retention times on kinetic models. To accomplish the objectives of this study, a 50 litre laboratory scaled membrane anaerobic system (MAS) combining ultrafiltration (UP) membrane with anaerobic reactor was used to treat raw sewage sludge, which collected from Taman Tun Dr. Ismail, sewage treatment plant (T.T.Dr.Ismail). Six steady states were attained as a part of a kinetic study. The results of all SIX . steady states were successfully fitted, above 98%, by Monod, Contois, and Chen and Hashimoto models. Contois model appeared to be the best at 99.7%. The microbial kinetic constants were Y = 0.74 gVSS/gCOD and b = 0.20 day-I. The minimum solids retention time, Be min obtained from the three kinetic models ranged from 5-16.9 days. The total gas yield obtained ranged from 0.28 VgCOD/d to 0.81 VgCOD/d at organic loading rate (OLR) of between 0.1kgCOD/m3.d to 10 kgCOD/m3d. The solids retention time (SRT) decreased from 1250 days to 16.1 days, (from SSI to SS6). The composition of methane gas, CH4 varied from 66.3% to 76.3%. At CH4, 66.3%, the solids retention time and hydraulic retention time were found to be 16.1 day and 7.8 days, with the COD removal efficiency range of between 96.5 - 99 % as well as high solids retention time 8c• The range of mixed liquor suspended solids was from 12760 mg/l to 21800 mg/l. The two methods of membrane cleaning (Mild brushing , flush with water and soak membrane in 0.1 M NaOH for day), were very important in order to increase the permeate flux and flowrate. The flux recovery time was 15 to 18 days. The maximum and minimum level of the flux were found to be 62. 1 Vm2 /hr and 6.9 Vm2/hr respectively.

The application of membrane bioreactors as decentralised systems for removal of endocrine disrupting chemicals and pharmaceuticals

The concentrations of some important endocrine disrupting chemicals and pharmaceuticals after various stages of wastewater treatment were investigated. The endocrine disrupting chemicals included natural and synthetic estrogenic and androgenic steroids. The pharmaceuticals included a series of sulfonamide antibiotics and trimethoprim. The removal efficiency of a membrane bioreactor (MBR) was investigated and compared with a conventional activated sludge (CAS) system. Samples were analysed by liquid chromatography tandem mass spectrometry. Results showed that the MBR and CAS systems effectively removed steroidal estrogens and androgens, but only partially eliminated the target antibiotics from wastewater. The MBR was shown to be more effective than the CAS system which was possibly attributed to the high solid retention time and concentration of biosolids in the MBR. The results highlight the potential wider application of MBRs for the removal of trace chemical contaminants in wastewater and their potential for use as decentralised wastewater treatment systems.

Evaluation of potential integration of entrapped mixed microbial cell and membrane bioreactor processes for biological wastewater treatment/reuse

The challenge of biological wastewater treatment process is the design and operation of effective retention of mixed microbial cells within the reactor. Entrapped mixed microbial cell (EMMC) technology is designed to entrap the mixed microbial cells in polymeric carriers; membrane bioreactor (MBR) process utilizes membrane sheets/fibers to effectively retain the biomass in the reactor. These two biotechnologies are considered potential alternatives for conventional biological treatment/reuse because of their capability of retaining high concentration of biomass in the reactor, or in other words increasing the solid retention time (SRT). The simultaneous removal of organics and nitrogen were investigated using a modified EMMC system design. The modified EMMC system demonstrated higher organic and nitrogen removal performance due to high SRT. Compared to single-stage MBR process operated at similar conditions, the modified EMMC system was able to achieve slightly lower organic removal, comparable nitrification, and higher total nitrogen removal. One limitation in applying an EMMC only treatment process regime for potential reuse of treated wastewater is that such an operation requires the removal of pathogens and large particles if disinfection and solid/liquid separation were not followed. The major challenge of MBR process to overcome is membrane fouling, and the high energy consumption associated with fouling control. The intrinsic features of EMMC process including high SRT, low, and stabilized effluent suspended biomass concentration may significantly reduce the chance and extent of membrane fouling; while the membrane filtration can further polish the effluent quality from EMMC process. Therefore, integrating MBR and EMMC is strongly recommended because it may be a “break-through” for solving the membrane fouling problem and in improving effluent quality for potential reuse.

A modified Activated Sludge Model to estimate solids production at low and high solids retention time

In this paper, a modified version of the IWA-ASM1 model capable of correctly simulating the production of solids over a wide range of solids retention time (SRT) is presented. The parameters of the modified model have been estimated by integrating the results of respirometric and titrimetric tests with those of studies conducted on pilot scale plants that treat industrial wastewaters of differing characteristics. On the basis of the experimental results and their subsequent processing, it appears that the production of solids may be satisfactorily estimated using the modified model in which fractions X P and X I are supposed to be hydrolysable with a first-order kinetic. In the cases that were examined, the constant of the aforementioned kinetics was estimated to be k i = 0.012 d −1 and k i = 0.014 d −1, for tannery and textile wastewater respectively. A reliable calibration of the parameter k i was possible when data relative to the experiment conducted in the pilot plants for no less than 60 d and in conditions of complete solid retention was utilized.

Study of the acclimation stage and of the effect of the biodegradability on the performance of a microbial fuel cell

In this work, it has been studied the production of electricity and the oxidation of the pollutants contained in a synthetic wastewater fed with glucose and peptone of soybean as carbon sources, using a mediator-less microbial fuel cell (MFC). Special attention has been paid to the acclimation stage, in which it was found that with high hydraulic and solid retention times it is possible to obtain a very efficient process with a 90% COD removal and practically total conversion of COD into electricity (considering the typical stoichiometric yield of heterotrophic biomass). The influence of concentration sludge was studied working with three different amounts of suspended solids, from 120 to 14000 mg. The maximum power density increased exponentially with the concentration sludge from 2.1 mW m −2 to 11 mW m −2 at the highest concentration sludge. More over, the percentage of the influent COD used to produce electricity was higher than 100% when the highest sludge concentration was used. This was explained taking into account the endogenous metabolism of micro-organisms presented in the system. Moreover, wastewater with two different compositions, but with the same COD concentration, were studied. One with 50% of glucose and 50% of peptone of soybean and the other, with 80% of peptone and 20% of glucose. Spite of the wastewater with 50% of glucose is more biodegradable than the other composition used, the microbial fuel cell performance obtained was lower than with the other (2.1 mW m −2 with respect to 6.8 mW m −2 when 80% of peptone was used). This means that the degradation of peptone occurs through the production of intermediates that favour electricity.

Evaluation of simultaneous nutrient removal and sludge reduction using laboratory scale sequencing batch reactors

The treatment and disposal of excess sludge has been a rising challenge for wastewater treatment plants worldwide. In this study, simultaneous sludge reduction and nutrient removal was evaluated in laboratory scale sequencing batch reactors (SBRs). Two SBRs were operated alongside for a duration of 370 d. One SBR was operated to achieve nutrient removal (control-SBR) at 10 d solids retention time (SRT), while the other (modified-SBR) was operated to achieve nutrient removal along with sludge reduction. Sludge reduction in the modified-SBR was accomplished by subjecting the recycled biomass to feasting and fasting at sufficiently high SRT close to infinity (phase I and II) and finite SRT (phase III). The observed biomass yield in the modified-SBR was estimated to be 0.17 mg TSS mg −1 COD, representing 63% sludge reduction compared to the control-SBR. The NH 3 levels in the effluents from both SBRs always remained below detection limit. The average dissolved phosphorus removal efficiencies in the control-SBR and the modified-SBR were 87% and 84%, respectively, during phase II. However, the biomass of the modified-SBR increased during phase II. To control this, biomass wastage was initiated directly from the modified-SBR during phase III at a rate equivalent to the observed biomass accumulation rate in the system in phase II. This resulted in an overall 100 d SRT for the modified-SBR system. Following this change, biomass accumulation in the modified-SBR was controlled, and a net 63% sludge reduction could be sustained along with 90% phosphorus and 100% NH 3 removal. Consistent denitrification activities were also noticed in both SBRs despite the absence of any carbon source during the anoxic phase of every cycle.

Removal and degradation characteristics of natural and synthetic estrogens by activated sludge in batch experiments

The removal and degradation characteristics of natural and synthetic estrogens by activated sludge were investigated by a series of batch experiments using the activated sludge samples of four actual wastewater treatment plants and synthetic wastewater spiked with estrogen. The rapid removal and degradation of 17β-estradiol (E2) and estrone (E1) were observed by the activated sludge samples of the oxidation ditch process which operated at higher solids retention time (SRT). On the other hand, E1 tended to remain both in the water phase and the sludge phase in the activated sludge samples of the conventional activated sludge process which operated at lower SRT. The anoxic condition was considered to be not favorable to the effective removal of estrogens as compared with the aerobic condition. The removal and degradation of EE2 showed the lag phase, which neither E2 nor E1 showed, but EE2 was finally removed and degraded completely after 24 h. The removal of estrogens in the water phase did not follow the first-order-rate reaction because a large part of the spiked estrogen was immediately removed from the water phase to the sludge phase by adsorption.

Digestion of thermally hydrolyzed sewage sludge by anaerobic sequencing batch reactor

Laboratory experiments were conducted to investigate the performance of an anaerobic sequencing batch reactor (ASBR) for the digestion of thermally hydrolyzed sewage sludge. Both mesophilic ASBR and continuous-flow stirred tank reactors (CSTR) were evaluated with an equivalent loading rate of 2.71 kg COD/m 3 day at 20-day hydraulic retention time (HRT) and 5.42 kg COD/m 3 day at 10-day HRT. The average total chemical oxygen demand (TCOD) removals of the ASBR at the 20-day and 10-day HRT were 67.71% and 61.66%, respectively. These were 12.38% and 27.92% higher than those obtained by CSTR. As a result, the average daily gas production of ASBR was 15% higher than that of the CSTR at 20-day HRT, and 31% higher than that of the CSTR at 10-day HRT. Solids in thermally hydrolyzed sludge accumulated within ASBR were able to reach a high steady state with solid content of 65–80 g/L. This resulted in a relatively high solid retention time (SRT) of 34–40 days in the ASBR at 10-day HRT. However, too much solid accumulation resulted in the unsteadiness of the ASBR, making regular discharge of digested sludge from the bottom of the ASBR necessary to keep the reactor stable. The evolution of the gas production, soluble chemical oxygen demand (SCOD) and volatile fatty acids (VFAs) in an operation cycle of ASBR also showed that the ASBR was steady and feasible for the treatment of thermally hydrolyzed sludge.

Application of a membrane sequencing batch reactor for landfill leachate treatment

A bench-scale membrane sequencing batch reactor (MSBR) was used for the treatment of “mature” landfill leachate, originated in the municipal solid waste landfill of Thessaloniki (North Greece). A hollow fiber ultrafiltration membrane module was used for the separation of the biomass from the treated effluent. The average quality of the raw leachate (feed) was 2456 mg/L COD, 375 mg/L total nitrogen (TN) and 8.2 mg/L PO 4-P. Hydraulic retention time (HRT) was kept constant at 10 days for most experimental runs whereas the solids retention time (SRT) was practically infinite, as almost no waste sludge removal took place during an overall MSBR operational period of 4 months. The initial concentration of suspended solids in the mixed liquor (MLSS) was 7000 mg/L and it was increased up to 15,300 mg/L. Several operational patterns were investigated in order to obtain maximum organic carbon and nutrients removal. Specific attention was given to the denitrification step. COD removal efficiency was as low as 40% and it was always below 60%. The poor MSBR performance in terms of COD removal was attributed to the high SRT, which had an apparent effect on the activity of the system's drastic biomass. On the other hand, TN removal efficiency was very satisfactory during the most stages of the MSBR operation, reaching a maximum value of 88%. Finally, PO 4-P removal efficiency varied between 35 and 45% in the first 50 days of the MSBR operation. The subsequent PO 4-P accumulation in the treated effluent was due to the direct addition of KH 2PO 4/K 2HPO 4 solutions in the feed leachate, aiming to improve the C:N:P ratio.