Conventional Activated Sludge System Research Articles

Two advanced biological approaches for sludge minimization from municipal wastewater treatment

Two advanced biological solutions for sludge minimization in wastewater treatment are tested. The first solution, particularly suitable for new installations, is based on the application of the sequential batch biofilter granular reactor (SBBGR). The second one, mostly appropriate for existing plants, is the alternate cycles process applied in the sludge line (ACSL) of conventional activated sludge systems. The results of treating raw municipal wastewater show that the SBBGR system is able to reduce the quantity of sludge up to 80%. Furthermore, the produced excess sludge requires no longer stabilization compared with the usual aerobic/anaerobic one. As regards the ACSL process, the results obtained in the full scale have shown an observed sludge yield reduction up to 54% with an increase in the specific oxygen uptake rate up to 20 mgO2/gVSS/h. Finally, applying the ACSL process low specific consumption of energy is required.

The effect of iron dosing on reducing waste activated sludge in the oxic-settling-anoxic process

This study evaluates the biological solid reduction in a conventional activated sludge system with an anoxic/anaerobic side stream reactor receiving 1/10 of return sludge mass. Influent iron concentrations and feeding modes were changed to explore the consistency between the influent iron concentration and yield values and to assess the impact of feeding pattern. The results indicated that sludge reduction occurs during alternately exposure of sludge to aerobic and anoxic/anaerobic conditions in a range of 38–87%. The sludge reduction values reached a maximum level with the higher iron concentrations. Thus, it is concluded that this configuration is more applicable for plants receiving high iron concentrations in the wastewaters.

Separation of organic matter from domestic sewage in a UASB-ABF system with anoxic bio-flocculation

An anoxic sewage treatment process, an upflow anaerobic sludge blanket (UASB) followed by an aerated bio-filter (ABF), was investigated for the reduction of oxygen demand and the separation efficiency of the organic matter. After recycling the nitrified effluent, complete denitrification occurred in the UASB, with an enhancement in both turbidity characteristics and TCOD removal rates. Low turbidity and COD in the effluent of the UASB reduced the oxygen demand and improved the nitrification efficiency in the subsequent ABF. Both with and without the recycling of the nitrified effluent in the UASB, 95 and 63% of the TCOD values, respectively, were removed. Compared with a conventional activated sludge system, approximately 11.74% of the TCOD was converted to CO2 in both the UASB and the ABF, generating an approximately 60% reduction in the amount of CO2. After accumulation in the UASB, 84% of the influent TCOD could be sequestered for use as a marginal energy source in a subsequent anaerobic digester.

Effect of Anaerobic Digestion on Rheological Parameters and Dewaterability of Aerobic Sludges from MBR and Conventional Activated Sludge Plants

The paper investigates and compares the characteristics of sludges produced by membrane Bio-Reactor (MBR) and Conventional Activated Sludge (CAS) systems. Stability and dewaterability of full-scale MBR and CAS treatment plants are measured and compared. Obtained results show that specific methane production is higher in CAS sludge compared to MBR sludge because of the higher solid retention time of the MBR. Nonetheless MBR sludge results to be characterized by a non-negligible biometanation potential (BMP). Methane production measured during BMP tests is around 250 NmL/gVS for MBR sludge, equaling 2/3 of methane production obtained, in similar condition, for CAS sludge. Dewaterability is evaluated by Specific Resistance to Filtration (SRF), Capillary Suction Time (CST) and Time To Filter (TTF) tests. Data obtained before and after the anaerobic digestion of the sludges show that the stabilization process affects much more the resistance to filtration of CAS sludge than the resistance to filtration of MBR sludge. The same trend is observed, for the other measured parameters. This is attributed to the variation of the rheological characteristics of the two sludges during the stabilization process, and to the different initial rheological properties of them due to the different selection process of the biomass deriving from the different biomass retention methods of MBR and CAS systems.

Impact of substrate feed patterns on solids reduction by the Cannibal process.

A biological solids reduction process that uses a side-stream anaerobic reactor (Cannibal process) was investigated to better understand why variations in solids reduction occur in field installations. Field observations indicated that single stage, high sludge age processes would frequently not provide a low solids yield. To study this, laboratory sequencing batch reactors were operated, one as a conventional activated sludge system and the other two with a side-stream anaerobic reactors. The control and one anaerobic side-stream system were operated with a 5-minute feed cycle to mimic a plug flow system. The other anaerobic side-stream system operated with a 4-hour feed cycle to simulate complete mix. The 5 minutes feed Cannibal system generated up to 80%. less biological solids than the control. The 4 hours feed Cannibal system produced less biological solids than the conventional activated sludge system but remained higher than for system with a 5-minute feed time.

Determination of Microbial Growth by Protein Assay in an Air-Cathode Single Chamber Microbial Fuel Cell

Microbial fuel cells (MFCs) have gathered attention as a novel bioenergy technology to simultaneously treat wastewater with less sludge production than the conventional activated sludge system. In two different operations of the MFC and aerobic process, microbial growth was determined by the protein assay method and their biomass yields using real wastewater were compared. The biomass yield on the anode electrode of the MFC was 0.02 g-COD-cell/g- COD-substrate and the anolyte planktonic biomass was 0.14 g-COD-cell/g-COD-substrate. An MFC without anode electrode resulted in the biomass yield of 0.07 ± 0.03 g-COD-cell/g-COD-substrate, suggesting that oxygen diffusion from the cathode possibly supported the microbial growth. In a comparative test, the biomass yield under aerobic environment was 0.46 ± 0.07 g-COD-cell/g-COD-substrate, which was about 3 times higher than the total biomass value in the MFC operation.

An experimental study on the impact of bioflocculation on activated sludge separation techniques

Membrane bioreactors have become a well-established alternative for conventional activated sludge systems with a filtration instead of a sedimentation step to separate the sludge from the purified water. Both separation processes, however, depend on good bioflocculation for their performance. Small activated sludge constituents are known to be detrimental in activated sludge filtration, while poor settling can often be attributed to improper floc formation. Therefore, the aim of this paper is to further unravel the relationship between bioflocculation and activated sludge separation processes. To this end, synthetic influents with different mono-over-polyvalent (M/P) cationic compositions are used to create activated sludges with different morphologies and bioflocculation states during a 178days experiment. The activated sludges were characterized by microscopic image analysis, relative hydrophobicity, extracellular polymeric substances, and were subjected to sedimentation, dead-end and submerged crossflow filtration tests. The deflocculating effect of a high M/P ratio is affirmed, inducing the release of colloidal material that caused severe fouling. Sludge flocs became empty and did not settle well. In the low M/P reactor, overall a better separation performance was observed but a non-expected filamentous bulking episode caused bad settleability and bad crossflow filterability, while dead-end filterability was good since a porous, well structured fouling layer was formed. Hence, it is concluded that dead-end filtration tests cannot asses sludge filterability in MBRs and that big activated flocs cannot contribute to the cake layer and cannot ensure its porosity in this way. Hydrophobicity was generally higher in the low M/P reactor but this might be attributed to an abundance of filaments during part of the testing period. No conclusive information could be extracted regarding the role of EPS, thus the use of EPS as a fouling or bioflocculation indicator is (again) questioned.

Superior Removal of Disinfection Byproduct Precursors and Pharmaceuticals from Wastewater in a Staged Anaerobic Fluidized Membrane Bioreactor Compared to Activated Sludge

A recently developed, staged anaerobic fluidized membrane bioreactor (SAF-MBR) system has demonstrated performance comparable to that of conventional activated sludge systems at similar hydraulic residence times. The low biosolid generation rate, small footprint, and energy-positive potential (from captured methane) suggest the possible advantages of using the SAF-MBR upstream of advanced treatment for decentralized water reuse. Previous research conducted on laboratory-scale systems indicated that the SAF-MBR system is capable of removing pharmaceutical compounds. The purpose of this study was to compare the removals of disinfection byproduct precursors as well as pharmaceuticals by parallel pilot-scale SAF-MBR and full-scale aerobic systems treating municipal wastewater. Significantly better removals of antivirals (acyclovir and lamivudine), β-blockers (atenolol, metoprolol, and propranolol), a β-blocker aerobic biotransformation product (metoprolol acid), an anticonvulsant (carbamazepine), antibiotics ...

Near zero energy wastewater treatment plants for the Greek islands

Conventional wastewater treatment is an energy-intensive process, requiring electric energy of about 1.1–2.4 MJ/m3, being higher in plants with relatively small hydraulic capacity (<5,000 m3/d). Solar energy can be used to supply electric energy, particularly for the Greek islands; however, the huge energy demand of the wastewater treatment is a drawback for the use of photovoltaic energy, due to the additional capital cost and electricity storage requirements during the night. Here, a novel approach for wastewater treatment is proposed, based on enhanced primary solids removal, by the use of advanced sieving and filtration processes, utilizing a proprietary rotary fabric belt microscreen followed by a proprietary continuous backwash upflow media filter. Raw municipal wastewater treated with the above process results in about 80–90% reduction in TSS and 60–70% reduction in BOD5. The overall electrical power requirements for a novel configuration plant with hydraulic capacity of 1,000 m3/d (typical capacity for small settlements in the Aegean islands) have been calculated to about 10 kW, as compared to over 25 kW for a conventional activated sludge system with the same capacity.

Efficiency of conventional activated sludge in the removal of linear alkylbenzene sulfonate from municipal sewage

Background: Linear Alkylbenzene Sulfonate (LAS) is an anionic detergent that is abundantly produced in different countries and discharged into natural environment through wastewater collection systems. Wastewater treatment systems play an important role in the removal of this contaminant. The purpose of this study was to determine the efficiency of a conventional activated sludge system in removing linear alkylbenzene sulfonate from wastewater in Kermanshah. Methods: This cross-sectional descriptive study was conducted on the wastewater in Kermanshah for 10 months. In order to determine the efficiency of the influent activated sludge process, 60 wastewater samples were taken after primary sedimentation and effluent of wastewater treatment system and LAS concentration was measured. All the sampling and testing methods were performed according to the standard method guidelines. Results: The results showed the means of LAS removal in the winter and warm seasons were 90.8% and 96.5%, respectively. The total mean of LAS removal in this system was 93.9 ±3.6. In addition, COD and TSS removal levels in winter were 88.3% and 72.3%; these values, however, in the summer were 86.9%, and 83.3%, respectively. Conclusion: The results showed that the conventional activated sludge process plays a pivotal role in the removal of LAS concentration. Moreover, the effluent concentration of LAS is less than the environmental standards for discharge into surface water in warm seasons.

Effect of dissolved oxygen and temperature on macromolecular composition and PHB storage of activated sludge

The macromolecular composition of activated sludge (lipids, intracellular proteins and intracellular polysaccharides) was studied together with its capacity to store macromolecules such as polyhydroxybutyrate (PHB) in a conventional activated sludge system fed with synthetic sewage water at an organic load rate of 1.0 kg COD/(m3·d), varying the dissolved oxygen (DO) and temperature. Six DO concentrations (0.8, 1.0, 1.5, 2.0, 2.5 and 8 mg/L) were studied at 20°C with a sludge retention time (SRT) of 6 days. In addition, four temperatures (10ºC, 15ºC, 20ºC and 30ºC) were assessed at constant DO (2 mg/L) with 2 days SRT in a second experimental run. The highest lipid content in the activated sludge was 95.6 mg/g VSS, obtained at 30°C, 2 mg/L of DO and a SRT of 2 days. The highest content of intracellular proteins in the activated sludge was 87.8 mg/g VSS, obtained at 20°C, 8 mg/L of DO and a SRT of 6 days. The highest content of intracellular polysaccharides in the activated sludge was 76.6 mg/g VSS, which was achieved at 20°C, a SRT of 6 days and a wide range of DO. The activated sludge PHB storage was very low for all the conditions studied.

Comparison of behaviour and fate of tetrabromobisphenol A (TBBPA) in membrane bioreactors and conventional activated sludge process

Research was completed on the behaviour and fate of tetrabromobisphenol A (TBBPA) in pilot-scale membrane bioreactors and conventional activated sludge (CAS) process at the City of Guelph Wastewater Treatment Plant. Emphasis was on the physicochemical processes, biological transformation and sorption kinetics. Measurement of TBBPA at the environmental level can be challenging, so the main focus of this paper was the development of an analytical method for the nano-level determination of TBBPA. The approach used was gas chromatography-negative ion chemical ionization-mass spectrometry in select ion monitoring mode. For the wastewater matrix studied, the developed method had a low method detection level (MDL) of 2.4 ng/L. The matrix effects for influent and activated sludge mixed liquor were determined to be 36.5 ± 5.9% (n = 10) and 79.2 ± 4.5% (n = 10), respectively. Preliminary results show that the CAS system demonstrates better performance for TBBPA removal, with removal at 83%. This was corroborated by the batch experiments that showed that the synergistic effects of biodegradation and biosorption potentially provide more removal of the TBBPA through the activated sludge process.

Modeling energy consumption in membrane bioreactors for wastewater treatment in north Africa.

Two pilot-scale membrane bioreactors were operated alongside a full-sized activated sludge plant in Tunisia in order to compare specific energy demand and treated water quality. Energy consumption rates were measured for the complete membrane bioreactor systems and for their different components. Specific energy demand was measured for the systems and compared with the activated sludge plant, which operated at around 3 kWh m(-3). A model was developed for each membrane bioreactor based on both dynamic and steady-state mass balances, microbial kinetics and stoichiometry, and energy balance. Energy consumption was evaluated as a function of mixed-liquor suspended solids concentration, net permeate fluxes, and the resultant treated water quality. This work demonstrates the potential for using membrane bioreactors in decentralised domestic water treatment in North Africa, at energy consumption levels similar or lower than conventional activated sludge systems, with the added benefit of producing treated water suitable for unrestricted crop irrigation.

Monitoring benzotriazoles: a 1 year study on concentrations and removal efficiencies in three different wastewater treatment plants

Benzotriazole (BTri), 4- and 5-tolyltriazole (4-TTri, 5-TTri) were monitored over 1 year in three wastewater treatment plants (WWTPs) with a membrane bioreactor (MBR-MH) and two conventional activated sludge systems (CAS-E, CAS-M). The influent/effluent concentrations and treatment stages removal efficiencies were monitored. 5-TTri was removed best (mean removal 80%) in the WWTP mainly by biodegradation followed by BTri (mean removal 45%) and 4-TTri (mean removal 15%) that showed a significant lower elimination. High removal fluctuations for all three benzotriazoles occurred over the four seasons with lowest removal during winter. All three WWTPs constituted a point source for BTs in the aquatic environment as concentration measurements in the receiving rivers upstream and downstream of the WWTP proved. While MBR-MH and CAS-M significantly increased the downstream concentrations, CAS-E only slightly increased the downstream concentrations as the receiving river was already contaminated with benzotriazoles from hydropower. 5-TTri was detected in lowest concentrations due to its good removal compared to BTri and 4-TTri that contribute to high effluent concentrations with the potential to accumulate due to insufficient self-purification.

Dairy wastewater treatment using an activated sludge-microalgae system at different light intensities.

A microalgae-bacteria system was used for dairy industry wastewater treatment in sequenced batch mode in a photobioreactor. The research investigated the influence of two light intensities: 360 and 820 μmol m(-2)s(-1) on treatment performances, microalgal cell recovery and dynamics of the protozoan community. Results showed that the light intensity of 360 μmol m(-2)s(-1) was found to be insufficient to support photosynthetic activity after the increase of bacterial biomass leading to the decrease of organic matter and ammonium removal efficiencies from 95 to 78% and 95 to 41%, respectively. Maximum microalgal cells recovery was about 63%. Continuous modification in the protozoan community was also noticed during this test. Increasing the light intensity to 820 μmol m(-2)s(-1) led to better microalgal cells recovery (up to 88%) and improved treatment performances. However, the decrease of protozoan richness to small flagellates and free-swimming ciliates was noticed. Moreover, the developed protozoan trophic network was found to be different from that identified in the conventional activated sludge system. The study emphasized that high increase of bacterial biomass promoted in nutrient- and organic matter-rich wastewater can strongly affect the treatment performances as a result of the shadow effect produced on the photoautotrophic microalgae aggregates.

Variance-based sensitivity analysis for wastewater treatment plant modelling

Global sensitivity analysis (GSA) is a valuable tool to support the use of mathematical models that characterise technical or natural systems. In the field of wastewater modelling, most of the recent applications of GSA use either regression-based methods, which require close to linear relationships between the model outputs and model factors, or screening methods, which only yield qualitative results. However, due to the characteristics of membrane bioreactors (MBR) (non-linear kinetics, complexity, etc.) there is an interest to adequately quantify the effects of non-linearity and interactions. This can be achieved with variance-based sensitivity analysis methods. In this paper, the Extended Fourier Amplitude Sensitivity Testing (Extended-FAST) method is applied to an integrated activated sludge model (ASM2d) for an MBR system including microbial product formation and physical separation processes. Twenty-one model outputs located throughout the different sections of the bioreactor and 79 model factors are considered. Significant interactions among the model factors are found. Contrary to previous GSA studies for ASM models, we find the relationship between variables and factors to be non-linear and non-additive. By analysing the pattern of the variance decomposition along the plant, the model factors having the highest variance contributions were identified. This study demonstrates the usefulness of variance-based methods in membrane bioreactor modelling where, due to the presence of membranes and different operating conditions than those typically found in conventional activated sludge systems, several highly non-linear effects are present. Further, the obtained results highlight the relevant role played by the modelling approach for MBR taking into account simultaneously biological and physical processes.

Dynamic Assessment of the Floc Morphology, Bacterial Diversity, and Integron Content of an Activated Sludge Reactor Processing Hospital Effluent

The treatment of hospital effluents (HE) is a major concern, as they are suspected of disseminating drugs and antibiotic resistance determinants in the environment. In order to assess HE influence on wastewater treatment plant biomass, lab-scale conventional activated sludge systems (CAS) were continuously fed with real HE or urban effluent as a control. To gain insights into the main hurdles linked to HE treatment, we conducted a multiparameter study using classical physicochemical characterization, phase contrast and confocal laser scaning microscopy, and molecular biology (i.e., pyrosequencing) tools. HE caused erosion of floc structure and the production of extracellular polymeric substances attributed to the development of floc-forming bacteria. Adaptation of the sludge bacterial community to the HE characteristics, thus maintaining the purification performance of the biomass, was observed. Finally, the comparative metagenomic analysis of the CAS showed that HE treatment resulted in an increase of class 1 resistance integrons (RIs) and the introduction of Pseudomonas spp. into the bacterial community. HE treatment did not reduce the CAS process performance; nevertheless it increases the risk of dissemination into the environment of bacterial species and genetic determinants (RIs) involved in antibiotic resistance acquisition.

Performance of a hybrid activated sludge/biofilm process for wastewater treatment in a cold climate region: Influence of operating conditions

The main aim of the study was to investigate a hybrid MBBR process, mostly in terms of organic matter removal and nitrification, when operating with different values of the mixed liquor sludge retention time (SRT), and highlighting the influence of temperature on the process. Based on experience in practice it was hypothesized that nitrification could be maintained at far lower SRT's than in conventional activated sludge systems and with high organic loading rates applied. A field gathering campaign has been carried out on a hybrid activated sludge/biofilm. The obtained results highlighted that the pilot plant was capable to remove the organic matter at loading rates up to 3.00kgTCODm−3day−1, also showing very high nitrification activity. Ammonia uptake rate (AUR) batch test showed that biofilm nitrification activity increased when the mixed liquor SRT decreased. The final suggestion is that it is possible to run a hybrid reactor with low mixed liquor SRT values, as well as low temperatures, still having a high ammonium removal efficiency, since a large fraction of nitrification activity will take place in the biofilm.

Foaming in membrane bioreactors: Identification of the causes

Membrane bioreactors (MBRs) represent by now a well established alternative for wastewater treatment. Their increasing development is undoubtedly related to the several advantages that such technology is able to guarantee. Nevertheless, this technology is not exempt from operational problems; among them the foaming still represents an “open challenge” of the MBR field, due to the high complexity of phenomenon. Unfortunately, very little work has been done on the foaming in MBRs and further studies are required. Actually, there is not a distinct difference between conventional activated system and MBR: the main difference is that the MBR plants can retain most Extracellular Polymeric Substances (EPSs) in the bioreactor. For these reason, unlike conventional activated sludge systems, MBRs have experienced foaming in the absence of foam-forming micro-organisms. Nevertheless, the actual mechanisms of EPS production and the role of bacteria in producing foam in activated sludge in MBRs are still unclear. In this paper, the authors investigated the roles of EPS and foam-forming filamentous bacteria by analyzing samples from different pilot plants using MBRs. In particular, in order to define the macroscopic features and the role of EPS and filamentous bacteria, a Modified Scum Index (MSI) test was applied and proposed. Based on the MSI and the foam power test, the causes of biological foaming were identified in terms of the potential for foaming, the quality and the quantity of the foam. The results indicated that the MBR foaming was influenced significantly by the concentration of bound EPSs in the sludge. In addition, the quantity and stability of MBR scum increased when both bound EPSs and foam-forming filamentous bacteria were present in the activated sludge.

A comparison between extended aeration sludge and conventional activated sludge treatment for removal of linear alkylbenzene sulfonates (Case study: Kermanshah and Paveh WWTP)

Linear alkylbenzene sulfonates (LASs) are anionic surfactants with extensive application as detergents. Discharge of wastewater containing these chemicals results in negative environmental impacts. In the present study, the process performance of an extended aeration activated sludge (EAAS) system in Paveh’s wastewater treatment plant (WWTP) and a conventional activated sludge (CAS) system in Kermanshah’s WWTP removing LAS and chemical oxygen demand (COD) were compared. The amounts of LAS removal in the CAS and the EAAS systems were calculated as 93.73 and 96.7%, respectively. It was concluded that EAAS system may be used for treating municipal wastewater with better LAS, COD, and total suspended solids (TSS) removal efficiency compared with CAS system.