Completely Mixed Activated Sludge Research Articles

Oxygen transfer parameters and oxygen uptake rates revisited

This paper provides a review of the essential equations and parameters that are used to design bioreactor aeration systems. Major objectives were to determine if the log-deficit method (LDM) and non-linear regression method (NLRM) yielded different results for the volumetric oxygen mass transfer coefficient (KL a)T and dissolved oxygen (DO) saturation concentration (Cs)T . Another objective was to compare and evaluate oxygen uptake rates (OURs) from the traditional BOD bottle technique with OURs determined from mass balances around completely-mixed activated sludge (CMAS) reactors. Full- and bench-scale, non-steady state reaeration tests were performed using sodium sulfite and cobalt chloride in addition to operating bench-scale CMAS reactors. Full-and bench-scale reaeration testing indicated there was no significant difference in the estimate of (KL a)T in tap water using the LDM or NLRM. Similarly, (Cs)T values were the same for the LDM and NLRM for 70 out of 119 reaeration tests. Actual oxygen uptake rates (AOURs) measured using the BOD bottle technique versus calculated oxygen uptake rates (COURS) were the same for 26 out of 51 data sets evaluated. The alpha-factor (α) increased as the solids retention time (SRT) was increased.

Activated sludge process simulator ASP-Sim, Part-2: Bod removal, nitrification, do, and sequencing batch reactor

The most commonly used biological treatment of municipal and industrial wastewater is the Activated Sludge Process. The design of activated process is usually done manually; this is often difficult and susceptible to errors due to the large number of equations involved (which call for approximations and round off errors). Also, the determination of the effects of changes in design parameters are very difficult and time-consuming for manual computations. This paper therefore employs the Visual Basic 6.0 programming for design and simulation of completely mixed activated sludge (CMAS) process. Algorithm and flow chart based on kinetic model were used for developing the Visual Basic code, ASPSIM. The code was tested using a design problem from standard text book. Results obtained proves that the Visual BASIC 6.0 program ASP-SIM is a useful tool for predicting the effect of temperature on design and operation of completely mixed activated plant design and operation.

Immobilized-Cell-Augmented Activated Sludge Process for Treating Wastewater Containing Hazardous Compounds

A novel bioaugmentation scheme called immobilized-cell-augmented activated sludge (ICAAS) was developed. Offline enricher reactors were used to maintain immobilized acclimated cells applied to augment completely mixed activated sludge (CMAS) treating a pentachlorophenol (PCP) pulse loading. Cellulose triacetate (CA) and powder activated carbon (PAC) combined with CA (PAC + CA) were the two media types used for entrapping the PCP-degrading culture. With ICAAS at 5% by volume augmentation, PCP removal of 73.1 and 75.1% via biodegradation, volatilization, and adsorption onto suspended cells, entrapped cells, and media was achieved for the systems with CA and PAC + CA media, respectively, while PCP removal in a control CMAS, which had a comparable level of combined PCP adsorption onto suspended cells and volatilization as the ICAAS, was 48.7%. Results further showed that the immobilized cells retained their PCP-degrading ability when they were fed with the inducer (PCP) once every 20 days.

Biodegradation of pyridine in a completely mixed activated sludge process

A potential bacterial culture (P1), isolated from garden soil and identified as Pseudomonas pseudoalcaligenes–KPN, was used as a starter seed to develop the biomass in a completely mixed activated sludge (CMAS) reactor and the system was evaluated for treatment of wastewater containing pyridine. The results of this study indicate that pyridine could be degraded efficiently at a loading of 0.251 kg pyridine kg MLSS −1 d −1 (0.156 kg TOC kg MLSS −1 d −1) and at an optimal hydraulic retention time (HRT) of 24 h. Pyridine was used as the sole source of carbon and nitrogen by the biomass. Ammonia-nitrogen (NH 3-N) was formed due to the metabolism of the pyridine ring. In the present investigation, the performance of CMAS with reference to pyridine biodegradation and the bio-kinetic constants for the biodegradation of pyridine, in a continuous system, were computed. The results indicate that a CMAS system inoculated with P. pseudoalcaligenes–KPN, under optimum conditions of HRT and pyridine loading, gives a yield coefficient of ( Y) 0.29, decay coefficient ( K d) 0.0011 d −1, maximum growth rate constant ( μ max) 0.108 d −1 and saturation rate constant ( K s) 5.37 mg L −1 for pyridine.

A simple method to estimate the contribution of biological floc and reactor‐solution to mass transfer of oxygen in activated sludge processes

In this study, the mass transfer coefficient of biological floc (K(L)a(bf)) was estimated from the mass transfer coefficient of the mixed-liquor (K(L)a(f)) and the reactor-solution (K(L)a(e)). The biological floc resistance (BFR) and reactor-solution resistance (SR) were defined as the reciprocal of K(L)a(bf) and K(L)a(e), respectively, by applying the concept of serial-resistance originally presented in two-film theory (Lewis and Whitman (1924) Ind Eng Chem 16:1215-1220). The specific biological floc resistance (SBFR) was defined as biological floc resistance per unit biomass concentration. The data indicated that an activated sludge process yielding low BFR/MLR and BFR/SR tended to produce higher oxygen transfer efficiency. Surprisingly, the reactor-solution posed the same level of resistance as clean water in all experiments, except in a 5-day SRT, non-nitrifying, completely mixed activated sludge (CMAS) process run. Furthermore, SBFR successfully represented biological floc and showed a positive correlation to sludge volume index (SVI). In addition, SBFR/SR and oxygen transfer efficiency (OTE(f)) followed an exponential relationship for the complete data set. The method of separating the mixed-liquor into biological floc and reactor-solution improved the understanding of oxygen transfer under process conditions, without resorting to intrusive techniques or direct handling of fragile biological floc.

Membrane bioreactor operation at short solids retention times: performance and biomass characteristics

This study investigated the performance and biomass characteristics of a membrane bioreactor (MBR) and a completely mixed activated sludge (CMAS) system operated at short solids retention times (SRT) ranging from 0.25 to 5 d and hydraulic retention times of 3 and 6 h. The lab-scale reactors were fed with synthetic wastewater to ensure consistency in feed composition. Results show the MBR was capable of achieving excellent quality effluent regardless of the extremely short SRT. The MBR removal efficiencies ranged from approximately 97.3-98.4% (TCOD) in the MBR, compared to 77.5-93.8% (TCOD) and 94.1-97.0% (SCOD) in the CMAS. Nitrification completely ceased when SRT was < 2.5 d. The MBR biomass was composed of small, weak and uniform-sized flocs with large mass of short filamentous organisms and mainly dispersed microorganisms at SRT of 5 and 0.25 d, respectively. In contrast, the CMAS sludge was composed of large flocs with filamentous organisms as a backbone at SRT > 2.5 d. The CMAS flocs were smaller and weaker at shorter SRT. The MBR sludge contained a much higher fraction of non-flocculating microorganisms. This fraction increased significantly with decreasing SRT. It was found that the concentrations of protein and carbohydrates in the exocellular polymeric substances for both the MBR and the CMAS decreased with increasing F/M ratio or decreasing SRT. The combination of increasing amounts of non-flocculating microorganisms and a reduction of EPS at shorter SRT in both reactors contributed to deteriorating sludge settling properties. A significant presence of dispersed biomass and small flocs in MBR contributed to better reactor performance probably due to less mass transfer resistance.

Relative efficacy of intrinsic and extant parameters for modeling biodegradation of synthetic organic compounds in activated sludge: dynamic systems.

The utility of intrinsic and extant kinetic parameters for simulating the dynamic behavior of a biotreatment system coupled with a distributed, unstructured, balanced microbial growth model were evaluated against the observed response of test reactors to transient loads of synthetic organic compounds (SOCs). Biomass from a completely mixed activated-sludge (CMAS) system was tested in fed-batch reactors, while a sequencing batch reactor (SBR) was tested by measuring SOC concentrations during the fill and react period. Both the CMAS system and the SBR were acclimated to a feed containing biogenic substrates and several SOCs, and the transient loading tests were conducted with biogenic substrates along with one or more SOCs. Extant parameters more closely reflect the steady-state degradative capacity of activated-sludge biomass than intrinsic parameters and, hence, were expected to be better predictors of system performance. However, neither extant nor intrinsic parameters accurately predicted system response and neither parameter set was consistently superior to the other. Factors that may have contributed to the inability of the model to predict system response were identified and discussed. These factors included the role of abiotic processes in SOC removal, disparity in the bases used to evaluate parameter estimates (substrate mineralization) and reactor performance (substrate disappearance), inhibitory substrate interactions under the severe loading conditions of the SBR, changes in the physiological state of the biomass during the transient loading tests, and the presumed correlation between the competent biomass concentration and the influent SOC concentration.

MEMBRANE BIOREACTOR PERFORMANCE AT SHORT MEAN CELL RESIDENCE TIMES

MEMBRANE BIOREACTOR PERFORMANCE AT SHORT MEAN CELL RESIDENCE TIMESThis study discusses the performance of a membrane bioreactor (MBR) and a completely mixed activated sludge (CMAS) system operated at short MCRT ranging from 0.25 to 5 d and hydraulic retention times of 3 and 6 h treating synthetic wastewater. Even at the shortest MCRT of 0.25 d, the MBR was able to produce excellent quality effluent free of suspended solids and total chemical oxygen demand (TCOD)...Author(s)How Y. NgSlawomir W. HermanowiczSourceProceedings of the Water Environment FederationSubjectSession 73 Research: Membrane BioreactorsDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2003ISSN1938-6478SICI1938-6478(20030101)2003:5L.144;1-DOI10.2175/193864703784606675Volume / Issue2003 / 5Content sourceWEFTECFirst / last page(s)144 - 155Copyright2003Word count210

Estimation of the competent biomass concentration for the degradation of synthetic organic compounds in an activated sludge culture receiving a multicomponent feed

Biomass samples from two completely mixed activated sludge (CMAS) systems fed an array of biogenic and synthetic organic chemicals (SOCs) were subjected to microbial enumeration using substrate-specific most probable number techniques to quantify the biomass fractions degrading isophorone, phenol, 4-chlorophenol, 4-nitrophenol, m-toluate, and m-xylene, which individually constituted 1.88–4.30% of the total chemical oxygen demand (COD) in the CMAS feed. The competent biomass fractions ranged from 3.1–6.4%, showing that only a small fraction of the total biomass was responsible for degrading the test SOCs. The efficacy of the influent COD fraction as a predictor of the competent biomass fraction was evaluated. Generally, the influent COD fraction underestimated the competent biomass fraction by a factor of about two. The convergence of degradation pathways for different SOCs could contribute to the discrepancy between the influent COD fraction and the competent biomass fraction for each SOC. An accurate estimate of the competent biomass fraction is essential for correctly predicting system response to transient SOC loads. Using the influent COD fraction to approximate the competent biomass fraction, despite the twofold difference, is still more realistic than assuming that the total biomass is responsible for SOC removal.

Effect of simultaneous biodegradation of multiple substrates on the extant biodegradation kinetics of individual substrates

A respirometric technique was developed to quantify the extant kinetics for biodegradation of an organic compound by biomass that is simultaneously degrading a complex mixture of substrates. The technique uses a fed‐batch feed to supply the mixture of substrates at a rate consistent with the organic loading on the completely mixed activated sludge (CMAS) bioreactor from which the biomass was obtained. The results were compared to results from a batch test wherein the target compound served as the sole carbon and energy source. Results from the fed‐batch technique showed that simultaneous biodegradation of multiple substrates caused an increased ability for the removal of the target compound, phenol, although the increase was relatively small. A 7 to 12% increase in the maximum specific growth rate was observed for biomass receiving a fed‐batch feed at a rate of one to three times the specific mass flow rate to the CMAS system. An increase in the fed‐batch feed rate to six times the CMAS rate caused a decrease from the maximum enhancement, which was observed at the threefold rate, indicating that there was an upper limit on the stimulatory effect. Less extensive studies with acrylamide, 4‐chlorophenol, ethylene glycol, and m‐toluate gave similar results. Because the effects of simultaneous substrate biodegradation were small, single‐substrate extant kinetic tests should be adequate for describing the capabilities of a biomass for degrading a particular substrate in a multisubstrate environment.

Estimation of the competent biomass concentration for the degradation of synthetic organic compounds in an activated sludge culture receiving a multicomponent feed

Biomass samples from two completely mixed activated sludge (CMAS) systems fed an array of biogenic and synthetic organic chemicals (SOCs) were subjected to microbial enumeration using substrate-specific most probable number techniques to quantify the biomass fractions degrading isophorone, phenol, 4-chlorophenol, 4-nitrophenol, m-toluate, and m-xylene, which individually constituted 1.88-4.30% of the total chemical oxygen demand (COD) in the CMAS feed. The competent biomass fractions ranged from 3.1-6.4%, showing that only a small fraction of the total biomass was responsible for degrading the test SOCs. The efficacy of the influent COD fraction as a predictor of the competent biomass fraction was evaluated. Generally, the influent COD fraction underestimated the competent biomass fraction by a factor of about two. The convergence of degradation pathways for different SOCs could contribute to the discrepancy between the influent COD fraction and the competent biomass fraction for each SOC. An accurate estimate of the competent biomass fraction is essential for correctly predicting system response to transient SOC loads. Using the influent COD fraction to approximate the competent biomass fraction, despite the twofold difference, is still more realistic than assuming that the total biomass is responsible for SOC removal.

Microbial degradation of heterocyclic bases in a completely mixed activated sludge process

Completely mixed activated sludge (CMAS) process seeded with acclimatized mixed culture of Nocardia sp. was employed for the treatment of simulated wastewater containing heterocyclic bases, viz. pyridine, methyl pyridincs and quinoline. CMAS was operated for assessment of hydraulic loadings and organic loadings for treatment of these bases. The results indicate that all the bases could be degraded efficiently at loading ranges of 0.08g to 0.12g TOC per got mixed liquor suspended solids per day with an optimal HRT of 4S hours. The treated effluent was non toxic to the fish Lebestes reticulatus. Further, the biooxidation kinetic constants for the CMAS were evaluated. The findings showed values of yield co‐efficient y.0.488, decay co‐efficient Kd:0.028d−1, maximum growth rate umax:0.0450 d−1 and saturation rate constant Ks: 176.2mgl.−1.

Changes in measured biodegradation kinetics during the long-term operation of completely mixed activated sludge (CMAS) bioreactors

Two completely mixed activated sludge (CMAS) bioreactors, one with an aerobic selector and one without, were operated for approximately twelve and sixteen months, respectively. Extant biodegradation kinetics for several compounds were periodically tested using a batch respirometric procedure. Kinetic parameters from the CMAS unit without a selector showed considerable variability (standard deviation of ± 50%) even though it was operated at steady state (i.e. constant HRT, SRT, organic loading, etc.) for the duration of the study. At first, there was a large discrepancy between the kinetic parameters of the two bioreactors. Phenol and 4-chlorophenol were biodegraded according to Monod kinetics in the selector system and Andrews (inhibitory) kinetics in the non-selector system, and the μ^ and KS values were significantly greater in the selector system. The kinetic parameter values of the two systems converged, however, when the xenobiotic compounds were no longer fed to the selector in that system but were fed to the main bioreactor. After this switch, phenol and 4-chlorophenol followed inhibitory kinetics in both systems. The lack of inhibition when phenol and 4-chlorophenol were fed to the selector suggests that, contrary to conventional wisdom, bioreactors which have a concentration gradient (e.g. plug flow, sequencing batch, and tanks in series bioreactors) may be more resistant to inhibition.

Changes in measured biodegradation kinetics during the long-term operation of completely mixed activated sludge (CMAS) bioreactors

Two completely mixed activated sludge (CMAS) bioreactors, one with an aerobic selector and one without, were operated for approximately twelve and sixteen months, respectively. Extant biodegradation kinetics for several compounds were periodically tested using a batch respirometric procedure. Kinetic parameters from the CMAS unit without a selector showed considerable variability (standard deviation of ± 50%) even though it was operated at steady state (i.e. constant HRT, SRT, organic loading, etc.) for the duration of the study. At first, there was a large discrepancy between the kinetic parameters of the two bioreactors. Phenol and 4-chlorophenol were biodegraded according to Monod kinetics in the selector system and Andrews (inhibitory) kinetics in the non-selector system, and the μ^ and Ks values were significantly greater in the selector system. The kinetic parameter values of the two systems converged, however, when the xenobiotic compounds were no longer fed to the selector in that system but were fed to the main bioreactor. After this switch, phenol and 4-chlorophenol followed inhibitory kinetics in both systems. The lack of inhibition when phenol and 4-chlorophenol were fed to the selector suggests that, contrary to conventional wisdom, bioreactors which have a concentration gradient (e.g. plug flow, sequencing batch, and tanks in series bioreactors) may be more resistant to inhibition.

Quantification of the kinetic differences between communities isolated from completely mixed activated sludge systems operated with or without a selector using a novel respirometric method

This study quantified the kinetic differences in microbial communities isolated from completely mixed activated sludge (CMAS) systems that were operated either with or without an aerobic selector preceding the main reactor. A new respirometric method was employed that allowed the determination of biodegradation kinetics from single oxygen consumption curves, thereby minimizing physiological changes to the examined communities during the assay. Results indicated that increased values for Ks and μmax for acetate, phenol, and 4-chlorophenol degradation were measured in the CMAS system operated with a selector. The biomass yields on acetate, phenol, and 4-chlorophenol were very similar in both systems. These findings indicate that the operation of CMAS systems with aerobic selectors may result in the selection for degrading populations with higher Ks and μmax values for both biogenic and xenobiotic organic compounds, and that substrate storage in the selector only partially contributes to increased substrate removal rates.

Bacterial growth kinetics in the completely mixed activated sludge treatment of fellmongery effluent

Fellmongery effluent was successfully treated (± 85% OA reduction) in a laboratory scale completely mixed activated sludge (CMAS) reactor with sludge recycle and controlled wasting. Varying the sludge age was found to have a significant effect on effluent quality. A mathematical model useful in the design and operation of effluent treatment plants was used to describe microbial growth in the treatment process. In the development of the model it is shown how a clear distinction must be made between the volatile suspended solids and the active volatile suspended solids for the model to accurately describe microbial growth kinetics.