Model-based analysis on growth of activated sludge in a sequencing batch reactor

A mathematical model is developed to describe the growth of multiple microbial species such as heterotrophs and autotrophs in activated sludge system. Performance of a lab-scale sequencing batch reactor involving storage process is used to evaluate the model. Results show that the model is appropriate for predicting the fate of major model components, i.e., chemical oxygen demand, storage polymers (X _STO), volatile suspended solid (VSS), ammonia, and oxygen uptake rate (OUR). The influence of sludge retention time (SRT) on reactor performance is analyzed by model simulation. The biomass components require different time periods from one to four times of SRT to reach steady state. At an SRT of 20 days, the active bacteria (autotrophs and heterotrophs) constitute about 57% of the VSS; the remaining biomass is not active. The model established demonstrates its capacity of simulating the reactor performance and getting insight in autotrophic and heterotrophic growth in complex activated sludge systems.     

Modeling predation processes in activated sludge

Predation by protozoa plays an important role in activated sludge. In this work, the kinetics for protozoan predation of active bacteria (X_H), extracellular polymeric substances (EPS), and intracellular storage products (X_STO) are added into a previously expanded unified model that describes the dynamics of EPS, X_STO, and soluble microbial products (SMP). The new biomass growth–decay–predation model describes the biomass fractions, soluble organic components, and oxygen‐uptake rates considering EPS, X_STO, and predators during dynamic operating conditions in activated sludge. Model calibration using batch experimental data provides the new parameter values for predation processes and insights into mechanisms involving predators. The calibrated value of the maximum specific growth rate for the predators is much slower than for the bacteria, confirming that predators are relatively slow growers. However, the predators and bacteria have similar decay rates and dissolved oxygen affinities. Model testing with results independent of the calibration data shows two things. First, the model and calibrated parameters accurately simulate the independent results when predators are present. Second, eliminating predation by high salinity significantly lowers the OUR, and this is captured by the model. Biotechnol. Bioeng. 2010;105: 1021–1030. © 2009 Wiley Periodicals, Inc.     

Effect of low solids retention time and focused pulsed pre-treatment on anaerobic digestion of waste activated sludge

The interacting effects of Focused Pulsed (FP) treatment and solids retention time (SRT) were evaluated in laboratory-scale digesters operated at SRTs of 2-20 days. Anaerobic digestion and methanogenesis of waste activated sludge (WAS) were stable for SRT ? 5 days, but the effluent soluble organic compounds increased significantly for SRT = 2 days due to a combination of faster hydrolysis kinetics and washout of methanogens. FP treatment increased the CH₄ production rate and TCOD removal efficiency by up to 33% and 18%, respectively, at a SRT of 20 days. These effects were the result of an increase in the hydrolysis rate, since the concentrations of soluble components remained low for SRT ? 5 days. Alternately, FP pre-treatment of WAS allowed the same conversion of TCOD to CH₄ with a smaller SRT and digester size: e.g., 40% size savings with a CH₄ conversion of 0.23 g CH₄-COD/g COD_in.     

Practical design equations for trickling-filter process

The concept of solid retention time (SRT) was applied in the trickling-filter process. A rational model of the trickling-filter process employing activated-sludge-process operational parameters was presented. The design equation was developed as follows; 1/SRT = [(S₀ ? S_n)/X ]·(F/V)·Y ? k_d, where SRT is the sludge retention time, S₀ is the influent substrate concentration; S_n is the effluent substrate concentration; X is the total cell mass retained per unit filter volume; V is the total volume of the filter; F is the influent flow rate; Y is the cell yield, and k_d is the cell decay rate. A laboratory-scale trickling-filter pilot plant treating synthetic sucrose waste-water was studied to verify the present design equation. The solid retention time was evaluated from the total slime mass (active and inactive) retained and the sludge wasted daily. It was found that the present design equation could be applied for designing the trickling-filter process by the application of SRT employed in the activated sludge process. Also, the SRT could be related to the hydraulic loading and influent substrate concentration for a given filter medium. The variation of SRT by the hydraulic loading at constant organic loading was observed and could be expressed by the mechanistic model. When SRT was maintained more than 12 days, it provided the highest five-day biological oxygen demand (BOD₅) removal, minimum sludge production, and lowest sludge volume index (SVI) value. The present model does include both microbial growth kinetic concepts, which can be more practical and meaningful for the design of a trickling filter.     

Effect of sludge age on methanogenic and glycogen accumulating organisms in an aerobic granular sludge process fed with methanol and acetate

The influence of sludge age on granular sludge formation and microbial population dynamics in a methanol- and acetate-fed aerobic granular sludge system operated at 35°C was investigated. During anaerobic feeding of the reactor, methanol was initially converted to methane by methylotrophic methanogens. These methanogens were able to withstand the relatively long aeration periods. Lowering the anaerobic solid retention time (SRT) from 17 to 8 days enabled selective removal of the methanogens and prevented unwanted methane formation. In absence of methanogens, methanol was converted aerobically, while granule formation remained stable. At high SRT values (51 days), γ-Proteobacteria were responsible for acetate removal through anaerobic uptake and subsequent aerobic growth on storage polymers formed [so called metabolism of glycogen-accumulating organisms (GAO)]. When lowering the SRT (24 days), Defluviicoccus-related organisms (cluster II) belonging to the α-Proteobacteria outcompeted acetate consuming γ-Proteobacteria at 35°C. DNA from the Defluviicoccus-related organisms in cluster II was not extracted by the standard DNA extraction method but with liquid nitrogen, which showed to be more effective. Remarkably, the two GAO types of organisms grew separately in two clearly different types of granules. This work further highlights the potential of aerobic granular sludge systems to effectively influence the microbial communities through sludge age control in order to optimize the wastewater treatment processes.     

The effects of hydraulic retention time and sludge retention time on the fate of di-(2-ethylhexyl) phthalate in a laboratory-scale anaerobic-anoxic-aerobic activated sludge system

A laboratory-scale anaerobic-anoxic-aerobic (AAA) activated sludge wastewater treatment system was employed to investigate the effects of hydraulic retention time (HRT) and sludge retention time (SRT) on the removal and fate of di-(2-ethylhexyl) phthalate (DEHP). In the range from 5 to 14h, HRT had no significant effect on DEHP removal. However, longer HRT increased DEHP accumulation in the system and DEHP retention in the waste sludge. When SRT was increased from 15 to 25d, DEHP removal efficiency stayed above 96%. Compared to the removal of only 88% at SRT of 10d, longer SRT enhanced DEHP degradation efficiency. The optimal HRT and SRT for both nutrients (nitrogen and phosphorus) and DEHP removal were 8h and 15d. At these retention times, about 71% of DEHP was degraded by the activated sludge process, 26% was accumulated in the system, 2% was released in the effluent, and 1% remained in the waste sludge. The anaerobic, anoxic and aerobic reactors were responsible for 15%, 19% and 62% of the overall DEHP removal, respectively.     

Disturbance and temporal partitioning of the activated sludge metacommunity

The resilience of microbial communities to press disturbances and whether ecosystem function is governed by microbial composition or by the environment have not been empirically tested. To address these issues, a whole-ecosystem manipulation was performed in a full-scale activated sludge wastewater treatment plant. The parameter solids retention time (SRT) was used to manipulate microbial composition, which started at 30 days, then decreased to 12 and 3 days, before operation was restored to starting conditions (30-day SRT). Activated sludge samples were collected throughout the 313-day time series in parallel with bioreactor performance (‘ecosystem function''). Bacterial small subunit (SSU) rRNA genes were surveyed from sludge samples resulting in a sequence library of >417 000 SSU rRNA genes. A shift in community composition was observed for 12- and 3-day SRTs. The composition was altered such that r-strategists were enriched in the system during the 3-day SRT, whereas K-strategists were only present at SRTs⩾12 days. This shift corresponded to loss of ecosystem functions (nitrification, denitrification and biological phosphorus removal) for SRTs⩽12 days. Upon return to a 30-day SRT, complete recovery of the bioreactor performance was observed after 54 days despite an incomplete recovery of bacterial diversity. In addition, a different, yet phylogenetically related, community with fewer of its original rare members displaced the pre-disturbance community. Our results support the hypothesis that microbial ecosystems harbor functionally redundant phylotypes with regard to general ecosystem functions (carbon oxidation, nitrification, denitrification and phosphorus accumulation). However, the impacts of decreased rare phylotype membership on ecosystem stability and micropollutant removal remain unknown.     

Comparison of biological removal via nitrite with real-time control using aerobic granular sludge and flocculent activated sludge

The process of nitrification–denitrification via nitrite for nitrogen removal under real-time control mode was tested in two laboratory-scale sequencing batch reactors (SBRs) with flocculent activated sludge (R1) and aerobic granular sludge (R2) to compare operational performance and real-time control strategies. The results showed that the average ammonia nitrogen, total inorganic nitrogen (TIN), and chemical oxygen demand (COD) removal during aeration phase were 97.6%, 57.0%, and 90.1% in R2 compared with 98.6%, 48.7%, and 88.1% in R1. The TIN removed in both SBRs was partially due to the presence of simultaneous nitrification–denitrification via nitrite, especially in R2. The specific nitrification and denitrification rates in R2 were 0.0416 mgNH₄⁺–N/gSS-min and 0.1889 mgNO_X⁻–N/gSS-min, which were 1.48 times and 1.35 times that of R1. The higher rates for COD removal, nitrification, and denitrification were achieved in R2 than R1 with similar influent quality. Dissolved oxygen (DO), pH, and oxidization reduction potential, corresponding to nutrient variations, were used as diagnostic parameters to control the organic carbon degradation and nitrification–denitrification via nitrite processes in both SBRs. The online control strategy of granular SBR was similar to that of the SBR with flocculent activated sludge. However, a unique U-type pattern on the DO curve in granular SBR was different from SBR with flocculent activated sludge in aerobic phase.     

Relationship between protozoan and metazoan communities and operation and performance parameters in a textile sewage activated sludge system

The present study aims at investigating the possibility of assessing performance and depuration conditions of an activated sludge wastewater treatment plant through an exploration of the microfauna. The plant, receiving textile industrial (70%) and domestic (30%) sewage, consists of a two-step biological depurating plant, with activated sludge followed by a percolating system. A total of 35 samples were analyzed during five months, and 30 taxa of protozoa and small metazoa were found. Epistylis rotans, Vorticella microstoma, Aspidisca cicada and Arcella sp. were the most frequent protozoa identified. Several significant correlations between biological, physical–chemical and operational parameters were determined, but no significant correlations could be established between biological parameters and removal efficiencies. The Sludge Biotic Index (SBI) reflected the overall state of the community but only presented statistically significant correlations with the influent total suspended solids (TSS), total suspended solids in mixed-liquor (MLTSS) and dissolved oxygen (DO). The determination of key groups and taxa along with general community parameters showed to have potential value as indicators of the depuration conditions. Despite the impossibility of correlating biological parameters and the removal efficiencies, the present study attests the value of the microfauna to assess the operation of the activated sludge systems even in the case of non-conventional plants and/or plants receiving industrial sewage.     

Isolation,identification of sludge-lysing strain and its utilization in thermophilic aerobic digestion for waste activated sludge

A strain of sludge-lysing bacteria was isolated from waste activated sludge (WAS) in this study. The result of 16S rRNA gene analysis demonstrated that it was a species of new genus Brevibacillus (named Brevibacillus sp. KH3). The strain could release the protease with molecule weight of about 40 kDa which could enhance the efficiency of sludge thermophilic aerobic digestion. During the sterilized sludge digestion experiment inoculated with Brevibacillus sp. KH3, the maximum protease activity was 0.41 U/ml at pH 8 and 50 °C, and maximum TSS removal ratio achieved 32.8% after 120 h digestion at pH 8 and 50 °C. In the case of un-sterilized sludge digestion inoculated with Brevibacillus sp. KH3, TSS removal ratio in inoculated-group was 54.8%, increasing at 11.86% compared with un-inoculation (46.2%). The result demonstrated that inoculation of Brevibacillus sp. KH3 could help to degrade the EPS and promote the collapse of cells and inhibit the growth of certain kinds of microorganisms. It indicated that Brevibacillus sp. KH3 strain had a high potential to enhance WAS-degradation efficiency in thermophilic aerobic digestion.     

Simulating a cyclic activated sludge system by employing a modified ASM3 model for wastewater treatment

To interpret the biological nutrient removal in a cyclic activated sludge system (CAS), a modified model was developed by combining the process of simultaneous storage and growth, and the kinetics of soluble microbial product (S _SMP) and extracellular polymeric substance (X _EPS) with activated sludge model no. 3 (ASM3). These most sensitive parameters were initially selected whilst parameters with low sensitivity were given values from literature. The selected parameters were then calibrated on an oxygen uptake rate test and a batch CAS reactor on an operational cycle. The calibrated model was validated using a combination of the measurements from a batch CAS reactor operated for 1 month and the average deviation method. The simulations demonstrated that the modified model was capable of predicting higher effluent concentrations compared to outputs of the ASM3 model. Additionally, it was also shown that the average deviation of effluent S _COD, S _NH, S _SMP and X _EPS simulated with the modified model was all less than 1 mg L⁻¹. In summary, the model could effectively describe biological processes in a CAS reactor and provide a wonderful tool for operation.

     

Effects of combining biological treatment and activated carbon on hexavalent chromium reduction

The objectives of the present work were: (a) to analyze the Cr(VI) removal by combining activated sludge (AS) with powdered activated carbon (PAC), (b) to analyze the effect of PAC and Cr(VI) on the growth kinetics of activated sludge, and (c) to determine if the combined method (AS-PAC) for Cr(VI) removal can be considered additive or synergistic with respect to the individual processes. Chromate removal was improved by increasing PAC concentrations in both PAC and AS-PAC systems. Cr(VI) removal using the AS-PAC system was higher than using AS or PAC. The increase of Cr(VI) caused longer lag phase and lower observed specific growth rate (μ_obs), biomass yield (Y_X/S), and specific growth substrate consumption rate (q_S) of activated sludge; additionally, PAC did not enhance the growth kinetic parameters (μ_obs, Y_X/S, q_S). Cr(VI) reduction in AS-PAC system was the result of the additive effect of each individual Cr(VI) removal process.     

Comparison of polyhydroxyalkanoates production by activated sludges from anaerobic and oxic zones of an enhanced biological phosphorus removal system: effect of sludge retention time

This study compared the PHAs production behavior of sludges from the anaerobic and oxic phases of an enhanced biological phosphorus removal (EBPR) system. This was accomplished by using the kinetics and stoichiometric coefficients obtained from aerobic batch tests to evaluate the performance of these two sludges. Experimental results indicated that the metabolic behavior of the sludges for PHAs production depend significantly on the operating sludge retention time (SRT) of the EBPR system. The oxic sludge with 5 days of SRT exhibited better PHAs production performance than anaerobic sludge. Conversely, the anaerobic sludge with 15 days of SRT had superior PHAs production capability compared to oxic sludge. These comparisons suggest that whether anaerobic or oxic sludge should be employed for PHAs production depends mainly on the operating SRT of the EBPR system.     

Effects of Cr(VI) on the performance and kinetics of the activated sludge process

The substrates removal performance, removal kinetics and the electron transport system (ETS) of sludge were investigated by sequencing batch reactors (SBR) and batch assays, respectively. Compared to the control system, significant decreases were observed in substrate removal efficiency with the Cr(VI)-feeding concentration up to 5 mg L⁻¹ in SBR system. And the recovery for NH₄⁺-N removal were more difficult than that of COD after the termination of Cr(VI)-feeding. Significant inhibitory effects of Cr(VI) on the ETS activity and substrate removal kinetics were observed in the batch assays. The inhibitory effects of Cr(VI) would be overestimated on COD removal and underestimated on NH₄⁺-N removal by the short-term batch assay as compared to the long-term operations. Additionally, significant correlations between the ETS activity and the inhibitory rates of Cr(VI) on substrate removal indicated the ETS activity can provide effective predictions on the potential performance of substrate removal in activated sludge.     

Removal of Cu(II) ions by biosorption onto powdered waste sludge (PWS) prior to biological treatment in an activated sludge unit: A statistical design approach

Biological treatment of synthetic wastewater containing Cu(II) ions was realized in an activated sludge unit with pre-adsorption of Cu(II) onto powdered waste sludge (PWS). Box-Behnken experimental design method was used to investigate Cu(II), chemical oxygen demand (COD) and toxicity removal performance of the activated sludge unit under different operating conditions. The independent variables were the solids retention time (SRT, 5–30 d), hydraulic residence time (HRT, 5–25 h), feed Cu(II) concentration (0–50 mg L^?1) and PWS loading rate (0–4 g h^?1) while percent Cu(II), COD, toxicity (TOX) removals and the sludge volume index (SVI) were the objective functions. The data were correlated with a quadratic response function (R² = 0.99). Cu(II), COD and toxicity removals increased with increasing PWS loading rate and SRT while decreasing with the increasing feed Cu(II) concentration and HRT. Optimum conditions resulting in maximum Cu(II), COD, toxicity removals and SVI values were found to be SRT of 30 d, HRT 15 h, PWS loading rate 3 g h^?1 and feed Cu(II) concentration of less than 30 mg L^?1.     

Overall functional gene diversity of microbial communities in three full-scale activated sludge bioreactors

Understanding microbial community composition is thought to be crucial for improving process functioning and stabilities of full-scale activated sludge reactors in wastewater treatment plants (WWTPs). However, functional gene compositions of microbial communities within them have not been clearly elucidated. To gain a complete picture of microbial community, in this study, GeoChip 4.2 was used to profile the overall functional genes of three full-scale activated sludge bioreactors, the 16S rRNA gene diversities of which had been unveiled by 454-pyrosequencing in our previous investigation. Triplicate activated sludge samples from each system were analyzed, with the detection of 38,507 to 40,654 functional genes. A high similarity of 77.3–81.2 % shared functional genes was noted among the nine samples, verified by the high 16S rRNA gene similarity with shared operational taxonomic units (OTUs) constituting 66.4–70.0 % of the detected sequences in each system. Correlation analyses showed that the abundances of a wide array of functional genes were associated with system performances. For example, the abundances of carbon degradation genes were strongly correlated to chemical oxygen demand (COD) removal efficiencies (r?=?0.8697, P?<?0.01). Lastly, we found that sludge retention time (SRT), influent total nitrogen concentrations (TN inf), and dissolved oxygen (DO) concentrations were key environmental factors shaping the overall functional genes. Together, the results revealed vast functional gene diversity and some links between the functional gene compositions and microbe-mediated processes.     

Microbial composition of the activated sludge of Moscow wastewater treatment plants

The contribution of the major technologically important microbial groups (ammonium- and nitrite-oxidizing, phosphate-accumulating, foam-inducing, and anammox bacteria, as well as planctomycetes and methanogenic archaea) was characterized for the aeration tanks of the Moscow wastewater treatment facilities. FISH investigation revealed that aerobic sludge were eubacterial communities; the metabolically active archaea contributed insignificantly. Stage II nitrifying microorganisms and planctomycetes were significant constituents of the bacterial component of activated sludges, with Nitrobacter spp. being the dominant nitrifiers. No metabolically active anammox bacteria were revealed in the sludge from aeration tanks. The sludge from the aeration tanks using different wastewater treatment technologies were found to have differing characteristics. Abundance of the nitrifying and phosphate-accumulating bacteria in the sludge generally correlated with microbial activity in microcosms and with efficiency of nitrogen and phosphorus removal from wastewater. The highest microbial numbers and activity were found in the sludge of the tanks operating according to the technologies developed in the universities of Hannover and Cape Town. The activated sludge from the Novokur’yanovo facilities, where abundant growth of filamentous bacteria resulted in foam formation, exhibited the lowest activity. The group of foaming bacteria included Gordonia spp. and Acinetobacter spp utilizing petroleum and motor oils, Sphaerotilus spp. utilizing unsaturated fatty acids, and Candidatus ‘Microthrix parvicella’. Thus, the data on abundance and composition of metabolically active microorganisms obtained by FISH may be used for the technological control of wastewater treatment.     

Properties of extracellular polymer having an effect on expression of activated sludge

Extracellular polymer in activated sludge was found to affect expression of these sludge by raising the value of the average specific resistance (α_av) during filtration and lowering the value of the modified consolidation coefficient (C_e) during consolidation. Further, the addition of extracellular polymer to the sludges caused higher values of α_av and lower values of C_e. The logarithmic plots of α_av and C_e against the quantity of extracellular polymer was linear. When these two values (α_av and C_e) of each activated sludge were plotted against the quantity of extracellular polymer extracted from each sludge, they were correlated by a single straight line with two exceptions. These were the return sludge of a textile factory and the anaerobically digested sludge of a sewage treatment plant. These exceptions indicate that the values of α_av of these sludges are higher than those of other sludges containing the same amount of extracellular polymer and that of C_e are lower than those of other sludges. It was confirmed that the extracellular polymer of these two sludges contains a lot of polysaccharides of the molecular weight 600–100,000 (with a standard of polyethylene glycol) by gel chromatography. The effects of the flocculant on dewatering were investigated. The effects of the cationic polymer flocculant addition to the extracellular polymer were analyzed using an ultrafiltration cell (molecular weight cut-off 10,000). The ultrafiltration rate of extracellular polymer was much improved by adding a cationic polymer flocculant. The removal of extracellular polymer by adding cationic polymer flocculant was ascertained by measuring the molecular weight distribution of the polysaccharides. On the other hand, no flocculations of extracellular polymer and sludge by adding anionic and nonionic polymer flocculant could be observed.     

Ozonation strategies to reduce sludge production of a seafood industry WWTP

In this work, several alternatives related to the application of ozone in different streams of a seafood industry WWTP were evaluated to minimize the production of waste sludge. The WWTP was composed of two coagulation-flocculation units and a biological unit and generated around of 6550 kg/d of sludge. Ozone was applied to sludge coming from flotation units (110 g TSS/L) at doses up to 0.03 g O(3)/g TSS during batch tests, no solids solubilization being observed. Ozone doses ranging from 0.007 to 0.02 g O(3)/g TSS were also applied to the raw wastewater in a bubble column reaching a 6.8% of TSS removal for the highest ozone dose. Finally, the effect of the pre-ozonation (0.05 g O(3)/g TSS) of wastewater coming from the first flotation unit was tested in two activated sludge systems during 70 days. Ozonation caused a reduction of the observed yield coefficient of biomass from 0.14 to 0.07g TSS/g COD(Tremoved) and a slight improvement of COD removal efficiencies. On the basis of the capacity for ozone production available in the industry, a maximum reduction of sludge generated by the WWTP of 7.5% could be expected.     

Validity of Monod kinetics at different sludge ages – Peptone biodegradation under aerobic conditions

The study presented an evaluation of the effect of culture history (sludge age) on the growth kinetics of a mixed culture grown under aerobic conditions. It involved an experimental setup where a lab-scale sequencing batch reactor was operated at steady-state at two different sludge ages (θ_X) of 2 and 10 days. The system sustained a mixed culture fed with a synthetic substrate mainly consisting of peptone. The initial concentration of substrate COD was selected around 500 mg COD/L. Polyhydroxyalkanoate (PHA) storage occurred to a limited extent, around 30 mg COD/L for θ_X = 10 days and 15 mg COD/L for θ_X = 2 days. Evaluation of the experimental data based on calibration of two different models provided consistent and reliable evidence for a variable Monod kinetics where the maximum specific growth rate, was assessed as 6.1/day for θ_X = 2 days and 4.1/day for θ_X = 10 days. A similar variability was also applicable for the hydrolysis and storage kinetics. The rate of storage was significantly lower than the levels reported in the literature, exhibiting the ability of the microorganisms to regulate their metabolic mechanisms for adjusting the rate of microbial growth and storage competing for the same substrate. This adjustment evidently resulted in case-specific, variable kinetics both for microbial growth and substrate storage.