Floc-forming Bacteria Research Articles

Use of image analysis in the study of competition between filamentous and non-filamentous bacteria

One of the most common problems in the operation of activated sludge plants is the inability to separate solids from treated effluents in the clarifiers caused by the overgrowth of filamentous micro-organisms (FM) with respect to floc-forming bacteria. In order to develop kinetic models that help to predict bulking events, growth kinetics of the FMs and non-filamentous bacteria (NFB) should be known. This paper addresses the competition of a FM and a NFB in a continuous stirred tank reactor. Experimental observations of the effect of the dilution rate on the composition of the mixed culture were compared to simulated results. Image analysis was used to measure NFB and FM fractions in order to evaluate the proposed mathematical model. Experimental results and numerical simulations showed that low D values favored the growth of FM; on the contrary, when high D values were applied a rapid overgrowth of the NFB were observed. Thus, high D values favored the growth of NFB minimizing the risk of filamentous bulking.

Effect of chlorine on filamentous microorganisms present in activated sludge as evaluated by respirometry and INT-dehydrogenase activity

Activated sludge technology is more used than any other for biological treatment of wastewater. However, filamentous bulking is a very common problem in activated sludge plants, chlorine being the chemical agent normally used to control it. In this work the effect of chlorine on microorganisms present in activated sludge flocs was assessed by a respirometric technique (oxygen uptake rate, OUR) and by the INT-dehydrogenase activity test (DHA) measured by two techniques: spectrophotometry (DHAa) and image analysis (DHAi). Both DHA tests were optimized and correlated with the respirometric technique (OUR) using pure cultures of a filamentous microorganism (Sphaerotilus natans) under chlorine inhibition. Using these correlations the tested methods were applied to determine the action of chlorine on respiratory activity in activated sludge. The OUR and the DHAa quantifies the action of chlorine on the total respiratory activity (RA) of flocs (filamentous and floc-forming bacteria); in contrast, the DHAi test evaluates specific action of chlorine on the RA of filamentous microorganisms. In activated sludge flocs containing filamentous microorganisms, a chlorine dose of 4.75 mg Cl2 (gVSS)−1 with a contact time of 20 min reduced about 80% of the RA of filamentous bacteria while affecting only 50-60% of the total RA of flocs. Besides, a chlorine dose of 7.9 mg Cl2 (gVSS)−1 produced the total respiratory inactivation of filamentous microorganisms after 10 min contact, however, with this dose the total RA of activated sludge flocs was reduced only about 45–65%; controlling filamentous bulking without affecting too much floc-forming bacteria. At the tested chlorine concentrations the inhibition of filamentous microorganisms was higher than in the whole activated sludge. Although floc-forming microorganisms were demonstrated to be more susceptible to chlorine than filamentous in pure cultures, results obtained in the present work confirmed that it is the location of the filamentous microorganisms in the flocs and the presence of extracellular polymer substances which largely determines their higher susceptibility to chlorine; consequently this feature plays a critical role in bulking control.

Effect of chlorine on filamentous microorganisms present in activated sludge as evaluated by respirometry and INT-dehydrogenase activity

Activated sludge technology is more used than any other for biological treatment of wastewater. However, filamentous bulking is a very common problem in activated sludge plants, chlorine being the chemical agent normally used to control it. In this work the effect of chlorine on microorganisms present in activated sludge flocs was assessed by a respirometric technique (oxygen uptake rate, OUR) and by the INT-dehydrogenase activity test (DHA) measured by two techniques: spectrophotometry (DHA a) and image analysis (DHA i). Both DHA tests were optimized and correlated with the respirometric technique (OUR) using pure cultures of a filamentous microorganism ( Sphaerotilus natans) under chlorine inhibition. Using these correlations the tested methods were applied to determine the action of chlorine on respiratory activity in activated sludge. The OUR and the DHA a quantifies the action of chlorine on the total respiratory activity (RA) of flocs (filamentous and floc-forming bacteria); in contrast, the DHA i test evaluates specific action of chlorine on the RA of filamentous microorganisms. In activated sludge flocs containing filamentous microorganisms, a chlorine dose of 4.75 mg Cl 2 (gVSS) −1 with a contact time of 20 min reduced about 80% of the RA of filamentous bacteria while affecting only 50-60% of the total RA of flocs. Besides, a chlorine dose of 7.9 mg Cl 2 (gVSS) −1 produced the total respiratory inactivation of filamentous microorganisms after 10 min contact, however, with this dose the total RA of activated sludge flocs was reduced only about 45–65%; controlling filamentous bulking without affecting too much floc-forming bacteria. At the tested chlorine concentrations the inhibition of filamentous microorganisms was higher than in the whole activated sludge. Although floc-forming microorganisms were demonstrated to be more susceptible to chlorine than filamentous in pure cultures, results obtained in the present work confirmed that it is the location of the filamentous microorganisms in the flocs and the presence of extracellular polymer substances which largely determines their higher susceptibility to chlorine; consequently this feature plays a critical role in bulking control.

The application of different techniques to determine activated sludge kinetic parameters in a food industry wastewater

In the present work, a continuous-feed system under steady state conditions (classical method) and a respirometric technique based on oxygen consumption measurements, were used to compare microbial parameters using a wastewater model system of a potato processing plant. The effects of the operating conditions in the continuous aerobic reactor on microbial growth and flora composition were also analysed. Continuous-feed experiments allowed for the calculation of the following biodegradation parameters: maximum substrate consumption specific rate (qSmax) = 0.19 mgCOD (mgVSS)-1·h-1, saturation constant (Ks) = 8.3 mgCOD·l-1, biomass yield (YX/S) = 0.61 mgVSS (mgCOD)-1, biomass decay constant (kd) = 0.04·h-1 and maximum oxygen consumption specific rate (qO2max) = 0.03 mgCOD (mgVSS)-1·h-1. The respirometric technique also allowed for the calculation of kinetic parameters. However, these parameters depended on dilution rate (D) and / or on dissolved oxygen concentration (DO). Values of qO2max and Ks increased with D; qO2max ranged between 0.05 and 0.13 mgO2 (mgVSS)-1·h-1 and Ks between 2 and 26 mgCOD·ℓ-1 for D values varying between 0.01 and 0.15·h-1. Microscope observations showed that sludge composition was a function of dilution rate. Low D values (low soluble organic matter (Ss) and high DO concentrations) yielded sludges mainly formed by floc-forming bacteria; high D values (high Ss and low DO concentrations) yielded sludges mainly formed by filamentous micro-organisms. Since the low substrate / biomass ratios used in the respirometric method limit the growth of micro-organisms maintaining the initial physiological state of the original biomass, the observed differences in the respirometric parameters reflected the actual microbial composition. WaterSA Vol.27(2) 2001: 169-176

Micromanipulation and further identification of FISH‐labelled microcolonies of a dominant denitrifying bacterium in activated sludge

The activated sludge process relies on the formation of strong microbial flocs. The knowledge about dominant floc-forming bacteria is at present very limited, especially from a phylogenetic perspective. In this study, numerous microcolonies in the activated sludge flocs were found to be targeted by a Betaproteobacteria-group-specific oligonucleotide probe using fluorescence in situ hybridization (FISH). Some of these were micromanipulated and further identified by reverse transcriptase polymerase chain reaction (RT-PCR) and sequencing to belong to the Aquaspirillum genus in the Neisseriaceae family. A specific oligonucleotide probe, Aqs997, was designed to target the identified bacteria. A survey in nine different wastewater treatment plants with nutrient removal (WWTP) showed a high abundance of bacteria hybridizing to the oligonucleotide probe developed. Microautoradiography (MAR) combined with FISH on activated sludge incubated with radiolabelled substrate showed uptake of substrate with oxygen, nitrate and nitrite as electron acceptor demonstrating a denitrifying potential of the bacteria investigated. The Aquaspirillum-related bacteria seemed to be abundant denitrifiers in WWTPs with nitrogen removal and they were particularly numerous in plants mainly receiving domestic wastewater, where they constituted up to 30% of all bacteria.

Linking floc structure and settling properties to activated sludge population dynamics in an SBR

Over a period of 227 days properties of activated sludge grown in an sequencing batch reactor (SBR) operated under stable conditions were analyzed. Settling properties (sludge volume index (SVI)) of the activated sludge were compared with on-line measurements of floc size and size distribution obtained by using a laser light scattering technique (Malvern Mastersizer/S, Malvern, UK), and with measurements of microbial community dynamics analyzed by denaturing gradient gel electrophoresis (DGGE) patterns of 16S rRNA genes. In addition, microscopical observations were used to confirm the results. Three distinct stages in the SBR evolution were observed. In the first stage the structural floc properties showed predominant presence of floc-forming bacteria in the activated sludge. A good correlation between floc size, properties and microbial community evolution was observed. The second stage showed a good balance between floc-forming and filamentous bacteria, with good settling properties and a highly dynamic community in the SBR. In the third stage, an increase in the filamentous bacteria, which became predominant in the system was observed. Again, a good correlation between settling properties and floc size distribution was obtained and a new dominant species was observed in the DGGE patterns, which can be assumed to be a filamentous organism.

Modeling of chlorine effect on floc forming and filamentous micro-organisms of activated sludges

Chlorination is the most economical, non-specific method to control the excessive growth of filamentous micro-organisms causing bulking in activated sludge systems in the treatment of food industrial wastewaters; it was one of the first methods used to control filamentous bulking and is still widely employed. Considering that chlorination affects both floc-forming and filamentous micro-organisms and leaves undesirable disinfection by-products, it is necessary to define the adequate doses to control bulking, minimizing the effect on floc-forming bacteria. In the present work the effect of biomass concentration and type of micro-organism on chlorine decay kinetics was evaluated; the inactivation of either a filamentous ( Sphaerotilus natans) or a floc-forming ( Acinetobacter anitratus) micro-organism due to chlorination was also analyzed. For chlorine decay assays, the samples were treated in a batch system with sodium hypochlorite ranging between 9.8 and 56.6 mg Cl 2 (gVSS) −1. Respirometric assays were used to evaluate the effect of chlorine on micro-organisms respiratory activity; in these cases, sodium hypochlorite doses ranged between 2.5 and 18 mgCl 2 (gVSS) −1. A model that allowed to predict simultaneously chlorine consumption and respiratory activity decay for both micro-organisms as a function of time was proposed. The model includes three coupled differential equations corresponding to respiratory inhibition, readily organic matter oxidation by chlorine and chlorine decay. The rate of chlorine decay depended on both, type and concentration of the micro-organisms in the system. Chlorine consumption rate due to S. natans was 2–4 times faster than A. anitratus. Using the proposed model initial critical chlorine doses (the lowest initial dose that leads to a total inhibition of the respiratory activity) were calculated for both micro-organisms and values of 11.9 mgCl 2 (gVSS) −1 for S. natans and 4.5 mgCl 2 (gVSS) −1 for A. anitratus were obtained. These critical doses indicated that in non flocculated pure cultures, floc-former bacteria A. anitratus was more susceptible to chlorine action than S. natans.

Analysis of the microbial community structure and function of a laboratory scale enhanced biological phosphorus removal reactor.

A laboratory scale sequencing batch reactor (SBR) operating for enhanced biological phosphorus removal (EBPR) and fed with a mixture of volatile fatty acids (VFAs) showed stable and efficient EBPR capacity over a four-year-period. Phosphorus (P), poly-beta-hydroxyalkanoate (PHA) and glycogen cycling consistent with classical anaerobic/aerobic EBPR were demonstrated with the order of anaerobic VFA uptake being propionate, acetate then butyrate. The SBR was operated without pH control and 63.67 +/- 13.86 mg P l-1 was released anaerobically. The P% of the sludge fluctuated between 6% and 10% over the operating period (average of 8.04 +/- 1.31%). Four main morphological types of floc-forming bacteria were observed in the sludge during one year of in-tensive microscopic observation. Two of them were mainly responsible for anaerobic/aerobic P and PHA transformations. Fluorescence in situ hybridization (FISH) and post-FISH chemical staining for intracellular polyphosphate and PHA were used to determine that 'Candidatus Accumulibacter phosphatis' was the most abundant polyphosphate accumulating organism (PAO), forming large clusters of coccobacilli (1.0-1.5 micro m) and comprising 53% of the sludge bacteria. Also by these methods, large coccobacillus-shaped gammaproteobacteria (2.5-3.5 micro m) from a recently described novel cluster were glycogen-accumulating organisms (GAOs) comprising 13% of the bacteria. Tetrad-forming organisms (TFOs) consistent with the 'G bacterium' morphotype were alphaproteobacteria, but not Amaricoccus spp., and comprised 25% of all bacteria. According to chemical staining, TFOs were occasionally able to store PHA anaerobically and utilize it aerobically.

A modified method to determine biomass concentration as COD in pure cultures and in activated sludge systems

A simple technique to determine biomass concentration as chemical oxygen demand (COD) was developed as an alternative to the standard volatile suspended solid (VSS) method. The proposed technique for biomass measurement as COD is based on the determination of the biomass COD (CODB) as the difference between total COD (CODT) and the soluble COD (CODS) of the sample. The obtained results show that this technique was quicker and simpler than the traditional VSS method. The validity of the proposed methods was tested with pure cultures of a filamentous micro-organisms (Sphaerotilus natans), a floc-forming bacteria and activated sludges. The method was also used for estimating the conversion factor (fCV) from VSS to COD units. A modification of the standard VSS technique was also proposed using two membranes in the filtration device; this technique allowed the biomass determination in 1 ?m size bacteria cultures that cannot be detected by the standard VSS method because cells are not retained by the 1.5 mm diameter pore glass-fibre filter. WaterSA Vol.28(4) 2002: 463-468

Toxicity Studies in a Tobacco Industry Biological Treatment Plant

Tobacco industry wastewater contains some toxic contaminantswhich inhibit the microbial consortium in biological treatmentplants. Investigations carried out showed that the most important sources of these toxic contaminants are nicotine,flavoring chemicals containing glycogen and alcohol, absorbableorganic halogens (AOX), and pesticides from tobacco leaves. Shock loadings of these toxic contaminants inhibit the bacterialactivity during biological treatment. Input of wastewater containing such toxic substances causes decreases in bacteria numbers and reduce the treatment efficiency in the treatmentplant. Viable numbers of floc forming, total and faecal coliform bacteria which are living in a biological treatmentunit (i.e. Biopac) were monitored. Conventional short-term bioassays and enrichment toxicity tests were performed to determine potential toxicity. If the ratio of bacteria numbersgrowing in the enrichment medium compared to control is between0.8 and 1.2, it may be assumed that toxic substances are absent.If this ratio exceeds 1.2, it may be assumed that growth stimulating substances are present. If this ratio is below 0.8,this indicates the presence of toxic substances. The results ofenrichment toxicity studies showed that enough nutrients canstimulate the bacterial growth, or known/unknown sources of toxicity can be inhibit the microbial growth. Furthermore, results of chemical coagulation/flocculation test experimentsshowed that chemical treatment is effective at reducing toxicityand is proposed for biological treatment.

Toxicity Studies of Tobacco Wastewater

Abstract Investigations carried out on tobacco industry wastewater showed that important sources of toxic contaminants are flavoring agents containing glycogen and alcohol, absorbable organic halogens and pesticides from tobacco leaves. Shock loading of these contaminants inhibited bacterial activity during biological treatment. Input of wastewater containing such toxic substances caused decreases in bacterial number and reduced the efficiency of the treatment plant. Viable numbers of floc-forming, total and faecal coliform bacteria within the biological treatment unit were monitored. Mortalities in bacterial populations and low chemical oxygen demand removal efficiencies were detected when absorbable organic halogens and pesticide concentrations were above 100 and 120 mg l-1, respectively. Short-term bioassays (using bacteria, protozoa or algae) and enrichment toxicity tests (including bacteria) performed to determine potential toxicity demonstrated that 90% mortalities in microbial populations were detected when absorbable organic halogens and pesticide concentrations were as high as 200 and 290 mg l-1, respectively. Enrichment toxicity tests indicated whether toxic, growth limiting or stimulation conditions were present. These tests showed that sufficient nutrients can stimulate bacterial growth, and high organic halogens and pesticide concentrations and chemical to biological oxygen demand ratios may inhibit microbial growth. Furthermore, results of chemical coagulation/flocculation test experiments showed that chemical treatment is effective at reducing toxicity.

Reclamation of an activated-sludge microbial consortium by selective biostimulation.

Our previous study showed that an activated-sludge process broke down at the phenol-loading rate of 1.5 g l(-1) day(-1), when non-flocculating bacteria (called R6T and R10) overgrew the sludge, resulting in a sludge washout. In this study, we attempted to circumvent this breakdown problem by reclaiming the consortium structure. Activated sludge was fed phenol, and the phenol-loading rate was increased stepwise from 0.5 g l(-1) day(-1) to 1.0 g l(-1) day(-1) and then to 1.5 g l(-1) day(-1). Either galactose or glucose (at 0.5 g l(-1) day(-1)) was also supplied to the activated sludge from the phenol-loading rate of 1.0 g l(-1) day(-1). Pure culture experiments have suggested galactose to be a preferential substrate for a floc-forming bacterium (R6F) that predominantly degrades phenol under low phenol-loading conditions. Supplying galactose allowed sustainment of the R6F population and suppression of the overgrowth of R6T and R10 at the phenol-loading rate of 1.5 g l(-1) day(-1). This measure allowed the activated-sludge process to treat phenol at a phenol-loading rate up to 1.5 g l(-1) day(-1), although it broke down at 2.0 g l(-1) day(-1). In contrast, supplying glucose reduced the R6F population and allowed the activated-sludge process to break down at the phenol-loading rate of 1.0 g l(-1) day(-1). This study demonstrated that reclamation of the activated-sludge consortium by selective biostimulation of the floc-forming population improved the phenol-treating ability of the process.

Modeling the competition between floc-forming and filamentous bacteria in activated sludge waste water treatment systems—I. Evaluation of mathematical models based on kinetic selection theory

One of the main reasons for failing of the sedimentation process in activated sludge waste water treatment systems is the phenomenon of filamentous bulking. This occurs when filamentous bacteria overgrow floc-forming bacteria. In this paper mathematical models are discussed which describe the competition of flocs and filaments based on the kinetic selection theory. It is proven with a stability analysis for a continuous reactor, that, in order to control filamentous bulking, the dilution rate plays a crucial role. Moreover, it is shown that coexistence of both organisms is generically not possible. Afterwards the continuous reactor model is extended in order to describe a standard waste water treatment system. Due to the complexity of the extended model it is difficult to perform the stability analysis analytically. It is proven that the model can be reduced without loss of stability properties. For the reduced model it is proven with a stability analysis that the transport terms play a crucial role in the survival of one or the other organism. As for the continuous reactor model it is shown that coexistence of both organisms is generically not possible.

Modeling the competition between floc-forming and filamentous bacteria in activated sludge waste water treatment systems—II. A prototype mathematical model based on kinetic selection and filamentous backbone theory

In this paper a prototype mathematical model is introduced which describes the growth of flocs and filaments in activated sludge waste water treatment systems. The model is based on both kinetic selection and filamentous backbone theory. A stability analysis of the model is performed and it is shown that this model, in contrast with mathematical models only based on kinetic selection theory, is able to describe coexistence of both flocs and filaments for a wide range of dilution rates. Afterwards the model is extended in order to describe a standard activated sludge waste water treatment system.

Application of a floc-forming genetically engineered microorganism to a sequencing batch reactor for phenolic wastewater treatment

For enhancing the survival of genetically engineered microorganisms (GEMs) in activated sludge processes, the use of a floc-forming bacterium as the host for a recombinant plasmid was proposed. The floc-forming and phenol-degrading GEM Sphingomonas paucimobilis 551 (pS10-45) was cultured to demonstrate this proposal. Although the maximum growth rate of the host strain S. paucimobilis 551 was low and the recombinant plasmid pS10-45 was unstable in the host, the resultant GEM S. paucimobilis 551 (pS10-45) was difficult to wash out together with the effluent, and it maintained population 3–4 times higher than the non-floc-forming GEM Escherichia coli HB101 (pS10-45) in a model activated sludge process operated in a sequencing batch mode. In the long run, the GEM-inoculated activated sludge process showed better phenol removal ability by the recombinant plasmid and better sludge settlement by the host strain.

Substrate utilization characteristics of the predominant filamentous and floc-forming bacteria isolated from a chemical fiber factory wastewater treatment plant

To supply theoretical verification of the function of a selector to control aerobic activated sludge bulking in the wastewater treatment plant for a chemical fiber factory in Taiwan, the filamentous and floc-forming bacteria in the aeration tank in the full-scale plant were examined microscopically and isolated. The kinetic characteristics of filamentous and floc-forming bacteria were also investigated. The predominant filamentous organism was Sphaerotilus natans. In addition to this organism, 21 strains of non-filamentous bacteria were isolated using the plate count method. These included Aeromonas jandaei DNA group 9, Acinetobacter johnsonii/genospecies 7, Bacillus pasteurii and Bacillus sp. (using the Biolog identification system). Nine strains showed the ability to form flocs when cultivated in glucose mineral salts medium. With glucose and acetate as sole substrates, the 4 floc-forming bacteria tested showed different substrate utilization characteristics. The 4 strains could be divided into 3 groups. The first group was the substrate degrading bacteria, the second group was the acid degrading bacteria and the other strains were those that had the highest substrate degradation rates at low substrate concentrations (below 800 mg/l). None of the floc-formers could utilize ethylene glycol, which is the major wastewater component. The kinetic characteristics of filamentous bacterium S. natans (Km = 4.0 mg glucose/l, Vm = 0.43 μl O2/l) and the floc-forming bacterium Aeromonas jandaei DNA group 9 (Km = 34.8 mg glucose/l, Vm = 0.59 μl O2/l) provided information for selector design.

Substrate utilization characteristics of the predominant filamentous and floc-forming bacteria isolated from a chemical fiber factory wastewater treatment plant

To supply theoretical verification of the function of a selector to control aerobic activated sludge bulking in the wastewater treatment plant for a chemical fiber factory in Taiwan, the filamentous and floc-forming bacteria in the aeration tank in the full-scale plant were examined microscopically and isolated. The kinetic characteristics of filamentous and floc-forming bacteria were also investigated.The predominant filamentous organism was Sphaerotilus natans. In addition to this organism, 21 strains of non-filamentous bacteria were isolated using the plate count method. These included Aeromonas jandaei DNA group 9, Acinetobacter johnsonii/genospecies 7, Bacillus pasteurii and Bacillus sp. (using the Biolog identification system). Nine strains showed the ability to form flocs when cultivated in glucose mineral salts medium.With glucose and acetate as sole substrates, the 4 floc-forming bacteria tested showed different substrate utilization characteristics. The 4 strains could be divided into 3 groups. The first group was the substrate degrading bacteria, the second group was the acid degrading bacteria and the other strains were those that had the highest substrate degradation rates at low substrate concentrations (below 800 mg/l). None of the floc-formers could utilize ethylene glycol, which is the major wastewater component. The kinetic characteristics of filamentous bacterium S. natans (Km = 4.0 mg glucose/l, Vm = 0.43 μl O2/l) and the flocforming bacterium Aeromonas jandaei DNA group 9 (Km = 34.8 mg glucose/l, Vm = 0.59 μl O2/l) provided information for selector design.

Isolation and Characterization of a Floc-Forming Bacterium Sphingomonas paucimobilis 551 from Activated Sludge.

Floc-forming bacteria were isolated from samples of a variety of wastewater treatment processes. At the shorter sludge retention times, a laboratory-scale activated sludge process maintained floc-forming bacteria at a higher percentage, though the number of total heterotrophic bacteria in the process became smaller. This result suggests that floc-forming bacteria are difficult to be washed out in the effluent. Among the 347 isolated heterotrophic bacterial strains, 10 strains showed high floc-forming ability. Sphingomonas paucimobilis 551 isolated from the activated sludge process, showed the most desirable properties for a floc-forming host of recombinant DNAs; (i) it is agram-negative bacterial strain into which we can easily introduce recombinant DNAs by using well-developed versatile plasmid vectors; (ii) it maintains good floc-formation for arelatively long time; (iii) it can grow in a relatively wide range of pHs, temperatures, and nutritional concentrations and on a wide variety of organic compounds.

Influence of storage on kinetic selection to control aerobic filamentous bulking

In various activated sludge systems, sludge grows under transient (unbalanced) conditions and storage of internal polymers becomes important. Differences in storage capacity under transients are often used to explain kinetic control of bulking, but storage is neither studied in detail nor usually included in modelling. For this reason, the transient response of different aerobic mixed cultures was studied by experimentally determining the role of storage. Two different mixed cultures (bulking and non-bulking) were selected in an acetate-limited medium, by continuous or intermittent feeding of a CSTR, respectively. Batch tests were used to investigate the transient response of the selected cultures as a function of the starvation time and of the ratio of the initial concentration of the substrate and sludge biomass (So/Xo). In most experimental conditions, both cultures showed that the storage of poly-hydroxybutyrate (PHB) is in general the prevailing mechanism of substrate removal. In particular, the culture dominated by floc-formers showed very fast response to the substrate spike with a high observed yield. Storage was practically the only metabolism occurring. The ratio So/Xo did not have a major role in determining the type and extent of the response. Starvation did not affect the response of the floc-formers to transient conditions. For the filamentous bacteria, both the growth response and, even more significantly, the storage response were negatively affected. Hence, the difference in storage capacity between filamentous and floc-forming bacteria was further increased.

Influence of storage on kinetic selection to control aerobic filamentous bulking

In various activated sludge systems, sludge grows under transient (unbalanced) conditions and storage of internal polymers becomes important. Differences in storage capacity under transients are often used to explain kinetic control of bulking, but storage is neither studied in detail nor usually included in modelling. For this reason, the transient response of different aerobic mixed cultures was studied by experimentally determining the role of storage. Two different mixed cultures (bulking and non-bulking) were selected in an acetate-limited medium, by continuous or intermittent feeding of a CSTR, respectively. Batch tests were used to investigate the transient response of the selected cultures as a function of the starvation time and of the ratio of the initial concentration of the substrate and sludge biomass (So/Xo). In most experimental conditions, both cultures showed that the storage of poly-bydroxybutyrate (PHB) is in general the prevailing mechanism of substrate removal. In particular, the culture dominated by floc-formers showed very fast response to the substrate spike with a high observed yield. Storage was practically the only metabolism occurring. The ratio So/Xo did not have a major role in determining the type and extent of the response. Starvation did not affect the response of the floc-formers to transient conditions. For the filamentous bacteria, both the growth response and, even more significantly, the storage response were negatively affected. Hence, the difference in storage capacity between filamentous and floc-forming bacteria was further increased.