Filamentous Sludge Bulking Research Articles

Kinetic Selection of Microorganisms by Means of a Selector – Twenty Years of Progress: History, Practice and Problems

Historical development, as well as the main principles of a kinetic selection theory (KST) of non-filamentous microorganisms is reviewed, and the original definition of a selector for filamentous bulking sludge control is given. The idea of a selector based on KST originated from experiments with a compartmentalized system at the beginning of seventies. Since that time, KST has been completed and confirmed by additional experiments with both intermittent substrate feeding and influence of degree of longitudinal mixing on sludge bulking. Nevertheless, numerous examples of practical use of a selector showed both positive and negative results on filamentous bulking control. Consequently, it is concluded that due to both a large diversity of filamentous organism's physiology and a complexity of bulking phenomena, there is no universal method to control the growth of all filamentous species. For each case, a specific analysis is necessary before choosing the specific solution. In the case of low F/M bulking with filaments like Microthrix p., types 0041 or 0092, the application of KST using contact zone has given good results for several full scale treatment plants.

The case of both energetic uncoupling and metabolic selection of microorganisms in the OSA activated sludge system

Abstract A modified activated sludge system with anaerobic treatment of returned sludge, named OSA (Oxic‐Settling‐Anaerobic) was studied. The main objective of the research was to create conditions of a physiological stress on microbial growth, resulting in uncoupling of catabolic processes from anabolic ones. This phenomenon, largely described in the literature, is accompanied by energy wastage resulting in biomass yield reduction. The experimental results obtained with a mixed culture (activated sludge) fed on synthetic substrate showed that the anaerobic conditions exerted a kind of selection pressure on microbial metabolism of the activated sludge microorganisms, resulting in metabolic selection of the species being able to favour catabolic pathways. These bacteria, capable of storing polyphosphates under oxic conditions and using them as energy reserves under anaerobic conditions are called poly‐P bacteria. The OSA mixed culture contained about 50 to 60% of these poly‐P bacteria. It has been concluded that the insertion of an anaerobic zone into the returned sludge circuit changed both composition and behaviour of activated sludge microorganisms. The typical phenomena observed in the OSA system were enhanced phosphate removal, reduction of the excess activated sludge production and the suppression of activated sludge filamentous bulking.

Floc-Load as It Relates to Enzymatic Transfer of Soluble Substrate and Sludge Bulking Control

Floc-load and enzymatic transfer of soluble substrate from a wastewater into microorganisms in an activated sludge floe are related. In the absence of other metabolic selectivity pressures, this rapid sequestering of available soluble organics is required for the avoidance of filamentous sludge bulking in various reactor configurations. The specific oxygen utilization rate response to floc-loading is a measure of the magnitude of the enzymatic transfer capacity of a biomass and can be used to determine its maximum sorptive capacity. During the transfer process the biomass Adenosine triphosphate (ATP) content undergoes a rapid depletion for 20-30 minutes before increasing again. Relevant parameters associated with soluble substrate removal and floc-forming selectivity mechanisms are summarized.

Effect of anaerobic conditions on activated sludge filamentous bulking in laboratory systems

The effect of anaerobic conditions on the occurrence of filamentous microorganisms in the biocenosis of activated sludge has been studied in laboratory systems. These systems consisted of an anaerobic completely mixed tank followed by an oxic one. A single oxic completely mixed tank served as a control unit. Synthetic wastewater was used incorporating glucose and ethanol as a carbon source in order to support the growth of filamentous microorganisms in completely mixed tanks. It has been demonstrated that the growth of some filamentous organisms, for instance Type 021N and Sphaerotilus natans, was suppressed under anaerobic conditions as a result of lower rates of polyphosphate depolymerization under anaerobic conditions. However, the positive effect of the anaerobiosis may be eliminated if simultaneous dissimilatory sulphate reduction occurs. In this case filamentous bulking caused by presence of such microorganisms like Thiothrix is possible.

Comment on “Control of activated sludge filamentous bulking—VI. Formulation of basic principles“ by J. Chudoba

Comment on “Control of activated sludge filamentous bulking—VI. Formulation of basic principles“ by J. Chudoba

Control of activated sludge filamentous bulking: Experimental verification of a kinetic selection theory

Two laboratory activated sludge systems, completely-mixed and selector-type, were continuously fed with a multi-component substrate. With excess biomass from both systems, the maximum substrate removal rates, r x.m, and the half-velocity coefficients, K s, were determined by means of a simple respirometric method. The following substrates were tested: glucose, galactose, acetic acid, valeric acid, citric acid, glutamic acid, alanine, tyrosine, methylalcohol, ethylalcohol and phenol. It has been found that both the r x.m and the K s are basically lower with the mixed cultures cultivated in a completely-mixed reactor (filamentous) than with those cultivated in a selector-type reactor (non-filamentous). The results have experimentally verified a kinetic selection theory in mixed cultures published previously.

Control of activated sludge filamentous bulking—VI. Formulation of basic principles

Overgrowing of filamentous micro-organisms in activated sludge is affected by the following factors: (a) composition of treated waste water; (b) actual concentration of dissolved oxygen in an aeration tank; (c) actual concentration of soluble substrate under which micro-organisms grow; (d) technological parameters of the process (sludge loading and age). All these factors were analysed and discussed in the light of the formulated basic principles for control of filamentous bulking.

Variation in Sludge Settleability with Temperature

Abstract This study examined the effect of temperature on the settleabi1ity of activated sludge at various organic loading rates. Five completely mixed, bench-scale, activated sludge plants, operating under similar conditions at 5, 10 and 19°C, were continuously fed diluted, settled sewage supplemented with carbohydrate (sucrose). Hydraulic loading rates, MLSS and pH were maintained at constant levels during the experiments to eliminate these factors are variables. Dissolved oxygen was kept in excess of 3 mg/1 so that it would not be a limiting factor. Sludge Volume Indices (SVI ) and zone settling velocities were used to indicate changes in sludge settleability. Microscopic examination of the activated sludge indicated significant differences in the morphological features of filamentous microorganisms present at the two temperature extremes. At 19°C, the predominant forms were characterized by long curving trichomes, occasionally falsely branching, containing short cylindrical cells. At 5 °C, much smaller straight filaments, composed of long, narrow, rod-shaped cells appeared to be the principal microorganisms responsible for bulking. Various other filamentous forms were always present at each of the temperatures studied. Stirred sludge settling tests of moderately bulking sludges generally exhibited much higher settling velocities and lower SVI's than unstirred bulking samples. However, extremely filamentous bulking sludge exhibited comparable stirred and unstirred settling velocity and SVI values. The standard SVI test was found to be an inadequate indicator of the extent of bulking when trying to correlate the SVI failures from bench-scale performance with the results from continuous units. Lower temperature had no appreciable effect on COD removal efficiency as long as bulking did not cause a loss of solids in the effluent. However, results suggested that less than half the organic load could be accepted at 5°C, that could be handled at 19°C, before filamentous bulking occurred. A plot of loading versus temperature for various SVI's provided a visual indication of the safe loading limit below which bulking was unlikely to occur. The study clearly demonstrated that temperature can have a significant effect on sludge settleability.

Control of activated sludge filamentous bulking—III. Effect of sludge loading

Previous experiments have indicated a possibility of controlling the filamentous bulking of activated sludge by cultivating a mixed culture in a system with the plug flow pattern or in a system with a selector. These experiments, however, were conducted at approximately constant sludge load [0·3–0·4 kg BOD 5(kg MLVSS day) −1] and sludge age (4·5–5·0 days) so that the results obtained had only a limited validity. Further experiments were, therefore, carried out in order to find if the previous findings hold also for activated sludge systems with high sludge loads and low sludge ages. The experiments were conducted on four activated sludge systems with different flow patterns: two of them as complete-mixing reactors and the other two as reactors with the plug flow pattern, having dispersion numbers of 0·08 and 0·07. A mixture of starch and peptone was used as the substrate. All systems were operated at sludge ages 2 and 3 days and at sludge loads from 0·5 to 2·3 kg BOD 5(kg MLVSS day) −1. It was found that at sludge loads above 0·5 kg kg −1 day −1 mixed cultures developed in the systems with the plug flow pattern possessed high values of the SVI and contained high portions of filamentous microorganisms. This was found in spite of sufficiently high concentration gradients of the substrate along these systems. It means that plug flow and, consequently, the concentration gradient of the substrate along the activated sludge system may not be the only factor in controlling the selection of microorganisms in a mixed culture. An additional factor must exert an influence on the population dynamics and selection of microorganisms in mixed cultures at high sludge loads and low sludge ages. The experiments have confirmed findings of others that values of the SVI depend on the sludge load. However, different trends of dependence were observed in the systems with different flow patterns. While in the complete-mixing systems the SVI decreased with increasing sludge loading in the systems with the plug flow pattern it increased. A greater phosphorus demand was observed in plug flow systems than in those with complete mixing.

Control of activated sludge filamentous bulking—I. Effect of the hydraulic regime or degree of mixing in an aeration tank

Laboratory experiments were conducted on four activated sludge systems with various flow patterns. The systems were characterized by dispersion numbers which are a numerical expression of the degree of mixing. Perfect mixing has a dispersion number of infinity. The dispersion numbers in the four systems were ∞, 1·06, 0·17 and 0·033. All systems were operated at the same detention period of 8 h and approximately at the same sludge loadings. A mixture of starch and peptone was used as the substrate. The mixed cultures developed in the systems mentioned possessed average SVI values of 517, 300, 91 and 51 ml g−1 respectively. The high SVI values were caused by a high content of the filamentous microorganisms, which consisted mainly of Leucothrix and Sphaerotilus, in the developed mixed cultures. It is concluded that the complete mixing systems tend to lead to excessive growth of the filamentous microorganisms. Aeration systems with a low degree of the axial mixing, i.e. with low dispersion numbers and higher concentration gradients of the substrate along the systems, lead to the growth of non-filamentous and suppress the growth of filamentous microorganisms. It is possible to control the growth of filamentous microorganisms in the mixed cultures by maintaining the concentration gradient of the substrate along the aeration system. The concentration gradient of the substrate depends on the degree of mixing in the aeration system.